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. Nature of Length MeasurementNature of Length Measurement

4. Derived MeasurementDerived Measurement

5. Length Measurement TechniquesLength Measurement Techniques
Mechanical length-measuring Instrument
Electrical
Radiation

6. 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

7. 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

8. 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

9. 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)

10. 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.

11. Mutual Inductance ChangeMutual Inductance Change

12. Application of Reluctance variation withApplication of Reluctance variation with
length changelength change
As Proximity switch

14. 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

15. Magneto-electric sensorMagneto-electric sensor
Faradays laws

16. 3. Electrical-capacitance sensors3. Electrical-capacitance sensors

17. Selection Capacitive or Magnetic???Selection Capacitive or Magnetic???