This chapter has three goals: explain what stepper motors are, present the main types of steppers, and show how steppers can be controlled by a circuit. The first goal is straightforward. A stepper motor is a motor intended to turn at a precise angle (the step angle) and halt. Torque is usually more of a concern than speed, and the torque exerted to hold the rotor’s position is called the holding torque.
The first of three types of stepper motor discussed in this chapter is the permanent magnet (PM) stepper. These motors have almost exactly the same structure as the inrunner brushless DC motors discussed in Chapter 3. One significant difference is that PM steppers have many more windings in the stator and magnets in the rotor. These additional windings and magnets make it possible for the PM stepper to turn at step angles such as 15° and 7.5°.
The second stepper type is the variable reluctance (VR) stepper. Like PM steppers, these have windings in the stator. But instead of having magnets on the rotor, the rotor of a VR stepper has teeth. A rotor can support many more teeth than magnets, so the rotor of a VR stepper turns at smaller angles than that of a PM stepper. However, because the teeth aren’t magnetized, the rotor is less attracted to the stator’s windings. This reduces the stepper’s torque to such an extent that VR steppers are rarely encountered in practical systems.
The last stepper type combines the advantages of PM steppers and VR steppers. The rotor of a hybrid (HY) stepper is divided into two or more sections called rotor poles. Each rotor pole is magnetized to behave like a north or south pole, and each has a set of teeth around its perimeter. These teeth are attracted to similar teeth on the stator. Because of the rotor’s magnetization, the HY stepper has torque similar to that of the PM stepper. Because of the rotor’s teeth, the HY stepper has angular resolution similar to that of the VR stepper. Common step angles of an HY stepper are 1.8° and 0.9°.
When you’re designing a control circuit for a stepper, it’s important to know whether the motor is bipolar or unipolar. A bipolar stepper has four wires that correspond to the A, B, A’, and B’ windings. These require H bridges to deliver current in the forward and reverse directions. Unipolar steppers have additional wires that deliver power to the windings. Unipolar steppers are easier to control than bipolar steppers but are less efficient.
The drive mode identifies how the controller energizes the stepper’s windings. The simplest drive mode is full-step (one phase on), in which only one winding is energized at a time. For increased torque, the full-step (two phases on) mode energizes two windings at a time. For twice the angular resolution, the half-step mode alternates between energizing one and two windings.
The fourth drive mode is microstep mode. In this mode, the controller divides its control signals into multiple signals of sinusoidal shape. This turns the rotor in tiny step angles to ensure that the rotation is as smooth as possible. Microstepping has been analyzed by many engineers and researchers, but if your system needs smooth motion control, you may want to consider a servomotor instead of a stepper motor. The next chapter presents this fascinating topic.