In a message dated 21/01/00 23:41:48 GMT Standard Time, twleiper@........ 
writes:

C>>the Lehman.... is an 'open loop' device.... 

T>There is no feedback, just damping (resistive)

C>> How about devising an auto-zero circuit for the seismometer, so that you 
never have to readjust it?
 
 T> Basically, a force balanced instrument (using a DISPLACEMENT SENSOR to 
create an "error" signal used to apply a balancing force to "hold" the mass 
in a fixed position relative to earth) would never need adjustment, except 
perhaps for optimum performance.

C>> Agreed, but you described a moving coil sensor, which gives a signal 
proportional to the rate of movement of the coil in the magnetic field, but 
it is not sensitive to the position of the coil in that field....

 T> All one has to do is pass a sample of that error voltage through a high 
pass filter .... and on to the signal processing circuitry.... The first 
integration of acceleration would give velocity and then integrating velocity 
gives displacement...I think. See what I mean?

C>> Suppose that you have a Lehman arm offset from zero and it then moves a 
bit. A coil pickup moving in a magnetic field will give you the 
differentiated movement signal, which could then be integrated to give the 
approximate movement, less any non linearity's, integrator drifts etc., or in 
a force balance system, used to hold the arm in a fixed position with respect 
to the base. What the integrated signal cannot give you is the initial offset 
(or integration constant). Unless you have some sort of position signal 
available, even if it is quite rough and ready, you can't correct for slow 
drifts.
    You used to be able to buy photo potentiometers which could have done 
this job nicely for your +/- 0.75" movements, but I haven't seen any about 
recently. They were made on an alumina strip ~2" long x ~0.15" wide. On one 
face, a conductive strip was deposited along one long edge and a resistance 
element on the other long edge. The strips were completely bridged by CdS 
photo conductor. If you put a voltage across the resistance and shone a 
narrow spot of light on the element, the CdS conducted only under the light 
spot and the conducting strip picked up that voltage. Does anyone know if 
they are still available, please?
    In the S-G seismometers, the arm is stabilised by gravity and you need to 
measure micron movements. In Karl Cunningham's force balance seismometer, the 
position is sensed by an +/- 0.1" LVDT transducer and used for the feedback. 
It would be nice to have an 'add on' device for a mechanical garden gate 
Lehman which would stabilise the arm, particularly for people who are 
interested in seismology but do not have access to a highly stable site.   
 
> I just want to make something that is simple, cheap, and gets good 
squiggles for others.

>> It is the 'good' which may be the difficult bit, but since you have done 
such an admirable job with the standard Lehman design.... and I do mean that 
most sincerely.

    Regards Chris

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