A stepper motor is a DC motor that turns in discrete steps. At each step, the motor holds its position without requiring power. This eliminates the need for feedback positioning sensors as long as the motor is sized correctly and no steps are skipped. Removing the feedback requirement and the digital nature of a stepper motor makes it easy to integrate in to digital systems.
While DC brush motors turn continuously when power is applied, steppers motor only move one discrete step.
This is because of the unique construction of a stepper motor and the use of multiple 'toothed' electromagnets around a gear-shaped magnetically attractive portion of the drive shaft. The stepping motion is achieved by using two sets of teeth that are offset a short distance apart. When one set of teeth is energized it pulls the drive shaft gear in to alignment with them.
When the next set of teeth is energized they pull the gear in to alignment with them, moving a step in the process. By controlling the spacing between the teeth, the amount of rotation in a step can be controlled.
The discrete steps in a stepper motor allow the motor to run without feedback in an open loop configuration. The danger of running without feedback is that the position can be lost if the load becomes greater that what the motor can hold and additional steps are taken without a command. This happens when the inertia of moving fast loads becomes greater than the holding strength of the motor and if the motor is switched too fast to respond. For these reasons, stepper motors are often over engineered for their application rather, or additional feedback sensors or encoders are used to ensure that the position is not lost.
Some stepper motors incorporate a rotary encoder or resolver in to their construction which prevents the loss of position and can also be used to optimize the torque generated by the stepper.
Types of Stepper Motors
There are three main types of stepper motors, the permanent magnet stepper motor, variable reluctance stepper motor, and hybrid stepper motor.
Permanent Magnet Stepper Motors
The permanent magnet stepper motor uses a permanent magnet on the motor shaft (rotor) and electromagnets around the perimeter (stator). Steps are achieved by using the attractive and repulsive interactions with the magnet and electromagnets. This gives the permanent magnet motor high torque but without the resolution that dedicated teeth provide. Permanent magnet stepper motors have the coarsest step sizes, generally from 7.5 to 15 degrees per step (24-48 steps per revolution).
Variable Reluctance Stepper Motors
Variable reluctance stepper motors have a magnetically, gear shaped rotor that are attracted to the toothed magnet poles. The teeth help to greatly increase the resolution of the stepper motor at the cost of a reduction in torque.
Hybrid Stepper Motors
Hybrid stepper motors combine the benefits of both permanent magnet and variable reluctance stepper motors to achieve the highest torque in a small package. The geared rotor of the variable reluctance stepper motor is combined with an axially aligned permanent magnet which increases the performance of the teeth and increases the holding torque of the motor. Hybrid stepper motors have step sizes from 0.9 to 3.6 degrees (100-400 steps per revolution), with 1.8 degree steps being the most common.
Stepper Motor Phases
Stepper motors are available in two basic wiring configurations, bipolar and unipolar. Unipolar motors have one winding with a center tap for each phase. This allows the motor direction to be reversed easily by changing which section of the phase is powered rather than reversing the flow of current. This allows the control circuitry to be very simple. Unipolar motors typically have six leads, three for each phase, but can also be found with five leads, with the center tap of both phases internally connected. Unipolar motors can be easily controlled with a microcontroller or stepper motor controller and are very affordable.
Bipolar motors have one or two windings without a center tap for each phase. In order for the direction of rotation to be reversed on a bipolar motor, the current direction needs to be reversed. This requirement makes the driving circuitry more complicated and is generally implemented with an H-bridge control arrangement or an H-bridge motor driver. While more complicated to drive, bipolar motors are much stronger for the same weight and size. Bipolar motors can be configured with series or parallel windings, allowing them to be driven with lower current in series or higher inductance and greater torque in parallel. Bipolar motors generally have four or eight leads, two or four per phase, allowing them to be distinguished from the five and six lead unipolar motors.