Rotary Actuators

6.3. Rotary Actuators#

Many of the moving parts in mechatronic systems rotate: wheels, fans, propellers, gears, joints, and pulleys all rely on rotary motion. Rotary actuators are devices that convert electrical energy into rotational movement. They are among the most common and versatile types of actuators, used everywhere from toy robots to industrial robots and CNC machines.

Although all rotary actuators produce spinning motion, they differ in how that motion is generated and controlled. The most common types are DC motors, BLDC motors, stepper motors, and servo motors. Each has different strengths and is suited to different tasks.

Hint

We’ve already covered DC motors and BLDC motors in Motors. This section expands on stepper motors and servo motors, since they provide precise control that is associated with actuators.

6.3.1. Stepper Motors#

Stepper motors are designed for precise positioning. Instead of spinning freely like a DC or BLDC motor, they move in small, exact steps. Inside, they have multiple coils arranged around the rotor. By energising these coils in sequence, the controller makes the rotor jump from one fixed position to the next. Because each step is a known angle (often 1.8° or 0.9°), the position of the shaft can be tracked just by counting steps.

Stepper motors do not need feedback sensors for open-loop control, which makes them ideal for applications where the motor always starts from a known position and does not need to resist external forces. However, if they are overloaded or accelerated too quickly, they can skip steps, which causes them to lose track of position.

They are commonly used where accurate, repeatable motion is needed, such as 3D printers, CNC machines, and camera sliders.

Advantages

Very precise control of angle and position
Can hold position when powered
No feedback sensors needed in open-loop systems

Examples

3D printer and CNC machine axis drives
Automated camera sliders and turntables
Laboratory instruments needing repeatable movement

6.3.2. Servo Motors#

Servo motors combine a DC motor, a gearbox, a position sensor, and a small control circuit inside one unit. Instead of just spinning freely, they are designed to rotate to a specific angle when commanded. The controller sends a PWM signal that encodes the target angle. The servo’s internal electronics compare this command to the sensor’s reading and drive the motor until the shaft reaches the correct position.

Because the feedback system is built in, servo motors are very easy to control from microcontrollers. They are widely used wherever a mechanism needs to move to a position and hold it, such as steering, pointing, or gripping.

Hobby servos are usually limited to about 180° of rotation, while industrial servos can rotate continuously like DC motors but still provide position feedback.