Bichop R.H. (Ed.) Mechatronic Systems, Sensors, and Actuators: Fundamentals and Modeling

Подождите немного. Документ загружается.

21-2 Mechatronic Systems, Sensors, and Actuators

with the load or physical environment. In addition, we will look at the electronic components that are

essential for modulating the necessary electrical energy.

From an energy perspective, a mechatronic system can be separated into a relatively higher energy

(power) portion that interacts with the physical world and a relatively low energy (power) portion that

process the data, see Figure 21.1. Sensor and actuators are the interfacing devices that accomplish the

tasks of energy modulation and energy conversion. Therefore, an actuator can be viewed as having the

structure depicted in Figure 21.2. Typically, actuators are considered only as energy conversion devices.

However, with the proliferation of power electronics, we will take a more inclusive view of actuators that

also includes power amplification. An electrohydraulic linear actuator, see Figure 21.3, can also be similarly

classified, where the spool valve is the power amplification/modulation block with spool position as the

FIGURE 21.1 Mechatronic system.

FIGURE 21.2 Actuator functional diagram.

FIGURE 21.3 Electrohydraulic and electromechanical actuators.

Communication Operator interface

High power

Low power

Physical process

mixture of mechanical, fluid,

thermal, chemical, electrical

components/subsystems

Information processing

electronics, software

Actuators

power modulation,

energy conversion

Sensors

energy conversion,

signal processing

Control

signal

Load

environment

Actuator

Low power

High power

Unmodulated

energy source

Power

amplification

& modulation

Energy

conversion

LOAD

Energy conversion

Power

amplification

& modulation

Low

energy

Control

signal

9258_C021_Sect001.fm Page 2 Wednesday, October 10, 2007 7:09 PM

Actuators 21-3

control signal and the hydraulic pressure/flow is the energy source. The hydraulic cylinder acts as the

energy conversion device that converts fluidic energy to mechanical energy. For a typical electromechanical

actuator, such as a DC motor (Figure 21.3), the power amplification block is the motor driver that

amplifies signal level (low current/power) control signal to the higher power (large current) signal that

is used to convert electrical energy to mechanical energy through the electromagnetic principle.

In this chapter, we will first present an overview of common types of electromechanical actuators.

They will be classified by the respective energy conversion mechanism. The power electronic components,

such as diodes, thyristors, and transistors, which are used for power amplification and modulation, will

be presented followed by discussion of common power amplification building blocks. We will conclude

by discussing some issues related to interfacing with electromechanical actuators.

21.1.2 Type of Electromechanical Actuators—Operating Principles

Converting electrical energy to mechanical energy is the common thread among different electrome-

chanical actuators. Physics provided us with many different mechanisms either through direct conversion

such as piezoelectric or through an intermediate medium such as a magnetic field. We will present an

overview of the more common electromechanical actuators by their energy conversion mechanism:

electromagnetic, electrostatic, and piezoelectric. The following discussion is intended to provide intro-

ductory information about the types of electromechanical actuation and is by no means exhaustive.

Detailed discussion of each can be found in subsequent chapters, where they will be discussed in more

detail.

21.1.2.1 Electromagnetics—Magnetic Field

Electromagnetic is the most widely utilized method of energy conversion for electromechanical actuators.

One of the reasons for using magnetic fields instead of electric fields is the higher energy density in magnetic

fields. The air gap that separates a stationary member (stator) and a moving member of an electrome-

chanical actuator is where the electromechanical energy conversion takes place. The amount of energy

per unit volume of air gap for magnetic fields can be five orders of magnitude higher than that of electric

fields.

Lorentz’s law of electromagnetic forces and Faraday’s law of electromagnetic induction are the two

fundamental principles that govern electromagnetic actuators. Before going into the detail of electro-

magnetics, we will first introduce the concept of magnetic field and flux.

Magnetic flux

exists due the presence of a magnetic field. The magnetic field strength (in A/m)

and the magnetic flux density (in tesla [T]) are related by the permeability of the material. In a vacuum,

the magnetic flux density is directly proportional to the magnetic field strength and is expressed by

(21.1)

where

= 4[T m/A] is the permeability constant. For other magnetic or ferromagnetic materials the

relationship is given by

(21.2)

where

() is the relative permeability of the material. Figure 21.4 shows typical B-H and

-H curves.

Lorentz’s Law of Electromagnetic Force

When a current carrying conductor is placed in a magnetic field, it will be subjected to an induced force

given by

(21.3)

⋅=

H()

⋅

H⋅=

×=

9258_C021_Sect001.fm Page 3 Wednesday, October 10, 2007 7:09 PM

21-4 Mechatronic Systems, Sensors, and Actuators

where is the force vector, is the current vector, and is the magnetic flux density. The force is called

the electromagnetic force or the Lorentz force. If a conductor of length L carrying constant current i is

placed in a constant (independent of location) field B, as shown in Figure 21.5, the magnitude of the

resultant Lorentz force exerted by the field B on the conductor is

(21.4)

Faraday’s Law of Electromagnetic Induction

The motion of a conductor in a magnetic field will produce an electromotive force (emf), or electric

potential, across the conductor given by

(21.5)

where

is the magnetic flux. For a conductor of length L moving at a constant speed v in a

constant (independent of location) magnetic field that is perpendicular to the area A, as shown in

Figure 21.6, the magnitude of the induced electromotive force (electric potential) is

(21.6)

There are two methods to generate a desired magnetic field , or equivalently, a desired magnetic

flux density . One is to use a permanent magnet and the other is to utilize the Boit–Savart law.

Boit–Savart Law

A long (infinite), straight, current carrying conductor induces a magnetic field around the conductor, see

Figure 21.7. The flux density at a perpendicular distance r from the conductor is

(21.7)

FIGURE 21.4

-H diagram and B-H diagram.

FIGURE 21.5 Lorentz’s electromagnetic force.

H H

Electromagnetic

(Lorentz) force

→

Electric

current

→

Flux

denity

→

F |

| BLi==

emf E −

------

⋅

emf EBLv==

----------

i⋅=

9258_C021_Sect001.fm Page 4 Wednesday, October 10, 2007 7:09 PM

Actuators 21-5

where i is the electric current. If we bend the straight current carrying conductor into a helical coil

(solenoid) with N turns, it will induce a corresponding magnetic field as depicted in Figure 21.8. If the

length of the coil L is much greater than its diameter D, the flux density follows the right-hand rule and

the magnitude inside the coil is approximately

(21.8)

where

is the permeability of the material inside the coil and i is the current through the

winding. This field can be intensified by inserting a ferromagnetic core into the solenoid by increasing

the permeability. Coil induced magnetic fields is widely utilized in electromagnetic devices for generating

controlled magnetic fields and are often referred to as electromagnets.

FIGURE 21.6 Motion induced electromotive force.

FIGURE 21.7 Magnetic field generated by current carrying conductor.

FIGURE 21.8 Coil (solenoid) induced magnetic field.

Electromotive

force (emf)

Velocity

→

Flux

denity

→

Electric

current

→

----

i⋅=

9258_C021_Sect001.fm Page 5 Wednesday, October 10, 2007 7:09 PM

21-6 Mechatronic Systems, Sensors, and Actuators

Solenoid Type Devices

Solenoids, see Figure 21.9, is the simplest electromagnetic actuators that are used in linear as well as

rotary actuations for valves, switches, and relays. As the name indicates, a solenoid consists of a stationary

iron frame (stator), a coil (solenoid), and a ferromagnetic plunger (armature) in the center of the coil,

see Figure 21.10.

As the coil is energized, a magnetic field is induced inside the coil. The movable plunger moves to

increase the flux linkage by closing the air gap between the plunger and the stationary frame. The magnetic

force generated is approximately proportional to the square of the applied current i and is inverse

proportional to the square of the air gap

, which is the stroke of the solenoid, i.e.,

(21.9)

As shown in Figure 21.11, for strokes less than 0.060 in., the flat face plunger is recommended with a

pull or push force three to five times greater than 60° plungers. For longer strokes up to 0.750 in., the

60° plunger offers the greatest advantage over the flat face plunger. When the coil is de-energized, the

field decreases and the plunger will return to the original location either by the load itself or through a

return spring.

All linear solenoids basically pull the plunger into the coil when energized. Push-type solenoids are

implemented by extending the plunger through a hole in the back-stop, see Figure 21.12. Therefore, when

energized, the plunger is still pulled into the coil, but the extended producing a pushing motion from

the back end of the solenoid. Return motion, upon de-energizing the coil, is provided by the load itself

(i.e., the weight of the load) and/or by a return spring, which can be provided as an integral part of the

solenoid assembly.

FIGURE 21.9 Assorts of solenoid actuators. (Courtesy of Shih Hsing Industrial Co., Ltd.)

FIGURE 21.10 A typical solenoid.

Plunger

Frame

Coil

----

∝

9258_C021_Sect001.fm Page 6 Wednesday, October 10, 2007 7:09 PM

Actuators 21-7

Rotary solenoids utilize ball bearings that travel down inclined raceways to convert linear motion to

rotary motion. When the coil is energized, the plunger assembly is pulled towards the stator and rotated

through an arc determined by the coining of the raceways, see Figure 21.13. An electromechanical relay

(EMR) is a device that utilizes a solenoid to close or open a mechanical contact (switch) between high

power electrical leads. A relay performs the same function as a power transistor in that relatively small

electrical energy is used to switch a large amount of currents. The difference is that a relay has the

capability of controlling much larger current level. Variations on this mechanism are possible: some relays

have multiple contacts, some are encapsulated, some have built-in circuits that delay contact closure after

actuation, and some, as in early telephone circuits, advance through a series of positions step by step, as

they are energized and de-energized.

Design/Selection Considerations. Force, stroke, temperature, and duty cycle are the four major design/

selection considerations for solenoids. A linear solenoid can provide up to 30 lb of force from a unit

less than in. long. A rotary solenoid can provide well over 100 lb of torque from a unit also less than

in. long. As shown in Figure 21.11, the relationship between force and stroke can be modified by

changing the design of some internal components. Higher performance, e.g., force output, can be

FIGURE 21.11 Typical force-stroke curve of solenoids. (Courtesy of Magnetic Sensor Systems.)

FIGURE 21.12 Push and pull type solenoids. (Courtesy of Ledex® & Dormeyer® Products.)

FIGURE 21.13 Rotary solenoid. (Courtesy of Ledex® & Dormeyer® Products.)

Plunger

Back stop

Stroke

Conical 60°

Conical 90°

Flat

Force

De-energizedDe-energized

Energized

pull type

Energized

pull type

Energized

(CW rotation)

De-energized

9258_C021_Sect001.fm Page 7 Wednesday, October 10, 2007 7:09 PM

21-8 Mechatronic Systems, Sensors, and Actuators

achieved by increasing the current to the coil winding. However, higher current tends to increase the

winding temperature. As the winding temperature increases, the wire resistance increases. This will

reduce the output force level. Solenoids are often rated as operating under continuous duty cycle or

intermittent duty cycle. A solenoid rated for 100% duty cycle may be energized at its rated voltage

continuously because its total coil temperature will not exceed maximum allowable ratings, while an

intermittent duty cycle solenoid has an associated allowable “on” time which must not be exceeded.

Intermittent duty coils provide considerably higher forces than continuous duty solenoids. The maxi-

mum operating temperature for a solenoid is determined by the rated temperature of the insulation

material used in the winding (see Table 21.1).

Voice-Coil Motors (VCMs)

As the name indicates, the voice-coil motor was originally developed for loudspeakers. It is now exten-

sively used in moving read/write heads in hard disk drives. Since the coil is in motion, VCM is also

referred to as a moving-coil actuator. The VCM consists of a moving coil (armature) in a gap and a

permanent magnet (stator) that provides the magnetic field in the gap, see Figure 21.14. When current

flows through the coil, based on the Lorentz law, the coil experiences electromagnetic (Lorentz) force F

Since most voice-coils are designed so that the flux is perpendicular to the current direction, the resultant

Lorentz force can be written as

(21.10)

where l is the coil length per turn, B is the flux density, N is the number of turns in the coil, i is the

current, and

is a coil utilization factor. It is important to know that the force is proportional to the

applied current amplitude and the proportional constant K

is often called the force constant.

TABLE 2 1. 1 Temprature Rating for Electrical Insulations

Insulation Classification Temperature Rating

Class A Class 105 105°C 221°F

Class E Class 120 120°C 248°F

Class B Class 130 130°C 266°F

Class F Class 155 155°C 311°F

Class H Class 180 180°C 356°F

Class N Class 200 200°C 392°F

FIGURE 21.14 Voice-coil motor.

Moving coil

Iron

Magnetic flux

Lorentz

force

Permanent magnet

×=

VCM

BNl i⋅ K

VCM

i∝⋅==

9258_C021_Sect001.fm Page 8 Wednesday, October 10, 2007 7:09 PM

Actuators 21-9

The coil is usually suspended in the gap by springs and attached to the load such as the diaphragm

of an audio speaker, the spool of a hydraulic valve, or the read/write head of the disk drive. The linear

relationship between the output force and the applied current and the bidirectional capability makes the

voice coil more attractive than solenoids. However, since the controlled output of the voice coil is force,

some type of closed loop control or some type of spring suspension is needed.

Design/Selection Consideration. From Equation 21.10 we see that the force constant depends on the

flux density and the amount of wires that can be packed into the gap. There are two options to increase

the force constant. One is to increase the flux density, which can be achieved by using stronger magnetic

material and the other is to increase either N or l, i.e., to pack more turns and/or make a larger diameter

coil.

Given a fixed gap volume, using higher gauge (thinner) wires is the only way to increase the number

of turns. However, higher gauge wires have larger resistance, which will increase the resistive heating of

the winding and limit the allowable current. In addition, the additional insulation will also occupy more

volume and tends to reduce the effect of increasing N. In summary, to improve the performance of the

voice coil, a designer can either choose a better magnetic material or to make the motor bigger by either

making the coil wider (increase D) or longer (increase N).

Electric Motors

Electric motors are the most widely used electromechanical actuators. They can either be classified based

on functionality or electromagnetic characteristics. The differences in electric motors are mainly in the

rotor design and the method of generating the magnetic field. Figure 21.15 shows the composition of a

permanent magnet DC motor. Some common terminologies for electric motors are:

Stator is the stationary outer or inner housing of the motor that supports the material that generates

the appropriate stator magnetic field. It can be made of permanent magnet or coil windings.

Field coil (system) is the portion of the stator that is responsible for generating the stator (field) magnetic

flux.

Rotor is the rotating part of the motor. Depending on the construction, it can be a permanent magnet

or a ferromagnetic core with coil windings (armature) to provide the appropriate armature field to

interact with the stator field to create the torque.

Armature is the rotor winding that carries current and induces a rotor magnetic field.

Air gap is the small gap between the rotor and the stator, where the two magnetic fields interact and

generate the output torque.

Brush is the part of a DC motor through which the current is supplied to the armature (rotor). For

synchronous AC motors, this is done by slip rings.

FIGURE 21.15 Permanent magnet DC motor. (From T. Keujo and S. Nagamori, Permanent-Magnet and Brushless

DC Motors, 1985, by permission of Oxford University Press.)

Commutator

Housing

Stator

(permanent magnent for filed

flux)

Brush

Rotor (armature)

9258_C021_Sect001.fm Page 9 Wednesday, October 10, 2007 7:09 PM

21-10 Mechatronic Systems, Sensors, and Actuators

Commutator is the part of the DC motor rotor that is in contact with the brushes and is used for

controlling the armature current direction. Commutation can be interpreted as the method to control

the current directions in the stator and/or the armature coils so that a desired relative stator and rotor

magnetic flux direction is maintained. For AC motors, commutation is done by the AC applied current

as well as the design of the winding geometry. For stepping motors and brushless DC (BLDC) motors,

commutations are done in the drive electronics and/or motor commands.

Torque generation in an electric motor is either through the interaction of the armature current

and the stator magnetic field (Lorentz Law) or through the interaction of the stator field and the

armature field. Table 21.2 summarizes the common classification of electric motors. The next chapter

will give a detailed discussion of the operation of various electric motors and the associated design

considerations.

TABLE 2 1. 2 Electric Motor Classification

Classification

Command

Input Magnetic Field Description

DC motors Permanent magnet Permanent magnets are used to generate the stator magnetic field

Electrical current is supplied directly into the armature winding of

the rotor through the brushes and commutators

Electro-

magnets

Shunt wound A stator (field) winding is used as electromagnet. Stator winding is

connected in parallel with the armature winding

Series wound A stator (field) winding is used as electromagnet. Stator winding is

connected in series with the armature winding

Compound wound Two stator (field) windings are used as electromagnet. The stator

windings are connected, one in series and one in parallel, with the

armature winding

Separate wound A stator (field) winding is used as electromagnet. Both the stator and

armature fields are individually energized

AC motors Single-phase Induction Single stator winding with squirrel-cage rotor. No external connection

to the rotor. Torque generation is based on the electromagnetic

induction between the stator and rotor. AC current provides the

commutation of the fields. Rotor speed is slightly slower than the

rotating stator field (slip)

Synchronous Permanent magnet rotor or rotor winding with slip ring commutation.

Rotating speed is synchronized with the frequency of the AC source

Poly-phase Induction Similar to single-phase induction motor but with multiple stator

windings. Self-starting

Synchronous Similar to single-phase synchronous motor but with multiple stator

windings for smoother operation

Universal Essentially a single-phase AC induction motor with similar electrical

connection as a series wound DC motor. Can be driven by either AC

or DC source

Stepper

Motors

Permanent magnet Permanent magnet rotor with stator windings to provide matching

magnetic field. By applying different sequence (polarity) of coil

current, the rotor PM field will align to match induced stator field

Variable reluctance Teethed ferromagnetic rotor with stator windings. Rotor motion is the

result of the minimization of the magnetic reluctance between the

rotor and stator poles

Hybrid Multi-toothed rotor with stator winding. The rotor consists of two

identical teethed ferromagnetic armatures sandwiching a permanent

magnetic

Brushless

motors

Poly-phase Synchronous Essentially a poly-phased AC synchronous motor but using electronic

commutation to match rotor and stator magnetic fields. Electronic

commutation enables using a DC source to drive the synchronous

motor

9258_C021_Sect001.fm Page 10 Wednesday, October 10, 2007 7:09 PM

Actuators 21-11

21.1.2.2 Electrostatics—Electrical Field

Since electrical fields have lower energy density than magnetic fields, typical applications of electrical field

forces are limited to measurement devices and accelerating charge particles, where the required energy

density is small. Recently, with the proliferation of microfabrication technology, it is possible to apply

the small electrostatic forces to microelectromechanical actuators, such as comb actuators (see Figure

21.16). The advantage of electrostatic actuation is the higher switching rate and less energy loss as

compared to the electromagnetic actuation. However, the limitation in force, travel, and high operating

voltage still needs to be addressed. Electrostatic actuation is the main actuation for moving charged toner

particles in electrophotographic (xerographic) processes, e.g., laser printers.

21.1.2.3 Piezoelectric

Piezoelectric is the property of certain crystals that produces a voltage when subjected to mechanical

deformation, or undergoes mechanical deformation when subjected to a voltage. When a piezoelectric

material is under mechanical stress, it produces an asymmetric displacement in the crystal structure

and in the charge center of the affected crystal ions. The result is charge separation. An electric potential

proportional to the mechanical strain can be measured. This is called the direct piezoelectric effect.

Conversely, the material will have deformation without volume change when electric potential is

applied. This reciprocal piezoelectric effect can be used to produce mechanical actuation. There are two

categories of piezoelectric materials: sintered ceramics, such as lead-zirconate-titinate (PZT), and

polymers, such as polyvinylidence fluoride (PVDF). Piezoceramics have a larger force output and are

used more as actuators. PVDFs tend to generate larger deformation and are used more for sensor

applications.

FIGURE 21.16 MEMS comb actuator uses electrostatic actuation. (Courtesy of Sandia National Laboratories,

MEMS and Novel Si Science and Technology Department, SUMMIT Technologies, www.mems.sandia.com.)

9258_C021_Sect001.fm Page 11 Wednesday, October 10, 2007 7:09 PM