Length Measurement Length 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
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Length MeasurementLength MeasurementAlternate names of Length:DisplacementMovementMotion Position
Units of Length fm,pm,nm,µm, mm, cm, mLength is often the intermediate stage of systems used to measure other parameters. Ex. Pressure, Force
Nature of Length Nature of Length MeasurementMeasurement
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
Electrical Length Electrical Length Measurement 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
Rotary form also available Advantages: Inexpensive, Simple to use & manufactureAccuracy 1/1000No driver ckt required as signal having
good strength.Disadvantages: due to sliding wear & tear of wire Nonlinearity Errors & Finite Force requireCritical PointsInductance of wire comes at higher f.Limited to lower freq. operation
Electrical Length Electrical Length Measurement 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)
Change in Mutual Inductance LVDT
Excitation Electromagnet or Permanent MagnetExcitation Electromagnet or Permanent MagnetVariable-reluctance methods Variable-reluctance methods short range short range poor linearity over longer ranges, and the poor linearity over longer ranges, and the possible possible restricted dynamic performance. restricted dynamic performance.
Mutual Inductance Mutual Inductance ChangeChange
Application of Reluctance variation Application of Reluctance variation with with length changelength change
As Proximity switch
Advantages: +/- 250mm. robust, quite linear, very reliable extremely sensitive if well designed.Limitation: RF interference so shielding Stray Magnetic field pick upConstrains: Mechanical Resonance Carrier freq. > 50KHz