piclist 2002\01\28\072143a >
Thread: Force sensors
www.piclist.com/techref/io/sensors.htm?key=sensor
picon face BY : Russell McMahon email (remove spam text)



> Hey, can anyone recommend some force sensors, capable of being interfaced
to
> a Pic? I need a few for the wind tunnel we're building (which is coming
along
> quite nicely, BTW). I'm not sure how big the forces are we'll be
measuring,
> but they shouldn't be too great. Also, any ideas for a pitot tube? If not,
we
> can make our own I suppose, though I don't know how accurate it will be.

If you only want modest performance you can get a ready made load cell in
some electronic scales.
I bought one here on special for about $NZ20 - usually about $NZ30 = about
$US14. These had a "proper" S type load cell with a machined alumium block
and 2 (AFAIR) strain gauges attached. 2 rather than 4 means that they aren'r
doing a full bridge with strain gauges and will be using 2 passive resistors
to make up the full bridge. Works the same but will need calibrating more
frequently.
This load cell was rated at 2 kg but you could apply the load at a different
point, within reason, to increase it or contemplate stiffening the beam
externally - naughty but workable.

You can also make your own with load cells epoxied to a suitable beam.
Details on one man's efforts in this area and his results, plus formulae for
calculating beam design parameters can be found at.

   http://members.aol.com/ricnakk//index.html

Richard is an experimental rocketry guru specialising in "Candy" motors
(fuel is various types of sugar) and documents all his work superbly.
I think Richard's design uses one or two starin gauges only which makes it
cheaper than a full 4 gauge design.

Any load cell has an output in the mV range and will need amplification
before processing with a PIC.
An A2D is preferable although for lowest cost (& low acquisition rates)
somehting like a VCO could be used.

Another family of approaches which would allow low cost in exchange for more
effort would be to convert deflection against a spring/bellows/balloon/...
into electrical signal using a variable resistor/capacitor/inductor to alter
DC voltage/frequency. eg a sping loaded pot to A2D or VCO, spring driven
variable capacitor in an oscialltor etc Horrendously mechanical could even
include a shaped vane with a light source and photocell or even perhaps a
LED approaching a sensor to incrase illumination with movement.

The tradeoff is cost and accuracy against time (and how much you enjoy
electro/electronic-mechanical fiddling).



regards



               Russell McMahon



> Hey, can anyone recommend some force sensors, capable of being interfaced
to
> a Pic? I need a few for the wind tunnel we're building (which is coming
along
> quite nicely, BTW). I'm not sure how big the forces are we'll be
measuring,
> but they shouldn't be too great. Also, any ideas for a pitot tube? If not,
we
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