Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 13

Moreover, the carriage plate to which the coil assembly isattached must have effective heat-sinking capability.Stepper MotorsA stepper or stepping motor is an AC motor whose shaft is indexedthrough part of a revolution or step angle for each DC pulse sent to it.Trains of pulses provide input current to the motor in increments that can“step” the motor through 360º, and the actual angular rotation of theshaft is directly related to the number of pulses introduced. The positionof the load can be determined with reasonable accuracy by counting thepulses entered.The stepper motors suitable for most open-loop motion control applications have wound stator fields (electromagnetic coils) and iron or permanent magnet (PM) rotors.

Unlike PM DC servomotors with mechanical brush-type commutators, stepper motors depend on externalcontrollers to provide the switching pulses for commutation. Steppermotor operation is based on the same electromagnetic principles ofattraction and repulsion as other motors, but their commutation providesonly the torque required to turn their rotors.Pulses from the external motor controller determine the amplitude anddirection of current flow in the stator’s field windings, and they can turnthe motor’s rotor either clockwise or counterclockwise, stop and start itquickly, and hold it securely at desired positions. Rotational shaft speeddepends on the frequency of the pulses.

Because controllers can stepmost motors at audio frequencies, their rotors can turn rapidly.Between the application of pulses when the rotor is at rest, its armature will not drift from its stationary position because of the steppermotor’s inherent holding ability or detent torque. These motors generatevery little heat while at rest, making them suitable for many differentinstrument drive-motor applications in which power is limited.3738Chapter 1Motor and Motion Control SystemsThe three basic kinds of stepper motors are permanent magnet, variable reluctance, and hybrid. The same controller circuit can drive bothhybrid and PM stepping motors.Permanent-Magnet (PM) Stepper MotorsPermanent-magnet stepper motors have smooth armatures and include apermanent magnet core that is magnetized widthwise or perpendicularto its rotation axis.

These motors usually have two independent windings, with or without center taps. The most common step angles for PMmotors are 45 and 90º, but motors with step angles as fine as 1.8º perstep as well as 7.5, 15, and 30º per step are generally available.Armature rotation occurs when the stator poles are alternately energizedand deenergized to create torque. A 90º stepper has four poles and a 45ºstepper has eight poles, and these poles must be energized in sequence.Permanent-magnet steppers step at relatively low rates, but they canproduce high torques and they offer very good damping characteristics.Variable Reluctance Stepper MotorsVariable reluctance (VR) stepper motors have multitooth armatures witheach tooth effectively an individual magnet. At rest these magnets alignthemselves in a natural detent position to provide larger holding torquethan can be obtained with a comparably rated PM stepper.

Typical VRmotor step angles are 15 and 30º per step. The 30º angle is obtained witha 4-tooth rotor and a 6-pole stator, and the 15º angle is achieved with an8-tooth rotor and a 12-pole stator. These motors typically have threewindings with a common return, but they are also available with four orfive windings. To obtain continuous rotation, power must be applied tothe windings in a coordinated sequence of alternately deenergizing andenergizing the poles.If just one winding of either a PM or VR stepper motor is energized,the rotor (under no load) will snap to a fixed angle and hold that angleuntil external torque exceeds the holding torque of the motor. At thatpoint, the rotor will turn, but it will still try to hold its new position ateach successive equilibrium point.Hybrid Stepper MotorsThe hybrid stepper motor combines the best features of VR and PM stepper motors.

A cutaway view of a typical industrial-grade hybrid stepperChapter 1Motor and Motion Control Systems39Figure 1-30 Cutaway view of a5-phase hybrid stepping motor. Apermanent magnet is within therotor assembly, and the rotor segments are offset from each otherby 3.5°.motor with a multitoothed armature is shown in Figure 1-30. The armature is built in two sections, with the teeth in the second section offsetfrom those in the first section.

These motors also have multitoothed stator poles that are not visible in the figure. Hybrid stepper motors canachieve high stepping rates, and they offer high detent torque and excellent dynamic and static torque.Hybrid steppers typically have two windings on each stator pole sothat each pole can become either magnetic north or south, depending oncurrent flow. A cross-sectional view of a hybrid stepper motor illustrating the multitoothed poles with dual windings per pole and the multitoothed rotor is illustrated in Figure 1-31. The shaft is represented by thecentral circle in the diagram.The most popular hybrid steppers have 3- and 5-phase wiring, andstep angles of 1.8 and 3.6º per step.

These motors can provide moretorque from a given frame size than other stepper types because either allor all but one of the motor windings are energized at every point in thedrive cycle. Some 5-phase motors have high resolutions of 0.72° per step(500 steps per revolution). With a compatible controller, most PM andhybrid motors can be run in half-steps, and some controllers are designedto provide smaller fractional steps, or microsteps. Hybrid stepper motorscapable of a wide range of torque values are available commercially.This range is achieved by scaling length and diameter dimensions.40Chapter 1Motor and Motion Control SystemsFigure 1-31 Cross-section of ahybrid stepping motor showingthe segments of the magneticcore rotor and stator poles withits wiring diagram.Hybrid stepper motors are available in NEMA size 17 to 42 frames, andoutput power can be as high as 1000 W peak.Stepper Motor ApplicationsMany different technical and economic factors must be considered inselecting a hybrid stepper motor.

For example, the ability of the steppermotor to repeat the positioning of its multitoothed rotor depends on itsgeometry. A disadvantage of the hybrid stepper motor operating openloop is that, if overtorqued, its position “memory” is lost and the systemmust be reinitialized. Stepper motors can perform precise positioning insimple open-loop control systems if they operate at low accelerationrates with static loads. However, if higher acceleration values arerequired for driving variable loads, the stepper motor must be operated ina closed loop with a position sensor.Chapter 1Motor and Motion Control Systems41DC and AC Motor Linear ActuatorsActuators for motion control systems are available in many different forms,including both linear and rotary versions.

One popular configuration is thatof a Thomson Saginaw PPA, shown in section view in Figure 1-32. Itconsists of an AC or DC motor mounted parallel to either a ballscrewor Acme screw assembly through a reduction gear assembly with a slipclutch and integral brake assembly. Linear actuators of this type canperform a wide range of commercial, industrial, and institutionalapplications.One version designed for mobile applications can be powered by a 12, 24-, or 36-VDC permanent-magnet motor.

These motors are capable ofperforming such tasks as positioning antenna reflectors, opening andclosing security gates, handling materials, and raising and lowering scissors-type lift tables, machine hoods, and light-duty jib crane arms.Other linear actuators are designed for use in fixed locations whereeither 120- or 220-VAC line power is available. They can have either ACor DC motors. Those with 120-VAC motors can be equipped withoptional electric brakes that virtually eliminate coasting, thus permittingpoint-to-point travel along the stroke.Where variable speed is desired and 120-VAC power is available, alinear actuator with a 90-VDC motor can be equipped with a solid-staterectifier/speed controller.

Closed-loop feedback provides speed regulation down to one tenth of the maximum travel rate. This feedback systemcan maintain its selected travel rate despite load changes.Figure 1-32 This linear actuatorcan be powered by either an ACor DC motor. It containsballscrew, reduction gear, clutch,and brake assemblies. Courtesy ofThomson Saginaw.42Chapter 1Motor and Motion Control SystemsThomson Saginaw also offers its linear actuators with either Halleffect or potentiometer sensors for applications where it is necessary ordesirable to control actuator positioning. With Hall-effect sensing, sixpulses are generated with each turn of the output shaft during which thestroke travels approximately 1⁄32 in.

(0.033 in. or 0.84 mm). These pulsescan be counted by a separate control unit and added or subtracted fromthe stored pulse count in the unit’s memory. The actuator can be stoppedat any 0.033-in. increment of travel along the stroke selected by programming. A limit switch can be used together with this sensor.If a 10-turn, 10,000-ohm potentiometer is used as a sensor, it can bedriven by the output shaft through a spur gear. The gear ratio is established to change the resistance from 0 to 10,000 ohms over the length ofthe actuator stroke. A separate control unit measures the resistance (orvoltage) across the potentiometer, which varies continuously and linearly with stroke travel.

The actuator can be stopped at any positionalong its stroke.Stepper-Motor Based Linear ActuatorsLinear actuators are available with axial integral threaded shafts and boltnuts that convert rotary motion to linear motion. Powered by fractionalhorsepower permanent-magnet stepper motors, these linear actuators arecapable of positioning light loads. Digital pulses fed to the actuatorcause the threaded shaft to rotate, advancing or retracting it so that a loadcoupled to the shaft can be moved backward or forward.