Capacitance-based proximity with interference rejection apparatus and methods

Apparatus and method for a capacitance-based proximity sensor with interference rejection. A pair of electrode arrays establish a capacitance on a touch detection pad, the capacitance varying with movement of a conductive object near the pad. The capacitance variations are measured synchronously with a reference frequency signal to thus provide a measure of the position of the object. Electrical interference is rejected by producing a reference frequency signal which is not coherent with the interference.

This invention relates generally to apparatus and methods for touch 
sensitive input devices, and more particularly, to apparatus and methods 
for capacitance-based touch detection wherein electrical interference is 
effectively rejected from the detection system. 
BACKGROUND OF THE INVENTION 
Numerous prior art devices and systems exist by which tactile sensing is 
used to provide data input to a data processor. Such devices are used in 
place of common pointing devices (such as a "mouse" or stylus) to provide 
data input by finger positioning on a pad or display device. These devices 
sense finger position by a capacitive touch pad wherein scanning signals 
are applied to the pad and variations in capacitance caused by a finger 
touching or approaching the pad are detected. By sensing the finger 
position at successive times, the motion of the finger can be detected. 
This sensing apparatus has application for controlling a computer screen 
cursor. More generally it can provide a variety of electrical equipment 
with information corresponding to finger movements, gestures, positions, 
writing, signatures and drawing motions. 
In U.S. Pat. No. 4,698,461, Meadows et al., a touch surface is covered with 
a layer of invariant resistivity. Panel scanning signals are applied to 
excite selected touch surface edges so as to establish an alternating 
current voltage gradient across the panel surface. When the surface is 
touched, a touch current flows from each excited edge through the 
resistive surface and is then coupled to a user's finger (by capacitance 
or conduction), through a user's body, and finally coupled by the user's 
body capacitance to earth ground potential. Different scanning sequences 
and modes of voltage are applied to the edges, and in each case the 
currents are measured. It is possible to determine the location of touch 
by measuring these currents. In particular, the physical parameter which 
indicates touch location is the resistance from the edges to the point of 
touch on the surface. This resistance varies as the touch point is closer 
or farther from each edge. For this system, the term "capacitive touch 
pad" may be a misnomer because capacitance is involved as a means of 
coupling current from the surface touch point through the user's finger 
but is not the parameter indicative of finger position. A disadvantage of 
this invention is that accurate touch location measurement depends on 
uniform resistivity of the surface. Fabricating such a uniformly resistive 
surface layer can be difficult and expensive, and require special 
fabrication methods and equipment. 
The panel of the Meadows '461 patent also includes circuitry for "nulling", 
or offsetting to zero, the touch currents which are present when the panel 
is not touched. This nulling can be accomplished while the panel operates, 
and allows touches which generate a relatively weak signal, such as from a 
gloved finger, to be more accurately determined. The Meadows '461 panel 
also includes circuitry for automatically shifting the frequency of panel 
scanning signals away from spectra of spurious signals, such as those 
developed by cathode-ray tube transformers, which may be present in the 
environment. The panel seeks to avoid interference from the spurious 
signals, which could happen if the frequency of scanning was nearly equal 
to that of the spurious signals. A microcontroller determines whether the 
scanning frequency should be shifted by monitoring the rate at which 
adjustments are required in nulling of the touch currents, as described 
above. The only means described for generating frequency control signals 
is based on this nulling adjustment. 
U.S. Pat. No. 4,922,061, Meadows et al., is similar to the Meadows '461 
patent in that the touch panel determines touch location based on 
variations in resistance, not capacitance. This is particularly evident 
from FIG. 2 where the resistances from edge to touch point are shown as Kx 
times Rx, where Kx is corresponds to the distance indicated by 76A. The 
apparatus uses a measurement signal of a frequency that varies in a 
substantially random manner, thus reducing susceptibility to interference 
from spurious electromagnetic spectra. 
U.S. Pat. No. 4,700,022, Salvador, describes an array of detecting 
conductive strips, each laid between resistive emitting strips. The finger 
actually makes electrical contact from an emitting strip to detecting 
strip. Touch location is determined from resistance variation (as with 
Meadows '461 and '061 above) in the strip contacted by the finger. 
Averages are taken of a certain number of synchronous samples. A design 
formula is presented to choose a sampling frequency so that it is not a 
multiple of the most undesired predetermined interfering signal. No 
suggestion is made that sampling frequency is adjusted or adapts 
automatically. 
In U.S. Pat. No. 5,305,017, Gerpheide, touch location is determined by true 
capacitance variation, instead of resistance variation, using a plurality 
of electrode strips forming virtual electrodes. This approach eliminates 
the necessity of a coating having uniform resistance across a display 
screen. However, such a capacitance-based detection device may suffer from 
electrical background interference from its surroundings, which is coupled 
onto the sensing electrodes and interferes with position detection. These 
spurious signals cause troublesome interference with the detection of 
finger positioning. The device operator may even act as an antenna for 
electrical interference which may cause a false charge injection or 
depletion from the detecting electrodes. 
Accordingly, there is a need for a touch detection system which has the 
following characteristics: 
(1) the touch location is determined without the need of resistance 
variation so as to avoid the high cost of requiring uniform resistance 
during fabrication; 
(2) the touch location is measured in a manner independent of resistance of 
the electrodes or their connecting wiring, thus broadening the range of 
materials and processes which may be used for fabrication; and 
(3) electrical interference signals are rejected and eliminated from the 
detection system regardless of their frequency and without requiring 
possibly expensive nulling apparatus. 
SUMMARY OF THE INVENTION 
The present invention employs a touch location device having true 
capacitance variation by using insulated electrode arrays to form virtual 
electrodes. The capacitance variation is measured by means independent of 
the resistance of the electrodes, so as to eliminate that parameter as a 
fabrication consideration. The electrical interference is eliminated 
regardless of frequency to provide a clear detection signal. 
An illustrative embodiment of the present invention includes an electrode 
array for developing capacitances which vary with movement of an object 
(such as finger, other body part, conductive stylus, etc.) near the array, 
a synchronous capacitance measurement element which measures variation in 
the capacitances, such measurements being synchronized with a reference 
frequency signal, and a reference frequency signal generator for 
generating a reference frequency signal which is not coherent with 
electrical interference which could otherwise interfere with capacitance 
measurements and thus position location. 
Interference rejection is carried out by generating a reference frequency 
signal whose frequency is different from the interference frequency. 
Alternately, the reference signal is generated with random frequencies so 
as not to be coherent with the interference frequencies and thus the 
electrical interference is effectively rejected.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows the essential elements of a capacitance variation finger (or 
other conductive body or non-body part) position sensing system 10, made 
in accordance with the invention. An electrode array 12 includes a 
plurality of layers of conductive electrode strips. The electrodes and the 
wiring connecting them to the device may have substantial resistance, 
which permits a variety of materials and processes to be used for 
fabricating them. The electrodes are electrically insulated from one 
another. Mutual capacitance exists between each two of the electrodes, and 
stray capacitance exists between each of the electrodes and ground. A 
finger positioned in proximity to the array alters these mutual and stray 
capacitance values. The degree of alteration depends on the position of 
the finger with respect to electrodes. In general, the alteration is 
greater when the finger is closer to the electrode in question. 
A synchronous electrode capacitance measurement unit 14 is connected to the 
electrode array 12 and determines selected mutual and/or stray capacitance 
values associated with the electrodes. To minimize interference, a number 
of measurements are performed by unit 14 with timing synchronized to a 
reference frequency signal provided by reference frequency generator 16. 
The desired capacitance value is determined by integrating, averaging, or 
in more general terms, by filtering the individual measurements made by 
unit 14. In this way, interference in the measurement is substantially 
rejected except for spurious signals having strong frequency spectra near 
the reference frequency. 
The reference frequency generator 16 attempts to automatically select and 
generate a reference frequency which is not coherent with the most 
undesirable frequency of spurious signals. This approach substantially 
eliminates interference even though its frequency is likely to be 
initially unknown and may even change during operation. 
A position locator 18 processes the capacitance measurement signal from the 
synchronous electrode capacitance measurement unit 14 and provides 
position signals for use by a host computer, for example, and to the 
reference frequency generator 16. The position locator unit 18 determines 
finger position signals based on the capacitance measurements. Several 
different systems are commonly known in the art for determining finger 
position based on measurements of capacitance associated with electrodes 
in an array. Position locators may provide one-dimensional sensing (such 
as for a volume slider control), two-dimensional sensing with contact 
determination (such as for computer cursor control), or full 
three-dimensional sensing (such as for games and three-dimensional 
computer graphics.) An example of a prior art position locator unit is 
shown in the Gerpheide '017 patent mentioned above, as units 40 and 50 of 
FIG. 1 of the patent. 
Electrode Array 
FIG. 2A illustrates the electrodes in a preferred electrode array 12, 
together with a coordinate axes defining X and Y directions. One 
embodiment includes sixteen X electrodes and twelve Y electrodes, but for 
clarity of illustration, only six X electrodes 20 and four Y electrodes 22 
are shown. It is apparent to one skilled in the art how to extend the 
number of electrodes. The array is preferably fabricated as a multilayer 
printed circuit board 24. The electrodes are etched electrically 
conductive strips, connected to vias 26 which in turn connect them to 
other layers in the array. Illustratively, the array 12 is approximately 
65 millimeters in the X direction and 49 millimeters in the Y direction. 
The X electrodes are approximately 0.7 millimeters wide on 3.3 millimeter 
centers. The Y electrodes are approximately three millimeters wide on 3.3 
millimeter centers. 
FIG. 2b illustrates the electrode array 12 from a side, cross-sectional 
view. An insulating overlay 21 is an approximately 0.2 millimeters thick 
clear polycarbonate sheet with a texture on the top side which is 
comfortable to touch. Wear resistance may be enhanced by adding a textured 
clear hard coating over the top surface. The overlay bottom surface may be 
silk-screened with ink to provide graphics designs and colors. 
The X electrodes 20, Y electrodes 22, ground plane 25 and component traces 
27 are etched copper traces within a multilayer printed circuit board. The 
ground plane 25 covers the entire array area and shields the electrodes 
from electrical interference which may be generated by the parts of the 
circuitry. The component traces 27 connect the vias 26 and hence the 
electrodes 20, 22, to other circuit components of FIG. 1. Insulator 31, 
insulator 32 and insulator 33 are fiberglass-epoxy layers within the 
printed circuit board 24. They have respective thicknesses of 
approximately 1.0 millimeter, 0.2 millimeters and 0.1 millimeters. 
Dimensions may be varied considerably as long as consistency is 
maintained. However, all X electrodes 20 must be the same size, as must 
all Y electrodes 22. 
One skilled in the art will realize that a variety of techniques and 
materials can be used to form the electrode array. For example, FIG. 3A 
illustrates an alternative embodiment in which the electrode array 
includes an insulating overlay 40 as described above. Alternate layers of 
conductive ink 42 and insulating ink 43 are applied to the reverse surface 
by a silk screen process. The X electrodes 45 are positioned between the 
insulating overlay 40 and X electrode conductive ink layer 42. Another 
insulating ink layer 43 is applied below layer 42. The Y electrodes 46 are 
positioned between insulating ink layer 43 and conductive ink layer 44. 
Another insulating ink layer 47 is applied below conductive ink layer 44, 
and ground plane 48 is affixed to Y electrode conductive ink layer 47. 
Each layer is approximately 0.01 millimeters thick. 
The resulting array is thin and flexible, which allows it to be formed into 
curved surfaces. In use it would be laid over a strong, solid support. In 
other examples, the electrode array may utilize a flexible printed circuit 
board, such as a flex circuit, or stampings of sheet metal or metal foil. 
A variety of electrode geometries and arrangements are possible for finger 
position sensing. One example is shown in FIG. 3b. This is an array of 
parallel electrode strips 50 for one-dimensional position sensing which 
could be useful as a "slider volume control" or "toaster darkness 
control". Other examples include a grid of diamonds, or sectors of a disk. 
It is desired that the electrode array of the present invention be easily 
fabricated by economical and widely available printed circuit board 
processes. It is also desired to allow use of various overlay materials 
selected for texture and low friction, upon which logo art work and 
coloration can be economically printed. A further preference is that the 
overlay may be custom printed separately from fabrication of the 
electrode-containing part of the array. This allows an economical 
standardized mass production of that part of the array, and later affixing 
of the custom printed overlay. 
Synchronous Electrode Capacitance Measurement 
FIG. 4 shows one embodiment of the synchronous electrode capacitance 
measurement unit 14 in more detail. The key elements of the synchronous 
electrode capacitance measurement unit 14 are (a) an element for producing 
a voltage change in the electrode array synchronously with a reference 
signal, (b) an element producing a signal indicative of the displacement 
charge thereby coupled between electrodes of the electrode array, (c) an 
element for demodulating this signal synchronously with the reference 
signal, and (d) an element for low pass filtering the demodulated signal. 
Unit 14 is coupled to the electrode array, preferably through a 
multiplexor or switches. The capacitances to be measured in this 
embodiment are mutual capacitances between electrodes but could be stray 
capacitances of electrodes to ground or algebraic sums (that is sums and 
differences) of such mutual or stray capacitances. 
FIG. 4 shows one specific embodiment of a synchronous electrode capacitance 
measurement unit 14 connected to the electrode array 12, in which 
algebraic sums of mutual capacitances between electrodes are measured. The 
components are grouped into four main functional blocks. A virtual 
electrode synthesis block 70 connects each of the X electrodes to one of 
the wires CP or CN, and each of the Y electrodes to one of the wires RP or 
RN. The electrodes are selectively connected to the wires by switches , 
preferably CMOS switches under control of the position locator apparatus 
18 (FIG. 1) to select appropriate electrodes for capacitance measurement. 
(See Gerpheide '017 which describes such electrode selection by signal S 
of FIG. 1 of the patent.) All electrodes connected to the CP wire at any 
one time are considered to form a single "positive virtual X electrode". 
Similarly, the electrodes connected to CN, RP, and RN form a "negative 
virtual X electrode", a "positive virtual Y electrode", and a "negative 
virtual Y electrode", respectively. 
The reference frequency signal is preferably a digital logic signal from 
the reference frequency generator 16 (FIG. 1). The reference frequency 
signal is supplied to unit 14 via an AND gate 72 also having a "drive 
enable" input, supplied by the reference frequency generator 16 (FIG. 1). 
The AND gate output feeds through inverter 74 and noninverting buffer 76 
to wires RP and RN respectively which are part of a capacitive measurement 
element 78. In the preferred embodiment, the drive enable signal is always 
TRUE, to pass the reference frequency signal. In further preferred 
embodiments, it is asserted FALSE by the reference frequency generator 
when interference evaluation is to be performed as described later. When 
the drive enable signal is FALSE, the drive signal stays at a constant 
voltage level. When the drive signal is TRUE, it is a copy of the 
reference frequency signal. 
The capacitance measurement element 78 contains a differential charge 
transfer circuit 80 as illustrated in FIG. 4 of Gerpheide, U.S. Pat. 
5,349,303, incorporated herein by reference. Capacitors Cs1 and Cs2 of 
FIG. 4 of that patent correspond to the stray capacitances of the positive 
and negative virtual electrodes to ground. The CHOP signal of that FIG. 4 
is conveniently supplied in the present invention as a square wave signal 
having half the frequency of the reference frequency signal, as generated 
by the divide-by-2 circuit 81 shown herein. As described in the Gerpheide 
'303 patent, the circuit maintains CP and CN (lines 68 and 72 therein) at 
a constant virtual ground voltage. 
The capacitance measurement element 78 also contains a non-inverting drive 
buffer 76 which drives RN and negative virtual Y electrodes to change 
voltage levels copying the drive enable signal transitions. The inverting 
buffer 74 drives RP and the positive virtual Y electrodes to change 
voltage levels opposite the drive enable signal transitions. Since CP and 
CN are maintained at virtual ground, these voltage changes are the net 
voltage changes across the mutual capacitances which exist between virtual 
Y and virtual X electrodes. Charges proportional to these voltage changes 
multiplied by the appropriate capacitance values are thereby coupled onto 
nodes CP and CN (the "coupled charges"). The charge transfer circuit 
therefore supplies a proportional differential charges at outputs Qo1 and 
Qo2, which are proportional to the coupled charges and thus to the 
capacitances. 
In short, this differential charge is a proportionality factor K times the 
"balance" L, which is a combination of these capacitances given by the 
equation: 
EQU L=M(xp,yn)+M(xn,yp)-M(xp,yp)-M(xn,yn) 
where M(a,b) is the notation for the mutual capacitance between virtual 
electrode "a" and virtual electrode "b". Changes in balance are indicative 
of finger position relative to virtual electrode position as described in 
Gerpheide, U.S. Pat. No. 5,305,017. The proportionality factor K has a 
sign which is the same as the drive enable signal transition direction. 
Referring again to FIG. 4, the synchronous electrode capacitance 
measurement element 78 is connected via lines carrying charges Qo1 and Qo2 
to a synchronous demodulator 82 which may be a double-pole double-throw 
CMOS switch 84 controlled by the reference frequency signal. The 
synchronous demodulator 82, which among other things functions to rectify 
the charges Qo1 and Qo2, is connected to a low-pass filter 86 which may be 
a pair of capacitors C1, C2 configured as an integrator for differential 
charges. (An integrator illustratively is a low pass filter with 6 db per 
octave frequency roll off.) Charges Qo1 and Qo2 are integrated onto 
capacitors C1 and C2, respectively, when the reference frequency signal 
has just transitioned positive, and K is positive. The charges are 
integrated onto opposite capacitors when K is negative. In this way, a 
differential charge proportional to the balance L is accumulated on C1 and 
C2. 
FIG. 5 shows another embodiment of the synchronous electrode capacitance 
measurement unit 14. In this embodiment, each electrode in an electrode 
array 90 is connected to a dedicated capacitance measurement element 92, 
each of which is connected to a synchronous demodulator 94 and then to a 
low pass filter 96. This configuration has the advantage of continuously 
providing capacitance measurements for each electrode, whereas the prior 
preferred embodiment measures a single configuration of electrodes at any 
one time. The disadvantage of the embodiment of FIG. 5 is the greater 
expense which may be associated with the duplicated elements. This is a 
common tradeoff between providing multiple elements to process 
measurements at the same time versus multiplexing a single element to 
process measurements sequentially. There is obviously a wide spectrum of 
variations applying this trade off. 
Also, many of the elements can be implemented in digital form using 
analog-digital converters and digital signal processing. While the 
preferred embodiments currently use substantial analog processing, future 
digital processing costs may be expected to become relatively cheaper. 
FIG. 6 provides a number of preferred alternatives for the capacitance 
measurement element 78 (FIG. 4) or 92 (FIG. 5). FIGS. 6A and 6B show 
circuits adapted for measuring mutual capacitances between electrodes 
(which may be physical or virtual electrodes), represented by Cmp, Cmn, 
and Cm. FIGS. 6C and 6D show circuits adapted for measuring electrode 
capacitance to ground represented by Cg. Each of these provides an output 
voltage change indicative of the capacitance being measured. These voltage 
changes are given by the following formulas: 
For FIG. 6A: 
EQU .DELTA.Vout=.DELTA.Vdrive.times.(Cmp-Cmn)/Cr 
For FIG. 6B: 
EQU .DELTA.Vout=.DELTA.Vdrive.times.Cm/Cr 
For FIG. 6C: 
EQU .DELTA.Vout=.DELTA.Vdrive.times.Cg/(Cg+Cr) 
For FIG. 6D: 
EQU .DELTA.Vout=.DELTA.Vdrive.times.(Cg+Cr)/Cg 
The formulas depend on a known reference capacitance represented by Cr and 
a known drive voltage change represented by .DELTA.Vdrive. Further 
capacitance measurement elements may be based on charge balance techniques 
as described in Meyer, U.S. Pat. No. 3,857,092. Synchronous demodulators 
may be implemented using an analog or digital multiplier, or a 
"double-balanced mixer" integrated circuit (such as part number LM1496) 
from National Semiconductor Company. There are different implementations 
known in the art for low pass filter elements, such as switched capacitor 
integrators and filters, high-order analog filters, and digital filters. 
Reference Frequency Generator 
FIG. 7 illustrates a preferred embodiment of reference frequency generator 
16 (FIG. 1). The generator observes position signals to evaluate the 
extent of interference at some reference frequency. In the event that 
substantial interference is detected, the generator 16 selects a different 
frequency for further measurements. The generator 16 seeks to always 
select a reference frequency away from frequencies which have been found 
to result in measurement interference, as described below. 
The generator 16 includes an oscillator 100 which is, for example, set at 
four MHz, driving a microcontroller 102 and a divide-by-(M+N) circuit 104. 
Value N is a fixed constant, approximately 50. Value M is specified by the 
microcontroller 102 to be, for example, one of four values in the range 61 
KHz to 80 KHz as specified by the microcontroller 102. 
The microcontroller 102 performs the functions of interference evaluation 
106 and frequency selection 108. It may perform other functions associated 
with the system such as position location. The preferred interference 
evaluation function 106 produces a measure of the interference in the 
position signals generated by the position location unit 18 (FIG. 1). This 
is based on the principle that interference produces a spurious, 
relatively large magnitude high-frequency component of a position signal, 
and operates according to the following code description. It assumes 
position data points X, Y, and Z occur approximately every ten 
milliseconds. In brief, it calculates an interference measure, IM, as the 
sum of the absolute values of the second differences of X and Y together 
with the absolute values of the first differences of Z over 32 data 
points. Differencing a stream of data has the effect of applying a 
high-pass filter to it. 
In detail, for each data point the interference evaluation function 106 
executes the following steps, where ABS() means the absolute value 
function: 
______________________________________ 
XD=X-XLAST ;computes first differences 
YD=Y-YLAST 
ZD=Z-ZLAST 
XDD=XD-XDLAST ;computes second differences 
YDD=YD-YLAST 
IM = IM + ABS(XDD) + ABS(YDD) + ABS(Z) ;sum 
IF EVERY 32ND SAMPLE 
{EXECUTE FREQUENCY SELECT FUNCTION 108 
(See description below) 
IM = O} 
XLAST=X ;move current values to last 
YLAST=Y 
ZLAST=Z 
XDLAST=XD 
YDLAST=YD 
______________________________________ 
In another embodiment, the interference evaluation function 106 is not 
based on position signals. Instead the function asserts the drive enable 
signal described above to a FALSE state and reads a resulting synchronous 
capacitance measurement. This measures charge coupled to the electrodes 
when no voltage is being driven across the electrodes by the apparatus. 
Such charge must be the result of interference, and so this interference 
(from spurious signals) is directly measured. This is another way to 
generate the interference measure, IM. 
The preferred frequency select function 108 generates a table of historical 
interference measurements for each frequency which may be selected. On 
system initialization, each entry is set to zero. Thereafter, the 
frequency select function is activated approximately every 32 data points 
by the interference evaluation function 106. The current interference 
measure, IM, is entered as the entry for the currently selected frequency 
in the table. Then all table entries are scanned. The frequency having the 
lowest interference measure entry is selected as the new current 
frequency, and the corresponding M value is sent to the divide-by-(M+N) 
element 104. Approximately every 80 seconds, every entry in the table is 
decremented by an amount corresponding to approximately 0.05 mm of 
position change. In this way, if a frequency is flagged as "bad" by having 
strong interference one time, it will not be flagged as "bad" permanently. 
The functions described above for the different embodiments could be 
carried out by a microprocessor such as part no. MC 68HC705P6 manufactured 
by Motorola, Inc. serving as the microcontroller 102. 
FIG. 8 shows an alternate preferred embodiment of the reference frequency 
generator 16 (FIG. 1). It generates a reference frequency signal that 
varies randomly. Each cycle of the signal has a different and 
substantially random period. It is extremely unlikely that a spurious 
signal would coherently follow the same sequence of random variation. 
Hence the spurious signal is substantially rejected by capacitance 
measurements synchronous to the reference frequency. The degree of 
rejection is not as great as in the former embodiment, but the generator 
is simpler because interference evaluation and frequency selection 
functions are not needed. 
The generator of FIG. 8 includes an oscillator 110 and a divide-by-(N+M) 
circuit 112. The value M supplied to the divider comes from a 
pseudo-random number generator (PRNG) 114 which generates numbers in the 
range 0 to 15. Each cycle of the reference frequency clocks the PRNG 114 
to produce a new number. PRNGs are well known in the art. 
For either embodiment in FIGS. 7 or 8, the range of values for M in 
relation to the value of N can be increased or decreased to give a greater 
or lesser range of possible frequencies. The value of N or the oscillator 
frequency can be adjusted to change the maximum possible frequency. A 
phase-locked frequency synthesizer such as the Motorola MC145151-2, or a 
voltage controlled oscillator driven by a D/A converter, could also 
preferably be employed instead of the divide-by-(M+N) circuit. 
It should be understood that other variations of the preferred embodiments 
described above fall within the scope of this invention. Such variations 
include different electrode array geometry, such as a grid of strips, a 
grid of diamonds, parallel strips and various other shapes. Also included 
are different variations of electrode array fabrication, such as printed 
circuit board (PCB), flex PCB, silk screen, sheet or foil metal stampings. 
Variations of the kinds of capacitance utilized are included, such as full 
balance (see Gerpheide '017), stray, mutual, half balance. 
The above description has provided certain preferred embodiments in 
accordance with this invention. It is apparent by those skilled in the art 
that various modifications can be made within the spirit and scope of the 
invention, which are included within the scope of the following claims.