An older application of capacitive sensing is a computer graphic input tablet manufactured by Shintron Co., Concord, Mass. The tablet was developed in 1967, but the principles of its operation are still used today. It measured the x, y, and z movements of a small, capacitively coupled pickup stylus relative to a square-wave-driven resistive sheet that generated an electrostatic field over the 11-in. square tablet. Two different methods of producing the orthogonal field lines were investigated, and a phase-locked-loop demodulator was used with ratiometric response. Performance was good at large stylus-to-tablet separations.
The tablet had a 1024-by-1024 resolution, 1% accuracy, and a sample rate of 100 x-y samples/s. The output format was 10-bit parallel digital, and maximum paper thickness was 0.5 in.
Driving a resistive sheet to measure a single axis is simple. If metallic electrodes A and B are placed along opposite sides of the sheet and fed with a 5-V square wave, the sheet will generate a linear ac voltage field just over its surface. A stylus using a small electrode will pick up a signal proportional to the y displacement when moved near the surface of the sheet. A circuit that measures signal amplitude can also measure the z position.
Two-Dimensional Complications
A two-dimensional system is more complicated. If metallic electrodes C and D are added to the remaining two sides, electrodes A and B will be shorted at the corners, or at minimum a very nonlinear field will be produced. One alternative is to drive the corners instead of the sheet's edges, and to use a medium-resistivity material on on the edges and a high-resistivity material for the sheet to produce an orthogonal, linear field.
To measure position in the y axis on this resistive tablet, electrodes A and B are connected together and driven with 100-kHz, 0º signal and electrodes C and D are connected together and driven with 100-kHz, 90º signal. As the stylus is moved in the y axis, the electrical angle will change from 0º to 90º, but displacement in x will not affect the signal. The x-axis position is determined by driving electrodes A and D together and B and C together. Changes in z will change amplitude, but not phase.
Excluding small fringe effects, the field produced by this tablet will be linear and orthogonal if the ratio of the resistives is large. In practice, a large resistivity ratio may be difficult to obtain and a geometric compensation, a slight pincushion shape, will be needed to correct the nonlinearity caused by a low ratio. A linearity correction table can handle a large nonlinearity. The low-resistance strips can be dispensed with, for example, and the resulting over-50% nonlinearity measured and stored.
The stylus pickup used a guarded, coaxial construction with a 2-mm point exposed for writing and sensing (Fig. 7a). It performed well up to 10 cm stylus-to-tablet spacing, at which distance its capacitance to the resistive sheet was less than 0.05 pF, and the signal amplitude was reduced to 10% of the amplitude at the surface due to fringe effects and unguarded stray capacitance. To achieve this performance, the amplifier's input capacitance had to be less than 0.1 pF, a level that attenuated the signal by 3X. The use of ratiometric position detection made this unimportant.
The pickup circuitry used discrete transistors and both guarding and neutralization techniques to minimize input capacitance (Fig. 7b). The guard in the follower circuit did most of the input-capacitance cancellation. A small amount remained due to the finite amplifier gain and the FET's gate-to-drain capacitance. The adjustable neutralizing capacitor, with a value of 0 to 0.1 pF, nulled this residue. Parasitic capacitance is reasonably repetitive and stable, so the neutralizing adjustment is not sensitive to environmental factors once adjusted.