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BASIC SENSORS AND PRINCIPLES

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① Strain gages Measurement of extremely small displacement ① Potentiometers Translational and Rotational displacement ② FSR and so on…

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

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; Strain (unitless) L F F

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① Brittle material (ex. glass) rupture Broken : Brittle material’s property ; Non-linear Linear region ; elastic region (like spring) Not linear ; over a wide range

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② Ductile material (ex. Al, Steel) For elastic region ( Linear region ) ( < ) Young’s modules (cf. y = kx) (modules of elasticity) rupture ; Steel; Al rupture elastic regionplastic region elastic region plastic region

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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 L F

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

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

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

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Strain Gages Resistor Strain Measurable size

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

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

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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].

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Problem (3) Load cell : force sensor (Structure + Strain gage) ① ② ③ ④ ① ② Top view

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① ② ③ ④ And LPF Problem (3) Part of it is your design 5 X 10 Your design Given by structure and material RegisterVariable Register Resistive Sensor

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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. (a)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.

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Problem (4) P-type Si Strain gage S 1 & S 2 : G = 100 N-type Si Strain gage S 3 & S 4 : G = -100 S1S1 S2S2 S3S3 S4S4 S1S1 S4S4 S3S3 S2S2

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S1S1 S4S4 S3S3 S2S2 And LPF Problem (4)

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Calibration F : 0 – 100N Change f by using different mass to measure V 0 Use Least Square Method to find the Calibration Eqution.

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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. CatheterLiquidSensor Diaphragm S1S1 S4S4 S3S3 S2S2

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

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Design a bridge circuit included 4 strain gages and specify each strain gage in the circuit. S1S1 S4S4 S3S3 S2S2

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Add instrumentation Amplifier to vary from 0V to 1V with dc 1V. Specify its gain. Problem (6) S1S1 S4S4 S3S3 S2S2 And LPF

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Problem (6)

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FYI: All three types of stress are measured in newtons / meter2 but all have different effects on solids. Materials Solids are often placed under stress.

FYI: All three types of stress are measured in newtons / meter2 but all have different effects on solids. Materials Solids are often placed under stress.

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