05-30-2012 10:14 AM - edited 05-30-2012 10:15 AM
I've been using Labview to acquire signals from a full bridge strain gauge circuit on a steel bar, which thus outputs a signal when I bend it. The circuit is amplified by an instrumental amplifier (ina125P). My problem lies with acquiring any signal at all with a quarter bridge arrangement. I've tried placing one strain gauge in a bridge with three matching resistors (whose resistances are 120 ohms with 1% accuracy).
This is all done on the arduino breadboard, with the strain gauge placed on a rod. Now, the strain gauge is not defective, nor is it placed on the rod incorrectly - I have managed to acquire a signal through a different means, using a strain gauge card and some older drivers.
When I try this with the arduino and Labview, however, my VI's graph behaves as if nothing is connected - absolutely no variation in signal. The ina125P amplifier has a good range of gain, so it is not the problem either.
Electronics forums on Stack Exchange maintain that I re check my circuit (which I have done many times) because apparently it should work. Any ideas why a full bridge arrangement works flawlessly but a quarter bridge does not?
EDIT: Added attachment of how I wired the entire thing on the breadboard.
06-01-2012 05:01 AM
How are you coding your acquisition. In DAQmx acquisitions you need to define you connection type be it differential or referenced single ended.
Have you thought about this in your code?
This may be where to look as you are sure your wiring is correct and a signal is being produced.
Failing that is it possible to verify that you are producing a signal using a DMM or oscilloscope.
06-01-2012 10:07 AM
Well in both cases described (full bridge and quarter bridge) I am using a standard piece of code called Analog Read Pin Example, which I downloaded from NI. It simply reads the A0 input on the arduino. Is there anything that could be missing in it? The code is quite simple, I attached it here.
06-06-2012 03:52 AM
Ok the question is then are you seeing anything at raw signal or not. If not then you need to look at where you are taking your signal from. I know you say you have double checked your circuit many times over but if you are not seeing anything on the input in this configuration then there isnt anything to read. You may need to think about some other circuitry in your system as the signal may be small and need amplifying.
06-06-2012 03:27 PM
I actually tried doing the quarter bridge arrangement again, and a half bridge arrangement (two strain gauges on the rod). I used a multimeter to sense the mV signal, and I powered the circuit with the arduino - so instead of reading across two terminals through the arduino on LabView, I attached it to the multimeter, but the source voltage came from the arduino. I sensed about 0.2 to 0.4 mV when I twisted the rod - the amplifier I have can multiply this by a maximum of 60,000 when R_g = 1 ohm, which should give me a massive signal. However, there is STILL no raw signal. I am completely baffled.
So I verified that the circuit is fine again. Could it be a small mistake in the code? If what I did worked for a full bridge there is no reason for it not to work for a quarter bridge or half bridge other than the amplification which is large enough.
06-10-2012 04:45 PM
From the spec on the INA125, it shows an internal excitation referrence. Bet you don't have that with the Arduino. For a bridge, either full or completed quarter, you must provide a stable bridge excitation voltage, or you will never get an output signal.