Step Motor

Step Motors are a specific type of Motor whose rotation is separated into "steps" and are capable of rotating a specified number of "steps". Step motors are often used in robotics or when any time specific movements or rotations are needed. For instance, a "200 step" step motor is capable of rotating in 1.8deg (360deg / 200) increments or steps. A user who needs the motor to rotate 90deg could tell this motor to rotate 50 steps to achieve this.

Step motors can rotate in either direction without changing electrical polarity and are able to hold their position steady when not rotating. To achieve this, a step motor is always fully powered, and its speed/direction is adjusted not by changing voltage or polarity, but by moving from one step to another at faster speeds, and in a specified direction.

The motor's Stator is typically wound with two phases, although motors from some manufacturers may be wired with up to five. Modern stepper motors have 200 "teeth" on the rotor, and thus are capable of 200 full steps per revolution. Typically they are sold as "1.8 degree step motors" because at 200 steps per 360-degree revolution, each step works out to 1.8 degrees.

Anatomy A step motor is composed of two parts, a rotor and a stator.

The rotor is the part that spins, and is composed of a North-South set of magnets notched to give a total of 200 "teeth". Large, so-called "multi-stack" motors simply have a longer rotor containing multiple North-South sets of permanent magnets. The rotor is mounted inside the stator, and is supported by bearings on either end. The part of the rotor that extends beyond the housing of the motor is called the "shaft", and is coupled to other parts of the machine to cause useful motion.

The stator is the body of the motor, and houses the motor windings. While the number of phases is unimportant for this discussion, all phases are wound into the motor's stator. They are energized in a specific sequence, as specified by the manufacturer. The motor windings use insulated magnet wire to create a series of electromagnets. These electromagnets interact with the permanent magnets on the rotor to cause motion.

Operation
The theory of operation is quite simple: phases are energized in a specific sequence to bring the rotor to rest at each step. This step-and-rest move is very simple to configure, and thus a step motor may be controlled by basic electronics. However, the trade-off is that this produces a very rough move. Step motors create a significant amount of audible noise, and are not the best choice if you require very smooth motion.

To combat this, a technique known as Microstepping was developed. Microstepping motor drivers are capable of partially energizing the motor's phases, stopping the motor between phases. For example, in a typical Bipolar motor configuration, adjacent phases are fully energized at opposite polarities. Since opposites attract with magnets, the rotor will be locked on one full step. If however, both phases are energized at the same polarity, the rotor will be "pushed" away from both adjacent full step positions, and will come to rest between them. This would be considered a "half step". High-end step motor drivers are capable of many thousands of steps per revolution, made possible by very tight control of the motor's phases.

Ease of use
Step motors are a good choice for beginners, as they can be operated with very simple electronics. A controller may be constructed from a few transistors or purchased as a single integrated circuit at various power levels. Some simple step drivers have been designed specifically for use in hobby systems, and are incredibly simple to use. One such example is the EasyDriver [], available from SparkFun[]. There are many other options available, varying widely in capability, complexity, and price.

Performance
Step motors are the ideal choice for low speed, high torque applications. Step motors will produce something close to their rated torque up to a specific speed, usually around 300-500RPM. Beyond this speed the available torque drops off significantly. This value depends on the specific motor used, of course, but in general a step motor is best suited for low-speed applications.

Reference

 * Step Motor: http://www.sparkfun.com/commerce/product_info.php?products_id=9238
 * EasyDriver: http://www.sparkfun.com/commerce/product_info.php?products_id=9402
 * http://en.wikipedia.org/wiki/Integrated_circuit
 * PMinMO.com: http://pminmo.com/PMinMOwiki/index.php5?title=Motors
 * RepRap: http://reprap.org/wiki/StepperMotor
 * Stepper Motor Basics: http://www.solarbotics.net/library/pdflib/pdf/motorbas.pdf