Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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431

CHAPTER

Actuators

his chapter describes various actuators important in mechatronic system

design. ■

INPUT SIGNAL

CONDITIONING

AND INTERFACING

- discrete circuits

- amplifiers

- filters

- A/D, D/D

OUTPUT SIGNAL

CONDITIONING

AND INTERFACING

- D/A, D/D

PWM

- power transistors

- power op amps

GRAPHICAL

DISPLAYS

- LEDs

- digital displays

- LCD

- CRT

SENSORS

- switches

- potentiometer

- photoelectrics

- digital encoder

- strain gage

- thermocouple

- accelerometer

- MEMs

ACTUATORS

solenoids, voice coils

DC motors

stepper motors

servo motors

hydraulics, pneumatics

MECHANICAL SYSTEM

system model dynamic response

DIGITAL CONTROL

ARCHITECTURES

- logic circuits

- microcontroller

- SBC

- PLC

- sequencing and timing

- logic and arithmetic

- control algorithms

- communication

amplifiers

CHAPTER OBJECTIVES

After you read, discuss, study, and apply ideas in this chapter, you will:

1. Be able to identify different classes of actuators, including solenoids, DC

motors, AC motors, hydraulics, and pneumatics

2. Understand the differences between series, shunt, compound, permanent mag-

net, and stepper DC motors

3. Understand how to design electronics to control a stepper motor

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Internet Lin

10.1Actuator

online resources

and vendors

432 CHAPTER 10 Actuators

4. Be able to select a motor for a mechatronics application

5. Be able to identify and describe the components used in hydraulic and

pneumatic systems

10.1 INTRODUCTION

Most mechatronic systems involve motion or action of some sort. This motion or

action can be applied to anything from a single atom to a large articulated struc-

ture. It is created by a force or torque that results in acceleration and displacement.

Actuators are the devices used to produce this motion or action.

Up to this point in the book, we have focused on electronic components and

sensors and associated signals and signal processing, all of which are required to

produce a specific mechanical action or action sequence. Sensor input measures

how well a mechatronic system produces its action, open loop or feedback control

helps regulate the specific action, and much of the electronics we learned about is

required to manipulate and communicate this information. Actuators produce physi-

cal changes such as linear and angular displacement. They also modulate the rate

and power associated with these changes. An important aspect of mechatronic sys-

tem design is selecting the appropriate type of actuator. This chapter covers some

of the most important actuators: solenoids, electric motors, hydraulic cylinders and

rotary motors, and pneumatic cylinders. Putting it poetically, this chapter is “where

the rubber meets the road.” Internet Link 10.1 provides links to vendors and online

resources for various commercially available actuators and support equipment.

10.2 ELECTROMAGNETIC PRINCIPLES

Many actuators rely on electromagnetic forces to create their action. When a current-

carrying conductor is moved in a magnetic field, a force is produced in a direc-

tion perpendicular to the current and magnetic field directions. Lorentz’s force law,

which relates force on a conductor to the current in the conductor and the external

magnetic field, in vector form is

FIB×=

(10.1)

where

is the force vector (per unit length of conductor),

is the current vector,

and

is the magnetic field vector. Figure 10.1 illustrates the relationship between

these vectors and indicates the right-hand rule analogy, which states that if your

right-hand index finger points in the direction of the current and your middle finger

is aligned with the field direction, then your extended thumb (perpendicular to the

index and middle fingers) will point in the direction of the force. Another way to

apply the right-hand rule is to align your extended fingers in the direction of the

vector and orient your palm so you can curl (flex) your fingers toward the direction

of the

vector. Your hand will then be positioned such that your extended thumb

points in the direction of

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Figure 10.1 Right-hand rule for magnetic force.

extended right hand

index finger

direction

extended

right hand thumb

direction

flexed right hand

middle finger

direction

10.3 Solenoids and Relays 433

Another electromagnetic effect important to actuator design is field intensifica-

tion within a coil. Recall that, when discussing inductors in Chapter 2, we stated that

the magnetic flux through a coil is proportional to the current through the coil and

the number of windings. The proportionality constant is a function of the permeabil-

ity of the material within the coil. The permeability of a material characterizes how

easily magnetic flux penetrates the material. Iron has a permeability a few hundred

times that of air; therefore, a coil wound around an iron core can produce a magnetic

flux a few hundred times that of the same coil with no core. Most electromagnetic

devices we will present use iron cores of one form or another to enhance magnetic

flux. Cores are usually laminated (made up of insulated layers of iron stacked par-

allel to the coil-axis direction) to reduce the eddy currents induced when the cores

experience changing magnetic fields. Eddy currents, which are a result of Faraday’s

law of induction, result in inefficiencies and undesirable core heating.

10.3 SOLENOIDS AND RELAYS

As illustrated in Figure 10.2 , a solenoid consists of a coil and a movable iron core

called the armature. When the coil is energized with current, the core moves to

increase the flux linkage by closing the air gap between the cores. The movable core

is usually spring-loaded to allow the core to retract when the current is switched off.

The force generated is approximately proportional to the square of the current and

inversely proportional to the square of the width of the air gap. Solenoids are inex-

pensive, and their use is limited primarily to on-off applications such as latching,

locking, and triggering. They are frequently used in home appliances (e.g., washing

machine valves), automobiles (e.g., door latches and the starter solenoid), pinball

machines (e.g., plungers and bumpers), and factory automation. Video Demos 10.1

through 10.3 show examples of interesting student projects using solenoids in cre-

ative ways.

An electromechanical relay is a solenoid used to make or break mechanical con-

tact between electrical leads. A small voltage input to the solenoid controls a poten-

tially large current through the relay contacts. Applications include power switches

and electromechanical control elements. A relay performs a function similar to a

power transistor switch circuit but has the capability to switch much larger currents.

Video Demo

10.1Magic piano

10.2Automated

melodica

10.3LED

fountain system

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Figure 10.2 Solenoids.

(a) plunger type

(b) nonplunger type

movable

armature

core

coil

stationary

iron core

spring

434 CHAPTER 10 Actuators

Relays, because they create a mechanical connection and don’t require voltage bias-

ing, can be used to switch either DC or AC power. Also, the input circuit of a relay

is electrically isolated from the output circuit, unlike the common-emitter transistor

circuit, where there is a common ground between the input and output. Because

the relay is electrically isolated, noise, induced voltages, and ground faults occur-

ring in the output circuit have minimal impact on the input circuit. One disadvan-

tage of relays is that they have slower switching times than transistors. And because

they contain contacts and mechanical components, they wear out much faster. Video

Demo 10.4 demonstrates how relays and transistors respond to different switching

speeds.

As illustrated in Figure 10.3 , a voice coil consists of a coil that moves in a

magnetic field produced by a permanent magnet and intensified by an iron core.

Figure 10.4 shows the coil and iron core of a commercially available voice coil,

which can be used as either a sensor or an actuator. When used as an actuator, the

force on the coil is directly proportional to the current in the coil. The coil is usually

attached to a movable load such as the diaphragm of an audio speaker, the spool of

a hydraulic proportional valve, or the read-write head of a computer disk drive. The

linear response, small mass of the moving coil, and bidirectional capability make

voice coils more attractive than solenoids for control applications.

Video Demos 10.5 and 10.6 show how a computer disk drive functions, where a

voice coil is used to provide the pivoting motion of the read-write head. Video Demo

10.7 shows a super-slow-motion clip, filmed with a special high-speed camera,

which dramatically demonstrates the accuracy and speed of the voice coil motion.

The read-write head comes to a complete stop on one track before moving to another.

In real-time (e.g., in Video Demo 10.6), this motion is a total blur.

Video Demo

10.4Relay

and transistor

switching circuit

comparison

10.5Computer

hard-drive with

voice coil

10.6Computer

hard-drive

track seeking

demonstration

10.7Computer

hard-drive super-

slow-motion video

of track finding

Figure 10.3 Voice coil.

movable

coil

stationary

iron core

permanent

magnet

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10.4 ELECTRIC MOTORS

Electric motors are by far the most ubiquitous of the actuators, occurring in virtually

all electromechanical systems. Electric motors can be classified either by function or

by electrical configuration. In the functional classification, motors are given names

suggesting how the motor is to be used. Examples of functional classifications

include torque, gear, servo, instrument servo, and stepping. However, it is usually

necessary to know something about the electrical design of the motor to make judg-

ments about its application for delivering power and controlling position. Figure 10.5

provides a configuration classification of electrical motors found in mechatronics

applications. The differences are due to motor winding and rotor designs, resulting

in a large variety of operating characteristics. The price-performance ratio of electric

motors continues to improve, making them important additions to all sorts of mecha-

tronic systems from appliances to automobiles. AC induction motors are particularly

important in industrial and large consumer appliance applications. In fact, the AC

induction motor is sometimes called the workhorse of industry. Video Demos 10.8

through 10.10 show some examples and describe how the motors function.

Video Demo

10.8AC

induction motor

(single phase)

10.9AC

induction motor

with a soft start for

a water pump

10.10AC

induction motor

variable frequency

drive for a building

air handler unit

■ CLASS DISCUSSION ITEM 10.1

Examples of Solenoids, Voice Coils, and Relays

Make a list of common household and automobile devices that contain solenoids,

voice coils, and relays. Describe why you think the particular component was

selected for each of the devices you cite.

Figure 10.4 Photograph of a voice coil iron core and coil.

10.4 Electric Motors 435

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motors

single

phase

induction

synchronous

shaded pole

hysteresis

reluctance

permanent magnet

polyphase

synchronous

universal motor

motors

wound rotor

squirrel cage

induction

wound rotor

squirrel cage

brushed

brushless

permanent magnet

variable reluctance

series wound

shunt wound

compound wound

permanent magnet

variable reluctance

Figure 10.5 Configuration classification of electric motors.

436 CHAPTER 10 Actuators

Figure 10.6 illustrates the construction and components of a typical electric

motor. The stationary outer housing, called the stator, supports radial magnetized

poles. These poles consist of either permanent magnets or wire coils, called field

coils, wrapped around laminated iron cores. The purpose of the stator poles is to pro-

vide radial magnetic fields. The iron core intensifies the magnetic field inside the coil

due to its permeability. The purpose for laminating the core is to reduce the effects

of eddy currents, which are induced in a conducting material (see Class Discussion

Item 10.2). The rotor is the part of the motor that rotates. It consists of a rotating

shaft supported by bearings, conducting coils usually referred to as the armature

windings, and an iron core that intensifies the fields created by the windings. There

Figure 10.6 Motor construction and terminology.

stator

shaft/rotor

air gap

shaft

commutator

segment

laminated iron core pole

winding

bearing

rotor

stator (end view section)

winding

laminated

iron core

pole

rotor

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is a small air gap between the rotor and the stator where the magnetic fields interact.

In many DC motors, the rotor also includes a commutator that delivers and con-

trols the direction of current through the armature windings. For motors with a com-

mutator, “brushes” provide stationary electrical contact to the moving commutator

conducting segments. Brushes in early motors consisted of bristles of copper wire

flexed against the commutator, hence the term brush; but now they are usually made

of graphite, which provides a larger contact area and is self-lubricating. The brushes

are usually spring-loaded to ensure continual contact with the commutator. Video

Demo 10.11 shows a small, brushed, permanent-magnet DC motor disassembled so

you can see the various components and how they function.

A brushless DC motor has permanent magnets on the rotor and a rotating field

in the stator. The permanent magnets on the rotor eliminate the need for a commuta-

tor. Instead, the DC currents in the stator coils are switched in response to proximity

sensors that are triggered as the shaft rotates. Video Demos 10.12 and 10.13 show

two examples of brushless DC motors. One advantage of a brushless motor is that

it does not require maintenance to replace worn brushes. Also, because there are no

rotor windings or iron core, the rotor inertia is much smaller, sometimes making

control easier. There are also no rotor heat dissipation problems, because there are no

rotor windings and hence no I

R heating. Another advantage of not having brushes

is that there is no arcing associated with mechanical commutation. Therefore, brush-

less motors create less EMI and are more suitable in environments where explosive

gases might be present. One disadvantage of brushless motors is that they can cost

more due to the sensors and control circuitry required.

Figure 10.7 shows examples of commercially available assembled motors. In

the top figure, the motor on the left is an AC induction motor with a gearhead speed

reduction unit attached. The motor on the right is a two-phase stepper motor. Motors

come in standard sizes with standard mounting brackets, and they usually include

nameplates listing some of the motor’s specifications. The bottom figure shows the

internal construction of a permanent-magnet-rotor stepper motor. Video Demo 10.14

shows other examples of commercially available regular DC motors and stepper

motors.

Video Demo

10.11DC motor

components

10.12Brushless

DC motor from a

computer fan

10.13Brushless

DC motor gear

pump

10.14DC and

stepper motor

examples

■ CLASS DISCUSSION ITEM 10.2

Eddy Currents

Describe the causes of eddy currents that are induced in a conducting material expe-

riencing a changing magnetic field. The iron core in a motor rotor is usually lami-

nated. Explain why. What is the best orientation for the laminations?

Torque is produced by an electric motor through the interaction of either stator

fields and armature currents or stator fields and armature fields. We illustrate both

principles starting with the first. Figure 10.8 illustrates a DC motor with six armature

windings. The direction of current flow in the windings is illustrated in the figure.

10.4 Electric Motors 437

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438 CHAPTER 10 Actuators

As a result of Equation 10.1 , the interaction of the fixed stator field and the currents

in the armature windings produce a torque in the counterclockwise direction. You

can verify this torque direction by applying the right-hand rule to the armature cur-

rent and stator field directions. To maintain the torque as the rotor rotates, the spatial

arrangement of the armature currents relative to the stator field must remain fixed.

Figure 10.7 Examples of commercial motors. (Courtesy of Oriental Motor,

Torrance, CA)

(a) AC induction and stepper motor

(b) exploded view of stepper motor with a

permanent magnet rotor

Case

FlangeStatorRotorBracket

Figure 10.8 Electric motor field-current interaction.

stator

stator field

current out

current in

torque

rotor

armature

windings

armature

field

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A commutator accomplishes this by switching the currents in the armature windings

in the correct sequence as the rotor turns.

Figure 10.9 illustrates an example commutator. It consists of a ring of alternat-

ing conductive and insulating materials connected to the rotor windings. Current is

directed through the windings via the brushes, which slide on the surface of the com-

mutator as it rotates. In the position shown, the current flows through windings A,

B, and C in the clockwise direction and through F, E, and D in the counterclockwise

direction.

When the rotor turns clockwise one sixth of a full rotation from the position

shown, the currents in windings C and F switch directions. As the brushes slide over

the rotating commutator, this process continues in sequence. With appropriate wind-

ing configurations, the commutator maintains a consistent spatial arrangement of the

currents relative to the fixed stator fields. This continually maintains the torque in the

desired direction.

Another method by which electric motors can create torque is through the inter-

action of stator and rotor magnetic fields. The torque is produced by the fact that

like field poles attract and unlike poles repel. Figure 10.10 illustrates this principle

of operation with a simple two-pole DC motor. The stator poles generate fixed mag-

netic fields with permanent magnets or coils carrying DC current. The winding in the

rotor is commutated to cause changes in direction of its magnetic field. The interac-

tion of the changing rotor field and the fixed stator fields produce a torque on the

shaft, causing rotation. With the rotor in position i, the right brush contacts commu-

tator segment A and the left brush contacts segment B, creating a current in the rotor

winding, resulting in the magnetic poles as shown. The rotor magnetic poles oppose

the stator magnetic poles, creating a torque causing clockwise motion of the rotor.

In position ii, the stator poles both oppose and attract the rotor poles to enhance

the clockwise rotation. Between positions iii and v the commutator contacts switch,

changing the direction of the rotor current and hence the direction of the magnetic

field. In position iv, both brushes temporarily lose contact with the commutator, but

Figure 10.9 Electric motor six-winding commutator.

armature

windings

out

brush

conductor

insulator

10.4 Electric Motors 439

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Figure 10.10 Electric motor field-field interaction.

stator

commutator

segment

brush

stator

pole

torque

(i)

N S

(iii)

N S

(iv)

N S

(v)

N S

(ii)

Video Demo

10.11DC motor

components

440 CHAPTER 10 Actuators

the rotor continues to move due to its momentum. In position v reversed magnetic

field in the rotor again opposes the stator field, continuing the clockwise torque and

motion.

■ CLASS DISCUSSION ITEM 10.3

Field-Field Interaction in a Motor

Does the armature field in Figure 10.8 have any effect on the torque produced by

the motor?

A problem with the simple two-pole design illustrated in Figure 10.10 is that

starting would not occur if the motor happens to be in position iv, where the brushes

are located over the commutator gaps. This problem can be avoided by designing the

motor with more poles and more commutator segments with overlapping switching.

This allows the brushes to always contact two active segments, even while switching

(see Video Demo 10.11 and Class Discussion Item 10.4).

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