Stepper Motor: Types Of Stepper Motors

Stepper Motor

A stepper motor, or stepping motor converts electronic pulses into proportionate mechanical movement. Each revolution of the stepper motor's shaft is made up of a series of discrete individual steps. A step is defined as the angular rotation produced by the output shaft each time the motor receives a step pulse. These types of motors are very popular in digital control circuits, such as robotics, because they are ideally suited for receiving digital pulses for step control. Each step causes the shaft to rotate a certain number of degrees. A step angle represents the rotation of the output shaft caused by each step, measured in degrees. Figure 1-1 illustrates a simple application for a stepper motor. Each time the controller receives an input signal, the paper is driven a certain incremental distance. In addition to the paper drive mechanism in a printer, stepper motors are also popular in machine tools, process control systems, tape and disk drive systems, and programmable controllers.

Figure 1-1

The most popular types of stepper motors are permanent-magnet (PM) and variable reluctance (VR). Today we're going to discuss about permanent-magnet (PM) stepper motor.

Permanent-magnet (PM) Stepper Motors

The permanent-magnet stepper motor operates on the reaction between a permanent-magnet rotor and an electromagnetic field. Figure 1-2 shows a basic two-pole PM stepper motor. The rotor shown in Figure 1-2(a) has a permanent magnet mounted at each end. The stator is illustrated in Figure 1-2(b). Both the stator and rotor are shown as having teeth. The teeth on the rotor surface and the stator pole faces are offset so that there will be only a limited number of rotor teeth aligning themselves with an energized stator pole. The number of teeth on the rotor and stator determine the step angle that will occur each time the polarity of the winding is reversed. The greater the number of teeth, the smaller the step angle.

Figure 1-2 Components of a PM stepper motor: (a) Rotor; (b) stator

When a PM stepper motor has a steady DC signal applied to one stator winding, the rotor will overcome the residual torque and line up with that stator field. The holding torque is defined as the amount of torque required to move the rotor one full step with the stator energized. An important characteristic of the PM stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. When no power is applied to the windings, a small magnetic force is developed between the permanent magnet and the stator. This magnetic force is called a residual, or detent torque. The detent torque can be noticed by turning a stepper motor by hand and is generally about one-tenth of the holding torque. Figure 1-3(a) shows a permanent magnet stepper motor with four stator windings. By pulsing the stator coils in a desired sequence, it is possible to control the speed and direction of the motor. Figure 1-3(b) shows the timing diagram for the pulses required to rotate the PM stepper motor illustrated in Figure 1-3(a). This sequence of positive and negative pulses causes the motor shaft to rotate counterclockwise in 90° steps. The waveforms of Figure 1-3(c) illustrate how the pulses can be overlapped and the motor made to rotate counterclockwise at 45° intervals.

Figure 1-3 (a) PM stepper motor; (b) 90 step; (c) 45 step.

A more recent development in PM stepper motor technology is the thin-disk rotor. This type of stepper motor dissipates much less power in losses such as heat than the cylindrical rotor and as a result, it is considerably more efficient. Efficiency is a primary concern in industrial circuits such as robotics, because a highly efficient motor will run cooler and produce more torque or speed for its size. Thin-disk rotor PM stepper motors are also capable of producing almost double the steps per second of a conventional PM stepper motor. Figure 1-4 shows the basic construction of a thin-disk rotor PM motor. The rotor is constructed of a special type of cobalt-steel, and the stator poles are offset by one-half a rotor segment.