Medical Instrumentation Ⅱ

동서의료공학과2005200431 이건우

-Sensor-

X

Y

M

F = MG

Fig1.1

AA = X·Y

“On the surface, the (average) force per unit area is denoted

as σ. F is sometimes called ‘load’.”

A

F : “Stress” (solid material), pressure – liquid, air …

unit: [N/ ]2m

▶ (Axial) Stress

L

F

Wall

Fig1.2

δ

▶ Strain

: “Strain” , unitless ( [m/m] )L

ε

▶ σ & ε curve 1. Brittle material (glass)

σu

linear region(elastic region)

rupture

nonlinear region

σu : ultimate stress

ε

“Not linear over a wide

range.”

Fig1.3

▶ σ & ε curve 2. Ductile material (Al)

σu

σys

σPl

elastic region

plastic region

ruptureσys : Yield stress

σPl < σys < σu

Fig1.4

▶ σ & ε curve 2. Ductile material (steel)

σPl

elastic region

for elastic region (linear region)

(σ ≤σPl)

rupture

σ = E·ε

Fig1.5

E: Young’s modulus(modulus of elasticity)

L

FWall

Fig1.6

δ

Negligible

▶ Cantilever

LE

A

F

σ = E·ε

AL

AEF £ε

“If you know ε, you can find force.”

E: constantA&L are almost constant

L

Wall

Fig1.7

F

L+δ

F = α·ε

“If force is applied,the subject on the

cantilever is stretched.”

▶ Strain gage

“A strain gage is a device used to measure the strain of an object. The gage is attached to the object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its electrical resistance to change.”

Fig1.8

Substrate

Electrical wire

Fig1.9 Strain gage

Soldering connections

Fig1.10 change of resistance

L

A ρ

ρ : resistivity

Fig1.11

▶ Resistance

“The Length is stretched,

ρ ,L,A are changed.But volume is

constant.”

A

LR

A

LR

A

LR

LA

RL

AL

R

2A

L

A

R A

A

LRA

2

AL RRRR

AA

LL

AA

LR

2

A

A

L

L

R

R

Strain ε

L

L 2

=

LD

Fig1.11

∆L

D- ∆D

▶ Poisson’s ratio, μ

L

L

D

D

D: diameter

L

L

R

R

)21(

piezoresistive effect

dimensional effect

▶ Problem

Pi

Po

catheterliquid sensor

diaphragm

There are two p-type Si strain gage and two n-type Si strain gage.Strain gage’s sensitivity is . When pressure is not applied, it’s resistance is 50Ω.

mmHg/%10 5

▶ Answer(a) If pressure vary from 0 to 500mmHg, what is each strain gage’s resistance?

☞ p-type strain gage’s resistance : 50+ ΩP510

☞ n-type strain gage’s resistance : 50- ΩP510

When pressure is not applied, it’s resistance is 50Ω. And sensitivity is . therefore,mmHg/%10 5

▶ Answer(b) Draw bridge circuit. And declare each strain gage’s position.

S1

S3

S2

S4

S1S2

S3S4

a b-

+

1V

Vo

▶ Answer(c) Design flowing circuit. Vdc = 1V, Vo = 0~1V, Av = ?

1131

1

RRRR

RR

RR

RVa R

RR

2

1142

2

RRRR

RR

RR

RVb R

RR

2

VbaO AVVV )(

VAR

R)(

When Pressure(∆R) is zero, Vo=0V.When P=500mmHg,

VAV VO 150

50010 5

410 VA

Thank youfor your attention!

:D