Stepper Motor & Driver

Stepper Motor: 

A stepper motor is a brushless synchronous motor that splits 360-degree shaft movement into multiple steps, allowing for precise speed control. It gets its name because the motor shaft moves in discrete steps with each electrical pulse. Typically, a stepper motor works in conjunction with a driver unit that generates electrical pulses to rotate the motor shaft. The speed of the stepper motor is determined by the frequency of these electrical pulses. The following picture shows the interior of a stepper motor. 

 

 

Stepper motors are usually classified into three types: variable reluctance, permanent magnet, and hybrid. The motor features a magnetic or toothed soft iron rotor that rotates within an electromagnetic field (see picture). By energizing the stators through a motor driver unit, a torque is applied to the rotor, causing it to rotate while maintaining a minimum gap between the stator coils and the rotor teeth or magnetic poles. When the stator coils are energized in a fixed sequence, the stepper motor shaft exhibits continuous rotary movement. The following pictures illustrate this phenomenon. The coils A-A', B-B', C-C', and D-D' are successively energized (indicated by the dark color), resulting in a 15-degree step movement of the motor shaft. 

 

Stepper Motor Driver:

A stepper motor driver is an electronic device used to control a stepper motor. The driver generates the appropriate signals from the input data to move the motor axis. The most common method for driving a stepper motor is using an H-bridge. This circuit consists of four FET transistors that have low resistance between the drain and source contacts in an inactive state. Since the stepper motor requires a minimum of two coils, at least two H-bridges are needed. By controlling the current direction through the motor coil and providing individual signals to the FET gates, the motor axis can be shifted. Most stepper motor drivers receive Step and Direction input signals from the controller, requiring only two signals for each driver. Each pulse indicates that the stepper motor will move one step, while the direction signal determines the direction (clockwise or counterclockwise) in which the stepper motor will turn. The driver is controlled by the controller using a standard communication protocol such as MODBUS or USB. The following picture illustrates the different connections between the stepper motor and the driver unit.

  

Stepper Motor Usage in CNC Machines:

Stepper motors are widely used in CNC machines to control various axes. They are commonly employed in CNC routers, CNC plasma cutters, and other machines operating on an open-loop control system, where precise positioning is not critical. In such cases, the stepper motor's lower positioning accuracy is acceptable. For precise CNC machining operations, servo motors are preferred. However, due to their lower cost compared to servo motors, stepper motors are often used in low-budget CNC machines. The picture below illustrates the different connections between stepper motors, drivers, and controllers in a CNC router machine.