The most complete analysis of DC motor working principle and control circuit (brushless + brush + servo + stepper)

Modern multi-pole, multi-tooth stepper motors have a step accuracy of less than 0.9 degrees (400 pulses per revolution) and are mainly used in highly accurate positioning systems, such as those for magnetic heads in floppy disks/hard drives, printers/plotters or robotic applications. The most commonly used stepper motor is the 200-step-per-revolution stepper motor. It has a 50-tooth rotor, a 4-phase stator and a step angle of 1.8 degrees (360 degrees/(50×4)).

The structure and control of stepper motor

In the simple example of a variable reluctance stepper motor above, the motor consists of a central rotor surrounded by four electromagnetic field coils labelled A, B, C and D. All coils with the same letter are connected together so that applying power to, for example, a coil labelled A will cause the magnetic rotor to align with that set of coils.

By energizing each set of coils in sequence, the rotor can be rotated or "stepped" from one position to an angle determined by its step angle configuration, and by energizing the coils in sequence, the rotor will produce rotational motion.

A stepper motor driver controls the step angle and speed of the motor by energizing the field coils in a set sequence (e.g. “ADCB, ADCB, ADCB, A…”, etc.) and the rotor will rotate in one direction (forward), reversing the pulse sequence to “ABCD, ABCD, ABCD, A…”, etc. and the rotor will rotate in the opposite direction (reverse).

So in the simple example above, the stepper motor has four coils, making it a 4-phase motor, and the number of poles on the stator is eight (2 x 4), with each phase spaced 45 degrees apart. The number of teeth on the rotor is six, spaced 60 degrees apart.

The rotor then has 24 (6 teeth x 4 coils) possible positions or "steps" to complete one full revolution. Therefore, the above step angle is: 360° / 24 = 15°.

Obviously, more rotor teeth and/or stator coils will result in more control and smaller step angles. Likewise, by connecting the motor's electrical coils in different configurations, full-step, half-step and micro-step angles can be achieved. However, to achieve micro-stepping, the stepper motor must be driven by a (quasi-)sinusoidal current, which is expensive to achieve.

The speed of a stepper motor's rotation can also be controlled by varying the time delay (frequency) between the digital pulses applied to the coils. The longer the delay, the slower the speed for one full revolution. By applying a fixed number of pulses to the motor, the motor shaft will rotate a given angle.

The advantage of using a time-delayed pulse is that no additional feedback of any kind is required, since by counting the number of pulses supplied to the motor the final position of the rotor is known accurately. This response to a set number of digital input pulses allows the stepper motor to be operated in an "open loop system", making control easy and cheap.

For example, let's say our stepper motor above has a step angle of 3.6 degrees per step. To make the motor rotate, for example, 216 degrees and then stop at the desired position, a total of: 216 degrees / (3.6 degrees / step) = 80 pulses are required to be applied to the stator coils.

There are many stepper motor controller ICs available which can control stepping speed, rotation speed and motor direction. The SAA1027 is one such controller IC which has all the necessary counters and code conversion functions built in and can automatically drive 4 fully controlled bridge outputs to the motor in the correct sequence.

The direction of rotation can also be selected with single-step mode or continuous (stepless) rotation in the selected direction, but this places some burden on the controller. When using an 8-bit digital controller, 256 microsteps per step can also be achieved

SAA1027 stepper motor control chip

In this tutorial about Rotary Actuators, we looked at brushed and brushless DC motors, DC servomotors, and stepper motors, which are electromechanical actuators that can be used as output devices for position or velocity control.