Automating the monitoring of Wheatstone Bridge Output

Feb 12, 2004 3 Replies
Q

Purpose: To automate the data collection of pre-wired full Wheatstone bridge (bonded foil strain gage) load cells. With an excitation of 10Vdc.



Strain Gage Resistance: 4500 Ohm or 1000 Ohm Desired resolution: to be able to accurately be able to detect a change of voltage of 1 uV



I would like to be able to put multiple load cells (Qty 25-50) inside an environmental chamber and monitor the output of each load cells through ambient --> hot (125F) and back to ambient.



I was thinking of using an Agilent 34420A. Software: LabView With a precision 10V power supply. It is desired to keep excitation on at all times to eliminate any possible self heating issues. I would keep the precision power & meter outside of the environmental chamber & keep cable length as short as possible ( ~5ft)



Somehow the multiple cells would be switched with a multiplexing unit (haven't researched yet)


Does anyone see any types of flaws


Thank you for your comments



Qu1nn


Vote
Twitter Facebook LinkedIn
R

Your 1uV spec is not too feasable; thermoelectric effects are large compared to that, and RFi can also fiddle readings as well. If you could arrange a method to modulate the bridge imbalance (ie: alter a bridge arm in a regular manner), then the output will be modulated, and a synchronous detector would give a chance of getting better than normal resolution (but still will not take care of the thermoelectric problem).

Vote
Twitter Facebook LinkedIn
G

Could use AC to excite bridge, amp and filter to recover signal, or switch the bridge to produce an AC signal:

+10V | o------o-----o | | .-. .-. _/ sw1 | | | | o------o/ o-------o | | | | | | '-' '-' | | | | | _/ sw2 o--------o o-------------------o------o/ o----o | | | | | | | _/ sw3| | OUT | o------o------o/ o----|--o | | | | .-. .-. | o------------o | | | | | _/ sw4| | | | | o------o/ o----o '-' '-' | | o------o-----o | | GND sw 2,3 alternated with sw1,4
Vote
Twitter Facebook LinkedIn
R

That is what i said.... But thermocouple effects will still give problems. And it is very doubtful that 1uV resolution or accuract is needed.

Vote
Twitter Facebook LinkedIn

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required