S T E P P E R M O TO R S
Thomson Industries stepper-motor linear actuators include models with built-in anti-rotational guidance. The manufacturer’s motorized leadscrew actuator (MLA) saves designers of high-precision and shorter-stroke applications the cost, time, and maintenance worries related to designing and building externally guided systems themselves. Combining lead screws and stepper motors is a simple and common setup for precise linear motion.
The low-speed torque of a stepper motor varies directly with current. How quickly the torque falls off at higher speeds depends on a number of factors such as the winding inductance and drive circuitry including the drive voltage. Steppers are generally sized according to torque curves, which are specified by the manufacturer. Sufficient documentation of a stepper motor includes a torque-curve that shows both pull-in and pullout torque, two critical stepper motor parameters . Cogging torque basics Cogging torque is a product of the magnetic interaction between the poles of the rotor’s permanent magnets and the steel laminations of the stator’s teeth. In other words, when the poles of the rotor line up with the teeth of the stator, a force is required to break the attraction, and this force is referred to as cogging torque. Cogging torque is positiondependent, according to the location of the stator teeth relative to the permanent magnets, as the magnets constantly search for a position of minimum reluctance. A motor’s cogging torque profile depends on the number of permanent magnets in the rotor and the number of teeth in the stator and can be minimized through mechanical means by optimizing the number of magnetic poles and teeth, or by skewing or shaping the permanent magnets to make their transition between stator teeth more gradual.
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Stepper Motor – Motion Control HB 08.18 V3 FINAL.indd 115
Cogging in stepper motors Stepper motors also exhibit cogging torque, also called detent torque. Like cogging torque, detent torque is a result of magnetic equilibrium in a nonenergized motor. This magnetic equilibrium must be overcome before the motor will rotate, which means that the detent torque reduces the amount of running torque the motor can produce. The amount of detent torque a stepper motor experiences is proportional to the motor’s speed, so the effect of detent torque on running torque is more substantial at higher motor speeds. PM and hybrid stepper motors (which use a permanent magnet rotor) exhibit detent torque. VR steppers (which use a non-magnetized soft iron rotor) do not. Of PM and hybrid types, hybrid steppers have higher detent torque due to their toothed rotors, which allows them to better manage the magnetic flux between the stator and the rotor. Detent torque is typically 5 to 20% of the motor’s holding torque, which is the amount of torque the motor can produce when the windings are energized but the rotor is stationary. But detent torque is not always a liability: When the motor is decelerating, it counters the motor’s momentum and helps it to stop more quickly.
Stepper motor construction
Shown here is a typical stepper motor and its parts ... including the motor shaft, rotor with teeth, and the stator — composed of pairs of poles which represent a single phase. Some stepper motors (such as PMX series motors from Kollmorgen) have several advantages over servo systems ... including design flexibility in a high torque-to-dollar package. Image courtesy Kollmorgen
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DESIGN WORLD — MOTION
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8/23/18 9:45 AM