Stepper and Servo motors

Stepper and Servo motors find key applications in motion control.

Generally a motion control system consists of:

• The mechanical part being moved
• The motor (stepper or servo) with a feedback to provide an indication of actual position
• Intelligent controller
• The motion driver unit
• Programming and operator interface software

Driving a controlled current through a number of coils within the motor generates the magnetic forces in a motor. Depending on their design, motors have many coils oriented in specific magnetic positions within their housing. By pulsing or steady control of current through different motor coils in a known pattern, electromagnetic fields develop in the motor, causing incremental or continuous motion.

The current and voltage that drives a motor typically comes from a power electronics device, known as an amplifier or power drive. Stepper and servo motors are located between the motion controller elements.

So what are stepper motors?
Stepper motors are discrete motion devices that move to positions that relate directly to the number of input control pulses, at a velocity that relates directly to the pulse rate.

Stepper motors rely on the principle of commutation or alternating magnetic forces to provide predictable controlled motion.

Commutation in motion applications is the controlled sequencing of drive currents and voltages in motor coil winding to provide torque and therefore, movement.

In a stepper motor system, individual signals from a motion controller are converted into an energizing pattern for the motor.

As the commutation pattern varies, the motor moves from one discrete position to another.

When the pattern is held in a single state, the stepper motor holds its position with a known torque also called holding torque. These single-state locations are known as the full-step locations of a stepper motor. One key stepper motor specification is the number of full steps per revolution (rotary motion) or full steps per unit length (linear motion).

The steps/revolution parameter of a stepper motor indicates the basic resolution of the motor.

Let’s consider a stepper motor with a resolution of 200 steps/revolution, which can also be referred to as a 1.8 degree/step motor. If the motion controller outputs 200 steps to a full-step motor driver connected to a 1.8 degree stepper motor, the resulting movement would be a full 360° of the movement or one revolution of the motor. If those 200 steps were generated evenly over a period of one minute, the speed of rotation of the motor would be one revolution per minute (RPM).

Having looked at the stepper motors, let us now learn more about Servo motors.

Servomotors are continuous motion devices that use feedback signals to provide position and velocity control in a closed loop system.

The primary types of Servomotors are DC brush servo and brushless servo.

An open loop servomotor rotates or moves uncontrolled as long as the power is applied to it.

By implementing a control loop around a servomotor using a PID controller and feedback from an encoder device mounted on the motor, it is possible to accurately and reliably move to the desired position at well controlled velocities following your specified motion trajectory paths.

All servomotor systems use a motor driver pattern power unit to control the voltage and current that flows through the motor armature and motor winding.

The basic principle of motion in servo motors is based on the flow of current through wire coil, generating a magnetic field that reacts with permanent magnets in the motor to cause attraction and repelling forces that cause movement.

You can also read: Closed Loop Control System

DC brush servomotor

This is the simplest servomotor design. Actually it is cost effective for its performance and power in general servo applications.

DC brush servo motors are self-commutating motion devices that rotate continuously while current is applied to the motor brush contacts. The current flows through the brushes to the armature and then through the motor coils, creating the magnetic forces that cause motion. Changing the direction of the current flow through the motor reverses the direction of rotation.

Encoder feedback to the motion board is required to provide accurate control of position and velocity with a DC brush servomotor. Encoders are mounted on the shaft of a motor or on the coupled mechanical unit as a linear or rotary device, directly translating movement into feedback data.

Don't miss out on key updates, join our newsletter list here.