Quote:
Originally Posted by Argus25
Geophone sensors are very good. These use a magnet on a spring moving inside a coil. It generates voltage proportional to the velocity of the magnet, different from a MEMS sensor that produces a signal proportional to acceleration,
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Agree that the magnet-on-a-spring will produce a signal proportional to magnet velocity, but how does that magnet velocity arise?
If the sensor is moved at constant velocity, steady state there will be no force needed to be applied to the magnet. So it will stay in its rest position with regard to its supports, the coil, etc. Zero output.
If the sensor is moved with constant acceleration, then the magnet will need a constant force applied to it to accelerate it. So it will be displaced from its rest position such that the spring displacement applies this constant force (Hooke's law, force is proportional to displacement). Since the magnet displacement from its rest position is constant, there is again no movement between it and its supports, the coil, etc.
Only if there is a rate of change of acceleration will there be a change of spring displacement, hence a non-zero relative velocity between magnet and coil. So this sensor gives an output proportional to rate of change of acceleration, whereas the MEMS is proportional to acceleration itself.
This might of course make it ideal as a shock sensor of course, but on reading your post it came across that the magnet/spring is a velocity sensor as against the MEMS being acceleration. It isn't - to get an output from the magnet/spring you have to differentiate velocity twice more!