Detecting signal for Strain Gages for bending

[mrelet]

Background: I have fabricated a device to isolate component forces to an applied load. The device uses pairs of cantilevered beams mounted such that both end conditions are fixed for bending but the ends move relative to each other-- forming a (nearly) linerar guide. I would like to record the component forces using strain gages and a look-up table, but with bench apparatus and at a low cost (I'm doing a feasibility test). I have read application notes and it appears my best bet is to use a full-bridge setup, because transverse, axial loads and temperature effects are conpensated. My question is this: can I use a battery for V(in) accross the strain gage and a voltmeter to measure V(out) if my tests are static and I record V(out)/V(in) for each
data point? Are there any (many) other points that I should know?

 

[madddog]

First off a full bridge will give you the temp compensation benefits as you said and will also give you the highest output per volt input. You can use a battery to drive the bridge but I would suggest using the battery to drive a voltage regulator. As the battery discharges the regulator will still maintain a constant voltage across the bridge. Your battery voltage must be higher than the regulator voltage by a minimum of 2-3 volts to take into account the voltage drop across the regulator. If I had to guess you are using gages with a resistance of 350 ohms so your power source will have a 350 ohm load across it. This load will tend to drain a small battery quickly, ie you will not have a constant voltage driving the bridge during the duration of your tests.

As far a measuring the output with a voltmeter, you must remember that a strain gage measures microstrain.
Your output will be in the range of microvolts per microstrain so you will need an amplifier with a high input impedance to booste the bridge output to a level you can read on a voltmeter. Also any load that you put on the bridge output will give you an error in your readings, the higher the impedance of the device you are measuring the bridge's output the better. A voltmeter has a relatively low impedance.

Good luck

rbrooks@hydril.com

 

[morizabal]

Hello,

You could use a battery to measure your output. Since the output of your bridge is a ratio of Vin/Vout, it doesn't matter what your Vin is. But as your battery drains your overall output is decreased and you will lose resolution. Typically a bridge is excitated with a 5 or 10 volt input. A 6.5 or 7.5 digit voltmeter is more than adequate to measure the strain. But since you are concerned about cost, this type of voltmeter will be expensinve. Is there a reason you need to use a battery? If not my company sells strain gage indicators you might be interested in

http://www.futek.com/product.asp?product=FP10846-00199-F

Mario