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Microstepping technology can be used to improve resolution in position-control applications. A drive-control scheme, however, requires consideration of some designs tradeoffs to achieve an optimum design. Discussed are: matching and accuracy requirements for microstepping control; H-bridge power-stage operating modes; the impact of the PWM switching frequency on systems operation; single supply operation; sign-magnitude-vs-bipolar inputs; and undervoltage, overcurrent, and overtemperature protection.

The specifications, functions, and performance of selected integrated circuits (ICs) dedicated to controlling speed and position of stepper and brushless DC motors are analyzed. The motors find key applications in computer peripherals, mostly disk and tape drives and printers. Stepper motors dominate position-control applications. Rifa's PBL3717 is a successful stepper-control motor and driver circuits. Upgraded, high-performance, and smaller versions of the 3717 design are also described. Rifa's PBL3771 and PBM3960 and SGS's L6217 and L6217A can optimize microstepping performance on many standard stepper motors to allow small two-phase stepper motors to be used for precise positioning and smooth operation. Sprague Electric's UCN-5804B translator and driver circuit is designed for four-phase unipolar stepper motors. Brushless DC motors dominate speed-control applications. National Semiconductor's LM621, Motorola's MC33034, Silicon Systems' SSI-590, and Integrated Power Semiconductors' IP3M05 are discussed.

New microstepping-drive techniques allow stepper motors to be used increasingly in computer-peripheral and industrial-robotic applications. These techniques use electronic proportioning to divide a stepper motor's step angle into smaller sub steps. Some microstepping drives can subdivide full-step pulses into as many as 125 microsteps per step. Microstepping provides the following benefits: improved positioning resolution, torque ripple reduction, and, in most cases, elimination of resonance. In the area of positioning resolution, for example, a 200-step-per-revolution motor with a 32-step-per-step microstepping drive can achieve 6,400 steps per revolution. Manufacturers incorporating microstepping drive techniques into their stepper motors are Airpax Corp, Parker-Hannifin Corp, PMI Motion Technologies, and Superior Electric Co.

Controller integrated circuits for computer-peripheral and robotics-systems applications are discussed. The two areas are similar in that they require many motors, have similar power requirements, and have a lack of standards for controllers. Controller ICs fall into two categories. digital controllers are used for stepper motors and analog controllers are used for spindle (dc-commutator) motors. Peripherals such as floppy-disk drives and printers make greatest use of stepper motors. Controllers are suggested for a range of motor applications in both categories. An accompanying diagram and text explains stepper-motor controller and three circuit diagrams are included. Also shown is a list of manufacturers from whom more information is available.

Circuitry is described that is capable of controlling a 2K x 8 EPROM without the aid of a microprocessor. Based on the 2716 EPROM, the circuit only needs motor drivers, a timer, a counter, and a few latches to become operational. A circuit diagram is included.

The CY525 is a newly released third-generation, intelligent ramping motor controller from Cybernetic Micro Systems. Providing control of motor position, direction, speed and acceleration, the device's twenty-seven commands provide a performance of 10,000 steps per second when driven by an II-MHz clock. The device is priced at $80 (100).

Motorola is introducing an integrated circuit for driving a two-phase stepper-positioning motor with the ability to provide up to 350 mA to each of a two-phase motor's two coils. The MC3479P chip consists of four input sections, two drive stages, and an output to indicate the phase-A drive state. The integrated circuit features selectable clockwise or counter-clockwise motor operation and the ability to rotate either one full step or one half step.

A new algorithm, developed by International Memories Inc. for their 5600 Winchester drives, cuts in half the average access time of these 5.25 inch units. Applying voice-coil techniques to stepper-motor technology, read-write heads have a tendency to pick up speed as they move from track to track, thus spending less time stopping at each track. Average access time is reduced from 85ms to 49ms.

Engineers are using digital techniques more and more when designing angular and linear motion control systems. Systems are easier to control by programming, and there are a rising number of digital- control-system components ranging from peripheral devices to robotic devices. Many are off-the-shelf components such as motion controllers, synchro-to-digital convertors, digital shaft encoders, and stepper-motor drive electronics. Digital-based systems are not necessarily more complicated than a similar analog system. In fact, many elements of a digital-motion-control system are identical. Many photographs are provided, as well as a table of digital- motion-control product and systems manufacturers.

The L293, L293E and L298 dual bridge ICs were designed to minimize problems when designing stepper-motor drive circuitry. Performance is improved, with the single chip replacing the transistors and predriver stages. Circuits described include an L-R Drive, a dual supply, a chopper circuit, a fixed frequency chopper, and controller circuitry. Flow charts and circuit schematics are given for these circuits as well.

A circuit using an MC6809 microprocessor and MC640 programmable timer controls up to three stepping motors simultaneously. The motors' speed and number of revolutions can be controlled separately using the timers as square-ware generators and single-shots. The circuit is illustrated and an assembly language program for simultaneous control of two motors is given.

The article features a BASIC program that predicts when each step of a varying stream of steps should begin. Four values are needed for input: 1) initial speed, 2) desired speed, 3) acceleration time, and 4) shape factor of the ramping curve. It is an iterative routine written for an open-loop control scheme using no positional feedback information.