Strain gauges & other tranducers Erik Mueller

 Forces are present in all load-bearing materials  When pressure is exerted on a material, it deforms – this is strain  Force can be measured directly, or through the deformation of the material

 Resistive film  Piezo-electric  Optical/Interferometry  Mechanical/Frequency shift

 Longitudinal and transverse strain  Unitless, defined by percentage, or parts per notation ( μstrain)

 As the surface deforms, the electrical path changes  More sensitive vertical than horizontal (in this orientation)  Fragile

 More commonly used as pressure transducers  Sensitive to any strain, especially vibration  Very rugged

 Uses the Fabry-Perot phenomenon of interferometry  Light path changes, and so does the interference pattern produced  Specialized readout device is needed  Immune to transverse strain, EM fields

 Measures frequency shifts in a vibrating wire to determine strain  Very accurate

 Piezoelectric pressure transducer – 0.1% Full Scale  The vibrating wire is by far the most sensitive of the strain gauges, with about 0.1% F.S. accuracy.  Film resistor – 0.5% F.S.  Fiber-optic – 3% F.S.

 Based on preliminary findings – standard film strain gauges are ideal Low cost, straightforward data acquisition  Second choice – fiber optic interferometer Immune to transverse strain, less false readings If used while testing an Ion thruster, no possibility of EM interference from the ions.