Robotics and Motor Control

By: Mouser Electronics

When robots assemble computers, they are actually different automatic control devices that handle every step of the assembly, packaging, and even delivery of printed circuit boards through pick-and-place actions. In the electronics world, semiconductors are like movie stars, attracting all the media's attention, but behind the scenes, they are assembled by many robots. Robots cannot stop assembling printed circuit boards because of the heat generated, they must keep moving, and this is where precision motor control comes into play.

Depending on the application, different types of motors and control circuits are created. A robot designed to lift heavy objects may use a high-torque AC induction motor that does not require precise control of speed or positioning.

Robotic surgery, on the other hand, requires extremely precise control of speed and positioning. Usually, brushless DC (BLDC) servo motors are used with closed-loop positioning feedback to provide precise control, or stepper motors with a large number of poles are used to achieve the accuracy required for surgical grade.

BLDC Motor

A typical BLDC motor requires sophisticated electronic control circuitry and some method of continuously determining the rotor position. The rotor position can be determined by using Hall Effect sensors or by measuring the change in the Back-EMF of each armature coil as the rotor rotates. The speed of a brushed DC motor is determined by the applied voltage, while the speed of a BLDC motor is determined by the switching frequency. The motor is driven by PWM pulses as shown in Figure 1.

Figure 1: PWM driven BLDC motor

There are three basic types of BLDC motors: single-phase, two-phase, and three-phase. Each type operates on the same principle. Instead of using a mechanical current inverter to change the magnetic polarity of the rotor coil, a transistor is used to continuously change the phase of the stator coil to keep the motor turning. Single-phase BLDC motors are often seen in low-power applications. Two-phase motors are more common in medium-power applications. Three-phase BLDC motors are commonly used to drive disk drives and DVD players. As shown in the table below, each motor type has different characteristics and is best suited for specific applications.

Stepper Motor

Not all applications require a motor to spin freely. A stepper motor rotates a rotor in finite increments and expects the shaft to stop moving until it starts moving again. Stepper motors have no brushes or contacts and are synchronous DC motors with an electronically switched magnetic field to rotate the armature magnet, converting digital pulses into mechanical shaft rotation, acting as a mechanical version of a DAC, converting digital steps into analog angular displacement.

A stepper is a synchronous DC motor that divides a full rotation into a discrete number of steps, or "phases," which is equal to the number of electromagnets arranged around a central gear-shaped core. Controlling the current sent to these magnets turns the motor to a precise angle.

Stepper motors are usually driven by an H-bridge with one coil per stepper motor, and the output is controlled by PWM (Figure 2). The rotation angle is proportional to the number of pulses, and the rotation speed is proportional to the frequency of the pulses.

Figure 2: Simple stepper motor with H-bridge driver

Providing power to robots

If you are considering a robotics development project, Mouser's library offers tutorials, design articles, and white papers for a variety of vertical market applications and technologies. When it comes time to design, refer to Mouser's diverse range of AC, DC, and servo motors; stepper motors; sensors; motor driver PMICs; and complete robotics kits. The Product Knowledge Center is the best place to start a design project.

No matter what type of robot you're developing, Mouser has the components and expertise you need to make your design project a success.