Sensors and Actuators Introduction to sensors

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1 Sensors and Actuators Introduction to sensors Sander Stuijk Department of Electrical Engineering Electronic Systems

2 INDUCTIVE SENSORS (Chapter 3.4, 7.3)

3 3 Inductive sensors

4 4 Inductive sensors damping control wheel speed sensor (ABS) crankshaft position sensor pedal position sensor speedometer (eddy current)

5 5 Sensor classification type / quantity measured Quantity Position, distance, displacement Flow rate / Point velocity Force Temperature Resistive Magnetoresistor Thermistor Strain gage RTD S e n s o r Potentiometer Capacitive Differential capacitor Capacitive strain gage Inductive and electromagnetic Thermistor Capacitor Eddy currents LVDT Load cell + LVDT LVDT Hall effect Magnetostriction t y p e Selfgenerating LVDT Magnetostriction Thermal transport + thermocouple Piezoelectric sensor Pyroelectric sensor Thermocouple PN junction Photoelectric sensor Diode Bipolar transistor reactance variation sensors (capacitive and inductive sensors) typically require no physical contact eert minimal mechanical loading

6 6 Magnetic reluctance electrical circuit may offer resistance to charge flow resistor: R resistor dissipates electrical energy current follows path of least resistance total resistance R tot R R V r R R magnetic circuit may offer reluctance to magnetic flu reluctance: reluctant circuit stores magnetic energy magnetic flu follows path of least reluctance total reluctance computed in similar way as resistance in electrical circuit tot 3 4

7 7 Magnetic reluctance reluctance depends on physical properties of the device l A l length of the device A cross-sectional area μ permeability of free space (4-7 H/m) μ relative permeability of the material soft ferromagnetic material (typically to ) permeability of air (appro. ) options to vary reluctance modify length l (variable gap sensor) modify magnetic permeability μ (moving core sensor) modify cross-sectional area A (not frequently used)

8 8 Magnetic reluctance reluctance depends on physical properties of the device l A sensor requires conversion of magnetic signal to electric signal Faraday s law relates magnetic reluctance to electric current v N di dt di L dt change in reluctance changes output voltage self-inductance L and reluctance are related: L N device can also be used as sensor without changing reluctance changing magnetic field causes electrons to move induces additional (eddy) current (eddy current sensor)

9 9 Variable gap sensor what is the output voltage (in terms of ) of a sensor with N windings? l l core object, object, air l l core object core A object A aira total core A object A aira total core object air reluctance of core and object are constant core reluctance of the circuit self-inductance of the circuit L l N total N k output voltage of the sensor v core di L dt l A core N di k dt object object A A total air k

10 Variable gap sensor output voltage of the sensor v di L dt N k highly non-linear relation between output and displacement use of sensor limited to proimity sensor di dt

11 Linear displacement transformer two coils in series, moving object increases reluctance in one coil decreases reluctance in other coil v e circuit is differential voltage divider impedance of coil is equal to Z jl N L l A N j N j A l Z v e Z /(-) changing l with a relative amount Z N j l A jl Z Z /(+)

12 Linear displacement transformer two coils in series, moving object increases reluctance in one coil decreases reluctance in other coil v e circuit is differential voltage divider output of the voltage divider v o Z / Z / ve ve Z / linear relation between output voltage and displacement offset voltage present v e Z /(-) displacement () should be small sensor often not practical Z /(+)

13 3 Mutual inductance self-inductance induced voltage due to change in own current di v L dt mutual inductance induced voltage due to change in current in neighboring circuit di di v L M dt dt depends on reluctance of the space between the coils changing reluctance between coils alters mutual inductance device usable as sensor two coil solution still not practical (large offset, small fluctuation) i M v L L i v

14 4 Linear Variable Differential Transformer Linear Variable Differential Transformer (LVDT) two secondary coils in series-opposition linear relation between output voltage and core displacement operation based on mutual inductance M v L v L L M linear range

15 5 Linear Variable Differential Transformer assume sinusoidal ecitation of primary circuit output voltage of secondary circuit v ( t) V sin t ( t) S V sin t S ω sensitivity at frequency ω displacement of the core from center φ phase shift (in voltage) from primary to secondary circuit M L v L S ω and φ depend on load R L of measurement circuit ecitation frequency ω M L phase shift can be compensated

16 6 Signal conditioning for LVDT sensors output signal of LVDT is amplitude modulated ac signal ( = ) t ( = ) t ( = - ) t amplitude indicates magnitude of displacement phase indicates direction of displacement M L v L L M

17 7 Linear Variable Differential Transformer output voltage (no load connected to secondary winding) no current in secondary circuit (I = ) V o I V I R jl jl R I jm jm jm M I V primary current I independent of core position output voltage V o proportional to core position V o j M M jl R V M M k V o jk I jk jl V R M M v i L R L R L v L M 3 R L L M M

18 8 Linear Variable Differential Transformer output voltage (no load connected to secondary winding) V o jk sensitivity V o jk k k k S jl R R R L L R j j L sensitivity increases with increasing frequency phase shift output voltage 9 out of phase with primary current phase shift between V and V 9 V I jk jl L arctan R V R consider phase shift when recovering position v M R L i L M 3 M L R R

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