Motors for Makers: Introduction to Stepper Motors for Motion Control
In this and the following chapter, the primary concern is motion control—making sure the motor turns with a specific angle and/or speed. This book discusses two types of motors intended for motion control: stepper motors and servomotors. I’ll refer to them as steppers and servos, respectively, and this chapter focuses on steppers.
A stepper’s purpose is to rotate through a precise angle and halt. The speed and torque of the rotation are secondary concerns. As long as the stepper rotates through the exact angle and stops, its mission is accomplished. Each turn is called a step, and common step angles include 30°, 15°, 7.5°, 5°, 2.5°, and 1.8°.
Due to their simplicity and precision, steppers are popular in electrical devices. Analog clocks, manufacturing robots, and printers (2D and 3D) rely on steppers for motion control. An important advantage is that the controller doesn’t have to read the stepper’s position to determine its orientation. If the stepper is rated for 2.5°, each control signal will turn the rotor through an angle of 2.5°.
For many applications, we want the step angle to be as small as possible. The smaller the motor’s step angle, the greater its angular resolution. Another important figure of merit is torque, particularly holding torque. A stepper is expected to hold its position when it comes to a halt, and holding torque identifies the maximum torque it can exert to maintain its position.
Modern steppers can be divided into three categories:
- Permanent motor (PM)—High torque, poor angular resolution
- Variable reluctance (VR)—Excellent angular resolution, low torque
- Hybrid (HY)—Combines structure of PM and VR steppers, provides good torque and angular resolution
The first part of this chapter examines these categories in detail. In each case, I’ll discuss the motor’s fundamental operation and present its advantages and disadvantages. The last part of the chapter explains how steppers can be controlled with electrical circuits.
4.1 Permanent Magnet (PM) Steppers
Small and reliable, permanent magnet (PM) steppers are popular in embedded devices such as disk drives and computer printers. Figure 4.1 depicts the ST-PM35 stepper from Mercury Motor.
Figure 4.1 A permanent magnet (PM) stepper motor
PM steppers have a lot in common with the brushless DC (BLDC) motors discussed in the preceding chapter. In fact, you can think of a PM stepper as a BLDC whose windings are energized to provide discrete rotation instead of continuous rotation.
The preceding chapter introduced the brushless DC motor and its two subcategories: inrunners and outrunners. PM steppers are similar to inrunners in many respects, and a good way to introduce them is to compare and contrast them with inrunner BLDCs. Figure 4.2 illustrates the internal structure of a simple PM stepper.
Figure 4.2 Internal structure of a permanent magnet (PM) stepper motor
There are five important similarities between PM steppers and inrunner BLDCs:
- Neither motor has a brush or a mechanical commutator (all steppers discussed in this book are brushless).
- The rotor is on the inside, with permanent magnets mounted on its perimeter.
- The stator is on the outside, with electromagnets (called windings) inside slots.
- The controller energizes the windings with pulses of DC current.
- Many of the windings are connected together. Each group of connected windings forms a phase.
PM steppers are brushless and receive DC pulses from the controller. For this reason, they could be classified as BLDCs. But in this book, as in other literature, we’ll only employ the term BLDC for motors that aren’t specifically intended for motion control.
Let’s look at the differences between the two types of motors. Table 4.1 contrasts the characteristics of PM steppers with those of inrunner BLDCs.
Table 4.1 Contrasting Characteristics of PM Steppers and Inrunner BLDCs
Intended for discrete rotation.
Intended for continuous rotation.
Almost always has two phases.
Almost always has three phases.
Controller energizes one or two phases at a time.
Controller energizes two phases at a time and leaves third phase floating.
Many windings and rotor magnets.
Few windings and rotor magnets.
From a structural perspective, the primary difference between PM steppers and inrunners is that PM steppers have more windings and rotor magnets. As it turns out, this is necessary to make the angular resolution as small as possible. The following discussion explains why this is the case.
To understand how a PM stepper operates, it’s crucial to see how its step angle is determined by the number of windings and rotor magnets. This discussion focuses on the motor depicted in Figure 4.2. Its stator has 12 windings and its rotor has six magnets mounted on its perimeter.
PM steppers are generally two-phase motors. In the figure, the different phases are denoted A and B. The windings labeled A’ and B’ receive the same current as those labeled A and B, but in the opposite direction. That is, if A behaves as a north pole, A’ behaves as a south pole.
Each winding has one of three states: positive current, negative current, and zero current. For this discussion, positive current implies a north pole and negative current implies a south pole.
Now let’s see how these motors operate. Figure 4.3 illustrates a single turn of a PM stepper. In the windings, a small “N” implies that the winding behaves like a north pole due to positive current. A small “S” implies that the winding behaves like a south pole due to negative current. If a winding doesn’t have an N or S, it isn’t receiving current.
Figure 4.3 30° rotation of a PM stepper motor
In Figure 4.3a, A is positive (north pole), A’ is negative (south pole), and Phase B isn’t energized. The rotor aligns itself so that its south poles are attracted to the A windings and its north poles are attracted to the A’ windings.
In Figure 4.3b, B is positive (north pole), B’ is negative (south pole), and Phase A isn’t energized. The rotor rotates so that its poles align with the B and B’ windings. The rotation angle equals the angle between the A and B windings, which means the rotor turns exactly 30° in the clockwise direction. This arrangement of eight windings and six poles is common for PM stepper motors, though others turn at angles of 15° and 7.5°.
In case this isn’t clear, let’s look at a second movement. Figure 4.4 presents another 30° rotation of a PM stepper motor.
Figure 4.4 Further rotation of a PM stepper motor
In Figure 4.4a, B is negative (south pole), B’ is positive (north pole), and A isn’t energized. The rotor is positioned so that its poles align with the B windings.
In Figure 4.4b, A is positive (north pole), A’ is negative (south pole), and B isn’t energized. The rotor turns exactly 30° in the clockwise direction to align itself between the A windings.
The controller’s job is to deliver current to the windings so the rotor continues turning in 30° increments. The difference in control signaling is a major difference between steppers and BLDCs. The last part of this chapter discusses the circuitry needed to govern a stepper’s operation.