Stepper Motors
Microstepping
Step motors have several qualities that make them particularly suited for control systems. They are stiff when stopped, produce high torque for their physical size, and are brushless -- making them virtually maintenance free.
Step motors have a serious drawback, however, when used for precise positioning. Standard hybrid step motors have a relatively large step size, usually 1/200 of a revolution or 1.8°. Such large step size can also cause motor-shaft oscillations at resonance points that occur at low speeds. But there is a widely used technique that retains the advantages of step motors and overcomes low speed roughness and low resolution. The technique is called microstepping.
Microstepping increases the position resolution and smoothness of conventional hybrid step motors. This is done with electronic control in the drive circuits. The drive subdivides each full step electronically into a large number of smaller steps. For example, a microstepping drive that subdivides each full step of a 200-step/rev motor into 125 microsteps produces 25,000 steps/rev (200 X 125 = 25,000).
Motors and drives must provide high positional resolution in applications such as semiconductor fabrication. A 25,000 step/rev system attached to a 10-pitch leadscrew on an X-Y table can position a silicon wafer to one part in 250,000/in. This high positional resolution often eliminates gearboxes (and gear backlash) or other mechanical reducers otherwise needed to place wire bonds or test probes on exposed IC wafers. But many applications that do not need high resolution can also benefit from microstepping.
The biggest advantage of microstepping is smooth operation and the elimination of resonance over its entire speed range, typically 0 to 3,000 rpm. Smooth operation permits full torque utilization and freedom from rattling and mechanical wear.
The true accuracy of a microstepping system is usually less than its resolution. System accuracy is a complex function of motor accuracy, electronic tolerances, and errors in the mechanical transmission. But the combination of micropositioning and smooth operation has enabled microstepping systems to become standard in X-Y positioning systems requiring 1 ° in. to 0.001 in. of resolution, and precision grinding, turning, and surface-finishing machines. Other precision motion-control applications include optical scanning and inspection, disk memory media manufacturing, and fiber-optics manufacturing.
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