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Measuring Forces

This article explores the foundational concepts of force and strain in load-bearing materials. It examines the various types of sensors used to measure strain, including resistive film, piezoelectric, optical interferometry, and vibrating wire transducers. The piece outlines how strain is measured in both longitudinal and transverse manners, detailing each sensor's sensitivity and accuracy. The conclusion highlights the advantages of standard film strain gauges and fiber optic interferometers for practical applications, especially in challenging environments.

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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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