A Stepper Motor is a digital device. Digital information is processed by the Stepper Motor to accomplish an end result, in this case, controlled motion. One may assume that a Stepper Motor will dependably follow digital instructions just as a computer is expected to. This is the distinguishing feature of a step motor.

The Stepper Motor is an electrical motor that is driven by digital pulses rather than a continuously applied voltage. Inherent in this concept is open-loop control, wherein a train of pulses translates into so many shaft revolutions, with each revolution requiring a given number of pulses. Each pulse equals one rotary increment, or step (hence, stepper motor), which is only a portion of one complete rotation.

Therefore, counting pulses can be applied in the Stepper Motor to achieve a desired amount of shaft rotation. The count automatically represents how much movement has been achieved, without the need for feedback information, as would be the case in servo systems.

Although the Stepper motor has been overshadowed in the past by servo systems for motion control, it now is emerging as the preferred technology in more and more areas. The major factor in this trend towards the Stepper Motor is the prevalence of digital control, and the emergence of the microprocessor.

Today we have many Stepper Motor applications all around us. The Stepper Motor is used in printers (paper feed, print wheel), disk drives, photo-typesetting, X-Y plotters, clocks and watches, factory automation, aircraft controls, and many other applications. The ingenuity and further advances in digital technology from researchers will continue to extend the list of applications in which the Stepper Motor will be used.

There are three basic types of Stepper Motor. These Stepper Motor types vary by construction and in how they function. Each of these types of Stepper Motor offers a solution to an application in a different way. The three basic types of Stepper Motor include the Variable Reluctance, Permanent Magnet, and Hybrid.

Variable Reluctance (VR) Stepper Motor
Variable Reluctance Stepper Motors are known for having soft iron multiple rotor and a wound stator. The Variable Reluctance Stepper Motor generally operates in step angles from 5 to 15 degrees at relatively high step rates. They also possess no detent torque. In Figure 5, when phase A is energized, four rotor teeth line up with the four stator teeth of phase A by magnetic attraction. The next step is taken when A is turned off and phase B is energized, rotating the rotor clockwise 15 degrees; Continuing the sequence, C is turned on next and then A again. Counter clockwise rotation is achieved when the phase order is reversed.

Permanent Magnet (PM) Stepper Motor
The Permanent Magnet Stepper Motor differs from the Variable Reluctance Stepper Motor by having permanent magnet rotors with no teeth. These rotors are magnetized perpendicular to the axis. When the four phases are energized in sequence, the rotor rotates as it is attracted to the magnetic poles. The motor shown in Figure 6 will take 90 degree steps as the windings are energized in sequence ABCD. The Permanent Magnet Stepper Motor generally has step angles of 45 to 90 degrees and tends to step at relatively low rates, but produce high torque and excellent damping characteristics.

Hybrid Stepper Motor
The Hybrid Stepper Motor combines qualities from the permanent magnet and variable reluctance steppers. The Hybrid Stepper Motor has some of the desirable features of each. These Stepper Motors have a high detent torque, excellent holding and dynamic torque, and they can operate in high stepping speeds. Step angles of 0.9 to 5.0 degrees are normally seen in the Hybrid Stepper Motor. Bi-filar windings are generally supplied to these Stepper Motors so a single power supply can be used to power the Stepper Motor. The rotor will rotate in increments of 1.8 degrees if the phases are energized one at a time in the order they are indicated at. This Stepper Motor can be driven in two phases at a time to yield more torque. The Hybrid Stepper Motor can also be driven by one then two then one phase to produce half steps of 0.9 degree increments.

There are three excitation modes that are commonly used with Stepper Motors. These Stepper Motor modes are the full-step, half-step- and micro-step.

Stepper Motor - Full-Step
In full step operation, the Stepper Motor steps through the normal step angle e.g. 200 step/revolution motors take 1.8 steps while in half step operation, 0.9 steps are taken. There are two kinds of full-step modes. Single phase full-step excitation is where the Stepper Motor is operated with only one phase energized at-a-time. This mode should only be used where torque and speed performance are not important, e.g. where the motor is operated at a fixed speed and load conditions are well defined. Problems with resonance can prohibit operation at some speeds. This type of mode requires the least amount of power from the drive power supply of any of the excitation modes. Dual phase full-step excitation is where the Stepper Motor is operated with two phases energized at-a-time. This mode provides good torque and speed performance with a minimum of resonance problems. Dual excitation, provides about 30 to 40 percent more torque than single excitation, but does require twice the power from the drive power supply.

Stepper Motor - Half-Step
Stepper Motor half-step excitation is alternate single and dual phase operation resulting in steps one half the normal step size. This mode provides twice the resolution. While the motor torque output varies on alternate steps, this is more than offset by the need to step through only half the angle. This mode has become the predominately used mode by Anaheim Automation because it offers almost complete freedom from resonance problems. The Stepper Motor can be operated over a wide range of speeds and used to drive almost any load commonly encountered.

Stepper Motor - Micro-Step
In the Stepper Motor micro-step mode, a Stepper Motor's natural step angle can be divided into much smaller angles. For example, a standard 1.8 degree motor has 200 steps/revolution. If the motor is micro-stepped with a 'divide-by-10', then each micro-step would move the motor 0.18 degrees and there would be 2,000 steps/revolution. Typically, micro-step modes range from divide-by-10 to divide-by-256 (51,200 steps/rev for a 1.8 degree motor). The micro-steps are produced by proportioning the current in the two windings according to sine and cosine functions. This mode is only used where smoother motion or more resolution is required.

The Stepper Motor is typically controlled by a driver and indexer. The amount, speed, and direction of rotation of a Stepper Motor is determined by the right configuration of digital control devices. The main types of Stepper Motor control devices are: Stepper Motor Drivers, Stepper Motor Control Links, and Stepper Motor Controllers. These devices are set up in figure 8. The Stepper Driver accepts the clock pulses and direction signals and translates these signals into appropriate phase currents for the Stepper Motor. The Stepper Indexer creates the clock pulses and the direction signals for the Stepper Motor. The computer or PLC (Programmable Logic Controller) sends out commands to the indexer.

Anaheim Automation offers a variety of options to customize the Stepper Motor. The list of modifications includes, but is not limited to: shaft, brake, oil seal for an IP65 rating, mounting dimensions, speed, torque, and voltage. Please give Anaheim Automation a call for any custom Stepper Motor applications.