PCIT_Chap05 Mechanical Sensors2013-10-18

Chapter 5

Mechanical Sensors

Chapter 5 Mechanical Sensors5.1 Introduction5.2 Displacement, Location, or Position Sensors

5.3 Strain Sensors5.4 Motion Sensors

5.5 Pressure Sensors5.6 Flow Sensors

5.1 Introduction

5.2 Displacement, Location, or Position Sensors

5.2 Displacement, Location, or Position Sensors5.2.1 Potentiometric Sensors converts linear or angular motion into a changing resistance

that may be converted directly to voltage Problems: mechanical wear, friction in wiper action

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5.2 Displacement, Location, or Position Sensors5.2.1 Potentiometric Sensors Example 5.1 A potentiometric displacement sensor is to be used to measure work-piece motion form 0 to 10cm. The resistance

changes linearly over this range from 0 to 1kΩ. Developsignal conditioning to provide a linear, 0 to 10V output.

5.2 Displacement, Location, or Position Sensors5.2.2 capacitive and inductive sensors Capacitive sensor: K = dielectric constant; ε0 = permittivity (8.85pF/m); A = plate common area; d = plate separationC K 0 A d

5.2 Displacement, Location, or Position Sensors5.2.2 capacitive and inductive sensorsEXAMPLE 5.2

Fig. shows a capacitive-displacement sensordesigned to monitor small changes in workpiece position. The two metal cylinders are separated by a plastic sheath/bearing of thickness 1mm and dielectric constant at 1 kHz

of 2.5. If the radius is 2.5 cm, find the sensitivityin pF/m as the upper cylinder slides in and out the lower cylinder. What is the range of capacity if h varies from 1.0 to 2.0 cm?

5.2 Displacement, Location, or Position Sensors5.2.2 capacitive and inductive sensors Inductive sensor: Principle: change in displacement results in changes in inductance Signal conditioning: ac bridge

5.2 Displacement, Location, or Position Sensors5.2.3 Variable-Reluctance Sensors variable-reluctance displacement sensors differs from the inductive sensors in that a moving core is used to vary the magnetic flux coupling between two or more coils measurement: translational or angular displacement LVDT (linear variable differential transformer) Structure: hollow form, permeable material, slide freely, primary coil, secondary coil, AC source.

5.2 Displacement, Location, or Position Sensors5.2.3 Variable-Reluctance SensorsPrinciple: Flux formed by the primary is linked to the two secondary, inducing an AC voltage in each coil. When reluctance changes, the induced voltage changes in amplitude and phase. If the two secondary coils are wired in series opposition, the two voltages will subtract, differential voltage is formed. Centrally located, output is zero moved to one side, net voltage amplitude increase. In addition, there is a change in phase with respect to the source when the core is moved.

5.2 Displacement, Location, or Position Sensors5.2.3 Variable-Reluctance Sensors Signal conditioning requirement: output = DC magnitude indicates the displaceme

nt amount polarity indicates the direction of the displacement simple example:

5.2 Displacement, Location, or Position Sensors5.2.3 Variable-Reluctance SensorsPractical detection scheme: typically provided as an IC.

Broad range: ±25cm to 1mm Time response: dependent on the equipment to which the core is connected

5.2 Displacement, Location, or Position Sensors5.2.4 Level Sensors(1) Mechanical Figure 5.10 a) (2) Electrical

Figure 5.10 b)(3) Ultrasonic Figure 5.11

5.5 Pressure Sensors5.5.1 Pressure PrinciplesPressure: the force per unit area that a fluid exerts on its surroundings. Fluid: liquid or gas For a gas ----- uniform on all walls, for a liquid----bottom (greatest),

top (0).Static Pressure: no motion occurring in the fluid

Dynamic Pressure: fluid is in motione.g. pressure of water in a hose, when nozzle closed, 40 lb/in2; when nozzle open, 30 lb/in2

5.5 Pressure Sensors5.5.1 Pressure Principles

Units:SI system: N/m2 Pa, Pascal, kPa, MPa English system: lb/in2 (psi) 1 psi=6.895kPa Others: atmosphere (atm) , bar

1atm=101.325kPa =14.7psi1bar=100kPa

5.5 Pressure Sensors5.5.1 Pressure Principles Gauge pressure Net pressure (gauge pressure): Pg=Pabs-Pat Pg = gauge pressure Pabs = absolute pressure Pat = atmospheric pressure If a closed vessel contained a gas at 1atm, there is no net pressure on the walls because the same pressure from outside.Head pressure The static pressure by the weight of the liquid above the point at which the pressure is being described

P= ρgh

P = pressure in Pa ρ = density in kg/m3 g = acceleration due to gravity (9.8m/s) h = depth in liquid in m

5.5 Pressure Sensors5.5.2 Pressure Sensors (P>1 atm)

The design of pressure sensors higher than 1 atm (nearly 1atm, 10atm, …, etc) differs from those less than 1 atm (approaching vacuum) Principle: Pressure->Displacement->Electrical signal Diaphragm: net force: F=(P1-P2)A P1, P1 = pressure; A = diaphragm area Like a spring, Hooke’s law, pressure difference force balance

5.5 Pressure Sensors5.5.2 Pressure Sensors (P>1 atm)

Bourdon Tube :material: bronze, brass

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5.5 Pressure Sensors5.5.2 Pressure Sensors (P>1 atm)

Bellows: The accordion-shaped sides of the bellows are made from thin metal . (1) Straight-line expansion; (2) LVDT is used to measure the displacement.

5.5 Pressure Sensors5.5.2 Pressure Sensors (P>1 atm)Solid-state pressure sensors IC is used to measure pressure 0-100kPa Three connections: dc power, ground and sensor output Basic sensing element: small wafer of silicon acting as a diaphragm Signal conditioning circuitry is used on the wafer

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