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Measuring Forces

Measuring Forces. Strain gauges & other tranducers Erik Mueller. Introduction. 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.

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Measuring Forces

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  1. Measuring Forces Strain gauges & other tranducers Erik Mueller

  2. Introduction 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

  3. Types of measuring sensors Resistive film Piezo-electric Optical/Interferometry Mechanical/Frequency shift

  4. How is strain measured? • Longitudinal and transverse strain • Unitless, defined by percentage, or parts per notation (μstrain)

  5. Resistive Film • As the surface deforms, the electrical path changes • More sensitive vertical than horizontal (in this orientation) • Fragile

  6. Piezoelectric • More commonly used as pressure transducers • Sensitive to any strain, especially vibration • Very rugged

  7. Optical/Interferometry • 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

  8. Vibrating wire • Measures frequency shifts in a vibrating wire to determine strain • Very accurate

  9. Sensitivity 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.

  10. Conclusion • 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.

  11. Q & A

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