MOTORS, DRIVES & CONTROLS
actual shaft position deviates from the expected position, and the motor can correct itself as needed. Te exception is when the load torque exceeds the rated torque of the stepper. In this case, the motor will stall and not be able to correct itself. Steppers do not retain any sort of
known positioning when they are powered down. Terefore, they need to complete a homing routine every time they are powered up. You can see this when you turn on your printer at home. You will see the motors complete a routine that resets to a known ‘home’ position. In short, stepper motors are ideal for a
wide variety of industries and small power applications, ranging from 3D printers, adjustable conveyors and small metering pumps.
WHAT IS A SERVO MOTOR? While servo motors simply mean a motor controlled with feedback, in the world of automation it is accepted to specifically mean an AC permanent magnet synchronous motor. Tese motors have a stator similar to that of an induction motor, but their rotors are constructed with rare earth magnets. Tis eliminates the slip found in induction motors and allows the servo motor to run at the synchronous speed defined by the frequency and number of poles (rpm = 120*frequency# of poles).
ADVANTAGES OF SERVO MOTORS AC servo motors use rare earth permanent magnets, so they offer best-in-class torque density. Tis is preferred for very dynamic motor applications that must accelerate and decelerate quickly. Servo motors include some sort of feedback, whether it is a resolver or encoder. Te shaft feedback is critical because it is used in the servo controller to control rotor position and torque. Unlike a stepper motor, servo motors
are capable of providing momentary peak torque across its speed range. A servo motor’s peak torque can be very important in certain applications and can result in an optimised motor selection. In addition, AC servo motors operate at much higher speeds compared to stepper motors. Critical motion control applications
that involve functional safety and the handling of critical or expensive products (e.g. semiconductor machines) should use servo motors with feedback. Te low
cost benefit of a stepper motor/control simply does not justify the risk of injury or product failure. Common examples for their best use are in packaging machines, material handling and robotics. KEB servo motors can be equipped with absolute encoders, which retain their position information even when the system is powered down. Tis can be very important in applications where re- homing is not possible.
LIMITATIONS OF SERVO MOTORS Te main disadvantage of servo motors compared to stepper motors is going to be the cost of the motor and the associated drive that controls it. Tis is particularly true for smaller power applications (e.g. 500W and below). If the application does not require the precision and high torque overload that a servo motor offers, then a stepper motor is likely to be the better choice.
Servo motors are well suited to a range of industries and applications such as powering conveyor systems for packaging machines, metal cutting and forming machines, as well as robotics. KEB offers a range of products to
support both stepper and servo motors. On the stepper motor side, KEB stepper drive module offers control for stepper motors up to 5A, while utilising CiA 402 motion control profiles. Te company also offers both servo motors and drives to control them. Its servo motors offer speeds up to 9,000 rpm and nominal torques as high as 70Nm. To control the motors, the Combivert S6 and Combivert F6 drives combine flexibility and performance to make them ideal for servo applications.
Mike Keefe is with KEB America.
www.kebamerica.com
www.engineerlive.com 47
The C6 Smart from KEB
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