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

Derived MeasurementDerived Measurement

Length Measurement Length Measurement TechniquesTechniques

Mechanical length-measuring Instrument

Electrical Radiation

Mechanical length-measuring Mechanical length-measuring InstrumentInstrument

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

Magneto-electric sensorMagneto-electric sensorFaradays laws

3. Electrical-capacitance 3. Electrical-capacitance sensors sensors

Selection Capacitive or Selection Capacitive or Magnetic???Magnetic???