U.S. patent application number 12/100408 was filed with the patent office on 2008-10-16 for sensing device having capacitive force sensor with saturated output at minimum capacitance.
This patent application is currently assigned to Silverbrook Research Pty Ltd. Invention is credited to Gary Shipton.
Application Number | 20080252615 12/100408 |
Document ID | / |
Family ID | 39852503 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080252615 |
Kind Code |
A1 |
Shipton; Gary |
October 16, 2008 |
SENSING DEVICE HAVING CAPACITIVE FORCE SENSOR WITH SATURATED OUTPUT
AT MINIMUM CAPACITANCE
Abstract
A sensing device is provided for interacting with a substrate.
The sensing device has a force sensor for detecting whether the
sensing device is interacting with the substrate and an image
sensor for sensing coded data disposed on the substrate. The force
sensor has a capacitive sensor circuit incorporating a sensor
capacitor having a variable capacitance C.sub.sen and a reference
capacitor relatively arranged so that the capacitive sensor circuit
is near saturation when C.sub.sen is at a minimum expected
value.
Inventors: |
Shipton; Gary; (Balmain,
AU) |
Correspondence
Address: |
SILVERBROOK RESEARCH PTY LTD
393 DARLING STREET
BALMAIN
2041
AU
|
Assignee: |
Silverbrook Research Pty
Ltd
|
Family ID: |
39852503 |
Appl. No.: |
12/100408 |
Filed: |
April 10, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60911260 |
Apr 11, 2007 |
|
|
|
Current U.S.
Class: |
345/174 |
Current CPC
Class: |
G01D 5/3473 20130101;
G01D 5/34746 20130101; G01L 1/146 20130101; G01L 1/144 20130101;
G06F 3/0383 20130101; G06F 3/03545 20130101 |
Class at
Publication: |
345/174 |
International
Class: |
G06F 3/041 20060101
G06F003/041 |
Claims
1. A sensing device for interacting with a substrate, said sensing
device comprising: a force sensor for detecting whether the sensing
device is interacting with the substrate; and an image sensor for
sensing coded data disposed on the substrate, said force sensor
comprising a capacitive sensor circuit incorporating a sensor
capacitor having a variable capacitance C.sub.sen and a reference
capacitor relatively arranged so that the capacitive sensor circuit
is near saturation when C.sub.sen is at a minimum expected
value.
2. A sensing device as claimed in claim 1 comprising a processor
configured for generating interaction data using the sensed coded
data.
3. A sensing device as claimed in claim 1 configured such that the
image sensor senses the coded data only when the force sensor
detects that the sensing device is interacting with the
substrate.
4. A sensing device as claimed in claim 1 selected from the group
comprising: a clicker lacking a nib or stylus, a pointer having a
non-marking stylus; and a pen having a marking nib.
5. A sensing device as claimed in claim 1, wherein the sensor
capacitor is a parallel plate capacitor having one fixed plate and
one movable plate.
6. A sensing device as claimed in claim 5, wherein the movable
plate is configured to move relative to the fixed plate due to a
pressure force exerted on the movable plate thereby altering
separation of the plates and said capacitance C.sub.sen.
7. A sensing device as claimed in claim 5, wherein a compression
spring is connected to the movable plate.
8. A sensing device as claimed in claim 1, wherein the capacitive
sensor circuit is a sigma-delta convertor circuit.
9. A sensing device as claimed in claim 8, wherein correlated
double sampling is applied to an integrator of the sigma-delta
convertor circuit.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to force sensors. It has been
developed primarily to provide force sensors for use in optical
sensing devices, such as pens, pointers and the like.
CO-PENDING APPLICATIONS
[0002] The following applications have been filed by the Applicant
simultaneously with the present application: [0003] NPS142US
NPS143US NPS144US
[0004] The disclosures of these co-pending applications are
incorporated herein by reference. The above applications have been
identified by their filing docket number, which will be substituted
with the corresponding application number, once assigned.
CROSS REFERENCES
[0005] The following patents or patent applications filed by the
applicant or assignee of the present invention are hereby
incorporated by cross-reference.
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BACKGROUND OF THE INVENTION
[0006] The Applicant has developed the Netpage system discussed in
detail below. As the invention is particularly well suited to
sensing devices for this system, it will be described in a Netpage
context. However, it will be appreciated that force sensors have
uses in many applications. In particular, hand-held optical sensors
have broad ranging applications in many different fields and the
invention is not limited to its use within the Netpage system or,
indeed, optical sensing devices.
[0007] The Netpage system involves the interaction between a user
and a computer network (or stand alone computer) via a pen and
paper based interface. The `pen` is an electronic stylus with a
marking or non-marking nib and an optical sensor for reading a
pattern of coded data on the paper (or other surface).
[0008] One of the primary features of the Netpage pen is its
ability to `click` on interactive elements on a Netpage in the same
way a mouse can click on screen-based interactive elements (e.g.
hyperlinks and so on). However, with a Netpage pen, the user simply
puts the nib on the interactive element in order to click on it.
The optical sensor identifies the element via its unique page and
location ID while a force sensor registers a `pen down` condition
when the nib is pressed against the page. Registering `pen down`
and `pen up` is also fundamental to capturing the user's
handwriting on Netpage input fields. Non-binary force signals are
also captured for reproducing hand-drawn strokes with varying
force-related width and opacity. Force variation can also be used
as one of the dimensions examined during signature
verification.
[0009] The force sensor is required to sense accurately relatively
light forces (such as the force of handwriting on a nib). It should
also require minimal power and be suitable for incorporation into a
handheld sensing device.
SUMMARY OF THE INVENTION
[0010] In a first aspect the present invention provides a force
sensor comprising a capacitive sensor circuit incorporating a
sensor capacitor having variable capacitance C.sub.sen arranged so
that an output of the capacitive sensor circuit is proportional to
1/C.sub.sen.
[0011] Optionally, the sensor capacitor is a parallel plate
capacitor having one fixed plate and one movable plate.
[0012] Optionally, the movable plate is configured to move relative
to the fixed plate due to a pressure force exerted on the movable
plate thereby altering separation of the plates and said
capacitance C.sub.sen.
[0013] Optionally, a compression spring is connected to the movable
plate.
[0014] Optionally, the sensor capacitor is positioned within a
capacitive feedback circuit of the capacitive sensor circuit such
that one of the plates of the sensor capacitor is connected
directly to the capacitive sensor circuit.
[0015] Optionally, the capacitive sensor circuit is a sigma-delta
convertor circuit.
[0016] In a second aspect the present invention provided a sensing
device for interacting with a substrate, said sensing device
comprising: a force sensor for detecting whether the sensing device
is interacting with the substrate; and an image sensor for sensing
coded data disposed on the substrate, said force sensor comprising
a capacitive sensor circuit incorporating a sensor capacitor having
variable capacitance C.sub.sen arranged so that an output of the
capacitive sensor circuit is proportional to 1/C.sub.sen.
[0017] In a further aspect there is provided a sensing device
comprising a processor configured for generating interaction data
using the sensed coded data.
[0018] In another aspect there is provided a sensing device
configured such that the image sensor senses the coded data only
when the force sensor detects that the sensing device is
interacting with the substrate.
[0019] In a further aspect there is provided a sensing device
selected from the group comprising: a clicker lacking a nib or
stylus, a pointer having a non-marking stylus; and a pen having a
marking nib.
[0020] Optionally, the sensor capacitor is a parallel plate
capacitor having one fixed plate and one movable plate.
[0021] Optionally, the movable plate is configured to move relative
to the fixed plate due to a pressure force exerted on the movable
plate thereby altering separation of the plates and said
capacitance C.sub.sen.
[0022] Optionally, a compression spring is connected to the movable
plate.
[0023] Optionally, the sensor capacitor is positioned within a
capacitive feedback circuit of the capacitive sensor circuit such
that one of the plates of the sensor capacitor is connected
directly to the capacitive sensor circuit.
[0024] Optionally, the capacitive sensor circuit is a sigma-delta
convertor circuit.
[0025] In a third aspect the present invention provided a force
sensor comprising a capacitive sensor circuit incorporating a
sensor capacitor having a variable capacitance C.sub.sen and a
reference capacitor relatively arranged so that the capacitive
sensor circuit is near saturation when C.sub.sen is at a minimum
expected value.
[0026] Optionally, the sensor capacitor is a parallel plate
capacitor having one fixed plate and one movable plate.
[0027] Optionally, the movable plate is configured to move relative
to the fixed plate due to a pressure force exerted on the movable
plate thereby altering separation of the plates and said
capacitance C.sub.sen.
[0028] Optionally, a compression spring is connected to the movable
plate.
[0029] Optionally, the capacitive sensor circuit is a sigma-delta
convertor circuit.
[0030] Optionally, correlated double sampling is applied to an
integrator of the sigma-delta convertor circuit.
[0031] In a fourth aspect the present invention provided a sensing
device for interacting with a substrate, said sensing device
comprising: a force sensor for detecting whether the sensing device
is interacting with the substrate; and an image sensor for sensing
coded data disposed on the substrate, said force sensor comprising
a capacitive sensor circuit incorporating a sensor capacitor having
a variable capacitance C.sub.sen and a reference capacitor
relatively arranged so that the capacitive sensor circuit is near
saturation when C.sub.sen is at a minimum expected value.
[0032] In a further aspect there is provided a sensing device
comprising a processor configured for generating interaction data
using the sensed coded data.
[0033] In another aspect there is provided a sensing device
configured such that the image sensor senses the coded data only
when the force sensor detects that the sensing device is
interacting with the substrate.
[0034] In a further aspect there is provided a sensing device
selected from the group comprising: a clicker lacking a nib or
stylus, a pointer having a non-marking stylus; and a pen having a
marking nib.
[0035] Optionally, the sensor capacitor is a parallel plate
capacitor having one fixed plate and one movable plate.
[0036] Optionally, the movable plate is configured to move relative
to the fixed plate due to a pressure force exerted on the movable
plate thereby altering separation of the plates and said
capacitance C.sub.sen.
[0037] Optionally, a compression spring is connected to the movable
plate.
[0038] Optionally, the capacitive sensor circuit is a sigma-delta
convertor circuit.
[0039] Optionally, correlated double sampling is applied to an
integrator of the sigma-delta convertor circuit.
BRIEF DESCRIPTION OF DRAWINGS
[0040] Optional embodiments of the invention will now be described,
by way of non-limiting example only, with reference to the
accompanying drawings, in which:
[0041] FIG. 1 shows an embodiment of basic netpage
architecture;
[0042] FIG. 2 is a schematic of a the relationship between a sample
printed netpage and its online page description;
[0043] FIG. 3 shows an embodiment of basic netpage architecture
with various alternatives for the relay device;
[0044] FIG. 3A illustrates a collection of netpage servers, Web
terminals, printers and relays interconnected via a network;
[0045] FIG. 4 is a schematic view of a high-level structure of a
printed netpage and its online page description;
[0046] FIG. 5A is a plan view showing a structure of a netpage
tag;
[0047] FIG. 5B is a plan view showing a relationship between a set
of the tags shown in FIG. 5a and a field of view of a netpage
sensing device in the form of a netpage pen;
[0048] FIG. 6A is a plan view showing an alternative structure of a
netpage tag;
[0049] FIG. 6B is a plan view showing a relationship between a set
of the tags shown in FIG. 6a and a field of view of a netpage
sensing device in the form of a netpage pen;
[0050] FIG. 6C is a plan view showing an arrangement of nine of the
tags shown in FIG. 6a where targets are shared between adjacent
tags;
[0051] FIG. 6D is a plan view showing the interleaving and rotation
of the symbols of the four codewords of the tag shown in FIG.
6A;
[0052] FIG. 7 is a flowchart of a tag image processing and decoding
algorithm;
[0053] FIG. 8 is a perspective view of a netpage pen and its
associated tag-sensing field-of-view cone;
[0054] FIG. 9 is a perspective exploded view of the netpage pen
shown in FIG. 8;
[0055] FIG. 10 is a schematic block diagram of a pen controller for
the netpage pen shown in FIGS. 8 and 9;
[0056] FIG. 11 is a schematic view of a pen class diagram;
[0057] FIG. 12 is a schematic view of a document and page
description class diagram;
[0058] FIG. 13 is a schematic view of a document and page ownership
class diagram;
[0059] FIG. 14 is a schematic view of a terminal element
specialization class diagram;
[0060] FIG. 15 is a schematic view of a static element
specialization class diagram;
[0061] FIG. 16 is a schematic view of a hyperlink element class
diagram;
[0062] FIG. 17 is a schematic view of a hyperlink element
specialization class diagram;
[0063] FIG. 18 is a schematic view of a hyperlinked group class
diagram;
[0064] FIG. 19 is a schematic view of a form class diagram;
[0065] FIG. 20 is a schematic view of a digital ink class
diagram;
[0066] FIG. 21 is a schematic view of a field element
specialization class diagram;
[0067] FIG. 22 is a schematic view of a checkbox field class
diagram;
[0068] FIG. 23 is a schematic view of a text field class
diagram;
[0069] FIG. 24 is a schematic view of a signature field class
diagram;
[0070] FIG. 25 is a flowchart of an input processing algorithm;
[0071] FIG. 25A is a detailed flowchart of one step of the
flowchart of FIG. 25;
[0072] FIG. 26 is a schematic view of a hyperlink request class
diagram;
[0073] FIG. 27 is a schematic view of a raw digital ink class
diagram;
[0074] FIG. 28 is a schematic of a parallel plate capacitor with
fixed and moveable plates;
[0075] FIG. 29 is a schematic of a conventional (proportional to
Csen) sensor;
[0076] FIG. 30 is a schematic of a 1/C sensor;
[0077] FIG. 31 illustrates output code versus applied force;
[0078] FIG. 32 illustrates measurement error due to
quantization;
[0079] FIG. 33 is a schematic of a second order 1/C CDC;
[0080] FIG. 34 is a schematic of a conventional improvement to the
second order circuit;
[0081] FIG. 35 is a schematic of an offset second order
circuit;
[0082] FIG. 36 illustrates CDS operation;
[0083] FIG. 37 shows error in C.sub.in1;
[0084] FIG. 38 shows error in C.sub.can;
[0085] FIG. 39 shows output of integrator 1 with leakage of .+-.50
nA and clk/2 chopping;
[0086] FIG. 40 shows the effect of sensor parallel resistance (18
Mohms);
[0087] FIG. 41 shows common mode voltage;
[0088] FIG. 42 shows summing point voltage; and
[0089] FIG. 43 shows a sensor equivalent circuit.
DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS
[0090] Note: Memjet.TM. is a trade mark of Silverbrook Research Pty
Ltd, Australia.
[0091] In the preferred embodiment, the invention is configured to
work with the netpage networked computer system, a detailed
overview of which follows. It will be appreciated that not every
implementation will necessarily embody all or even most of the
specific details and extensions discussed below in relation to the
basic system. However, the system is described in its most complete
form to reduce the need for external reference when attempting to
understand the context in which the preferred embodiments and
aspects of the present invention operate.
[0092] In brief summary, the preferred form of the netpage system
employs a computer interface in the form of a mapped surface, that
is, a physical surface which contains references to a map of the
surface maintained in a computer system. The map references can be
queried by an appropriate sensing device. Depending upon the
specific implementation, the map references may be encoded visibly
or invisibly, and defined in such a way that a local query on the
mapped surface yields an unambiguous map reference both within the
map and among different maps. The computer system can contain
information about features on the mapped surface, and such
information can be retrieved based on map references supplied by a
sensing device used with the mapped surface. The information thus
retrieved can take the form of actions which are initiated by the
computer system on behalf of the operator in response to the
operator's interaction with the surface features.
[0093] In its preferred form, the netpage system relies on the
production of, and human interaction with, netpages. These are
pages of text, graphics and images printed on ordinary paper, but
which work like interactive webpages. Information is encoded on
each page using ink which is substantially invisible to the unaided
human eye. The ink, however, and thereby the coded data, can be
sensed by an optically imaging sensing device and transmitted to
the netpage system. The sensing device may take the form of a
clicker (for clicking on a specific position on a surface), a
pointer having a stylus (for pointing or gesturing on a surface
using pointer strokes), or a pen having a marking nib (for marking
a surface with ink when pointing, gesturing or writing on the
surface).
[0094] In one embodiment, active buttons and hyperlinks on each
page can be clicked with the sensing device to request information
from the network or to signal preferences to a network server. In
one embodiment, text written by hand on a netpage is automatically
recognized and converted to computer text in the netpage system,
allowing forms to be filled in. In other embodiments, signatures
recorded on a netpage are automatically verified, allowing
e-commerce transactions to be securely authorized. In other
embodiments, text on a netpage may be clicked or gestured to
initiate a search based on keywords indicated by the user.
[0095] As illustrated in FIG. 2, a printed netpage 1 can represent
a interactive form which can be filled in by the user both
physically, on the printed page, and "electronically", via
communication between the pen and the netpage system. The example
shows a "Request" form containing name and address fields and a
submit button. The netpage consists of graphic data 2 printed using
visible ink, and coded data 3 printed as a collection of tags 4
using invisible ink. The corresponding page description 5, stored
on the netpage network, describes the individual elements of the
netpage. In particular it describes the type and spatial extent
(zone) of each interactive element (i.e. text field or button in
the example), to allow the netpage system to correctly interpret
input via the netpage. The submit button 6, for example, has a zone
7 which corresponds to the spatial extent of the corresponding
graphic 8.
[0096] As illustrated in FIGS. 1 and 3, netpage sensing device 101,
such as the pen shown in FIGS. 8 and 9 and described in more detail
below, works in conjunction with a netpage relay device 601, which
is an Internet-connected device for home, office or mobile use. The
pen is wireless and communicates securely with the netpage relay
device 601 via a short-range radio link 9. In an alternative
embodiment, the netpage pen 101 utilises a wired connection, such
as a USB or other serial connection, to the relay device 601.
[0097] The relay device 601 performs the basic function of relaying
interaction data to a page server 10, which interprets the
interaction data. As shown in FIG. 3, the relay device 601 may, for
example, take the form of a personal computer 601a, a netpage
printer 601b or some other relay 601c.
[0098] The netpage printer 601b is able to deliver, periodically or
on demand, personalized newspapers, magazines, catalogs, brochures
and other publications, all printed at high quality as interactive
netpages. Unlike a personal computer, the netpage printer is an
appliance which can be, for example, wall-mounted adjacent to an
area where the morning news is first consumed, such as in a user's
kitchen, near a breakfast table, or near the household's point of
departure for the day. It also comes in tabletop, desktop, portable
and miniature versions. Netpages printed on-demand at their point
of consumption combine the ease-of-use of paper with the timeliness
and interactivity of an interactive medium.
[0099] Alternatively, the netpage relay device 601 may be a
portable device, such as a mobile phone or PDA, a laptop or desktop
computer, or an information appliance connected to a shared
display, such as a TV. If the relay device 601 is not a netpage
printer 601b which prints netpages digitally and on demand, the
netpages may be printed by traditional analog printing presses,
using such techniques as offset lithography, flexography, screen
printing, relief printing and rotogravure, as well as by digital
printing presses, using techniques such as drop-on-demand inkjet,
continuous inkjet, dye transfer, and laser printing.
[0100] As shown in FIG. 3, the netpage sensing device 101 interacts
with the coded data on a printed netpage 1, or other printed
substate such as a label of a product item 251, and communicates,
via a short-range radio link 9, the interaction to the relay 601.
The relay 601 sends corresponding interaction data to the relevant
netpage page server 10 for interpretation. Raw data received from
the sensing device 101 may be relayed directly to the page server
10 as interaction data. Alternatively, the interaction data may be
encoded in the form of an interaction URI and transmitted to the
page server 10 via a user's web browser. Of course, the relay
device 601 (e.g. mobile phone) may incorporate a web browser and a
user display.
[0101] In appropriate circumstances, the page server sends a
corresponding message to application computer software running on a
netpage application server 13. The application server may in turn
send a response which is displayed on a user display device
associated with the relay 601, or printed on the originating
netpage printer.
[0102] The netpage relay device 601 can be configured to support
any number of sensing devices, and a sensing device can work with
any number of netpage relays. In the preferred implementation, each
netpage sensing device 101 has a unique identifier. This allows
each user to maintain a distinct profile with respect to a netpage
page server 10 or application server 13.
[0103] Digital, on-demand delivery of netpages 1 may be performed
by the netpage printer 601b, which exploits the growing
availability of broadband Internet access. Netpage publication
servers 14 on the netpage network are configured to deliver
print-quality publications to netpage printers. Periodical
publications are delivered automatically to subscribing netpage
printers via pointcasting and multicasting Internet protocols.
Personalized publications are filtered and formatted according to
individual user profiles.
[0104] A netpage pen may be registered with a netpage registration
server 11 and linked to one or more payment card accounts. This
allows e-commerce payments to be securely authorized using the
netpage pen. The netpage registration server compares the signature
captured by the netpage pen with a previously registered signature,
allowing it to authenticate the user's identity to an e-commerce
server. Other biometrics can also be used to verify identity. One
version of the netpage pen includes fingerprint scanning, verified
in a similar way by the netpage registration server.
Netpage System Architecture
[0105] Each object model in the system is described using a Unified
Modeling Language (UML) class diagram. A class diagram consists of
a set of object classes connected by relationships, and two kinds
of relationships are of interest here: associations and
generalizations. An association represents some kind of
relationship between objects, i.e. between instances of classes. A
generalization relates actual classes, and can be understood in the
following way: if a class is thought of as the set of all objects
of that class, and class A is a generalization of class B, then B
is simply a subset of A. The UML does not directly support
second-order modelling--i.e. classes of classes.
[0106] Each class is drawn as a rectangle labelled with the name of
the class. It contains a list of the attributes of the class,
separated from the name by a horizontal line, and a list of the
operations of the class, separated from the attribute list by a
horizontal line. In the class diagrams which follow, however,
operations are never modelled.
[0107] An association is drawn as a line joining two classes,
optionally labelled at either end with the multiplicity of the
association. The default multiplicity is one. An asterisk (*)
indicates a multiplicity of "many", i.e. zero or more. Each
association is optionally labelled with its name, and is also
optionally labelled at either end with the role of the
corresponding class. An open diamond indicates an aggregation
association ("is-part-of"), and is drawn at the aggregator end of
the association line.
[0108] A generalization relationship ("is-a") is drawn as a solid
line joining two classes, with an arrow (in the form of an open
triangle) at the generalization end.
[0109] When a class diagram is broken up into multiple diagrams,
any class which is duplicated is shown with a dashed outline in all
but the main diagram which defines it. It is shown with attributes
only where it is defined.
1 Netpages
[0110] Netpages are the foundation on which a netpage network is
built. They provide a paper-based user interface to published
information and interactive services.
[0111] A netpage consists of a printed page (or other surface
region) invisibly tagged with references to an online description
of the page. The online page description is maintained persistently
by the netpage page server 10. The page description describes the
visible layout and content of the page, including text, graphics
and images. It also describes the input elements on the page,
including buttons, hyperlinks, and input fields. A netpage allows
markings made with a netpage pen on its surface to be
simultaneously captured and processed by the netpage system.
[0112] Multiple netpages (for example, those printed by analog
printing presses) can share the same page description. However, to
allow input through otherwise identical pages to be distinguished,
each netpage may be assigned a unique page identifier. This page ID
has sufficient precision to distinguish between a very large number
of netpages.
[0113] Each reference to the page description is encoded in a
printed tag. The tag identifies the unique page on which it
appears, and thereby indirectly identifies the page description.
The tag also identifies its own position on the page.
Characteristics of the tags are described in more detail below.
[0114] Tags are typically printed in infrared-absorptive ink on any
substrate which is infrared-reflective, such as ordinary paper, or
in infrared fluorescing ink. Near-infrared wavelengths are
invisible to the human eye but are easily sensed by a solid-state
image sensor with an appropriate filter.
[0115] A tag is sensed by an area image sensor in the netpage
sensing device, and the tag data is transmitted to the netpage
system via the nearest netpage relay device. The pen is wireless
and communicates with the netpage relaye device via a short-range
radio link. Tags are sufficiently small and densely arranged that
the sensing device can reliably image at least one tag even on a
single click on the page. It is important that the pen recognize
the page ID and position on every interaction with the page, since
the interaction is stateless. Tags are error-correctably encoded to
make them partially tolerant to surface damage.
[0116] The netpage page server 10 maintains a unique page instance
for each unique printed netpage, allowing it to maintain a distinct
set of user-supplied values for input fields in the page
description for each printed netpage.
[0117] The relationship between the page description, the page
instance, and the printed netpage is shown in FIG. 4. The printed
netpage may be part of a printed netpage document 45. The page
instance may be associated with both the netpage printer which
printed it and, if known, the netpage user who requested it.
2 Netpage Tags
2.1 Tag Data Content
[0118] In a preferred form, each tag identifies the region in which
it appears, and the location of that tag within the region. A tag
may also contain flags which relate to the region as a whole or to
the tag. One or more flag bits may, for example, signal a tag
sensing device to provide feedback indicative of a function
associated with the immediate area of the tag, without the sensing
device having to refer to a description of the region. A netpage
pen may, for example, illuminate an "active area" LED when in the
zone of a hyperlink.
[0119] As will be more clearly explained below, in a preferred
embodiment, each tag contains an easily recognized invariant
structure which aids initial detection, and which assists in
minimizing the effect of any warp induced by the surface or by the
sensing process. The tags preferably tile the entire page, and are
sufficiently small and densely arranged that the pen can reliably
image at least one tag even on a single click on the page. It is
important that the pen recognize the page ID and position on every
interaction with the page, since the interaction is stateless.
[0120] In a preferred embodiment, the region to which a tag refers
coincides with an entire page, and the region ID encoded in the tag
is therefore synonymous with the page ID of the page on which the
tag appears. In other embodiments, the region to which a tag refers
can be an arbitrary subregion of a page or other surface. For
example, it can coincide with the zone of an interactive element,
in which case the region ID can directly identify the interactive
element.
TABLE-US-00002 TABLE 1 Tag data Field Precision (bits) Page
ID/Region ID 100 Tag ID/x-y coordinates 16 Flags 4 Total 120
[0121] Each tag contains 120 bits of information, typically
allocated as shown in Table 1. Assuming a maximum tag density of 64
per square inch, a 16-bit tag ID supports a region size of up to
1024 square inches. Larger regions can be mapped continuously
without increasing the tag ID precision simply by using abutting
regions and maps. The 100-bit region ID allows 2100
(.about.10.sup.30 or a million trillion trillion) different regions
to be uniquely identified.
2.2 Tag Data Encoding
[0122] The 120 bits of tag data are redundantly encoded using a
(15, 5) Reed-Solomon code. This yields 360 encoded bits consisting
of 6 codewords of 15 4-bit symbols each. The (15, 5) code allows up
to 5 symbol errors to be corrected per codeword, i.e. it is
tolerant of a symbol error rate of up to 33% per codeword.
[0123] Each 4-bit symbol is represented in a spatially coherent way
in the tag, and the symbols of the six codewords are interleaved
spatially within the tag. This ensures that a burst error (an error
affecting multiple spatially adjacent bits) damages a minimum
number of symbols overall and a minimum number of symbols in any
one codeword, thus maximising the likelihood that the burst error
can be fully corrected.
[0124] Any suitable error-correcting code can be used in place of a
(15, 5) Reed-Solomon code, for example a Reed-Solomon code with
more or less redundancy, with the same or different symbol and
codeword sizes; another block code; or a different kind of code,
such as a convolutional code (see, for example, Stephen B. Wicker,
Error Control Systems for Digital Communication and Storage,
Prentice-Hall 1995, the contents of which a herein incorporated by
cross-reference).
2.3 Physical Tag Structure
[0125] The physical representation of the tag, shown in FIG. 5a,
includes fixed target structures 15, 16, 17 and variable data areas
18. The fixed target structures allow a sensing device such as the
netpage pen to detect the tag and infer its three-dimensional
orientation relative to the sensor. The data areas contain
representations of the individual bits of the encoded tag data.
[0126] To achieve proper tag reproduction, the tag is rendered at a
resolution of 256.times.256 dots. When printed at 1600 dots per
inch this yields a tag with a diameter of about 4 mm. At this
resolution the tag is designed to be surrounded by a "quiet area"
of radius 16 dots. Since the quiet area is also contributed by
adjacent tags, it only adds 16 dots to the effective diameter of
the tag.
[0127] The tag includes six target structures. A detection ring 15
allows the sensing device to initially detect the tag. The ring is
easy to detect because it is rotationally invariant and because a
simple correction of its aspect ratio removes most of the effects
of perspective distortion. An orientation axis 16 allows the
sensing device to determine the approximate planar orientation of
the tag due to the yaw of the sensor. The orientation axis is
skewed to yield a unique orientation. Four perspective targets 17
allow the sensing device to infer an accurate two-dimensional
perspective transform of the tag and hence an accurate
three-dimensional position and orientation of the tag relative to
the sensor.
[0128] All target structures are redundantly large to improve their
immunity to noise.
[0129] The overall tag shape is circular. This supports, amongst
other things, optimal tag packing on an irregular triangular grid.
In combination with the circular detection ring, this makes a
circular arrangement of data bits within the tag optimal. To
maximise its size, each data bit is represented by a radial wedge
in the form of an area bounded by two radial lines and two
concentric circular arcs. Each wedge has a minimum dimension of 8
dots at 1600 dpi and is designed so that its base (its inner arc),
is at least equal to this minimum dimension. The height of the
wedge in the radial direction is always equal to the minimum
dimension. Each 4-bit data symbol is represented by an array of
2.times.2 wedges.
[0130] The 15 4-bit data symbols of each of the six codewords are
allocated to the four concentric symbol rings 18a to 18d in
interleaved fashion. Symbols are allocated alternately in circular
progression around the tag.
[0131] The interleaving is designed to maximise the average spatial
distance between any two symbols of the same codeword.
[0132] In order to support "single-click" interaction with a tagged
region via a sensing device, the sensing device must be able to see
at least one entire tag in its field of view no matter where in the
region or at what orientation it is positioned. The required
diameter of the field of view of the sensing device is therefore a
function of the size and spacing of the tags.
[0133] Assuming a circular tag shape, the minimum diameter of the
sensor field of view is obtained when the tags are tiled on a
equilateral triangular grid, as shown in FIG. 5b.
2.4 Tag Image Processing and Decoding
[0134] The tag image processing and decoding performed by a sensing
device such as the netpage pen is shown in FIG. 7. While a captured
image is being acquired from the image sensor, the dynamic range of
the image is determined (at 20). The center of the range is then
chosen as the binary threshold for the image 21. The image is then
thresholded and segmented into connected pixel regions (i.e. shapes
23) (at 22). Shapes which are too small to represent tag target
structures are discarded. The size and centroid of each shape is
also computed.
[0135] Binary shape moments 25 are then computed (at 24) for each
shape, and these provide the basis for subsequently locating target
structures. Central shape moments are by their nature invariant of
position, and can be easily made invariant of scale, aspect ratio
and rotation.
[0136] The ring target structure 15 is the first to be located (at
26). A ring has the advantage of being very well behaved when
perspective-distorted. Matching proceeds by aspect-normalizing and
rotation-normalizing each shape's moments. Once its second-order
moments are normalized the ring is easy to recognize even if the
perspective distortion was significant. The ring's original aspect
and rotation 27 together provide a useful approximation of the
perspective transform.
[0137] The axis target structure 16 is the next to be located (at
28). Matching proceeds by applying the ring's normalizations to
each shape's moments, and rotation-normalizing the resulting
moments. Once its second-order moments are normalized the axis
target is easily recognized. Note that one third order moment is
required to disambiguate the two possible orientations of the axis.
The shape is deliberately skewed to one side to make this possible.
Note also that it is only possible to rotation-normalize the axis
target after it has had the ring's normalizations applied, since
the perspective distortion can hide the axis target's axis. The
axis target's original rotation provides a useful approximation of
the tag's rotation due to pen yaw 29.
[0138] The four perspective target structures 17 are the last to be
located (at 30). Good estimates of their positions are computed
based on their known spatial relationships to the ring and axis
targets, the aspect and rotation of the ring, and the rotation of
the axis. Matching proceeds by applying the ring's normalizations
to each shape's moments. Once their second-order moments are
normalized the circular perspective targets are easy to recognize,
and the target closest to each estimated position is taken as a
match. The original centroids of the four perspective targets are
then taken to be the perspective-distorted corners 31 of a square
of known size in tag space, and an eight-degree-of-freedom
perspective transform 33 is inferred (at 32) based on solving the
well-understood equations relating the four tag-space and
image-space point pairs (see Heckbert, P., Fundamentals of Texture
Mapping and Image Warping, Masters Thesis, Dept. of EECS, U. of
California at Berkeley, Technical Report No. UCB/CSD 89/516, June
1989, the contents of which are herein incorporated by
cross-reference).
[0139] The inferred tag-space to image-space perspective transform
is used to project (at 36) each known data bit position in tag
space into image space where the real-valued position is used to
bilinearly interpolate (at 36) the four relevant adjacent pixels in
the input image. The previously computed image threshold 21 is used
to threshold the result to produce the final bit value 37.
[0140] Once all 360 data bits 37 have been obtained in this way,
each of the six 60-bit Reed-Solomon codewords is decoded (at 38) to
yield 20 decoded bits 39, or 120 decoded bits in total. Note that
the codeword symbols are sampled in codeword order, so that
codewords are implicitly de-interleaved during the sampling
process.
[0141] The ring target 15 is only sought in a subarea of the image
whose relationship to the image guarantees that the ring, if found,
is part of a complete tag. If a complete tag is not found and
successfully decoded, then no pen position is recorded for the
current frame. Given adequate processing power and ideally a
non-minimal field of view 193, an alternative strategy involves
seeking another tag in the current image.
[0142] The obtained tag data indicates the identity of the region
containing the tag and the position of the tag within the region.
An accurate position 35 of the pen nib in the region, as well as
the overall orientation 35 of the pen, is then inferred (at 34)
from the perspective transform 33 observed on the tag and the known
spatial relationship between the pen's physical axis and the pen's
optical axis.
2.5 Alternative Tag Structures
[0143] The tag structure described above is designed to support the
tagging of non-planar surfaces where a regular tiling of tags may
not be possible. In the more usual case of planar surfaces where a
regular tiling of tags is possible, i.e. surfaces such as sheets of
paper and the like, more efficient tag structures can be used which
exploit the regular nature of the tiling.
[0144] FIG. 6a shows a square tag 4 with four perspective targets
17. It is similar in structure to tags described by Bennett et al.
in U.S. Pat. No. 5,051,746. The tag represents sixty 4-bit
Reed-Solomon symbols 47, for a total of 240 bits. The tag
represents each one bit as a dot 48, and each zero bit by the
absence of the corresponding dot. The perspective targets are
designed to be shared between adjacent tags, as shown in FIGS. 6b
and 6c. FIG. 6b shows a square tiling of 16 tags and the
corresponding minimum field of view 193, which must span the
diagonals of two tags. FIG. 6c shows a square tiling of nine tags,
containing all one bits for illustration purposes.
[0145] Using a (15, 7) Reed-Solomon code, 112 bits of tag data are
redundantly encoded to produce 240 encoded bits. The four codewords
are interleaved spatially within the tag to maximize resilience to
burst errors. Assuming a 16-bit tag ID as before, this allows a
region ID of up to 92 bits.
[0146] The data-bearing dots 48 of the tag are designed to not
overlap their neighbors, so that groups of tags cannot produce
structures which resemble targets. This also saves ink. The
perspective targets therefore allow detection of the tag, so
further targets are not required. Tag image processing proceeds as
described in section 1.2.4 above, with the exception that steps 26
and 28 are omitted.
[0147] Although the tag may contain an orientation feature to allow
disambiguation of the four possible orientations of the tag
relative to the sensor, it is also possible to embed orientation
data in the tag data. For example, the four codewords can be
arranged so that each tag orientation contains one codeword placed
at that orientation, as shown in FIG. 6d, where each symbol is
labelled with the number of its codeword (1-4) and the position of
the symbol within the codeword (A-O). Tag decoding then consists of
decoding one codeword at each orientation. Each codeword can either
contain a single bit indicating whether it is the first codeword,
or two bits indicating which codeword it is. The latter approach
has the advantage that if, say, the data content of only one
codeword is required, then at most two codewords need to be decoded
to obtain the desired data. This may be the case if the region ID
is not expected to change within a stroke and is thus only decoded
at the start of a stroke. Within a stroke only the codeword
containing the tag ID is then desired. Furthermore, since the
rotation of the sensing device changes slowly and predictably
within a stroke, only one codeword typically needs to be decoded
per frame.
[0148] It is possible to dispense with perspective targets
altogether and instead rely on the data representation being
self-registering. In this case each bit value (or multi-bit value)
is typically represented by an explicit glyph, i.e. no bit value is
represented by the absence of a glyph. This ensures that the data
grid is well-populated, and thus allows the grid to be reliably
identified and its perspective distortion detected and subsequently
corrected during data sampling. To allow tag boundaries to be
detected, each tag data must contain a marker pattern, and these
must be redundantly encoded to allow reliable detection. The
overhead of such marker patterns is similar to the overhead of
explicit perspective targets. One such scheme uses dots positioned
a various points relative to grid vertices to represent different
glyphs and hence different multi-bit values (see Anoto Technology
Description, Anoto April 2000).
[0149] Additional tag structures are disclosed in U.S. Pat. No.
6,929,186 ("Orientation-indicating machine-readable coded data")
filed by the applicant or assignee of the present invention.
2.6 Tag Map
[0150] Decoding a tag results in a region ID, a tag ID, and a
tag-relative pen transform. Before the tag ID and the tag-relative
pen location can be translated into an absolute location within the
tagged region, the location of the tag within the region must be
known. This is given by a tag map, a function which maps each tag
ID in a tagged region to a corresponding location. The tag map
class diagram is shown in FIG. 22, as part of the netpage printer
class diagram.
[0151] A tag map reflects the scheme used to tile the surface
region with tags, and this can vary according to surface type. When
multiple tagged regions share the same tiling scheme and the same
tag numbering scheme, they can also share the same tag map.
[0152] The tag map for a region must be retrievable via the region
ID. Thus, given a region ID, a tag ID and a pen transform, the tag
map can be retrieved, the tag ID can be translated into an absolute
tag location within the region, and the tag-relative pen location
can be added to the tag location to yield an absolute pen location
within the region.
[0153] The tag ID may have a structure which assists translation
through the tag map. It may, for example, encode Cartesian (x-y)
coordinates or polar coordinates, depending on the surface type on
which it appears. The tag ID structure is dictated by and known to
the tag map, and tag IDs associated with different tag maps may
therefore have different structures.
2.7 Tagging Schemes
[0154] The preferred coding scheme uses "location-indicating" tags
as already discussed. An alternative coding scheme uses
object-indicating tags.
[0155] A location-indicating tag contains a tag ID which, when
translated through the tag map associated with the tagged region,
yields a unique tag location within the region. The tag-relative
location of the pen is added to this tag location to yield the
location of the pen within the region. This in turn is used to
determine the location of the pen relative to a user interface
element in the page description associated with the region. Not
only is the user interface element itself identified, but a
location relative to the user interface element is identified.
Location-indicating tags therefore trivially support the capture of
an absolute pen path in the zone of a particular user interface
element.
[0156] An object-indicating tag contains a tag ID which directly
identifies a user interface element in the page description
associated with the region. All the tags in the zone of the user
interface element identify the user interface element, making them
all identical and therefore indistinguishable. Object-indicating
tags do not, therefore, support the capture of an absolute pen
path. They do, however, support the capture of a relative pen path.
So long as the position sampling frequency exceeds twice the
encountered tag frequency, the displacement from one sampled pen
position to the next within a stroke can be unambiguously
determined.
[0157] With either tagging scheme, the tags function in cooperation
with associated visual elements on the netpage as user interactive
elements in that a user can interact with the printed page using an
appropriate sensing device in order for tag data to be read by the
sensing device and for an appropriate response to be generated in
the netpage system.
3 Document and Page Descriptions
[0158] A preferred embodiment of a document and page description
class diagram is shown in FIGS. 12 and 13.
[0159] In the netpage system a document is described at three
levels. At the most abstract level the document 836 has a
hierarchical structure whose terminal elements 839 are associated
with content objects 840 such as text objects, text style objects,
image objects, etc. Once the document is printed on a printer with
a particular page size, the document is paginated and otherwise
formatted. Formatted terminal elements 835 will in some cases be
associated with content objects which are different from those
associated with their corresponding terminal elements, particularly
where the content objects are style-related. Each printed instance
of a document and page is also described separately, to allow input
captured through a particular page instance 830 to be recorded
separately from input captured through other instances of the same
page description.
[0160] The presence of the most abstract document description on
the page server allows a a copy of a document to be printed without
being forced to accept the source document's specific format. The
user or a printing press may be requesting a copy for a printer
with a different page size, for example. Conversely, the presence
of the formatted document description on the page server allows the
page server to efficiently interpret user actions on a particular
printed page.
[0161] A formatted document 834 consists of a set of formatted page
descriptions 5, each of which consists of a set of formatted
terminal elements 835. Each formatted element has a spatial extent
or zone 58 on the page. This defines the active area of input
elements such as hyperlinks and input fields.
[0162] A document instance 831 corresponds to a formatted document
834. It consists of a set of page instances 830, each of which
corresponds to a page description 5 of the formatted document. Each
page instance 830 describes a single unique printed netpage 1, and
records the page ID 50 of the netpage. A page instance is not part
of a document instance if it represents a copy of a page requested
in isolation.
[0163] A page instance consists of a set of terminal element
instances 832. An element instance only exists if it records
instance-specific information. Thus, a hyperlink instance exists
for a hyperlink element because it records a transaction ID 55
which is specific to the page instance, and a field instance exists
for a field element because it records input specific to the page
instance. An element instance does not exist, however, for static
elements such as textflows.
[0164] A terminal element 839 can be a visual element or an input
element. A visual element can be a static element 843 or a dyamnic
element 846. An input element can be a hyperlink element 844 or a
field element 845, as shown in FIG. 14. A static element 843 can be
a style element 847 with an associated style object 854, a textflow
element 848 with an associated styled text object 855, an image
element 849 with an associated image element 856, a graphic element
850 with an associated graphic object 857, a video clip element 851
with an associated video clip object 858, an audio clip element 852
with an associated audio clip object 859, or a script element 853
with an associated script object 860, as shown in FIG. 15.
[0165] A page instance has a background field 833 which is used to
record any digital ink captured on the page which does not apply to
a specific input element.
[0166] In the preferred form of the invention, a tag map 811 is
associated with each page instance to allow tags on the page to be
translated into locations on the page.
4 The Netpage Network
[0167] In a preferred embodiment, a netpage network consists of a
distributed set of netpage page servers 10, netpage registration
servers 11, netpage ID servers 12, netpage application servers 13,
and netpage relay devices 601 connected via a network 19 such as
the Internet, as shown in FIG. 3.
[0168] The netpage registration server 11 is a server which records
relationships between users, pens, printers and applications, and
thereby authorizes various network activities. It authenticates
users and acts as a signing proxy on behalf of authenticated users
in application transactions. It also provides handwriting
recognition services. As described above, a netpage page server 10
maintains persistent information about page descriptions and page
instances. The netpage network includes any number of page servers,
each handling a subset of page instances. Since a page server also
maintains user input values for each page instance, clients such as
netpage relays 601 send netpage input directly to the appropriate
page server. The page server interprets any such input relative to
the description of the corresponding page.
[0169] A netpage ID server 12 allocates document IDs 51 on demand,
and provides load-balancing of page servers via its ID allocation
scheme.
[0170] A netpage relay uses the Internet Distributed Name System
(DNS), or similar, to resolve a netpage page ID 50 into the network
address of the netpage page server handling the corresponding page
instance.
[0171] A netpage application server 13 is a server which hosts
interactive netpage applications.
[0172] Netpage servers can be hosted on a variety of network server
platforms from manufacturers such as IBM, Hewlett-Packard, and Sun.
Multiple netpage servers can run concurrently on a single host, and
a single server can be distributed over a number of hosts. Some or
all of the functionality provided by netpage servers, and in
particular the functionality provided by the ID server and the page
server, can also be provided directly in a netpage appliance such
as a netpage printer, in a computer workstation, or on a local
network.
5 The Netpage Pen
[0173] The active sensing device of the netpage system may take the
form of a clicker (for clicking on a specific position on a
surface), a pointer having a stylus (for pointing or gesturing on a
surface using pointer strokes), or a pen having a marking nib (for
marking a surface with ink when pointing, gesturing or writing on
the surface). A pen 101 is described herein, although it will be
appreciated that clickers and pointers may be of similar
construction. The pen 101 uses its embedded controller 134 to
capture and decode netpage tags from a page via an image sensor.
The image sensor is a solid-state device provided with an
appropriate filter to permit sensing at only near-infrared
wavelengths. As described in more detail below, the system is able
to sense when the nib is in contact with the surface, and the pen
is able to sense tags at a sufficient rate to capture human
handwriting (i.e. at 200 dpi or greater and 100 Hz or faster).
Information captured by the pen may be encrypted and wirelessly
transmitted to the printer (or base station), the printer or base
station interpreting the data with respect to the (known) page
structure.
[0174] The preferred embodiment of the netpage pen 101 operates
both as a normal marking ink pen and as a non-marking stylus (i.e.
as a pointer). The marking aspect, however, is not necessary for
using the netpage system as a browsing system, such as when it is
used as an Internet interface. Each netpage pen is registered with
the netpage system and has a unique pen ID 61. FIG. 11 shows the
netpage pen class diagram, reflecting pen-related information
maintained by a registration server 11 on the netpage network.
[0175] When the nib is in contact with a netpage, the pen
determines its position and orientation relative to the page. The
nib is attached to a force sensor, and the force on the nib is
interpreted relative to a threshold to indicate whether the pen is
"up" or "down". This allows an interactive element on the page to
be `clicked` by pressing with the pen nib, in order to request,
say, information from a network. Furthermore, the force is captured
as a continuous value to allow, say, the full dynamics of a
signature to be verified. The force sensor is described in more
detail in Section 8.
[0176] The pen determines the position and orientation of its nib
on the netpage by imaging, in the infrared spectrum, an area 193 of
the page in the vicinity of the nib. It decodes the nearest tag and
computes the position of the nib relative to the tag from the
observed perspective distortion on the imaged tag and the known
geometry of the pen optics. Although the position resolution of the
tag may be low, because the tag density on the page is inversely
proportional to the tag size, the adjusted position resolution is
quite high, exceeding the minimum resolution required for accurate
handwriting recognition.
[0177] Pen actions relative to a netpage are captured as a series
of strokes. A stroke consists of a sequence of time-stamped pen
positions on the page, initiated by a pen-down event and completed
by the subsequent pen-up event. A stroke is also tagged with the
page ID 50 of the netpage whenever the page ID changes, which,
under normal circumstances, is at the commencement of the
stroke.
[0178] Each netpage pen has a current selection 826 associated with
it, allowing the user to perform copy and paste operations etc. The
selection is timestamped to allow the system to discard it after a
defined time period. The current selection describes a region of a
page instance. It consists of the most recent digital ink stroke
captured through the pen relative to the background area of the
page. It is interpreted in an application-specific manner once it
is submitted to an application via a selection hyperlink
activation.
[0179] Each pen has a current nib 824. This is the nib last
notified by the pen to the system. In the case of the default
netpage pen described above, either the marking black ink nib or
the non-marking stylus nib is current. Each pen also has a current
nib style 825. This is the nib style last associated with the pen
by an application, e.g. in response to the user selecting a color
from a palette. The default nib style is the nib style associated
with the current nib. Strokes captured through a pen are tagged
with the current nib style. When the strokes are subsequently
reproduced, they are reproduced in the nib style with which they
are tagged.
[0180] The pen 101 may have one or more buttons 209, which are
pressed by the user to select a mode of the pen. The button(s) are
used to determine a behavior of the pen, which, in turn, determines
how a stroke is interpreted by the page server 10.
[0181] Whenever the pen is within range of a relay device 601 with
which it can communicate, the pen slowly flashes its "online" LED.
When the pen fails to decode a stroke relative to the page, it
momentarily activates its "error" LED. When the pen succeeds in
decoding a stroke relative to the page, it momentarily activates
its "ok" LED.
[0182] A sequence of captured strokes is referred to as digital
ink. Digital ink forms the basis for the digital exchange of
drawings and handwriting, for online recognition of handwriting,
and for online verification of signatures.
[0183] The pen is wireless and transmits digital ink to the relay
device 601 via a short-range radio link. The transmitted digital
ink is encrypted for privacy and security and packetized for
efficient transmission, but is always flushed on a pen-up event to
ensure timely handling in the printer.
[0184] When the pen is out-of-range of a relay device 601 it
buffers digital ink in internal memory, which has a capacity of
over ten minutes of continuous handwriting. When the pen is once
again within range of a relay device, it transfers any buffered
digital ink.
[0185] A pen can be registered with any number of relay devices,
but because all state data resides in netpages both on paper and on
the network, it is largely immaterial which relay device a pen is
communicating with at any particular time.
[0186] One embodiment of the pen is described in greater detail in
Section 7 below, with reference to FIGS. 8 to 10.
6 Netpage Interaction
[0187] The netpage relay device 601 receives data relating to a
stroke from the pen 101 when the pen is used to interact with a
netpage 1. The coded data 3 of the tags 4 is read by the pen when
it is used to execute a movement, such as a stroke. The data allows
the identity of the particular page to be determined and an
indication of the positioning of the pen relative to the page to be
obtained. Interaction data comprising the page ID 50 and at least
one position of the pen, is transmitted to the relay device 601,
where it resolves, via the DNS, the page ID 50 of the stroke into
the network address of the netpage page server 10 which maintains
the corresponding page instance 830. It then transmits the stroke
to the page server. If the page was recently identified in an
earlier stroke, then the relay device may already have the address
of the relevant page server in its cache. Each netpage consists of
a compact page layout maintained persistently by a netpage page
server (see below). The page layout refers to objects such as
images, fonts and pieces of text, typically stored elsewhere on the
netpage network.
[0188] When the page server receives the stroke from the pen, it
retrieves the page description to which the stroke applies, and
determines which element of the page description the stroke
intersects. It is then able to interpret the stroke in the context
of the type of the relevant element.
[0189] A "click" is a stroke where the distance and time between
the pen down position and the subsequent pen up position are both
less than some small maximum. An object which is activated by a
click typically requires a click to be activated, and accordingly,
a longer stroke is ignored. The failure of a pen action, such as a
"sloppy" click, to register may be indicated by the lack of
response from the pen's "ok" LED.
[0190] There are two kinds of input elements in a netpage page
description: hyperlinks and form fields. Input through a form field
can also trigger the activation of an associated hyperlink.
6.1 Hyperlinks
[0191] A hyperlink is a means of sending a message to a remote
application, and typically elicits a displayed or printed response
in the netpage system.
[0192] A hyperlink element 844 identifies the application 71 which
handles activation of the hyperlink, a link ID 54 which identifies
the hyperlink to the application, an "alias required" flag which
asks the system to include the user's application alias ID 65 in
the hyperlink activation, and a description which is used when the
hyperlink is recorded as a favorite or appears in the user's
history. The hyperlink element class diagram is shown in FIG.
16.
[0193] When a hyperlink is activated, the page server sends a
request to an application somewhere on the network. The application
is identified by an application ID 64, and the application ID is
resolved in the normal way via the DNS. There are three types of
hyperlinks: general hyperlinks 863, form hyperlinks 865, and
selection hyperlinks 864, as shown in FIG. 17. A general hyperlink
can implement a request for a linked document, or may simply signal
a preference to a server. A form hyperlink submits the
corresponding form to the application. A selection hyperlink
submits the current selection to the application. If the current
selection contains a single-word piece of text, for example, the
application may return a single-page document giving the word's
meaning within the context in which it appears, or a translation
into a different language. Each hyperlink type is characterized by
what information is submitted to the application.
[0194] The corresponding hyperlink instance 862 records a
transaction ID 55 which can be specific to the page instance on
which the hyperlink instance appears. The transaction ID can
identify user-specific data to the application, for example a
"shopping cart" of pending purchases maintained by a purchasing
application on behalf of the user.
[0195] The system includes the pen's current selection 826 in a
selection hyperlink activation. The system includes the content of
the associated form instance 868 in a form hyperlink activation,
although if the hyperlink has its "submit delta" attribute set,
only input since the last form submission is included. The system
includes an effective return path in all hyperlink activations.
[0196] A hyperlinked group 866 is a group element 838 which has an
associated hyperlink, as shown in FIG. 18. When input occurs
through any field element in the group, the hyperlink 844
associated with the group is activated. A hyperlinked group can be
used to associate hyperlink behavior with a field such as a
checkbox. It can also be used, in conjunction with the "submit
delta" attribute of a form hyperlink, to provide continuous input
to an application. It can therefore be used to support a
"blackboard" interaction model, i.e. where input is captured and
therefore shared as soon as it occurs.
6.2 Hyperlink Activation Protocol
[0197] A preferred embodiment of a hyperlink activation protocol is
shown in FIG. 42.
[0198] When a user clicks on a netpage with a netpage pen, the pen
communicates the click, in the form of interaction data, to the
nearest netpage relay device 601. The click identifies the page and
a location on the page. The relay device 601 already knows the ID
61 of the pen from the pen connection protocol.
[0199] The relay device 601 determines, via the DNS, the network
address of the page server 10 handling the particular page ID 50.
The address may already be in its cache if the user has recently
interacted with the same page. The relay device 601 then forwards
the pen ID, its own device ID 62, the page ID and click location to
the page server.
[0200] The page server loads the page description 5 identified by
the page ID and determines which input element's zone 58, if any,
the click lies in. Assuming the relevant input element is a
hyperlink element 844, the page server then obtains the associated
application ID 64 and link ID 54, and determines, via the DNS, the
network address of the application server hosting the application
71.
[0201] The page server uses the pen ID 61 to obtain the
corresponding user ID 60 from the registration server 11, and then
allocates a globally unique hyperlink request ID 52 and builds a
hyperlink request 934. The hyperlink request class diagram is shown
in FIG. 26. The hyperlink request records the IDs of the requesting
user and relay device, and identifies the clicked hyperlink
instance 862. The page server then sends its own server ID 53, the
hyperlink request ID, and the link ID to the application.
[0202] The application produces a response document according to
application-specific logic, and obtains a document ID 51 from an ID
server 12. It then sends the document to the page server 10b
responsible for the document's newly allocated ID, together with
the requesting page server's ID and the hyperlink request ID.
[0203] The second page server sends the hyperlink request ID and
application ID to the first page server to obtain the corresponding
user ID and device ID 62. The first page server rejects the request
if the hyperlink request has expired or is for a different
application.
[0204] The second page server allocates document instance and page
IDs 50, returns the newly allocated page IDs to the application,
adds the complete document to its own database, and finally sends
the page descriptions to the requesting relay device.
[0205] The hyperlink instance may include a meaningful transaction
ID 55, in which case the first page server includes the transaction
ID in the message sent to the application. This allows the
application to establish a transaction-specific context for the
hyperlink activation.
[0206] If the hyperlink requires a user alias, i.e. its "alias
required" attribute is set, then the first page server sends both
the pen ID 61 and the hyperlink's application ID 64 to the
registration server 11 to obtain not just the user ID corresponding
to the pen ID but also the alias ID 65 corresponding to the
application ID and the user ID. It includes the alias ID in the
message sent to the application, allowing the application to
establish a user-specific context for the hyperlink activation.
6.3 Forms
[0207] A form defines a collection of related input fields used to
capture a related set of inputs through a printed netpage. A form
allows a user to submit one or more parameters to an application
software program running on a server.
[0208] A form 867 is a group element 838 in the document hierarchy.
It ultimately contains a set of terminal field elements 839. A form
instance 868 represents a printed instance of a form. It consists
of a set of field instances 870 which correspond to the field
elements 845 of the form. Each field instance has an associated
value 871, whose type depends on the type of the corresponding
field element. Each field value records input through a particular
printed form instance, i.e. through one or more printed netpages.
The form class diagram is shown in FIG. 19.
[0209] Each form instance has a status 872 which indicates whether
the form is active, frozen, submitted, void or expired. A form is
active when first printed. A form becomes frozen once it is signed
or once its freeze time is reached. A form becomes submitted once
one of its submission hyperlinks has been activated, unless the
hyperlink has its "submit delta" attribute set. A form becomes void
when the user invokes a void form, reset form or duplicate form
page command. A form expires when its specified expiry time is
reached, i.e. when the time the form has been active exceeds the
form's specified lifetime. While the form is active, form input is
allowed. Input through a form which is not active is instead
captured in the background field 833 of the relevant page instance.
When the form is active or frozen, form submission is allowed. Any
attempt to submit a form when the form is not active or frozen is
rejected, and instead elicits an form status report.
[0210] Each form instance is associated (at 59) with any form
instances derived from it, thus providing a version history. This
allows all but the latest version of a form in a particular time
period to be excluded from a search.
[0211] All input is captured as digital ink. Digital ink 873
consists of a set of timestamped stroke groups 874, each of which
consists of a set of styled strokes 875. Each stroke consists of a
set of timestamped pen positions 876, each of which also includes
pen orientation and nib force. The digital ink class diagram is
shown in FIG. 20.
[0212] A field element 845 can be a checkbox field 877, a text
field 878, a drawing field 879, or a signature field 880. The field
element class diagram is shown in FIG. 21. Any digital ink captured
in a field's zone 58 is assigned to the field.
[0213] A checkbox field has an associated boolean value 881, as
shown in FIG. 22. Any mark (a tick, a cross, a stroke, a fill
zigzag, etc.) captured in a checkbox field's zone causes a true
value to be assigned to the field's value.
[0214] A text field has an associated text value 882, as shown in
FIG. 23. Any digital ink captured in a text field's zone is
automatically converted to text via online handwriting recognition,
and the text is assigned to the field's value. Online handwriting
recognition is well-understood (see, for example, Tappert, C., C.
Y. Suen and T. Wakahara, "The State of the Art in On-Line
Handwriting Recognition", IEEE Transactions on Pattern Analysis and
Machine Intelligence, Vol. 12, No. 8, August 1990, the contents of
which are herein incorporated by cross-reference).
[0215] A signature field has an associated digital signature value
883, as shown in FIG. 24. Any digital ink captured in a signature
field's zone is automatically verified with respect to the identity
of the owner of the pen, and a digital signature of the content of
the form of which the field is part is generated and assigned to
the field's value. The digital signature is generated using the pen
user's private signature key specific to the application which owns
the form. Online signature verification is well-understood (see,
for example, Plamondon, R. and G. Lorette, "Automatic Signature
Verification and Writer Identification--The State of the Art",
Pattern Recognition, Vol. 22, No. 2, 1989, the contents of which
are herein incorporated by cross-reference).
[0216] A field element is hidden if its "hidden" attribute is set.
A hidden field element does not have an input zone on a page and
does not accept input. It can have an associated field value which
is included in the form data when the form containing the field is
submitted.
[0217] "Editing" commands, such as strike-throughs indicating
deletion, can also be recognized in form fields.
[0218] Because the handwriting recognition algorithm works "online"
(i.e. with access to the dynamics of the pen movement), rather than
"offline" (i.e. with access only to a bitmap of pen markings), it
can recognize run-on discretely-written characters with relatively
high accuracy, without a writer-dependent training phase. A
writer-dependent model of handwriting is automatically generated
over time, however, and can be generated up-front if necessary,
[0219] Digital ink, as already stated, consists of a sequence of
strokes. Any stroke which starts in a particular element's zone is
appended to that element's digital ink stream, ready for
interpretation. Any stroke not appended to an object's digital ink
stream is appended to the background field's digital ink
stream.
[0220] Digital ink captured in the background field is interpreted
as a selection gesture. Circumscription of one or more objects is
generally interpreted as a selection of the circumscribed objects,
although the actual interpretation is application-specific.
[0221] Table 2 summarizes some typical pen interactions with a
netpage.
TABLE-US-00003 TABLE 2 Summary of typical pen interactions with a
netpage Object Type Pen input Action Hyperlink General Click Submit
action to application Form Click Submit form to application
Selection Click Submit selection to application Form field Checkbox
Any mark Assign true to field Text Handwriting Convert digital ink
to text; assign text to field Drawing Digital ink Assign digital
ink to field Signature Signature Verify digital ink signature;
generate digital signature of form; assign digital signature to
field None -- Circumscription Assign digital ink to current
selection
[0222] The system maintains a current selection for each pen. The
selection consists simply of the most recent stroke captured in the
background field. The selection is cleared after an inactivity
timeout to ensure predictable behavior.
[0223] The raw digital ink captured in every field is retained on
the netpage page server and is optionally transmitted with the form
data when the form is submitted to the application. This allows the
application to interrogate the raw digital ink should it suspect
the original conversion, such as the conversion of handwritten
text. This can, for example, involve human intervention at the
application level for forms which fail certain application-specific
consistency checks. As an extension to this, the entire background
area of a form can be designated as a drawing field. The
application can then decide, on the basis of the presence of
digital ink outside the explicit fields of the form, to route the
form to a human operator, on the assumption that the user may have
indicated amendments to the filled-in fields outside of those
fields.
[0224] FIG. 25 shows a flowchart of the process of handling pen
input relative to a netpage. The process consists of receiving (at
884) a stroke from the pen; identifying (at 885) the page instance
830 to which the page ID 50 in the stroke refers; retrieving (at
886) the page description 5; identifying (at 887) a formatted
element 839 whose zone 58 the stroke intersects; determining (at
888) whether the formatted element corresponds to a field element,
and if so appending (at 892) the received stroke to the digital ink
of the field value 871, interpreting (at 893) the accumulated
digital ink of the field, and determining (at 894) whether the
field is part of a hyperlinked group 866 and if so activating (at
895) the associated hyperlink; alternatively determining (at 889)
whether the formatted element corresponds to a hyperlink element
and if so activating (at 895) the corresponding hyperlink;
alternatively, in the absence of an input field or hyperlink,
appending (at 890) the received stroke to the digital ink of the
background field 833; and copying (at 891) the received stroke to
the current selection 826 of the current pen, as maintained by the
registration server.
[0225] FIG. 25A shows a detailed flowchart of step 893 in the
process shown in FIG. 25, where the accumulated digital ink of a
field is interpreted according to the type of the field. The
process consists of determining (at 896) whether the field is a
checkbox and (at 897) whether the digital ink represents a
checkmark, and if so assigning (at 898) a true value to the field
value; alternatively determining (at 899) whether the field is a
text field and if so converting (at 900) the digital ink to
computer text, with the help of the appropriate registration
server, and assigning (at 901) the converted computer text to the
field value; alternatively determining (at 902) whether the field
is a signature field and if so verifying (at 903) the digital ink
as the signature of the pen's owner, with the help of the
appropriate registration server, creating (at 904) a digital
signature of the contents of the corresponding form, also with the
help of the registration server and using the pen owner's private
signature key relating to the corresponding application, and
assigning (at 905) the digital signature to the field value.
7 Netpage Pen and Printer Description
7.1 Pen Mechanics
[0226] Referring to FIGS. 8 and 9, the pen, generally designated by
reference numeral 101, includes a housing 102 in the form of a
plastics moulding having walls 103 defining an interior space 104
for mounting the pen components. Mode selector buttons 209 are
provided on the housing 102. The pen top 105 is in operation
rotatably mounted at one end 106 of the housing 102. A
semi-transparent cover 107 is secured to the opposite end 108 of
the housing 102. The cover 107 is also of moulded plastics, and is
formed from semi-transparent material in order to enable the user
to view the status of the LED mounted within the housing 102. The
cover 107 includes a main part 109 which substantially surrounds
the end 108 of the housing 102 and a projecting portion 110 which
projects back from the main part 109 and fits within a
corresponding slot 111 formed in the walls 103 of the housing 102.
A radio antenna 112 is mounted behind the projecting portion 110,
within the housing 102. Screw threads 113 surrounding an aperture
113A on the cover 107 are arranged to receive a metal end piece
114, including corresponding screw threads 115. The metal end piece
114 is removable to enable ink cartridge replacement.
[0227] Also mounted within the cover 107 is a tri-color status LED
116 on a flex PCB 117. The antenna 112 is also mounted on the flex
PCB 117. The status LED 116 is mounted at the top of the pen 101
for good all-around visibility.
[0228] The pen can operate both as a normal marking ink pen and as
a non-marking stylus. An ink pen cartridge 118 with nib 119 and a
stylus 120 with stylus nib 121 are mounted side by side within the
housing 102. Either the ink cartridge nib 119 or the stylus nib 121
can be brought forward through open end 122 of the metal end piece
114, by rotation of the pen top 105. Respective slider blocks 123
and 124 are mounted to the ink cartridge 118 and stylus 120,
respectively. A rotatable cam barrel 125 is secured to the pen top
105 in operation and arranged to rotate therewith. The cam barrel
125 includes a cam 126 in the form of a slot within the walls 181
of the cam barrel. Cam followers 127 and 128 projecting from slider
blocks 123 and 124 fit within the cam slot 126. On rotation of the
cam barrel 125, the slider blocks 123 or 124 move relative to each
other to project either the pen nib 119 or stylus nib 121 out
through the hole 122 in the metal end piece 114. The pen 101 has
three states of operation. By turning the top 105 through
90.degree. steps, the three states are: [0229] Stylus 120 nib 121
out; [0230] Ink cartridge 118 nib 119 out; and [0231] Neither ink
cartridge 118 nib 119 out nor stylus 120 nib 121 out.
[0232] A second flex PCB 129, is mounted on an electronics chassis
130 which sits within the housing 102. The second flex PCB 129
mounts an infrared LED 131 for providing infrared radiation for
projection onto the surface. An image sensor 132 is provided
mounted on the second flex PCB 129 for receiving reflected
radiation from the surface. The second flex PCB 129 also mounts a
radio frequency chip 133, which includes an RF transmitter and RF
receiver, and a controller chip 134 for controlling operation of
the pen 101. An optics block 135 (formed from moulded clear
plastics) sits within the cover 107 and projects an infrared beam
onto the surface and receives images onto the image sensor 132.
Power supply wires 136 connect the components on the second flex
PCB 129 to battery contacts 137 which are mounted within the cam
barrel 125. A terminal 138 connects to the battery contacts 137 and
the cam barrel 125. A three volt rechargeable battery 139 sits
within the cam barrel 125 in contact with the battery contacts. An
induction charging coil 140 is mounted about the second flex PCB
129 to enable recharging of the battery 139 via induction. The
second flex PCB 129 also mounts a force sensor 143 for detecting
displacement in the cam barrel 125 when either the stylus 120 or
the ink cartridge 118 is used for writing, in order to enable a
determination of the force being applied to the surface by the pen
nib 119 or stylus nib 121. Exemplary arrangements for the force
sensor 143 are discussed in detail in Section 8.
[0233] Rubber grip pads 141 and 142 are provided towards the end
108 of the housing 102 to assist gripping the pen 101, and top 105
also includes a clip 142 for clipping the pen 101 to a pocket.
7.2 Pen Controller
[0234] The pen 101 is arranged to determine the position of its nib
(stylus nib 121 or ink cartridge nib 119) by imaging, in the
infrared spectrum, an area of the surface in the vicinity of the
nib. It records the location data from the nearest location tag,
and is arranged to calculate the distance of the nib 121 or 119
from the location tab utilising optics 135 and controller chip 134.
The controller chip 134 calculates the orientation of the pen and
the nib-to-tag distance from the perspective distortion observed on
the imaged tag.
[0235] Utilising the RF chip 133 and antenna 112 the pen 101 can
transmit the digital ink data (which is encrypted for security and
packaged for efficient transmission) to the computing system.
[0236] When the pen is in range of a relay device 601, the digital
ink data is transmitted as it is formed. When the pen 101 moves out
of range, digital ink data is buffered within the pen 101 (the pen
101 circuitry includes a buffer arranged to store digital ink data
for approximately 12 minutes of the pen motion on the surface) and
can be transmitted later.
[0237] In Applicant's U.S. Pat. No. 6,870,966, the contents of
which is incorporated herein by reference, a pen 101 having an
interchangeable ink cartridge nib and stylus nib was described.
Accordingly, and referring to FIG. 27, when the pen 101 connects to
the computing system, the controller 134 notifies the system of the
pen ID, nib ID 175, current absolute time 176, and the last
absolute time it obtained from the system prior to going offline.
The pen ID allows the computing system to identify the pen when
there is more than one pen being operated with the computing
system.
[0238] The nib ID allows the computing system to identify which nib
(stylus nib 121 or ink cartridge nib 119) is presently being used.
The computing system can vary its operation depending upon which
nib is being used. For example, if the ink cartridge nib 119 is
being used the computing system may defer producing feedback output
because immediate feedback is provided by the ink markings made on
the surface. Where the stylus nib 121 is being used, the computing
system may produce immediate feedback output.
[0239] Since a user may change the nib 119, 121 between one stroke
and the next, the pen 101 optionally records a nib ID for a stroke
175. This becomes the nib ID implicitly associated with later
strokes.
[0240] Cartridges having particular nib characteristics may be
interchangeable in the pen. The pen controller 134 may interrogate
a cartridge to obtain the nib ID 175 of the cartridge. The nib ID
175 may be stored in a ROM or a barcode on the cartridge. The
controller 134 notifies the system of the nib ID whenever it
changes. The system is thereby able to determine the
characteristics of the nib used to produce a stroke 175, and is
thereby subsequently able to reproduce the characteristics of the
stroke itself.
[0241] The controller chip 134 is mounted on the second flex PCB
129 in the pen 101. FIG. 10 is a block diagram illustrating in more
detail the architecture of the controller chip 134. FIG. 10 also
shows representations of the RF chip 133, the image sensor 132, the
tri-color status LED 116, the IR illumination LED 131 and the force
sensor 143.
[0242] The pen controller chip 134 includes a controlling processor
145. Bus 146 enables the exchange of data between components of the
controller chip 134. Flash memory 147 and a 512 KB DRAM 148 are
also included. An analog-to-digital converter 149 is arranged to
convert the analog signal from the force sensor 143 to a digital
signal.
[0243] An image sensor interface 152 interfaces with the image
sensor 132. A transceiver controller 153 and base band circuit 154
are also included to interface with the RF chip 133 which includes
an RF circuit 155 and RF resonators and inductors 156 connected to
the antenna 112.
[0244] The controlling processor 145 captures and decodes location
data from tags from the surface via the image sensor 132, monitors
the force sensor 143, controls the LEDs 116 and 131, and handles
short-range radio communication via the radio transceiver 153. It
is a medium-performance (.about.40 MHz) general-purpose RISC
processor.
[0245] The processor 145, digital transceiver components
(transceiver controller 153 and baseband circuit 154), image sensor
interface 152, flash memory 147 and 512 KB DRAM 148 are integrated
in a single controller ASIC. Analog RF components (RF circuit 155
and RF resonators and inductors 156) are provided in the separate
RF chip. The image sensor is a 215.times.215 pixel CCD (such a
sensor is produced by Matsushita Electronic Corporation, and is
described in a paper by Itakura, K T Nobusada, N Okusenya, R
Nagayoshi, and M Ozaki, "A 1 mm 50 k-Pixel IT CCD Image Sensor for
Miniature Camera System", IEEE Transactions on Electronic Devices,
Volt 47, number 1, January 2000, which is incorporated herein by
reference) with an IR filter.
[0246] The controller ASIC 134 enters a quiescent state after a
period of inactivity when the pen 101 is not in contact with a
surface. It incorporates a dedicated circuit 150 which monitors the
force sensor 143 and wakes up the controller 134 via the power
manager 151 on a pen-down event.
[0247] The radio transceiver communicates in the unlicensed 900 MHz
band normally used by cordless telephones, or alternatively in the
unlicensed 2.4 GHz industrial, scientific and medical (ISM) band,
and uses frequency hopping and collision detection to provide
interference-free communication.
[0248] In an alternative embodiment, the pen incorporates an
Infrared Data Association (IrDA) interface for short-range
communication with a base station or netpage printer.
[0249] In a further embodiment, the pen 101 includes a pair of
orthogonal accelerometers mounted in the normal plane of the pen
101 axis. The accelerometers 190 are shown in FIGS. 9 and 10 in
ghost outline.
[0250] The provision of the accelerometers enables this embodiment
of the pen 101 to sense motion without reference to surface
location tags, allowing the location tags to be sampled at a lower
rate. Each location tag ID can then identify an object of interest
rather than a position on the surface. For example, if the object
is a user interface input element (e.g. a command button), then the
tag ID of each location tag within the area of the input element
can directly identify the input element.
[0251] The acceleration measured by the accelerometers in each of
the x and y directions is integrated with respect to time to
produce an instantaneous velocity and position.
[0252] Since the starting position of the stroke is not known, only
relative positions within a stroke are calculated. Although
position integration accumulates errors in the sensed acceleration,
accelerometers typically have high resolution, and the time
duration of a stroke, over which errors accumulate, is short.
8 Force Sensor
[0253] A force sensor suitable for connection to the nib of the pen
is a parallel plate capacitor having one fixed plate and one
moveable plate that is attached to at least one spring, as
illustrated in FIG. 28. An applied force compresses the spring
changing the distance between the plates and thus the capacitance.
By measuring the change in capacitance the amount of applied force
can be known.
[0254] In FIG. 28, two springs are depicted in order to assist in
balanced alteration of the plate separation, however one spring or
more than two springs can be used. Further, in FIG. 28 the springs
are depicted as being located between the fixed and movable plates
merely for illustrative purposes, and the spring(s) can be
externally located with respect to the plates. Furthermore, one of
ordinary skill in the art understands that other parallel capacitor
arrangements, or other mechanical arrangements, are possible for
the force sensor. One of ordinary skill in the art also understands
that this exemplary force sensor can be applied to situations and
environments other than a Netpage system.
[0255] The variable capacitance of such the parallel plate
capacitor of FIG. 28 scan be approximated by:
C sen = K 1 d ( EQ 1 ) ##EQU00001##
where K.sub.1 is a constant for a given sensor and d is the
distance between the plates. The distance d is controlled by the
applied force f to a spring and one arrangement is for the force to
be measured to be applied to the spring which compresses it and
reduces the distance d in proportion to the applied force.
Then:
d = K 2 - f K 3 ( EQ 2 ) ##EQU00002##
where K.sub.2 is the initial distance between the plates with no
applied force and K.sub.3 is related to the spring constant.
Then:
C sen = K 1 K 3 ( K 2 K 3 - f ) ( EQ 3 ) ##EQU00003##
[0256] One way to measure capacitance C.sub.sen, to thereby
determine the applied force f, is to use a capacitive sensor
circuit, such as a delta-sigma modulator or convertor. A
conventional delta-sigma convertor is illustrated in FIG. 29 and a
first example of a delta-sigma modulator in accordance with an
embodiment of the present invention is illustrated in FIG. 30. Both
cases are illustrated in simplified form, i.e., switches of the
switched capacitor circuits are not shown. This is also the case
for the below described further circuit diagrams.
[0257] In the circuit of FIG. 29, an excitation signal Vex is
applied to a parallel plate capacitor Csen and a signal bs is a
bitstream whose mean value, Vout, represents the output of the
convertor. A decimation filter is used to convert the single (or
multi) bit signal bs to a multi-bit digital signal which is the
output of the convertor.
[0258] When the circuit is in balance (i.e., C.sub.sen=0), the
delta-sigma convertor adjusts its bitstream duty cycle to be 50:50.
The gain of an integrator int is controlled with a feedback
capacitor Cf which allows the output to remain in a linear range.
The sensitivity to capacitance change in the capacitor Csen is
controlled by a reference capacitor Cref. When the squarewave
excitation signal Vex and the output Vout of a comparator have the
same voltage swing, the conversion gain becomes independent of the
swing. The convertor output may then be expressed as:
V dig = K dig ( C sen C ref ) ( EQ 4 ) ##EQU00004##
where K.sub.dig is the gain of the decimation filter at the output.
Then:
f meas = K 4 - K 5 V dig ( EQ 5 ) ##EQU00005##
where K.sub.4=K.sub.2K.sub.3 and
K.sub.5=K.sub.1K.sub.3K.sub.dig/C.sub.ref.
[0259] The calculation of the measured force f.sub.meas from
V.sub.dig then requires a division and subtraction.
[0260] In the embodiment of the present invention, this calculation
is avoided by placing the parallel plate capacitor C.sub.sen in a
feedback path of a delta-sigma convertor, as illustrated FIG. 30.
In particular, the positions of the parallel plate capacitor Csen
and the reference capacitor Cref are reversed from the positions in
the conventional convertor circuit such that one of the plates of
the capacitor Csen is connected directly to the output of the
convertor and the reference capacitor Cref is at the input of the
convertor.
[0261] Now with a decimation filter at the output of the convertor
which produces V.sub.dig as a uni-polar signal, the bitstream can
be inverted to provide subtraction so that:
V dig = K dig ( 1 - C ref C sen ) then : ( EQ 6 ) f meas = K 6 V
dig where ( EQ 7 ) K 6 = f max - f min V digmax - V digmin ( EQ 8 )
##EQU00006##
[0262] If C.sub.ref=C.sub.senmin which occurs when f=0, then:
K 6 = K 2 K 3 K dig ( EQ 9 ) ##EQU00007##
[0263] As can be seen from EQs 4 and 6, the conventional convertor
arrangement produces an output which is proportional to C whereas
the arrangement of the present invention produces an output which
is proportional to 1/C which is proportional to distance.
Accordingly, the arrangement of the present invention allows a
larger change in capacitance to be measured for smaller changes in
distance as compared to the conventional arrangement. As a result
the force sensor of the present invention is able to accurately
sense relatively light forces, such as the force of handwriting on
the nib of the sensing device whilst requiring minimal power.
[0264] Results of an exemplary comparison are illustrated in FIGS.
31 and 33. In this example, a 10 bit convertor in which the gains
have been adjusted such that both types of convertor cover the same
force and output code ranges is used. FIG. 31 shows the non-linear
output code versus applied force for the proportional to C
convertor and FIG. 32 shows that the error due to quantization
increases with decreasing force for the proportional to C convertor
whereas the error remains constant for the 1/C convertor. That is,
the proportional to C arrangement loses resolution at the low force
end of the range (which is the most important for accuracy in the
Netpage pen application).
[0265] The above-described arrangement of the present invention is
a first order sigma-delta capacitive sensor. However, higher order
sensors are possible and within the scope of the present invention.
For example, a possible second order sigma-delta capacitive sensor
is illustrated in FIG. 33.
[0266] The second order sensor employs zero force trimming through
the addition of capacitor Ccan as illustrated in FIG. 33, which
allows any parasitic capacitance across the capacitor Csen to be
cancelled. This arrangement has more parameters that can be
optimized than the first order arrangement. The low pass response
of the convertor to input signals becomes more obvious and the
cut-off frequency is proportional to Csen, i.e., with the minimum
value of Csen, at nib-up, the convertor provides the most
filtering.
[0267] A further improvement to the first (or second) order
arrangement is explained in relation to FIGS. 34 and 35. A
conventional circuit is illustrated in FIG. 34, in which correlated
double sampling (CDS) has been applied to reduce offsets. As
C.sub.sen increases the conventional delta-sigma convertor will
reach a balanced state with a 50% duty cycle on the signal bs.
Thus, only half (i.e., 0% to 50%) of the conversion range is
available.
[0268] On the other hand, in the further arrangement of the present
invention illustrated in FIG. 35, CDS is again applied but the
signal bs at the capacitor Csen is replaced by 0 (zero), which
changes the balanced state of the convertor to a 100% duty cycle.
Accordingly, the entire conversion range is made available.
[0269] In particular, the capacitor Cref is used to set the offset
which must be adjusted so that the sigma-delta convertor is near
saturation when C.sub.sen is at the minimum expected value. When
CDS is used in the integrator int in conjunction with the zero
input signal at the capacitor Csen, the bitstream applied to the
capacitor Csen is changed in a way that allows substantially the
full range of the convertor to be used.
[0270] CDS operates on two phases as demonstrated in FIG. 36, being
sample and evaluate, and the input to the integrator int (through
the capacitor Csen) is V.sub.int=V.sub.eval=V.sub.samp. It is
assumed that the signal bs can take the values 0 and 1 and that the
average V.sub.int=0 for the circuit to be balanced. V.sub.int is
calculated in Table 3 for the conventional circuit of FIG. 34 and
in Table 4 for the offset circuit of FIG. 35.
TABLE-US-00004 TABLE 3 Calculation for conventional circuit Bs cds
= 1 cds = 0 V.sub.int bsb - bs 0 1 - 0 1 1 0 - 1 -1
Thus for balance, the signal bs requires a duty cycle of 50% to
maintain the average V.sub.int=0.
TABLE-US-00005 TABLE 4 Calculation for offset circuit Bs cds = 1
cds = 0 V.sub.int bsb - 0 0 1 - 0 1 1 0 - 0 0
Thus for balance, the signal bs=1 (100% of the time) to maintain
the average V.sub.int=0.
8.1 Behavioral Modeling
[0271] Some requirements of the force sensor for the Netpage system
are:
[0272] Pen down delay <1.5 ms;
[0273] Final sampling rate 50 Hz to 150 Hz;
[0274] Output 10 bits monotonic;
[0275] Power consumption <100 uW (55 uA@1.8V);
[0276] Operating temperature range -10.degree. C. to 55.degree.
C.;
[0277] Pen down detection at 6 grams; and
[0278] Accuracy <20%.
[0279] VHDL modeling was carried out with these requirements as
considerations.
8.1.1 First Order VHDL
[0280] A VHDL model written for a first order sigma-delta
capacitive sensor explores the proportional to distance (1/C)
arrangement of the present invention.
[0281] A test bench calculated EQ 1, with d linearly stepped in
increments of five distance units (except the first value was 0.5
instead of 0). This simulation resulted in a set of linear steps in
response to C.sub.sen with C.sub.f=8 pF and C.sub.ref=0.49 pF. The
amplitude of the output of the integrator int is controlled by the
capacitor Csen and is saturated when C.sub.sen=20 pF.
[0282] The output voltage is given by C.sub.ref/C.sub.sen (e.g.,
0.08 pF/0.1 pF=0.8V). This configuration only allows the offset to
be set (the feedback capacitor Cf only controls the integrator
gain). The gain could be reduced by connecting another capacitor in
parallel with the capacitor Csen but this will introduce
non-linearity.
[0283] The feedback capacitor Cf allows the integrator dynamic
range to be optimized. Provided the integrator is well behaved if
clipping is allowed, the minimum value for C.sub.f should be used
so that clipping occurs at the maximum value of C.sub.sen.
[0284] Both arrangements could be implemented with a select bit to
control the mode. Due to mechanical tolerances, any practical
sensor will require two point calibration to set the offset and
gain.
8.1.2 Second Order VHDL
[0285] As previously discussed, the second order arrangement of
FIG. 33 has more parameters that can be optimized than the first
order design.
[0286] The dc level at the output of integrator int1 can be
controlled by the ratio C.sub.bs2/C.sub.in2. This allows some
freedom in the setting the gain of the integrator int1 which should
be as high as possible without clipping. It is important that no
clipping occurs with pen-up otherwise the margin to the pen-down
threshold is reduced. C.sub.in1 is set so that the convertor is as
near to the lower end of the conversion range without clipping. Too
low a duty cycle in the bitstream results in very large excersions
at integrator int2. Some allowance needs to be made for long term
drift in the zero force capacitance of the sensor to ensure it
always stays within the range of the ADC.
[0287] In the model, the sensor is assumed to provide a 3:1
capacitance change with plate distance of the capacitor Csen in the
range of 50 um to 150 um. The capacitance at 50 um was 1 pF. Then
capacitance (in pF) is given by:
C sen = 50 d ( EQ 10 ) ##EQU00008##
where d(um) is the plate distance. The value of d is given by:
d = 150 - f 5 Then : ( EQ 11 ) C sen = 250 ( 750 - f ) ( EQ 12 )
##EQU00009##
[0288] The value of C.sub.can is adjusted to cancel the parasitic
capacitance across the sensor. The remaining error, due to
quantization of C.sub.can, results in a gain error. On the other
hand, the gain accuracy requirement is <20%. If the resolution
of C.sub.can were 100 fF and the remaining offset taken out with
C.sub.in1, the convertor would read 455 g instead of 500 g or a
gain error of -9% (which could be corrected for in software).
[0289] Three decimators were tried; normally a third order
decimator should be used with a second order convertor for optimum
noise filtering for which the step response takes about three
samples to settle. To meet the pen-down detection time requirement
of <2 ms, the output sample rate was set to 1.5 kHz. A
decimation factor of 42 was used with an input sampling clock of 64
kHz. A first order decimator cannot provide the required 10 bits of
resolution with this decimation factor. A second order decimator
can provide 10.8 bits of resolution and could, on average, detect
pen-down one sample earlier at 1.33 ms but with less certainty.
[0290] The implementation uses CDS to cancel OTA offsets, the most
important being that of the integrator intl. CDS should also
attenuate the 1/f noise coming from the capacitor Csen and
capacitor Cin1. However, there is a problem when adding an
anti-alias filter to the input of integrator 1 as it must pass
chopped CDS signals. Even with a normalized cut-off of three, ADC
gain is reduced by about 5%. Furthermore, unwanted signals at the
input will be aliased to dc when they occur at harmonics of the
sampling frequency. This is an unusual switched capacitor circuit
since one of the switched capacitors is external and the anti-alias
filter can only be placed inside the switched capacitor circuit.
The same is true for a non-CDS implementation, the anti-alias
filter must allow the integrator input current to settle within
half a clock cycle. Therefore, the function of this filter can only
be to attenuate rf signals well above the sampling frequency. It
cannot be used to attenuate signals just above the Nyquist
frequency.
8.1.3 Digital Output Calculation
[0291] This section gives analytical expressions to calculate the
CDC output code.
8.1.3.1 Decimator Gain
[0292] The decimator gain, which is the same for both modes of the
convertor, is given by:
K dig = dec n scale ( EQ 13 ) ##EQU00010##
where dec=decimation factor, n=order of decimator, scale=scale
factor at decimator output and must be a power of 2. The digital
output is 10 bits and scale must be adjusted to make the useful
data fit within these 10 bits (or allow some clipping if this is
acceptable). Typically the sigma-delta CDC should only be operated
between 10% and 90% of full scale to avoid internal clipping so the
decimator output will not reach full scale.
[0293] For example, with F.sub.in=64 kHz and F.sub.out=1.5 kHz,
dec=42 and n=3. This would give a maximum output of 42.sup.3=74088
but the maximum allowable output code is 1023. If scale=128, the
maximum output would be 579 which is <1023. However, assuming
the maximum analogue signal range is up to 80% of full scale, scale
can be set to 64 so then 74088*0.8/64=926<1024 (range) making
better use of the digital range. In this case,
K.sub.dig=74088/64=1157.625.
[0294] A digital offset is provided to centre the CDC range within
the output range, so the lower 10% of the CDC range which will not
be used can be removed.
8.1.3.2 Proportional to C
[0295] The following applies for the second order:
V dig = K dig ( ( C sen - C can ) C in 1 ) ( EQ 14 )
##EQU00011##
[0296] C.sub.in1 sets the conversion gain and C.sub.can allows the
range of C.sub.sen to be centred in the available conversion range.
V.sub.dig represents the scaled and offset sensor capacitance. The
term within the outer parenthesis should lie between 0.1 and
0.9.
[0297] The force is calculated as:
f = K 1 - K 2 V dig ( EQ 15 ) ##EQU00012##
[0298] from EQ 12, C.sub.sen in terms of force is given by:
C sen = K 4 ( K 5 - f ) ( EQ 16 ) ##EQU00013##
[0299] Substituting for C.sub.sen in EQ 14 and assuming no offset,
K.sub.1=K.sub.5 and K.sub.2=K.sub.4K.sub.dig/C.sub.in1 so that:
f = K 5 - K 4 K dig C in 1 V dig ( EQ 17 ) ##EQU00014##
8.1.3.3 Proportional to 1/C
[0300] The following applies for the second order:
V dig = K dig ( 1 - C in 1 ( C sen - C can ) ) ( EQ 18 )
##EQU00015##
[0301] The term within the outer parenthesis should lie between 0.1
and 0.9. The force is calculated from (assuming no offset):
f=K.sub.3V.sub.dig (EQ 19)
[0302] K.sub.3 is the slope of the CDC given by:
K 3 = f max - f min V dig max - V dig min ( EQ 20 )
##EQU00016##
where f.sub.min (0 g) is the minimum force and f.sub.max (500 g)
the maximum.
[0303] Substituting for C.sub.sen from EQ 16 in EQ 17 and for
V.sub.dig in EQ 20 yields:
K 3 = K 4 K dig C in 1 ( EQ 21 ) ##EQU00017##
[0304] For example, assuming C.sub.in1=C.sub.sensmin=1/3 pf, then
K.sub.3=250.times.64.times.3/74088=0.64788. Note that in practice,
C.sub.in1<C.sub.sensmin so that the CDC does not clip.
[0305] An error in the value of C.sub.in1 results in both offset
and gain errors at the CDC output. Modifying EQ12 to allow an error
e in the value of C.sub.in1 gives:
K 3 e = K 4 K dig ( C in 1 + e ) ( EQ 22 ) ##EQU00018##
[0306] Then using the following approximation:
1 ( K + x ) .apprxeq. 1 K - x K 2 ( EQ 23 ) then : K 3 e = K 4 K
dig ( 1 C in 1 - e C in 1 2 ) ( EQ 24 ) ##EQU00019##
the slope error is then given by:
K 3 - K 3 e = K 4 e K dig C in 1 2 ( EQ 25 ) ##EQU00020##
or expressed as a percentage error as:
er = 100 e C in 1 ( EQ 26 ) ##EQU00021##
[0307] If e=1 fF, the gain error is 0.3% for C.sub.in1=1/3pF.
8.1.4 MatLab
[0308] The graphs plotted shown in FIGS. 37 and 38 are from the
equations derived in Section 8.1.3 and show the measured force
error due to the specified capacitor value error.
[0309] The simulation assumes a 2 point calibration for gain and
offset after which an error of up to +10 fF in steps of 2 fF is
added to C.sub.in1.
8.1.5 Noise Analysis
[0310] The nib is allowed to move up to 100 u so a range of 50 u to
150 u would be practical. The sensor capacitance is expected to be
in the range of 0.333 pF to 1 pF. Then with Cref=0.333 pF, the
output would be in the range of 1V down to 0.333V (-0.34V with the
full range variant). The S/N loss due to not being able to reach
full scale is only 20*log(2/3)=3.5 dB.
[0311] The incremental gain of the ADC is -3 V/pf when Csen=0.333
pF and -1 V/pf when Csen=1 pF.
[0312] Assuming the dominant noise source is the sensor capacitor,
the rms noise voltage is given by:
V n = kT C sen ( EQ 27 ) ##EQU00022##
where k=1.38.times.10.sup.-23, T=300, C.sub.sen=0.333 pF then
V.sub.n=117 uVrms.
[0313] Assuming all of this noise is folded into the passband of
the sigma-delta convertor, a proportion of the noise will be
removed by the decimation filter. Since this noise is present at
the input, no noise shaping takes place and the improvement in
signal to noise ratio at the filter output is given by 10 log(OSR)
where OSR is the over sampling ratio. With a clock of 64 kHz and an
output sample rate of 1.3 kHz, OSR=49.23 giving an improvement of
16.92 dB.
[0314] The voltage gain of the convertor from Csen to the output
was checked in the behavioural model. For Csen=0.333 pF, the gain
is 0.5 (for full scale=0 to +1V). Thus, the noise generated by the
capacitor is reduced by 6 dB at the output. However Cref will
contribute an equal amount of noise as it is also 0.333 pF thus
increasing the noise by 3 dB at the output. Note that the exemplary
convertor implements CDS which reduces the low frequency capacitor
noise.
[0315] The pre-decimation filter output noise with Csen=0.333
pF=117 uV/2=58.5 uVrms, output signal=1V. Output with Csen=1
pF=0.333V. S/N=20 log((1-0.333)/58.5 uV)=81.1 dB. After the
decimation filter, S/N=81.1+16.92=98 dB or 16.3 ENOB. The convertor
output needs to be thresholded to detect pen down so it is
appropriate to consider the peak noise. Assuming a pk to rms ratio
of 13 dB for the noise, the resolution of the convertor will be
reduced to 14.2 ENOB.
[0316] Note that with Csen=0.333 pF, the overall S/N will be
limited by the OSR of the sigma-delta convertor to about 70 dB
(11.6 ENOB) rather than kT/C noise.
8.2 Design Criteria
[0317] Circuit simulation has shown some of the criteria for
implementing the capacitive interface.
8.2.1 Input Impedance
[0318] The stage connected to the sensor must be able to overcome
the effects of parasitic capacitance to ground at this node. The
input resistance is given by 1/gm so forms an RC filter with a time
constant of C.sub.par/gm. This time constant dominates in the
settling time of the integrator and can be used to determine the
minimum gm requirement of the first stage. The voltage step
response is given by:
V = 1 - e t CR ( EQ 28 ) Then : gm min = - 2 C par F s ln ( 1 - V )
( EQ 29 ) ##EQU00023##
[0319] For a 1% final error with C=50 pF and F.sub.s=64 kHz,
gm.sub.min=29 u Mhos. It should be noted that the power consumption
of the first stage is proportional to the sampling rate and
C.sub.par.
[0320] Although it is possible to connect the sensor to the first
integrator, the requirements to provide a low input impedance to
overcome parasitic capacitance require high current in the first
integrator. It is much better to use a buffer amplifier which
allows the buffer and sigma-delta convertor to be optimised
independently. The first integrator current can be substantially
reduced and the overall current consumption reduced by adding the
buffer.
[0321] With parasitic capacitance to ground present, the gm of the
buffer substantially controls the settling time of the circuit and
the open loop voltage gain determines the accuracy. Some trade-off
can be made between gm and Ro maintaining the same low frequency
Av. A good target would be gm=100 u Mhos and Ro=100 M ohms so that
Av=80 dB.
[0322] The gain boosted folded cascode, when operated at 4.3 u tail
current (1 uA into the reference circuit) provides gm=55 u Mhos and
Ro=140 M giving Av=78 dB. Since the input from the sensor is single
ended, only a single ended output OTA is required which should save
power. The non-inverting input can then be tied directly to vcm
thus ensuring the lowest input impedance.
8.2.2 Connection of Inn to Vcm
[0323] If the first integrator is used to directly interface to the
capacitive sensor, it would be desirable to connect the unused
differential input directly to the common mode voltage, vcm, to
obtain the lowest resistance at the virtual earth summing node.
However, because of offsets in the OTA, this connection must be
made with a capacitor so that the offset can be stored during the
sample phase and cancelled during evaluation.
8.2.3 Capdac Unit Capacitor
[0324] 20 fF may be too small. Need careful layout and good
extraction.
8.2.4 Multiplexers
[0325] Some multiplexers may be directly connected to the summing
points of the integrator. In this case, leakage due to the body
source diodes and sub-threshold current must be minimised. Tee
switches may be used for both the capdacs and modulator input
multiplexers to obtain sufficient isolation.
[0326] When Tee switches are used, the mid point of the Tee switch
must be biased to vcm and not to one of the supplies. This ensures
sub-threshold current only flows when the output node of the
multiplexer is not at vcm. The integrator summing points are biased
at vcm thus minimising sub-threshold leakage.
[0327] The front end multiplexers which provide the excitation
signals are three levels deep and could be replaced with more logic
and one level of muxing.
8.2.5 Sensor Node Leakage
[0328] DC leakage current into the sensor node degrades the
accuracy of the circuit. The analogue input pad is expected to be
the largest source. 1 nA of leakage current results in 1
nA*15.625us/3 pF=5.2 mV of error per integrator step. In both
modes, this current results in a pen up force error of 20 g. This
can be solved by chopping the excitation signals and modulator
inputs. The chopping signal must contain an even number of cycles
during the output period of the decimation filter. This leakage
current uses up part of the conversion range so it must be ensured
that the convertor remains within its operating range. The feedback
within the sigma-delta modulator will minimise the range loss
provided the convertor does not saturate (which is most likely to
happen at pen-up).
[0329] The pad leakage current may be expected to be up to 50 nA.
It may be necessary to use both NMOS and PMOS protection devices so
that not all the leakage current flows into the convertor.
[0330] FIG. 39 shows the cancellation of 50 nA of leakage current
with the corrected result being obtained every second cycle.
[0331] The chop signal should change at the same time as ph0 not
when the input clock of the 2-phase clock generator changes as this
results in large errors. The best solution is to have an enabled
toggle clocking on ph0 and taking its enable from a programmable
divider that operates on the same clock as the decimation
filter.
8.2.6 Sensor Parallel Resistance
[0332] Sensor parallel resistance cannot be corrected by chopping
and provides a time dependant charge injection. This looks like a
capacitor in parallel with the sensor but one whose value depends
on the clock frequency. Taking a resistor that would leak 50 nA of
value 1.8/(2*50 nA)=18 Mohms; FIG. 40 shows that after a few
cycles, the bit pattern deviates from the reference with no
parallel resistor.
[0333] The equivalent capacitance is given by:
C eq = 1 4 F s R p ( EQ 30 ) ##EQU00024##
where F.sub.s is the sampling frequency and R.sub.p the parallel
resistance. The factor of 4 occurs because of 2 factors of 2. The
first is that integration of the leakage current only takes place
during the evaluation phase of the integrator which is half the
cycle time. Secondly, because of the CDS, charge is taken into
account on both phases of the clock for the capacitor. Effectively,
the sensor capacitor is subject to a voltage swing of vdd but the
resistor only to vdd/2. This is because the virtual earth point is
held at vdd/2 and the resistor driven with 0 or vdd. For F.sub.s=64
kHz and R.sub.p=1 Gohm, C.sub.eq=3.9 fF.
[0334] This is confirmed by the Analog Devices datasheet, which
infers at least 1 Gohm to be equivalent to a capacitance change of
8 fF or 6 gm (with F.sub.s=32 kHz).
[0335] If the pen down threshold were set to 12 gm, the capacitance
change would be 5.42 fF.
[0336] Then if the error due to R.sub.p was allowed to be 10% or
0.54 fF, R.sub.p needs to be >7.2 Gohm. R.sub.p can be
calibrated out so then it is the change in R.sub.p that becomes
important.
[0337] Since C.sub.eq is a sampling frequency dependant capacitor,
it would be possible to make measurements at 2 different
frequencies and isolate it from the real parasitic capacitance. The
use of the highest possible sampling frequency would minimise its
effect but this is contrary to minimising power consumption.
8.2.7 Sensor Series Resistance
[0338] This is another of the design criteria which may be
considered for the arrangement of the present invention.
8.2.8 Driver Parasitic Capacitance
[0339] The driver output must settle within half a clock cycle.
Assuming constant current drive in the following:
I.sub.min=2F.sub.sC.sub.parV.sub.d (EQ 31)
then for F.sub.s=64 kHz, C.sub.par=50 pF, V.sub.d=1.8V,
I.sub.min=11.5 uA.
[0340] Assuming resistive drive, then:
R max = 1 2 F s C par ln ( 1 - tol ) ( EQ 32 ) ##EQU00025##
[0341] To cancel the sensor parallel parasitic capacitance, the
drive waveform needs to settle to high accuracy. An error of 1%
when this capacitance is 2 pF would result in a cancellation error
of 20 fF. Simulation indicates that a source resistance of up to 10
Kohms should be acceptable at the voltage reference allowing for
additional resistance in the switches.
8.2.9 RF Filter
[0342] This is not an anti-aliasing filter. Since it is inside a
switched capacitor circuit, its cut-off frequency directly affects
the settling time. As an approximation, the cut-off frequency
should be about 10 F.sub.s. With a fixed cut-off and highest
sampling frequency of 1 MHz, it should be set to 10 MHz.
8.2.10 Power Saving
[0343] By using clocked bias, tt is possible to save current during
the sample phase of the integrators (clock ph0=1) as a lower
voltage gain is acceptable when the outputs are connected to the
inputs. A reduction in current consumption to 10% is possible
during the sample phase.
[0344] There was a problem with the common mode amplifier as it had
to adapt its output each time the bias current changed. The common
mode voltage at the outputs was dropping to 600 mV during low
current, as illustrated in FIG. 41; there was insufficient time for
it to recover.
[0345] The original OTA relied on the common mode amplifier
offsetting a bias current in the output branches of the OTA. This
offset was dependant on the bias current so needed correction each
time the bias current was switched. This was solved by making the
common mode amplifier push-pull and balancing currents in the
output branches of the main OTA. The sigma-delta modulator now
performs very well at low current.
TABLE-US-00006 TABLE 5 Supply Current ph0 = 1 0 average 33 uA 85 uA
60 uA
[0346] Table 5 above shows the current consumption during the 2
clock phases and the average with a 64 kHz clock. Clocked biassing
was only applied to the sigma-delta convertor as no benefit could
be obtained from the buffer amplifier. Further current will be
required by the bandgap reference and low power oscillator.
8.2.11 PSRR
[0347] At DC, there is no dependence on vdd other than the
operating range of the circuits as it cancels in the equations of
the interface. However, the feedback loop cannot cancel variation
from one clock cycle to the next although some averaging out may
occur in the decimation filter. This can be seen in the VHDL model
but the amount of supply rejection is not simple to estimate as it
varies with the sensor capacitance.
[0348] The solution is to provide regulated reference levels in the
CDC. This will reduce slightly the SNR of the CDC as the reference
levels must each be some 400 mV away from the power supplies.
[0349] The common mode voltage, vcm, also needs to be regulated and
not derived directly from VDD. Vcm must be set for optimum
operation at VDD.sub.min or about 800 mV.
[0350] Register control over the reference and vcm voltages should
be provided.
[0351] The Analog Devices datasheet gives no information on supply
rejection other than at DC, 50 Hz and 60 Hz. AD7745 board
measurements may be performed.
8.2.12 Integrator Reset
[0352] Mainly for simulation. All switches are closed to save
adding more.
8.2.13 Summing Point
[0353] With an input swing of 1.8V, the summing point voltage shown
in FIG. 42 can reach 0.9V.+-.1.8V instantaneously before the OTA
can respond. This will result in clipping and loss of charge giving
rise to large errors at the output of the integrator. It can be
solved by either increasing the rise and fall times of the drive
waveform but this is undesirable as a low impedance slew rate
limited driver would be needed. Alternatively, a capacitor can be
added to ground at the summing point. If this were equal to the
input capacitor, the voltage swing would be limited to
0.9V.+-.0.9V. This will be provided by the pad and ground parasitic
capacitances.
[0354] The direct path through C.sub.ref and C.sub.f to the output
explains the glitches that are seen there.
8.2.14 Connection of Inn to Vcm
[0355] The average voltage at the buffer summing point should be
vcm when the sensor is operating correctly. If a short circuit
between the plates occurs, the voltage will toggle between GND and
VDD. This can be detected by a couple of comparators with reference
levels of vcm.+-.V.sub.t and a simple low pass filter. This will
also detect any short of the sensing input to GND or VDD.
[0356] Or simply detect if the summing point goes below GND+V.sub.t
or above VDD-V.sub.t. Minimise current consumption.
8.3 Sensor Requirements
[0357] FIG. 43 illustrates a sensor equivalent circuit for
determining sensor requirements.
[0358] In Table 6 below, only one impairment is applied at a
time.
TABLE-US-00007 TABLE 6 Capacitive sensor requirements (F.sub.s = 64
kHz) Parameter Sym Comments Min Typ Max Units Capacitance at
C.sub.senmin Excluding C.sub.par 0.33 pF minimum force Capacitance
at C.sub.senmax Excluding C.sub.par 1 pF maximum force Parallel
C.sub.par Can be cancelled with C.sub.can 0 5.1 pF parasitic
capacitance Excitation node C.sub.gnd1 50 pF parasitic capacitance
Sensing node C.sub.gnd2 1fF error when C.sub.sen = 0.5 pF 15 pF
parasitic capacitance Series R.sub.ser 10 Kohm resistance Parallel
R.sub.par 1fF error when C.sub.sen = 0.5 pF 4 Gohm resistance.sup.a
Excitation node R.sub.gnd1 1fF error when C.sub.sen = 0.5 pF 30
Mohm parasitic resistance Sensing node R.sub.gnd2 1fF error when
C.sub.sen = 0.5 pF; 30 Mohm parasitic chopping on. resistance
.sup.aWithout dynamic zero force correction
8.4 Calibration Procedure
[0359] The required decimation factor and corresponding decimator
shift should be set first. The ADC offset should be set to 0. Both
modes of operation require 2 point calibration to take out gain and
offset errors. Only one calibrated force of 500 gm needs to be
applied.
[0360] The gain of the CDC depends on absolute capacitor values to
is subject to process variation (typically .+-.10% for MiM
capacitors). The offset will be dominated by sensor parasitic
capacitance, C.sub.par and the mechanical tolerances of the sensor.
Also, R.sub.par could be significant and must be maximised by good
design. If dynamic calibration is used, only C.sub.can should be
used to offset R.sub.par. so that the converter remains in its
normal operating range.
8.4.1 1/C Mode
[0361] The following procedure elements are used.
[0362] The nominal zero force capacitance, C.sub.sensmin, must be
known; this is a design parameter of the force sensor.
[0363] C.sub.in1 must be set so that the convertor operates at 10%
of full scale under zero force conditions. This value can be
calculated given C.sub.sensmin. Using EQ 18 and assuming C.sub.par
has been cancelled, C.sub.in1=0.9 C.sub.sensmin. For example, with
C.sub.sensmin=0.3333 pF, set C.sub.in1=0.3 pF.
[0364] Cancel C.sub.par by adjusting C.sub.can until the converter
reads 10% of scale. The typical value for C.sub.par will be known
and should get the converter out of saturation. Using the example
of Section 8.1.3.1, with scale=64, the reading at 10% will be
116.
[0365] Set the ADC offset to zero the converter. (Since the
converter output cannot be negative, perform this step now and not
before setting the 10% point).
[0366] Apply the calibrated force and record the output; this sets
the gain calibration factor.
8.4.2 C Mode
[0367] The following procedure elements are used.
[0368] The parameters K.sub.4 and K.sub.5 used in EQ 17 will have
been determined so the sensor and converter need to be calibrated
to match it.
[0369] Set C.sub.par and C.sub.in1 to their typical values to get
the converter into its operating range.
[0370] Calculate the force from the current converter output.
[0371] Apply the calibrated force and record the calculated force;
the difference between the two calculations sets the gain
calibration.
[0372] Calculate the value for C.sub.in1 that gives the required
gain correction and set this value.
[0373] With no applied force, calculate the force and calculate the
required value for C.sub.can that will set the force to zero and
set this value.
8.4.3 Dynamic Calibration
[0374] The step size of C.sub.can is likely to be about 20 fF which
represents a force change of 42 gm. A combination of adjustment of
C.sub.can and the ADC offset must be used so that the pen-down
threshold remains accurate. In the case of 1/C operation, the gain
calibration factor needs to be recalculated. Without recalculation,
the C.sub.can step size will result in a 10% gain error (which may
be acceptable).
8.5 AD7745 Measurements
[0375] Some measurements to evaluate interfering signal immunity of
an AD7745 have been made. An unwanted signal was injected into the
CIN1 pin, which is the output side of the sensor capacitor, via a
39 pF capacitor. The addition of this capacitor has negligible
effect on the measured sensor capacitor since it is connected to
ground via the output impedance of the signal generator. The sensor
capacitance was mostly parasitic capacitance of about 60 fF.
[0376] The board was set up to output the maximum sampling rate of
90 Hz and both CAPDAC's switched off. The excitation level was the
maximum of .+-.VDD/2 and operated in single ended mode. The
excitation frequency was about 33 kHz.
[0377] The signal level (measured on CIN1) required to alter the
measured capacitor was adjusted to give a -20 fF error. See Table 7
below:
TABLE-US-00008 TABLE 7 F(MHz) level mVpk-pk 2 120 2.4 150 3 180 4
220 5 300 6 440 >7 <600
8.5.1 Observations
[0378] Above 2 MHz, the injected signal reduces the measured
capacitance. [0379] Below 1 MHz, the injected signal increases the
measured capacitance. [0380] Between 1 MHz and 2 MHz, depending on
the frequency and amplitude, the capacitance error can go in either
direction. [0381] Frequencies which are near harmonics of the
excitation frequency result in the most sensitivity (these will be
aliased down to the passband). [0382] With chop enable selected,
the immunity increased by a factor of about 3 times. The maximum
output sampling rate was 49.8 Hz.
[0383] There appears to be a pole at about 3 MHz which is either
implemented as an input RC filter or due to the integrator op-amp
bandwidth. The data sheet recommends no more than 1K input series
resistance implying an RC filter with C=53 pF (rather large) could
be added.
[0384] The most likely source of interference is the serial
interface at 35 MHz and Bluetooth at 2.4 GHz; well away from 3
MHz.
8.5.2 Noise Measurements
[0385] These were made with a mechanical mock up of a capacitive
proximity detector.
[0386] CAPDACA was set to the maximum of 16.87 pF to offset the
parasitic capacitance of the sensor. A capacitance change of about
0.75 pF was seen in the linear region and 1.75 pF using sticky tape
as the dielectric at the end of the range. With 90 Hz sampling and
.+-.VDD/2 excitation, the noise levels were measured with 1000
samples in "pen-up" as 0.00036 pFrms and 0.0032 pFpk-pk.
[0387] Assuming white noise, the scaling factor from 90 Hz to 1.3
kHz is 10*log(1300/90)=11.6 dB.
[0388] For a range of 0.75 pF, pk-pk signal to rms noise
S/N=20*log(0.75/0.00036)=66.4 dB. (note that the AD7745 calculates
its S/N for a capacitance range of 8 pF giving an rms resolution of
13 bits or 78 dB S/N). Scaling for 1.3 kHz, S/N=66.4 dB-11.6
dB=54.8 dB (9.1 ENOB).
[0389] With the sensor disconnected and CAPDACA off, the measured
capacitance was 0.0052 pF. The rms noise measured reduced to
0.0000654 pFrms which is a 14.8 dB improvement.
8.6 Conclusions
[0390] The sigma-delta capacitive interface arrangements of the
present invention provide a robust solution to the problem of force
sensing, particularly for the nib of the Netpage sensing device.
The above-described arrangements have the advantages of providing
convertor output directly proportional to applied force, avoiding
hardware division in the calculation of force from the convertor
output, maintaining constant accuracy over the force range, and
making use of the full range of the convertor.
[0391] Further advantages of the specific and modeled examples are:
[0392] Low impedance drive (excitation) into the sensor capacitor,
voltage pickup immunity; [0393] Sensor capacitor drives a virtual
earth, voltage pickup immunity; [0394] High tolerance to parasitic
capacitance to ground on both sides of the sensor capacitor; [0395]
No active guard required; [0396] Can be designed to provide an
output proportional to 1/C (compressing the output as C increases
and conversely providing enough resolution for small values of C);
[0397] Good immunity to RF (>10 MHz) induced voltages; [0398]
Does not need an accurate clock; [0399] Does not need an accurate
voltage reference so the maximum excitation .+-.VDD/2 can be used;
[0400] The usual benefits of sigma-delta conversion apply; [0401]
If power consumption can be made low enough (in processor sleep
mode), pen-down offset can be applied in the digital domain; and
[0402] Sampling rate can be increased when the processor is on to
improve unwanted signal rejection.
[0403] It will, of course, be appreciated that the present
invention has been described purely by way of example and that
modifications of detail may be made within the scope of the
invention as described herein.
* * * * *