Medical Instrumentation II

Resistive Sensor

Strain gage : changes in L & ρ Force sensing resistor : changes

in A Thermistor : changes in L Potentiometer : changes in L

Medical Instrumentation II

FSR(Force sensing resistor)

Medical Instrumentation II

F

A & F is proportion.R & 1/F is inverse proportion.

R

Thermistor

R

F

R

T

R∞

NTC(Negative Temperature Coefficient)

R s R

O PAMP

+

-

O U TIA

Vo = - R/Rs E

E

Design.

Medical Instrumentation II

R

R s

-

+

Vo

(A)

(B)

oV

ERR

RV

s

so

V0 vs F or T : Non - linear

V 10V dc

U 1

O PAMP

+

-

O U TE[v]

S3

R1 S2

S4

reference

in = 0

ip = 0

IAVa Vb

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(C) Bridge circuit

(D) Linearigation

R p

R s

R = RsRp / Rs + Rp

Rs

Rp

Linear RangeR

Capacitive Sensor

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A

x x

AC ko

k

Most capacitor sensor are based on changes in x

Inductive Sensor Self Inductance Mutual Inductance Differential Inductance

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(1) Self Inductance

x

F

(2) Mutual Inductance

-b

Vs Vo

+a

- D

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(3) LVDT (Linear Variable Differential Transformer)

D

X = 0

b

a c

d

e

δ

- δ

efcdcfo VVVV

Vs Vo

(b) When core is at X = + δ

(a) When core is at X = 0

(c) When core is at X = + δ

Vcd is Blue, Vef is Red and Vo is Black

Vo = 0

f

Problem 13.

Using LVDT, A object that is a reciprocasting motion verticality direction measure location. This is y(t), changing by next page figure. 1st coil’s voltage is V(t) = sin(2π x 1000t) [v].

(a) Design the LVDT output signal in 0 ~ 10ms. 2nd coil’s phase isn’t delay, Suppose LVDT

is linear in ± 20mm. (b) Write verticality direction y(t) by LVDT output signal. (c) Write calibration method for output voltage represent [mm] unit

Medical Instrumentation II

Problem 13. LVDT

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40mm

F

X=20mm

X=0

X=-20mm

20mm

-20mm 5ms 10ms

180 G Phase change

])[10002sin()( vttVs

msTkHzf 1,1

t

y(t)

Method 1. Rectifier + LPFNo Phase

change!

RC

FWRLPF V(t)

Vo(t)

V(t)

t

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Method 2. Phase-sensitive demodulation

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XLPF

Vo(t)

sin(2πx10³t)

V

(t)

IVo(t)I

t

g X g

Matlab 3. 5~10ms 사이에 0.01ms 씩 샘플링하여

그래프 표현 filter 함수 배열 사용 .

Medical Instrumentation II

Piezoelectric sensor

x

kxeChq )arg(

dt

tdxk

dt

tdqAi

)()(][

I A

O PAMP

+

-

O U TR Vo(t)x(t)

(1)

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O PAMP+

-

O U T

C(2)

CjR

CjR

IIo

1

1

RCj

Rj

RCj

RIIo

11

I H I

Tfc = 1 / 2πRC

Medical Instrumentation II

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

Medical Instrumentation II