Step Motor

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

The Step 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, which is only a portion of one complete rotation. Therefore, counting pulses can be applied in the Step 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 Step 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 Step Motor is the prevalence of digital control, and the emergence of the microprocessor.

Today we have many Step Motor applications all around us. The Step 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 Step Motor will be used.

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

Variable Reluctance (VR) Step Motor
The Variable Reluctance Step Motor is known for having soft iron multiple rotor and a wound stator. The Variable Reluctance Step 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) Step Motor
The Permanent Magnet Step Motor differs from the Variable Reluctance Step 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 Step 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 Step Motor
The Hybrid Step Motor combines qualities from the permanent magnet and variable reluctance steps. A Hybrid Step Motor has some of the desirable features of each. This Step Motor has 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 Step Motor. Bi-filar windings are generally supplied to this type of Step Motor so a single power supply can be used to power the Step 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 Step Motor can be driven in two phases at a time to yield more torque. The Hybrid Step 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 the Step Motor. These Step Motor modes are the full-step, half-step- and micro-step.

Step Motor - Full-Step
In full step operation, the Step 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 Step 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 Step 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.

Step Motor - Half-Step
Step 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 Step Motor can be operated over a wide range of speeds and used to drive almost any load commonly encountered.

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

Anaheim Automation offers a variety of options to customize the Step 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 Step Motor applications.

Step Motor: A basic summary of characteristics and uses

Step motor products are used all around the world because of their wide range of applications. The three types of Step Motor products that exist consist of variable-reluctance, the permanent-magnet, and the hybrid step motor. The different types of Step Motors make it useful for a variety of applications as a constant power device.

The Step Motor is often put to use for:
•Positioning objects
• Conveyor belts
•Assembly Lines
•Grinding and drilling machines
•Lathes
•Laser Cutting


Suppose you need a machine that performs precise positioning operations; a Step Motor is ideal for handling both complex positions and moments. It also offers a high level of power at a convenient size. An escalator demonstrates how a Step Motor can be effectively used in operating a conveyor belt by keeping the escalators speed constant throughout the entire time of operation. The torque of the Step Motor is capable of maintaining this speed despite carrying heavy loads, sometimes totaling over a thousand pounds. The Step Motor achieves this torque by running at low rpms. The Step Motor is able to achieve its full torque by energizing the windings at stand still.

Assembly lines today often use the Step Motor because of its precision. The Step Motor runs off of an open-loop system, meaning it keeps track of the input step pulses so no feedback info on the position is needed. Another reason the Step Motor is useful on assembly lines is because they have an accurate response time when starting, stopping and reversing.

All of these characteristics of the Step Motor make it a common choice for many different applications. In addition, the machines are both cost effective by using open-loop control and reliable because they don’t contain brushes. The Step Motor has been around for many years and will continue to be put into use for many years to come.

How to Measure Torque Requirements

The primary question Anaheim Automation needs answered regarding the application of a Step Motor is, "What is the torque requirement of the mechanism driven?" Accurate measurements of the torque requirements will facilitate in the selection of a Step Motor and driver/controller. A torque wrench is perhaps the easiest method to determine torque. The wrench's gauge will indicate the torque measurement in units of ounce-inches or pound-inches. A torque watch can also be used; it is an instrument that works similarly to a torque wrench and attaches to the end of a shaft. A torque watch can be purchased at locations where precision instruments are sold.

Another way to measure torque is the old "fish scale method," which involves purchasing a spring scale and a pulley to fit the desired shaft. First mount the pulley onto the shaft, securing a strong string to the pulley. Wind the string around the pulley a few times, attaching the other end to the scale. Continue to pull on the string until the shaft begins to turn. The torque in pound inches is determined by multiplying the force in pounds displayed on the scale, with the radius of the pulley in inches. The radius of the pulley is the distance between the shaft’s center and the string wound around the pulley. This crude method can be used to determine both the starting and running torque; depending on how precisely the test is conducted.

The same task can be performed without a spring scale, simply by getting the shaft to turn by adding more weight. Then place the weight on a scale and multiply the pounds or ounces times the radius in inches to determine the torque. If unsure in how to select an appropriate Step Motor for your application, contact our Applications Department.

Flexibility is the Key to Successful Automation

Despite the familiarity Anaheim Automation, Inc. has with helping in the design of automating equipment in today’s world, there is often an instance where automated controls are not used to perform a designated task without the presence of human judgment. Consequently, this type of procedure can limit the amount of alternative methods of production, and be labor intensive. This was present in the case of the oil drilling lathe project that came to us.

A large lathe was used to thread drill pipe for oil well drilling operations. A drill pipe’s diameter varies, up to two feet in diameter. Therefore, it is essential to keep close thread tolerances to ensure even stress points during drilling. Anaheim Automation needed to coordinate the programming, the electronics, and other disciplines (i.e. hardware and software support), required for the development of this automated control system. Together with the customer – a machinery builder - we achieved an automated control system that incorporates the skill of the operator, into an automated performance using Step Motor Drives.

This was a large endeavor; many features were built into the machine, including the depth of cut, number of passes, safety cutoff with automatic retract, and rapid traverse. Anaheim Automation succeeding in providing maximum performance in this application using high-performance step motors, Step Motor Drives, and stepper controllers. When completed, essentially all the customer needed to do was to press a button, and the lathe cut and finished one perfect thread. The machine was a success, due to its relatively easy operation.

Unfortunately, this first lathe design was automated to the point where everyone that used it was forced to mold to its abilities; as result a new criteria was developed. The new lathe design provided customers with the ability to perform a complete threading operation without mediating the process, and still allowed them to have the luxury of customization found in manual machines. The customer can disable the automated process and utilize the machine completely manually, if desired. The system could even be installed on an already existing manual machine.

Due to their ability to increase production speed, as well as the fact that they can be operated safely and easily, larger lathes can be found nationwide at drilling sites being operated by relatively inexperienced workers. In this application, Anaheim Automation found a way to maximize the performance of Step Motor Drives automated threading pipe by incorporating human judgment and control, while avoiding obsolescence among older technology.