Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 12

Each of these linear servomotors has characteristics and features that are optimal in different applicationsThe coils of steel-core motors are wound on silicon steel to maximizethe generated force available with a single-sided magnet assembly orway. Figure 1-28 shows a steel-core brushless linear motor. The steel inthese motors focuses the magnetic flux to produce very high force density.

The magnet assembly consists of rare-earth bar magnets mountedon the upper surface of a steel base plate arranged to have alternatingpolarities (i.e., N, S, N, S)Chapter 1Motor and Motion Control Systems33Figure 1-28 A linear iron-corelinear servomotor consists of amagnetic way and a mating coilassembly.The steel in the cores is attracted to the permanent magnets in a direction that is perpendicular (normal) to the operating motor force. Themagnetic flux density within the air gap of linear motors is typically several thousand gauss.

A constant magnetic force is present whether or notthe motor is energized. The normal force of the magnetic attraction canbe up to ten times the continuous force rating of the motor. This flux rapidly diminishes to a few gauss as the measuring point is moved a fewcentimeters away from the magnets.Cogging is a form of magnetic “detenting” that occurs in both linearand rotary motors when the motor coil’s steel laminations cross the alternating poles of the motor’s magnets. Because it can occur in steel-coremotors, manufacturers include features that minimize cogging. The highthrust forces attainable with steel-core linear motors permit them toaccelerate and move heavy masses while maintaining stiffness duringmachining or process operations.The features of epoxy-core or ironless-core motors differ from thoseof the steel-core motors.

For example, their coil assemblies are woundand encapsulated within epoxy to form a thin plate that is inserted in theair gap between the two permanent-magnet strips fastened inside themagnet assembly, as shown in Figure 1-29. Because the coil assembliesdo not contain steel cores, epoxy-core motors are lighter than steel-coremotors and less subject to cogging.The strip magnets are separated to form the air gap into which the coilassembly is inserted. This design maximizes the generated thrust forceand also provides a flux return path for the magnetic circuit.

Con-34Chapter 1Motor and Motion Control SystemsFigure 1-29 A linear ironlessservomotor consists of an ironlessmagnetic way and an ironless coilassembly.sequently, very little magnetic flux exists outside the motor, thus minimizing residual magnetic attraction.Epoxy-core motors provide exceptionally smooth motion, makingthem suitable for applications requiring very low bearing friction andhigh acceleration of light loads.

They also permit constant velocity to bemaintained, even at very low speeds.Linear servomotors can achieve accuracies of 0.1 µm. Normal accelerations are 2 to 3 g, but some motors can reach 15 g. Velocities are limited by the encoder data rate and the amplifier voltage.

Normal peakvelocities are from 0.04 in./s (1 mm/s) to about 6.6 ft/s (2 m/s), but thevelocity of some models can exceed 26 ft/s (8 m/s).Ironless linear motors can have continuous force ratings from about 5to 55 lbf (22 to 245 N) and peak force ratings from about 25 to 180 lbf(110 to 800 N). By contrast, iron-core linear motors are available withcontinuous force ratings of about 30 to 1100 lbf (130 to 4900 N) andpeak force ratings of about 60 to 1800 lbf (270 to 8000 N).CommutationThe linear motor windings that are phased 120º apart must be continually switched or commutated to sustain motion. There are two ways tocommutate linear motors: sinusoidal and Hall-effect device (HED), ortrapezoidal.

The highest motor efficiency is achieved with sinusoidalcommutation, while HED commutation is about 10 to 15% less efficient.Chapter 1Motor and Motion Control SystemsIn sinusoidal commutation, the linear encoder that provides positionfeedback in the servosystem is also used to commutate the motor. Aprocess called “phase finding” is required when the motor is turned on,and the motor phases are then incrementally advanced with each encoderpulse. This produces extremely smooth motion. In HED commutation acircuit board containing Hall-effect ICs is embedded in the coil assembly. The HED sensors detect the polarity change in the magnet track andswitch the motor phases every 60º.Sinusoidal commutation is more efficient than HED commutationbecause the coil windings in motors designed for this commutationmethod are configured to provide a sinusoidally shaped back EMF waveform.

As a result, the motors produce a constant force output when thedriving voltage on each phase matches the characteristic back EMFwaveform.Installation of Linear MotorsIn a typical linear motor application the coil assembly is attached to themoving member of the host machine and the magnet assembly ismounted on the nonmoving base or frame. These motors can be mountedvertically, but if they are they typically require a counterbalance systemto prevent the load from dropping if power temporarily fails or is routinely shut off. The counterbalance system, typically formed from pulleys and weights, springs, or air cylinders, supports the load against theforce of gravity.If power is lost, servo control is interrupted. Stages in motion tend tostay in motion while those at rest tend to stay at rest.

The stopping timeand distance depend on the stage’s initial velocity and system friction.The motor’s back EMF can provide dynamic braking, and friction brakescan be used to attenuate motion rapidly. However, positive stops andtravel limits can be built into the motion stage to prevent damage in situations where power or feedback might be lost or the controller or servodriver fail.Linear servomotors are supplied to the customer in kit form formounting on the host machine.

The host machine structure must includebearings capable of supporting the mass of the motor parts while maintaining the specified air gap between the assemblies and also resistingthe normal force of any residual magnetic attraction.Linear servomotors must be used in closed loop positioning systemsbecause they do not include built-in means for position sensing.Feedback is typically supplied by such sensors as linear encoders, laserinterferometers, LVDTs, or linear Inductosyns.3536Chapter 1Motor and Motion Control SystemsAdvantages of Linear vs.

Rotary ServomotorsThe advantages of linear servomotors over rotary servomotors include:• High stiffness: The linear motor is connected directly to the movingload, so there is no backlash and practically no compliance betweenthe motor and the load. The load moves instantly in response tomotor motion.• Mechanical simplicity: The coil assembly is the only moving part ofthe motor, and its magnet assembly is rigidly mounted to a stationarystructure on the host machine. Some linear motor manufacturersoffer modular magnetic assemblies in various modular lengths.

Thispermits the user to form a track of any desired length by stacking themodules end to end, allowing virtually unlimited travel. The forceproduced by the motor is applied directly to the load without anycouplings, bearings, or other conversion mechanisms. The onlyalignments required are for the air gaps, which typically are from0.039 in. (1 mm) to 0.020 in.

(0.5 mm).• High accelerations and velocities: Because there is no physical contact between the coil and magnet assemblies, high accelerations andvelocities are possible. Large motors are capable of accelerations of 3to 5 g, but smaller motors are capable of more than 10 g.• High velocities: Velocities are limited by feedback encoder data rateand amplifier bus voltage.

Normal peak velocities are up to 6.6 ft/s (2m/s), although some models can reach 26 ft/s (8 m/s). This compareswith typical linear speeds of ballscrew transmissions, which are commonly limited to 20 to 30 in./s (0.5 to 0.7 m/s) because of resonancesand wear.• High accuracy and repeatability: Linear motors with position feedback encoders can achieve positioning accuracies of ±1 encodercycle or submicrometer dimensions, limited only by encoder feedback resolution.• No backlash or wear: With no contact between moving parts, linearmotors do not wear out.

This minimizes maintenance and makesthem suitable for applications where long life and long-term peakperformance are required.• System size reduction: With the coil assembly attached to the load,no additional space is required. By contrast, rotary motors typicallyrequire ballscrews, rack-and-pinion gearing, or timing belt drives.• Clean room compatibility: Linear motors can be used in clean roomsbecause they do not need lubrication and do not produce carbonbrush grit.Chapter 1Motor and Motion Control SystemsCoil Assembly Heat DissipationHeat control is more critical in linear motors than in rotary motorsbecause they do not have the metal frames or cases that can act as largeheat-dissipating surfaces.

Some rotary motors also have radiating fins ontheir frames that serve as heatsinks to augment the heat dissipation capability of the frames. Linear motors must rely on a combination of highmotor efficiency and good thermal conduction from the windings to aheat-conductive, electrically isolated mass. For example, an aluminumattachment bar placed in close contact with the windings can aid in heatdissipation.