Spread Spectrum Capacitive Sensor
I have developed an extremely sensitive capacitive sensor which can
detect a person over 1 meter away. It operates using a
pseudo-random excitation signal, which is novel for a capacitive
sensor. This allows high sensitivity to be achieved in the
(inevitable) presence of interfering signals. The sensor also has
seven measurement channels, which allow the location and motion of
objects to be detected.
This sensor was developed primarily to be used in safety systems which
detect any people that are close to a car, truck or bus, and might be
in danger of being run over. However, the technology could be
used in many other ways.
I developed this on my own time during 2004, working in my home
electronics lab. My primary goals
were to:
- Demonstrate my electronic system design competence: analog,
digital, firmware and DSP,
- Show that I can develop an innovative hardware system all the way
from concept to completion,
- Have fun, and
- Gain design experience.
I wanted to play with the capacitive sensor principle, and
spread-spectrum appeared to be a novel approach to interference
rejection, which was one of the major weakness of past long-range
capacitive sensors.
This was my effort to make the best capacitive sensor I could within a
modest budget. In the end, I came close enough to the ideal
sensor that the dominant weaknesses seen in practice are intrinsic in
the measurement principle. In particular, minute changes in the
near sensor environment such as thermal expansion of the mounting can
create capacitance drifts well above the electronic noise floor.
For most practical sensing purpose, this design is overkill.
In the process, I came up with some cool hacks that resulted in a
simple
high-performance design. To me, this was engineering finesse as
an end in itself, but it did also result in a system that is highly
manufacturable.
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