[Engineering Feature]
Smart Motion Makes For A Smarter Design
Motor control is more than turning a switch on and off. Today's microcontrollers deliver improved performance, efficiency, and control.
If it moves, jumps, rotates, or vibrates, it usually contains a motor. Electric motors come in all types of devices, from tiny hard-disk drives to hybrid vehicles to locomotives. Intelligent motor control can be employed over this wide range of devices, delivering improved efficiency, longer life, and better fault control compared to simply applying power to a motor. Key to greater use of intelligent motor-control systems are low-cost microcontrollers and digital-signal-processing chips that target this market.
Unfortunately, simply throwing a basic microcontroller at the problem is rarely an option. Digital design is hard. Analog design is harder. Combine the two disciplines and you've got the doubly difficult design task of digital motor control. Intensifying the problem is the need for a microcontroller with analog peripherals. On top of that, motor-control-oriented microcontrollers typically include a significant timing component.
Finally, there's the issue of power. Microcontrollers usually sink enough current for only the smallest of motors. Connect one directly to a one-horsepower (HP) electric motor, and the chip will burn out faster than a popping fuse. This is why most microcontroller-based solutions are coupled with external power transistors and often complex power control systems. We'll concentrate on the microcontroller aspects of the system here.
Motor-control microcontrollers must address a wide range of motor types (see "Motor Basics" at www.elecdesign.com, Drill Deeper 11291). A particular microcontroller tends to be tuned to a particular type of motor, such as a brushless dc (BLDC) motor. Given the need to support so many different motors with different performance characteristics, a wide variety of microcontroller solutions exists. Likewise, numerous microcontroller vendors supply motorcontrol microcontrollers (see "Need More Information?" p. 44).
SMART APPLICATIONS Motors and electronics are ubiquitous. As embedded designers, we know that everything from kitchen appliances to MP3 players hosts them. Often, many intelligent motors are used in combination.
For example, take Segway's Human Transporter (HT) (Fig. 1). It uses a pair of Texas Instruments TMS320C2000 DSP platforms for closed-loop motor control and balance computation. This people mover is remarkably easy to ride because the electronics do the real work.
The Human Transporter has a simple design. It has only two moving parts—the wheels—with a fixed axle. The controller board is underfoot, and it includes the power transistors needed for motor control (Fig. 2). It works with the sensor platform, which contains a pair of gyroscopes and three accelerometers (Fig. 3). These sensors, which are checked 100 times/s, are as much a key to the Human Transporter's success as the software.
Dean Kamen, the Human Transporter's inventor, first developed the Independence IBot Mobility System for Independence Technology. This chair has more functionality than the Human Transporter, including the ability to climb stairs and raise the rider to the eye level of a standing adult. It's designed to give the disabled unprecedented mobility.
At the other end of the motor spectrum are tiny, mobile hard disks (see "Mobile Storage: Chips Served With Hard-Disk Salsa" at www.elecdesign.com, ED Online 10184). These devices employ one motor to rotate the disk and another to move the heads. In addition to the normal motor-control chores, the electronics in these tiny devices also use accelerometers to determine whether or not the drive is moving. That's because the heads must be locked down to prevent problems due to shock and vibration.
Designers of kitchen appliances with motors are turning to microcontrollers even for the least expensive products, due to their low cost and improvements to the product. For example, ramping speed up and down increases motor life and overall reliability. It's also easier to add motor-related features with no changes to the hardware.
Hybrid cars are a popular example for regenerative braking. Power recovery also is becoming more important for a range of products. For example, portable devices can extend battery life if it's possible to recover power when a motor is turned off.
SMART CONTROL Motor-control-system configurations are relatively simple and consistent (Fig. 4). The differences typically-involve the number of phases, which dictates the number of power connections on the motor and the number of sensors within the system. Very simple systems don't employ a feedback mechanism.
Systems that employ feedback usually implement a PID ( Proportional, Integral, Derivative) feedback device, based on the relative rotational position of a motor's rotor. Optical sensors, Hall-effect sensors, and sensorless back EMF (electromotive force) systems are the most common feedback mechanisms.
Optical sensors use a rotating disk with slots that can be detected. The optical sensor digital outputs are fed into a quadrature encoder found on many microcontrollers. The encoder provides tachometer information as well as rotational direction.
Hall-effect sensors detect magnets mounted in the rotor or shaft. They provide positional and tachometer information via comparators.
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