U.S. patent number RE37,878 [Application Number 09/174,811] was granted by the patent office on 2002-10-15 for pointing device with differential optomechanical sensing.
This patent grant is currently assigned to Logitech Europe, S.A.. Invention is credited to Xavier Arreguit, Marc Bidiville, Eric Vittoz.
United States Patent |
RE37,878 |
Bidiville , et al. |
October 15, 2002 |
Pointing device with differential optomechanical sensing
Abstract
A pointing device including a ball engaging one or more shaft
encoders, each shaft encoder having an encoding wheel, including a
monolithic photosensitive array for detecting light pulses
representative of rotary movement of the ball. The photosensitive
array provides a plurality of resolutions and permits accurate
tracking of rotary movement of the ball without need for a mask.
Comparator and latch means operate on the output of the array and
provides digital hysteresis.
Inventors: |
Bidiville; Marc (Monaco,
MC), Arreguit; Xavier (Le Mont-sur-Lausanne,
CH), Vittoz; Eric (Cernier Neuchatel, CH) |
Assignee: |
Logitech Europe, S.A.
(Romanel-sur-Morges, CH)
|
Family
ID: |
25454590 |
Appl.
No.: |
09/174,811 |
Filed: |
October 19, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
927334 |
Aug 10, 1992 |
05680157 |
Oct 21, 1997 |
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Current U.S.
Class: |
345/165;
345/163 |
Current CPC
Class: |
G06F
3/0312 (20130101); G06F 3/0317 (20130101); G06F
3/03543 (20130101); G06F 2203/0333 (20130101) |
Current International
Class: |
G06F
3/033 (20060101); G09G 005/08 () |
Field of
Search: |
;345/157,158,163,165,166,167 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"An Incremental Optical Shaft Encoder Kit with Integrated
Optoelectronics", Howard C. Epstein, Hewlett-Packard Journal Oct.
1981, pp. 10-15. .
Tenth Annual Symposium--incremental motion control systems and
devices, Jun. 1981, B.C. Kuo, Editor, "Optical and Mechanical
Design Trade-Offs in Incremental Encoders", Howard C.
Epstein,Hewlett-Packard, pp. 57-64. .
"Push-Pull Optical Detector Integrated Circuits", Mark Leonard,
IEEE Journal of Solid-State Circuit, vol. SC-15, No. 6, Dec. 1980,
pp. 1087-1089. .
Hewlett-Packard Journal, Oct. 1981 vol. 32, No. 10..
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Primary Examiner: Chang; Kent
Attorney, Agent or Firm: Townsend and Townsend and Crew
LLP
Claims
What is claimed is:
1. A cursor control device for control the position of a cursor on
a video display screen wherein the cursor control device uses a
rotatable ball in engagement with at least two shaft encoders to
convert rotational movement of the ball into digital signals
representing movement of the cursor comprising a housing, means for
supporting the ball in engagement with the at least two shaft
encoders, each of said shaft encoders having thereon an encoder
wheel having slots therein, and the at least two shaft encoders
arranged orthogonally to permit one shaft encoder to detect
movement in an X direction and another to detect movement in the Y
direction, light emitting means on one side of each encoding wheel
for emitting light in the direction of said encoding wheel, a
plurality of light sensitive means on the other side of each
encoding wheel for detecting when the slots in the encoding wheel
permit light from the light emitting means to impinge upon at least
one of the light sensitive means and for generating at least one
sensor output signal having a magnitude in response thereto, with
no mask interposed between the light emitting means and the light
sensitive means, and a plurality of comparator means responsive to
a plurality of said at least one sensor output signals for
comparing at least first and second of said sensor output signals
and generating at least one comparator output signal representative
of the relative magnitudes of said sensor output signals, each
comparator means providing a comparator output signal on a first
output port if the first sensor output signal is greater than the
second sensor output signal, and providing a signal on a second
output port if the second sensor output signal is greater than the
first sensor output signal, processor means responsive to the
comparator output signal for providing a cursor control output
representative of the movement of the ball in one dimension for
each of said shaft encoders.
2. The invention of claim 1 wherein each of the plurality of
comparator means and light sensitive means operatively associated
with a shaft encoder are fabricated within a single integrated
circuit..Iadd.
3. A control device for controlling a position on a display screen
wherein the control device uses an encoder wheel to convert
rotational movement into digital signals representing movement of
the position comprising: a light emitter for emitting light in the
direction of said encoding wheel; a plurality of light sensors for
detecting when the encoding wheel permits light from the light
emitter to impinge upon at least one of the light sensors and for
generating at least one sensor output signal having a magnitude in
response thereto, with no mask interposed between the light emitter
and the light sensors; and a plurality of comparators, each
responsive to sensor output signals from two of said sensors for
comparing at least first and second of said sensor output signals
and generating at least one comparator output signal representative
of the relative magnitudes of said sensor output signals, each
comparator providing a first comparator output signal if the first
sensor output signal is greater than the second sensor output
signal, and providing a second comparator output signal if the
second sensor output signal is greater than the first sensor output
signal..Iaddend..Iadd.
4. The device of claim 3 further comprising: a processor responsive
to the comparator output signals for providing a position control
output representative of the movement of said encoder
wheel..Iaddend..Iadd.
5. The device of claim 3 further comprising: at least one line
driver for transmitting said comparator output signals to a
computer..Iaddend..Iadd.
6. The device of claim 3 further comprising: a processor responsive
to the comparator output signals for providing a position control
output representative of the movement of said encoder
wheel..Iaddend..Iadd.
7. The device of claim 3 wherein said encoding wheel includes a
plurality of slits, and said light emitter is mounted on a first
side of said encoding wheel and said light sensors are mounted on a
second side of said encoding wheel..Iaddend..Iadd.
8. A pointing device comprising: an encoder rotatable in response
to a user movement; a light emitter for emitting light in the
direction of the encoder; a plurality of light sensors for
detecting when the encoder permits light from the light emitter to
impinge upon at least one of the light sensors, each of the light
sensors generating a sensor output signal; and a differential
sampling circuit coupled to said plurality of sensors for
generating digital signals corresponding to a difference between
two of said sensor output signals, said digital signals having
first and second digital values, and being said first digital value
when a first of said two of said sensor outputs is greater than a
second of said sensor outputs, and being said second digital value
when said second of said sensor outputs is greater than said first
of said sensor outputs..Iaddend..Iadd.
9. The pointing device of claim 8 wherein said differential
sampling circuit includes at least one
comparator..Iaddend..Iadd.
10. The pointing device of claim 8 wherein said plurality of light
sensors and said differential sampling circuit are fabricated
within a single integrated circuit..Iaddend..Iadd.
11. The pointing device of claim 8 further comprising: a processor
coupled to said differential sampling circuit for providing a
position control output representative of the movement of said
encoder..Iaddend.
Description
FIELD OF THE INVENTION
The present invention relates to pointing devices for cursors on
video display screens for personal computers and workstations, and
more particularly relates to optomechanical sensors for translating
rotation of a ball into digital signals representative of such
movement.
BACKGROUND OF THE INVENTION
Pointing devices, such as mice and trackballs, are well known
peripherals for personal computers and workstations. Such pointing
devices allow rapid relocation of the cursor on a display screen,
and are useful in many test, database and graphical programs.
Perhaps the most common form of pointing device is the electronic
mouse.
With a mouse, the user controls the cursor by moving the mouse over
a reference surface; the cursor moves a direction and distance
proportional to the movement of the mouse. Although some electronic
mice use reflectance of light over a reference pad, most mice use a
ball which is on the underside of the mouse and rolls over the
reference surface (such as a desktop) when the mouse is moved. In
such a device, the ball contacts a pair of shaft encoders and the
rotation of the ball rotates the shaft encoders, which includes a
mask having a plurality of slits therein. A light source, often an
LED, is positioned on one side of the mask, while a photosensor,
such as a phototransistor, is positioned substantially opposite the
light source. Rotation of the mask therebetween causes a series of
light pulses to be received by the photosensor, by which the
rotational movement of the ball can be converted to a digital
representation useable to move the cursor.
In conventional electronic mice, a quadrature signal representative
of the movement of the mouse is generated by the use of two pairs
of LED's and photodetectors. However, the quality of the quadrature
signal has often varied with the matching of the sensitivity of the
photosensor to the light output of the LED. In many instances, this
has required the expensive process of matching LED's and
photodetectors prior to assembly. In addition, varying light
outputs from the LED can create poor focus of light onto the
sensor, and extreme sensitivity of photosensor output to the
distance between the LED, the encoding wheel, and the
photosensor.
There has therefore been a need for a photosensor which does not
require matching to a particular LED or batch of LED's, while at
the same time providing good response over varying LED-to-sensor
distances.
In addition, many prior art mice involve the use of a mask in
combination with an encoder wheel to properly distinguish rotation
of the encoder wheel. Because such masks and encoder wheels are
typically constructed of injection molded plastic, tolerances
cannot be controlled to the precision of most semiconductor
devices. This has led, effectively, to a mechanical upper limit
imposed on the accuracy of the conventional optomechanical mouse,
despite the fact that the forward path of software using such mice
calls for the availability of ever-increasing resolution. There has
therefore been a need for a cursor control device for which
accuracy is not limited by the historical tolerances of injection
molding.
In addition, in some instances it is desirable to offer cursor
control devices with different resolutions. Thus, for example, in
some applications a cursor control device having a resolution of
200 dots per inch is appropriate, while in other applications a
cursor control device having a resolution of 400 dots per inch is
desired. In such circumstances, different mechanical components are
needed to implement such different resolutions, leading to
increased complexity and expense. This increased expense is
necessarily passed on to the consumer, creating more expensive
products. There has therefore been a need for an optomechanical
implementation for a cursor control device which can operate at
different resolutions, when combined with appropriate other
components.
SUMMARY OF THE INVENTION
The present invention substantially overcomes the foregoing
limitations of the prior art by providing an optical sensor
employing a differential sensing arrangement. Such an approach, as
described in greater detail hereinafter, substantially eliminates
the need to match LEDs and the associated photosensors.
Further, by appropriately locating multiple sensors on a single
substrate, and providing associated microprocessor control, it is
possible to eliminate the need for a mechanical mask. Such
elimination of the mechanical mask permits increased resolution by
removing the constraints on accuracy associated with injection
molding of plastics as compared to fabrication of
semiconductors.
In addition, the sensor may comprise multiple sensors on a single
wafer of silicon, permitting use at different resolutions simply by
altering a single mechanical component and reselecting the sensors
being monitored.
The present invention is also less sensitive to LED-to-sensor
distances than the prior art.
The pointing device of the present invention, which is operable
with electronic mice, trackballs, or other pointing devices which
convert rotational movement to digital signals, includes at least
one and typically two shaft encoders positioned to be rotated by
movement of a rotational member, such as a ball. The shaft encoder
includes a mask or encoding wheel having slits therethrough
conforming to the resolution of the pointing device in dots per
inch. Typical resolutions vary between two hundred and four hundred
dots per inch, although substantially higher resolutions are not
uncommon.
Positioned on either side of the encoding wheel for each shaft
encoder are two pairs of LED's and photosensors. The pairs are
arranged not to be along a diameter of the wheel.
A differential sampling circuit detects motion of the wheel past
the LED's, which causes the generation of a quadrature signal. The
quadrature signal is then provided to a microprocessor, where the
signal is sampled and manipulated as described in U.S. patent
application Ser. No. 07/717,187, now U.S. Pat. No. 5,256,913 ,
entitled Low Power Optoelectronic Device and Method and assigned to
the assignee of the present invention. A conventional cursor
control signal is then provided as the output of the
microprocessor, although appropriate line drivers and related
circuitry may be interposed. In particular, LED pulsing may be used
to save power, among other techniques described in the
aforementioned application.
In particular, the differential sensor of the present invention may
be implemented as a single chip on which a plurality of
photodetectors, such as photodiodes or phototransistors, may be
disposed. In a typical arrangement, two pairs of photosensors
spaced precise distances from one another are laid out on the
semiconductor, although in a presently preferred embodiment, six
sensors are fabricated into the semiconductor, with two of the
sensors used only for low resolution, two used for both low and
high resolution, and two used only for high resolution. In this
manner the same sensor may be used for, for example, 200 dpi and
400 dpi resolution. In either event, two pairs of sensors are used
at once.
The semiconductor bearing the photodetectors is positioned within
the cursor control device so that the photodetectors are spaced
apart from a pair of light source such as LEDs, with an encoding
wheel placed therebetween. Depending on the desired resolution of
the pointing device, the encoding wheel will have greater or fewer
slots therethrough by which the light (usually but not necessarily
infrared) is allowed to strike the photodetectors to indicate
movement. Greater numbers of slots typically translates into
increased resolution; the radial arrangement of slots about the
center of the encoding wheel is precisely managed to ensure that
light from the LEDs strikes the sensors only at the appropriate
times; more particularly, a period of light striking one pair of
detectors corresponds to a period of darkness at the adjacent
photodetector. By this technique, the photodetectors permit current
flow when the encoding wheel properly lines up, but are effectively
open circuits when not struck by light. Accordingly, the output of
the LEDs is a series of poorly shaped current pulses of different
phase.
To provide improved detection, a comparator circuit comprising a
plurality of current comparators receives on its A and B inputs the
pulse train from a respective pair of photodetectors. In the
presently preferred embodiment, the current comparators are
fabricated on the same chip as the photodetectors, although such an
arrangement is not in all cases required. In the presently
preferred embodiment, four comparators are used, with the A and B
inputs provided to each comparator being from selected ones of the
six photodetectors. Each comparator then generates signal on a
first output if A>B, and a signal on a second output if B>A.
These outputs provide the inputs to an associated four RS latches,
and in turn the output of the latches may be provided to a
microprocessor for sampling and manipulation as required to provide
an accurate representation of movement of the pointing device. In a
presently preferred embodiment, the RS latches are also fabricated
on the same substrate as the photodetectors and comparators.
It is therefore one object of the present invention to provide a
cursor control device having optomechanical sensors which do not
require matching of LEDs and photodetectors.
It is another object of the invention to provide a cursor control
device using a ball having differential sensors for detecting
rotational movement of the ball.
It is a further object of the present invention to provide a
monolithic photosensor having a plurality of photodetectors
disposed thereon for providing different resolutions of optical
sensing.
It is yet another object of the present invention to provide a
cursor control device which requires only an encoding wheel, a
light source and a photodetector for detecting the rotation of a
ball indicative of movement of the cursor control device.
These and other objects of the present invention will be better
appreciated from the following Detailed Description of the
Invention, taken in combination with the appended Figures.
FIGURES
FIG. 1 shows in exploded perspective view a pointing device, and in
particular a mouse, according to the present invention.
FIG. 2 shows in perspective view a cursor control device, and in
particular a trackball, in accordance with the present
invention.
FIG. 3 shows in schematic diagram form the electronics associated
with the cursor control or pointing devices of FIGS. 1 and 2.
FIG. 4 shows in block diagram form a photodetector, comparator
circuit and latch in accordance with the present invention.
FIG. 5A shows in greater detail in block diagram form the current
comparator and latch circuit of the present invention.
FIG. 5B shows at component level the circuitry shown in FIG.
5A.
FIG. 5C shows the output waveform of the delayed current mirror of
FIGS. 5A-5B.
FIG. 5D shows the hysteresis generated by the circuit of FIGS.
5A-5B.
FIG. 6 shows a series of waveforms as generated by the
photodetectors of the present invention in response to rotation of
the ball of the cursor control device.
FIG. 7 shows the layout of photodetectors on a monolithic
semiconductor for use at different resolutions.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, an electronic pointing device, and in
particular an electronic mouse 10, is shown in exploded perspective
view. The mouse 10 includes an upper housing 20, a printed circuit
board 30 close to which a ball cage 40 is juxtaposed, a lower
housing 50 (onto which the ball cage is sometimes mounted), a ball
60, and a belly door 70 which connects into the lower housing for
retaining the ball within the ball cage 40.
The printed circuit board 30 typically includes circuitry for
converting the analog movement of the ball 60 into digital signals,
and in particular typically includes a pair of shaft encoders 80A-B
which are maintained in engagement with the ball 60. The shaft
encoders 80A-B each include an encoder wheel 90A-B of the type
described in U.S. patent application Ser. No. 07/768,813, entitled
Integral Ball Cage for Pointing Device and commonly assigned with
the present invention, and incorporated herein by reference. Thus,
movement of the mouse causes rotational movement of the ball, and
that rotational movement is in turn converted into digital signals
which control the cursor on the screen of an associated personal
computer, terminal or workstation. In serial port mice, the printed
circuit board will typically include a microprocessor and related
driver circuitry for sending and receiving standard serial
communications, such as RS232 signals. Alternatively, if the mouse
is a bus device, the intelligence will typically be found on a
circuit board installed within the PC, and the circuit board in the
mouse will simply comprise photodetectors and associated signal
shaping circuitry, together with line drivers for transmitting the
signal to the board in the PC.
Referring next to FIG. 2, a trackball 200 is shown in perspective
view. While trackballs are typically similar to electronic mice in
terms of the optomechanical aspects, the physical aspects of
supporting the ball can be and usually are quite different. Thus,
the trackball 200 includes an upper housing 210 and a lower housing
220, between which is sandwiched a ball 230. Also enclosed within
the upper and lower housings are ball supporting element such as
described in U.S. Pat. No. 5,008,528, entitled Invertible
Trackball, which convert rotational movement of the ball into
cursor control signals just as with the mouse of FIG. 1.
Referring next to FIG. 3, the circuitry included on the printed
circuit board 30 of FIG. 1 may be better appreciated. In
particular, a pair of LED's 300A-300B generate photons in
accordance with a control signal applied along a line 302 to the
base of a current amplifier transistor 304, the collector of which
is connected to V.sub.cc. The control line 302 is controlled by a
processor 306, which typically pulses the LEDs 300A-B to save power
although such pulsing is not required. One method for pulsing such
LEDs is described in U.S. patent application Ser. No. 07/717,187,
now U.S. Pat. No. 5,256,913, referenced earlier, and which is
incorporated herein by reference.
In appropriate circumstances, light from the LEDs 300A-B strikes
photodetector circuits 308A-B. As will be discussed in connection
with FIG. 4, each of the photodetector circuits 308A-B in fact
comprises an array of photodetectors and related circuitry,
including associated comparators and latches. It will be
appreciated by those skilled in the art that, although not shown in
FIG. 3, the encoder wheels 90A-B shown in FIG. 1 are in fact
interposed between the LEDs 300 and the photodetector circuits 308,
and light from the LEDs reach the photodetectors only when the
slots in the encoder wheels provide a path from the LED to the
photodetector.
The output of the photodetector circuits 308A-B is provided to the
processor 306, where it can be sampled and manipulated in the
manner taught by the aforementioned U.S. patent application Ser.
No. 07/717,187, now U.S. Pat. No. 5,256,913.
In addition, control signals may be provided to the processor by
means of user operated switches 310A-C. Finally, for the embodiment
shown in FIG. 3, the processor receives and outputs conventional
RS-232 signals through a plurality of lines 320 which comprise a
serial port 330.
Turning next to FIG. 4, the photodetector circuits 308 are shown in
schematic block diagram form. In particular, each circuit 308
includes an array of photodetectors 400, and in the presently
preferred embodiment comprises six such photodetectors 400A-F. In
some instances it is not necessary to provide such a plurality of
sensors; however, this arrangement has the advantage of permitting
different resolutions depending on the type of encoding wheel 90
used and which photodetectors 400A-F are selected for
monitoring.
In the exemplary embodiment show in FIG. 4, for example,
photodetectors 400A and 400C may be used for both 200 dpi and 400
dpi resolution, while photodetectors 400B and 400D are used only
for 400 dpi resolution and photodetectors 400E and 400F are used
only for 200 dpi resolution. Regardless which resolution is chosen,
the operation of the photodetectors is to generate a pulse train on
output lines 410A-H in response to light received from the LEDs 300
through the encoder wheels 90.
It should be noted that the photodetectors 400 can be either a
photodiode or a phototransistor. In a presently preferred
embodiment, a photodiode is used because of its faster response
times. However, phototransistors, and particularly Darlington
pairs, are also acceptable in many instances.
The output(s) of each photodetector 400A-F is paired with output of
another photodetector of the same resolution, and each pair of
output 410 provides the inputs to one of an array of current
comparators 420A-D. Thus, for example, outputs 410A and 410D
provide the paired inputs to comparator 420A, while outputs 410C
and 410F provide the paired inputs to comparator 420B. For purposes
of example, these pairings may be considered to represent the
higher resolution. Thus, pairs 410B/410G and 410E/410H and their
associated comparators 420C and 420D may be considered to represent
the lower resolution. It will be apparent to those skilled in the
art that numerous additional resolutions could be provided simply
by providing additional photodetectors 400 and associated
circuitry.
The current comparators 420A-D each compare their respective A and
B inputs, and provide a comparator output signal on a first output
422A-D if the A input signal is greater than the B input signal.
Similarly, a comparator output signal is provided on a second
output 424A-D if the B input signal is greater than the A input
signal.
The output signals 422A-D each provide a set input to an associated
one of four RS latches 430A-D, while the output signals 424A-D each
provide a Reset input to the associated RS latch 430A-D. For each
resolution, two latches are operable, such that two of the outputs
of the latches 430A-D are provided to the processor 306 (FIG. 3)
from each photodetector circuit 308, as shown in FIG. 3.
Taking FIGS. 5A and 5B in combination with FIG. 4, the operation of
one channel (i.e., one comparator 420 and one associated latch 430)
can be better appreciated. For purposes of illustration, the
photodetectors 400A and 400C, comparator 420A and latch 430A have
been arbitrarily selected. Referring first to FIG. 5A and
concurrently to the more detailed diagram of FIG. 5B, the
photodiodes 400A and 400C generate a current I.sub.x and I.sub.y
respectively, in response to impinging light from the LEDs 300.
The currents I.sub.x and I.sub.y are each mirrored in respective
current mirror circuits 510A and 510B, in a circuit which is
symmetrical for each input. In addition, delayed current mirror
circuits 512A-B, respectively, are provided. The result is that the
currents I.sub.x and I.sub.y are mirrored with a ratio of b>1
and a<1. The output S' of the delayed current mirror 512A is the
result of the comparison of I.sub.x with b1.sub.y +a1.sub.x ;
similarly, the output R' of the delayed current mirror 512B is the
result of the comparison of I.sub.y with b1.sub.x +a1.sub.y ; the b
mirror factor is provided by a pair of circuits 514A-B
cross-connected to I.sub.y and I.sub.x, respectively. It can thus
be appreciated that the output S' is driven to ground when the
current in the diode 400A is b/(1-a) bigger than in the diode 400C,
while the output R' is driven to ground when the current in the
diode 400C is b/(1-a) bigger than in the diode 400A. The outputs
are typically inverted in inverters 516A-B before feeding the RS
latch 430A.
To ensure proper operation of the RS latch, some precautions are
appropriate. More particularly, maintaining a ratio b>1 ensures
that for the same current I.sub.x =I.sub.y, the signals S' and R'
are high; that is, S'=R'=1. Additionally, a ratio a<1 in the
delayed mirror circuits ensures that the outputs return to 1 after
the LED is turned off and before the next flash occurs. The time
constant associated with the output nodes S' and R' is proportional
to the capacitance at the nodes and inversely proportional to the
current charging or discharging the capacitance--that is, the time
constant depends on the how much light illuminates the
photodetectors. When the LED is turned off, only dark currents are
available through the photodiodes. To maintain a high current in
this branch, so that the capacitance can be charged in a time t,
I.sub.x and I.sub.y are stored in the delayed mirror circuits
512A-B. This ensures that the outputs return to 1 with a time
constant much smaller than the pulsing frequency of the LEDs. The
output waveform of the delayed current mirror can be better
appreciated from FIG. 5C.
In addition, the implementation shown in FIGS. 5A-5B using the
current comparator and RS latches introduces an hysteresis
function, with the hysteresis thresholds determined by the values
of a and b. Referring to FIG. 5D, the hysteresis of the comparator
can be better appreciated. The first plot of FIG. 5C shows the
outputs of the current comparators as a function of the current
ratio, whereas the second plot shows the outputs of the current
comparators as a function of the values of the diode currents.
Finally, the third plot of FIG. 5C shows the output of the RS latch
as a function of the current ratio.
To ensure sufficient hysteresis to avoid noise and component
mismatch problems, yet small enough for the available semiconductor
area and current consumption, values of b=3 and a=1/3 have been
implemented successfully. This yields a ratio of b/(1-a)=4.5. The
ratios can be implemented by designing three parallel transistors
in one branch of the mirror, and only one transistor in the other
branch, as best seen from FIG. 5B. It will be appreciated by those
skilled in the art that the entire detector 308 comprises a
plurality of the circuits shown in FIG. 5A-5B; in particular, the
presently preferred embodiment comprises four such circuits which
effectively operate independently.
Referring next to FIG. 6, the waveforms at the outputs of the
photodetectors 400 and the comparators 420 can be better
appreciated. More particularly, the appearance of the waveforms
with movement of the ball to the left or right can be understood.
In particular, given that four photodetectors are operable for
either resolution, four photodetector outputs are shown as D1, D2,
D3 and D4, with rotation to the left and to the right as shown at
the top of the figure. In turn, the final output of the associated
comparators 420 are designated C1 and C2.
From the foregoing, it can be appreciated that the current
invention provides a simplified optomechanical encoder for use with
cursor pointing device in that it eliminates the need for a mask
while at the same time making it possible to have high
resolution.
In addition, the differential sensing of the present encoder
substantially eliminates the need to match LEDs and photodetectors,
thus substantially simplifying automated assembly.
In addition, the use of an array of photodetectors in a single
sensor, with specific sensors monitored based on desired
resolution, provides simplified assembly more conducive to
automation.
To better appreciate the spatial relationships, reference is made
to FIG. 7, in which the upper diagram shows the spatial
relationship between disk slots 700 and photodiodes 400 in a 200
dpi implementation, where the values of d.sub.space and d.sub.slot
are on the order of 0.623 mm and 0.2 mm, respectively. In the lower
diagram of FIG. 7 is show the corresponding spatial relationship
between disk slots 710 and photodiodes 400 for a 400 dpi
implementation, where the distances d.sub.space and d.sub.slot are
on the order of 1.044 mm and 0.47 mm.
Having fully described a preferred embodiment of the present
invention together with alternatives, it will be apparent to those
of ordinary skill in the art that numerous alternatives and
equivalents exist which do not depart from the invention set forth
above. It is therefore to be understood that the invention is not
to be limited by the foregoing description, but only by the
appended claims.
* * * * *