Editorial Feature

Intermediate Robotics: Stepper Motors

A stepper motor is a permanent magnet or variable reluctance DC motor that is capable of performing digital control, holding torque at zero speed, rotation in both directions, and repetition of accurate motion or velocity profiles.

Construction and function of a DC motor/stepper motor. Video by Sharpe Edge Learning.

Accurate angular increments demonstrate the rotational capabilities of this motor type, which is called steps based on the application of digital pulses from a digital controller to an electric drive circuit.

The speed and position of a motor shaft can be controlled with respect to the rate and number of digital pulses.

This motor is powered by DC sources, and does not require feedback for control. However, the accuracy of the motor can be improved by the use of an encoder.

The stepper motor tends to consume more power, and hence it is primarily used in stationary robots. It is also ideal for rotational movements that are accurate and larger than 180°.

Types of Stepper Motors

There are two types of stepper motors: bipolar and unipolar.

Unipolar steeping motors consist of 5 to 6 wires that are wound on each of the two windings with center taps.

The ends of the windings are alternately grounded to reverse the field direction, and the center tap is linked to the positive supply.

Based on the current flowing from one winding to a terminal, the top stator pole is considered to be a north pole while the bottom stator pole is considered to be a south pole. The motor starts to rotate upon the application of power to the two windings in sequence.

Bipolar stepper motors are operated based on the principle similar to that of hybrid motors and bipolar permanent magnets.

The two windings are linked without any center taps. Although, the motor appears to be simple, the drive circuitry required for reversing the polarity is very complex. Hence, an H- bridge control circuit is required for each winding to control the polarity at each end of the winding.

Working Principle

There are permanent magnets that are attached to the shaft of the stepper motor.

The body of the motor consists of a series of magnetic coils for creating magnetic fields that causes the rotor to move.

When the coils are turned off and on in a sequence known as phase pattern, the motor can rotate in forward or reverse direction. Some of the common types of phase pattern include half-step, full-single phase and a full-double phase.

The motor can also be rotated by energizing one coil alone. This energized coil takes up all of the electrical current even when the motor is not turning. The torque generated at this standstill position is called the holding torque.

The stepper motor can be easily controlled by using microcontroller chips and digital circuitry. The controller can energize the coils in a specific way to enable the motor to move in a desired pattern.

A computer will track the number of steps implemented to rotate the move in order to determine the position of the motor. In addition, an encoder can be used to check the position of the motor.

Applications

Some of the applications of a stepper motor in robotics are listed below:

  • It is used for motion control
  • It acts as an actuator for activating the robotic joints
  • It can accelerate payload up to the desired speed.

References

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