Actuators

An actuator is responsible for moving and controlling a mechanism or system by converting energy into motion. Examples include rotating a motor or opening a valve. When an actuator receives a controlling signal, it converts the energy source into mechanical motion. In mechatronics systems, actuators enable various mechanical actions. There are two main types of actuators based on the type of movement: linear actuators, which move in a straight path, and rotary actuators, which provide rotating motion. Actuators can also be classified into three categories based on the type of energy used for motion conversion: electrically operated, hydraulically operated, and pneumatically operated. The chart below shows the categorization of different actuators.

Electrically operated actuator

Electrically operated actuators are typically represented by various types of motors. In mechatronics systems, electrically operated actuators interact with other devices associated with different mechanisms. The following is a brief list of different actuators: 

a) AC and DC motors
b) Servo motors
c) Stepper motors
d) Electric Linear Actuator or ELAs
e) Solenoid valve coils
f) Electromagnetic brakes

a) AC and DC motor

Different AC or DC motors are used in mechatronics systems depending on the system requirements. AC motors are generally preferred over DC motors due to the absence of commutation processes and carbon brushes. AC motors are available in single-phase or three-phase configurations, while DC motors are typically single-phase. Three-phase AC motors are self-starting, while single-phase AC motors require starting torque. DC motors are also self-starting. Despite the challenges, small DC motors are still used in certain applications because they provide higher starting torque compared to AC motors, and miniaturizing AC motors can be inconvenient. 3-phase induction motors of various capacities are commonly used in different machinery and mechatronics systems. The following pictures show a DC motor and an AC 3-phase induction motor.

b) Servo motor

Servo motors are special electromechanical devices used for precise and controlled movements in mechatronics systems, as well as for rotating objects at controlled speeds. These motors deliver high torque, operate at maximum efficiency with low current consumption, and are smaller in size. Servo motors are typically driven by a specialized unit called a servo drive or servo amplifier. Both DC and AC servo amplifiers can be used in mechatronics systems. Servo motors are also known as control motors because they control mechanical systems and operate on a closed-loop servo system. Earlier mechatronics systems utilized both DC and AC servo motors, but currently, AC servo motors are predominantly used due to their added advantages over DC motors. The following picture shows an AC servo motor and its interior.

A feedback sensor is always used with a servo motor. Commonly employed feedback sensors include an encoder, Inductosyn, and Resolver. In the past, servo motors used separate sensors, with encoders for position feedback and tacho generators for velocity feedback. However, encoders, resolvers, and Inductosyns can provide both position and velocity feedback as a single feedback device for servo motors. Sometimes, a braking arrangement is also incorporated inside a servo motor. The operating principle of a servo motor is explained in a separate chapter.

c) Stepper motor

A stepper motor is a brushless synchronous motor that divides a 360-degree shaft movement into multiple steps and allows accurate control of the motor shaft's speed. The name "stepper motor" comes from the fact that each electrical pulse causes a step movement of the motor shaft. Similar to a servo motor, a stepper motor is driven by a driver unit that generates the necessary electrical pulses to rotate the motor shaft. The speed of a stepper motor typically depends on the frequency of the electrical pulses. The following picture shows a stepper motor and its interior. 

Stepper motors are usually classified into three types: variable reluctance, permanent magnet, and hybrid stepper motors. These motors consist of a magnetic or toothed soft iron rotor that rotates within an electromagnetic field (see the picture). When the stators are energized by a motor driver unit, a torque is generated and applied to the rotor, causing it to start rotating while maintaining a minimum gap between the stator coils and rotor teeth or magnetic poles. If the stator coils are energized in a fixed sequence, continuous rotary movement is obtained in the stepper motor shaft. This phenomenon can be explained with the following pictures, where A-A', B-B', C-C', and D-D' coils are successively energized (dark color signifies coil excitation). As a result, the motor shaft rotates in steps of 15 degrees.

d) Electric Linear Actuator or ELA

An electric linear actuator (ELA) is typically powered by a 12V DC motor (although other types of motors can also be used) and consists of a lead-screw and nut system with a gear assembly. The rotation of the lead-screw causes linear movement on a shaft attached to the nut assembly. Compared to hydraulic or pneumatic linear actuators, electric linear actuators have the advantage of a compact design and do not require valves, pumps, pressure lines, etc., to achieve linear movement. The image below depicts an electric linear actuator.

ELAs serve various functions in mechatronics systems, with the stroke length usually dependent on the length of the lead screw. Clockwise and anticlockwise movements of the motor drive the extended or retracting movement of the shaft in the ELA. Small snap switches are typically placed inside an ELA to terminate the motor supply when it reaches both ends.

e) Solenoid valve coil

A solenoid valve coil is an electromagnetic actuator that directs hydraulic or pneumatic pressure lines when electrical power is applied. Usually, the coil is made of insulated copper wire wrapped around a hollow cylinder, creating a magnetic field inside the cylinder when an electric current passes through it. The coil is attached to a solenoid valve (hydraulic or pneumatic) in a way that a Ferro-magnetic core inside the solenoid valve, known as the Valve Plunger, fits correctly into the cylindrical part.

When a magnetic field is generated inside the cylinder, the valve plunger becomes an electromagnet and moves outward, opening an orifice inside the valve to influence the pressure line in a specific direction. Solenoid valve coils come in two different types based on actuating voltage: DC and AC voltage type. Commonly used voltages are 24V DC, 110V AC, and 220/240V AC for different mechatronics systems. Pneumatic solenoid valves typically use 24V DC coils, while hydraulic valves use 110/220V AC coils. The image below shows a 24V DC pneumatic solenoid valve in its energized and de-energized states.

f) Electromagnetic brake

Mechatronics systems use electromagnetic brakes to stall or delay the movement of electrically operated actuators. These brakes create mechanical friction or resistance through an electromagnetic force, earning them the name "electromagnetic brakes." There are different types of electromagnetic brakes, and one common type is the electrical power-off brake, which is typically fitted to a motor's shaft.

The electrical power-off brake consists of a strong permanent magnet and an electromagnetic coil attached to the motor body. A friction disk is coupled with the motor shaft, along with springs to create balance. In a normal situation, the friction plate is rigidly fastened to the permanent magnet, preventing movement of the motor shaft. When voltage is applied to the brake coil, the electromagnet neutralizes the magnetic fluxes of the permanent magnet, loosening the friction plate from spring tension. This allows the motor shaft to rotate freely. If the supply voltage is withdrawn from the braking coil, the friction plate re-attaches to the permanent magnet, halting the movement of the motor shaft. The electromagnetic brake can be energized along with the motor coil supply and de-energized by turning off the motor supply voltage. The image below illustrates a typical electromagnetic braking system commonly used with induction motors.

Hydraulic operated actuator

Hydraulic actuators convert pressurized hydraulic fluid energy into mechanical motion. A typical hydraulic system consists of a hydraulic pump driven by an induction motor, generating pressurized hydraulic fluid that passes through various valves to operate different hydraulic actuators. Hydraulic actuators perform mechanical functions such as blocking, clamping, ejecting, and power transmissions. In mechatronics systems, multiple tasks are accomplished using various hydraulically operated actuators. The functioning of a hydraulic actuator depends on factors such as hydraulic fluid pressure, flow rate, and pressure drop within the actuator. There are two basic types of hydraulic actuators: linear actuators and rotary actuators.

a) Linear hydraulic actuator

A linear actuator is used to transfer or displace an element in a straight line. The displacement depends on the stroke length of the actuator. A hydraulic cylinder is the most commonly used linear actuator in machinery. It is usually made of steel to withstand high hydraulic pressures. The movement is exerted by a piston rod inside the hydraulic cylinder, driven by pressurized fluid. The piston rod is connected to an external load and produces a pulling or pushing force in a straight line. Hydraulic cylinders are mainly categorized into two types: single-acting and double-acting cylinders.

Single-acting cylinder: A single-acting cylinder consists of a cylindrical housing, also known as a barrel, with a piston placed inside (refer to the picture below). The piston is connected to a solid rod, allowing it to move back and forth. To prevent pressurized fluid from entering the upper part of the cylinder, a rubberized piston seal is placed near the piston's diameter. The cylindrical housing features a pressure port on the side opposite to the rod and piston, enabling the entry or exit of pressurized fluid into the cylinder. The piston in the cylinder moves in only one direction due to the pressurized fluid, while a spring tension helps return the piston to its initial position. Please refer to the schematic diagram below to visualize a single-acting cylinder.

In the front of the cylindrical housing, there is a small port called the Vent port, which serves to release the accumulated air from the upper part of the cylinder into the atmosphere. The incoming pressure line, controlled by a valve, pushes the cylinder piston outward and also returns the fluid to the tank when the piston retracts. This means that the hydraulic fluid accumulated in the lower part of the cylinder's piston is also returned to the hydraulic tank through the control valve. Single-acting cylinders come in two varieties: Push-type and Pull-type. The basic function of these two cylinders is the same, with the only difference being the position of the Pressure port and Vent port, which are located in opposite directions (refer to the pictures below).

Double-acting cylinder: A double-acting cylinder operates similarly to a single-acting cylinder in terms of piston movement. However, it incorporates an additional pressure line for the reverse movement of the cylinder piston instead of relying on a spring action. As a result, a double-acting cylinder possesses two separate pressure ports at the top and bottom of the cylinder, allowing the piston to be actuated in both directions (see the picture below).

Double-acting cylinders are further categorized into two types: one with a piston rod on one side and another with a piston rod on both sides. This means that the piston can be located on either side of the cylinder or on both sides. In most cases, a double-acting cylinder with a one-sided piston rod is used in mechatronics systems and specific applications, while a double-acting cylinder with a piston rod on both sides is more commonly seen. The image below illustrates a double-acting hydraulic cylinder with a piston rod on one side, commonly found in various mechanisms for linear movement.

b) Rotary hydraulic actuator

When high torque and heavy-duty rotary motions are required in a mechatronics system, a hydraulic rotary actuator is preferable over an induction motor. Hydraulic actuators are more efficient for shifting, rotating, or indexing heavy loads. There are two types of hydraulic rotary actuators: limited movement rotary actuators and continuous movement rotary actuators, also known as hydro-motors.

Limited movement rotary actuators come in various types, including rack and pinion, crank-lever, vane, parallel piston, etc., depending on the specific movement and application. The images below illustrate a rack and pinion type and a vane type of limited movement hydraulic rotary actuator, commonly used in different mechatronics systems.

For applications requiring slow and continuous rotary motion of heavy loads, a continuous movement hydraulic rotary actuator or hydro-motor is typically employed. Hydro-motors offer advantages over induction motors in terms of size, as they are smaller while performing the same work. Different types of hydro-motors are available, including gear, piston, and vane types. The image below shows a vane-type hydro-motor commonly found in various mechatronics systems.

Pneumatic operated actuator

A pneumatic actuator converts compressed air energy into mechanical motion. When a gaseous substance like air is compressed, its volume decreases, causing an increase in pressure. This enhanced pressure can be utilized to perform various mechanical tasks. Compressed air can be stored in a reservoir for later use. Pneumatic actuators are used in applications such as automatic machine door opening and closing, and arm movement of cutting tools. Pneumatic actuators are designed to handle light loads, typically between 5 to 7 kg of pneumatic pressure. To handle larger loads, a pneumatic actuator requires a cylinder piston with a bigger diameter. The body of a pneumatic cylinder is usually made of aluminum or its alloy, which makes it lighter compared to a hydraulic cylinder. Pneumatic actuators can be either linear or rotary, similar to hydraulic actuators.

a) Pneumatic linear actuator

Pneumatic linear actuators refer to a range of pneumatic cylinders. Pneumatic cylinders come in different types and structures depending on their function. Since compressed air pressure is lower than hydraulic pressure, the mechanical power available with a pneumatic cylinder is also less. Therefore, the structure of a pneumatic cylinder is lighter compared to a hydraulic cylinder. If the same amount of work is performed using a pneumatic cylinder instead of a hydraulic cylinder, the barrel diameter of the pneumatic cylinder will be larger. Pneumatic cylinders are more convenient to use in mechatronics systems. However, when a steady force is required or when dealing with fluctuating loads, a hydraulic cylinder is preferable over a pneumatic cylinder. The following picture shows a pneumatic cylinder and its interior, commonly used in mechatronics systems.

The response time of a pneumatic cylinder is instantaneous, but it also has some disadvantages. When the cylinder completes its stroke, the piston thrusts extensively against the end covers of the cylinder, which can lead to damage. To overcome this problem, most pneumatic cylinders are equipped with a cushioning system. The cushioning system reduces the piston movement when it strikes the cylinder edge. Pneumatic cylinders have two types of cushioning systems: fixed type and adjustable type cushioning. The fixed cushioning system is commonly found in lower-diameter pneumatic cylinders, while the adjustable cushioning system is used when the cylinder piston speed is higher. Different types of cylinders are usually found in various mechatronics systems.

• Single-acting pneumatic cylinder
• Double-acting pneumatic cylinder
• Rodless pneumatic cylinder

Single-acting Pneumatic Cylinder - The functioning of a single-acting pneumatic cylinder is similar to a single-acting hydraulic cylinder. It consists of a piston placed inside a cylindrical barrel with a rod attached to it, allowing it to move with the piston. The displacement of the cylinder's piston is achieved by air pressure and retracted by compression or expansion of spring tension. These cylinders also have a fixed or adjustable cushioning system. Like hydraulic cylinders, pneumatic single-acting cylinders are available in two varieties: push-type and pull-type. The following picture shows a push-type single-acting pneumatic cylinder.

Double-acting Pneumatic Cylinder - Double-acting pneumatic cylinders function similarly to hydraulic cylinders. They can have a piston rod on one side or on both sides. Both types of cylinders are equipped with a cushioning system. The following pictures show both types of double-acting pneumatic cylinders. 

Rodless Pneumatic Cylinder - The function of a rodless pneumatic cylinder differs from that of a basic cylinder. In this type of cylinder, there is no piston rod connected to the cylinder piston. Instead, the piston is coupled with an outer load-carrying cartridge through a magnetic or mechanical coupling. Rodless pneumatic cylinders come in three types: cable cylinder, sealing band cylinder with a slotted cylinder barrel, and magnetically coupled slide cylinder. Among them, sealing bands and magnetically coupled cylinders are commonly used in mechatronics systems. The following pictures show a magnetically coupled rodless pneumatic cylinder and its interior. 

b) Pneumatic rotary actuator

A pneumatic rotary actuator utilizes pneumatic energy or air pressure to achieve rotary movement. Pneumatic rotary actuators are available in two types: continuous rotary movement and limited rotary movement. A continuous rotary motion pneumatic actuator is sometimes referred to as a pneumatic motor. This motor delivers constant rotary motion by utilizing a pneumatic pressure line. According to their structure and working principles, pneumatic motors can be categorized into three types: piston motor, sliding vane motor, and gear motor. The following picture shows a sliding vane pneumatic motor.

A limited movement pneumatic rotary actuator allows for higher torque. The standard rotations of these actuators are usually 90°, 180°, and 270°. There are three types of rotary actuators available: vane type, rack & pinion type, and helix spine type pneumatic rotary actuator. The following pictures show a rack & pinion type limited movement rotary actuator.