Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 9
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It is essentially a computerdedicated to motion control that has been programmed by the end userfor the performance of assigned tasks. The motion controller produces alow-power motor command signal in either a digital or analog format forthe motor driver or amplifier.Significant technical developments have led to the increased acceptanceof programmable motion controllers over the past five to ten years: Theseinclude the rapid decrease in the cost of microprocessors as well as dramatic increases in their computing power. Added to that are the decreasingcost of more advanced semiconductor and disk memories.
During the pastfive to ten years, the capability of these systems to improve product quality, increase throughput, and provide just-in-time delivery has improvedhas improved significantly.The motion controller is the most critical component in the systembecause of its dependence on software. By contrast, the selection of mostmotors, drivers, feedback sensors, and associated mechanisms is less critical because they can usually be changed during the design phase or evenlater in the field with less impact on the characteristics of the intendedsystem.
However, making field changes can be costly in terms of lost productivity.The decision to install any of the three kinds of motion controllersshould be based on their ability to control both the number and types ofmotors required for the application as well as the availability of the software that will provide the optimum performance for the specific application. Also to be considered are the system’s multitasking capabilities, thenumber of input/output (I/O) ports required, and the need for such features as linear and circular interpolation and electronic gearing and camming.In general, a motion controller receives a set of operator instructionsfrom a host or operator interface and it responds with corresponding com-1516Chapter 1Motor and Motion Control Systemsmand signals for the motor driver or drivers that control the motor ormotors driving the load.Motor SelectionThe most popular motors for motion control systems are stepping or stepper motors and permanent-magnet (PM) DC brush-type and brushless DCservomotors.
Stepper motors are selected for systems because they can runopen-loop without feedback sensors. These motors are indexed or partiallyrotated by digital pulses that turn their rotors a fixed fraction or a revolution where they will be clamped securely by their inherent holding torque.Stepper motors are cost-effective and reliable choices for many applications that do not require the rapid acceleration, high speed, and positionaccuracy of a servomotor.However, a feedback loop can improve the positioning accuracy of astepper motor without incurring the higher costs of a complete servosystem.
Some stepper motor motion controllers can accommodate a closedloop.Brush and brushless PM DC servomotors are usually selected forapplications that require more precise positioning. Both of these motorscan reach higher speeds and offer smoother low-speed operation withfiner position resolution than stepper motors, but both require one or morefeedback sensors in closed loops, adding to system cost and complexity.Brush-type permanent-magnet (PM) DC servomotors have woundarmatures or rotors that rotate within the magnetic field produced by aPM stator.
As the rotor turns, current is applied sequentially to the appropriate armature windings by a mechanical commutator consisting of twoor more brushes sliding on a ring of insulated copper segments. Thesemotors are quite mature, and modern versions can provide very high performance for very low cost.There are variations of the brush-type DC servomotor with its ironcore rotor that permit more rapid acceleration and deceleration because oftheir low-inertia, lightweight cup- or disk-type armatures. The disk-typearmature of the pancake-frame motor, for example, has its mass concentrated close to the motor’s faceplate permitting a short, flat cylindricalhousing. This configuration makes the motor suitable for faceplatemounting in restricted space, a feature particularly useful in industrialrobots or other applications where space does not permit the installationof brackets for mounting a motor with a longer length dimension.The brush-type DC motor with a cup-type armature also offers lowerweight and inertia than conventional DC servomotors.
However, the tradeoff in the use of these motors is the restriction on their duty cycles becauseChapter 1Motor and Motion Control Systemsthe epoxy-encapsulated armatures are unable to dissipate heat buildup aseasily as iron-core armatures and are therefore subject to damage ordestruction if overheated.However, any servomotor with brush commutation can be unsuitablefor some applications due to the electromagnetic interference (EMI)caused by brush arcing or the possibility that the arcing can ignite nearbyflammable fluids, airborne dust, or vapor, posing a fire or explosion hazard. The EMI generated can adversely affect nearby electronic circuitry.In addition, motor brushes wear down and leave a gritty residue that cancontaminate nearby sensitive instruments or precisely ground surfaces.Thus brush-type motors must be cleaned constantly to prevent the spreadof the residue from the motor.
Also, brushes must be replaced periodically, causing unproductive downtime.Brushless DC PM motors overcome these problems and offer the benefits of electronic rather than mechanical commutation. Built as insideout DC motors, typical brushless motors have PM rotors and wound stator coils. Commutation is performed by internal noncontact Hall-effectdevices (HEDs) positioned within the stator windings. The HEDs arewired to power transistor switching circuitry, which is mounted externallyin separate modules for some motors but is mounted internally on circuitcards in other motors.
Alternatively, commutation can be performed by acommutating encoder or by commutation software resident in the motioncontroller or motor drive.Brushless DC motors exhibit low rotor inertia and lower winding thermal resistance than brush-type motors because their high-efficiency magnets permit the use of shorter rotors with smaller diameters. Moreover,because they are not burdened with sliding brush-type mechanical contacts, they can run at higher speeds (50,000 rpm or greater), providehigher continuous torque, and accelerate faster than brush-type motors.Nevertheless, brushless motors still cost more than comparably ratedbrush-type motors (although that price gap continues to narrow) and theirinstallation adds to overall motion control system cost and complexity.Table 1-1 summarizes some of the outstanding characteristics of stepper,PM brush, and PM brushless DC motors.The linear motor, another drive alternative, can move the loaddirectly, eliminating the need for intermediate motion translation mechanism.
These motors can accelerate rapidly and position loads accuratelyat high speed because they have no moving parts in contact with eachother. Essentially rotary motors that have been sliced open and unrolled,they have many of the characteristics of conventional motors. They canreplace conventional rotary motors driving leadscrew-, ballscrew-, orbelt-driven single-axis stages, but they cannot be coupled to gears thatcould change their drive characteristics. If increased performance is1718Chapter 1Motor and Motion Control SystemsTable 1-1 Stepping and Permanent-Magnet DC ServomotorsCompared.required from a linear motor, the existing motor must be replaced with alarger one.Linear motors must operate in closed feedback loops, and they typically require more costly feedback sensors than rotary motors.
In addition, space must be allowed for the free movement of the motor’s powercable as it tracks back and forth along a linear path. Moreover, theirapplications are also limited because of their inability to dissipate heat asreadily as rotary motors with metal frames and cooling fins, and theexposed magnetic fields of some models can attract loose ferrousobjects, creating a safety hazard.Motor Drivers (Amplifiers)Motor drivers or amplifiers must be capable of driving their associatedmotors—stepper, brush, brushless, or linear. A drive circuit for a steppermotor can be fairly simple because it needs only several power transistors to sequentially energize the motor phases according to the numberof digital step pulses received from the motion controller. However,more advanced stepping motor drivers can control phase current to permit “microstepping,” a technique that allows the motor to position theload more precisely.Servodrive amplifiers for brush and brushless motors typically receiveanalog voltages of ±10-VDC signals from the motion controller.
Thesesignals correspond to current or voltage commands. When amplified, thesignals control both the direction and magnitude of the current in theChapter 1Motor and Motion Control Systemsmotor windings. Two types of amplifiers are generally used in closedloop servosystems: linear and pulse-width modulated (PWM).Pulse-width modulated amplifiers predominate because they are moreefficient than linear amplifiers and can provide up to 100 W.
The transistors in PWM amplifiers (as in PWM power supplies) are optimized forswitchmode operation, and they are capable of switching amplifier output voltage at frequencies up to 20 kHz. When the power transistors areswitched on (on state), they saturate, but when they are off, no current isdrawn. This operating mode reduces transistor power dissipation andboosts amplifier efficiency. Because of their higher operating frequencies, the magnetic components in PWM amplifiers can be smaller andlighter than those in linear amplifiers. Thus the entire drive module canbe packaged in a smaller, lighter case.By contrast, the power transistors in linear amplifiers are continuouslyin the on state although output power requirements can be varied. Thisoperating mode wastes power, resulting in lower amplifier efficiencywhile subjecting the power transistors to thermal stress.
However, linearamplifiers permit smoother motor operation, a requirement for some sensitive motion control systems. In addition linear amplifiers are better atdriving low-inductance motors. Moreover, these amplifiers generate lessEMI than PWM amplifiers, so they do not require the same degree of filtering. By contrast, linear amplifiers typically have lower maxi-mumpower ratings than PWM amplifiers.Feedback SensorsPosition feedback is the most common requirement in closed-loopmotion control systems, and the most popular sensor for providing thisinformation is the rotary optical encoder. The axial shafts of theseencoders are mechanically coupled to the drive shafts of the motor. Theygenerate either sine waves or pulses that can be counted by the motioncontroller to determine the motor or load position and direction of travelat any time to permit precise positioning.
Analog encoders produce sinewaves that must be conditioned by external circuitry for counting, butdigital encoders include circuitry for translating sine waves into pulses.Absolute rotary optical encoders produce binary words for themotion controller that provide precise position information. If they arestopped accidentally due to power failure, these encoders preserve thebinary word because the last position of the encoder code wheel acts asa memory.Linear optical encoders, by contrast, produce pulses that are proportional to the actual linear distance of load movement. They work on the1920Chapter 1Motor and Motion Control Systemssame principles as the rotary encoders, but the graduations are engravedon a stationary glass or metal scale while the read head moves along thescale.Tachometers are generators that provide analog signals that aredirectly proportional to motor shaft speed.
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