Medical Instrumentation Ⅱ 동서의료공학과 이건우 -Sensor-

X Y M F = MG Fig1.1 A A = X·Y “On the surface, the (average) force per unit area is denoted as σ. F is sometimes called ‘load’.” : “Stress” (solid material), pressure – liquid, air … unit: [N/ ] ▶ (Axial) Stress

L F Wall Fig1.2 δ ▶ Strain : “Strain”, unitless ( [m/m] ) ε

▶ σ & ε curve 1. Brittle material (glass) σuσu linear region (elastic region) rupture nonlinear region σ u : ultimate stress ε “Not linear over a wide range.” Fig1.3

▶ σ & ε curve 2. Ductile material (Al) σuσu σ ys σ Pl elastic region plastic region rupture σ ys : Yield stress σ Pl < σ ys < σ u Fig1.4

▶ σ & ε curve 2. Ductile material (steel) σ Pl elastic region for elastic region (linear region) ( σ ≤ σ Pl ) rupture σ = E · ε Fig1.5 E: Young’s modulus (modulus of elasticity)

L F Wall Fig1.6 δ Negligible ▶ Cantilever σ = E · ε £ε£ε “If you know ε, you can find force.” E: constant A&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.” Strain ε =

L D Fig1.11 ∆L D- ∆D ▶ Poisson’s ratio, μ D: diameter piezoresistive effect dimensional effect

▶ Problem PiPi PoPo catheter liquid 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.

▶ Answer (a) If pressure vary from 0 to 500mmHg, what is each strain gage’s resistance? ☞ p-type strain gage’s resistance : 50+ Ω ☞ n-type strain gage’s resistance : 50- Ω When pressure is not applied, it’s resistance is 50Ω. And sensitivity is. therefore,

▶ Answer (b) Draw bridge circuit. And declare each strain gage’s position. S1 S3 S2 S4 S1 S2 S3 S4 a b - + 1V Vo

▶ Answer (c) Design flowing circuit. Vdc = 1V, Vo = 0~1V, Av = ? When Pressure(∆R) is zero, Vo=0V. When P=500mmHg,

Thank you for your attention! :D