AC Servo Amplifiers

The amplifier for AC three-phase motors includes a pulse-width modulation circuit for voltage, current and frequency control. The transistors in this amplifier are connected in an H-bridge configuration, and the motor windings are connected as a three-phase wye with no external wires connected to the wye point.

The AC Servo Amplifier has velocity amplifier that get the original command signal for both the amplifier and the velocity feedback. The op amp provides an output that represents the difference between the command and the feedback signals. The output from this amp is sent to the drive logic and PWM circuit block where the output acts as the command signal. The PWM switching controller and drive logic have six circuits for each of the six transistors.

The current-sensing part of the amplifier uses a recirculating chopper system. This controls the current similar to the chopper circuit in DC amplifiers. These signals come from the voltage that develops across the series resistors connected between the transistor section and the motors. The position encoder provides feedback signals; this means that the velocity and position amplifiers are actually a closed-loop system within another closed-loop system. To get best torque response and smoothest acceleration and deceleration, the gain for each of these amplifiers must be turned.

Ultra-High Speed Brushless Motors and Generators

The reluctance motor principle has been improved because of the new range of high efficiency reluctance machine for various applications. This allows reluctance machines to attain high speeds without problems of self-destruction. Additionally, Pulsed Synchronous Reluctance Machines' controllability are simpler than the majority of inverted fed AC machines.

•  A starter, or generator, for a micro-gas turbine has typical shaft speeds of up to 80,000 rpm with an output power of 3kW.
•  The physical robustness of the rotor and low cost construction are key features in the design of this prototype.

DC Motors

Limited-Angle Torque Motors
The limited-angle torquer (LAT) is a special type of brushless motor that produces torque through a rotation angle of less than 180 °. They are also used to operate servovalves, direct laser mirrors, position missile-guidance radar antennas, open shutters for heat-seeking sensors, as well as other systems that rotate through small angles.

The LATs rotor carries field magnets and the stator supports armature windings. This is similar to the construction of conventional brushless motors, but the LATs are wound single phase, unlike usual brushless types which are typically wound for two or three-phase operations. The single-phase construction of LATs eliminates the need for commutation circuitry.

•  When conventional three-phase brushless motors are used as limited-angle torquers, two of the three leads are used.
•  Conventional brushless motors can be used for limited-angle service
•  Armature windings in some limited-angle torquers are embedded in slots around the inside periphery of a laminated stator, a construction similar to that used with conventional brushless motors.
•  Some stators are laminated and others are solid.

Because a larger number of conductors can be exposed to the magnetic field, Slot-sound LATs exhibit higher motor constant K m than corresponding toroidally wound types. Heat is more easily conducted from the armature core to the outer housing in slot-wound LATs than in toroidal versions, which rely only on the mounting tabs for heat conduction. Thus, Slot-wound types are generally able to carry heavier loads than corresponding toroidally wound motors. Slot-wound LATs, however, exhibit more torque ripple (cogging) and generate greater friction and hysteresis losses.

•  Cogging is essentially zero in toroidally wound LATs, a result of non-varying reluctance path and relatively large air gaps.
•  Toroidally wound armatures are typically molded onto the stator which protects the windings from damage and holds them in place.
•  Toroidally wound LATs are suitable for use as limited-angle tachometers because of uniform reluctance paths.
•  The motors are often used in pairs, one as a torquer and the other as a tachometer which provides a reference speed signal for the motor-control circuit.
•  LATs produce torque through a rotation angle determined by the number of motor poles. Current of one polarity produces clockwise torque, and vice versa.

The typical characteristic curve for LATs is represented by the positive lobe of a cosine function that is

T = T p cos(? N /2)

Where ? = angle of rotation and N = number of poles .

This equation approximates torque values only for the roll-off portions of the curves. The actual torque-position characteristics may vary somewhat from that shown in the curves. The curves do not reflect the effects of armature reaction which depends on both armature current level and field magnet. A similar curve can be used to show the general torque characteristic for toroidally wound motors, but it may also have a flat portion.

The rotational range of a LAT is generally specified in terms of an excursion angle which represents the difference between the rotor position that produces maximum torque and the zero-torque point on the characteristic curve. The limited-angle torquers are generally specified with a set of factors similar to those used for conventional brushless motors. The performance of the motor is determined with an identical set of equations.

Limited-angle torquers are available in ratings from 2.8 to 1,000 oz-in

The 2.8 oz-in limited-angle torquer has

LATs that have much lower and higher torque ratings or excursion angles smaller than 18° are feasible, but the maximum possible excursion is 180°.

LATs are generally controlled through single-phase servoamplifiers. Single-phase PWM amplifiers are widely used for the application, but LATs rated up to a few hundred watts are more often powered by linear amplifiers. In this range, linear amplifiers are often simpler and less costly than PWM types.

Other Configurations

Some brushless motors have a cup-shaped rotor that rotates around a wound stator. These "inside-out" motors power spindles in hard disk drives, some high-speed air-conditioning and ventilation systems, and in other equipment calling for high inertia. Very precise speed regulations are made possible by the high inertia.

Applications that need high torque and low speed need brushless motors with a large number of poles - in some cases up to 64. The magnetic gearing, or arrangement, is an alternative to speed reducers in slow equipment and eliminates the friction, stiction, compliance, and backlash that speed-reducing systems normally exhibit.

•  Ring motors, also known as ring torquers, are high pole-count motors that exhibit low torque-ripple.

Pancake motors are a type of multipole motors that often power robots, transfer machines and other equipment that needs high torque at moderate speeds. The speed of a pancake motor is constant with peak torque up to about 67%, at which point speeds begin to fall. These motors have:

•  a large diameter ring magnet which contains from 8 - 16 poles
•  two windings
•  a disk-shaped rotor

Permanent-magnet , disc-type stepping motors can also perform like dc brushless motors. They exhibit high torque and low inertia resulting in a high power rate, and eddy-current and hysteresis losses are low allowing operation at high speeds. Disc motors are appreciably smaller and lighter than conventional types for a given power output.

Permanent-magnetic steppers also operate like dc brushless motors. The motors develop more torque than hybrid steppers, and at speeds up to about 3,500 rpm, they produce more torque than dc servomotors. Permanent-magnetic steppers contain a high-resolution position resolver that costs little and imposes no size penalty.

Magnetless version

Variable-reluctance (VR) or switched-reluctance (SR) motors can operate as brushless dc motors.

VR motors have salient poles on a soft-iron rotor. Interaction between the rotor poles and rotating field set up by the stator windings results in motor action. High torque-to-inertia ratios are yielded through these motors' construction. VR motors cost less than corresponding permanent-magnet brushless types, and because the variable-reluctance motors call for unidirectional current, amplifiers for them cost less than those for conventional types.

These motors are increasingly being used in motion-control systems that require high torque or high horsepower where the cost of magnets in conventional motors becomes excessive. The 150-pole version of a variable-reluctance motor is widely used for powering robots and other machines calling for slow speed and precise positioning. A 32-bit microprocessor-based adaptive controller adjusts frequency response in real time. These motors also exhibit a much wider bandwidth - typically over 80Hz - than conventional motors.

The Servo Drive

  Servo Control

Servo Control is the regulation of velocity and position of a motor based on a feedback signal. The most basic servo loop is the velocity loop which produces a torque command in order to minimize the error between velocity command and velocity feedback.

Most servo systems require position control and velocity control. The most common way to provide position control is to add a position loop in "cascade" or series with a velocity loop. Though sometimes a single PID position loop is used to provide position and velocity control without an explicit velocity loop.

Servo loops have to be "tuned" for each application. Tuning is the process of setting servo gains. Higher servo gains provide higher levels of performance, but they move the system closer to instability. Low-pass filters, which must be tuned at the same time as the servo loops, are commonly used in series with the velocity loop to reduce high-frequency stability problems.

Some drive manufacturers provide advanced control algorithms to deal with demanding applications. The algorithms are necessary in some cases because the mechanics of the system do not allow the use of standard servo loops or because the performance requirements of the application may not be satisfied with standard servo control loops.

Motor Control

The process of producing actual torque in response to the torque command from the servo control loop is the motor control process. For brush motors, motor control is simply the control of current in motor winding because the torque produced by the motor is approximately proportional to the current in the winding.

Most industrial servo controllers rely on current loops. Current loops are similar in structure to velocity loops, but they operate at much higher frequencies. A current loop takes a current command and compares it to a current feedback signal and generates an output which is essentially a voltage command. If the system needs more torque, the current loop responds by increasing the voltage applied to the motor until the right among of current is produced.

Tuning current loops is complicated so manufacturers usually tune current loops for motors.

  Power Conversion

•  Servo drivers provide power to the motor.

•  Control algorithms rely on the ability of the power stage to produce the current that will make the torque that will satisfy the speed and position loops.

•  Power transistors provide current to the motor windings through a process called modulation.

The amount of power that can be delivered to the motor is a function of the voltage applied and current rating of the drive

For more information vist. www.automotioninc.com