PSN-L Email List Message

Subject: Re: Etc.
From: barry lotz gbl@.......
Date: Sun, 23 Jan 2000 13:29:11 -0800

   I'm kind of entering this thread in the middle. I have a force balance
horizontal and lehman adjacent to each other. I use the output of the FB sensor
to tell when to check my Lehman position (due to slab tilt). I previously used
infared emitter detector combination (radio shack). I attached  narrow target to
the end of my Lehman. It shadowed the emitter from hitting the detector. However
when the Lehman drifted 1/2 the target width the emitter hit the detector and lit
an led in my house. It didnt't tell which direction but did tell when attention
was required.

ChrisAtUpw@....... wrote:

> 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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Larry Cochrane <cochrane@..............>