Path: utzoo!mnetor!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Pressure sensing
Message-ID: <2473@kitty.UUCP>
Date: 12 Apr 88 02:25:33 GMT
References: <1368@PT.CS.CMU.EDU> <8266@oberon.USC.EDU>
Organization: Recognition Research Corp., Clarence, NY
Lines: 46
Keywords: transducer, pressure, sensor, circuit
Summary: Load cells

In article <8266@oberon.USC.EDU>, blarson@skat.usc.edu (Bob Larson) writes:
>> I want to build a circuit that senses pressure someone's foot is exerting on 
>> a surface, without using springs, rheostats etc, because it has to be small
>> & simple. Is there such a thing as a small (e.g. < 0.5" tall) transducer or
>> some such component whose resistance/capacitance/whatever varies with the
>> pressure exerted on it?
> 
> Commercailly there are things called "pressure transducers".  I
> remember seeing data sheets in one of my old National Semiconducter
> catalogs.  They don't seem to be readily available on the hobby
> market, (so you get to pay distributer price) and may not be able to
> handle the pressure you want.

	Pressure transducers are used for measuring pressure of a fluid
(i.e., gas or liquid).  Unless one used a hydraulic cylinder or hydraulic
bladder of some sort whose pressure was transmitted to the pressure
transducer,  a pressure transducer would be of no value.  Assuming that
a liquid was used, some rather simple math determines how the surface
area (or piston area) of the bladder (or cylinder) determines the
range and sensitivity of the device when connected to a suitable pressure
transducer.
	A more useful transducer for this application is called a load
cell.  It transforms mechanical pressure to an electrical signal, which
may be in the form of a 1,2 or 4-arm resistive strain gage bridge,
LVDT, etc.  Piezoelectric load cells exist, but they measure only
dynamic pressure changes - not static pressure.
	For those with experience, a strain gage element may be directly
installed upon the measuring surface to measure deflection; knowing the
elastic modulus of the measuring surface plus some relatively simple
math determines pressure in engineering units.  Strain gage elements
themselves are inexpensive, but they are extremely fragile and require
suitable protection for direct use.  A load cell is most commonly a
strain gage whose element is protected within an enclosure.

> A cheap, easy to make pressure sensitive resistor can be made with
> conductive foam.  (The black foam used in shipping static sensitive
> ICs.)  Just hook contacts on opposite corners.

	This is true; however, the output of such a device is highly
non-linear and will change over time as the foam undergoes permanent
deformation through repeated use!

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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