DSP Prospects in Motor Control

There are several approaches to motor control. In some cases, the concern is simply to control the speed, acceleration, torque or other characteristic of a motor based on human input, as from a control panel. This manual control is termed open-loop control. In other cases, the motor should automatically respond to real-time stimuli to take some action. The response to the stimuli is monitored, and necessary adjustments are automatically made. Thus, it becomes a closed-loop motor control environment. Such closed-loop controls are called servomechanisms or simply servos.

Servo control is a signal processing process that involves control of a motor based on dynamic input signals like position, velocity or torque. Analog circuitry has been used to implement servo control in the past, but the adaptive capability of DSP is making it the technology of choice for many applications. Since bandwidths for straightforward speed and torque control are relatively narrow, compared to, say, speech coding, MCUs are often fast enough for the DSP processing chores. In some cases, though, they have to rely on look-up tables to approximate results. But MCUs don't have the speed to execute advanced control algorithms needed in the highest-performance hard disk drives, CD-ROMs and DVD drives. However, those disk DSPs are called on to perform more than just motor speed control; they must also provide servo position control of read (and write) mechanisms which are often linear motors or voice coil mechanisms. Adaptive control is necessary to compensate for thermal changes, disk aging and physical acceleration forces.

Other computer DSP servo applications include tape drives and pen plotters. Non-computer servo applications include robot arm/axis control and other motorized position-control functions that require tracking of analog phenomena. Automotive applications of servo control include anti-skid braking (performed mostly with MCUs, but that is likely to change) and even new (non-hydraulic) power steering systems. The military employs servo control for platform stabilization, launcher positioning and missile guidance. Algorithms commonly employed for servo control include Kalman filtering as well as deadbeat, optimal and adaptive control.

Answering the demand for low-frequency motor control capability, MCUs like Motorola's 68HC16 and Intel's 80196 have been beefed up with math enhancements to make them attractive for such DSP operations. The Motorola product includes an on-board parallel multiplier and DSP instructions added to an otherwise conventional 68HC11 (the unit's 8-bit predecessor) instruction set. The Intel 80C196 family keeps getting its MAC capability bumped up. The MAC times for the '196 have gone from 2.3 s to 640 ns and in 1996 (as the new 80296) to 80 ns, finally getting into the DSP chip execution space. Both the Motorola and Intel MCUs mentioned are being employed in disk controller applications, but are also employed for other motor control functions, as well.

DSP has long been employed for stepper motor control and closed-loop DC servo motors. Both MCUs and DSPs have been employed in some high-end three-phase motors, mostly for digital commutation applications; but the single-phase motor market has been a challenge for DSP. But now, DSP application to single-phase AC induction motor control is becoming practical.

To better address the motor-control market, Analog Devices has invested heavily in development of simultaneous sampling A/D converters (for three-phase power systems), a motion coprocessor (the ADMC201) designed for use with ADI's 21xx family of DSP chips and, most recently, the ADMC330 a specialized motor-control IC which includes the 21xx DSP core, a seven-channel A/D (synchronized to the PWM switching frequency to minimize motor current ripple), two auxiliary PWM timers for power factor correction and conventional MCU peripherals. The ADMC330 began sampling in late 1996, with pricing at $10.00 (@100K). ADI has a joint venture with Infosys Technologies (Bangalore, India) that began in 1993 to develop algorithms for AC induction motor control. A number of proprietary algorithms have resulted from the venture, including rotor time compensation and sensorless control. To exploit the motor control market more fully, ADI has set up an independent P&L group to serve this market.

Texas Instruments has also begun to address the industrial motor market. TI is dominant in the hard disk drive controller market, but introduced a new chip targeted for industrial motor control late last year. The TMS320C240 DSP Controller combines the company's 'C2xLP core with a motor-control-optimized event manager, dual A/D converters and MCU-type peripherals. The C240 is priced at $10.00 (@100K), but future reduced-function versions are expected in the $5.00 neighborhood. A flash-memory version is available for development and prototyping purposes. Sampling should now be underway.

The most aggressive developments in DSP for motor control appear to be in Europe and Japan, locations which seem to place high values on smaller, quieter and more efficient motors for household appliances. The European automotive market, for example, is ripe for power steering based on DSP-controlled electric motors. The majority of present European cars have only manual steering mechanisms and they tend to have small engines which lack power for hydraulic power steering in low-speed situations like parking. European gasoline prices are also very high compared to the U.S. and hydraulic power steering exacts a penalty in gas mileage—even when the vehicle is pointed straight ahead on the highway. TRW has indicated that it will make electrically powered steering systems for the automotive industry, a market which can absorb millions of DSPs annually.

But DSP motor control development cannot take place in a vacuum. Consequently, development of motors that are more amenable to DSP control is also taking place. For example, the switched reluctance motor (a brushless DC motor which has no rotor windings and no permanent magnets) controlled by DSP could eventually be the cheapest solution for many applications. Further, due to the simplicity and ruggedness of its rotor, the SR can operate at speeds above that of the induction motor and can increase torque production through electromagnetic gearing. The SR's high energy efficiency and high low-speed torque have created high interest in the technology, but until recently the electronics associated with the SR motor cost as much as the mechanical parts. That made it impractical for high-volume applications. SR motors demand a sophisticated commutation strategy which may be best addressed through DSP chips. With the potential availability of inexpensive DSP chips, the future of SR motors looks very bright.