Motion System Design
Two for one

Two for one

Servos and steppers overlap in low power applications. But there is still a way to choose between the two

While there have been many answers to the “Stepper or servo?” question, statements of the case are often oversimplified. There is, without exaggeration, a complex array of variables involved, and the separating line grows more blurred – the potential overlap broadens – as both motor types’ abilities continue to advance.

An “apples and oranges” approach has characterized many a design engineer’s choice: assess both technologies with a sweeping glance and settle on one exclusively.

But it’s not as basic as that. For instance, a machine may have multiple axes, some requiring servo, some stepper, and some able to run either. There are a handful of fundamental concepts that should be noted before making a decision.

Power requirement is always the first consideration. Step motors are impractical above 1 kW. This eliminates at least 50% of the applications. (Along with the fact that higher power carries a higher cost, the broader applicability of servos makes that market’s revenue approximately five times that of the stepper market.)

If the required power doesn’t rule out a stepper, it’s time to delve into other parameters as explained in the following discussion.

Conditions requiring servomotors

• Speeds over 3,000 rpm. The high pole count of a step motor means there is virtually no torque available beyond this speed. Servo systems sidestep this speed limitation thanks to their current control and low pole count.
• Dynamic loads. No matter what kind of motor, variable loading is rarely cause for celebration, but programmable tuning parameters and access to actual motor position help a servo system manage such loads very effectively.
• Actual motor position is needed at the controller. For many applications, accurate machine function depends on knowing the exact motor position; synchronized multiple-axis motion often requires dead-on rotor position information at the control. A servo system is the most practical choice to ensure near-absolute position feedback.
• Extreme acceleration. When a closed-loop system is commanded to move with too high an acceleration, it does the best it can. When an open-loop system is asked to accelerate beyond its capacity, it will stall. Thus, with very high accelerations, servos promise more stability.

Conditions requiring stepper motors

• Applications requiring minimal velocity ripple. Open-loop step motor systems can provide extremely steady rotor velocity. Advances in damping techniques continue to improve the stepper motor system’s velocity accuracy, and the lack of a servo loop around position or speed results in very little dithering.
• Certain point-to-point applications. The ability of step motors to step and settle, especially with the newer techniques for end-of-move damping, makes them the best choice for high-tact applications with rapid and definite stepping. A step motor is also very stable at zero speed, which may be crucial if, say, an object needs to be held still for inspection.

One or the other

Of the systems with power requirements in the range of both servo and stepper (1 kW or less), an estimated 55% can actually use either. Conventional wisdom in the field, with all other things being equal – especially price – is to choose the servo system. This is sound reasoning, and is advisable for that 55% of applications that can accept either technology. But price is usually not the same.

In general, it can be assumed that there is not a large difference in cost between the drive electronics for a step motor and the drive electronics for a servomotor. The motor design, however, and the presence of a feedback device, influence overall system expense considerably. Machine builders can usually reduce cost by 10 to 20% per axis using a step motor system.

The actual use of servos in this “overlap” category is unnecessarily high because machine builders often fail to view step motors as a viable option. In such cases, a servo is a wasted expense. Rather than asking, “Is the step motor the right motor?” designers should ask, “When can I take advantage of the low-cost stepper solution?” The money-saving opportunities are there; they just need to be brought into the open.

Here are a few circumstances where a stepper motor can be put to use, even though a servo would also fit.

• Predictable loads. (Most applications fit this category.) If a step motor is sufficient to drive a load, and the load doesn’t change, it will move with very high repeatability and reliability.
• Insignificant external forces. Step motors are very stable at zero speed and are able to accommodate a low level of external forces; but as a rule, go with the servo system where there are large external forces.
• High resolution. With its high stiffness and stability, a step motor system can get good resolution in an open-loop mode. Servo systems require very expensive feedback devices to achieve comparable resolution. (Remember that resolution does not equate to accuracy, although step motors tend to have higher accuracy than servos.)
• Continuous power output. Step motors deliver constant power (within their range) without problems. But it’s wise to use a servo system for high-power, intermittentduty applications.

Certainly, step motors are less flexible and more finicky than a servo system. That’s the nature of open-loop systems. However, if matched to the application correctly, a step motor system can not only cost less, it can be easier to apply and, by virtue of eliminating the feedback device, more reliable.

Risk is obviously a considerable concern of machine builders. The risk that a step motor will be inappropriate is often higher than the potential benefits. But many of the criteria stated here are not fully understood during the design phase, and improper selection does occur. However, it’s not necessarily incurable.

Some motor manufacturers ease the upgrade process by using a common drive platform (irrespective of step motor or servomotor) that allows a switchover to servo with minimal change to the set-up and wiring of the machine. And, as drive electronics become more sophisticated, applications will be able to use open- and closed-loop systems with both low and high pole count motors; each axis of a machine will easily accept the optimal motor type.

John Walewander is a manager with Parker- Hannifin, Compumotor Div., Rohnert Park, Calif.

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