Graphics input tablet with three-dimensional data

A graphics input tablet has a conductive layer and a resistive layer with contacts (16,17,23,24) arranged one along each edge of the resistive layer. The conductive layer is held at a negative potential relative to the resistive layer so current flows in each contact when localized pressure is applied to a region of the tablet (20) to bring the layers into electrical contact. The currents vary with both position and magnitude of the localized pressure. The position of the pressure is detected from the relative currents in opposed contacts and the magnitude of the pressure is detected from the total current in the contacts. The currents are measured by current sense amplifiers and processed in a computer. Analogue front-end processing of the currents is also possible.

BACKGROUND OF THE INVENTION 
This invention relates to a graphics input tablet. The invention is 
particularly useful in the data processing field where it may be employed 
to input data of a graphical nature into a data processing machine. 
BACKGROUND ART 
In the data processing field one of the major problems to users of a system 
is the rapid inputting of data. This has traditionally been achieved with 
a keyboard but when it is necessary to input graphical data, rather than 
character strings, a keyboard is grossly inefficient. There are various 
examples in the prior art of attempts to provide a more efficient and 
easier to use input means for graphical data. 
One such example is disclosed in EP 5,996 (Quest Automation Ltd.) which 
shows an electrographic apparatus with two resistive layers, one overlying 
the other, held apart by a framework but arranged to be brought into 
contact by the pressure of a stylus or similar. Excitation voltages are 
applied to the resistive layer at 90 degrees to each other and two 
analogue voltages related to the position of the stylus are obtained in an 
unspecified manner. Another example is disclosed in GB 2,088,063 (Robert 
Branton). This shows a conductive layer overlying a rectangular resistive 
layer with an insulating mesh between the two. The resistive layer has a 
contact at each corner and one opposed pair of contacts is energized at 
any one time. A stylus (ball-point pen or similar) is pressed onto the 
upper, conductive layer which then makes contact, through a gap in the 
insulating mesh, with the resistive layer. In consequence, the potential 
of the conductive layer equals thatof the resistive layer at the point of 
contact; since this varies with the relative distances of the point from 
each of the polarized contacts, the potential of the conductive layer 
provides information to identify the position of the stylus. Once the 
position with respect to these polarized contacts is established they are 
de-energized and the other pair is polarized to give information regarding 
the position of the stylus along a different axis. Thus the position of 
the stylus in two dimensions may be identified. 
SUMMARY OF THE INVENTION 
The prior art techniques only permit two dimensional input directly from 
the tablet; if a third dimension is required it must be input by other 
means, e.g., a potentiometer arranged for manual control. The ability to 
input three dimensional data using the tablet alone would be very useful, 
for example (a) to define the color and/or intensity of a location on a 
graphic terminal or (b) for signature verification where the profile of 
the pressure applied across the signature would be a useful additional 
check, above and beyond a two dimensional check of the appearance of the 
signature. Further, the prior art techniques do not permit the pressure 
information to be detected at all; this information could be useful, for 
example in the case of a one dimensional position-detecting tablet in the 
form of a strip. This would provide two dimensional input capability 
without occupying the large area taken up by a prior art 2-D input tablet. 
Accordingly, the present invention provides a graphics input tablet 
comprising a layer of electrically resistive material supported 
co-extensively with a layer of electrically conductive material to provide 
a flat tablet surface, the resistive material having the property that the 
electrical resistance between the layers in the region of localized 
pressure applied thereto changes monotonically with the applied pressure, 
first and second conductors connected respectively to first and second 
portions of the resistive layer said portions being spaced apart on a 
first notional line across the tablet surface, the construction and 
arrangement being such that with an electrical potential applied between 
the resistive layer and the conductive layer substantially no current 
flows through said first and second conductors, but with said localized 
pressure applied to said tablet surface currents flow through said first 
and second conductors, the relative magnitudes of the currents being 
related to the respective distances of the region of applied pressure from 
the first and second portions and the total current flowing between the 
resistive sheet and the conductive sheet being related to the magnitude of 
the applied pressure. 
This provides the facility to detect the position of a stylus in two 
dimensions, i.e., one linear dimension and one pressure dimension. 
Preferably, the graphics input tablet further comprises third and fourth 
conductors connected respectively to third and fourth portions of the 
resistive layer, said portions being spaced apart on a second notional 
line, intersecting said first notional line, across the tablet surface, 
such that when said localized pressure is applied in said region, currents 
flow in said third and fourth conductors, the relative magnitudes of the 
currents in the third and fourth conductors being related to the 
respective distances of said third and fourth portions from the area of 
applied localized pressure. 
This provides the facility to detect the position of a stylus in three 
dimensions, i.e., two linear dimensions and one pressure dimension. 
Preferably, the graphic input tablet has electrical sensing means arranged 
to evaluate said total current by summing the individual currents measured 
in each of said conductors. 
Alternatively, the graphics input tablet has electrical sensing means 
arranged to evaluate said total current from a measurement of the current 
flowing between the electrical excitation means and the conductive layer. 
The resistive layer may be in the form of a single sheet of material. 
Alternatively, the resistive layer may comprise two sheets, each of a 
resistive material, the conductors being connected to a first of said 
sheets and the second of said sheets having the property as aforesaid that 
the electrical resistance between the layers in the region of localized 
pressure applied thereto changes monotonically with the magnitude of the 
pressure applied to the layers.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1a shows a graphics input tablet according to the present invention. 
Layer 10 is a compressible resistive sheet known as Vermahide (Trade 
Mark). This material has the property that its localized electrical 
resistance reduces with increased pressure exerted on it as the 
electrically conducting fibers which make it up are forced into more 
intimate contact. Beneath layer 10 is layer 11 which is an electrically 
conducting coat applied to insulating substrate 13. Beneath insulating 
substrate 13 is conductive coat 50 which may be electrically grounded to 
give electrostatic screening. Layers 11,13,50 may conveniently be provided 
by double-sided unetched printed circuit board. Overlying layers 
10,11,13,50 is protective sheet 12 which is electrically insulating, 
physically hard-wearing and yet locally elastically deformable. Electrical 
connection is made to conductive layer 11 by conductor 30 and to opposed 
edges of resistive layer 10 by conductors 14,15,21,22 (see FIG. 2). 
FIG. 1b shows an alternative tablet comprising two resistive sheets, one 
compressible 10a and one rigid 10b. Conductive layer 11 overlies the 
resistive sheets and is locally elastically deformable. 
FIG. 2 shows a plan view of the tablet in FIG. 1a. Contact pads 16,17, 
23,24 are shown, attached to conductors 14,15,21,22 respectively. Each 
contact pad makes electrical contact with a portion of the resistive 
layer: 16,17,23,24 contacting 18,19,25,26 respectively. This leaves the 
remainder 20 of layer 10 as the input region where a user applies 
localized pressure with a stylus, causing currents to flow in conductors 
14,15,21,22. 
FIG. 3a shows one technique for producing signals x,y,z representing the 
position of pressure applied on region 20 (x,y) and the magnitude of 
pressure applied (z). Four current sense amplifiers (44) are provided for 
measuring the current flowing in the conductors 14,15,21,22. The outputs 
of the currents sense amplifiers are fed into two analogue summing 
circuits 35,37, one 35 for conductors 14,15 covering the x dimension and 
the other 37 for conductors 21,22 covering the y dimension. The outputs of 
these summers are fed into dividers 36 and 38, respectively, along with 
one of the current sense outputs in each case. The signal for x (and 
similarly for y) which this produces is not the simple ratio of currents 
in opposed conductors 14,15 (or 21,22); rather, if current in 14=I1 and 
current in 15=I2 then the value of x is: 
##EQU1## 
This function is selected since it provides a result which varies 
approximately rectilinearly between 0 and 1. 
The signal for z, representing the pressure applied by the stylus is simply 
a sum of the individual currents in conductors 14,15,21,22. This is 
obtained by summing in 39 the sums of x and y currents produced by summing 
circuits 35 and 37, respectively. 
FIG. 3b shows a circuit for inputting x, y and z to a computer. The 
multiplexer 40 selects each of x,y,z in turn to forward to the analogue to 
digital converter 41. The results are fed to buffer 42 and thence to data 
bus 43 which is connected to the computer. 
FIG. 4 shows an alternative arrangement to that of FIGS. 3a,3b. In this 
case, there is an additional current sense amplifier 45 for directly 
measuring the current in conductor 30. This avoids the expense and 
inaccuracy of analogue summing circuit 39 (used in conjunction with 
summing circuits 35 and 37). Further, the generation of the x and y 
functions is performed in the computer so that the tablet does not need 
the local intelligence provided by circuits 35,36,37,38. This approach has 
the disadvantage that more processing is necessary in the computer so that 
the maximum sampling rate will be lower but it does not require circuits 
35,36,37,38,39 so the hardware may well be cheaper to produce. 
In use, the conductive layer 11 of the tablet is held at a voltage equal to 
-10 Volts. The conductors 14,15,21,22 along the edges of resistive layer 
10 are held at 0 Volts. When no pressure is applied to the upper surface 
of the tablet, the physical contact between layers 10 and 11 is very 
slight and no significant current flows between them. However, when 
localized pressure is applied to the upper surface, the layers 10 and 11 
are pushed into physical and electrical contact so that a current flows 
from conductive layer 11 to the conductors 14,15,21,22 connected to 
resistive layer 10. The current through each of these conductors is in 
inverse relation to the distance from the respective contact pad to the 
point where pressure is applied. 
Another possibility is to have the arrangement as shown in FIG. 4 but 
without sense amplifier 45 (and its associated resistor). In this case, 
the computer must sum all four conductor currents digitally. This is a 
processing overhead which reduces the maximum sampling rate but the 
hardware will be still cheaper to produce. 
FIG. 5 shows the variation in the measured value of x versus the actual 
position at which pressure is applied. It can be seen that there are 
significant edge effects which mean that it may be necessary to process 
digitally the measurements once they are received by the computer in order 
to expand the measured x values to cover the entire range from 0 to 1 (0 
is the left-hand edge of area 20 and 1 is the right-hand edge). It may 
also be necessary to take account of the y displacement when expanding the 
x readings since the x readings are compressed when taken near y=0 or y=1 
compared to readings taken near y=0.5.This is shown by the two lines on 
FIG. 5, line A being taken at y=0.5 (i.e., across the center of the 
tablet) and line B being taken at y=0.9 (i.e., near the upper edge of the 
tablet). 
FIG. 6 shows the variation in measured total current (i.e., z) versus force 
applied to an area of tablet 1.5 millimeters square. It can be seen that 
this too would benefit from some digital processing, since the line 
produced is not as straight as ideally it should be. This would be 
straightforward to achieve if necessary, for example, by employing a 
look-up table correlating measured current to applied pressure. It will be 
noted that below a certain non-zero value for the pressure, the measured 
total current is zero. This is useful since it means that light pressure 
(e.g., from a person's hand) will not be detected by the tablet and will 
not interfere with the normal operation of the tablet. 
The variation in current with applied pressure stems from two effects. The 
first of these is that the resistive material compresses locally, so 
reducing the electrical resistance in the region of applied pressure since 
the fibers in the material are in better electrical contact. The second 
and more significant effect is that the contact area between the 
conductive and resistive layers increases. 
The increase in contact area with increased pressure can cause the measured 
position to be in error. This happens when the pressure is applied 
significantly closer to one contact pad than to the opposed contact pad 
(e.g., closer to 16 than to 17). The increase in contact area with 
pressure is uniform in all directions but since the contact area is closer 
to 16 than to 17, the distance from the contact area edge to 16 reduces by 
a greater percentage than the distance from the contact area edge to 17. 
This causes the measured position to appear further towards the near edge 
as more pressure is applied. If this effect is unacceptable, then it would 
be necessary to compensate for it digitally in the computer by weighting 
the position measurement towards the center, the level of weighting 
increasing with increasing pressure (i.e., increasing total current). 
Another compensation which may be required is to allow for the fact that, 
at a constant pressure, the total current increases as the contact point 
approaches any edge of the tablet. This is because the overall resistance 
through the resistive sheet from the contact point to the contact pads 
16,17,23,24 decreases as the contact point moves further from the center 
and closer to one or two of the contact pads. If this effect is too large 
to be ignored, then a suitable weighting could be applied when the data is 
processed. 
The rate at which the location and pressure of the stylus are sampled will 
depend on the requirements of the application and the circuitry and 
computer software employed. For graphics input to a terminal and for 
signature verification a sampling rate of around 10 kHz may be acceptable 
although a rate of around 20 kHz is preferable.