Toshio Takahashi
Director of Engineering
Digital Control IC Design Center
International Rectifier
El Segundo, Calif.

Circuitry for handling highperformance control of ac servos has been condensed onto one chip in the IRMCK201.

There is an incentive to control motor speed in a widening array of applications. Technological advances have now made it possible to devise a single integrated circuit that can handle the details of ac servocontrol, including manipulation of feedback from position encoders. These chips provide closed-loop current and velocity control in highperformance servodrives and sensorless systems. Such chips, designated IRMCK201 and IRMCK203, are implemented in 100 and 80-pin QFP packages, respectively, and require only an inexpensive crystal resonator to feed a 33-MHz clock.

These integrated circuits not only reduce component count but also eliminate complex software programming associated with precision motion control and automation. They also improve the performance of sinusoidal sensorless control by means of a built-in motion-control engine, or MCE.

The functions of such motor controllers can best be understood in the context of the control hardware normally needed to vary the torque and speed of ac servomotors. Thus it can be helpful to first review the component parts of a typical ac servocontroller.

Any servomotor-control system has two cascaded control loops. The outer motion loop controls the motor shaft position and velocity based on signals fed back from a position or velocity sensor. The output of this loop is a command for an increase or decrease in motor torque. This command signal feeds to the inner current loop of the system. The current loop generates signals for the power converter which supplies currents to the motor that produce the desired output torque.

Power from the supply to the motor is controlled by rapidly varying the on/off conduction time of power semiconductor switches such as IGBTs or power MOSFETs. These control signals are typically fixed-frequency, variable duty-cycle waveforms.

One tricky thing about controlling torque in ac motors is that each of the three-phase currents powering the motor must synchronize to the position of the rotating rotor field. A way to accomplish this is to first measure the three stator currents and the motor shaft position, and then use this information to transform the measured stator currents to a reference frame synchronized to the rotor field. The term applied to this transformation is field-oriented control (FOC).

This process results in two equivalent dc-motor current quantities: a torque-producing component and a field-control component. The ac-motor control system calculates the two quadrature voltages that force the current to both directly follow the torque command and maintain a constant rotor field. An inverse transformation is then used to transform the dc-motor voltages back to the stator reference frame to give the required winding voltages.

In practical implementations of ac servos, a resolverto-digital converter is often used to derive a digital version of the motor-shaft position fed back from a shaft-mounted resolver. Motor velocity is calculated from the position of the motor shaft using an estimation algorithm.

An a/d converter digitizes the measured motor stator currents and the resulting signals get passed on as an input to a vector transformation algorithm. Transformation algorithms convert motor currents and voltages between stator and rotor reference frames. The first transformation takes three stator current signals and the rotor electrical angle and calculates the torque and field current components. (In actuality, the calculation uses only two stator currents. It infers the third stator current signal because all three stator currents sum to zero.)

The current loop can be thought of as consisting of two component loops, a torque loop and a field loop. A controller generates the outputs of these loops, often after applying proportional and integral compensation. This is done because response of these loops can be improved by feeding forward the estimated winding backemf and winding impedance drops.

The outputs of the current loop calculations then get transformed in the vector transformation block to digital equivalents of the threephase stator voltages for driving the motor. Specifically, measured stator currents get converted to torque-producing and field-control components through what's called a reverse Park rotation. Similarly, a forward Park rotation followed by an inverse Clark transformation converts the corresponding rotor-referenced voltage to three-phase stator voltages. The voltage values are used to control the timing of digital circuits that set the duty cycle of the pulse-width modulation. The PWM module, in turn, drives the three-phase inverters actually powering the ac servomotor.