Stepmotors and servomotors are both used successfully for positioning in equipment worldwide. However, designers sometimes overlook their distinct characteristics. The two motor types offer different advantages that should be considered during motion design.

Stepping motors are widely used in applications requiring less than 1.5 hp. Simplicity through open-loop operation makes for cost-effective design; tuning adjustments (required by servos) are unnecessary. If the distance to be moved is relatively short and the load unchanging, a stepping motor provides sufficiently rapid moves.

In contrast, servomotors are widely used in myriad applications; high peak torque and speed capability are their strengths. Feedback allows monitoring and immediate correction for any errors; corrective controls also heighten accuracies. Whether the distance to be moved is long or short, a servomotor provides sufficiently fast speeds and rapid positioning.

Stepper technology

Stepmotors move or index in fixed angular increments. These motors are used with a command signal that appears as a series of digital pulses. One pulse causes the motor to increment or move through one angle of motion — so one pulse moves the stepping motor through 2.8°, for example. This specific angular movement is repeated for every pulse.

There are various stepmotor designs — those that step through 0.8°, 1.8°, and 2.8°, for example — and various types (such as variable reluctance, synchronous induction, hybrid, and permanent magnet) with the latter being most common.

Most stepping motors are used in an open-loop configuration: A command for movement is issued from the drive, to the motor, and then the motor moves. Here, the command is simply sent; no signal feedback is returned to inform the drive that any operation has, or has not, occurred. In other words, the open-loop control assumes that the movement or operation has taken place.

Stepping motors have two unique speed-torque characteristics. The first is that a stepmotor's peak torque is the same as its continuous torque capability — so there is no extra torque available. As a result, the stepper can stall, or lose steps, if the load becomes too high or inconsistent.

If this occurs, the machine is homed to a known reference position. To prevent this, it is usually recommended that steppers be oversized by applying a safety factor of at least two times the application torque.

The second unique stepmotor characteristic is the slope of the speed-torque curve. Consider the S-T plot on page 19: At 500 rpm, the available torque drops to about 30 to 40% of the stall torque value; as speed increases, available torque drops further.

This characteristic makes stepping motors better suited for applications with lower-speed operation.

Servomotor technology

Prime features that differentiate servomotors from others are:

1. Motor diameter
2. Feedback device
3. Closed-loop operation, and
4. Product design and manufacturing.

Servomotors are specifically designed to have small diameters, yet maintain a given output horsepower or torque. Consider typical one-horsepower motors: An induction motor with such power is 5.6 to 7.6 in. in diameter; a PMDC servomotor is 4.0 in.; and a brushless servomotor is 3.5-in. square. How is the servomotor made smaller? Its permanent magnets eliminate large bulky windings (as found in the stator of an ac induction motor) while maintaining the magnet field strength.