Detecting and Troubleshooting

In order to effectively assess the presence and magnitude of noise, the strain gage instrument selected for use should incorporate a simple, but very significant feature - a switch for removing the excitation from the Wheatstone bridge. With such a control, the instrument output can be easily checked for noise, independently of any strain signal. This represents a very powerful tool for evaluating the effectiveness of shields and ground, and for experimentally modifying these methods to minimize the effects of noise. All Vishay Measurements Group strain gage signal conditioners are equipped with this important feature.

The following procedure can be used to troubleshoot a system for noise:

1. If not already known, determine the tolerable levels of noise in output units (millivolts, inches of deflection, etc.) as observed on a readout such as an oscilloscope or recorder.
    
2. Consideration should be given first to noise sources affecting the strain indicator itself, isolated from all external circuits. For this purpose, disconnect any strain gage leads, and terminate the S+/S- amplifier input with about the same input impedance that the amplifier normally senses (typically between 120 and 1000 ohms). If excessive noise exists:
   
   

   a) Check for ground loops (more than one connection of the system to ground).
   
   b) Check for line- ("mains-") radiated noise.
   
   c) If feasible, reduce amplifier gain and compensate by increasing bridge voltage.
   
   

3. Having eliminated or satisfactorily minimized noise pickup by the instrument, turn next to the external circuitry. With the excitation switch set to OFF, connect the gage or transducer circuit (including leadwires) to the instrument, and observe noise. Of course, any additional noise picked up in this step is attributed to leadwire and/or gage pickup. If the output changes when the instrument chassis is touched with a finger, this is an indication of a poor ground and/or radio-frequency interference.
    
4. Apply a load to the part under test (with excitation still OFF). If additional noise is observed, the noise is due to something associated with the loading system such as a motor creating magnetic field, or the motion of the gage or wiring (generating emf).
    
5. If possible, remove the load from the test part and apply excitation voltage to the bridge circuit. After balancing the bridge, any subsequent change in output, if gradual, is zero drift, not noise. This may be due to gage self-heating effects (see Vishay Measurements Group Tech Note TN-502, Strain Gage Excitation Levels ) - or other time-dependent resistance changes.

The following sections of this publication give recommended noise-reduction procedures for electrostatic noise, and for magnetic noise.

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