MOTION SYSTEMS HANDBOOK
Stepper motors: the basics Integrated stepper motor drives such as the Sure Step series from AutomationDirect feature micro-stepping position and speed control.
A TYPICAL STEPPER MOTOR uses open-loop control as opposed to closed loop like many other motor types, including servomotors. One way that stepper motors are classified is by the number of steps they can be commanded to move. For instance, a 1.8˚ step motor is capable of 200 steps/revolution (1.8 x 200 = 360˚ or one full revolution) in full-step mode. If operated in half-step mode, each step becomes 0.9˚ and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements. Two common types of stepper motors are the permanent magnet (PM) motor and the variable reluctance (VR) type.
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Stepper Motor – Motion Control HB 08.18 V3 FINAL.indd 114
Traditional VR stepper motors have a large number of electromagnets arranged around a central gear-shaped piece of iron. When any individual electromagnet is energized, the geared iron tooth closest to that electromagnet will align with it. This makes them slightly offset from the next electromagnet so when it is turned on and the other switched off, the gear moves slightly to realign. This continues with the energizing and de-energizing of individual electromagnets, thus creating the individual steps of motion. Combining the best features of both PM and VR type stepper motors are hybrids. The rotor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide a path
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to help guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding, and dynamic torque characteristics of the motor when compared with both the variable reluctance and permanent magnet motor. Hybrid stepper motors are usually more expensive than PM stepper motors but can provide better performance with respect to step resolution, torque and speed. Control techniques such as half-stepping and microstepping let designers get even finer movements, which make for more exact output than that from VR stepper motors, which usually can’t be microstepped. Hybrid stepper motors also have higher torqueto-size ratios and higher output speeds than other stepper-motor types, and are also quieter than VR stepper motors.
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8/23/18 9:43 AM