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Short Note On Stepper Motors

Sep 20, 2017

                                            Short note on Stepper motors

A stepper motor's shaft has permanet magnets attached to it. Around the body of the motor is a series of coils that 

create a magnetic field that interacts with the permanet magnets. When these coils are turned on and off the 

magnetic field causes the rotor to move. As the coils are turned on and off in sequence the motor will rotate forward 

or reverse. This sequence is called the phase pattern and there are several types of patterns that will cause the motor 

to turn. Common types are full-double phase, full-single phase, and half step.

To make a stepper motor rotate, you must constantly turn on and off the coils. If you simply energize one coil the motor 

will just jump to that position and stay there resisting change. This energized coil pulls full current even though the 

motor is not turning. The stepper motor will generate a lot of heat at standstill. The ability to stay put at one position 

rigidly is often an advantage of stepper motors. The torque at standstill is called the holding torque.

Because steppers can be controlled by turning coils on and off, they are easy to control using digital circuitry and 

microcontroller chips. The controller simply energizes the coils in a certain pattern and the motor will move 

accordingly. At any given time the computer will know the position of the motor since the number of steps given can 

be tracked. This is true only if some outside force of greater strength than the motor has not interfered with the motion.

An optical encoder could be attached to the motor to verify its position but steppers are usually used open-loop 

(without feedback). Most stepper motor control systems will have a home switch associated with each motor that will 

allow the software to determine the starting or reference "home" position.

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