It takes two

April 1, 2005
Servomotors are the clear choice for today's motion applications. Then again, so are stepmotors. What's more, stepmotors are also less expensive than

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.

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.

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 steppers 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 (aluminum-nickel-cobalt). Each has its own advantages, but the first two produce higher-energy magnets, yielding more efficient and powerful motors.

Neodymium actually offers the highest energy, and can now operate in motors at temperatures to 180°C. As the price of Neodymium powder falls, and as magnet makers adopt alternative (injection and compression-molding) manufacturing techniques, industry will see increasing performance improvements with this material.

The use of high energy density magnets in existing motor designs is not without drawbacks, however. Such magnets will typically increase magnetic flux density in the iron, saturating the housing and armature laminations and increasing iron loss. To get around this, many manufacturers are designing new motor lines, reducing current draw and increasing efficiency in the process. Avoiding saturation is the key because once iron saturates, more current usually means only more heat and less efficiency to achieve a given output.

More powerful steppers

Whether to use a stepper or servo, as always, is an issue dictated by the application. Although it's natural to make decisions based on past experience and comfort levels, it's wiser to pick the right motor, be it a stepper or servo, for the right job.

In general, if an application requires high throughput, high-speed performance, or high bandwidth for disturbance correction — as in primary machine axes — servomotors are the way to go. If performance and speed requirements are modest — as in secondary axes, such as machine adjust and setup — steppers are usually the better choice.

Stepmotors are ideal for secondary axes because they tend to be easier to design into control systems and less expensive to operate. In most cases, steppers don't require tuning or feedback circuits. They're also less prone to failure because of their simplicity.

Modern steppers are also able to generate more power than earlier generations. One reason is that microprocessors can now sit inside stepmotor housings, tightly controlling current. Increasing rotor diameter is another reason. Stepmotors with “oversized” rotors generate more torque per unit volume, as well as higher inertia. With a 10:1 load-to-rotor inertia ratio being a good rule of thumb for stepper sizing, this also broadens the scope of applications suitable to a particular frame size.

Other factors that improve stepmotor performance include built-in feedback, microstepping, and end-of-move damping. Although most steppers are extremely accurate running open loop, built-in feedback provides additional accuracy without the cost of an external feedback device. Microstepping, a technique that reduces step size, results in smoother torque at low speeds and greater resolution at high speeds. End-of-move damping, as its name implies, reduces settling time while maximizing accuracy.

External feedback may still be used with steppers, but for different reasons today than a few years ago. Encoders were used at one time to help controllers avoid stalling by keeping current pulses synchronized with shaft angle. Today, this is usually not necessary as “stall detection” is handled by drive electronics. Where feedback is used, it's to help correct misalignment problems caused by other components; errors in a positioning table, for example. That said, many stepmotor applications requiring feedback begin to approach the cost of a servosystem, in which case the advantages and disadvantages of both types of systems should be considered.

Souped-up servos

Servomotors have two distinct advantages over steppers: They can generate high torque over a wide speed range, and they do it in a small package. They've also dropped in price over the past few years — more so than steppers — largely because of high-volume manufacturing.

Tuning problems, once the bane of servo users, are for the most part history. Some servosystems, in fact, tune themselves automatically and adapt to any mechanical system without a decrease in performance.

Although servomotors are designed to run at high speeds, they can run at extremely low speeds under precision control, even down to 0 rpm. Where precision is not an issue, however, stepmotors are usually a more economical solution for low-speed applications. Generally speaking, “low speed” is anything less than 1,000 rpm. Above 1,000 rpm, stepmotor torque begins to fall off, the result of energy losses and magnetic circuit time constants. In contrast, servomotors with comparable torque do not begin to fall off until around 2,500 to 3,000 rpm, or more.

Other factors, such as horsepower, torque, and repeatability, determine which motor type to use in the speed range between 1,000 to 3,000 rpm. Above 2 hp, for example, brushless servomotors typically get the call. Below 2 hp, servo and stepper performance tends to overlap.

At low speeds, during stall, or when holding a load, stepmotors (with oversized rotors) can produce more torque than servos for a given frame size. All that torque lets steppers produce extremely accurate and stiff low-speed motion without a gearbox or other mechanical advantage.

There is no movement when a stepper motor is at rest. In contrast, when enabled, a servomotor is never at rest due to the constant closed-loop error correction. This servomotor “dither,” typically amounting to no more than a few feedback counts, though unnoticeable in most applications, can be absolutely unacceptable in others.

Where repeatability and resolution are an issue — traditionally servomotor territory — steppers may now be considered. The requirement is that the load must be predictable, or subject to only small external forces. Here, steppers (running open loop) can save up to 30% over comparable servo solutions. msd

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Summary of selection criteria

Cost: Once the main factor in choosing between servos and steppers. At a time when both motor types were quite expensive, cost favored stepper systems because of their simplicity. Today, lower costs seem to favor servomotors more, widening the applications that can take advantage of their capabilities.

Load inertia: As a rule of thumb, stepmotors usually don't exceed a 10:1 ratio of load to motor inertia. On the other hand, direct-drive servosystems with high resolution feedback and no compliance, can run as high as 50+:1 with quicker response times relative to previous technology. For a typical servosystem with a drive train that requires high acceleration or deceleration, it is best to keep the ratio within a range of 1:1 to 5:1 for quick response. To achieve a good system bandwidth with higher inertia mismatches — compliance must be minimized or even eliminated, feedback resolution maximized, and current, velocity, and position-loop update rates made as fast as possible.

Torque: Consider a constant or variable load. Servosystems can recover from an overloaded condition, but stepper systems cannot. Steppers can give you a lot of torque in a small package, under 1,000 rpm. Servomotors, on the other hand, handle torque requirements well above 1,000 rpm (as well as below). Regarding torque, designers should select the motor that provides the higher value from speed-torque curves. For the same price, most designers prefer to use servomotors.

Complexity: One change that improves reliability and maintenance in servos has been a reduction in the number of wires necessary between the power and feedback devices. Manufacturers also have taken much of the guesswork out of tuning and determining when a system needs maintenance. Automated or calculated tuning techniques and built-in diagnostic programs help simplify these tasks. Most servodrives can use traditional “step” and “direction” (stepper) inputs, usually in “position” mode to eliminate the potential for loss or addition of steps.

Nonetheless, stepmotors are still simpler. They have fewer wires to connect and require minimal amounts of tuning and adjustment to get a system up and running.

Resolution: Servomotor resolution is theoretically infinite, but in closed-loop operation, system positioning depends primarily on the resolution of the feedback device, be it a sine encoder, resolver, or digital-type encoder.

With steppers, there's also a difference between theoretical and actual resolution. For example, a two-phase, full stepping, 1.8° step-angle motor may have 200 possible positions in one revolution (360°/1.8°), but whether or not it's achieved depends on the application. Same is true of half-stepping and microstepping motors; a 1.8° microstepper, though specified as having ten microsteps per each full step, cannot necessarily find any position within 0.18°. Several microsteps may be commanded before torque builds up enough to overcome friction and load inertia. In a real-world situation, the motor could easily jump one or more microsteps beyond the number commanded and stabilize there.

When positioning resolution must exceed 200 steps/rev, steppers may be used with a feedback encoder. In closed-loop mode, it's possible to go as high as 1,000 steps/rev. Five-phase motors and, with caution, microstepping motors, can improve on this as well.

Repeatability: Servomotors are extremely repeatable because they run closed loop. But steppers can be just as repeatable, especially when running in one direction. However, when friction load increases (as during direction reversal) the situation changes. Similar to how a gearbox must take up backlash, the stepper must also catch up to system command. During the first move in a new direction, motor accuracy is affected because the stepper is overcoming friction (the affects of the load). Once that happens, the system regains its specified repeatability. It should be noted, however, that servomotor repeatability has also improved, helped by high-resolution (219 to 221 counts/motor rev) sine encoders and resolvers.

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