What is a Capacitive Sensor?

A capacitive sensor is a proximity sensor that detects nearby objects by their effect on the electrical field created by the sensor.   Simple capacitive sensors have been commercially available for many years, and have found a niche in nonmetallic object detection, but are limited to short ranges, typically less than 1 cm.

Capacitive sensors have some similarities to radar in their ability to detect conductive materials, while seeing through insulating materials such as wood or plastic.   In practice, the differences are considerable; When compared to radar, capacitive sensors:
When used for detecting objects all around a vehicle, some of the disadvantages of the capacitive sensor are less problematic.  A practical system has many sensors regularly spaced around the outside of the vehicle.   This means that there is always a sensor close by, so no great range is required, and objects can be roughly localized by which sensor they are detected in. Non-directional response is actually desirable, since it can detect objects that are between sensors but very close to the vehicle.

What Can it Detect?

Due to its non-directional nature, the capacitive sensor measures some capacitance from objects in the environment that are always present and therefore not interesting.  When mounted on a car, the sensor detects the car itself and the ground.  Unknown objects are detected as increases in this background capacitance.

Commercial capacitive sensors typically operate at ranges of 1 cm or less.    At these ranges the object capacitance approaches the background capacitance.   However, at 1 meter the capacitance change is orders of magnitude smaller, and much less than the background capacitance.  It is necessary to determine what this background capacitance is so that it can be subtracted from the measurement. 

Since the background capacitance is large compared to the object capacitance, and is also subject to drift, it is much easier to use the sensor to detect change in the environment than to detect the absolute presence or absence of an unknown object.  The amount of background capacitance change depends on how stable the environment is.  In a relatively poorly controlled environment such as the outside of a car,  absolute presence detection of a person is probably limited to 30cm or less.

In this change detector mode, the sensor is not so much a presence detector as a change-of-presence detector,  somewhat like a passive infrared motion detector (PIR.)   However, because of its intrinsically short range, a capacitive motion detector can be used in situations where a PIR detector would falsely respond to apparent background changes.   This is true in the suggested vehicle safety application, where motion of the vehicle causes changes in the thermal background.

Spread Spectrum:

The spread spectrum operating concept is widely used in modern communication systems because it has numerous advantages over traditional narrow-band communication systems.  The approach discussed here is direct sequence spread spectrum, where a pseudo-random noise (PN) code is transmitted, and then the presence of the code is detected by the correlation between the received signal and the known code sequence.  Application of direct sequence spread spectrum to capacitive sensors is particularly simple because the transmitter and receiver are located in the same place, so synchronization of the transmit and receive code is trivial.

There is a great deal of good introductory material on the web which I will not duplicate.  Here are some links: The ABCs of Spread Spectrum, Spread Spectrum (SS) - Introduction , Spread Spectrum Techniques.

A key property of a spread spectrum system is the processing gain, which is a measure of how spread the spectrum is.  The processing gain is the ratio of the bandwidth of occupied spectrum of the spread signal to the actual signal bandwidth.  In RF communication systems, processing gains of 10's to 1000's are typical.  In this system, the bandwidth into the demodulator is approximately 100 kHz, and the output bandwidth is 1.5 Hz, so the processing gain is 67,0000, or 96 dB.

For capacitive sensors, spread spectrum has three major advantages:
  1. Substantial immunity to narrow-band interfering signals.  Any narrow band signal is attenuated by the processing gain, 96 dB.  In a narrow-band sensor, a narrow band interferer that happens to land in the passband is not attenuated at all.
  2. Automatic sharing of bandwidth between multiple users, with no coordination required for channel allocation.  In particular, a spread-spectrum sensor can operate in the presence of other similar spread-spectrum sensors  without requiring tuning to distinct frequencies.
  3. Very narrow and sharp effective bandwidths can be easily achieved using only a low frequency low-pass filter at the output of the demodulator.  This improves manufacturability because narrow-band RF filters are not required.  The entire circuit could be fabricated on a single chip.  (This is a benefit of synchronous demodulation or baseband processing, rather than spread spectrum per-se.)

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