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:
- Are simpler, so are potentially smaller, less expensive and less
power-hungry.
- Are proximity sensors, rather than range sensors. They do
not
give a direct indication of how far away the detected object is.
A
more distant strong target can give the same response as a nearby weak
target.
- Are non-directional and have a short range.
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:
- 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.
- 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.
- 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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