nterfacing a ensor-- use of ignal onditioners input Transducer (sensor signal conditioner -Deflection bridges -A.. arrier systems -nstrumentation Amplifiers -oltage to current onverters -urrent to voltage onverters -oltage to frequency onverters -Frequency to voltage onverters -Optical coupler -Grounding and shielding signal processor (u-computer display controller
ensor nterfacing to a control system sensor nstrumentation Amplifier - Analog filter /H Digital ontrol ircuit A/D Trigger trobe Timer Parallel nput port Data storage analysis control Display ontrol D/A Parallel Output port Deflection Bridge trobe
Deflection Bridge (Wheatstone Bridge A kind of signal conditioning resistive capacitive inductive sensors Bridge ckt voltage
Deflection Bridge ircuit Analysis
5 Deflection Bridge Output quation D B -i i i ( i ( i i i PADQ : loop i i i PABQ : loop
6 Design of resistive deflection bridges All four impedances are pure resistive f depends on e input measurand Parameters: Three
Balanced deflection bridge. Balanced Bridge: MN MN MAX when MAX MN MN MN MN MAX MAX MN This gives us e value for a given / 7
Maimum power dissipation of a resistive deflection bridge. lectrical power in sensor must be limited i Λ for MN ω ( Walts : ( MAX maimum power dissipation Λ ω This gives us a range of s wiout overheating e sensor device 8
Linearity of a resistive deflection bridge. Non-linearity of e overall relationship between and ideal ma ma min ma ma min (ideal straight line between and MN ma ideal Λ N for min ma 9
Determine e / resistor ratio of a resistive deflection bridge. /? According to e type of sensor ( MN MN s ( (
Design of a resistive deflection bridge MN s r r r r r or ( is non-linear depends on r Balanced bridge v
ample : train gage Δ Ge is very small( Design consideration: We required e sensitivity of e bridge to be as high as possible v ( r o o for a single strain gauge is determined by e power dissipation ( MN r r - ( (
train gage (cont. [ [ Δ MN MN Ge ] MN ]
Typical values 5 G e -5 s Geμ high amplification is required not for low level d.c. signal a.c. carrier system a.c. supply voltage
ample : Metal resistance sensor (TD T ( αt Ω ~ 5 5Ω MN Λ Design considerations: This device has small non-linearity N <% a linear bridge is required r( ( ( T r TMN r r r ( αt αt 5
ample : Four active strain gages (for elastic sensing elements ( Ge ( Ge in tension in compression ( Ge ( Ge 6
Four Active train Gages(cont.. For cantilever Load cell and Torque sensor ( Ge where ( Ge unstrained ( l wt ( Ge Ge e e F or e ( Ge ( Ge ( Ge gage resistance ( Ge T πa sensitivity Temperature compensation Ge ( ( Ge ( Ge 7
. Pillar load cell Four Active train Gages(cont. GνF ( A GF ( A 8
Assume Four Active strain gages(cont. GF A << GνF ( A GνF GF ( ( A A GνF GF ( ( A A ( ν GF F A ( GF A ( GF A ( GνF A Typical values: 5Ge -5 Geu High amplification is required! 9
θ 5 75 ampe :Thermistor k ep( β θ (98K (8K :. ~.7 r :.5 ~. non - linear kω kω Use e bridge non-linearity to partially compensate ermistor s.
Thermistor(cont. equire: output range: ~. (input range: 98~8K minimum non-linearity.5 at K θ 8 98..5.
Thermistor(cont. olve /
Two metal resistance ermometer to measure temperature difference ( T α ( T α Balanced bridge: when T -T T when T
Two metal resistance ermometer to measure temperature difference(cont. ( ( ( / ( ( T T T T choose T T >> α α α α α
eactive deflection Bridge A upply voltage Bridge: arms: reactive elements arms: resistive elements 5
apacitive level transducers h h πε log (b / a e jω [ ( ε h ] jω h 6
7 apacitive level transducers(cont. ( / / ( minimum level (h min min min min >> Λ h h h h h h if at
apacitive push-pull displacement sensor εεa d εεa d 8
9 apacitive push-pull displacement sensor ω εε εε εε ω ω of independent and is linear d d d d d d A d A d A j j
nductive push-pull displacement sensor L L ( ( r k k a L L a L L μ π α α α
nductive push-pull displacement sensor ω ω ω ω ω α α ω ω non - linear and dependent on output is alternatively ( e L j L j or j j f a L L L L j L j