Path: utzoo!attcan!uunet!cs.utexas.edu!usc!apple!bbn!mit-eddie!uw-beaver!tektronix!tekig5!brianr
From: brianr@tekig5.PEN.TEK.COM (Brian Rhodefer)
Newsgroups: sci.electronics
Subject: Re: How to convert weight to volts?
Summary: A few suggestions
Message-ID: <4496@tekig5.PEN.TEK.COM>
Date: 11 Jul 89 03:22:56 GMT
References: <7903@etana.tut.fi>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 52


Again the sad refrain, "the automagic e-mail system isn't".
Three attempts at private mail have bounced, so here are some
blue-sky guesses for everyone's amusement:

1) National Semiconductor used to sell pressure transducing modules.
   connect one to a small oil-filled hydraulic cylinder, and load the
   cylinder's piston with your test weight.  Force->pressure->volts.

2) Tape a strain gauge to a metal rod, and use the rod to suspend the
   unknown weight.  I don't remember much about the care and feeding
   of strain gauges, other than their signal outputs are fairly weak,
   and that they have fairly bothersome temperature and aging drifts.

3) Build a torsion balance:  Suspend the test weight by a cord wrapped
   around a wheel, attached to a shaft.  Support the shaft by bearings,
   and connect a torsion coil spring to it.  Anchor the other end of the
   coil spring to the same support the bearings use.  Add an optical or
   magnetic rotational encoder wheel to the shaft, and count the number
   of pulses produced when the weight is loaded onto the end of the cord.

4) Buy a run-of-the-mill spring scale.  Rig an LED/Phototransistor
   module up so as to get a thrill whenever the indicator is just past
   its "rest" position.  Attatch the object to be weighed to the scale's
   hook.  Shock the resultant system into mechanical oscillation with an
   external force, and measure the time intervals between the pairs of
   pulses from the phototransistor, which will give the period of the
   mechanical oscillator composed of the spring and the test mass, and
   will be a function of the test mass (provided the damping coefficient
   isn't too unreasonably large).


5) Build yourself a capacitor by sandwiching a squishable dielectric
   between two rigid conductors (probably metal plates).  The capacitance
   ought to vary as a function of how tightly the dielectric is squoze,
   which, if your test mass is placed atop the pile, will be a function
   of how heavy it is.  By incorporating this capacitor into an oscillator,
   you can measure weight as a function of frequency.  If you use two layers
   of the squishable dielectric, with foil in the middle, you can AC ground
   both the "outside" metal plates, to confine the capacitance changes to
   those induced by the weight.

   I think this last one's a winner.  Use a Bose (emitter-coupled astable)
   oscillator; as RC oscillators go, they're bulletproof, and unaffected by 
   temperature and supply voltage variations.  Best of all, if you're trying
   for computer-readable input, time-related phenomena are easier to interface
   than voltage-related ones.


Suggestively yours,

Brian Rhodefer