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.