MOTION SYSTEMS HANDBOOK
Servo versus
closed-loop stepper
motion controls
Servo motors and stepper motors both include rotors with permanent magnets and stators with coiled windings … so application of voltage to those windings prompts rotor rotation. But this is where construction similarities between these motors end. Stepper motors have 50 to 100 poles and common hybrid iterations are two-phase (bipolar) and four-phase (unipolar). In contrast, many common motors run under servo control have four to 12 poles and are three-phase units. What’s more, drives commonly paired with stepper motors generate sine waves with constant amplitude and varied frequency to change motor speed — or drive the motor with pulse-width modulation (PWM). In contrast, servo-motor drives produce sine waves with variable frequency and amplitude for full control over speed and torque. Regarding controls, traditional stepper motors move upon reception of some command to advance some pulses correlating to a distance but are openloop, so lack feedback to verify whether the target is reached. Servo motors also move on receipt of a controller command signal — but under closed-loop control with encoders tracking motion to inform motioncontroller corrections until target position or actuation is reached. So with traditional stepper designs, inadequate motor torque for overcoming
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a load will cause skipped steps or stalling even while receiving current from the drive — for a discrepancy between commanded and actual position. One solution is to oversize the stepper motor to ensure sufficient margin between highest expected torque required (to move the load) and available motor torque. Another solution is to add an encoder for closedloop stepper operation. Some closed-loop stepper systems include controls that compare expected position (based on the steps the motor should have taken) with actual position reached as reported by encoder feedback. Any difference between the two prompts corrective moves. Slightly more sophisticated closed-loop stepper systems continuously monitor any discrepancy between position steps and encoder feedback — and then command real-time corrections in the form of changes to drive pulse rate, current, or step angle. Still more sophisticated closed-loop stepper systems use drives that deliver sinusoidal commutation. When rotor and stator magnetic fields are improperly aligned, digital signal processors (DSPs) leveraging encoder feedback command changes to the current into the motor — to prompt torque that’s sufficient for moving or holding the axis load. Because the controls use feedback to control torque with motorcurrent changes, this stepper control mode is sometimes called servo control. Basically, the stepper motor functions as a highpole-count servo motor — sans the noise 8 • 2019
of traditional stepper-motor operation and with smooth and precise motion. Dynamic rather than constant current input minimizes motor-heating issues … and these closedloop stepper designs also exhibit less velocity ripple than open-loop vaeriations. Read Open-loop versus closed-loop stepmotor systems at motioncontroltips.com for more on the latter.
CHOOSING BETWEEN CLOSEDLOOP STEPPERS AND SERVOS Stepper motors under closed-loop control are a small but growing percentage of stepper applications — increasingly common where load on an axis may vary and even in machine designs involving advanced multi-axis coordination. Where appropriate for the application, it’s not uncommon for closed-loop stepper systems to be 25% less expensive than comparable servo motor solutions … and with common NEMA dimensions for simple integration. Closed-loop stepper motors are typically most suitable for applications in which: • • •
Position loss is unacceptable The axis needs high torque at relatively low rpm The design has a simple architecture or particularly tight cost constraints
In these situations, closed-loop stepper systems excel with relatively high torque output — for acceleration and top motioncontroltips.com | designworldonline.com
8/19/19 10:39 AM
