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PICList Thread
'[EE]:stain gauge'
2002\08\13@034815 by MATTHEWS, DEAN (D.)

picon face
Can anyone recommend a suitable strain gauge that can be interfaced to the PIC a/d converter (0-5v range).  I would like the gauge to detect tools breaking/missing/wearing on high volume automotive production line machines.

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2002\08\13@051521 by Dominic Stratten

Stain gauge ? ;-) Sounds like you're setting up an interesting project -
could be hooked up to sheets etc.

I'll stop there

{Original Message removed}

2002\08\13@093659 by Francisco Ares

 I use metal foil strain gauges, and they will deliver only 20 to 30 mV
(full Wheatstone bridge, always) with 10 V supply.

There are semiconductor based strain gauges, 200 to 250 mV full scale
signal at 5V supply.

To interface to a PIC, you must amplify these signals with an
"instrumentation operational amplifier" (try a google search for this),
preferably connected on the classical three op-amp high input impedance.
There are many ICs on the market (Analog Devices and BurrBrown/Texas
just to name a few) that already comes with this configuration, giving
you the option among some fixed gain resistors or to put your own gain

For strain gauges, see the Vishay website.

There are plenty of considerations when using strain gauges: the best
point to stick them on the tool (there are many types of bonding
chemicals, the best are based on epoxy), spetial surface treatment
before bonding, type and syze of the gauge grid, strain gauge impedance,
full / half Wheatstone bridge, cabling and shielding (noise is a big
issue due to the low signals), impedances, grid and soldering points
protection, life-time, temperature compensation, hysteresis, creep,
calibration, ..., none of them is easy to master.

For example, the strain gauge is like a piece of wire, its resistance
given by its material resistive constant, its cross section area
(assuming an uniform wire) and its length. If, by the effect of some
force, it is streched, the length and the cross section area will vary,
and so the final resistance, but the difference between the original
value to this new value is so small that only combining strain gauges
assembled in different parts of the structure (your tools) you may have
a combination of those effects to get a readable value. So you will have
to look for the best possible points of strain deflection (as a strain
gauge measures forces through the deflections) both positive and
negative (traction and compression) to bond the gauges on those places.

Not impossible, but you've got a lot of things to learn.

Hope this helps

MATTHEWS, DEAN (D.) wrote:

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