Medical Device Link.

Originally Published MEM Fall 2001

MOTOR TECHNOLOGY

Achieving Simple, Precise Motion Control with Hybrid Step Motors

Hybrid steppers provide smooth operation in medical electronic equipment.

Hasit Parikh

Designers of medical equipment such as analyzers, diagnostic instrumentation, and laboratory automation systems often face the challenge of implementing precise motion control at low cost. For most such equipment, three technology alternatives for achieving electronic motion control are available: permanent-magnet brush dc motors, brushless dc motors, and step motors. Permanent-magnet brush dc motors are a viable choice, but they have to be fitted with a feedback device such as an optical encoder in order to implement closed-loop feedback control. Brushless dc motors with their associated electronics also can be used for closed-loop control. They offer the advantage of not having brushes that need replacement.

A family of hybrid step motors can be easily integrated into a variety of designs.

Step motors—alternatively called stepping motors, stepper motors, or, simply, steppers—provide another option for position or speed control. Steppers are commonly used open-loop, without feedback. They are inherently digital: a pulse applied to the drive electronics results in a shaft movement of one step. The number of incoming pulses and the rate at which they are fed can be manipulated to produce highly precise, yet very simple, control of position, speed, and acceleration. As long as the speeds required by the application are not too high—typically <3000 rpm—step motors can be the best motion control alternative, being far simpler than the others, lower in cost, and maintenance free.

Three types of rotary step motors are used today: canned-stack, variable-reluctance (VR), and hybrid. Canned-stack, or permanent-magnet, steppers are made with claw-toothed (stamped) parts and with radially magnetized permanent magnets in the rotor. Unlike canned-stack steppers, VR step motors have no permanent magnets in the rotor; their operation depends on an induced magnetic field in their serrated rotor. A combination of the two technologies, manifested by both permanent magnets and reluctance serrations in the rotor and stator, results in hybrid step motors.

Hybrid steppers are generally made with precision-machined parts and offer finer resolutions than canned-stack steppers (step angles of 1.8° or 0.9° per step as compared with 3.6°–18° respectively). The canned-stack design, however, is less expensive. Hybrid steppers are the recommended choice for applications requiring low-cost yet precise shaft position control with fine resolution.

Hybrid Step Motor Construction

The operation of any electric motor can be understood as the interaction between its stator, or stationary component, and its rotor. In a hybrid stepper, electrical current in the coils around each stator slot creates an electromagnetic pole in the stator. (See Figure 1 for diagrams of hybrid stepper construction.) The serrated teeth in the rotor, which also incorporates a permanent-magnet ring for reinforcement, line up with the serrated teeth in the stator.

Figure 1. Radial (a) and longitudinal (b) cross sections of a hybrid step motor.

The force with which this alignment takes place produces the torque, or rotating moment, in the rotor shaft. Using switching electronics, the next coil is then energized and the rotor moves again to align itself with the new position of the magnetic pole in the stator. Smooth rotating movement is achieved as the coils are energized sequentially. If more torque is required, either the stator's magnetic pole has to be strengthened (by means of more coils, more current, or a larger diameter) or the rotor's magnetic pole has to be strengthened (via stronger magnets or a larger diameter).

Motor features that concern the medical equipment designer are the number of coils, the number of wire turns in each coil, the relative number of teeth in the stator and rotor, and the diameter and flux density of the magnet.

For application purposes, the geometry of the motor and, therefore, its step angle per step are fixed when the motor is chosen. However, a great deal of flexibility in the choice of windings facilitates trade-offs between speed and the torque produced for a given power output, which is itself a product of speed and torque; in the international system of units, speed in radians per second times torque in newton-meters equals power in watts.

Drive Methods

Unipolar and Bipolar. Step motors are generally available in two versions: unipolar, with six or eight leads and requiring only one power source, and bipolar, with four leads and requiring either two power sources or one switched-polarity power source (see Figure 2). The mechanical construction of each type is exactly the same. The only differences between them relate to how the coil ends are brought out and to the fact that, in bipolar motors, the polarity of the current reverses.

Figure 2. Wiring diagrams for unipolar (a) and bipolar (b) styles of step motor.

From an applications standpoint, it is unnecessary to be concerned with the logic sequence that is used to energize the windings. This is hardwired into the step motor logic chips that are offered. It is sufficient to remember that there are two kinds of windings available, unipolar and bipolar. The unipolar method of driving step motors produces less torque because not all of the windings are utilized all of the time; however, it offers higher speed. It is also a less-expensive method on account of its simple drive electronics. The four-lead bipolar method is ordinarily used with what is known as a chopper drive, where the energizing pulses into the windings are turned on and off (i.e., "chopped") at a high frequency rate—often as high as 20 kHz—in order to control the effective current and energy fed into the windings. The bipolar chopper method of driving is somewhat more complex, and therefore more expensive, than the unipolar method, but it produces more torque for a given motor.

Half-Stepping and Microstepping. An innovation appearing early in the development of step motor technology was the half-stepping method of driving motors. Today, the same drive chips that are used for full stepping can be used for half-stepping as well by means of a simple logic-level toggle switch built into the chip. In the half-stepping mode, not only are the windings energized sequentially as in the full-step mode, but the current level in the windings also is controlled at an intermediate level between fully on and fully off. The effect is that a null position is created in between steps; thus, half-stepping is achieved.

The obvious advantage of half-stepping is that positioning resolution is increased by a factor of two. Just as important, half-stepping improves the smoothness of shaft rotation. This is because of the intermediate current levels employed in increasing sequentially the energy in the windings, which contrasts with turning the windings completely on and off as with full stepping. For these reasons, motors should be driven in the half-stepping mode rather than in the full-stepping mode whenever possible.

Microstepping is an extension of the concept of half-stepping. The current levels are increased sequentially in the windings in smaller increments, thus improving position resolution further. Drivers are now available that can deliver 1/4 step per step, 1/8 step per step, 1/16 step per step, 1/64 step per step, and so on. Increments smaller than 1/64 step are essentially for design show, since such finer resolutions are beyond the mechanical accuracies of the motor.

Specification Terminology

The step motor industry, like most others, has its jargon. (See Figure 3 for the identification of common performance terminology.) A good understanding of the specialized terminology can assure effective communication with vendors in the motor sizing and selection process. Knowledge of the following essential terms will be useful in establishing a dialogue with the motor vendor's application engineering department.

Figure 3. Speed-torque curve for typical motor.

Holding torque is the maximum torque that the motor can produce with its shaft held firmly without movement, when the windings are energized. A motor's holding torque serves as a measure of relative merit in comparing one step motor with another. The relative value of a step motor can be judged by the torque delivered per unit volume.

Pull-in torque is the maximum torque a step motor can produce in a start-stop mode without stepping error. The torque produced depends greatly on the drive method. For this reason, the drive method should always be specified when calling out the pull-in torque—or, for that matter, any other type of torque.

Pull-out torque is the maximum torque a step motor can produce in a slewing mode, that is, without regard to starting and stopping. For any motor, the pull-out torque is always higher than the pull-in torque across the full range of speeds.

The pull-in (or start) speed range of a motor is its rotating-shaft speed range in pulses per second in a start-stop mode without step error.

Slewing speed range is the rotating-shaft speed range of the motor expressed in pulses per second (PPS) without regard to starting and stopping. The speed in revolutions per minute (RPM) can be calculated from the PPS according to the formula PPS times 60 divided by the number of steps per revolution. For example, for a 1.8° hybrid step motor (200 steps per revolution), RPM = (PPS x 60)/200.

Rotor inertia is the property of the motor that signifies its ability to accelerate rapidly. It determines the torque needed to accelerate the motor to a specified speed within a specified time. The rotor inertia is often not negligible in comparison with the inertia of the load. It is therefore a very important specification in the load calculations. Commercial software that application engineers for good motor vendors can use is available to make the otherwise tedious load calculations easy.

The last essential term worthy of discussion in this overview is the motor's maximum temperature limit. If the motor is undersized for the application, it will get too warm and eventually burn up. Dialogue with the vendor's engineering department will ensure that load calculation is performed adequately and that the size of the motor chosen will be appropriate for the equipment.

Application Fine Points

Hybrid step motors are very easy devices to integrate into designs, especially with the abundance of drive chips and board-level controllers available in the market. These motors may be simple, but the process of manufacturing them is very exacting. If production processes are not adequately controlled, small contaminants getting into the very tight air gaps (typically 0.002–0.003 in.), or the slightest out-of-roundness in the machining processes, can cause all sorts of problems that can result in stepping error, vibration, or excessive audible noise.

In operation, as energy is transferred from one coil of the motor to the next, the whole motor structure physically vibrates. The motor will vibrate with the load if care is not taken, leading to resonance, a nuisance to be avoided. The problem of resonance has been analyzed, and all kinds of schemes, including mechanical dampers and electrical filters, have been used to overcome it.

The best way to fight resonance is to begin by adjusting the speed, or acceleration, if this is an option. Alternatively, employing half-stepping or microstepping transfers the energy to adjacent windings in smaller increments, which usually does the trick. Dependence on a reputable vendor with expert application engineering capabilities once again puts the equipment manufacturer in a good position should resonance problems arise that are a challenge to solve.

Applications often require the generation of high torque at low speeds. In such cases, a gearhead attached to the motor becomes necessary. Fortunately, serious motor vendors also supply optional gearheads with a variety of gear ratios, which can be chosen to suit the application. If linear movement is desired, ball screws, acme screws, and linear bearings can be acquired from suppliers to help streamline the application engineering process.

Conclusion

Hybrid steppers facilitate the implementation of low-cost, reliable, and repeatable motion control into systems that can interface easily with digital electronics.

Hasit Parikh is a chief engineer of permanent-magnet motors at Thomson Airpax Mechatronics (Cheshire, CT). He can be reached at hparikh@thomsonmail.com.

Copyright © 2001 Medical Electronics Manufacturing