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To learn and understand the principle of operation of the stepper motors, to me is very important. A stepper motor is always the simplest, cheapest and lighter solution for accurate positioning systems. It this article, i will explain how the stepper motors are made, and how they work. It is necessary to have some very basic knowledge for the operation of DC motors to follow this article. 

First of all, a stepper motor is a motor. This means, that it converts electrical power into mechanical power. The main difference between them and all the other motors, is the way they revolve. Unlike other motors, stepper motors does not continuously rotate! Instead, they rotate in steps (from which they got the name). Each step is a fraction of a full circle. This fraction depends mostly from the mechanical parts of the motor, and from the driving method. The stepper motors also differs in the way they are powered. Instead of an AC or a DC voltage, they are driven (usually) with pulses. Each pulse is translated into a degree of rotation. For example, an 1.8^o stepper motor, will revolve its shaft 1.8^o on every pulse that arrives. Often, due to this characteristic, stepper motors are called also digital motors. 

 A very basic stepper motor

As all motors, the stepper motors consists of a stator an a rotor. The rotor carries a set of permanent magnets, and the stator has the coils. The very basic design of a stepper motor would be as follows:

There are 4 coils with 90^o angle between each other fixed on the stator. The way that the coils are interconnected, will finally characterize the type of stepper motor connection. In the above drawing, the coils are not connected together. The above motor has 90^o rotation step. The coils are activated in a cyclic order, one by one. The rotation direction of the shaft is determined by the order that the coils are activated. The following animation demonstrates this motor in operation. The coils are energized in series, with about 1sec interval. The shaft rotates 90^o each time the next coil is activated:

In this section, i will explain the various ways that the coils are energized, and the results on the motors shaft.

The first way is the one described previously. This is called Single-Coil Excitation, and means that only one coil is energized each time. This method is rarely used, generally when power saving is necessary. It provides less than half of the nominal torque of the motor, therefore the motor load cannot be high.

This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle.

The second and most often used method, is the Full step drive. According to this method, the coils are energized in pairs. According to the connection of the coils (series or parallel) the motor will require double the voltage or double the current to operate that needs when driving with Single-Coil Excitation. Yet, it produces 100% the nominal torque of the motor.

This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle.

This is a very interesting way to achieve double the accuracy of a positioning system, without changing anything from the hardware! According to this method, all coil pairs can be energized simultaneously, causing the rotor to rotate half the way as a normal step. This method can be single-coil or two-coil excitation as well. The following animations make this clear:

With this method, the same motor will have double the steps per revolutions, thus double the accuracy in positioning systems. For example, this motor will have 8 steps per cycle!

Microstepping is the most common method to control stepper motors nowadays. The idea of microstepping, is to power the coils of the motor NOT with pulses, but with a waveform similar to a sin waveform. This way, the positioning from one step to the other is smoother, making the stepper motor suitable to be used for high accuracy applications such as CNC positioning systems. Also, the stress of the parts connected on the motor, as well as the stress on the motor itself is significantly decreased. With microstepping, a stepper motor can rotate almost continuous, like simple DC motors.

The waveform that the coils are powered with, is similar to an AC waveform. Digital waveforms can also be used. here are some examples:

The microstepping method is actually a power supply method, rather than coil driving method. Therefore, the microstepping can be applied with single-coil excitation and full step drive. The following animation demonstrated this method:

Although it seems that the microstepping increases the steps even further, usually this does not happen. In high accuracy applications, trapezoidal gears are used to increase the accuracy. This method is used to ensure smooth motion.