In a message dated 25/05/01, mja6042@............. writes:

> It seems like the thing to do then is build Lehman with a displacement
> sensor similar to the one found on a Shackleford-Gundersen seismometer in
> place of the magnet and coil on the boom of the Lehman.
> 
> I have not heard of anyone doing this. Is there a reason? Something I
> 

Hi there Mark,

       Drift is likely to be the biggest problem. Lehmans are very sensitive 
to tilt effects. The longer the period, the greater the tilt sensitivity. 
This is not seen on a SG seis, which is a simple vertical pendulum with a 
strong tendency to re-centre. You can use a LVDT to measure the deflection on 
a Lehman. They can have much greater dynamic range than a variable separation 
capacitative plate transducer such as used on a SG. A variable area 
capacitative plate could be OK. Putting the whole apparatus inside a feedback 
loop to keep the weight in the same position would be advantageous. The 
normal coil assembly used on a Lehman is sensitive to the rate of change of 
position and isn't worried by small drifts.
       Have a look at:- A Force-Balance Seismometer by Karl Cunningham 
       on the PSN Website.

       Regards, Chris Chapman
In a message dated 25/05/01, mja6042@............. writes:



It see
ms like the thing to do then is build Lehman with a displacement

sensor similar to the one found on a Shackleford-Gundersen seismometer in

place of the magnet and coil on the boom of the Lehman.



I have not heard of anyone doing this. Is there a reason? Something I

have not taken into consideration?



Hi there Mark,



       Drift is likely to be the biggest problem. Lehmans are very sensitive 

to tilt effects. The longer the period, the greater the tilt sensitivity. 

This is not seen on a SG seis, which is a simple vertical pendulum with a 

strong tendency to re-centre. You can use a LVDT to measure the deflection on 

a Lehman. They can have much greater dynamic range than a variable separation 

capacitative plate transducer such as used on a SG. A variable area 

capacitative plate could be OK. Putting the whole apparatus inside a feedback 

loop to keep the weight in the same position would be advantageous. The 

normal coil assembly used on a Lehman is sensitive to the rate of change of 

position and isn't worried by small drifts.

       Have a look at:- A Force-Balance Seismometer by Karl Cunningham 

       on the PSN Website.



       Regards, Chris Chapman