Touch sensor and controller

A touch sensor is disclosed that senses the two-dimensional position and pressure of an object 34, such as a stylus or finger, that is touching its surface. The sensor is comprised of two insulating substrates 28 and 29 which extend over the area to be sensed. A first fixed resistor 33 establishes a potential gradient over the first substrate 29 in a first, or X, position dimension. A second fixed resistor 32 establishes a potential gradient over the second substrate 28 in a second, or Y, position dimension. Between the substrates is a force variable resistor 42 that changes its local resistance under the touch point as a function of the touch pressure. Two touch controllers are also disclosed that comprise the touch sensor 50 and an electronic circuit that accurately measures the resistance values of the touch sensor. The touch controllers output three signals: the X position, the Y position, and the pressure of the touch point. The first touch controller uses a current regulator 51 to supply a constant current through the touch sensor. The second touch controller uses a voltage regulator 67 to supply a constant voltage across the touch sensor.

FIELD OF THE INVENTION 
This invention relates to devices for providing information, in the form of 
electrical signals, about the position and pressure of a selected touch 
point on the surface of a touch sensor. In particular, these devices are 
well suited for use as human-computer interfaces, both as general computer 
input devices and as controllers for dedicated electronics systems. 
BACKGROUND OF THE INVENTION 
Many touch controllers (also known as "pads", "panels", "tablets", or 
"digitizers") are known in the prior art. U.S. Pat. No. 4,129,747 (Pepper, 
Jr., 1978) describes a two-axis pressure-sensitive touch controller that 
uses a resistive sheet with sinusoidal phase fields established across its 
surface. In one embodiment, the user directly contacts the resistive 
sheet, thereby changing the impedance of the resistive sheet at the touch 
point, however this gives unreliable and unpredictable results because the 
impedance of the user's body is unpredictable. Another embodiment uses a 
conductive pickup layer in a sandwich with the resistive layer, which 
relies only on the area of touch for sensing the pressure dimension, which 
is also an unreliable technique. Further, both embodiments require 
complicated electronics to generate two sinusoids in quadrature and 
accurately detect phase shifts. 
U.S. Pat. No. 4,293,734 (Pepper, Jr., 1981) improves on the first 
embodiment of U.S. Pat. No. 4,129,747, however this device requires 
rectifiers and analog dividers which reduce the accuracy of the position 
data. 
U.S. Pat. No. 4,644,100 (Brenner et. al., 1987) describes a 
pressure-sensitive touch panel based on surface acoustic wave propagation 
over a glass substrate. This technology is expensive to apply; it requires 
a specialized sensor with etchings and attached transducers, and 
complicated and expensive electronics. Further, it operates on the 
principle that the user's finger acoustically dampens the propagating 
signal, which produces an unpredictable pressure measurement. 
Many non-pressure-sensitive touch tablets based on resistive technologies 
have been described in the prior art, including U.S. Pat. Nos. 4,570,149 
(Thornburg et al., 1986); 4,587,378 (Moore, 1986); 4,752,655 (Tajiri et 
al., 1988); and 4,897,511 (Itaya et al., 1990). Although the present 
invention has similarities to some of these devices, they do not provide 
for the sensing of continuous variation in touch pressure. 
Of particular interest among non-pressure-sensitive touch tablets are U.S. 
Pat. Nos. 4,475,008 (Doi et al., 1984) and 4,775,765 (Kimura et al., 
1988). Both patents disclose multilayer devices that employ an 
intermediate pressure-sensitive layer which decreases in resistance 
locally at the point where applied pressure is increased. However, both 
devices only use this material in a switching mode and do not provide for 
continuous sensing of pressure. 
U.S. Pat. No. 4,798,919 (Miessler et al., 1989) describes a 
pressure-sensitive touch tablet based on a single semiconductive resistive 
sheet, facing a conductive sheet. Applied pressure causes the resistive 
sheet to reduce its resistance locally at the touch point, while its 
resistance remains constant everywhere else. The nature of the resistive 
sheet results in nonlinearity between the true touch position and the 
reported position. Further, the driving electronics require analog 
division which further reduce the device's accuracy, and further increase 
its cost and complexity. 
U.S. Pat. No. 4,739,299 (Eventoff et al., 1988) describes a 
pressure-sensitive touch pad using a pressure-sensitive resistor layer. 
The touch position is detected using fixed-value resistive sheets, which 
suffer the same problem of touch position nonlinearity as U.S. Pat. No. 
4,798,919. 
U.S. Pat. No. 4,810,992 (Eventoff, 1989) discloses another 
pressure-sensitive touch pad wherein the touch position is linearized 
within the sensor using a pattern of parallel conductive traces attached 
to a fixed resistor, in a similar fashion to the earlier 
non-pressure-sensitive resistive touch pads. However, this solution 
required the touch sensor to be divided into two overlapping but 
electrically isolated touch sensors, one sensor for each position 
dimension. This arrangement reduces the touch sensitivity and reliability 
of the device because the activating force must press through the upper 
sensor in order to activate the lower sensor. Further, current flows 
across the plane of the force variable resistor sheet, rather than 
perpendicular to the plane, which increases the mechanical contact noise; 
decreases the standoff resistance; and often exhibits an unnatural 
force-to-resistance curve. This embodiment also produces two pressure 
signals, which is more complicated and ambiguous to process than a single 
pressure signal. 
U.S. Pat. Nos. 4,734,034 (Maness et al., 1988) and 4,856,993 (Maness et 
al., 1989) describe a pressure-sensitive contact sensor which comprises a 
simple and reliable touch sensor, however the sensor is used in a scanning 
mode. While this has the advantage of detecting multiple independent touch 
points, it comes at the cost of requiring an enormous number of terminal 
contacts from the sensor to the connecting electronics; the scanning 
hardware is fairly complex and requires a high-speed analog-to-digital 
converter; the response time is slow due to the time required to complete 
a scan; very high data rates are produced which incur a computational and 
memory overhead to process and interpret the data; and the system cost and 
complexity increase exponentially for linear increases in touch sensor 
size or resolution. In many applications, only a single touch point need 
be detected, so that the overhead incurred by scanning is uneconomical. 
Copending U.S. patent application Ser. No. 07/497,691 (Asher, filed Mar. 
22, 1990), U.S. Pat. No. 5,008,497 describes a touch controller which 
comprises a current regulator and differential amplifier as an improved 
electronic circuit for measuring the position and pressure of a touch 
point on a touch sensor that uses a resistive membrane and force variable 
resistor. The present invention, however, discloses a touch sensor that 
represents significant improvements over the prior art of touch sensors. 
Accordingly, the present invention also discloses two touch controllers 
that comprise electronic circuits that are further improved and optimized 
for the new touch sensor. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
The principle object of the present invention is to provide a new touch 
sensor design that passively detects the two-dimensional position and 
pressure of an object touching its surface through changes in resistance, 
which changes may then be measured by an electronic circuit. Of particular 
significance is that, compared to the prior art, the new touch sensor 
maximizes performance while achieving very low production cost. 
The performance criteria include: position resolution, pressure resolution, 
wide dynamic range of pressure sensitivity, fast response time, position 
accuracy over the full dynamic range, simultaneous contact of X and Y 
dimensions, linearity of the position measurements to actual position, 
"natural" response of the pressure measurement to actual pressure, and low 
susceptibility to contact noise and external interference. 
Cost criteria include: amount of raw materials used, type of materials 
used, ease of manufacture, simplicity of electrical interconnection to 
external circuits, and inelasticity of circuit and system cost to changes 
in touch sensor size and resolution. 
Another object of the present invention is to provide numerous variations 
on touch sensor design and fabrication methods so that cost and 
performance may be optimized for particular applications. 
Another object of the present invention is to provide a touch controller 
that comprises the touch sensor and an electronic circuit which interfaces 
to the touch sensor, providing measurements of the two-dimensional 
position and pressure of the object touching the surface of the touch 
sensor. The outputs of the touch controller are direct, linear 
representations of the actual touch position and pressure. 
Another object of the present invention is to provide a touch controller 
that reports the two-dimensional position and pressure of the object, the 
measured outputs having high accuracy, resolution, and fast response time, 
and the electronic circuit having low cost and complexity. Numerous 
variations on touch controller circuit design are presented so that cost 
and performance may be optimized for particular applications. 
Another object of the present invention is to enable the user to activate 
the touch sensor with a finger or with a stylus while not requiring 
electrical connection to the finger or stylus. This feature allows a wide 
variety of surface coverings over the touch sensor, such as Lexan or 
Teflon, which protect the sensor, provide different tactile surfaces, and 
allow for limitless graphic overlays to support the user. 
In general computing applications, the present invention can be used as a 
cursor control pointing device, as well as many other ways. In the paper 
"Issues and Techniques in Touch-Sensitive Tablet Input", 1985, William 
Buxton et al. discuss human-factors issues in using touch tablets for 
human-computer interaction, and in particular cite the need for 
pressure-sensitive touch controllers. 
A pressure-sensitive touch controller can perform the input and cursor 
control functions of a mouse, joystick, trackball, light pen, or digitizer 
tablet, with the enhanced feature that the pressure signal provides an 
additional independent control dimension. The touch position can be mapped 
to absolute video screen coordinates, as with digitizer tablets and light 
pens. The touch position can also be mapped to relative video screen 
coordinates, as with trackballs and mice. The touch position can also be 
mapped to changes in screen coordinates, as with joysticks. Graphic 
overlays can also indicate multiple touch zones, or fixed-function areas. 
A small touch controller can be embedded directly into a laptop computer or 
notebook computer, or a larger touch controller can be used as a separate 
input peripheral to a desktop computer or terminal. 
In typical office software applications, such as word processors and 
spreadsheets, the pressure dimension could be used to modulate cursor 
speed, data entry, scrolling rates of file lists or document pages, etc. 
As a graphics input device, the present invention can be used as a drawing, 
painting, or calligraphy tool where the pressure dimension modulates line 
width or shape parameters. 
One specialized computer input application is interactive control of video 
games. A pressure-sensitive touch controller would enable the player to 
have three dimensions of control of a virtual on-screen object with a 
single, intuitive touch gesture: the player could fly a virtual plane or 
paint in a virtual coloring book. 
Another specialized computer input application is control of electronic 
musical instruments. Although touch devices, such as ribbon controllers, 
have been known in the industry for many years, the pressure dimension 
provides a critical control parameter. The pressure dimension can control 
the volume and timbre of a sound in such a musically intuitive way that 
the performance nuance of a clarinet, violin, or trombone can be imitated 
with simple finger gestures. 
In dedicated control applications, three-dimensional processes can be 
easily controlled with a pressure-sensitive touch controller, such as 
steering a robotic manipulator or controlling the imaging process of 
medical instrumentation. The pressure dimension can further act as a "dead 
man's handle" in safety-critical situations where the presence of touch 
must be unambiguously detected. 
The present invention represents a very general technology that has many 
specific applications in the growing field of human-computer interaction. 
Particularly, in the field of pressure-sensitive touch controllers, this 
invention achieves a new level of high performance and low cost that has 
not been demonstrated in the prior art. 
SUMMARY OF THE INVENTION 
In its most general form, the touch sensor of the present invention 
comprises a lower, or X, substrate; an X fixed resistor which establishes 
a potential gradient along an X dimension relative to the X substrate; two 
X terminals connected to the X fixed resistor; an upper, or Y substrate; a 
Y fixed resistor which establishes a potential gradient along a Y 
dimension relative to the Y substrate; two Y terminals connected to the Y 
fixed resistor; and a force variable resistor sandwiched between the upper 
(Y) and lower (X) substrates. 
When a disjunct member, such as a finger or stylus, presses on the upper 
substrate of the touch sensor, the local resistance of the force variable 
resistor under the touch point decreases, and further decreases as the 
disjunct member increases pressure. The touch point also temporarily 
divides the X fixed resistor into two segments, and similarly divides the 
Y fixed resistor into two segments; the fixed resistor segments forming 
electrical nodes with the force variable resistor. 
The touch sensor itself is passive; it only represents the position and 
pressure of the touch point through changes in resistance, which are 
electronically measured to produce distinct signals representing the touch 
point. The position of the disjunct member on the surface of the touch 
sensor can be determined by measuring the resistance of the fixed resistor 
segments, and the pressure can be determined by measuring the resistance 
of the force variable resistor. 
Many embodiments of the touch sensor are possible, including but not 
limited to: cartesian coordinate system position measurement; polar 
coordinate system position measurement; single-substrate assembly; 
two-sheet substrate assembly; integration into printed circuit boards; 
thin-film fixed resistors deposited on substrate; discrete fixed resistor 
networks; potential gradient established by a set of conductive traces; 
potential gradient established by a set of fixed resistor traces; 
potential gradient established by a resistive sheet; one or two force 
variable resistor layers; force variable resistor comprised of traces; 
force variable resistor comprised of a sheet; and multiple touch zones in 
the X and/or Y dimensions. 
A touch controller, according to the present invention, employs an 
electronic circuit connected to the touch sensor. The circuit measures the 
resistances of the fixed resistor segments and of the force variable 
resistor, and outputs three signals: the X position of the touch point, 
the Y position of the touch point, and the touch pressure. The outputs of 
the electronic circuit are analog voltages that represent the touch point. 
In a typical application, the analog voltages are converted to digital 
signals by an analog-to-digital converter, so that they may be read into 
and interpreted by a microprocessor or computer. 
A preferred embodiment of the electronic circuit of the touch controller 
comprises a current regulator and a differential amplifier connected to 
the touch sensor's terminals. The principle of operation is, no matter 
what the value of the force variable resistor might be, the current 
regulator guarantees a constant current flowing through the fixed 
resistors and through the force variable resistor. 
As a result of the current regulation through the touch sensor, the 
differential voltage across either fixed resistor is linearly proportional 
to the touch position and does not require any scaling adjustments to 
compensate for varying touch pressure. This differential voltage is 
detected and amplified with a differential amplifier. Similarly, the 
differential voltage across the force variable resistor is linearly 
proportional to the resistance of the force variable resistor. Typically, 
this resistance is inversely proportional to the applied pressure. 
There are many alternative construction techniques of the preferred 
embodiment of the touch controller, including but not limited to: 
ground-referenced current regulator with grounded touch sensor; floating 
current regulator with floating touch sensor; independent differential 
amplifiers for X and Y position measurement; pressure measurement derived 
from the current regulator output; pressure measurement derived from the 
voltage differential between common-mode position measurements; Wheatstone 
resistor bridges connected across each fixed resistor; current mirrors 
connected across each fixed resistor; single differential amplifier 
multiplexed across the touch sensor terminals with multiplexed X, Y, and 
pressure outputs; and additional analog multiplexers for calibration of 
the touch sensor's fixed resistors. 
An alternative embodiment of the electronic circuit of the touch 
controller, which trades lower component cost for lower performance, 
comprises a voltage source, a single-ended amplifier, a set of analog 
multiplexers, and a pulldown resistor. The multiplexers switch the 
configuration of the voltage source and the single-ended amplifier across 
the terminals of the touch sensor. Position is measured by switching the 
voltage source across a fixed resistor, and detecting the voltage at the 
touch point through (i.e. in series with) the force variable resistor. 
Pressure is measured by switching the voltage source across the whole 
touch sensor in series with the pulldown resistor, forming a voltage 
divider.

REFERENCE INDICIA OF THE DRAWINGS 
10,11 . . . X terminals (conductive traces) 
12,13 . . . Y terminals (conductive traces) 
20 . . . insulating substrate including X and Y substrates 
21 . . . substrate tab for electrical connectors 
22 . . . fold between X substrate and Y substrate 
23 . . . insulating film covering substrate traces 
24,25 . . . insulating film aperture exposing Y, X traces 
26 . . . connector from Y terminals onto X substrate 
27 . . . outline from Y substrate onto X substrate 
28,29 . . . Y, X insulating substrates 
30,31 . . . Y, X conductive traces over sensing area 
32,33 . . . Y, X fixed resistors 
34 . . . disjunct member (finger, stylus, etc.) 
40,41 . . . Y, X force variable resistor traces 
42 . . . force variable resistor sheet 
50 . . . touch sensor schematic symbol 
51 . . . current regulator 
52,53,54 . . . differential amplifiers: X, Y, pressure outputs 
55 . . . operational amplifier for current regulator 
56,57 . . . differential amplifiers with common mode outputs 
58,59 . . . 1.times.2 analog multiplexers 
60-63 . . . 1.times.4 analog multiplexers 
64 . . . differential amplifier, multiplexed output 
65 . . . single-ended amplifier, multiplexed output 
66 . . . analog single-pole single-throw switch 
67 . . . voltage regulator 
Q1 . . . current reference transistor 
Q2,Q3 . . . Y current mirror transistors 
Q4,Q5 . . . X current mirror transistors 
R1 . . . current reference resistor 
R2,R3 . . . Y Wheatstone Bridge resistors 
R4,R5 . . . X Wheatstone Bridge resistors 
R6 . . . pull-down resistor 
DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the basic fixed resistor and conductive trace pattern for a 
touch sensor that measures position in a cartesian coordinate system over 
a roughly square area. This basic touch sensor design is economical in 
that it is formed from a single substrate and therefore requires minimal 
assembly; all electrical terminations are brought forth from the touch 
sensor on the same side of a single connector lead, thereby simplifying 
interconnection to external circuits; and all conductive elements form a 
pattern that does not require an insulating layer to be deposited or 
inserted, which is often required to prevent such elements from 
electrically shorting together. 
An insulating film 20 forms the substrate for the touch sensor. The 
insulating film could be a polyester film such as Mylar, which provides a 
cost-effective substrate for many applications. Other films may be used, 
such as Kapton, which provide greater dimensional stability over a wide 
temperature range, but at higher cost. Other insulating films may also be 
used. 
Conductive traces 10-13, 30, and 31 are deposited on the substrate. The 
conductive traces may be directly printed on the substrate using 
electrically conductive inks. Other methods may also be used, such as 
photo-etching of a thin metallic film that is laminated to the substrate. 
See Flexible Circuits by Steve Gurley, 1984. 
One set of conductive traces 31 is arranged to detect position along the X 
dimension (horizontal) and another set of conductive traces 30 is arranged 
to detect position along the Y dimension (vertical). Two conductive traces 
10 and 11 form the electrical terminals for external circuitry that 
measures touch position along the X dimension, and two other conductive 
traces 12 and 13 form the electrical terminals for external circuitry that 
measures touch position along the Y dimension. The pressure measurement is 
also derived from these terminals, as will be subsequently demonstrated. 
Terminals 10-13 are brought forth from the touch sensor on a connector tab 
21 that is conveniently cut from the substrate 20, such that all interface 
connections from the touch sensor to external circuitry are positioned on 
the same surface of a single substrate, which simplifies the fabrication 
of products that employ this touch sensor. However, two individual 
substrates for the X and Y traces may also be used with the terminals 
brought forth on independent connector tabs. 
Two strips having fixed resistance 32 and 33 are also deposited on the 
substrate. The technology of formulating and applying conductive and 
resistive inks is well known. See Screen Printing Electronic Circuits by 
Albert Kosoloff, 1980 and 1984. 
One fixed resistor 33 physically overlays one end of each of the X 
conductive traces 31, and each end of this resistive strip is connected to 
terminals 10 and 11. The other fixed resistor 32 overlays one end of each 
of the Y dimension conductive traces 30, and each end of this resistive 
strip is connected to terminals 12 and 13. Typical values for these fixed 
resistors range from 10K ohm to 100K ohm, as measured from the terminals, 
although resistance values outside of this range may also be used and the 
resistance values may be optimized for particular applications. 
It can be seen in FIG. 1 that the X fixed resistor 33 with terminals 10 and 
11 establish a potential gradient that varies with position. Each of the X 
conductive traces 31 is placed at a unique position (and electrical 
potential) along the X fixed resistor 33. The same is true for the Y fixed 
resistor 32, Y terminals 12 and 13, and Y conductive traces 30. For most 
applications, it is preferred that the resistive strips 32 and 33 have a 
constant resistance gradient over their length, and that the conductive 
traces 30 and 31 have equidistant intertrace spacing, so that the position 
measurements vary linearly with the true position of the object being 
detected. However, non-linear relationships between true and measured 
position are easily achieved by modifying the resistance gradient or the 
spacing between adjacent conductive traces. 
To assemble the touch sensor, the substrate is simply folded in half along 
the dotted line 22, so that the X conductive traces 31 overlap, are 
oriented orthogonally to, and are facing the Y conductive traces 30. It is 
preferred that the touch sensor be glued or taped around the edges in 
order to keep it flat, although other methods of fastening may be used. 
Before the touch sensor is sealed, however, a force variable resistor is 
added between the X conductive traces 31 and the Y conductive traces 30. 
Several methods of accomplishing this will be demonstrated in subsequent 
figures. 
FIG. 2 shows the touch sensor of FIG. 1 with a set of X force variable 
resistor traces 41 deposited over the X conductive traces 31, and with a 
set of Y force variable resistor traces 40 deposited over Y conductive 
traces 30. It is also possible to deposit a continuous force variable 
resistor layer that covers all of the X conductive traces, and a second 
continuous force variable resistor layer that covers all of the Y 
conductive traces, however individual traces will exhibit lower resistive 
crosstalk between adjacent conductive traces and therefore provide for 
higher accuracy. 
It is also possible to cover only one side of the conductive traces (say, 
the X traces only) with a force variable resistor. However, it is 
preferred that both sets of conductive traces be covered with a force 
variable resistor so that the conductive traces are not openly exposed to 
the air, which can deteriorate them over time. Also, contact between two 
force-variable resistor layers exhibits lower mechanical contact noise 
than between a force-variable resistor and exposed conductive traces. 
Once the touch sensor is completed by folding along 22, pressure from an 
external object on the outside of the touch sensor substrate 20 will cause 
at least one X force variable resistor trace 41 to contact at least one Y 
force variable resistor trace 40, thereby allowing current to flow from 
the X terminals 10 and 11, through the X fixed resistor 33, through an X 
conductive trace 31, through an X force variable resistor trace 41, 
through a Y force variable resistor trace 40, through a Y conductive trace 
30, through the Y fixed resistor 32, and finally through the Y terminals 
12 and 13. 
In a preferred embodiment, the force variable resistors 40 and 41 are 
printed from a semiconductive ink that has the characteristics of behaving 
like an open circuit (infinite resistance) when there is no pressure 
applied, and monotonically decreasing in resistance as the applied 
pressure is increased. Preferably, the resistance of the force variable 
resistor is inversely proportional to the applied force, but other 
force-to-resistance curves may be formulated and preferred in particular 
applications. 
The distinction between the terms "force" and "pressure" should be noted in 
this context, as the field of art supports such terms as 
"pressure-sensitive resistor" and "force-sensing resistor". Usually, these 
resistors are implemented as thin films or sheets, and the sense of force 
is understood to be perpendicular to the plane of the film or sheet, as is 
the direction of current flow. The resistance will change as a function of 
the force applied over a constant-sized area, as distinct from some 
sensors that change resistance as a function of the area over which the 
force is applied. The present invention senses the total force, rather 
than the average pressure--the total force being the integral of all 
pressures over the sensor surface. 
Various force-sensitive resistive inks and coatings are known in the art, 
including: U.S. Pat. No. 4,745,301 (Michalchik 1988); U.S. Pat. No. 
4,790,968 (Ohkawa et al. 1988); U.S. Pat. No. 4,315,238 (Eventoff 1982); 
and U.S. Pat. No. 4,856,993 (Maness et al. 1989), which are hereby 
incorporated by reference. 
Another method of employing a force variable resistor is demonstrated in 
FIGS. 3 and 4. FIG. 3 shows the touch sensor of FIG. 1, with an insulating 
layer 23 having an aperture 25 for traces 31 and another aperture 24 for 
traces 30. The insulating layer 23 can be printed or otherwise deposited 
onto the substrate, or alternatively could be an insulating film, such as 
Mylar, which is simply inserted in the touch sensor when it is assembled. 
The insulating layer could be used to prevent the various conductive 
traces and fixed resistors from making unwanted electrical contact. 
A slightly thick insulating film, say 5 to 10 mils, could also be used to 
provide a standoff gap between the upper and lower substrate. This gap 
would ensure that no contact is made between traces of the upper and lower 
substrates when there is no external pressure on the touch sensor. 
However, when the force variable resistor exhibits infinite resistance 
with little or no applied force, such a gap is unnecessary. 
FIG. 4 shows a force variable resistor sheet 42 which is placed between the 
X conductive traces and Y conductive traces (as opposed to being printed, 
sprayed, coated, etc.). Various methods for producing force-sensitive 
resistor sheets are also known in the art, including: U.S. Pat. No. 
4,273,682 (Kanamori 1981); U.S. Pat. No. 4,302,361 (Kotani et al. 1981); 
U.S. Pat. No. 4,258,100 (Fujitani 1981); and U.S. Pat. No. 4,252,391 (Sado 
1981), which are hereby incorporated by reference. Of particular interest 
in this application is Sado's patent for an anisotropically 
electroconductive sheet, which has a force-variable resistance in the 
direction perpendicular to the plane of the sheet, and high resistivity in 
all directions within the plane of the sheet. 
FIG. 5 shows a different method of forming the potential gradient. The X 
terminals 10 and 11 are extended over the sensing area along the Y 
dimension, and Y terminals 12 and 13 are similarly extended over the 
sensing area along the X dimension. The X fixed resistor 33, rather than 
being a single resistor outside of the sensing area, is now comprised of 
many X fixed resistor traces, each trace touching the X terminals 10 and 
11, and the set of X fixed resistor traces being distributed along the Y 
dimension. Similarly, the Y fixed resistor 32 is comprised of a set of Y 
fixed resistor traces, each trace touching the Y terminals 12 and 13, and 
the set of Y fixed resistor traces being distributed along the X 
dimension. 
The touch sensor of FIG. 5 may have a force variable resistor applied as in 
any of the above stated methods: deposited traces, deposited sheet, or 
inserted sheet. This touch sensor functions similarly to the touch sensors 
of FIGS. 1-4, except that the sets of conductive traces extending over the 
sensing area are not required. 
FIG. 6 shows yet a different method of forming the fixed resistor in order 
to effect position-sensing. The X terminals 10 and 11 are extended over 
the sensing area along the Y dimension, and the Y terminals 12 and 13 are 
extended over the sensing area along the X dimension. The fixed resistors 
32 and 33, in this case, are thin film resistive sheets that are deposited 
on the substrate 20, each sheet contacting its respective terminals 10-13, 
and extending over the sensing area. 
The touch sensors of FIGS. 1-5 are usually preferred over the touch sensor 
of FIG. 6 because they provide a precisely linear relationship between the 
actual position of the object being detected and the resulting resistance 
changes. However, because those touch sensors detect position using 
discrete traces, there is some quantization noise in the measurements when 
a very small object is being measured, such as the tip of a stylus. The 
touch sensor of FIG. 6 provides a purely smooth relationship between the 
measured and actual position because the fixed resistors are continuous 
sheets that directly sense the touch point, although there is some 
nonlinearity introduced because the current paths from the touch point to 
the terminals are no longer constrained to linear traces. However, because 
the substrate tends to distribute the applied force over several traces, 
the effect of quantization noise is not normally detectable. 
FIG. 7 shows a touch sensor design that detects position in a polar 
coordinate system. The X fixed resistor 33 is a thin strip deposited on 
the substrate 20, and positioned with one end at the origin of the polar 
coordinate system. The X conductive traces 31 form a concentric circular 
pattern, centered about the origin of the polar coordinate system, and 
touching the X fixed resistor 33. The X position measurement will 
represent the radial distance of the touch point from the center of the 
system. 
The Y fixed resistor 32 is an annulus deposited on the substrate 20, and 
centered about the origin of the polar coordinate system. The Y fixed 
resistor traces 30 form a radial pattern, also centered about the origin 
of the polar coordinate system, and touching the Y fixed resistor 32. The 
Y position measurement will represent the angular distance around the 
system; in this case the measurements are symmetrical about the diameter, 
from 0.degree. to 180.degree. and back to 0.degree.. 
FIG. 8 shows the touch sensor of FIG. 7 with force variable resistor traces 
deposited over the conductive traces. The X force variable resistor traces 
41 are deposited over the X conductive traces, and the Y force variable 
resistor traces 40 are deposited over the Y conductive traces. With the 
exception of the shape of the conductive and force variable traces forming 
a polar coordinate system, this touch sensor functions identically to the 
touch sensor of FIG. 2. 
FIG. 9 shows the touch sensor of FIG. 7, but with a single force variable 
resistor sheet 42, and insulating layer 23. With the exception of the 
pattern of the conductive, resistive, and force variable traces forming a 
polar coordinate system, this touch sensor functions identically to the 
touch sensor of FIG. 4. Touch sensors that measure position in a polar 
coordinate system may also be constructed using the methods employed in 
FIGS. 5 and 6: i.e. using fixed resistive traces or fixed resistive sheets 
over the sensing area. 
FIG. 10 shows an alternative construction of a touch sensor. The substrate 
is divided into two sections: an X substrate 29 and a Y substrate 28. The 
X substrate 29 is a printed circuit board, with conductive traces 31 
deposited on the surface. The X fixed resistor 33 is comprised of many 
discrete resistors in a series interconnection, forming a resistor divider 
ladder, with each conductive trace electrically connected to a node in the 
resistor ladder. 
The Y substrate 28 is a thin flexible film, with conductive traces 30, Y 
fixed resistor 32, Y terminals 12 and 13, and Y force variable resistor 
traces 40, all deposited directly on the Y substrate. The Y substrate 28 
is positioned over the X substrate 29 so as to cover the X conductive 
traces 31, within the area marked by 27. A connector 26 attaches the Y 
terminals onto the X substrate. 
FIG. 11 shows a touch sensor, similar to the touch sensor of FIG. 2, except 
the Y fixed resistor 32 is divided into several segments, forming several 
Y touch zones (five, in this case). Each Y fixed resistor segment also has 
independent terminals 12 and 13, so that each Y touch zone may be scanned 
in succession and may be analyzed independently of the other touch zones. 
As a result, each touch zone on the touch sensor can detect an individual 
touch point independently of the other touch zones. A similar scheme may 
be used to divide the X fixed resistor into segments, thereby creating a 
matrix of X-Y touch zones. 
The commonality of structure and function between the touch sensors of 
FIGS. 1 through 11 will be demonstrated in FIGS. 12 and 13, which 
represent a generalized model for the touch sensor. 
FIG. 12 is a cross-sectional view of a touch sensor. The upper surface of 
the touch sensor is the Y substrate 28, and the lower surface is the X 
substrate 29, although the two substrates could be formed from the same 
material folded over itself. 
The Y substrate supports a Y resistive layer 32, which establishes a 
potential gradient that spans a Y spatial dimension over the surface of 
the Y substrate. The Y resistive layer 32 may comprise conductive traces, 
but must minimally comprise a fixed value resistor that establishes the 
potential gradient. There are also two Y terminals 12 and 13 that 
electrically connect to the Y resistive layer 32. 
Similary, the X substrate 29 supports an X resistive layer 33, which 
establishes a potential gradient that spans an X spatial dimension over 
the surface of the X substrate, and is electrically connected to X 
terminals 10 and 11. The physical area spanned by the resistive layers 32 
and 33 must minimally cover the entire area to be sensed. The X and Y 
dimensions refer only to the direction of the potential gradients 
established by the fixed resistors. 
Between the resistive layers 32 and 33 is a force variable resistor layer 
42, which must also physically span the area to be sensed. The force 
variable resistor layer 42 has the characteristics of having a high 
resistance when there is no force applied to it, and having a resistance 
that decreases as an applied force is increased. 
It is preferred that, with little or no applied force, the resistance of 
the force variable resistor layer 42 is practically infinite, so that 
there is no conduction between the X resistive layer 33 and the Y 
resistive layer 32. If a force variable resistor material is selected that 
does exhibit a significantly low resistance, then some other means should 
be employed to ensure a large resistance with no applied force. Such means 
could include inserting a spacer to create an air gap between the layers, 
or depositing minute insulating bumps on one of the layers to create a 
standoff. 
A finger, stylus, or other member 34 applies a downward force on the upper 
surface of the Y substrate 28. This downward force causes the resistance 
of the force variable resistor layer 42 to decrease locally within the 
area beneath the touch point. This drop in resistance causes current to 
flow from the X resistive layer 33, through the force variable resistor 
layer 42, to the Y resistive layer 32. The position of the touch point on 
the surface of the touch sensor will determine at what point along the X 
dimension the X resistive layer conducts current, and at what point along 
the Y dimension the Y resistive layer conducts current, into the force 
variable resistor. 
FIG. 13 is a schematic representation of the generalized touch sensor 
model. Rf is the value of the force variable resistor 42 at the touch 
point, which has a practically infinite resistance at all points other 
than the touch point. The decrease in resistance of the force variable 
resistor 42 will enable current to flow between the X fixed resistor 33 
and the Y fixed resistor 32. The position of the touch point temporarily 
divides the X fixed resistor 33 into two segments having resistance values 
Rxa and Rxb, the intersection between Rxa, Rxb, and Rf being an electrical 
node. The position of the touch point also temporarily divides the Y fixed 
resistor 32 into two segments having resistance values Rya and Ryb; the 
intersection between Rya, Ryb, and Rf also being an electrical node. 
Determination of the position of the touch point in the X dimension 
requires measuring the resistance values of Rxa or Rxb. This may be 
accomplished by measuring the relative value of Rxa to Rxb; by measuring 
the relative value of either Rxa or Rxb to the sum of Rxa and Rxb, which 
is a fixed value; or by measuring the absolute resistance of either Rxa or 
Rxb. Likewise, determination of the position of the touch point in the Y 
dimension requires measuring the resistance values of Rya or Ryb. 
Determination of the applied pressure at the touch point requires measuring 
the resistance of the force variable resistor, Rf, at that point. This 
typically involves either an absolute measurement of the resistor value, 
or a measurement that is relative to some external resistor. 
The difficulty in the determination of the position and pressure of the 
touch point arises from the fact that the internal electrical nodes that 
connect the fixed resistors 32 and 33 with the force variable resistor 42 
are not available outside the touch sensor, and hence are not directly 
observable. The subsequent figures demonstrate several electronic circuits 
that accomplish the required measurements according to the objects of the 
present invention. The combination of the touch sensor with such a circuit 
creates a touch controller that outputs three analog voltage signals, 
representing the X position, Y position, and pressure of the touch point. 
The signals are linear representations of the actual position and pressure 
so that no further manipulations or calculations are required. 
The outputs of the touch controller, being analog voltages, may be 
subsequently digitized by an analog-to-digital converter, which can then 
be read by a microprocessor or computer. Many choices for an 
analog-to-digital converter may be used to optimize a particular 
application, trading off cost for sampling rate, resolution, and circuit 
complexity. A review of analog-to-digital converters may be found in The 
Art Of Electronics, pages 415-428, Horowitz and Hill, Cambridge University 
Press, 1980, which is hereby included by reference. 
FIG. 14 shows a touch controller that uses a current regulator 51 to supply 
a constant current Io through the touch sensor 50. A comprehensive review 
of current regulator design is available in Implementation And 
Applications Of Current Sources And Current Receivers, from Burr-Brown 
Corporation, Tuscon, Ariz., 1989, and is hereby incorporated by reference. 
The basic principle of operation of this touch controller is that a voltage 
drop across a resistor is proportional to current flowing through that 
resistor, according to the well-known law: voltage=current x resistance 
(i.e. V=IR). Since the current through the various resistive elements of 
the touch sensor is maintained at a constant Io, the voltage changes 
measured across any resistor in the touch sensor will be linearly 
proportional to any changes in that resistor. Measuring those voltage 
changes then directly provides accurate measurements of the resistor 
values, without scaling. 
The touch controller comprises three differential amplifiers: 52 to measure 
the X position and having an X output, 53 to measure the Y position and 
having a Y output, and 54 to measure the pressure and having a P output. 
The differential amplifiers should have high impedance inputs, such as 
instrumentation amplifiers. 
Suitable integrated circuit instrumentation amplifiers include: the AMP-01 
from Precision Monolithics Inc., the LM363 from National Semiconductor 
Corp., and the LT1101 from Linear Technology Corp. Also, a review of the 
design of differential amplifiers can be found in The Art Of Electronics, 
pages 279-287, by Horowitz and Hill, Cambridge University Press, 1980, 
which is hereby incorporated by reference. 
Differential amplifier 52 is connected to the X terminals of the touch 
sensor, so that it measures the differential voltage across the X fixed 
resistor. Because the differential amplifier has high impedance inputs, 
virtually no current flows from the touch point into the noninverting (+) 
input, so that the voltage present at the noninverting (+) input is 
virtually equal to the voltage at the touch point on the X fixed resistor. 
Similarly, differential amplifier 53 is connected to the Y terminals of the 
touch sensor, so that it measures the differential voltage across the Y 
fixed resistor. Differential amplifier 54 is connected to one X terminal 
and to one Y terminal, which are also the noninverting (+) inputs of 52 
and 53, so that it measures the differential voltage across the force 
variable resistor. 
Since the current regulator output is constant Io, the voltage outputs 
relate to the resistances of the touch sensor as follows: 
EQU X=Gx Io Rxa 
EQU Y=Gy Io Rya 
EQU P=Gp Io Rf 
where Gx, Gy, and Gp are the voltage gains of the differential amplifiers 
for X (52), Y (53), and P (54), respectively. 
In some instances the resistance of the X fixed resistor will not be equal 
to the resistance of the Y fixed resistor, so it might be convenient to 
have different gains for each differential amplifier. Typically, the gains 
of these amplifiers are set so that the maximum range of voltage swings is 
matched to the full scale input range of an analog-to-digital converter. 
FIG. 15 is a touch controller that uses a variation of the circuit shown in 
the touch controller of FIG. 14. A floating current regulator is formed 
from operational amplifier 55 and resistor R1, with the touch sensor 50 
within the feedback loop of the operational amplifier. This arrangement 
can produce a very fast settling and response time to changes in touch 
position and pressure, provided that the operational amplifier 55 has 
sufficient bandwidth and output drive so that it does not oscillate with 
the touch sensor 50 in its feedback loop. Suitable operational amplifiers 
include the LF351 family from National Semiconductor Corp. and the TL071 
family from Texas Instruments. However, a resistor-capacitor compensation 
network can be used with operational amplifiers that might tend to 
oscillate. 
Although the pressure measurement can be effected by using a third 
differential amplifier, in the circuit of FIG. 15 it is taken directly 
from the output of the operational amplifier 55, i.e. the current 
regulator output. The tradeoff for the savings of an additional 
differential amplifier is a bias voltage in the pressure measurement. 
There is a fixed bias voltage resulting from the voltage across R1, and 
additional voltage drops across each of the X and Y fixed resistors which 
vary with position. In many circumstances, these voltage biases are very 
small compared to the range of differential voltage across the pressure 
resistor and can be ignored. For greater accuracy, these biases can be 
subtracted from the pressure measurement since they are all known or 
measurable quantities. 
FIG. 16 shows a touch controller that uses an electronic circuit that draws 
current across the whole X fixed resistor and across the whole Y fixed 
resistor, as opposed to connecting one of each of the X and Y terminals to 
a high impedance input that draws negligible current. The total current 
through the touch sensor 50 is Io, and is supplied by current regulator 
51. 
Two resistors of equal value R2 and R3 are connected to the two Y 
terminals, forming a Wheatstone bridge with the two segments of the Y 
fixed resistor, Rya and Ryb. Similarly, two other resistors of equal value 
R4 and R5 are connected to the two X terminals, forming a Wheatstone 
bridge with the two segments of the X fixed resistor, Rxa and Rxb. 
An instrumentation amplifier 57 is also connected to the Y terminals, 
across the Y fixed resistor, and outputs the Y position measurement. 
Instrumentation amplifier 57 measures the differential voltage across the 
Y fixed resistor, but also provides a common mode output that measures the 
average voltage between the Y terminals. The common mode voltage output is 
sometimes referred to as a "guard drive" output; it is available in some 
integrated circuit instrumentation amplifiers, such as the LH0036 from 
National Semiconductor Corp.; and is also described in The Art Of 
Electronics, as previously referenced. 
Similarly, an instrumentation amplifier 56 is connected to the X terminals, 
across the X fixed resistor, and outputs the X position measurement. 
Amplifier 56 also provides a common mode output that measures the average 
voltage across the X fixed resistor. Both of the common mode outputs from 
instrumentation amplifiers 56 and 57 are input into differential amplifier 
54, which differences the common mode measurements, providing the pressure 
output P. 
The outputs of the amplifiers 56, 57, and 54 are related to the resistors 
of touch sensor 50 as follows: 
EQU X=Gx Io R4 (Rxb-Rxa)/(Rx+2R4) 
EQU Y=Gy Io R2 (Ryb-Rya)/(Ry+2R2) 
EQU P=Gp Io(Rf+Rx/2+Ry/2) 
where: 
Rx=Rxa+Rxb 
Ry=Rya+Ryb 
are the constant values of the X and Y fixed resistors, respectively; 
R2=R3; R4=R5; and Gx, Gy, Gp, and Io are as previously defined. 
FIG. 17 shows a touch controller that uses an electronic circuit that draws 
equal currents through each of the terminals of the touch sensor 50. A 
dual current source is made from resistor R1 and NPN transistors Q1, Q4, 
and Q5; where Q4 and Q5 are in a current mirror configuration with Q1, and 
each draw equal currents. The value of resistor R1 and reference voltage 
VREF determine the constant current Io supplied by each of Q4 and Q5. 
The collectors of Q4 and Q5 are connected to the Y terminals of the touch 
sensor 50, so that each Y fixed resistor segment, Rya and Ryb, has the 
constant current Io flowing through it. PNP transistors Q2 and Q3 form a 
current mirror, with their collectors connected to the X terminals, so 
that each X fixed resistor segment, Rxa and Rxb, also has the constant 
current Io flowing through it. Because the force variable resistor forms a 
node with each of the fixed resistor segments, the force variable resistor 
has the constant current 2Io flowing through it. 
It is preferred that the NPN transistors Q1, Q4, and Q5 are fabricated on a 
common substrate, and that the PNP transistors Q2 and Q3 are also 
fabricated on a common substrate. This arrangement produces transistors 
with closely matched gains and temperature coefficients, so that the 
currents supplied by the current mirrors will be closely matched. The 
transistor array CA3096 from RCA conveniently contains three NPN and two 
PNP transistors on a single substrate. 
Two instrumentation amplifiers with common mode outputs 56 and 57 are 
connected to the X terminals and Y terminals and provide the X and Y 
position outputs, respectively, in a similar arrangement as FIG. 16. Also, 
differential amplifier 54 connects to the common mode outputs and provides 
the pressure (P) output. These outputs relate to the resistors within the 
touch sensor 50 as follows: 
EQU X=Gx Io (Rxb-Rxa) 
EQU Y=Gy Io (Ryb-Rya) 
EQU P=Gp Io (2 Rf+Rx/2+Ry/2) 
where: Rx, Ry, Gx, Gy, Gp, and Io are as previously defined. 
FIG. 18 shows a touch controller with an identical circuit as the touch 
controller of FIG. 14, except for the addition of two analog 1-of-2 
multiplexers, 58 and 59. The purpose of the analog multiplexers is to 
switch the current source between the X and Y terminals so that the 
absolute resistances of the X fixed resistor and the Y fixed resistor may 
be measured. Such measurements would be used as a calibration for 
subsequent touch position measurements, and would include any offsets and 
variations in the current source 51 and differential amplifiers 52 and 53. 
Suitable analog multiplexers include industry standard integrated circuits 
CD4053 and DG303. 
The CX input controls analog multiplexer 59, and the CY input controls 
analog multiplexer 58. When both analog multiplexers are switched to the 
first (upper) position, the touch controller behaves as does the touch 
controller of FIG. 14. 
When the CX signal causes analog multiplexer 59 to switch to the second 
(lower) position, the current source is switched from one of the Y 
terminals to one of the X terminals. Current is thereby prevented from 
flowing through the force variable resistor and the Y fixed resistor. 
Instead, the current flows through the entire length of the X fixed 
resistor, resulting in an X measurement that is the maximum possible for 
any touch point. 
Similarly, the CY input may cause the analog multiplexer 58 to switch to 
the second position, resulting in a Y measurement that is the maximum 
possible for any touch point. In general, it is not necessary to calibrate 
the force variable resistor for these circuits, since its full range of 
values is determined by the voltage compliance limits of the current 
source. 
FIG. 19 shows a touch controller with a circuit that functions using the 
same principles as the touch controller of FIG. 14, except a single 
differential amplifier 64 is used in conjunction with several analog 
1-of-4 multiplexers 60-63, rather than three separate differential 
amplifiers. Suitable analog multiplexers include the industry standard 
integrated circuits CD4052 and DG509. Current regulator 51 supplies 
current Io which is switched by the analog multiplexers 60-63 across touch 
sensor 50. 
This touch controller is most useful in cost-critical situations where 
continuously available outputs are not required and slower response time 
may be traded off for decreased circuit cost and complexity. The X 
position, Y position, and pressure signals are time-multiplexed from the 
output of the differential amplifier 64, and the selection of the 
measurement is controlled by the state of the SI (select current source) 
and SM (select measurement) inputs. The SI input controls analog 
multiplexers 60 and 61, which switch the current regulator and ground 
across the touch sensor's terminals. The SM input controls analog 
multiplexers 62 and 63, which switch the inputs of the differential 
amplifier 64 across the touch sensor's terminals. 
The desired measurement that is output from the touch controller is 
determined by the SI and SM inputs as follows: 
______________________________________ 
Measurement 
SM state SI state Result 
______________________________________ 
Ry 1 3 calibrate y 
Ryb 1 1 y position (b) 
Rya 1 2 y position (a) 
Rx 2 4 calibrate x 
Rxb 2 1 x position (b) 
Rxa 2 2 x position (a) 
Rf(b) 3 1 pressure (b side) 
Rf(a) 4 2 pressure (a side) 
______________________________________ 
For greatest accuracy using this touch controller, the following 
post-processing and measurement calibration is preferred. Typically, this 
processing is performed in software after the position and pressure 
signals are digitized, however it may also be performed in analog or 
digital hardware. 
EQU y position=(Ryb-Rya)/Ry 
EQU x position=(Rxb-Rxa)/Rx 
EQU pressure=(Rf(b)+Rf(a))/(2Rf(max)) 
Rf(max) is the value of the pressure measurement when there is no force 
applied to the touch sensor, which should equal the compliance limit of 
the current regulator 51. The resulting X and Y position values range from 
-1.0 to +1.0, with the coordinate point (0.0,0.0) being the center of the 
touch sensor's coordinate system. The resulting pressure value ranges from 
0.0 (no touch) to +1.0 (maximum touch pressure). 
FIG. 20 shows a touch controller with an electronic circuit that does not 
require a current regulator or a differential amplifier. Although this 
circuit has greater susceptibility to noise and interference, and does not 
offer continuously available outputs, its simplicity might be useful in 
extremely cost-critical situations. 
Four analog 1-of-4 multiplexers 60-63 are connected to the four terminals 
of the touch sensor 50. The SM (select measurement) input signal 
simultaneously controls the state of all of the analog multiplexers. The 
analog multiplexers can connect the touch sensor terminal to: a voltage 
regulator 67 which supplies a constant voltage Vo; circuit ground; a 
pull-down resistor R6; and the input of a single-ended amplifier 65. 
The pull-down resistor R6 is needed only for pressure measurements, and is 
grounded by switch 66, which could be a simple transistor or an analog 
switch such as industry standard integrated circuit CD4066. Switch 66 is 
controlled by the SP (select pressure) input signal. When not pulled down, 
resistor R6 does not affect the circuit. The single-ended amplifier 65 
could be a standard operational amplifier, or the input buffer to a 
sample-and-hold circuit or analog-to-digital converter. 
The principle of operation of this circuit is simply to treat each of the X 
and Y fixed resistors as a potentiometer, with the touch point and force 
variable resistor serving as the wiper. To measure pressure, the voltage 
drop across the entire touch sensor 50 is compared to the voltage drop 
across pull-down resistor R6, in a simple voltage-divider network. 
The position and pressure measurements are obtained from the output of the 
single-ended amplifier 65 in a time-multiplexed fashion, according to the 
state of the SM and SP inputs as follows: 
______________________________________ 
Measurement SM state SP state Result 
______________________________________ 
Ryb 1 off y position 
Rxb 2 off x position 
Rf 3 on pressure 
______________________________________ 
The implementations described above present just several examples of many 
possibilities for touch sensor design and implementations of a touch 
controller, adapted from the basic principles of this invention as shown 
in FIGS. 12 and 13. 
In particular, a touch sensor that accurately detects the position and 
pressure of a touch point on its surface may be constructed by 
establishing a first potential gradient across a lower substrate using a 
fixed value resistor; establishing a second potential gradient across an 
upper substrate using a second fixed resistor; and placing a force 
variable resistor layer in between the two substrates, so that current 
flows from the first fixed resistor through the force variable resistor 
and into the second fixed resistor at the touch point when pressure is 
applied. 
Two approaches to making a touch controller based on this touch sensor are 
presented. The first touch controller uses an electronic circuit that 
maintains a constant current through the touch sensor, and measures the 
touch position using a differential amplifier placed across each fixed 
resistor. The second touch controller uses an electronic circuit that 
maintains a constant voltage across the touch sensor, and measures the 
touch position using a single-ended amplifier that reads the voltage drop 
across each fixed resistor in series with the force variable resistor. 
The embodiments of touch sensors and touch controllers based on this 
invention provide many improvements over the prior art of 
position-and-pressure sensitive touch sensors. Unlike capacitive 
technologies and surface-acoustic wave technologies, as commonly used in 
computer touch screens, the touch sensor of the present invention is 
completely sealed so that many different coverings and graphic overlays 
may be used, and the present invention may be activated with a wide 
variety of control members, such as a finger or stylus. The simpler 
electronics of the present invention also result in less expensive and 
more reliable product designs. 
Technologies using force variable resistor inks in a scanning mode employ a 
touch sensor that has many of the advantages of the touch sensor of the 
present invention, however scanning-type systems require an enormous 
number of terminals to be brought forth from the sensor; the scanning 
hardware is complex and requires fast analog-to-digital converters; the 
response time is slow due to the time required to complete a scanning 
cycle; the data rates produced are high, requiring enormous computational 
and memory overhead to process and interpret the data; and the hardware 
and computational overhead increase exponentially for linear increases in 
sensor size or sensor resolution. In cost sensitive applications, such as 
high-volume consumer products, or in many applications that only require 
sensing a single touch point, the present invention suffers from none of 
the drawbacks exhibited by scanning technologies. 
The present invention overcomes many of the drawbacks of prior 
pressure-sensitive touch sensors that comprise fixed resistors and force 
variable resistors. Comprising only two substrate layers, the present 
invention minimizes the amount of material required for fabrication, and 
ensures maximum sensitivity because both position dimensions make contact 
simultaneously. This also simplifiers assembly of mass-produced devices 
because the touch sensor can be produced from a single film with only four 
terminals emerging on the same side of the same substrate. Only a single 
pressure signal is generated by the present invention, simplifying 
construction, reducing circuit cost and reducing post-processing overhead. 
The force variable resistor of the present invention conducts current 
perpendicular to the plane of the resistor, reducing contact noise and 
allowing a wide variety of force variable resistor materials to be used. 
Because interdigitated pickup traces are not required, the resolution of 
the touch sensor is only limited by the resolution of the printing 
technology used for sensor fabrication. 
A great many variations on the embodiments of the present invention are 
possible. The touch sensor can be constructed from a single substrate 
folded over itself, or from two independent substrates sandwiched 
together. One substrate may be stiff, such as a printed circuit board or 
glass. One substrate may also contain the touch controller electronic 
circuitry, such as a printed circuit board or flexible circuit laminates. 
Many flexible substrate materials may be used, such as polyester and 
polyimide films. Fixed resistors may be implemented in various ways, such 
as thin films directly deposited on the substrate, or discrete resistor 
networks. The potential gradients may be established over the substrate 
surfaces in various ways, such as conductive traces, resistive traces, and 
resistive sheets. Many force variable resistor materials may be used, 
including semiconductive inks and semiconductive rubber sheets. Many 
coordinate systems may be defined by changing the patterns of the 
potential gradient, such as a cartesian coordinate system or a polar 
coordinate system. 
The available electronics literature provides many suitable methods of 
constructing current regulators (sources, sinks, and mirrors) as well as 
differential (including instrumentation) amplifiers. The sense of voltage 
in each circuit is easily reversed (i.e. positive versus negative voltages 
and currents) without affecting operation. The circuits may be constructed 
using bipolar power supplies or single-sided power supplies. Various 
methods may be used to draw current across the fixed terminals while 
maintaining a linear relationship between the measured and actual touch 
position, including drawing current in only one direction over the fixed 
resistor, and using a resistive Wheatstone Bridge or current mirror across 
the fixed resistor. Analog multiplexers may be used to reduce hardware 
requirements, trading off response time of the touch sensor. Simple 
electronic means, using a voltage regulator and single-end amplifier, may 
be used to read the touch sensor, trading off sensitivity and noise 
immunity. 
It should be appreciated that the present invention as described herein may 
be modified or adapted in applying the principles of the present invention 
to different situations. Accordingly, the embodiments described herein 
should not be taken as a limitation on the scope of the present invention, 
but rather the invention should only be interpreted in accordance with the 
following claims.