New Developments in Commutation and Motor Control Techniques

Chuck Lewin, President & CEO of Performance Motion Devices (download as PDF)

Introduction

The field of motion control is not generally known for its headline-grabbing breakthroughs or fast-paced developments. Motor designs tend to evolve slowly, and engineers are understandably cautious when it comes to changing time-honored approaches. Lately though, there has been a beehive of activity around the use of advanced current control techniques, particularly field oriented control (FOC). This math-intensive technique for controlling brushless DC and AC induction motors has become a major focus of the motion control industry due to its potential for improved performance and lower energy consumption.

Field oriented control is a significant improvement over the standard approach for brushless DC motors of trapezoidal Hall-based commutation, and it has a speed range advantage over the more sophisticated technique of sinusoidal commutation. For AC induction motors, FOC is a significant improvement over standard variable speed drive techniques, and is a cousin of flux vector control, which is a somewhat similar technique for controlling inexpensive 3-phase AC induction motors to get them to perform as if they were more expensive brushless DC motors. In fact, many vendors use these two terms interchangeably.

Compared to other servo motor types such as DC brush, which are still used in a number of important applications, brushless DC and AC induction motors provide greater power density, much greater reliability, and in the case of the AC induction motor, lower cost. To gain full access to these capabilities, motion designers are utilizing fast algorithm platforms in the form of DSP (digital signal processors) and specialized microprocessors to improve performance, and increase efficiency.

In brushless DC motor applications, higher performance means smoother motion and greater operating speed. The potential for AC induction motors is even more exciting. Compared to simple “all on/off ” control, FOC means that motors can be run more efficiently, sized more optimally, and operated with less heat generation. It also allows features such as direction reversal, and may allow elimination of external hardware such as brakes or clutches. Considering that 60–65% of all energy in the US is used to drive electric motors, it is no wonder that the marketplace is increasingly demanding more efficient motors.