Voltage drifting

[E2005]

I have a strain gauge connected to a metallic rod hinged at one end. The system detects the bending of the beam(when I press it with my hand, as seen by increase in output voltage) but even before touching the rod, as I bring my hand close to the rod the output starts increasing (offsetting from zero point) and decreases to normal when I move away from it. The change in output voltage due to this offset is greater than that caused by bending. It seems almost as if the grounding of the system changes. I tried insulating and shielding the circuit, wires and beam(with aluminum foils, tapes etc) but still the systems offsets as soon as my hand or body comes close to the rod. I am trying to understand the reason for it and if there’s a way to solve this problem.

 

[MikeHalloran]

Probe the circuit with a good oscilloscope and measure the frequency of whatever your hand is coupling into it. That may give you a clue about where the extra signal is coming from, and what to do about it.





Mike Halloran
Pembroke Pines, FL, USA

 

[BrianR]

As MikeHalloran says, it sounds like capacitive coupling inducing (probably) power frequency voltage into the strain gauge amplifier sufficient to drive it into non linear operation, hence the steady output offset. This suggests that the amplifier is either not a differential amplifier or has insufficient common mode rejection ratio (CMRR).

Without direct knowledge of the setup it is difficult to comment but look at how you can bleed a capacitively coupled charge away from the amplifier input and ensure that a balanced connection is being used. Earth the rod as well.

 

[Higgler]

Sounds like you need to change your strain guage or check the source of power that runs it. Seems like somewhere there is a disconnected wire. No way that should happen.

kch

 

[kontiki99]

I'd guess its capacitave (displacement current) or your body acting as an antenna.
You also might try wearing a properly grounded ESD coat to eliminate the capacitance possibility. If it is displacement current, inserting a high value reisitor to ground in your signal conditioning circuit might bleed it off. Likewise if your coupling at a specific frequency you might be able to shunt that to ground with the right sized RC filter.