Increasing demand of Automation, Robots, 3D Printers, CNC Machining leading to a demand of powerful, quick responding and accurate positioning system. For this purpose close loop servo systems are broadly used. To obtain desired positioning and accuracy, these systems have a feedback mechanism with encoders and sensors. The close loop servo systems are expensive and complex. In past few decade an another alternative came in existence as stepper drive system. This system is capable to maintain positional accuracy without any feedback mechanism. Since it can work in an open loop system. There are other more benefits of using stepper drive system. Stepper Drives are inexpensive and simpler as compared to close loop servo system. One of the most important component of the system with which the system is named “Stepper” i.e. “Stepper Motor”. The Stepper Motor is the main component of the system. In this article we will discuss about Stepper Motors.
Stepper motors are brushless DC motors, unlike other motors these motor gives output in the form of discrete incremental steps. Each step is of same size. Every step movement results in a specific angular movement of motor shaft. These motors require electric pulse as Input signal. Electric pulse are generated and controlled by controller and sent to motor through motor driver circuit.
ON/OFF action of electric supply is known as electric pulse. Each ON/OFF cycle constitute a single electric pulse.
The angle covered by the motor shaft while moving a single step is termed as step angle. Suppose one revolution of motor is divided into 200 discrete steps then a single step movement will cause the motor shaft to rotate by 1.8o.
Advantages of Stepper Motors
Stepper drive system is a best inexpensive and simpler alternative for expensive and complex close loop servo systems. Servo system require feedback mechanism (A compulsory part of close loop system) to provide feedback to controller in order to obtain desired output. Sensors and encoders are the part of this feedback mechanism.While in stepper drive system a feedback mechanism is not compulsory, this system can work as an open loop system. The output result is tracked by controller by keeping the record of input pulse.
These motors are brushless motors, since their service life totally depends on the shaft bearings.
Motor speed is directly proportional to electric pulse frequency [Hz]. Since speed and acceleration is regulated with the help of controller by controlling pulse frequency.“Pulse frequency is defined as number of pulse produced in unit time.”Speed is achieved by increasing the pulse frequency and acceleration or de-acceleration can be done by increasing or decreasing pulse frequency with respect to time according to the requirement.
High precision and accuracy can be achieved by the motor as motor shaft moves with an accuracy of 3-5% a step. This error is non-cumulative from step to step. Weather a motor moves a single step or thousands of steps the output will be obtained with variation of 3-5% of a single step.
Unlike other motors these motors produce high torque at low rpm thus are best suited for applications demanding high torque.
Quite difficult to control at high speed.
Stepper motors have low rotor inertia due to which these motors show quick response to the command. Since are well suited for quick positioning.
These motors lock themselves at stop position without any application of mechanical brake. These motors have good torque holding capacity at stop position while having their windings energized.
Energized windings create a magnetic field inside the motor which powerfully restricts the movement of rotor even if it is holding torque.
Resonance occurs sometime.
Types of Stepper Motors
On the basis of mechanism there are three types of stepper motors as
1. Permanent Magnet Stepper Motor
2. Variable Reluctance Stepper Motors
3. Hybrid Stepper Motors
On the basis of polarity there are two types of stepper motors as
1. Unipolar Stepper Motors
2. Bipolar Stepper Motors