Considering both steppers and servos

Dec 5, 2007 9:09 AM, Hurley Gill

Both servo and stepper technologies continue to advance, so there's no clear "better" choice. Here we review the designs as they stands now

Servomotors are the clear choice for today s motion applications. Then again, so are stepmotors. What s more, stepmotors are also less expensive than they were a few years ago and easier to select, install, and operate. But for that matter, so are servomotors.

A one-sided view of the stepper versus servo issue is just as cloudy today as it s always been. Both motors are leveraging essentially the same technological developments and advancing at about the same pace. Compared to five years ago, stepmotors better fulfill stepping applications to the same extent servomotors better fulfill servo applications.

Permanent magnet servomotors come in dc brush and brushless styles. Brushed styles require less complex controllers than brushless and are most often used in low-power applications. Brushless dc servomotors (ac permanent magnet types) require complex electronic circuits for commutation. Their windings are near the outside of the motor housing, which aids cooling. They also have less rotor inertia, and they are more efficient and dependable due to a lack of brush friction and brush voltage drop.

Controller improvements

One area where the technological developments have been particularly advantageous is in motion controllers. Driven by the electronic revolution, the devices that operate both steppers and servos are getting better in almost every way. The biggest improvement, perhaps, is the development of easy-to-use interfaces running on common platforms such as Microsoft Windows. Standard interfaces and platforms save time and money, simplifying installation, programming, and troubleshooting.

Controllers have also become smaller and more powerful thanks to solid-state switches and intelligent power modules. Today s power-switching devices, with reduced voltage drop and switching losses, and the additional practicality of bus voltages in the range of 565-680 Vdc, are now so efficient they can transmit more than twice the power to the motor, often within a smaller package. And intelligent modules are now so flexible and robust that some manufacturers are placing them within the motor housing itself.

With added intelligence, modern motion controllers also provide multiple feedback options and control strategies, and they incorporate more functions than ever. Communication boards, for example, are becoming standard, implementing open bus structures SynqNet,, Signet, Sercos, CanOpen, Profibus, DeviceNet, Ethernet on controllers and intelligent drives.

A related trend involves the integration of PC and PLC architectures. In a growing number of motion controllers, the computer is right inside.. In other applications it makes more sense to do the opposite; motion controllers are being embedded in machine controllers, PCs, and PLCs. Either way, the long-standing boundaries between machine and motion controllers are rapidly falling.

A stepmotor is equivalent to an inductor in series with a resistance, and as a result, the current that produces torque requires time to rise. This time constant limits the speed at any given voltage, so higher voltages may be needed to increase the motor's speed in a given application.

For designers, this is all good news, but there is one caveat. Electronic technology is improving faster than the typical machine life cycle. As a result, any machine that cannot be easily upgraded will become obsolete rather quickly. Computer hardware and software engineers learned this the hard way: Backward compatibility is a must for nearly every electronic component today, especially in industrial machinery.

Magnetic materials

Like controllers, magnetic materials have come a long way in the past few years. Better processing and material formulations have given rare-earth magnets greater thermal capacity, letting suppliers produce more powerful motor magnets at a lower cost. The reach-through effect in motors means that today s high-power servos and step-pers are much more affordable than their predecessors.

What s more, for a given physical frame size, manufacturers can now install more windings than before because of better manufacturing techniques. Such motors can generate (and remove) more heat without deteriorating magnet performance. At a given efficiency, more heat dissipation in the same space equates to higher power and torque density.

Magnetic materials most commonly used in motors include Neodymium (neodymium-iron-boride), Samarium Cobalt, Ferrite, and Alnico (aluminumnickel-cobalt). Each has its own advantages, but the first two produce higher-energy magnets, yielding more efficient and powerful motors.