M350 Wk6 Intro_Sensors

8/12/2019 M350 Wk6 Intro_Sensors

1/24

MECH 350 Instrumentation and Design ofExperiments

Basic SensorsPage 147

Main topics: Measured Parameters

Sensor Fundamentals


2/24

Contents Sensor principles Predominant sensor technologies Overview of main sensor categories

Position / Speed Stress / Strain Temperature Vibration / Acceleration

Sampling of Specialty sensors


3/24

Objectives

Understand the fundamentals of simple sensors,including proximity sensor, potentiometers,linear variable differential transformers, optical

encoders, strain gages, thermocouples, andaccelerometers.

Be able to identify suitable sensor technologiesfor a given application.


4/24

Sensor Principles

Recall, from Lecture 1: A sensor is a device that takes in information

from the outside world. Based on theinformation, the sensor creates a signal on

which a system can base a decision. A sensor, thus, has an input and an output Usually, a sensor is tailored to a specific task

A sensor will measure some physical quantityand convert it into some electrical signal (e.g.,voltage, current)


5/24

Sensor Classifications

1. Position and Speed measurement : rate or

voltage of output signal changes with input2. Stress and Strain measurement : applied

force results in changes in resistance

3. Temperature measurement : resistance orvoltage of output signal changes with temp.

4. Vibration and Acceleration measurement :voltage created from piezoelectric materials

5. Pressure and Flow measurement : oftenderived from sensors listed above.


6/24

Sensor Technology (1)Mechanical sensor technology: Centrifugal governor as rotational velocity

increases, outward movement of weights causes alinear shaft movement closes steam valve

Bimetallic strip - differing coefficient of

expansion results in movement with temperature standard household thermostat Spring loaded valve / scale opposing pressure

or force can be read from a scale

Bourdon tube length and/or position changeswith pressure. Pitot pressure tube fluid velocity based on

pressure; aircraft airspeed indicator


7/24

Sensor Technology (2)

Electrical sensor technology: Resistive

change in length, L or cross-sectional area, A results inchanges in resistance for a given material resistivity,

Inductive - voltage based on time rate of changeof magnetic flux;

Movement of a magnetically permeable objectwithin the magnetic field of a coil

Movement of a magnet within a coil

A

L R


8/24

Sensor Technology (3)

Electrical sensor technology (continued): Capacitive energy stored between parallel

plates of a capacitor is related to distance betweenplates and the plate geometry.

Optical Photodiodes and photoresistors areaffected by amount of light striking the sensingsurface

Piezoelectric sensors made of materials thatproduce a voltage when subjected to strain; quartzis most common.


9/24

1. Position and SpeedPosition Measurements: Proximity sensors create a change in state

(switch) or vary an analog output when close to,but typically not touching an object.

Technology: Magnetic, capacitance, inductance. Applications: counting objects, limiting travel of a

mechanism

Potentiometers resistance varies with angular

or linear positioning; 5k pot Technology: resistance due to length of electrical travel

Applications: volume control, variable speed control


10/24

1. Position and SpeedPosition Measurements: Proximity sensors

Potentiometers


11/24

1. Position and Speed (cont.)

Speed Measurements: Digital optical encoder using light, the sensor

converts rotary motion to digital pulses Technology: LED and photodiode / photoresistor

Applications: speed of conveyor systems

Generator converts rotary motion to DC voltagepotential.

Technology: induced EMF by rotating armature Applications: speed of moving or rotating machinery


12/24

1. Position and Speed (cont.)

Speed Measurements: Digital optical encoder

Generator


13/24

2. Stress and StrainValues of stress, derived from strain Resistance strain gage resistance of a thin

metal foil grid changes with strainTechnology: electrical resistance

Applications: stress / force on any elastic member canbe measured planes, trains, automobiles

Load cell calibrated strain gages with circuitry ina specified package

Technology: Resistance, piezoelectric Applications: Instron and other tensile testing

equipment.


14/24

2. Stress and Strain Resistance strain gage

Load cell


15/24

3. Temperature

Thermocouple voltage created by contactingdissimilar metals varies with temperatureTechnology: Seebeck (thermoelectric) effect

Applications: industrial temperature monitoring where

control and/or recording is required

Resistance Temperature Detector (RTD) metallic coil wrapped on a ceramic core

Technology: Metallic resistance. Applications: faster response and/or greater accuracy

than thermocouple devices; limited to 600 oC


16/24

3. Temperature

Thermocouple

Unsheathed Thermocouple

Resistance Temperature

Detector (RTD)


17/24

4. Vibration and Acceleration

Accelerometers used to determine the rate ofchange in velocity of an object.Technology: inertial mass and displacement of mass

compared to surroundings; (position sensors)

Applications: seismic activity, car crash testing,pedometer, suspension damping effectiveness,camcorder image stabilization, Wii remote

Often used to measure continuous mechanicalvibrations bearings in machinery, resonantfrequency of structures.


18/24

4. Vibration and Acceleration

Accelerometers


19/24


20/24

5. Pressure and FlowPressure:

PiezoelectricPressure Transducer

Flow:


21/24

Specialty Sensors

Mass Airflow sensor automotive intake; vanemeter or hot wire sensor

Oxygen sensor ceramic and platinum; senses

O 2 concentration in the exhaust vs. outside air

Capacitive touch sensor appliance and othertouch control with no physical switch movement


22/24

Specialty Sensors (cont.)

PIR (Passive Infrared) Motion sensor convertsinfrared radiation into electrical signal

Microelectromechanical systems (MEMS)

micromachined silicon; acceleration, pressure

Light / Image sensors photoelectric, CCD

others - http://en.wikipedia.org/wiki/List_of_sensors


23/24


24/24

Homework Use pages 147 thru 188 as a guideChoose two of the applications listed below and provide

answers to the following:a) Identify the physical quantities that need to be measuredand why they are important.b) Identify the sensor category and sensor technology thatcould be used. Be sure to explain your reasoning.

c) Discuss any limitations and/or trade-offs that are involved.

Applications: Home burglar system, combined household heatingand cooling system, television with attached Nintendo Wii

console, automobile drivetrain, hospital recovery room, weatherforecasting data center.

Note: There are many other sensors in addition to those presentedin this lecture. Grading will be based on thoroughness.

M350 Wk6 Intro_Sensors

Documents

Transcript of M350 Wk6 Intro_Sensors