An actuator is a vital component of any physical system enabling movements by converting an energy source into another, primarily electrical, air, or hydraulic energy, into mechanical force [ 1 2 ] to modify the current system’s state. Some major applications include the automotive industry, furniture-ergonomics, automation, industrial machinery, maritime applications, the medical industry, and renewable energy systems. Various types of actuators are in use (as per the power sources used) such as electrical actuators [ 3 ], hydraulic actuators [ 4 ], pneumatic actuators [ 5 ], mechanical actuators, and a combination of these such as electro-hydraulic [ 6 ], electro-pneumatic [ 7 ], electro-mechanical [ 8 ], self driven thermo-mechanical [ 9 ], etc. An electric actuator creates a load movement or an action requiring a force such as clamping, using an electric motor to create the desired force, converting electricity into kinetic energy to automate valves, or damper actions using precise flow control [ 10 ]. Electric motors may work on AC or DC supplies depending on the requirements of the application, limit switches, brake mechanisms, resolvers, temperature sensors, etc. The desired force is generated from the motor’s torque capability and automates industrial valves, process plants, flow control, thermal power plants, irrigation systems, etc. [ 11 ].
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-axis motion) to clean the whole solar panel; small-scale wind systems [2,18,21,22,Renewable energy plays a pivotal role in tackling climate change by reducing greenhouse gas emissions and fossil fuel use. Energy conversion systems driven by actuators help ensure optimal conversion while transforming renewable sources into valuable forms. Rotary to linear motion is achievable using a mechanical transmission unit, called linear motion actuators, which are better in accuracy than rotary actuators, compact in size, and easy to use in many applications such as solar tracking systems [ 12 ], where solar panels can track the sun’s apparent motion; solar furnace systems [ 13 ], where the motion required to control the sun-rays by moving shutters; solar heater systems [ 14 ], where sun rays are concentrated on oil pipes (called active area) placed over the parabolic solar panels; solar cleaning systems [ 15 16 ], where two motions are performed (viz.,-axis and-axis motion) to clean the whole solar panel; small-scale wind systems [ 17 ], where actuators are used as protection devices in high wind speeds; wave systems [ 1 19 ], where actuators are used in the conversion of wave motion to linear and rotary motion for energy generation; and bio-energy systems [ 20 23 ], where actuators automate the operation of geologically distributed systems.
Energy optimization is achievable by increasing energy generation or reducing energy consumption. In solar panel cleaning, Williams et al. [ 15 ] use piezoceramic actuators for self-cleaning operation. The results show that dust moves away from high to low vibration velocity. The convectional solar panel’s mechanical cleaning structure is called a gantry structure [ 24 ] and is operated by two rotary actuators. Using telescopic actuators offers the possibility of reducing the use of one rotary actuator for reduced overall cost and energy consumption. In solar furnace systems [ 25 ], instead of rotary actuators by applying linear actuators, one may neglect the mechanical dead zone for decreased operation time due to the quick response increased testing quality. For solar tree applications, a 2DOF spherical actuator [ 26 ] is more beneficial than a linear actuator. There are two advantages: less space required and a mechanical latch arrangement which uses reduced power while moving. Such an appropriate selection of actuator ensures an optimal system operation.
Actuators are mainly classifiable into active and passive actuators. The active actuators need an electric energy source for functioning. In contrast, passive actuators do not require a source and function based on natural energy, such as thermal expansion material, and energy stored in the spring [ 27 ]. Various required motions are circular, rotational, oscillatory (seismic), and rectilinear. Therefore, electric actuators are classified according to their motion, degree of freedom (DOF), and excitation sources for particular functional motion. As per motion, actuators are rotary actuators, linear actuators, and a new one called a spherical actuator with multiple degrees of freedom [ 26 ]. Linear motion actuators are split into two categories: rotary-to-linear (hybrid motion) actuators involving mechanical transmission units (also called electromagnetically linear actuators as shown in Figure 1 ) and direct linear motion ones not requiring mechanical transmissions units.
∘ /h [Actuators are used in renewable energy sources such as solar tracking applications to drive solar panels, solar dishes, heliostats, and solar cookers moving towards the sun throughout the day [ 28 ]. Solar water pumping applications use linear actuators in a water pump system [ 29 ], and solar tracking, or cooker, applications use actuators (motors or shape memory alloys) to drive the rotation of panels towards the sun throughout the day at a rate of 15/h [ 27 30 ]. Depending on the temperature, certain materials in a thermo-mechanical actuator help control the collector’s angle of inclination to face the sun and provide increased production of about 39% compared to a fixed system. In addition, there are some bioenergy applications, where rotary actuators are used for agricultural machinery and the automation of geographically distributed biogas systems; portable wind energy applications [ 31 ] with actuator-controlled over-speeding; wave energy applications, where actuators are used in power-generation purposes [ 32 ]; smart grid or microgrid environment [ 33 ], where actuators control power generation and consumption; and geothermal power plants with electric valve actuators.
Rotary (also called electromechanical rotary) actuators help move large loads in angular increments. They include electrical motors with gearboxes used to change the direction of rotation or achieve the desired torque and speed with the desired adjustable rotation angle. Examples of the rotary actuators are induction motors, DC motors (brushed and brushless), synchronous motors, and special motors such as stepper and servo motors. Motors’ selection factors for actuation are speed, output torque, type of supply required, and motor duty cycle. However, induction motors, which give almost synchronous speed, are effective with high power requirements. Due to the low inertia, servo motor rotors working on AC or DC supply and controlled through pulse width modulation can quickly start and stop and are used to position mechanical parts precisely. Such precise motion control is needed in CNC machines and production assembly lines. In addition, DC servo, DC brushed, and BLDC motors are economical and less complex actuation devices. Unlike servos, a stepper motor that divides the rotary motion into equal small steps can provide 360-degree continuous rotation and is helpful for high torque output and high accuracy at low-cost [ 34 ]. However, due to electrical (cogging) and mechanical (dead zone) nonlinearities [ 35 ], these rotary actuators are relatively less efficient and have lower accuracy. Whenever we may face motor concussion problem [ 36 ], accuracy is improved by using special motors such as stepper motors and servo motors, which still face mechanical nonlinearities and, thus, low performance. In mechatronic systems, electric rotary actuators find extensive use.
± 360 ∘ ). Typical rotary solenoids have bi-stable positions that do not require power to hold either end position, minimizing energy consumption. It can drive camera shutters and mirror deflections, fluid/gas valves, the actuation of indicating instruments, application areas such as the packaging industry, medical devices, apparatus construction, and food technology for interlocking purposes. Rotary-to-linear actuators (also called electro-mechanical linear actuators) comprises electric motors and mechanical transmission elements. The rotational motion converts into linear motion using mechanical transmission elements, such as the ball and screw mechanism, the lead screw mechanism, the rack and pinion mechanism, the belt and pulley mechanism, and rigid chains. Such elements slow down electric rotations, thereby increasing torque such that the load on the transmission part moves smoothly. The ball and screw are made out of a cylinder to allow the re-circulation of the balls, causing linear movement, but are used for distinct purposes and are not interchangeable [A rotary solenoid is more suitable for small rotations and low torque applications than a costly and complex control circuitry stepper motor. It comprises an electromagnetic coil operated on a DC supply, rotor, and torsion spring. The change in supply polarity is easily handled in the direction of rotation, providing a meager angle rotational movement around a fixed axis (usually ⩽). Typical rotary solenoids have bi-stable positions that do not require power to hold either end position, minimizing energy consumption. It can drive camera shutters and mirror deflections, fluid/gas valves, the actuation of indicating instruments, application areas such as the packaging industry, medical devices, apparatus construction, and food technology for interlocking purposes. Rotary-to-linear actuators (also called electro-mechanical linear actuators) comprises electric motors and mechanical transmission elements. The rotational motion converts into linear motion using mechanical transmission elements, such as the ball and screw mechanism, the lead screw mechanism, the rack and pinion mechanism, the belt and pulley mechanism, and rigid chains. Such elements slow down electric rotations, thereby increasing torque such that the load on the transmission part moves smoothly. The ball and screw are made out of a cylinder to allow the re-circulation of the balls, causing linear movement, but are used for distinct purposes and are not interchangeable [ 37 ].
Unlike rotary to linear ones, direct linear motion actuators have no transmission elements and provide high accuracy, resolution, and speed. There are various types, viz., iron core linear motors, U-channel linear motors, linear shaft motors, voice coil motors, linear motion solenoids, screw-type linear piezoelectric actuators [ 38 ], and electromagnetic clutches. The stator and forcer (called rotor), with a permanent magnet and electromagnetic coils, respectively, create linear force. However, the iron core brings cogging and eddy currents, heat, magnetic saturation, and lateral forces. U-channel direct linear motor eliminates such disadvantages, providing increased force through an additional permanent magnet on the opposite side for a double-sided configuration, with electromagnetic coils in an epoxy-mounted non-ferrous forcer aluminum plate. The absence of a magnetic core eliminates cogging, but reduces heat dissipation, and a double-sided configuration increases cost. Although they do not generate the same forces as an iron core motor and U-channel linear motors, they do not have to deal with the additional issues with that design. Voice coil actuators, working on the principle of Lorentz force, are a direct drive mechanism that allows for excellent fine-positioning over small movements.
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An electromagnetic device called a “Linear Solenoid” turns electrical energy into a mechanical pushing or pulling force or motion for rapid ON and OFF operation and mainly comprises an electromagnetic coil, a plunger (armature), and a spring. When the coil energizes, the spring is compressed for the valve to open, and when the coil de-energizes, the spring moves downward to close the valve and control fluid/airflow as a solenoid valve. However, the disadvantages are that there are only two available positions, i.e., fully open or closed, generating a limited linear force for electrically opening doors and latches. Electric (also called electromagnetic) clutches have an electric actuation between two rotational gears (drive/input and driven/output) for mechanical transmission of torque through linear motion, i.e., operate electrically and transmit torque mechanically. The passage of a current through the clutch coil makes it an electromagnet producing magnetic lines of flux [ 39 ]. Although mainly used in biogas automation [ 20 ], clutches are also used in conveyor drives, packaging machinery, and factory automation. Apart from linear and rotary motion, the new spherical motion actuators [ 26 ] are multi-degree-of-freedom voice coil devices with a spherical-shaped stator, two coils, a ball-shaped rotor, and multiple permanent magnets (a plurality of magnets) [ 40 ] applied to robot eyes, joints, and solar trees.
This comprehensive literature survey focuses exclusively on renewable applications with electrical actuators. Particular actuators find a place in multiple renewable energy systems, while others have only specific or limited usage. Furthermore, there are certain advantages and disadvantages (provided subsequently in each Section) concerning actuator motion. We also suggest some alternative solutions for conventional actuators. However, the overall advantages of electric actuators against non-electrical actuators (such as hydraulic and pneumatic) are given as follows:
Lower response time: Electrical actuators are directly driven without auxiliary accessories, unlike hydraulic actuators, which comprise motors, hydraulic tanks, controlling valve pipes, and cylinders, and hence are fast operating.
Lightweight: Compared to non-electrical actuators, electric actuators are lightweight and easy to install, with no adverse effect on system performance. The cost of overall installation and material is reduced [ 15 41 ].
Precision control: The low tolerances compared to non-electrical actuators such as gear backlash, slack, and inherent flex provide highly precise positioning [ 42 ].
Compact in size: Electric actuators are compact due to their direct electromagnetic energy conversion system, and they are designed for any space and physical size [ 26 ].
Clean: Electrical actuators are a clean source of energy with no chance of leaking or dirty work areas [ 43 ].
IoT compatible: Motion and position are easily measurable due to inbuilt rotation sensors, and encoders and easily transfer to an IoT environment [ 44 45 ].
The paper’s structure is as follows. Section 2 reviews various types of electric actuators in different solar system applications such as solar tracking, furnace, heater, and solar panel cleaning. Section 3 presents a brief review of electric actuators used in wind applications, especially miniature rooftop wind turbines and wave energy applications. Then, Section 4 reviews actuators used in bioenergy applications, including agriculture and irrigation. Section 5 presents important performance parameters of actuators and evolution. Finally, Section 6 presents the conclusions and future direction.
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