13.3 Actuator types and control methods

Actuators are crucial components in embedded systems, converting electrical signals into physical actions. This section covers various types of actuators, including electric motors, solenoids, and relays, along with their control methods.

Understanding actuator types and control techniques is essential for designing effective embedded systems. From precise motor control to simple on-off switching, actuators enable devices to interact with the physical world, bridging the gap between digital commands and mechanical responses.

Electric Motors

DC Motor Fundamentals

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• DC motors convert electrical energy into mechanical energy through the interaction of magnetic fields and conductors
• Consist of a stator (stationary part) and a rotor (rotating part)
• Stator generates a stationary magnetic field using either permanent magnets or electromagnetic windings
• Rotor contains an armature winding that carries current, producing a magnetic field that interacts with the stator's field to generate torque
• Brushed DC motors use mechanical commutation (brushes and commutator) to switch the direction of current in the armature windings as the rotor turns
• Brushless DC (BLDC) motors use electronic commutation, where the stator windings are energized in a specific sequence to create a rotating magnetic field

Stepper and Servo Motors

• Stepper motors are brushless DC motors that divide a full rotation into a number of equal steps
• Move in precise increments (steps) when electrical pulses are applied to their windings in a specific sequence
• Commonly used in applications requiring precise positioning and speed control (3D printers, CNC machines)
• Servo motors are self-contained systems that include a motor, feedback device (encoder or potentiometer), and control circuitry
• Provide precise position, velocity, and torque control through a closed-loop feedback system
• Widely used in robotics, automation, and radio-controlled vehicles

Motor Control Techniques

• Pulse Width Modulation (PWM) is a technique used to control the speed and power of DC motors
• PWM varies the duty cycle of a square wave signal to adjust the average voltage supplied to the motor
• Higher duty cycles result in higher average voltage and faster motor speed, while lower duty cycles reduce speed
• H-bridge drivers are electronic circuits used to control the direction and speed of DC motors
• Consist of four switches (transistors or MOSFETs) arranged in an H configuration
• By controlling the states of the switches, the motor can be driven forward, reverse, or braked
• Motor controllers are devices that integrate various control functions, such as PWM generation, current sensing, and communication interfaces
• Provide a convenient way to control motors using microcontrollers or other control systems
• Feedback control techniques, such as PID (Proportional-Integral-Derivative) control, are used to improve the performance and accuracy of motor systems
• Feedback devices (encoders, tachometers) measure the motor's position, speed, or torque, and the control system adjusts the motor drive signals accordingly

Electromagnetic Actuators

Solenoids

• Solenoids are electromechanical devices that convert electrical energy into linear motion
• Consist of a coil of wire wound around a movable iron core (plunger)
• When current flows through the coil, a magnetic field is generated, which pulls the plunger into the center of the coil
• The plunger returns to its original position by spring force or gravity when the current is removed
• Used in applications requiring linear actuation (valves, locks, switches)

Relays

• Relays are electrically operated switches that use an electromagnet to control the switching of electrical contacts
• Consist of a coil, an armature, and a set of contacts (normally open, normally closed, or changeover)
• When current flows through the coil, a magnetic field is generated, which attracts the armature and changes the state of the contacts
• Relays provide electrical isolation between the control circuit and the switched circuit
• Used to control high-power devices using low-power control signals (automotive systems, industrial control panels)