New Tech Europe | Jan 2017 | Digital Edition

Stepper Motors Use Closed-Loop Technology To Step Into Servo Applications

Dr. Markus Erlich, Servotronix

INTRODUCTION Technological advancement is changing the performance-cost ratio between stepper motors and servo motors for a growing variety of demanding industrial automation applications. Thanks to the adoption of closed-loop technology, less expensive steppers are making inroads into applications that have been considered the exclusive domain of more expensive servos. STEPPERS VERSUS SERVOS Conventional wisdom states that servo control systems are superior in applications requiring speeds greater than 800 RPM as well as applications that require high dynamic response. Stepper motors are preferable in applications that run at lower speeds, produce low to

medium acceleration rates and/or require high holding torque. Where does the conventional wisdom concerning steppers and servos come from? Construction A stepper motor rotates in steps, using magnetic coils to pull a magnet in steps from one position to the next. To move the motor 100 positions in a given direction, the circuit steps the motor 100 times. The stepper moves incrementally using pulses and can be precisely positioned without any feedback sensor. The servo’s method of movement is different. It uses a magnetic rotor that is connected to a position sensor - an encoder - which continually senses the exact position of the motor. Servos monitor the difference

between the motor’s actual and commanded positions and adjust current accordingly. This closed- loop system enables the motor to stay on course. Simplicity and Cost Not only are steppers less expensive than servos, but they are also simpler to commission and maintain. Steppers are stable at rest and hold their position, even with dynamic loads. However, as the demands of certain applications increase, more expensive and complex servos must be applied. Positioning A crucial difference between steppers and servos is in applications that require knowledge of the precise position of the machine at every moment. In an open-loop, stepper-

22 l New-Tech Magazine Europe