Stepper Motor: Basics, Types, Applications
Special Purpose Motors: An Introduction
Industrial Motors are used to convert electrical energy to mechanical energy. They are neither precision speed nor precision positioning devices. For many automated systems, precise speed and/or precise positioning are required.
Normal Industrial Motors can not be used in these situations, in such cases special purpose motors are used. The stepper motor is one of the special purpose machine widely used in various automated systems.
Stepper Motor Tutorial:
The stepper motor is also called as stepping motor or step motor. This motor rotates through a fixed angular step in response to each input current pulse received by its controller. So it is named as stepper motor.
The Stepper Motor becomes very popular because of its major advantage that it can be controlled directly by computers, microprocessors and programmable controllers. Normally the steppers do not need feedback position indicators, as they step reliably and precisely to a given point when programmed properly. But in critical applications, feedback indication must be used for precise control. Special ICs have been developed for stepper motor control.
- The stepper motors develop torques ranging from 1 µN-m upto 40 N-m in a motor of 15 cm diameter suitable for machine tool applications. Their power output ranges from about 1W to maximum of 2500W.
- The only moving part in a stepper motor is its rotor. The rotor has no windings, commutator or brushes. Because of these feature the motor is quite robust and reliable.
- The torques of various sizes of stepper motors run from 0.5oz-in to 5000 oz-in.
- The rotors of the steppers are either permanent-magnet or variable-reluctance type.
The angle through which the motor shaft rotates for each command pulse is called the step angle. The relationship between steps per revolution and step angle is given by the following formula:
Step angle = 360° / [No. of steps per revolution]
Smaller the step angle, greater the number of steps per revolution and higher the resolution or accuracy of positioning obtained. The step angles can be as small as 0.72° or as large as 90°. But the most common step sizes are 1.8°, 2.5°, 7.5° and 15°.
How can the number of steps per revolution of a stepper motor be increased?
We know that to increase the steps means smaller step angle. We can not use gear train to achieve this. Because gear train is bulk and costly. In addition to that inaccuracies would be introduced by the gear trains inertia and backlash.
The right system for increasing the number of steps per revolution is microstepping system.
- The microstepping system involves the application of varying voltages to the rotor poles.
- For regular stepping the voltage to each pole is either ON or OFF.
- Varying ratios of voltage are applied to adjacent poles for microstepping.
- As these relative voltages vary, the rotor positions itself at various intermediate positions between stator poles.
Some stepper motor operates upto 20,000 steps per second. Still it remain fully in synchronism with the command pulses. When the pulse rate is high, the shaft rotation looks like continuous. Such an operation of stepper motor at high-speed is called 'slewing'.
1. Variable Reluctance Stepper Motor
2. Permanent Magnet Stepper Motor
3. Hybrid Stepper Motor
Applications of Stepper Motor:
- Stepper Motors are used in a variety of automation applications in which a relatively small amount of torque is needed.
- Typical applications include rotary table control, wire-harness assembly, laser or pen positioning, and office peripheral equipment control.
- Stepper motors can be used for precise positioning, without the need for a complicated position indicating feedback system.