Compared with competing optical, inductive, and piezoresistive transducers, capacitive sensors have many advantages, among them low cost and power usage, and good stability, resolution, and speed. They also have a near-zero temperature coefficient, can be optically transparent, and are easy to integrate into ICs or onto printed-circuit boards (pc boards). Capacitive sensors can detect motion, acceleration, flow, and many other variables, and are used in a wide range of applications.
But many engineers still distrust the technology. Some believe that capacitive sensors are affected by temperature and humidity, sensitivity to noise, difficulties in designing, instability, and nonlinearity. Capacitive sensors do need some specialized design know-how to avoid those hazards. Some sample designs and applications should help dispel this distrust.
Capacitive sensors come in one of three types. Fixed-plate versions maintain the relative position of the two plates, while the capacitive coupling changes as a result of different materials placed near the plates. A grounded conductive material will reduce coupling capacitance, and a high-dielectric material will raise it. These sensors are used for sensing wall studs or determining the composition of materials. An array of multiple fixed plates can form an x-y touch sensor to measure finger or stylus position in two dimensions or even to image fingerprints.
Another technique involves changing the spacing between the capacitor's parallel plates. This geometry's ability to accurately measure small motions down to 10-14 m makes it useful in electret microphones, tiltmeters, seismometers, and micrometers. Adding a third plate to sandwich the moving plate between two fixed plates, and driving the fixed plates while sensing the moving plate, increases the signal and provides shielding. This arrangement does not handle large motions well since capacitance drops to a difficult-to-measure value with large plate spacing.
The third type, which involves moving the parallel plates so their overlap area changes, can measure greater linear motions. Adding a second fixed plate above the moving plate again improves performance, as first-order spacing dependence is nulled out. To improve accuracy, multiple plate patterns are used, with a demodulator counting plates for a coarse position determination and interpolating between plates for a fine measurement, similar to optical encoders.
Ratiometric position sensing is better still. In this technique, the moving plate, C, is on one side of two fixed plates, A and B. The device measures the ratio of the capacitances CCA and CCB. This device is not sensitive to spacing. Adding two more fixed plates A' and B' on the other side of the moving plate, and then connecting A' to A and B' to B, makes the unit self-shielding and first-order insensitive to tilt.
Demodulation Methods
Several techniques are available to convert the capacitance or capacitance ratio to a voltage output. In the direct method, the plates are charged with a dc voltage and feed a very-high-impedance amplifier. This scheme is inexpensive and has good high-frequency response. But it doesn't work at dc unless the amplifier's impedance is infinite, and it is noisy because semiconductors are noisy at low frequencies.
Another method uses the sense plates to create the C in an RC oscillator, and then measures the oscillator's output frequency or period. The result is a simple, low-noise demodulator that rejects low-frequency noise. But if stray capacitance is not nulled, it may swamp the sensor's capacitance, causing a low-amplitude and unstable output.
In a synchronous demodulator, the sensor plates are driven by a square- or sine-wave signal at, for example, 5 V and 100 kHz, rather than a dc signal. Some systems use two square waves at 0°and 180° phases, and a phase-sensitive demodulator. In these systems, the sensor is usually configured for ratiometric measurements for improved stability and precision. The ratiometric synchronous demodulator is the most accurate circuit, but it has the highest part count.
Practical Circuits
In a practical system, the capacitance to be sensed usually ranges from 0.01 pF for ICs to 2-3 pF for a pc board of a square centimeter. Impedance ranges from 1.59 M? at 100 kHz with a 1-pF sensor to 1590 M? at 10 kHz with a 0.01-pF sensor. The amplifier's input impedance should be much larger than the sensor's to avoid shunting the signal, and since typical operational amplifiers have input capacitances of a few picofarads, special low-capacitance amplifiers are needed. These amplifiers can be used with any of the three types of demodulators.
At these extremely high impedances, system noise is usually dominated by amplifier current noise rather than voltage noise. Amplifier voltage noise is generally restricted to a relatively narrow range, between 3 nV/?*Hz and 60 nV/?*Hz. But current noise is much more variable, with extremes of 0.2 fA/?*Hz to 50 pA/?*Hz
The capacitive pickup amplifier will probably need a FET input stage, either a JFET or MOSFET, to get acceptably low current noise. MOSFET current noise is in the range of 0.2 fA-1 fA/?*Hz and will not contribute to output noise with reasonably high sensor plate capacitances (0.5 pF) and excitation frequencies (100 kHz). JFETs are almost as good, 2 to 40 fA/?*Hz, and usually have lower voltage noise and are less sensitive to electrostatic discharge. Bipolar transistor current noise can be over 50 pA/?*Hz. With a sensor impedance of 100 M? and a bandwidth of 10 kHz, this level of current noise will produce over 100 mV rms of input-referred noise.
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