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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