CHAPTER 21: Transducers and Actuators potx

Temperature Scales • Two commonly used temperature scales are Fahrenheit F and Celsius C • Fahrenheit is used in the United States.. Converting Temperature Scales If you know the temper

CHAPTER 21

Transducers

and Actuators

Introduction

• Transducers convert one form of energy into

another Examples: a microphone converts sound to AC voltage; a speaker converts AC voltage into sound

• Actuators are electromechanical devices that

move, rotate, push, pull, and in general

“make something happen” when electrical

signals are sent to them

A Generic Control System

Sensors and actuators are used to “close the loop”

Temperature

• Temperature measures average kinetic

energy at the atomic level

• Something is hot when its molecules are

banging into each other quickly

• Something is cold when its molecules bump

each other slightly

• At absolute zero temperature (0° Kelvin),

atoms stop moving

Temperature Scales

• Two commonly used temperature scales are

Fahrenheit (F) and Celsius (C)

• Fahrenheit is used in the United States The

rest of the world mostly uses Celsius

• Water freezes at 0° C and boils at 100° C

0° C = 32° F, and 100° C = 212° F

• A change of 1° K (Kelvin) = a change of 1° C

0° C = 273° K

Converting Temperature Scales

If you know the temperature on one scale, you can convert it to the other scale:

A test: is it true that –40° F = –40° C ?

Thermocouples

• A thermocouple is a device made of two different

kinds of metal “welded” together to form a

junction Electrons transfer from one metal to the other with an energy proportional to temperature

• The electron transfer produces a voltage

(Seebeck voltage) that is proportional to

temperature

• The voltage is very low, and requires conditioning

are reasonably linear

Thermocouples

Platinum and rhodium are expensive

RTDs

• Resistance Temperature Detectors (RTDs) rely

on the positive temperature coefficient of

resistance shown by metals Platinum is

commonly used

• RTDs have a very linear response to

temperature

• RTDs make stable and accurate transducers

• RTDs are fragile and expensive

• RTDs require special meters to read them

• Come in various shapes and sizes

• Made from semiconducting ceramic

IC Temperature Transducers

Very linear response

LVDTs

Can measure a 2-micrometer move

Strain Gages

Commonly used, inexpensive, linear, small signal

Pressure

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Pounds / in 2 or Newtons / m 2 (Pascals).

Pressure

Types of Pressure Measurements:

• Absolute Pressure: compared to a

vacuum

• Gage Pressure: compared to ambient

pressure (atmospheric pressure)

• Differential Pressure: pressure change

across a boundary

Flow Transducers

Measure how fast material moves

Some examples:

• Gallons per minute (for fluids)

• Cubic feet per minute (for gases)

• Pounds per minute (for solids)

• Feet per second (actually, a velocity)

Flow

<insert figure 21-20 here>

Bernoulli’s principle: velocity up  pressure down

Flow

• Bernoulli’s principle:

based on conservation of energy in a moving fluid

• Positive Flow Transducers:

based on rotation of a turbine caused

by direct contact with a moving fluid The “fluid” could be a slurry: a mixture

of solids and liquids

Acceleration

Newton’s Law: Force = Mass  Acceleration

Hall-Effect Sensors

Voltage from current through a magnetic field

Hall-Effect Sensors

Output proportional to magnetic field strength

Solenoid Actuators

Moves when coil is energized

Relays

Originally invented to relay telegraph signals

DC Motors

Torque from interacting static magnetic fields

Synchronous Motors

Rotor matches rotating magnetic field of stator

AC Induction Motors

Stator induces current in rotor

Induction Motors

• The “workhorse” of electric motors

• Torque from interacting magnetic fields of stator and rotor

• 3-phase version has high torque, easy

starting, and can be reversed by swapping two of the connections

• Single-phase versions have less torque (for their size) and need special starting circuits

Stepper Motors

Accurate movement without feedback (servo) loop