Stepper Motors: Explained.

 

Stepper Motor:

            A stepper motor is one of the most common motors used in motion control applications. These motors are used mostly in positioning applications because they can be accurately controlled down to fractions of a degree without the use of feedback devices such as encoders or resolvers. They are typically operated open-loop without the need for tuning parameters as in closed-loop servo systems.

Working Principle:

The principle of operation for stepper motors is fairly straightforward. Traditional variable reluctance 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 to realign. This continues with the energizing and D energizing of individual electromagnets. Thus creating the individual steps of motion steppers are generally classified by the number of steps they can be commanded to move. For instance, a one-point eight degrees step motor is capable of 200 steps per revolution in full-step mode. If operated in half-step mode, each step becomes 0.9 degrees and the motor can then turn 400 steps per revolution. Another mode called micro-stepping subdivides the degrees per step even further allowing for extremely precise movements.

Types:

            There are several different stepper motor technologies including permanent magnet motors, variable reluctance, and hybrid types, the hybrid step motor is more expensive than permanent magnets and different motors but delivers better performance concerning step resolution, torque, and speed. These motors combine the best features of both the permanent magnet and variable reluctance type stepper motors. The motor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet around its shaft. The rotor teeth provide a path to help guide the magnetic flux to preferred locations in the air gap. This increases the detent, holding, and dynamic torque characteristics of the motor.

 

Stepper motors are relatively inexpensive because the speed is proportional to the frequency of the input pulses. A range of speeds is attainable however while stepper motors are capable of producing high torque at low speeds. They generally are well suited for lower power applications but not for applications requiring lots of torque to move heavier loads. They are best for applications requiring the control rotation angle speed and position. Also, stepper motors are generally not easy to operate at extremely high speeds and as the motor speed increases, the torque decreases. Steppers are generally sized according to torque curves which are typically specified by the manufacturer.

 

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