Transducers and Sensors

Review questions (Q) Distinguish between Analog and digital Transducer Answer : Analog Transducer :Converts I/p quantity into an analog O/p which Is a continuous function of time Digital Transducer: Converts the I/p quantity into an electrical O/p which is in the form of pulses (Q) How capacitance value can be changed in Capacitance Transduction ? Answer : By changing i) Distance between the capacitor plates ii) Area of contact of two plates iii) Dielectric Constant 2 •(Q) What is an Electrical Transducer ? Answer: Converts measurand into electrical signals •(Q) What is the function of Potentiometric Resistance Transducer? Answer : Converts linear or rotational displacement into an emf •(Q) List any two advantages of Potetiometric Resistance Transducer Answer : i) High electrical efficiency ii) Inexpensive 3 Objectives On completion of this period you would be able to know • Types of inductive transducers and transducers’ construction and working with diagrams. • Capacitive Transducers • Construction and working of various capacitive transducers with neat diagrams 4 Inductive Transducer • Simple and most popular type of displacement transducer • Force or displacement causes Change in inductance ratio of pair of coils Or Change of inductance in a simple coil 5 Self inductance expression : • L= N² ∕ S • • N = No. of turns of the coil S = Reluctance of the coil • • S = l ∕ µA µ = Permeability • Substituting (2) in (1) • L = µAN²∕ l (1) (2) Henry 6 •L • = µAN²∕ l Henry Inductance L can be varied by varying the parameters • 1. N No of turns • 2. A Area of the core(m²) • 3. l length of coil 7 Types of inductive transducers • Single ended armature type • Differential armature type • Inductive transducer with a movable core • Linear variable differential Transformer 8 Working method • • • • • • • • • Apply force Movable core changes its position Reluctance of magnetic field varies Inductance of the coil changes This inductance is used as one of the components of an oscillator Oscillator frequency f =1∕2π√LC ‘ f ‘ varies corresponding to variation of ‘L’ Calibrated oscillator scale measures force 9 Single ended armature type Air gap force coil Change in air gap causes change in inductance ‘L’ of the coil Movable armature core 10 Differential Armature type Moving core armature coil Due to movement of armature Inductance of one coil increases and the other decreases Air gap EE- 304.53 11 Differential Armature type coil Moving core displacement D R R •Core centrally spaced : two halves of coil have equal inductance value •No detection on ‘D’ Bridge supply source •Deflector D detection depends on displacement of the coil 12 Displacement transducer or Linear Variable Differential transformer (LVDT) • It is an induction type displacement transducer • It measures force in terms of displacement of electromagnetic core of a transformer 13 Construction of LVDT transducer Transformer has • • • • • • • One primary winding Wound on central part of the former Two identical secondary windings Secondary windings have equal number of turns Secondary windings wounded on two ends of the former Secondary windings are connected in series opposition Secondary windings are placed on either side of the primary winding 14 Linear variable differential transformer Secondary winding motion Primary i/p voltage Primary winding former Soft iron core Output voltage 15 Basic circuit diagram of LVDT Input frequency ranges from 50 Hz to 20 KHz AC Input Primary winding displacement Second ary winding When core displaces then Es1≠ Es2, Er ≠ 0 Es2 Es1 Er SecondaryEE-voltage ∞ core 304 , 53 position Seconda ry winding When core is at centre Es1 = Es2 Then Er =0 16 Working of LVDT •Primary winding is supplied from ac I/p •Supply frequency ranges from 50 Hz to 20kHz • emfs induced in the secondary winding oppose each other •When core is at center position, emfs induced in secondary are • equal and opposite direction •When an external force moves the core in either direction, • difference in emfs occur in the secondary winding •The difference between the O/p voltage of the two secondary 17 • windings is a function of core displacement Advantages of LVDT • Linearity : Linear about displacement of 5 mm • High output :Reasonable O/p, hence afterwards less amplification required • High sensitivity: i. e., 1 mm displacement of core produces an O/p voltage of 300mV • Ruggedness : Withstand mechanical shock and vibration • Less friction : No sliding contact hence friction is very less 18 • Low Hysteresis : Hence repeatability is • extremely good •Low power consumption •Small •Simple •Light in weight •Stable •Easy to align •Easy to maintain 19 Disadvantages of LVDT • Large displacement required for appreciable differential O/p • Sensitive to magnetic field • Dynamic response is limited by mass of the core • Temperature affects the transducer 20 Capacitive Transducers • The capacitance of a parallel plate capacitor • Where • C=Kε˛ A Farads d K = Dielectric constant • ε˛ = permitivity (F/m) • A = plate common area(m²) • d = plate separation (m) • Capacitance C can be varied by changing K, A and d 21 WORKING METHODS OF CAPACITIVE TRANSDUCERS 22 Capacitive transducers : • The variation in capacitance is a function of displacement • Capacitance is inversely proportional to the distance of of separation (d) Distance of separation changes the capacitance d displacement 23 Capacitive transducers as a function of area (a) common to both the plates Displacement AREA CHANGES THEN CAPACITANCE CHANGES A A‫י‬ 24 Capacitive Transducers as a function of change in dielectric material between two plates When dielectric material (k) k Changes then capacitance changes 25 A variable position capacitive transducer • Employed for measurement of displacement construction : •The capacitor is made up of two plates • One plate is fixed • The other plate movable • A spring controlled Cantilever plate A is attached to the • movable plate of the capacitor •Air between plates acts as dielectric medium •O/p voltage is measured by a voltmeter 26 Working • Distance between plates decreases • capacitance of the capacitor increases Dielectric material •High dielectric constants brings linearity in measurement Variable spring O/p voltage displacement force Apply force X X X V output c Movable plate Capacitor plates Fixed plate 27 Quiz • • • • • In capacitive Transducers capacitance value changed by A) Change in dielectric medium B) Change in distance between plates C) Change in area of contact of two plates D) All above is • Number of secondary turns in LVDT • One • Two • Three • None Frequently asked questions • List out different types of Inductive Transducers • • • • Single ended armature type Differential armature type Inductive transducer with a movable core Linear variable differential Transformer • What is the full form of LVDT ? • Linear variable differential Transformer Essay type questions • How displacement is measured by LVDT ? • What is capacitive transducer ? 30