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Noise Control in Strain Gage Measurements

Noise Sources & Pickup Media

Virtually every electrical device which generates, consumes, or transmits power is a potential source for causing noise in strain gage circuits. And, in general, the higher the voltage or current level, and the closer the strain gage circuit to the electrical device, the greater will be the induced noise. Following is a list of common electrical noise sources:

  • ac power lines
  • motors and motor starters
  • transformers
  • relays
  • generators
  • rotating and reciprocating machinery
  • arc welders
  • vibrators
  • fluorescent lamps
  • radio transmitters
  • electrical storms
  • soldering irons
Electrical noise from these sources can be categorized into two basic types: electrostatic and magnetic. The two types of noise are fundamentally different, and thus require different noise-reduction measures. Unfortunately, most of the common noise sources listed above produce combinations of the two noise types, which can complicate the noise-reduction problem.

Electrostatic Noise
Electrostatic fields are generated by the presence of voltage with, or without current flow. Alternating electrical fields inject noise into strain gage systems through the phenomenon of capacitive coupling, by which charges of correspondingly alternating sign are developed on any electrical conductors subjected to the field (Fig. 501.1). Fluorescent lighting is one of the more common sources of electrostatic noise.


Fig. 501.1 - Electrostatic noise coupling.

Magnetic Noise
Magnetic fields are ordinarily created either by the flow of electric current or by the presence of permanent magnetism. Motors and transformers are examples of the former, and the earth's magnetic field is an instance of the latter. In order for noise voltage (emf) to be developed in a conductor, magnetic lines of flux must be "cut" by the conductor. Electric generators function on this basic principle. In the presence of an alternating field, such as that surrounding a 50/60-Hz power line, voltage will be induced into any stationary conductor as the magnetic field expands and collapses (Fig. 501.2). Similarly, a conductor moving through the earth's magnetic field has a noise voltage generated in it as it cuts the lines of flux. Since most irons and steels are ferro-magnetic, moving machine members redirect existing lines of flux, and may cause them to be cut by adjacent sensitive conductors. As a result, signal conductors in the vicinity of moving or rotating machinery are generally subject to noise voltages from this source.


Fig. 501.2 - Electromagnetic noise coupling.



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