Wheatstone Bridge with the Arduino

Well...I gave strain gages a shot with the arduino and the INA125 amplifier. When I was designing my circuit I referenced a few other people who tried to apply a wheatstone bridge with an arduino. They were utilizing it as a beam in bending to measure mass ranging from 0 - 500 g. I was trying to apply it in much more sensitive application picking up a force on the beam that was 0 - <1 lb. Most people had difficulty in amplifying the signal because they could not balance the bridge or utilized a quarter or half bridge. I opted to use a full wheatstone bridge but it was difficult to get the resolution I needed for a few reasons. They had some reasonable results with the amount of force they were trying to measure.

• The arduino can only handle a 5V signal. I was forced to use a 5 v source for my bridge which does not leave a large range of output signal for the forces I am reading.
• After mounting all the strain gages and wiring, there is some slight variation in the resistances of the bridge which shows up as an amplified signal. I was already forced to limit my output range to a few volts. 
• The resistance I had set for amplification was very touchy. I had to amplify the signal significantly because I was trying to read very small changes in force. A change of a few ohms on the resistance changed the amplification significantly and the amplifier would become saturated reading the maximum voltage.
• Wiring of the strain gage makes a BIG difference. I would jiggle and bend my wires and peg the output voltage. I probably could have done better with this but didn't want to invest anymore money.

Full Wheatstone Bridge with the Arduino

I  was able to notice some change but it was not consistent and responsive. It was a fun try but don't think I will invest much more time or money into the project. I might start looking for alternative methods of measuring very small changes in force with a common micro-controller. It was interesting working with the Wheatstone bridge and it was the first time I had used the INA125. 

INA125 wired to bridge and Arduino

I have some ideas so stay tuned for what I come up with next. If anyone has suggestions or experience using the INA125 or wheatstone bridges let me know and I'll give it a shot.