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Medical Instrumentation Ⅱ

Learn about stress and strain in medical instrumentation sensors, including types of materials, behavior under load, Young’s modulus, and strain gages. Explore how strain gages measure deformation and resistance changes. Solve a problem using strain gages in a catheter diaphragm sensor setup.

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Medical Instrumentation Ⅱ

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  1. Medical Instrumentation Ⅱ -Sensor- 동서의료공학과 2005200431 이건우

  2. ▶ (Axial) Stress Y X A = X·Y A M F = MG Fig1.1 “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/ ]

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

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

  5. ▶ σ & ε curve 2. Ductile material (Al) σu σys σPl rupture σys : Yield stress σPl <σys <σu elastic region plastic region Fig1.4

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

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

  8. L Wall “If force is applied, the subject on the cantilever is stretched.” F F = α·ε L+δ Fig1.7

  9. ▶ Strain gage Fig1.8 “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.”

  10. Substrate Electrical wire Soldering connections Fig1.9 Strain gage Fig1.10 change of resistance

  11. ▶ Resistance L ρ A ρ : resistivity Fig1.11 “The Length is stretched, ρ ,L,A are changed. But volume is constant.” = Strain ε

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

  13. ▶ Problem sensor liquid catheter diaphragm Pi Po 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.

  14. ▶ 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,

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

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

  17. Thank you for your attention! :D

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