RESISTIVE, CAPACITOR, INDUCTIVE, PIEZOELECTRIC SENSOR

2006200401 Tae-eui, Kim

Resistive SensorResistive Sensor

① Strain gage : changes in

② Force sensing resistor : changes in A

③ Thermistor : change in T

④ Potentiometer changes in L

Forcing Sensing ResistiveForcing Sensing Resistive

A F1RF

R

F

R

ThermisterThermister

ceramic R

NTC(Negative Temperature Coefficient) Thermistercf) PTC(Positive Temperature Coefficient)

(A) Voltage Divider

(B) Inverting Amplifier

so

s

RV ER R

oV F T

os

RV ER

R

sR

sR R

E

E

(C) Bridge Circuit

(D) Linearization

pR

sR

R

Linear region

R

sR

s P

s P

R RRR R

Capacitive SensorCapacitive Sensor

o RACx

A

Rx

Most capacitive sensors are based on changes in x

Inductive SensorInductive Sensor

① Self Inductance

② Mutual Inductance

③ Differential Inductance

Inductive SensorInductive Sensor

① Self Inductance

F

x

oVS

Inductive SensorInductive Sensor

② Mutual Inductance

Don’t know where it is

oV

sV

sV oV

Inductive SensorInductive Sensor

o cf cd efV V V V

cd feV V

③ Differential InductanceLVDT(Linear variable Differential Transformer)

sV

a

b

c

d

e

f

D

0x

D

Primary coil

Inductive SensorInductive Sensor

(a) When core is at 0x

cdV

feV

0oV ; cancelled out

Inductive SensorInductive Sensor

(b) When core is at x

cdV

feV

oV

Inductive SensorInductive Sensor

(c) When core is at x

cdV

feV

oV

Adventage of LVDT; You have wide of range of measuring

Problem 13Problem 13

( )x t

t

20mm

20mm

( ) sin(2 1000 )sV t t [ ]V1f kHz1t ms

40mm

F

20x mm

20x mm

Problem 13Problem 13

<Method I> Rectifier + LPF

( )oV t

( )V t

LPF

t

Problem 13Problem 13

<Method I> Rectifier + LPF

Problem 13Problem 13

<Method II> phase – sensitive demodulation

( )oV t

sin(2 1000 )t

( )V t

t

Problem 13Problem 13

<Method II> phase – sensitive demodulation

Problem 14Problem 14( ) ( )sin(2 1000 )oV t x t t

( ) ( ) sin(2 1000 ) sin(2 1000 )mV t x t t t

0

Problem 14Problem 1410

Problem 14Problem 1445

Problem 14Problem 1445

Piezoeletric SensorPiezoeletric Sensor

x

q kx

( ) ( )( ) dq t dx ti t kdt dt

0dxdt

0i if then

Piezoelectric sensor has no dc responseYou cannot measure static, but dynamic

Microphone , pulse

Piezoeletric SensorPiezoeletric Sensor

( )x t ( )i t R ( ) ( )sV t Ri t

( )( ) dx ti t kdt

( )oV t

( ) ( )si t V t

Piezoeletric SensorPiezoeletric Sensor

② Charge Amplifier

0V

0V

0i

R

C

( )i t( )oV t

1

1o

Rj CV I

Rj C

1 1R j RI k Xj RC j RC

( )( )( )

dx ti t kd t

I kj X

FFT

Piezoeletric SensorPiezoeletric Sensor

1oV j RHX j RC

f

1kC

H

12cf RC

If you plan to use the piezoelectric sensor for measuring pulse, the lowest frequency of pulse signal is assumed to be 0.1Hz