PSN-L Email List Message

Subject: Re: Lehman sensor friction
From: "Robert O. Green" rog@..........
Date: Sat, 18 Apr 2009 18:49:56 -0700


Thank you.  I appreciate your willingness to share your detailed 
experience with and understanding of Lehman sensors.

I am almost ready to start building one.

I have a question regarding the lower pivot.  I read the material you 
provided previously.  It seems a SS plate on the horizontal arm working 
against a SS ball bearing on the vertical is a good choice.  Is my 
understanding correct?

I am planing a crossed carbide drill shafts for the upper pivot.

Thanks again  for all your help.



ChrisAtUpw@....... wrote:
> In a message dated 18/04/2009, rog@.......... writes:
>     Gary,
>     I am curious.
>     What are typical times for Lehman half-life decay?
>     What are the main causes of decay?
>     Rob
> Hi Gary & Rob,
>     With a Lehman, you first need to set both the cross balance and 
> the period of the pendulum, but with NO external damping and with the 
> sensor coil DISCONNECTED. Put a peel off label on the mass and another 
> on a match box or similar. Mark lines on both with a fibre pen. Also 
> mark dots on the box at +/-1mm, +/-5mm and +/-10 mm and place it on 
> the frame just clear of the mass by 1~2mm. You place the line on the 
> box at the centre of the frame, centre the mass by hand to align the 
> marks, release it and see which way it drifts. You adjust the cross 
> balance screws so that it stays central.
>     Then you deflect the arm 10 mm, release it and time one FULL cycle 
> of oscillation when the arm swings past the central alignment and when 
> it swings past again going in the same direction. Then adjust the tilt 
> of the boom, usually with a screw on the base near the mass end, till 
> you get the chosen period, which should be a minimum of 15 seconds. I 
> use 20 seconds. If you have difficulty in getting a stable period of 
> 15 seconds, you will need to improve your suspension. You may have to 
> trim the cross balance while doing this. If you have lock nuts and 
> spring washers on the adjusting screws, tighten them and recheck the 
> cross balance.
>     You can then repeat the deflection to 10 mm and time how long it 
> takes until the swing is reduced to ~half amplitude, 5mm. You should 
> aim for a time of >4 minutes. If the time is much less than this, 
> you have a lossy / poor system which may need correction. You can get 
> problems if the top wire is not firmly clamped, if the suspension can 
> flex, if you are trying to use either a point or a knife edge lower 
> 'bearing', if the arm can rotate about it's longitudinal axis, if the 
> frame is not sufficiently rigid, or if base mountings are not firm. No 
> material is free of loss when the load on it is changed.  Air flow 
> damping may be significant if you are trying to use capacitative 
> position sensing, but it is usually very small for coil + magnet or 
> LVDT sensors.
>     Then plug the coil into the amplifier, switch on the power, 
> deflect the mass, release it and check that it swings at least one 
> full cycle. The amplifier input resistor / input circuit may add 
> damping. Progressively add more damping until on release from the 10 
> mm deflected position, the arm swings 0.5 mm PAST the zero and then 
> back to zero.  It is quite easy to judge 0.5 mm if you use a fixed x4 
> mounted magnifying glass to observe the balance position. Trying to do 
> this 'by eye' is more difficult.
>     I hope that this will help.
>     Regards,
>     Chris Chapman
> ------------------------------------------------------------------------
> Internal Virus Database is out of date.
> Checked by AVG -
> Version: 8.0.176 / Virus Database: 270.11.48/2048 - Release Date: 4/8/2009 7:02 PM


Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]