Hello All,

Please withdraw my posting of Tue, 24 Nov 2009 23:04:04 -0800 (attached) 
which contains a grievous error.  I incorrectly described the output of 
the displacement and velocity detectors as being identical but as Chris 
tried to tell me (following a previous reference to identity) (but I 
failed to understand), they are much different. 

My error came when I wrote "

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.

"
The output of the displacement detector is correctly described but the 
description of the output of the velocity sensor is grossly wrong.  
During the movement from position 1 to position 2, the seismometer must 
accelerate, reach a velocity, and then decelerate, stopping at position 
2.  Because the velocity detector is extracting energy from motion 
through a magnetic field, the velocity detector first measures the 
motion of the initial movement, then the motion caused by the return to 
a zero position forced by the return mechanism, then the motion caused 
by deceleration, and (finally) the motion of the final return to zero 
position caused by the return mechanism after the stop at position 2 
occurs.  For the velocity detector, the simple movement of the 
seismometer from position 2 to position 2 yields two positive peaks and 
two negative peaks. 

I write this before I have received any feedback.  I expect that several 
commentators will describe the differences between the two detector 
outputs as they point out my error, and will do a much better job than I 
did.  My apologies to all for the initial misdirection. 

Roger


psn-l-digest-request@.............. wrote:
> .------ ------ ------ ------ ------ ------ ------ ------ ------ ------.
> | Message 1                                                           |
> '------ ------ ------ ------ ------ ------ ------ ------ ------ ------'
> Subject: Re: Integrating in WinQuake
> From:    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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>
>
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