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Types of Actuators: Principles, Mechanisms, and Applications

Thermal actuators

Thermal actuators convert temperature changes into linear movement or “stroke” by utilising the expansion and contraction of thermally sensitive materials within them.

They integrate both temperature sensing and actuation, making them valuable for various applications, including:

1. Temperature control

2. Fluid mixing and diverting

3. Freeze protection

4. High-temperature shutoffs

, Types of Actuators: Principles, Mechanisms, and Applications

These actuators are central components in devices like thermostats and various valves, including:Thermal bypass valves which regulate fluid flow by temperature. Thermostatic mixing valves which blend hot and cold water to maintain a set temperature. Tempering valves which control hot water to prevent scalding. Thermostatic balancing valves which balance water temperature across systems.

A thermostatic actuator specifically controls valve positions based on a temperature setpoint. For instance, it can manage the flow of hot water in radiators to maintain room temperature.

Additionally, combining a thermal actuator with a positive temperature coefficient (PTC) thermistor creates a wax motor, often found in appliances like washing machines or dishwashers. Wax motors engage mechanical functions such as locking doors or triggering dispensers by heating a wax material, which expands and generates the necessary force to operate the mechanism.

Thermal actuators rely on a temperature-sensitive material that expands or contracts with temperature changes, driving a piston.

The temperature-sensing material, which can be liquid, gas, or wax-like substances, expands with rising temperature and contracts when cooled. In wax-based thermal actuators, the wax is engineered to undergo a phase change within a specific temperature range:

When heated above its melting point, the wax shifts from solid to liquid, expanding significantly. This expansion exerts force on a diaphragm, pushing the piston forward.

When cooled, the wax solidifies, contracting and releasing pressure on the diaphragm. A spring mechanism then retracts the piston to its initial position.

Typical stroke ranges for wax-based actuators are 0.05 to 0.5 inches, delivering forces between 5 and 75 pounds over temperatures from -150°F to 300°F.

Advantages: Thermal actuators are particularly valuable in thermostatic mixing/diverting temperature control valves, which automatically mix or divert fluids by temperature. Integrated temperature sensing, actuation, and fluid control functions make these valves efficient, especially compared to systems using solenoids and electronic sensors.

Benefits of thermal valves include: Fewer external parts so lower installation costs and simplify setup. Thermal valves operate independently of electricity, increasing reliability and safety by avoiding risks like power failures or short circuits. They deliver a high power-to-size ratio, require no calibration, and perform reliably in extreme environments.

Disadvantages: Hysteresis is a key limitation, leading to a lag in piston movement with rising and falling temperatures:

When temperatures rise, the piston moves forward as the material expands. However, upon cooling, the retraction lags, causing a slight difference in piston position at the same temperature depending on whether the actuator is heating up or cooling down.

Application Example: A custom thermal actuator can control coolant temperature in a jet engine for an unmanned aerial vehicle (UAV). Here, as engine coolant temperature rises, the actuator opens an air intake flap, allowing more airflow to cool the fluid. As the temperature falls, the actuator closes the flap, maintaining optimal coolant temperature for engine performance.

Electrical actuators

An electric actuator is a device that generates movement or applies force to a load using an electric motor. It is commonly used in applications requiring controlled movement or force, such as clamping, lifting, positioning, or opening and closing mechanisms. The electric motor drives a mechanical system, which can be linear or rotary, depending on the application, to perform the desired action.

How does an electric actuator work?

An electric actuator generates linear motion by converting the rotary motion of an electric motor into straight-line movement.

This conversion process works as follows:

1. The motor’s spindle (rotor) rotates, creating rotary motion.

2. The spindle is connected to a helical screw via a drive shaft, so as the motor turns, the helical screw also rotates.

3. A ball screw nut is mounted on the helical screw, moving forward or backward along the screw as it turns.

4. A hollow piston rod is attached to the ball screw nut. This rod extends out of or retracts into the actuator, creating the linear motion as the motor rotates clockwise or counter-clockwise.

Control and Feedback: The actuator’s motion is managed by an electric drive:

The electric drive controls the motor’s rotation speed, allowing precise control of the actuator’s linear speed. A feedback mechanism provides positional information, allowing the actuator to be programmed for specific movements. The actuator can be instructed to reach a target position, pause, and then continue moving or return to a designated resting position.

Power and Force: The motor’s power determines the torque produced, which in turn dictates the force output of the actuator. Higher torque enables the actuator to exert greater force, making electric actuators suitable for a wide range of applications, from delicate adjustments to high-force tasks. This setup, combined with precise electronic control, allows electric actuators to perform with high accuracy, reliability, and repeatability in various industrial and automated environments.

Selection of an actuator?

The actuator will be required for an application requiring force.

Other advantages are: higher axial force; high accuracy; low noise; flexibility through control characteristics; load stiffness and overall lower operating costs

Electric actuators come in various types and configurations tailored for specific applications. Key types include:

Types of Electric Actuators

Rod-style actuators: These actuators use a piston rod to generate linear motion, ideal for pushing or pulling loads.
Rodless actuators: These use an internal carriage that moves along the actuator body, allowing for compact designs in space-limited applications.

Motor configurations: Actuators can come with an integrated motor or be designed to accommodate an external motor and drive setup. Motors can be mounted axially behind the actuator body or parallel to it, with parallel mounting options available in four different positions for flexibility in fitting within confined spaces.

Applications of Electric Actuators: Electric actuators are widely used across various industries, including: Automotive Industry in driverless transport vehicles for precise navigation and load handling. For dispensing applications and various jointing methods like glueing, welding, and riveting. Food and Beverage Industry for PET bottle production, where precision and speed are key, in filling and labelling systems that require accurate positioning and robotic applications such as automated milking robots. Packaging Sector in materials handling tasks like servo pressing and clamping. Robotics, Electronics, and Machine Tools.

The electric actuators provide the accuracy, flexibility, and low operating costs crucial in these fields, contributing to tasks in electronic assembly, machine tools, and other specialised industrial applications

Mechanical Actuator

Mechanical actuators are devices that convert rotary motion into linear motion, commonly used in applications requiring precise linear positioning, elevation, and translation. They are essential in various fields, including manufacturing, transportation, and optics.

Types of Mechanical Actuators and Conversion Mechanisms

Mechanical actuators employ several mechanisms to achieve linear motion: Screw-Based Actuators such as lead screws, screw jacks, ball screws, and roller screws operate on the screw principle. The actuator’s nut rotates, causing the screw shaft to move in a linear path. Used in positioning applications, such as jackscrews in car jacks and linear displacement systems

Wheel and Axle-Based Actuators include hoists, winches, rack and pinions, chain drives, rigid chains, and rigid belts. The rotating wheel or axle moves a connected cable, rack, chain, or belt, resulting in linear motion.

Cam Actuators convert rotary motion to limited travel linear motion. As a cam rotates, its eccentric shape provides thrust along the base of a shaft.

Some mechanical actuators are designed to operate in only one direction:

Pull-only actuators: Hoists, chain drives, and belt drives.

Push-only actuators: Cam actuators.

Segmented Spindle Actuators convert rotation of a handle into linear motion, often used in jack heads.

Laser and Optics Applications in optics and lasers, mechanical actuators adjust linear stages, rotary stages, mirror mounts, goniometers, and other positioning instruments with high accuracy.

Precision Control includes mechanical actuators with index marks, encoders, or digital position readouts that allow for accurate and repeatable positioning. They resemble micrometre knobs but focus on position adjustment rather than measurement.

Example Applications

Elevation and Translation: Common in machinery that requires precise movement along a single axis.
Positioning: Used extensively in precision optical systems to fine-tune the alignment of components.
Manual Control: Mechanical actuators enable users to convert rotary input from control knobs or handles into precise linear displacement, making them valuable in fields where accurate manual adjustments are required.

Mechanical actuators are favoured for their simplicity, reliability, and precision, especially in applications requiring consistent, high-accuracy movement without the need for complex control systems.

Hydraulic actuators

Hydraulic actuators, or cylinders, use unbalanced pressure on a piston within a hollow cylinder to create linear motion. This pressure, controlled by a hydraulic pump, generates precise and powerful force, ideal for high-force applications due to the incompressibility of the hydraulic fluid. Movement is constrained along the piston’s axis, ensuring controlled displacement.

Advantages include high force output, smooth, precise control and reliability for heavy-duty tasks

Applications include automotive (e.g., jacks, brakes), industrial equipment (e.g., presses), and construction machinery (e.g., excavators), making hydraulic actuators essential in fields requiring substantial, accurate linear force.

Pneumatic actuators

Pneumatic actuators, or cylinders, use compressed air rather than liquid to generate linear force, similar to hydraulic actuators. Air is pumped into a chamber, moving a piston to create motion. While not typically used for heavy-duty tasks, pneumatic actuators are preferred for their simplicity, as they only require an air compressor. This makes them versatile for many applications. However, drawbacks include the bulkiness and noise of air compressors, difficulty in relocation, and potential air leaks, which reduce efficiency compared to mechanical actuators.

Piezoelectric actuators

The piezoelectric effect allows certain materials to expand when a voltage is applied, with high voltages causing only tiny expansions. This makes piezoelectric actuators ideal for achieving extremely fine positioning resolution, but they have a very limited range of motion. Additionally, hysteresis in piezoelectric materials complicates precise, repeatable control of their expansion, making consistent positioning challenging.

Electro-mechanical actuators

Examples of electromechanical Actuators

A miniature electromechanical linear actuator where the lead nut is part of the motor. The lead screw does not rotate, so as the lead nut is rotated by the motor, the lead screw is extended or retracted

Electro-mechanical actuators are similar to mechanical actuators except that the control knob or handle is replaced with an electric motor. Rotary motion of the motor is converted to linear displacement. They may also be used to power a motor that converts electrical energy into mechanical torque. There are many designs of modern linear actuators.

Simplified design: An electric motor in an actuator often rotates a lead screw with a continuous helical thread.

A lead nut or ball nut with matching threads is threaded onto the lead screw and prevented from rotating, so when the lead screw turns, the nut moves along its length, creating linear motion.

The direction of the nut’s movement depends on the rotation direction of the lead screw. This motion can be converted to usable linear displacement by linking other parts to the nut.

Modern actuators prioritise speed, force, or a balance between both, depending on the application. Key specifications to consider are travel, speed, force, accuracy, and lifetime. Many actuators are mounted on dampers or butterfly valves for applications in industrial and automation systems.

Standard vs compact construction:

In a linear actuator using standard motors, the motor is often positioned as a separate cylinder attached to the actuator, either parallel or perpendicular to it, or mounted at its end. The motor’s solid drive shaft connects through gearing to the actuator’s drive nut or screw, enabling linear motion.

Compact linear actuators utilise specially designed motors to minimise overall size. These designs often include: Hollow drive shafts, allowing the drive screw and nut to be positioned at the motor’s centre, eliminating the need for extra gearing. Small outside diameters with lengthwise-extended pole faces, maintaining high torque despite a compact diameter. These compact configurations make it possible to achieve high-performance torque within limited space, beneficial for applications requiring both power and minimal footprint.

Principles: Most linear actuators operate on the principle of an inclined plane, where the threads of a lead screw function as a continuous ramp. This design allows a small rotational force to be applied over a long distance to move a large load over a shorter distance. The lead screw’s threads effectively transform rotational motion into linear motion, providing mechanical advantage to handle heavy loads with minimal effort.

Static load capacity

Linear screw actuators often feature a static loading capacity, meaning they can lock in place when the motor stops, supporting loads that push or pull on the actuator. This static load capacity enhances mobility and speed, and can be improved with high-viscosity grease, though it largely depends on the actuator’s design.

The braking force of a screw actuator is influenced by the angular pitch and design of the screw threads. Acme threads provide high static load capacity, making them more resistant to movement under load. Ball screws, on the other hand, have low static load capacity, often resulting in a near-free-floating behaviour.

Additional static load capacity can be achieved with an electromagnetic brake system, which applies friction to the drive nut. A spring may apply brake pads to hold the drive nut in place when the actuator is off. When movement is needed, an electromagnet counteracts the spring, releasing the brake and allowing the drive nut to move freely. This setup allows precise control over the actuator’s position and increases safety by holding loads securely when power is off.

Dynamic load capacity:The dynamic load capacity of a linear actuator refers to the force it can exert during active operation. This force depends on both the screw type (as friction affects movement) and the motor driving the actuator. Dynamic load capacity is the primary specification for classifying actuators, offering insight into their suitability for various applications. A higher dynamic load capacity indicates that an actuator can handle heavier or more demanding tasks reliably.

Speed control: For electro-mechanical actuators, speed control is often essential. Speed controllers typically adjust the voltage supplied to the motor, which directly changes the rotation speed of the lead screw. Another method to control speed is by adjusting the gear ratio changing the gearing influences how fast the actuator operates. Many actuators come with multiple gearing options, allowing flexibility in speed and force adjustments to better match application requirements.

Duty cycle: The duty cycle of a motor defines the operating time limit before it requires a cooling period. Adhering to the duty cycle is crucial for maintaining the longevity and performance of the actuator. Exceeding the duty cycle can lead to overheating, loss of power, and even motor burnout over time, risking permanent damage to the actuator. Proper management of duty cycles helps prevent these issues and ensures reliable operation.

Linear motors: A linear motor functions like a rotary motor but with its rotor and stator components laid out in a straight line, creating a continuous magnetic field structure along the actuator’s length. Unlike rotary motors, linear motors don’t require a lead screw to convert motion, as they naturally produce linear motion.

Advantages: No mechanical contact between the two halves, enabling use in outdoor or dirty environments.

Electromagnetic coils can be sealed and waterproofed, protecting against moisture and corrosion for an extended lifespan.

Limitations: Low load capacity compared to other actuators, due to reliance solely on magnetic attraction and repulsion forces, which limits material and motor strength.

Linear motors are ideal for high-performance positioning systems requiring high velocity, precision, and force, commonly used in precision machinery and advanced robotics.

Selecting actuators

There are many factors to consider when selecting an actuator for an application. Some of these important parameters are mentioned below.

Note that there will be a trade-off between the parameters force, speed and power consumption. For example:

Higher speed –> lower force –> lower current consumption

Higher force –> lower speed –> higher current consumption

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