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Princess Sumaya Univ. Electronic Engineering Dept. 3441 Industrial Instruments 1 Chapter 5 Mechanical Sensors. Dr. Bassam Kahhaleh. Mechanical Sensors. Objective Understand how mechanical sensors work and how to interface them. − +. Mechanical Sensors.

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mechanical sensors
3441 - Industrial Instruments 1Mechanical Sensors

Objective

Understand how mechanical sensors work and how to interface them.

mechanical sensors1

+

3441 - Industrial Instruments 1

Mechanical Sensors

Displacement, Location, or Position Sensors

Potentiometric

V S

R D

R 1

R 3

VR

V out

R 2

R3 loading effect!!

mechanical sensors2
3441 - Industrial Instruments 1Mechanical Sensors

Displacement, Location, or Position Sensors

Capacitive

K = the dielectric constant

ε0 = permittivity = 8.85 pF/m

A = plate common area

D = plate separation

mechanical sensors3
3441 - Industrial Instruments 1Mechanical Sensors

Displacement, Location, or Position Sensors

Inductive

Permeable core

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3441 - Industrial Instruments 1Mechanical Sensors

Displacement, Location, or Position Sensors

Variable Reluctance

Linear Variable Differential Transformer (LVDT)

mechanical sensors5
3441 - Industrial Instruments 1Mechanical Sensors

Displacement, Location, or Position Sensors

LVDT

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3441 - Industrial Instruments 1Mechanical Sensors

Displacement, Location, or Position Sensors

Level Sensors

  • Mechanical
  • Electrical
  • Ultrasonic
  • Pressure
mechanical sensors7

A

A

F

F

F

F

l

l

3441 - Industrial Instruments 1

Mechanical Sensors

Strain Sensors

Stress: Forces applied to an object

Strain: the result (deformation) of stress

  • Tensile Stress-Strain
  • Compressional Stress-Strain

F = applied force in N

A = cross-sectional area in m2

Δl = change in length in m

l = original length in m

mechanical sensors8

F

l

A

F

3441 - Industrial Instruments 1

Mechanical Sensors

Strain Sensors

  • Shear Stress-Strain

Δx

F

l

F

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Stress-Strain Curve

Modulus of elasticity (Young’s modulus):

Strain

Breaks

Necking

Linear

region

Stress

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Strain Gauge (SG)

Resistance in metals:

Constant volume:

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Strain Gauge - Temperature effect

Example:

R0 = 120 Ω

Strain = 1000 μm / m

0 = 0.004 / °C

  • Strain ΔR = 0.24 Ω
  • Temp. ΔR = 0.48 Ω / °C
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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Metal Strain Gauges (SGs)

Gauge Factor:

Typically, GF ~ 2

Special alloys and carbon: GF = 10

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Signal Conditioning

R 1

R 2

D

VS

a

b

R D

R A

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Two-arm Bridge

D 1

D 2

D

VS

a

b

A 1

A 2

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3441 - Industrial Instruments 1Mechanical Sensors

Strain Sensors

Semiconductor Strain Gauges (SGs)

Typically, GF: −50 ~ −200

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3441 - Industrial Instruments 1Mechanical Sensors

Motion Sensors

Types of Motion

  • Rectilinear
  • Angular
  • Vibration
  • Shock
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3441 - Industrial Instruments 1Mechanical Sensors

Motion Sensors

Accelerometer

Spring-Mass System

k = spring constant in N/m

There is a natural frequency at which the system oscillates without acceleration.

Use it with frequency < 1 / (2.5 fN)

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3441 - Industrial Instruments 1Mechanical Sensors

Motion Sensors

Types of Accelerometers

  • Potentiometric
  • LVDT
  • Variable Reluctance

(small magnet / inductor)

  • Piezoelectric
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C

R 2

R

V in

R 1

V out

+

+

3441 - Industrial Instruments 1

Mechanical Sensors

Motion Sensors

Steady-State Acceleration

Example

sensor: 14 mV / g

Design: Vout = 0.25 V / (m/s)

R = 1 MΩ, C = 1 μF

  • VIntegral = −1.43 mV / (m/s)
  • GainInverter = −175
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3441 - Industrial Instruments 1Mechanical Sensors

Pressure Sensors

p > 1 atmosphere

  • Diaphragm
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3441 - Industrial Instruments 1Mechanical Sensors

Pressure Sensors

p > 1 atmosphere

  • Bourdon Tube
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3441 - Industrial Instruments 1Mechanical Sensors

Pressure Sensors

p > 1 atmosphere

  • Solid-State Pressure Sensors
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3441 - Industrial Instruments 1Mechanical Sensors

Pressure Sensors

p > 1 atmosphere

  • Solid-State Pressure Sensors
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3441 - Industrial Instruments 1Mechanical Sensors

Pressure Sensors

p < 1 atmosphere

In low-pressure environment, the heat loss is proportional

to the number of gas molecules per unit volume.

  • Pirani Gauge

Measure filament resistance (RTD)

  • Thermocouple
  • Ionization Gauge
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3441 - Industrial Instruments 1Mechanical Sensors

Flow Sensors

Solid-Flow Measurement

Where:

Q = flow (kg /min)

W = weight of material on section of length L

R = conveyor speed (m/min)

L = length of weighing platform (m)

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3441 - Industrial Instruments 1Mechanical Sensors

Flow Sensors

Liquid-Flow

Restriction Type

Where:

Q = Volume flow rate

K = constant (for the pipe & liquid type)

Δp = drop in pressure across the restriction

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3441 - Industrial Instruments 1Mechanical Sensors

Flow Sensors

Liquid-Flow

Obstruction Type

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3441 - Industrial Instruments 1Mechanical Sensors

Flow Sensors

Liquid-Flow

Magnetic Type

Conducting

liquids

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