Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 10

They are mechanically coupled to the motor shaft and can be located within the motor frame. Aftertachometer output is converted to a digital format by the motion controller, a feedback signal is generated for the driver to keep motor speedwithin preset limits.Other common feedback sensors include resolvers, linear variabledifferential transformers (LVDTs), Inductosyns, and potentiometers.Less common are the more accurate laser interferometers. Feedbacksensor selection is based on an evaluation of the sensor’s accuracy,repeatability, ruggedness, temperature limits, size, weight, mountingrequirements, and cost, with the relative importance of each determinedby the application.Installation and Operation of the SystemThe design and implementation of a cost-effective motion-control system require a high degree of expertise on the part of the person or persons responsible for system integration.

It is rare that a diverse group ofcomponents can be removed from their boxes, installed, and interconnected to form an instantly effective system. Each servosystem (andmany stepper systems) must be tuned (stabilized) to the load and environmental conditions. However, installation and development time canbe minimized if the customer’s requirements are accurately defined,optimum components are selected, and the tuning and debugging toolsare applied correctly. Moreover, operators must be properly trained informal classes or, at the very least, must have a clear understanding ofthe information in the manufacturers’ technical manuals gained by careful reading.SERVOMOTORS, STEPPER MOTORS, ANDACTUATORS FOR MOTION CONTROLMany different kinds of electric motors have been adapted for use inmotion control systems because of their linear characteristics.

Theseinclude both conventional rotary and linear alternating current (AC) anddirect current (DC) motors. These motors can be further classified intoChapter 1Motor and Motion Control Systemsthose that must be operated in closed-loop servosystems and those thatcan be operated open-loop.The most popular servomotors are permanent magnet (PM) rotary DCservomotors that have been adapted from conventional PM DC motors.These servomotors are typically classified as brush-type and brushless.The brush-type PM DC servomotors include those with wound rotorsand those with lighter weight, lower inertia cup- and disk coil-type armatures. Brushless servomotors have PM rotors and wound stators.Some motion control systems are driven by two-part linear servomotors that move along tracks or ways.

They are popular in applicationswhere errors introduced by mechanical coupling between the rotarymotors and the load can introduce unwanted errors in positioning. Linearmotors require closed loops for their operation, and provision must bemade to accommodate the back-and-forth movement of the attached dataand power cable.Stepper or stepping motors are generally used in less demandingmotion control systems, where positioning the load by stepper motors isnot critical for the application.

Increased position accuracy can beobtained by enclosing the motors in control loops.Permanent-Magnet DC ServomotorsPermanent-magnet (PM) field DC rotary motors have proven to be reliable drives for motion control applications where high efficiency, highstarting torque, and linear speed–torque curves are desirable characteristics. While they share many of the characteristics of conventional rotaryseries, shunt, and compound-wound brush-type DC motors, PM DC servomotors increased in popularity with the introduction of strongerceramic and rare-earth magnets made from such materials asneodymium–iron–boron and the fact that these motors can be driven easily by microprocessor-based controllers.The replacement of a wound field with permanent magnets eliminatesboth the need for separate field excitation and the electrical losses thatoccur in those field windings. Because there are both brush-type andbrushless DC servomotors, the term DC motor implies that it is brushtype or requires mechanical commutation unless it is modified by theterm brushless.

Permanent-magnet DC brush-type servomotors can alsohave armatures formed as laminated coils in disk or cup shapes. They arelightweight, low-inertia armatures that permit the motors to acceleratefaster than the heavier conventional wound armatures.The increased field strength of the ceramic and rare-earth magnetspermitted the construction of DC motors that are both smaller and lighter2122Chapter 1Motor and Motion Control SystemsFigure 1-17 Cutaway view of afractional horsepower permanent-magnet DC servomotor.than earlier generation comparably rated DC motors with alnico (aluminum–nickel–cobalt or AlNiCo) magnets.

Moreover, integrated circuitry and microprocessors have increased the reliability and costeffectiveness of digital motion controllers and motor drivers oramplifiers while permitting them to be packaged in smaller and lightercases, thus reducing the size and weight of complete, integrated motioncontrol systems.Brush-Type PM DC ServomotorsThe design feature that distinguishes the brush-type PM DC servomotor, asshown in Figure 1-17, from other brush-type DC motors is the use of a permanent-magnet field to replace the wound field.

As previously stated, thiseliminates both the need for separate field excitation and the electricallosses that typically occur in field windings.Permanent-magnet DC motors, like all other mechanically commutatedDC motors, are energized through brushes and a multisegment commutator.While all DC motors operate on the same principles, only PM DC motorshave the linear speed–torque curves shown in Figure 1-18, making themideal for closed-loop and variable-speed servomotor applications. Theselinear characteristics conveniently describe the full range of motor perform-Chapter 1Motor and Motion Control Systems23Figure 1-18 A typical family ofspeed/torque curves for a permanent-magnet DC servomotor atdifferent voltage inputs, withvoltage increasing from left toright (V1 to V5).ance. It can be seen that both speed and torque increase linearly withapplied voltage, indicated in the diagram as increasing from V1 to V5.The stators of brush-type PM DC motors are magnetic pole pairs.When the motor is powered, the opposite polarities of the energizedwindings and the stator magnets attract, and the rotor rotates to alignitself with the stator.

Just as the rotor reaches alignment, the brushesmove across the commutator segments and energize the next winding.This sequence continues as long as power is applied, keeping the rotor incontinuous motion. The commutator is staggered from the rotor poles,and the number of its segments is directly proportional to the number ofwindings. If the connections of a PM DC motor are reversed, the motorwill change direction, but it might not operate as efficiently in thereversed direction.Disk-Type PM DC MotorsThe disk-type motor shown exploded view in Figure 1-19 has a diskshaped armature with stamped and laminated windings.

This nonferrouslaminated disk is made as a copper stamping bonded betweenepoxy–glass insulated layers and fastened to an axial shaft. The statorfield can either be a ring of many individual ceramic magnet cylinders,as shown, or a ring-type ceramic magnet attached to the dish-shaped end24Chapter 1Motor and Motion Control SystemsFigure 1-19 Exploded view of apermanent-magnet DC servomotor with a disk-type armature.bell, which completes the magnetic circuit. The spring-loaded brushesride directly on stamped commutator bars.These motors are also called pancake motors because they are housedin cases with thin, flat form factors whose diameters exceed theirlengths, suggesting pancakes. Earlier generations of these motors werecalled printed-circuit motors because the armature disks were made by aprinted-circuit fabrication process that has been superseded.

The flatmotor case concentrates the motor’s center of mass close to the mountingplate, permitting it to be easily surface mounted. This eliminates theawkward motor overhang and the need for supporting braces if a conventional motor frame is to be surface mounted. Their disk-type motor formfactor has made these motors popular as axis drivers for industrial robotswhere space is limited.The principal disadvantage of the disk-type motor is the relativelyfragile construction of its armature and its inability to dissipate heat asrapidly as iron-core wound rotors. Consequently, these motors are usually limited to applications where the motor can be run under controlledconditions and a shorter duty cycle allows enough time for armature heatbuildup to be dissipated.Cup- or Shell-Type PM DC MotorsCup- or shell-type PM DC motors offer low inertia and low inductanceas well as high acceleration characteristics, making them useful in manyChapter 1Motor and Motion Control Systems25Figure 1-20 Cutaway view of apermanent-magnet DC servomotor with a cup-type armature.servo applications.

They have hollow cylindrical armatures made as aluminum or copper coils bonded by polymer resin and fiberglass to form arigid “ironless cup,” which is fastened to an axial shaft. A cutaway viewof this class of servomotor is illustrated in Figure1-20.Because the armature has no iron core, it, like the disk motor, hasextremely low inertia and a very high torque-to-inertia ratio. This permits the motor to accelerate rapidly for the quick response required inmany motion-control applications.

The armature rotates in an air gapwithin very high magnetic flux density. The magnetic field from the stationary magnets is completed through the cup-type armature and a stationary ferrous cylindrical core connected to the motor frame. The shaftrotates within the core, which extends into the rotating cup. Springbrushes commutate these motors.Another version of a cup-type PM DC motor is shown in the explodedview in Figure 1-21. The cup type armature is rigidly fastened to theshaft by a disk at the right end of the winding, and the magnetic field isalso returned through a ferrous metal housing.

The brush assembly ofthis motor is built into its end cap or flange, shown at the far right.The principal disadvantage of this motor is also the inability of itsbonded armature to dissipate internal heat buildup rapidly because of itslow thermal conductivity. Without proper cooling and sensitive controlcircuitry, the armature could be heated to destructive temperatures inseconds.26Chapter 1Motor and Motion Control SystemsFigure 1-21 Exploded view ofa fractional horsepower brushtype DC servomotor.Brushless PM DC MotorsBrushless DC motors exhibit the same linear speed–torque characteristics as the brush-type PM DC motors, but they are electronically commutated. The construction of these motors, as shown in Figure 1-22, differs from that of a typical brush-type DC motor in that they are“inside-out.” In other words, they have permanent magnet rotors insteadof stators, and the stators rather than the rotors are wound.