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# Length measurement

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### Length measurement

1. 1. Length MeasurementLength Measurement Alternate names of Length: Displacement Movement Motion Position  Units of Length fm,pm,nm,µm, mm, cm, m Length is often the intermediate stage of systems used to measure other parameters. Ex. Pressure, Force
2. 2. Nature of Length MeasurementNature of Length Measurement
3. 3. Derived MeasurementDerived Measurement
4. 4. Length Measurement TechniquesLength Measurement Techniques Mechanical length-measuring Instrument Electrical Radiation
5. 5. Mechanical length-measuring InstrumentMechanical length-measuring Instrument  Mechanical Gauges like  Vernier Calipers  Micrometer Screw Advantages: Easy to use & good accuracy compared cost Disadvantages:  Accuracy depends on users  Can not transmitted only 4 local measurement
6. 6. Electrical Length MeasurementElectrical Length Measurement A. Change in ResistanceA. Change in Resistance Sliding contact along fixed resistance unit Resistance change due to change of bulk properties of a resistance element induced by strain of the element. Made up of Carbon film Resistor. V=IR and R=(σl)/A
7. 7. Rotary form also available Advantages: Inexpensive, Simple to use & manufacture Accuracy 1/1000 No driver ckt required as signal having good strength. Disadvantages: due to sliding wear & tear of wire Nonlinearity Errors & Finite Force require Critical Points Inductance of wire comes at higher f. Limited to lower freq. operation
8. 8. Electrical Length MeasurementElectrical Length Measurement B. Change in Inductance due to GeometryB. Change in Inductance due to Geometry (I) Self Inductance and (II) Mutual Inductance(I) Self Inductance and (II) Mutual Inductance Turns variation with length change. Reluctance variation with length change R=(σl)/A Reluctance S= (1/µ0 * µr) * (l/A) Flux ᵩ = LI/N e = -N(dᵩ/dt) e = -L(dI/dt)
9. 9. Change in Mutual Inductance LVDT Excitation Electromagnet or Permanent MagnetExcitation Electromagnet or Permanent Magnet Variable-reluctance methodsVariable-reluctance methods short rangeshort range poor linearity over longer ranges, and the possiblepoor linearity over longer ranges, and the possible restricted dynamic performance.restricted dynamic performance.
10. 10. Mutual Inductance ChangeMutual Inductance Change
11. 11. Application of Reluctance variation withApplication of Reluctance variation with length changelength change As Proximity switch
12. 12. Advantages:  +/- 250mm.  robust, quite linear, very reliable  extremely sensitive if well designed. Limitation:  RF interference so shielding  Stray Magnetic field pick up Constrains: Mechanical Resonance  Carrier freq. > 50KHz
13. 13. Magneto-electric sensorMagneto-electric sensor Faradays laws
14. 14. 3. Electrical-capacitance sensors3. Electrical-capacitance sensors
15. 15. Selection Capacitive or Magnetic???Selection Capacitive or Magnetic???