Basic Introduction of Step Motors

Introduction

Step motors convert electrical energy into precise mechanical motion. These motors rotate a specific incremental distance per each step. The number of steps executed controls the degree of rotation of the motor’s shaft. This characteristic makes step motors excellent for positioning applications. For example, a 1.8° step motor executing 100 steps will rotate exactly 180° with some small amount of non-cumulative error. The speed of step execution controls the rate of motor rotation. A 1.8° step motor executing steps at a speed of 200 steps per second will rotate at exactly 1 revolution per second.

Step motors can be very accurately controlled in terms of how far and how fast they will rotate. The number of steps the motor executes is equal to the number of pulse commands it is given. A step motor will rotate a distance and at a rate that is proportional to the number and frequency of its pulse commands.

Basic Step Motor System

The diagram above shows a typical step motor based system. All of these parts must be present in one form or another. Each component’s performance will have an effect on the others.

First is the pulse generator, also known as a controller or indexer. The pulse generator will output command pulses that the motor will follow. By altering the frequency of the pulse train, the pulse generator can instruct the motor to accelerate, run at a speed, decelerate or stop. A pulse generator must be present otherwise the motor will not move.

Next is the motor driver. The driver’s function is to control the magnitude and direction of current flow into the motor windings. The driver takes the pulses from the pulse generator and determines how and when the windings should be energized. The windings must be energized in a specific sequence to generate motion.

Finally there is the step motor itself

A step motor has two primary parts; the rotor, the moving piece, and the stator, the stationary piece. The stator contains coils of wire called windings. The rotor spins on bearings or bushings inside the stator. All step motors operate through the principle of the rotor following a rotating magnetic field created by sequencing the flow of current through the stator windings. Each NMB step motor has two phases, which are groups of electrically connected windings. As current is passed through each phase, the motor takes “steps” or small movements to keep in synchronism with the magnetic field. The degree of rotation per step depends on the style of driver used and the construction of the motor.

Step Motor Advantages

Step motors have several advantages over other types of motors. One of the most impressive is their ability to position very accurately. NMB’s standard step motors have a step angle accuracy of +/-5%. The error does not accumulate from step to step. This means that a standard step motor can take a single step and travel 1.8° +/- 0.09°. Then it can take one million steps and travel 1,800,000° +/-0.09°. This characteristic gives a step motor almost perfect repeatability. In motor terms, repeatability is the ability to return to a previously held position. A step motor can achieve the same target position, revolution after revolution.

Step Motor Advantages:

Accuracy & Repeatability – Ability to position accurately.
Responsiveness & Quick Acceleration – Step motors have low rotor inertia, allowing them to get up to speed quickly. This makes step motors an excellent choice for short, quick moves.
Excellent torque for their size – Step motors have the highest torque per cubic inch of any motor.
Positioning Stability – Unlike other types of motors, step motors can be held completely motionless in their stopped position.
Open Loop Control – Open loop control is simpler, more reliable and less expensive than feedback based (closed loop) control. In closed loop systems, encoders are used to count the number of steps taken by the motor. The number of steps taken is compared to the number of step commands given. This feedback is used to make position corrections or initiate alarm signals. Encoders and their associated electronics add additional cost to a motion control system. Assuming that a step motor is properly sized for its load, it should never miss a step, making an encoder unnecessary.
Cost and Reliability – Step motor technology is reliable and proven. It is the most cost effective method of precision position control.