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
I am planing a crossed carbide drill shafts for the upper pivot.
Thanks again for all your help.
> In a message dated 18/04/2009, rog@.......... writes:
> I am curious.
> What are typical times for Lehman half-life decay?
> What are the main causes of decay?
> 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.
> Chris Chapman
> Internal Virus Database is out of date.
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