Stepper Motors

What is a stepper motor?

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements.

   

·         The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.

·         The motors rotation has several direct relationships to these applied input pulses.

·         The sequence of the applied pulses is directly related to the direction of motor shafts rotation.

·         The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied

Before going any further you should know the benefits this motor provides over other motors. So let’s have advantages of motors first.

Advantages

1. The rotation angle of the motor is proportional to the input pulse.

2. The motor has full torque at standstill (if the windings are energized)

3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step.

4. Excellent response to starting/stopping/reversing.

5. Very reliable since there are no contact brushes in the motor.

       Therefore the life of the motor is simply dependant on the life of the             bearing.

6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.

8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages

1. Resonances can occur if not properly controlled.

2. Not easy to operate at extremely high speeds.

Open Loop Operation

One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no feedback information about position is needed. This type of control eliminates the need for expensive sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the input step pulses. And this is achievable only when the external load doesn’t overcome its stall torque.

Torque v/s Speed Characteristics

You know what, torque v/s speed characteristics are the key to selecting the right motor and drive method for a specific application.  

These characteristics are dependent upon

-The motor

-Excitation mode and

-Type of driver or drive method.

A typical “speed – torque curve” is shown in above.

To get a better understanding of this curve it is useful to define the different aspect of this curve.

Holding torque:-   The maximum torque produced by the motor at standstill.

Pull-In Curve:- The pull-in curve defines a area refered to as the start stop region.                                

                                        This is the maximum frequency at which the motor can start/stop instantaneously, with a load applied, without loss of synchronism.

Maximum Start Rate:- The maximum starting step frequency with no load applied.

Pull-Out Curve:- The pull-out curve defines an area refered to as the slew region. It defines the maximum frequency at which the motor can operate without losing synchronism.

Maximum Slew Rate:- The maximum operating frequency of the motor with no load applied.

The pull-in characteristics vary, depending on the load. Generally, and as per i experienced, the larger the load the smaller the pull-in area. The decreasing torque output as the speed increases is caused by the fact that at high speeds the inductance of the motor is the dominant circuit element.

Drive Method

·         The shape of the speed – torque curve can change quite dramatically depending on the type of driver used.

·         The bipolar chopper type drivers which Ericsson Components produces will maximum the speed – torque performance from a given motor.

·         Most motor manufacturers provide these speed-torque curves for their motors.

·         It is important to understand what driver type or drive method the motor manufacturer used in developing their curves as the torque vs. speed characteristics of an given motor can vary significantly depending on the drive method used.

When to Use a Stepper Motor

A stepper motor can be a good choice whenever controlled movement is required. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Because of the inherent advantages listed previously, stepper motors have found their place in many different applications. Some of these include printers, plotters, high-end office equipment, hard disk drives, medical equipment, fax machines, automotive and many more.