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BASIC SENSORS AND PRINCIPLES

MEDICAL INSTRUMENTATION II. BASIC SENSORS AND PRINCIPLES. Introduction of Resistive Sensor. Strain gages Measurement of extremely small displacement Potentiometers Translational and Rotational displacement FSR and so on…. (Axial) Stress. On the surface,

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BASIC SENSORS AND PRINCIPLES

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  1. MEDICAL INSTRUMENTATION II BASIC SENSORS AND PRINCIPLES

  2. Introduction of Resistive Sensor • Strain gages • Measurement of extremely small displacement • Potentiometers • Translational and Rotational displacement • FSR • and so on…

  3. (Axial) Stress On the surface, The average force per area is denoted as A = xy ; Stress -solid (cf. Pressure) -liquid, gas F = mg F is sometimes called as “LOAD”

  4. Strain L F ; Strain (unitless) F

  5. & CURVE Brittle material (ex. glass) rupture Broken : Brittle material’s property ; Non-linear Linear region ; elastic region (like spring) Not linear ; over a wide range

  6. & CURVE Ductile material (ex. Al, Steel) rupture rupture rupture ; Al ; Steel elastic region plastic region elastic region plastic region Young’s modules (cf. y = kx) For elastic region ( Linear region ) ( < ) (modules of elasticity)

  7. In Summary, so far...

  8. Cantilever L F E : constant A & L are almost constant ; Provided that you know , it is possible to measure F Strain gage is widely used to measure F

  9. The Principle of Weighing Machine • None of our business, • All about for mechanics • in terms of a variety of structures. Able to derive ‘m’

  10. Strain Gages Substrate Electrical wire 8 X L Eight times Electrical length X L ; The relation between Strain and Resistor A : resistivity All variables changed for volume constant F Length ( elastic region ) F Length

  11. Strain Gages Partial Derivative in order to know the relation to each component

  12. Strain Gages Measurable size Strain Resistor

  13. Strain Gages Poisson’s ratio with L D Piezo-resistive effect Dimensional effect

  14. Strain Gages Gage factor • For metal strain gage G : ~1.6 • For semiconductor strain gage G : 100 ~ 170 (High temperature coefficient)

  15. Problem (3) • Four metal strain gages which gage factor is 10 are attached • on a plain. By forcing F to the plain, Gage1 and 2 are • expanded as long as ∆L, whereas Gage3 and 4 are shorten • in the same length. • It has a relation that ∆L/L = kf ,k is constant. • Design a bridge circuit getting output voltage in proportion to • F, describe output voltage as F. • Voltage source of the bridge circuit is dc 5[V].

  16. Problem (3) Load cell : force sensor (Structure + Strain gage) Top view

  17. Problem (3) Resistive Sensor Register Variable Register And LPF Given by structure and material Part of it is your design Your design 5 X 10

  18. Problem (4) • Consider to design a system measuring force by using both two P-type Si strain gages which gage factor is 100 and two N-type Si strain gages which one is -100. • Design a circuit including a bridge circuit having four strain • gages as well as instrumentation amplifier in order to magnify output. Specify the type of each strain gage composing the bridge circuit. • (b) Assuming that both top and bottom of cantilever is changed in the same length in case that forced. By forced F, maximum change of the length of strain gage is +0.05%, resistor is 200 without any load. Specify gain in order output to vary in the range between -5V to +5V. • (c) Derive to calibrate this kind of instrument.

  19. Problem (4) P-type Si Strain gage S1 & S2 : G = 100 N-type Si Strain gage S3 & S4 : G = -100 S3 S4 S1 S2 S1 S2 S3 S4 <Top View> <Bottom View>

  20. Problem (4) S3 S4 S2 S1 And LPF

  21. Problem (4)

  22. Problem (4) • Calibration F : 0 – 100N Change f by using different mass to measure V0 Use Least Square Method to find the Calibration Eqution.

  23. Problem (6) 2 P-type Si strain gages and 2 N-type Si strain gages are attached below diaphragm of Pressure sensor. In case of pressure on diaphragm, same strain occur at the each strain gage with its sensitivity of %/mmHg, its resistor is 50 without any pressure. Assuming that it is linear between pressure and strain. Catheter Liquid Sensor S3 S4 Diaphragm S2 S1

  24. Problem (6) • In variation of pressure from 0 to 500mmHg , how each resistor of P-type and N-type Si strain gages changed.

  25. Problem (6) Design a bridge circuit included 4 strain gages and specify each strain gage in the circuit. S3 S4 S2 S1

  26. Problem (6) Add instrumentation Amplifier to vary from 0V to 1V with dc 1V. Specify its gain. S3 S4 S2 S1 And LPF

  27. Problem (6)

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