Medical devices such as blood analyzers, diagnostic equipment, and lab-automation systems often require precise, yet low-cost motion (position, speed, and acceleration) control. When it comes to specifying motors for such applications, designers have three basic choices : brush dc, brushless dc, and stepper motors.

Brush and brushless dc motors are a viable option, but they require the addition of electronics such as optical encoders to implement closed-loop feedback control. This additional feedback adds to the total cost of the motion solution and adds complexity. Step motors, often called steppers, are typically used open-loop, without feedback, for a simpler and cost-effective solution.

Of the motor types, steppers provide a big advantage: They are inherently “digital” — a pulse applied to the drive electronics results in a precise shaft movement of one step. For applications with speeds under 3,000 rpm, steppers provide a maintenance-free technology.

Stepper motors come in a few flavors: canned stack, variable reluctance (VR), and hybrid. Canned-stack steppers contain a permanent magnet (PM) rotor. VR steppers have no permanent magnet — they rely on an induced magnetic field in a serrated rotor for operation. Hybrids combine elements of both types — a permanent magnet and “reluctance” serrations in the rotor and stator. Hybrid steppers are made with precise machined parts. The motors provide finer resolutions (usually 1.8 or 0.9° step angles) when compared to the 3.6 to 18° step angles of canned-stack steppers.

Inside a hybrid stepper

The operation of any electric motor is best understood by looking at the interaction between stator and rotor. In hybrid steppers, electrical current in the coils around each stator creates electromagnetic poles in the stator. The serrated teeth in the rotor — which also has a permanent magnet ring in it for reinforcement — line up with the serrated teeth in the stator. The force with which this alignment takes place produces the torque (or rotating moment) in the rotor shaft. Switching electronics cause the next coil to energize and the rotor turns (steps) again to align itself to the new position of the magnetic pole in the stator. The sequential energization of the coils causes smooth rotating movement. To provide more torque, either the stator's magnetic pole must be strengthened (more coils, more current, or larger diameter) or the rotor's magnetic pole must be strengthened (stronger magnets or larger diameter rotor).

Considerations in the design of steppers themselves include: number of coils, number of wire turns in each coil, relative number of teeth in the stator and rotor, and diameter and flux density of the magnet. There is generally a lot of flexibility in the choice of windings for an optimum trade-off between speed and torque for a given power output, although the geometry of the motor, and therefore its step angle, are fixed when the motor is selected.

Unipolar and bipolar

All step motors come in two versions: unipolar (with six or eight leads, requiring only one power source) and bipolar (with four leads, requiring two power sources or one switchable-polarity power source). The mechanical construction of the two motor types is the same. The main difference between the types is that the polarity of the current reverses in bipolar motors.

Designers need not worry about the logic sequence used to energize the motor windings; this data is hardwired into the logic chips. The unipolar method of driving step motors produces less torque because not all the windings are used all of the time. This method also provides higher speed and is the least expensive because of the simplicity of the drive electronics.

The bipolar method, on the other hand, is usually used with a “chopper” drive in which the energizing pulses are “chopped” (i.e., turned on and off) at a high rate, typically as high as 20 KHz, to control the amount of current and energy fed into the windings. The bipolar chopper method is somewhat more complex and therefore more expensive than the unipolar method, but it produces the most torque for a given motor.

Half stepping and microstepping

A clever innovation was that of the “half-stepping” method of driving motors. This mode uses the same chips as full stepping, but with a simple logic-level toggle switch built into the driver chip. The windings are energized sequentially as in full-step mode, while the current level in the windings is controlled at an intermediate level between “full on” and “full off.” The net effect is the creation of an extra step in-between steps. An obvious advantage is that positioning resolution is improved by a factor of two. Just as importantly, half stepping makes shaft rotation smoother since the energy in the windings is increased sequentially, as opposed to full on and full off. If half-stepping is available, always drive the motor in that mode.

It should be no surprise that microstepping is an extension of half stepping. Current levels are increased sequentially in the windings in smaller increments so position resolution is improved even further. It is common to see drivers that can deliver ¼ step per step, ⅛ step per step, 1/16 step per step, 1/64 step per step, and so on. However, resolutions more than 1/64 step are beyond the mechanical accuracies of most motors. Microstepping is declining in cost so it's wise to consider the method even in cost-sensitive applications because of the increased smoothness of operation.

Potential pitfalls

Hybrid stepper motors are simple to use, especially with the many drive chips and board-level controllers available. Over the years, suppliers have learned about potential pitfalls and how to avoid them. For example, if manufacturing is not adequately controlled, small contaminants in the tight air gaps (typically 0.002 to 0.003 in.) or the slightest out-of-roundness from machining can cause stepping errors, vibration, and excessive audible noise. Therefore, selecting a vendor with application-engineering support is an important consideration.

Another potential issue: As energy is transferred from one coil to the next, the motor structure vibrates. This can lead to resonance, a nuisance to be avoided. All kinds of schemes including mechanical dampers and electrical filters have been used to analyze and overcome the problem of resonance. The best way to avoid it is to adjust speed or acceleration to avoid the resonance band of the motor. Also, the use of half stepping or microstepping usually does the trick. Again, the selection of a reputable vendor with expert application-engineering help is important.

Understanding stepper motor specs

Like most industries, the stepper-motor industry has its own jargon. Here are some helpful terms to know:

Holding torque is the maximum torque that the motor can produce with the shaft held firmly without movement, when the windings are energized. Holding torque serves as a relative figure of merit useful in comparing one step motor against another. The relative value of a step motor can be judged by the torque delivered per unit volume (motor diameter by length).

Pull-in torque is the maximum torque a step motor can produce in a start-stop mode without stepping error. The torque generated depends mainly on the drive method. The drive method should always be specified when calling out pull-in or any other type of torque.

Pull-out torque is the maximum torque a step motor can produce in a slewing mode (i.e. without regard to starting and stopping). Pull-out torque is always higher than the pull-in torque for a given motor across the full range of speeds.

Pull-in (or start) speed range is the rotating shaft speed range of the motor in pulses per second (PPS) in a start-stop mode without step error.

Slewing speed range is the rotating shaft speed range of the motor expressed in PPS, without regard to starting and stopping. The speed in rpm can be calculated from the PPS as follows:

rpm = (PPS × 60) / 200 fora 1.8° (200-step per revolution)hybrid step motor.