PSN-L Email List Message

Subject: Re: Etc.
From: Ed Thelen ethelen@........
Date: Mon, 24 Jan 2000 02:09:27 -0800

twleiper@........ wrote:
> On Sun, 23 Jan 2000 13:29:11 -0800 barry lotz  writes:
> >Hi
> >   I'm kind of entering this thread in the middle. ...continued.
> The problem with all this stuff is differentiating drift from legitimate
> signal. In the example you cited, you simply have an indicator which
> alerts the human (you) when attention MIGHT be required. After all, those
> LEDs might be blinking due to a large quake.
> ...
> Drift of +/- .5" can be ignored, and that range should be readily
> achievable with a sensible and low cost design.
> HOWEVER, since not all are interested in such designs, let me tell you
> how I WOULD do it if I thought it was worth the effort:
> It is logical to assume that, over a long period, a mechanically centered
> boom will make as many excursions to the left of center as to the right.
> There will be short term exceptions much as a bridge with double tolls
> one way and toll free the other will carry "free" travelers who never
> return, and paying travelers who always return via some other route. With
> this in mind, I would simply put a "null" differentiator with adjustable
> hysteresis (one quad OpAmp) on the "pen motor" signal (signal in your
> design that is around 0 to +/- a volt or two) and have it clock a chip
> (such as 74191)... the count total
> (through use of divide by "N" counters), as well as the pitch and/or
> gearing of the stepper you would have a simple and cheap method that
> closely approximates the human operator.
> In normal operation the count may reach max/min in a couple days of slab
> tilt, adjust the weight a little and then again a couple days (or weeks)
> later. If you put the new piano in the room overhead and the boom swings
> permanently to one side, the "X" minutes increment will eventually roll
> over enough motor steps to correct for the condition.
> You want to try it? Go knock yourself out...

Ah - it was fun ;-)

I did roughly the same thing with a little BASIC STAMP
 computer (programmed in BASIC  :-)
 which can be set to:
  - turn on LEDs
  -  to do rough A-to-D (for the photo-transistors)
  - and programed to drive a stepper motor
     which drove something like a 36 to 1 gear box
     'cus the adjustment screws take so little turning,
     and the torque of the stepper not very much.

Added considerations are :
  - enable the actual adjustment only a fixed time of the day
     (to help prevent confusion about what might be 
  - short the pickup coil to
      - prevent massive signals from the 
          electrical and mechanical noise
          from scaring you
      - damp the pendulum way above critical
          so it restores more smoothly after
          adjustment (I use resistive damping).

It works - 

however, I still want to do a 3 axis force balance thing ;-)

  Ed Thelen


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