U.S. patent application number 12/508328 was filed with the patent office on 2009-11-19 for finger/stylus touch pad.
This patent application is currently assigned to SYNAPTICS INCORPORATED. Invention is credited to Federico Faggin, Richard Robert Schediwy.
Application Number | 20090283342 12/508328 |
Document ID | / |
Family ID | 22645214 |
Filed Date | 2009-11-19 |
United States Patent
Application |
20090283342 |
Kind Code |
A1 |
Schediwy; Richard Robert ;
et al. |
November 19, 2009 |
FINGER/STYLUS TOUCH PAD
Abstract
A touch pad module to implement user input functions to an
electronic device is disclosed. The touch pad module includes a
sensor layer which, when used in conjunction with an insulative
layer and contiguous conductive layer enable the touch pad module
to sense both finger and stylus input data to the electronic
device.
Inventors: |
Schediwy; Richard Robert;
(Union City, CA) ; Faggin; Federico; (Los Altos
Hills, CA) |
Correspondence
Address: |
INGRASSIA FISHER & LORENZ, P.C. (SYNA)
7010 E. Cochise Road
SCOTTSDALE
AZ
85253
US
|
Assignee: |
SYNAPTICS INCORPORATED
Santa Clara
CA
|
Family ID: |
22645214 |
Appl. No.: |
12/508328 |
Filed: |
July 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09176639 |
Oct 20, 1998 |
|
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12508328 |
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Current U.S.
Class: |
178/19.03 |
Current CPC
Class: |
G06F 3/0443
20190501 |
Class at
Publication: |
178/19.03 |
International
Class: |
G06F 3/041 20060101
G06F003/041 |
Claims
1. A capacitive touch pad module comprising: a first set of
conductive lines; a second set of conductive lines insulated from
the first set of conductive lines; an insulating layer disposed
over the first and second sets of conductive lines; a conductive
layer placed over the insulating layer, the conductive layer
configured such that it would spread out a ground image of a
grounded conductive stylus in contact with the conductive
layer.
2. The capacitive touch pad module of claim 1 wherein the touch pad
module further includes a means for distinguishing a characteristic
of the conductive object.
3. The capacitive touch pad module of claim 2, wherein the means
for distinguishing a characteristic of the conductive object
distinguishes between stylus and finger.
4. The capacitive touch pad module of claim 1 wherein the
conductive layer comprises a conductive material disposed in a
plastic carrier.
5. The capacitive touch pad module of claim 1 wherein the first set
of conductive lines runs in a first direction, wherein the second
set of conductive lines runs in a second direction generally
perpendicular to the first direction, and wherein the first set of
conductive lines and the second set of conductive lines form an
electrode grid.
6. The capacitive touch pad module of claim 1 wherein a first
subset of the first set of conductive lines is configured to have
applied to it an oscillating potential of a given frequency and a
first phase, and wherein a second subset of the first set of
conductive lines is configured to have applied to it an oscillating
potential of the given frequency and a second phase, wherein the
second phase is opposite to the first phase.
7. The capacitive touch pad module of claim 1 wherein the first set
of conductive lines comprise printed circuit board traces and the
second set of conductive lines comprise printed circuit board
traces.
8. The capacitive touch pad module of claim 1, wherein the first
set of conductive lines is transparent, the second set of
conductive lines is transparent, the insulating layer is
transparent, and the conductive layer is transparent.
9. The capacitive touch pad module of claim 1, wherein the touch
pad module is transparent, further comprising: a display screen
under the first set of conductive lines, the second set of
conductive lines, the insulating layer, and the conductive
layer.
10. A touch pad module comprising: a surface; a first set of
conductive lines located below the surface; a second set of
conductive lines insulated from the first set of conductive lines
and located below the surface, wherein the first and second set of
conductive lines are configured to detect changes in capacitance
resulting from a stylus in contact with the surface; an insulating
layer disposed over the first and second sets of conductive lines;
and a conductive layer placed over the insulating layer, the
conductive layer configured such that it would spread a ground
image of a grounded object in contact with the conductive
layer.
11. The touch pad module of claim 10 wherein the touch pad module
further includes a means for distinguishing between stylus contact
and finger contact.
12. The touch pad module of claim 10 wherein the conductive layer
comprises a conductive material disposed in a plastic carrier.
13. The touch pad module of claim 10 wherein the first set of
conductive lines and the second set of conductive lines form an
electrode grid.
14. The touch pad module of claim 10 wherein a first subset of the
first set of conductive lines is configured to have applied to it
an oscillating potential of a given frequency and a first phase,
and wherein a second subset of the first set of conductive lines is
configured to have applied to it an oscillating potential of the
given frequency and a second phase, wherein the second phase is
opposite to the first phase.
15. The touch pad module of claim 10 wherein the touch pad module
is transparent, further comprising: a display screen under the
first set of conductive lines, the second set of conductive lines,
the insulating layer, and the conductive layer.
16. A capacitive touch pad module comprising: a surface; a set of
conductive lines disposed under the surface; an insulating material
disposed over the set of conductive lines; a conductive layer
placed over the insulating layer, wherein the set of conductive
lines, the insulating layer, and the conductive layer are
configured to create a detectable capacitance change when a user
places a stylus in contact with the surface, the detectable
capacitance change determined in part by the conductive layer, and
wherein the conductive layer is configured to create an image of
the conductive object to thereby make the detectable capacitance
change greater than without the conductive layer.
17. The capacitive touch pad module of claim 16 wherein the set of
conductive lines, the insulating layer, and the conductive layer
are configured to distinguish between stylus and finger.
18. The capacitive touch pad module of claim 16 wherein the set of
conductive lines form an electrode grid.
19. The capacitive touch pad module of claim 16 wherein a first
subset of the set of conductive lines is configured to have applied
to it an oscillating potential of a given frequency and a first
phase, and wherein a second subset of the set of conductive lines
is configured to have applied to it an oscillating potential of the
given frequency and a second phase, wherein the second phase is
opposite to the first phase.
20. The capacitive touch pad module of claim 16 wherein the touch
pad module is transparent, and further comprising: a display screen
under the first set of conductive lines, the second set of
conductive lines, the insulating layer, and the conductive layer.
Description
PRIORITY DATA
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/176,639, filed Oct. 20, 1998, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention involves a touch pad module for use
with an electronic device, such as a notebook computer, which makes
use of such modules to implement user input functions. The touch
pad module is configured of certain insulative and conductive
layers as to enable the electronic device to sense input data from
both finger and stylus.
BACKGROUND OF THE INVENTION
[0003] Over the last several years, capacitive touch pad pointing
devices have entered widespread use in personal computers. There
are at least three distinct capacitive sensing technologies used in
touch pad devices today:
[0004] 1. The "Field Distortion" approach, used by Cirque and Alps
as described in PCT Application No. US90/04584, Publication No.
WO91/03039 to Gerpheide. Specifically, Gerpheide teaches the
application of an oscillating potential of a given frequency and
phase to all electrodes on one side of a virtual dipole, and an
oscillating potential of the same frequency and opposite phase to
those on the other side. Electronic circuits develop a "balanced
signal" which is zero when no finger is present, and which has the
polarity of a finger on one side of the center of the virtual
dipole, and the opposite polarity of the finger on the opposite
side. To characterize the position of the finger initially, the
virtual dipole is scanned sequentially across the tablet. Once the
finger is located, it is "tracked" by moving the virtual dipole
toward the finger once the finger has moved more than a row or
column of the matrix constituting the capacitive sensor touch pad.
Because the virtual dipole method operates by generating a balance
signal that is zero when the capacitance does not vary with
distance, it only senses the perimeter of the finger contact area,
rather than the entire contact area.
[0005] 2. The charge-detection approach used by the present
assignee and described in its U.S. Pat. No. 5,374,787 to Miller et
al. Specifically, the present assignee employs what is called a
"finger pointer" technique. This approach is to provide a position
sensing system including a position sensing transducer comprising a
touch-sensitive surface disposed on a substrate, such as a printed
circuit board, including a matrix of conductive lines. A first set
of conductive lines runs in a first direction and is insulated from
the a second set of conductive lines running in a second direction
generally perpendicular to the first direction. An insulating layer
is disposed over the first and second sets of conductive lines. The
insulative layer is thin enough to promote significant capacitive
coupling between a finger placed on its surface and the first and
second sets of conductive lines. Sensing electrodes respond to the
proximity of a finger to translate the capacitance changes of the
conductors caused by the finger proximity into position and touch
pressure information.
[0006] 3. An unrelated approach employed currently by Logitech.
[0007] All three of these technologies share an important common
feature: The finger is detected by a plurality of
horizontally-aligned sensor electrodes disposed on a first layer,
separated by an insulator from a plurality of vertically-aligned
sensor electrodes disposed on a second layer. Such sensor
electrodes are often formed as, but are not limited to, standard
copper printed circuit board traces.
[0008] An example of such an electrode arrangement is shown in FIG.
1. Specifically, reference is made to FIGS. 1A through D, top,
bottom, composite and cross-sectional views, respectively. Sensor
array 10 is provided comprising substrate 12 including a set of
first conductive traces 14 disposed on top of surface 16 thereof
and run in a first direction to comprise row positions of sensor
array 10. The set of second conductive traces 18 are disposed on a
bottom surface 20 thereof and run in a second direction preferably
orthogonal to the first direction to form the column positions of
the sensor array 10. The set of first and second conductive traces
14 and 18 are alternately in contact with periodic sense pads 22
comprising enlarged areas, shown as diamonds in FIGS. 1A-1C. While
sense pads 22 are shown as diamonds in FIGS. 1A-1C, any shapes such
as circles, which allows close packing the sense pads 22 is
equivalent for purposes of this discussion.
[0009] It is well recognized that capacitive touch pads, such as
those described above, work well with fingers, but are normally
unable to sense a pen or stylus. Capacitive touch pads are
typically used as pointing devices. Resistive touch pads work well
with pens, but require an uncomfortable amount of pressure when
used with fingers. Resistive touch pads are typically used as
writing or drawing input devices. To date, there has not been a
practical touch pad which would work well with both fingers and
pens along with a single input device to serve both functions. Such
a touch pad would be especially valuable in portable applications
where space is at a premium.
[0010] It is thus an object of the present invention to provide an
input device in the form of a touch pad module which will accept
both finger and stylus input, that is, having the desirable
attributes of both a capacitive touch pad for finer input and a
resistive touch pad for stylus input in the same module.
[0011] This and further objects will be more readily apparent when
considering the following disclosure and appended claims.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention is directed to a touch pad module to
implement user input functions to an electronic device. The module
comprises a sensor layer having a length and width for detecting
position of a conductive object in contact with a touch pad module.
An insulative layer is positioned over and contiguous with the
sensor layer and a moderately conductive layer is positioned over
and contiguous with the insulative layer to provide a touch pad
module which can be used as both capacitive and resistive elements
have been employed in the past to receive input information from
both a finger conductive stylus.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIGS. 1A through 1D are top plan and side views of
capacitive touch pads of the prior art.
[0014] FIGS. 2A and B show, in perspective, the effect of a finger
contacting a capacitive touch pad module and a graph illustrating
capacitance versus horizontal position on the pad.
[0015] FIGS. 3A and B show a depiction, in plan view, and in
graphical form, of the measurement of finger capacitance in one
dimension and the capacitance of various electrodes based upon
finger pressure.
[0016] FIGS. 4A and B show, in perspective, and in graphical form,
the effect of a stylus on a capacitive touch pad module and the
capacitance generated as a result.
[0017] FIGS. 5A and B are similar to the depictions shown in FIGS.
4A and B with the contact area of the stylus enlarged.
[0018] FIGS. 6A and B show, in perspective, a stylus used in
conjunction with the touch pad module of the present invention and
a capacitance graph generated as a result.
[0019] FIGS. 7A and B illustrate in perspective, and in graphical
form, the results of the application of a stylus to a touch pad
wherein the conductance of its top surface is too high.
[0020] FIGS. 8A and B illustrate in perspective, and in graphical
form, the results of the application of a stylus to a touch pad
wherein the conductance of its top surface is too low.
[0021] FIGS. 9A and B are similar to FIGS. 8A and B with a finger
employed in place of the conductive stylus.
[0022] FIGS. 10A and B are again similar to FIGS. 8A and B showing
the boundary effects of the conductive stylus contacting the touch
pad module of the present invention near its periphery.
[0023] FIG. 11 is the touch pad module of the present invention in
perspective showing the embodiment of providing the user with
visual feedback created by the application of suitable stylus.
[0024] FIG. 12 is a graph of capacitance versus time showing the
distinguishing characteristics between the use of stylus and finger
in discriminating these two objects in providing positional input
data to a suitable electronic device in using the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention involves a touch pad module for use
with an electronic apparatus which makes use of such a module to
implement all or part of its user input functions. Notebook and
desktop computers as well as copiers are typical examples of such
electronic apparatus having need for a touch pad device such as
that disclosed herein. When used in conjunction with a computer, a
touch pad allows the user to manipulate a graphics cursor on a CRT
display or allows a user to manipulate a stylus thereby allowing
input of written text. The touch pad comprises a sensitive planar
surface and a means for detecting the position of an object, such
as a finger or stylus, near or in contact with the sensitive planar
surface. The touch pad continuously communicates this position
information to the electronic apparatus typically at a rate of from
40 to 100 Hz.
[0026] As noted previously, the touch pad module of the present
invention can be used to implement user input functions to an
electronic device through the use of both the finger of a user as
well as through the use of a conductive stylus held by the user.
FIG. 2 shows the effect of a finger on a sensor of the prior art,
that is, capacitive sensor intended to accept positional data by
the application of a fingertip to the touch pad module. Above the
electrodes 202 is an insulating layer 201 which provides the
surface 203 over which the finger 204 is detected (see FIG. 2A). In
operation, each electrode on electrode layer 202 provides one plate
of a capacitor and the finger 204, if present, provides a second
plate, with the insulating layer 201 forming the dielectric between
them. The conductance of the human body, combined with the human
body's inherent capacitance to free space, causes the finger to
appear to be electrically grounded in terms of its capacitance to
the electrodes. Sensing electrodes scan the array of electrodes for
increased capacitance to ground caused by the presence of a finger
or other object over them. By measuring the capacitance on both the
horizontal and the vertical electrodes, the location of the finger
can be determined.
[0027] FIG. 2B shows a graph of capacitance versus horizontal
position on the pad. The capacitance is proportional to the
finger's circular area of contact. Hence, the capacitance is
highest near the center of the finger and tapers off toward the
edge of the region of contact. Away from the finger, the
capacitance is essentially zero, i.e., unaffected by the finger.
Touch pads measure the finger position by locating the peak 206 of
the curve 205 in FIG. 2B.
[0028] The position of the finger can be determined much more
accurately than the distance between the electrodes if the finger
is wide enough to provide a measurable signal on more than one of
the electrodes in each of the horizontal and vertical dimensions.
FIG. 3 shows the effects of fingers of various sizes on the
electrode matrix. For simplicity, electrode grid 351 is shown in
just the horizontal dimension, and the electrodes are shown as
linear wires when, in fact, a more complex pattern such as linear
strings of diamond shapes may be preferred in practice. The finger
(not shown) makes an approximately circular area of contact with
the surface. This circular region 352 is typically large enough to
cover several adjacent electrodes. The capacitance on an electrode
is proportional to the area of the electrode that is covered by the
finger. This area of overlap is largest near the center of the
finger, and tapers off toward the edge of the finger contact
region. FIG. 3B shows graph 355 of the capacitances of the various
electrodes. The capacitance 356 of the electrode nearest the center
of the finger is highest because that electrode has the greatest
overlap with the finger. Because the finger is large compared to
the electrode spacing, the adjacent electrodes sense a reduced but
non-zero capacitance. The relative magnitudes of the detected
capacitances on the nearby electrodes can be used to determine the
position of the finger accurately with sub-electrode resolution.
One popular method computes the centroid of the entire curve 355;
another method finds the electrode with maximum capacitance and
interpolates using a quadratic fit to the adjacent electrode
readings.
[0029] If finger 353 is narrower than the distance between
electrodes, then it may produce a signal on just one electrode 357
and high-resolution interpolation is impossible. If the finger 354
is extremely narrow, it may fall entirely between electrodes and
not register at all as shown at 358. Fortunately, real fingers are
wide enough to allow for good interpolation with a touch pad having
a feasible number of electrodes (e.g., 15 electrodes in each
dimension).
[0030] To use a stylus with such a capacitive sensing touch pad,
the stylus must have certain special properties. First, the stylus
must be conductive so as to form the required second plate of
detectable capacitance. The conductive stylus is grounded either by
direct contact with the skin of the effectively grounded human, or
by capacitive coupling to the human. Suitable materials for the
stylus include metals, and highly conductive plastics such as nylon
loaded with carbon fibers or carbon powder.
[0031] Second, the stylus must form a large enough signal on at
least two adjacent traces in each dimension to allow for accurate
position measurement. Traditional stylus designs feature a pointed
tip which is not large enough to form a signal on more than one
trace, as shown in FIG. 4. Stylus 301 has such a small contact area
302 that the resulting capacitance signal 303 is both too narrow
and too low in amplitude for effective position measurement.
[0032] Several designs for a wide stylus have been attempted. For
example, a ball of conductive foam may be attached to the end of
the stylus, or a small circular plate of metal can be attached by a
ball joint to the tip. FIG. 5 illustrates the latter design. Stylus
401 is tipped with plate 402, whose area has been chosen to mimic
the contact area of a typical finger. Hence, the capacitive signal
403 created by the plate on the electrodes is a good simulation of
the signal produced by a true finger (compare curve 403 to curve
205 of FIG. 2). Stylus designs of this kind have been built and
shown to work, but they are too clumsy, bulky and fragile to gain
wide acceptance among users.
[0033] For these reasons, the great majority of pen-actuated touch
pads currently manufactured use resistive, not capacitive, sensors.
In a resistive touch pad, pressure from the finger or stylus pushes
a flexible conductive membrane against another conductive surface
and thereby detects a measurable electrical signal. The resistive
touch pad works well with a pointed stylus, but because it requires
actual pressure, the resistive pad is uncomfortable to use with a
finger. Also, the large contact area of a finger reduces the
accuracy of a resistive pad. Finally, because the resistive touch
pad contains moving parts, it is more fragile than a capacitive
touch pad. Hence, a capacitive touch pad that could work with a
point-tipped stylus would be of considerable value in the
marketplace.
[0034] As noted previously, the present invention involves placing
a moderately conductive layer above the insulating layer, so that
the grounded conductive stylus makes contact with the moderately
conductive layer. The conductive layer effectively spreads out the
ground image of the tip of the stylus, forming a larger second
capacitor plate which can be sensed by more than one electrode on
each of the horizontal and vertical axes.
[0035] In FIG. 6, electrode 503 and insulating layer 502 have been
covered by moderately conductive layer 501. Layer 501 is made from
a conductive material durable enough to be exposed as the surface
of the touch pad with no protective coating. A suitable material
for this purpose is conductive carbon powder in a plastic carrier
material such as epoxy. A conductive stylus 504 is then touched to
the surface. Because stylus 504 is held by the human, the stylus is
effectively grounded as previously disclosed. The tip of stylus 504
makes electrical contact with conductive layer 501, causing a
grounded region 505 to form on the conductive layer. Because layer
501 is only moderately conductive, the grounding effect dissipates
with distance from the point of contact with the stylus. A sensing
circuit which measures capacitance to ground will measure a strong
signal in region 505, but little or no signal far away from region
505 and the stylus tip.
[0036] By controlling the conductivity of layer 501, the perceived
image size of the tip of the stylus can be adjusted to provide
sufficient signal on an appropriate number of electrodes. This
permits the stylus 504 to be formed in any convenient size and
shape, such as that of a familiar fine-tipped pen.
[0037] If the conductive layer is too conductive, then the image
will be very large, possibly even covering the entire surface of
the touch pad. In this case it may not be possible to determine the
location of the stylus by measuring the capacitance on each
electrode. In FIG. 7, layer 601 has such high conductance (i.e.,
such low resistance) that stylus 602 creates a grounded region 603
that covers a large fraction of the surface. Hence, the capacitance
graph 604 is so wide that it is hard to measure the peak of the
curve accurately. In the extreme case of a highly conductive layer
601, contact with a stylus anywhere on the surface would produce a
uniform grounding effect over the entire surface and no position
information could be gained.
[0038] If the conductive layer is not conductive enough, then the
image will not be much larger than the tip of the stylus, and it
may not be possible to determine the location of the stylus to a
resolution any higher than the electrode pitch. In FIG. 8, layer
701 has such low conductance (i.e., such high resistance) that
grounded region 703 is very small, producing a graph 704 which is
not much better than graph 303 with no conductive layer at all.
[0039] For best operation, the conductivity of the surface layer
should be chosen such that the image of the stylus is about the
same size as the image generated by a finger on a normal capacitive
sensor (note the similarity of capacitance graphs 205 of FIGS. 2
and 506 of FIG. 6).
[0040] A key benefit of the present invention is that the touch pad
can still be used effectively with a finger, as well as with a
stylus as previously disclosed.
[0041] The fundamental mechanism of the capacitive touch pad as
described above continues to detect fingers on touch pads with the
additional conductive layer. In FIG. 9, finger 802 touches the
surface and produces a grounded region 803 which is larger than the
image of a finger on a normal touch pad, but not so large as to
render the resulting capacitance graph 804 unusable for calculating
the finger position.
[0042] Thus, the addition of a conductive layer 801 allows the
touch pad to work well with either a stylus or a finger.
[0043] It was determined that when the stylus or finger nears the
edge of the sensor, the present invention can cause a noticeable
distortion in the measured position. Referring to FIG. 10, stylus
902 being close to edge 904 of the sensor, causes grounded region
903 to be truncated into a semicircular shape. The resulting
capacitance graph shows a truncated and lop-sided curve as seen in
FIG. 10B. The true peak of the curve, and thus the true stylus
position, is shown by arrow 906. The centroid method, if employed,
will calculate a different position 905 because the curve is
truncated on one side. The simplest solution to the boundary
distortion effect is to make the touch pad somewhat larger than
needed, then to cover the sensor with a bezel with a smaller
opening that prevents the finger or stylus from nearing the true
edge of the sensor.
[0044] Another solution is to compensate for the distortion in
later processing on the computed position data. This is possible
because the effect of the distortion is predictable and repeatable,
especially if the conductance of layer 901 is a well-controlled
manufacturing variable. To compensate for the distortion, a stylus
is placed at various positions across the sensor, and the
corresponding measured positions tabulated. The resulting table
describes a mathematical function. It is easy to see that the
effect shown in FIG. 10 produces a monotonic distortion in the
measured position, which means the tabulated function has an
inverse which can be computed by means well known in the art. The
distortion is compensated by applying this inverse function to each
measured position during operation of the touch pad.
[0045] By choosing appropriate materials for the stylus tip and
touch pad surface, the stylus can be made to leave a mark on the
surface of the touch pad, giving a visual feedback to the user. In
FIG. 11, the touch pad is made of the same electrode layer 1003,
insulating layer 1002, and conductive layer 1001, but conductive
layer 1001 is made of a material whose properties cause stylus 1004
to leave a visible trail 1005 on the surface. The material may be
pliant so that the stylus leaves a groove, or have other mechanical
or chemical properties that cause the stylus to leave a mark. Or,
the stylus can be made of a sacrificial material such as pencil
graphite which leaves a trail when moved across the surface. With
an appropriate surface material, the markings can be easily wiped
off so that subsequent marks are more easily visible.
[0046] It is possible to make materials which are both conductive
and transparent to visible light. In this case, layer 1001 may be
made transparent and layer 1002 may be made of a material which
changes color or reflectivity when mechanically disturbed. In yet
another approach, all three layers 1001, 1002, and 1003 may be made
transparent, and the whole assembly placed over a display screen
such as a liquid crystal display (LCD) which can provide visual
feedback under software control.
[0047] In some applications it may be useful to be able to
distinguish between stylus contact and finger contact on the touch
pad. Although there is no guaranteed way to make this distinction
given only the capacitance graph, it is possible to make a fairly
reliable heuristic guess by noting the differences between stylus
input graph 506 and finger input graph 804.
[0048] The conductive layer on the touch pad surface will expand
any grounded contact by a roughly constant distance which in the
preferred embodiment is comparable to the width of a finger. A
stylus tip, which is essentially a point of negligible size, is
expanded to be finger-sized by the conductive layer. A finger has a
finger-sized contact area, which is expanded to a much larger size
by the conductive layer. Thus, a finger can be expected to produce
a grounded region of approximately twice the width or diameter as
that of a stylus. With the diameter increased by a factor of two,
the total area of grounded contact is increased by a factor of
four. Hence, the system can measure either the total number of
electrodes reporting increased capacitance (the diameter of the
grounded region) or the total summed capacitance among all the
electrodes (the area of the grounded region) to guess whether the
contact is a stylus or a finger.
[0049] Another useful factor is that a capacitance signal produced
by a finger will tend to fluctuate as the angle of contact of the
finger on the surface changes, but a stylus signal will remain very
constant. The stylus signal is independent of the angle at which
the stylus is held because the contact area of the stylus tip
itself is negligible. Yet another factor is that the stylus will
produce no signal until contact is made with the surface, whereupon
the signal will jump immediately to full strength, whereas a finger
will begin producing a small signal as it approaches the surface
since a finger-sized conductor creates some capacitance merely by
proximity to a capacitance sensor.
[0050] FIG. 12 illustrates a graph of total summed capacitance "Z"
versus time. First, a stylus contact is made which is characterized
by a small, steady, sharp Z signal 1201. Then, a finger contact
occurs with a larger, more varying signal 1202 with a smoother rise
and fall.
[0051] In summary, the present invention recognizes, for the first
time, that the application of a conductive layer above the
insulating layer of a capacitive touch pad provides such an input
device which works well with both a finger and a conductive stylus.
In addition, it is noted that the size of the stylus tip can be
made as small as desired without impacting the ability of the touch
pad to accurately determine its location.
* * * * *