The Stepper-motor Explained The Application Of Rugged Trustworthiness!

A stepper-,motor is most frequently used in applications where cost effectiveness

must be maintained through the design, and where ruggedness and the ability to work with total predictability in a range of environments is paramount.

The construction of the motor is very simple, hence its ability to operate in a range of challenging environments. It is effectively a metal gear wheel surrounded by electromagnets. Each tooth on the wheel represents a step. The more teeth and the more magnets, the more precise the steps will be "" but the bigger the whole stepper-motor has to be too, in order to accommodate all the extra kit.

At all sizes, though, the motor is rugged and predictable. You turn on one magnet and the wheel moves around as it attracts the next tooth on the cog. Turn the magnets on in serial and the cog keeps on turning.

While this allows for a pretty good amount of precision in motion and direction control, there are one or two payoffs that the designer must consider. The first has to do with torque.

At low speeds, the torque of the stepper-motor is very high. This allows for exceptional accuracy and the robustness of operation already mentioned. Effectively the highest torque in the motor is found when the cog is at rest, so the slower the wheel turns the more torque you have holding the operation in place.

However, as the motor wheel speeds up, the torque diminishes, so at high speeds there is an appreciable loss of control over the wheel. As such the stepper-motor is far less suitable for operations where continual and rapid movement of the cog wheel is required.

This loss of torque at high speeds ties into another consideration the designer has to take under advisement "" namely the potential for gear wobble when steps are completed.

Because the cog teeth are attracted into a magnetic field there is a certain degree of vagueness involved before the wheel snaps to and the step is completed. This is because a magnetic field is broader than the cog tooth, with weaker influence in its outer frequencies and stronger influence at the centre. The step completes when the tooth falls into synch with the frequency of the electromagnetic field and stops moving "" but the movement of the tooth into that field normally involves that it goes past the point of maximal frequency, then rocks back into it. So there is a perceptible judder in the wheel action, which at high speeds may make proper control impossible.

The third consideration is one of size. The stepper-motor, as noted, has to get progressively larger the more teeth are added to the wheel. While this means a greater degree of precision thanks to the larger number of steps involved in a full turn, it also means there"s a theoretical upper limit to the size of the wheel against the power required to turn it and of course the finished size of the design.

The stepper-motor is perfect for low speed, high torque applications that need robustness and simplicity first and foremost.

by: Ewan Fisher

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