Features of Stepper Motors & How they are applied in Industrial Control

A stepper motor is a permanent magnet or variable reluctance DC motor that has the following characteristics:

• It is able to rotate in both directions
• It can move in precise angular increments
• It can sustain holding torque at zero speed
• Can be controlled with digital circuits

Stepper motor moves in accurate angular increments, known as steps in response to digital pulses sent to an electric drive circuit. The number and rate of pulses control the position and speed of the motor shaft.

The rotor of the stepper motor is usually a permanent magnet with several poles and a stator with several windings. A stepper with eight magnetic poles and six-section stator is shown in figure 1(a) below.

The stepper motor can be reversed by changing the sequence of the driving pulses. Stepper motors are available with stepping angles of 0.9°, 1.8°, 3.6°, 7.5°, 15°, 18°, etc.

Generally, stepper motors are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, which result in shaft increments of 30°, 15°, 5°, 2.5°, 2° and 1.8° per step. Special micro-stepping circuitry can be designed to allow many steps per revolution, often 10,000 steps per rev or more.

Stepper motors are either bipolar, requiring two power sources or a switchable polarity power source, or unipolar requiring only one power source. They are powered by DC sources and require digital circuitry to produce coil energizing sequences for rotation of the motor.

Since the motor steps are a known angle with each input pulse, feedback is not required. Nevertheless since only the relative position is known, loss of power will cause the loss of position information, to avoid this, systems using stepper motors require a position reference or to put in other way, applications where exact control is critical, encoders or other position sensors are employed to ensure accuracy.

In general, stepper motors produce less than 1 hp (746 W), therefore they are only used in low-power position control.

The variable reluctance stepper motor has a ferromagnetic rotor rather than a permanent magnetic rotor. Motion and holding result from the attraction of stator and rotor poles to positions with minimum magnetic reluctance (magnetic field resistance) that allow for maximum magnetic flux. A variable reluctance stepper motor has the advantage of a lower rotor inertia and hence faster dynamic response while the permanent magnet stepper motor has the advantage of a small residual holding torque called detent torque even when the stator is not energized.

Related: Pulse Width Modulation (PWM) Speed Control of a DC motor

Operation of the Stepper Motor

To understand how the stepper motor moves in incremental steps, consider a simple design of four stator poles and a permanent magnet rotor shown in figure below.

In step 0, the rotor is in equilibrium because opposite poles on the stator and rotor are adjacent to and attract each other. Unless the magnet polarities of the stator poles are changed, the rotor will remain in this position and can withstand an opposing torque up to a value called the holding torque. When the stator polarities are changed as illustrated in figure 1(b), a torque is applied to the rotor causing it to move 90° in clockwise direction to a new equilibrium position as illustrated in step 1. When the stator polarities are again changed as shown in step 1 and step 2, the rotor experiences a torque driving it to step 2. By successfully, changing the stator polarities this way, the rotor can move to successive equilibrium positions in the clockwise direction. The sequencing of the pole excitations is the manner by which the direction of rotation occurs. Counter clockwise motion can be achieved by applying the polarity sequence in the opposite direction. The motor torque is directly related to the field strength of the poles and the rotor.

Stepper motors are available in many different designs with a wide selection of the number of poles and drive requirements.

Stepper motor driver is a circuit that correctly applies signals to the poles of a stepper motor for rotation. The driver circuits come in form ICs like Allegro Micro systems UCN 5804B, E-lab’s EDE1200, Signetics SAA 1027, etc.

The block diagram below shows how a stepper motor used in an industrial position control.