Voltage drifting
Voltage drifting
(OP)
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.





RE: Voltage drifting
TTFN
RE: Voltage drifting
RE: Voltage drifting
istsmoked: I tried grounding the rod , that didnt help
RE: Voltage drifting
Presumably you are measuring the drift at the output of the amplifier? Have you looked at the output on a 'scope?
As well as the expected amplified d.c. from the strain gauge, it could be unstable and oscillating at a high frequency. If this is the case you can have a problem with "hand capacitance" tuning the oscillation and producing a variable output as you get closer or further away.
RE: Voltage drifting
RE: Voltage drifting
RE: Voltage drifting
Just because your system is running DC does not mean that there is no AC, nor does it mean that there are no common mode signals running around. In some respects, a battery-operated device that is not grounded can be more susceptible to stray signals, precisely because it's floating.
Besides grounding, you might also check to make sure that your gauge wires are identical in length, dressing, etc. They should also be twisted/shielded, with a minimum of 4 twists per inch. Coax will not work correctly in this type of application, since the outer shield is acting as a signal line and behaves differently from the internal signal wire.
TTFN
RE: Voltage drifting
If you use coax cable you can use both the signal and shield as inputs to a differential amplifier. The "noise," whether coupled in AC, RF, etc. will then likely show up as "common mode" (present on both inputs). The differential amplifier will give you the difference between the shield and signal cables, which essentially cancels the noise. Depending on the op-amp/instrument amp and component tolerances used, this technique can reduce common mode noise by 40-80 dB.
RE: Voltage drifting
RE: Voltage drifting
RE: Voltage drifting
How frequently should you read the bridge? Make a low pass filter to give a chance.
If I place my DMM in the DC millivolt range, leads floating, it will pickup any body movement around it.