New Tech Europe | Jan 2017 | Digital Edition

Figure 1. The mechanical structure of a hybrid stepping motor

maximum torque is exceeded. For these reasons, servos are preferred for most high-speed applications. Conversely, the stepper’s high pole count has a beneficial effect at lower speeds giving the stepper motor a torque advantage over the same size servo motor. Heat and Energy Consumption Open-loop stepper motors operate with constant current and give off a significant amount of heat. Closed- loop control avoids the heat problem by supplying only the current that is demanded by the velocity loop. Summary of Steppers versus Servos Servo control systems are best suited to high-speed applications that involve dynamic load changes like robot arms. Stepper control systems are preferred for applications that require low-to-medium acceleration and high holding torque such as 3D printers, conveyors, and accessory axes. Because they are less expensive, steppers are favored as they can lower the cost of automation systems whenever they can be used. Motion-control systems that require the properties of servos must justify the higher cost of these motors. ALTERING PERCEPTIONS What if the advantages of closed- loop servo technology could be adapted to steppers? Could we realize the cost benefits of steppers while achieving servo-like performance? By adopting closed- loop technology, steppers are able to deliver the combined benefits of servos and steppers in a low- cost stepper package. Because of their significant performance and energy-efficiency improvements, closed-loop steppers can replace more expensive servos in a growing variety of demanding applications.

Figure 2. PRO2 Servo motor from Servotronix

Figure 3. Position feedback in a closed-loop, servo system

controlled motion application, the control system assumes that the motor is always moving correctly. However, when a problem is encountered, such as jammed part that causes the motor to stall, the controller does not know the actual location of the machine, causing it to lose position. The servo’s inherent closed-loop system holds an advantage: should the machine snag on an object, it will be sensed immediately. The machine will stop operating and never lose position.

Speed and Torque Performance differences between steppers and servos derive from their dissimilar motor designs. Stepper motors have a lot more poles than servo motors, thus one complete rotation of a stepper motor requires many more current exchanges through the windings, causing its torque to fall off dramatically as speed increases. Furthermore, steppers can lose their step synchronization if the

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