Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 11

Although thisgeometry is required for brushless DC motors, some manufacturers haveadapted this design for brush-type DC motors.The mechanical brush and bar commutator of the brushless DCmotor is replaced by electronic sensors, typically Hall-effect devices(HEDs). They are located within the stator windings and wired to solidstate transistor switching circuitry located either on circuit cardsmounted within the motor housings or in external packages.

Generally,only fractional horsepower brushless motors have switching circuitrywithin their housings.The cylindrical magnet rotors of brushless DC motors are magnetizedlaterally to form opposing north and south poles across the rotor’s diameter. These rotors are typically made from neodymium–iron–boron orsamarium–cobalt rare-earth magnetic materials, which offer higher fluxdensities than alnico magnets. These materials permit motors offeringhigher performance to be packaged in the same frame sizes as earliermotor designs or those with the same ratings to be packaged in smallerframes than the earlier designs.

Moreover, rare-earth or ceramic magnetChapter 1Motor and Motion Control Systems27Figure 1-22 Cutaway view of abrushless DC motor.rotors can be made with smaller diameters than those earlier models withalnico magnets, thus reducing their inertia.A simplified diagram of a DC brushless motor control with one Halleffect device (HED) for the electronic commutator is shown inFigure 1-23. The HED is a Hall-effect sensor integrated with an ampli-Figure 1-23 Simplified diagramof Hall-effect device (HED) commutation of a brushless DCmotor.28Chapter 1Motor and Motion Control SystemsFigure 1-24 Exploded view of abrushless DC motor withHall-effect device (HED)commutation.fier in a silicon chip.

This IC is capable of sensing the polarity of therotor’s magnetic field and then sending appropriate signals to powertransistors T1 and T2 to cause the motor’s rotor to rotate continuously.This is accomplished as follows:1. With the rotor motionless, the HED detects the rotor’s north magnetic pole, causing it to generate a signal that turns on transistor T2.This causes current to flow, energizing winding W2 to form a southseeking electromagnetic rotor pole.

This pole then attracts therotor’s north pole to drive the rotor in a counterclockwise (CCW)direction.2. The inertia of the rotor causes it to rotate past its neutral position sothat the HED can then sense the rotor’s south magnetic pole. It thenswitches on transistor T1, causing current to flow in winding W1,thus forming a north-seeking stator pole that attracts the rotor’ssouth pole, causing it to continue to rotate in the CCW direction.The transistors conduct in the proper sequence to ensure that the excitation in the stator windings W2 and W1 always leads the PM rotor fieldto produce the torque necessary keep the rotor in constant rotation. Thewindings are energized in a pattern that rotates around the stator.There are usually two or three HEDs in practical brushless motors thatare spaced apart by 90 or 120º around the motor’s rotor. They send thesignals to the motion controller that actually triggers the power transistors, which drive the armature windings at a specified motor current andvoltage level.The brushless motor in the exploded view Figure 1-24 illustrates adesign for a miniature brushless DC motor that includes Hall-effect com-Chapter 1Motor and Motion Control Systems29mutation.

The stator is formed as an ironless sleeve of copper coilsbonded together in polymer resin and fiberglass to form a rigid structuresimilar to cup-type rotors. However, it is fastened inside the steel laminations within the motor housing.This method of construction permits a range of values for starting current and specific speed (rpm/V) depending on wire gauge and the number of turns. Various terminal resistances can be obtained, permitting theuser to select the optimum motor for a specific application. The Halleffect sensors and a small magnet disk that is magnetized widthwise aremounted on a disk-shaped partition within the motor housing.Position Sensing in Brushless MotorsBoth magnetic sensors and resolvers can sense rotor position in brushless motors.

The diagram in Figure 1-25 shows how three magnetic sensors can sense rotor position in a three-phase electronically commutatedbrushless DC motor. In this example the magnetic sensors are locatedinside the end-bell of the motor. This inexpensive version is adequate forsimple controls.In the alternate design shown in Figure 1-26, a resolver on the end capof the motor is used to sense rotor position when greater positioningaccuracy is required. The high-resolution signals from the resolver canFigure 1-25 A magnetic sensoras a rotor position indicator: stationary brushless motor winding(1), permanent-magnet motorrotor (2), three-phase electronically commutated field (3), threemagnetic sensors (4), and theelectronic circuit board (5).30Chapter 1Motor and Motion Control SystemsFigure 1-26 A resolver as arotor position indicator: stationary motor winding (1), permanent-magnet motor rotor (2),three-phase electronically commutated field (3), three magneticsensors (4), and the electronic circuit board (5).be used to generate sinusoidal motor currents within the motor controller.

The currents through the three motor windings are position independent and respectively 120º phase shifted.Brushless Motor AdvantagesBrushless DC motors have at least four distinct advantages over brushtype DC motors that are attributable to the replacement of mechanicalcommutation by electronic commutation.• There is no need to replace brushes or remove the gritty residuecaused by brush wear from the motor.• Without brushes to cause electrical arcing, brushless motors do notpresent fire or explosion hazards in an environment where flammableor explosive vapors, dust, or liquids are present.• Electromagnetic interference (EMI) is minimized by replacingmechanical commutation, the source of unwanted radio frequencies,with electronic commutation.• Brushless motors can run faster and more efficiently with electroniccommutation. Speeds of up to 50,000 rpm can be achieved vs.

theupper limit of about 5000 rpm for brush-type DC motors.Chapter 1Motor and Motion Control SystemsBrushless DC Motor DisadvantagesThere are at least four disadvantages of brushless DC servomotors.• Brushless PM DC servomotors cannot be reversed by simply reversing the polarity of the power source. The order in which the currentis fed to the field coil must be reversed.• Brushless DC servomotors cost more than comparably rated brushtype DC servomotors.• Additional system wiring is required to power the electronic commutation circuitry.• The motion controller and driver electronics needed to operate abrushless DC servomotor are more complex and expensive than thoserequired for a conventional DC servomotor.Consequently, the selection of a brushless motor is generally justifiedon a basis of specific application requirements or its hazardous operatingenvironment.Characteristics of Brushless Rotary ServomotorsIt is difficult to generalize about the characteristics of DC rotary servomotors because of the wide range of products available commercially.However, they typically offer continuous torque ratings of 0.62 lb-ft(0.84 N-m) to 5.0 lb-ft (6.8 N-m), peak torque ratings of 1.9 lb-ft (2.6N-m) to 14 lb-ft (19 N-m), and continuous power ratings of 0.73 hp(0.54 kW) to 2.76 hp (2.06 kW).

Maximum speeds can vary from 1400to 7500 rpm, and the weight of these motors can be from 5.0 lb (2.3 kg)to 23 lb (10 kg). Feedback typically can be either by resolver orencoder.Linear ServomotorsA linear motor is essentially a rotary motor that has been opened out intoa flat plane, but it operates on the same principles. A permanent-magnetDC linear motor is similar to a permanent-magnet rotary motor, and anAC induction squirrel cage motor is similar to an induction linear motor.The same electromagnetic force that produces torque in a rotary motoralso produces torque in a linear motor. Linear motors use the same controls and programmable position controllers as rotary motors.3132Chapter 1Motor and Motion Control SystemsFigure 1-27 Operating principles of a linear servomotor.Before the invention of linear motors, the only way to produce linearmotion was to use pneumatic or hydraulic cylinders, or to translate rotarymotion to linear motion with ballscrews or belts and pulleys.A linear motor consists of two mechanical assemblies: coil and magnet, as shown in Figure 1-27.

Current flowing in a winding in a magneticflux field produces a force. The copper windings conduct current (I ), andthe assembly generates magnetic flux density (B). When the current andflux density interact, a force (F ) is generated in the direction shown inFigure 1-27, where F = I × B.Even a small motor will run efficiently, and large forces can be createdif a large number of turns are wound in the coil and the magnets are powerful rare-earth magnets. The windings are phased 120 electrical degreesapart, and they must be continually switched or commutated to sustainmotion.Only brushless linear motors for closed-loop servomotor applicationsare discussed here. Two types of these motors are available commercially—steel-core (also called iron-core) and epoxy-core (also calledironless).