PSN-L Email List Message

Subject: Re: Integrating in WinQuake
From: RSparks rsparks@..........
Date: Tue, 24 Nov 2009 23:04:04 -0800

```Hello All,

Brett makes a particularly good point when he comments that "You can not
say that a ground motion is only a displacement, or a velocity, or an
acceleration, it is always all three.". Geoffrey makes the same point
when he offers the equations that relate the three metrics.

I am not familiar with the MEMS accelerometers but Wikipedia has an
informative article on Accelerometers
.  It is interesting to see
that capacitive detection (which is displacement sensing)  is a common
way for these micro seismometers to sense acceleration.  Other methods
of detection (that I am not familiar with) are also used.

Perhaps our experience with the automobile colors our thinking.  With
the car, first we accelerate, then we have velocity and finally,
distance is traveled.  A very time sequenced pattern.

If we examine this same series of events on a micro scale, we can only
find velocity (which may be zero) or change in velocity (acceleration).
There is nothing else.  Any non zero measurement will have a distance
component and a time component (back to the three equations).

Perhaps we should think of detection in terms of energy.  If we have
velocity, then the moving mass has kinetic energy.   The magnet/coil
system extracts energy from the velocity (and slows/opposes the relative
motion) with the result that power is sent to the amplifier.  On the
other hand, a capacitive or optic system uses external power to sense
position.

Knowing all the above, now let us make two seismometers, one using a
magnet/coil detector and one using a capacitive detector.  Move both
from position 1 to position 2.  Both will record a positive peak and
then a negative peak (or the reverse).  Each peak will correspond to the
maximum acceleration, the first peak to maximum initial acceleration and
the second peak to maximum deceleration (measured as negative
acceleration).

Apparently we can use either displacement or velocity detection to
measure acceleration.

Roger

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