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 step-per 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 step-pers 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.