Hello,
I'm currently working on a project, where I want to measure force using strain gauges. The strain gauges are configured in four point Wheatstone bridges. I'm using a four channel ADC. On each channel I feed in the amplified differential output voltage of a Wheatstone bridge. My question is how to properly excite the 4 Wheatstone bridges with very little error?
Here is more detailed version of my problem and my approaches regarding excitation the so far:
Typically one would use some linear regulator to power the bridge (see example). Additionally you would connect sense wires directly to the strain gauges and connect them to REF+, REF- of your ADC. Firstly this removes offsets due to voltage drop caused by the wire resistance. This can be really important, e.g. 5 V excitation voltage leads to 41 mA for 120 Ohm stain gauge. Secondly any error or ripple would be common to exication voltage and ADC reference.
Using 6-wire measurement like this is not possible for my case because I have 4 Wheatstone bridges and I could just connect Vref to a single bridge. Apart from that my ADC only has a single (Vref+) terminal.
At first I was thinking about current excitation, since one can easily build a high quality current source using a voltage reference, an op-amp, a series pass transistor and a shunt resistor. With current excitation, I would need to put all my Wheatstone bridges in series to ensure they all have the same current. I could then feed the voltage over the shunt resistor to the opamp, since it would be proportional to the exicitation current ripple. If it helps I can also post a picure of this.
One of my requirements is that my design should work for 120 Ohm, 350 Ohm, 1k Ohm strain gauges. Ofcourse sensitivity and also the total error of measurement is better with a higher exiciation current/voltage. But using 120 Ohm gauge would require 41 mA current source for good total error. On the other hand a fixed current source of 41 mA would mean a voltage drop of 166 V over the 4 Wheatstone bridges with R_nom=1k Ohm, which is obviously far out of the range of the rails I'm planning to use.
Furthermore I was thinking about using current exication but with all 4 Wheatstone bridges in parallel, in combination with current mirrors. I heard that current mirrors often have high error (even with matched pair transistors), so I'm not sure if that would be a good approach. A second option would be to use a high quality voltage reference for the ADC, use voltage exciation with the Wheatstone bridges in parallel and just use low resistance wire maybe even offset calibrated. But I'm not really happy with option 2 since it does not compensate for aging, temperature drift and you would need to calibrate every time you switch the wiring.
Finally I should also mention that an ac exciation is not really an option, since my application also requires higher kHz speed signals.
Are there any suggestions, how I could find a neat sollution to this problem? Thanks alot for reading this long question! I'm happy for any help
Greetings
Dave
I'm currently working on a project, where I want to measure force using strain gauges. The strain gauges are configured in four point Wheatstone bridges. I'm using a four channel ADC. On each channel I feed in the amplified differential output voltage of a Wheatstone bridge. My question is how to properly excite the 4 Wheatstone bridges with very little error?
Here is more detailed version of my problem and my approaches regarding excitation the so far:
Typically one would use some linear regulator to power the bridge (see example). Additionally you would connect sense wires directly to the strain gauges and connect them to REF+, REF- of your ADC. Firstly this removes offsets due to voltage drop caused by the wire resistance. This can be really important, e.g. 5 V excitation voltage leads to 41 mA for 120 Ohm stain gauge. Secondly any error or ripple would be common to exication voltage and ADC reference.
Using 6-wire measurement like this is not possible for my case because I have 4 Wheatstone bridges and I could just connect Vref to a single bridge. Apart from that my ADC only has a single (Vref+) terminal.
At first I was thinking about current excitation, since one can easily build a high quality current source using a voltage reference, an op-amp, a series pass transistor and a shunt resistor. With current excitation, I would need to put all my Wheatstone bridges in series to ensure they all have the same current. I could then feed the voltage over the shunt resistor to the opamp, since it would be proportional to the exicitation current ripple. If it helps I can also post a picure of this.
One of my requirements is that my design should work for 120 Ohm, 350 Ohm, 1k Ohm strain gauges. Ofcourse sensitivity and also the total error of measurement is better with a higher exiciation current/voltage. But using 120 Ohm gauge would require 41 mA current source for good total error. On the other hand a fixed current source of 41 mA would mean a voltage drop of 166 V over the 4 Wheatstone bridges with R_nom=1k Ohm, which is obviously far out of the range of the rails I'm planning to use.
Furthermore I was thinking about using current exication but with all 4 Wheatstone bridges in parallel, in combination with current mirrors. I heard that current mirrors often have high error (even with matched pair transistors), so I'm not sure if that would be a good approach. A second option would be to use a high quality voltage reference for the ADC, use voltage exciation with the Wheatstone bridges in parallel and just use low resistance wire maybe even offset calibrated. But I'm not really happy with option 2 since it does not compensate for aging, temperature drift and you would need to calibrate every time you switch the wiring.
Finally I should also mention that an ac exciation is not really an option, since my application also requires higher kHz speed signals.
Are there any suggestions, how I could find a neat sollution to this problem? Thanks alot for reading this long question! I'm happy for any help
Greetings
Dave