Actuators play a vital role in modern production process automation. They are often referred to as the hands and feet of production process control because they receive control signals from regulators in automated control systems, automatically change the controlled variables, and achieve the purpose of regulating the controlled parameters (such as temperature, pressure, flow rate, liquid level, etc.) to ensure that the production process proceeds normally according to predetermined requirements.
Actuator
An actuator consists of two parts: the actuator mechanism and the regulating mechanism. The actuator mechanism is the driving part of the actuator, generating thrust or displacement according to the signal sent by the controller. The regulating mechanism is a device that changes the energy or material conveying rate based on the output signal of the actuator mechanism; a common example is a control valve.
Actuators can be classified into three main categories based on the energy source used in their operation: pneumatic, electric, and hydraulic.
Actuator structure:
Servo amplifier:
The components include a preamplifier, trigger, thyristor main circuit, and power supply.
Function: To integrate the input signal and the feedback signal, and amplify the result signal to give it sufficient power to control the rotation of the servo motor.
Principle: Based on the polarity of the synthesized signal, the amplifier should output a signal of corresponding polarity to control the forward and reverse operation of the motor.
Executive agency
Servo motor: It is the power component of the actuator.
Speed reducer: Converts high speed and low torque into low speed and high torque.
Position transmitter: Based on the working principle of differential transformer, the displacement of the output shaft is used to change the position of the iron core in the differential coil to generate feedback signal and position signal.
The controller is used to perform tasks such as switching between manual and automatic modes, remote operation, and seamless automatic tracking.
Regulation mechanism
A control valve is a flow control element that adjusts flow rate by changing the valve core stroke according to the direction and magnitude of the control signal, thereby altering the valve's resistance coefficient. From a fluid mechanics perspective, a control valve is a throttling element with variable local resistance.
principle:
Ideal fluid: an incompressible fluid with no viscosity.
Steady flow: The velocity at every point in space does not change with time.
Bernoulli's equations: study the relationship between pressure and velocity in an ideal fluid undergoing steady motion.
Straight-through single-seat control valve: When the valve stem is lifted, the valve opening increases and the flow rate increases; conversely, when the valve stem is lowered, the opening decreases and the flow rate decreases.
Features: Tight sealing, reliable performance, simple structure, and low cost, but the valve stem thrust is relatively large, so the working torque requirement of the actuator is relatively high.
Suitable for: places with strict shut-off requirements and small pressure differentials, such as the control of ordinary air conditioning units, fan coil units, heat exchangers, etc.
Straight-through double-seat valve:
The valve body contains two valve seats and two valve cores. The valve stem moves up and down to change the position of the valve cores and valve seats.
Features: Lower torque requirements on the actuator when opening and closing the valve. Less tightness at the shut-off point compared to a single-seat valve. Relatively higher cost.
Suitable for: Suitable for places with large differential pressure control but relatively low requirements for tightness of shut-off. Typical applications include differential pressure control valves on the supply and return pipes of air conditioning chilled water.
Three-way regulating valve:
Three-way valves have three inlets and outlets connected to the pipeline. They are classified into two types according to their function: three-way mixing valves and three-way diverting valves.
Features: It can basically maintain a constant total water volume. Therefore, it is suitable for constant flow systems. Generally, the temperature of the three-way valve is required to be less than 150℃.
Suitable applications: Three-way control valves are commonly used for bypass regulation of heat exchangers, and can also be used for simple proportioning regulation.
Butterfly valve:
A valve whose opening and closing element is a disc-shaped butterfly plate that can rotate around the axis of the valve body.
Features: Small size, light weight, easy installation, and a relatively large allowable pressure difference between opening and closing the valve. However, its regulating performance and valve-closing tightness are both poor.
Suitable for: places with large pressure differentials and low requirements for regulation performance.
Actuator Classification
Based on the control medium used, actuators can be divided into three types: pneumatic, electric, and hydraulic. According to the form of output displacement, actuators can be categorized into rotary and linear types. Based on the action law, actuators can be classified into three types: switching, integral, and proportional. Based on the input control type, actuators can be classified into those that can input air pressure signals, DC current signals, electrical contact on/off signals, pulse signals, etc.
Pneumatic actuators and applications
Pneumatic actuators use compressed air as power to control valves. They offer advantages such as simple structure, reliable operation, stable performance, easy maintenance, fire and explosion protection, ease of manufacturing into actuators with high thrust, low cost, simple inspection and maintenance, and good environmental adaptability. Disadvantages include the need for dedicated air supply pipelines for control, and the inability of double-acting pneumatic actuators to return to the preset position after air supply interruption; single-acting pneumatic actuators, however, can return to the preset position via a spring after air supply interruption. Pneumatic actuators can be classified into diaphragm type, piston type, and rack and pinion type according to the method of converting control air pressure into displacement.
pneumatic diaphragm actuator
Pneumatic diaphragm actuators are the most commonly used actuators. They are simple in structure, reliable in operation, easy to maintain, and inexpensive, leading to their widespread application. They are available in two types: air-to-open and air-to-close.
When the input signal increases, the pneumatic actuator generates a thrust below the diaphragm, which overcomes the spring force and opens the valve. (See diagram.)
When the input signal increases, the pneumatic actuator generates a thrust on the diaphragm, which overcomes the spring force and closes the valve. (See figure.)
Piston pneumatic actuator
Pneumatic piston actuators use control air as power to push a piston within a cylinder, producing angular or linear displacement on the output shaft. Piston actuators can apply significant control air pressure to the piston, allowing for actuators with greater thrust or torque than diaphragm actuators, while maintaining a relatively smaller size. (See figure.)
rack and pinion pneumatic actuator
Rack and pinion (double piston rack and pinion) pneumatic actuators are characterized by their compact structure, elegant appearance, rapid response, stable operation, and long service life. All components utilize state-of-the-art anti-corrosion treatment technology, enabling them to withstand various harsh working conditions. Their high and low temperature actuators, as well as those with various special strokes, perform well in a wide range of applications.
The working principle diagram is as follows:
When compressed air enters the pneumatic actuator through nozzle A, the gas pushes the double pistons to move linearly towards both ends (cylinder head end). The rack on the piston drives the gear on the rotating shaft to rotate 90 degrees counterclockwise, thus opening the valve. At this time, the gas at both ends of the pneumatic actuator valve is discharged through nozzle B. Conversely, when compressed air enters both ends of the pneumatic actuator through nozzle B, the gas pushes the double pistons to move linearly towards the middle. The rack on the piston drives the gear on the rotating shaft to rotate 90 degrees clockwise, thus closing the valve. At this time, the gas in the middle of the pneumatic actuator is discharged through nozzle A.
Electric actuators and applications
Electric actuators are a crucial unit in industrial control systems, forming part of a combined electric instrument cluster. They typically consist of two completely independent circuit components: a control circuit and an actuator. Electric actuators receive control signals from a DCS system, linearly converting them into mechanical angular or linear displacement to operate regulating mechanisms such as dampers, baffles, and valves. Advantages include convenient energy access, fast signal transmission speed, long transmission distance, ease of centralized control, high sensitivity and accuracy, easy integration with electric regulating instruments, and simple installation and wiring. Disadvantages include complex structure, a higher average failure rate than pneumatic actuators, and suitability for locations with low explosion-proof requirements and limited air supply. They are classified into linear and angular stroke types based on the output shaft motion.
An angular stroke electric actuator has a rotation angle of less than 360 degrees. The output shaft of the angular stroke electric actuator is angular displacement, and the torque is used to represent the working capacity of the actuator. The rotation of the output shaft is less than one revolution, that is, less than 360 degrees, usually 90 degrees, to control the opening and closing process of the valve.
A linear electric actuator's output shaft moves linearly, and its thrust represents the magnitude of the actuator's working force.
The internal structure diagram is as follows:
Applications: Electric actuators are actuators powered by electric motors and are widely used in the automatic control of thermal equipment in power plants.
Hydraulic actuators and applications
A hydraulic actuator is an actuator that uses hydraulic oil as its power source to perform actions. Hydraulic actuators have the lowest practical application rate among the three types of actuators (electric, pneumatic, and hydraulic), and are only used in some large-scale work applications.
Advantages of hydraulic actuators:
Hydraulic actuators offer higher output driving force than pneumatic and electric actuators, and their output torque can be precisely adjusted according to requirements. Hydraulic actuators provide smoother and more reliable transmission, with cushioning and no impact, making them suitable for environments with high transmission requirements. They offer high adjustment precision and fast response, enabling high-precision control. Driven by hydraulic oil, which is incompressible, hydraulic actuators easily achieve good anti-deviation capabilities. Because they are hydraulically driven, they do not exhibit the arcing phenomenon common in electric actuators, thus offering higher explosion-proof performance than electric actuators.
Disadvantages of hydraulic actuators:
Hydraulic actuators require external hydraulic system support to operate. Operating hydraulic actuators necessitates the installation of hydraulic and oil pipelines, resulting in a higher initial investment and more installation work compared to electric and pneumatic actuators. Therefore, hydraulic actuators are only used in large-scale DEH systems.
Application: Hydraulic actuators. (See diagram.)
High-pressure oil is introduced into the lower chamber of the hydraulic actuator piston. The hydraulic actuator piston moves upward against the pressure of the spring, opening the valve via a lever or connecting rod. Alternatively, when the high-pressure oil in the lower chamber is discharged, the piston moves downward with the help of the spring force, closing the valve.
Development trend of electric actuators
The rapid development of power electronics, computer, and communication technologies will inevitably drive the development of electric actuators even faster. Mechatronics will replace split structures; intelligent communication will replace analog signals; control precision will become increasingly higher, and the application environment will become increasingly wider; functions will become more powerful, and reliability will be higher, in order to meet the ever-evolving requirements of automatic control.
1. Bus-based and networked
Abroad, factory automation engineering technology based on industrial local area network (LAN) technology has developed rapidly in the last decade. As one of the automated instruments in automatic control, electric actuators, to adapt to this development trend, should also have standard serial communication interfaces (such as RS-232 or RS-422 interfaces) and dedicated LAN interfaces to enhance their interconnection capabilities with other control devices. Only a single cable or fiber optic cable is needed to connect several, or even dozens, of electric actuators to a host computer to form an entire CNC system. Fieldbus is a serial, digital, multi-point communication data bus installed in the production process area between field devices/instruments and automatic control devices/systems in the control room. Fieldbus enterprise networks, as the future development direction of control systems, with their openness and networking advantages, make integration with the Internet possible, making the application of fieldbus technology to electric actuators an inevitable trend. The application of fieldbus technology replaces traditional 4-20mA analog signals, realizing remote monitoring of electric actuators, transmission of status, fault, and parameter information, completing remote parameterization work, improving its reliability, and reducing system and engineering costs. Currently, the most influential fieldbuses include PROFIBUS, FF, HART, and CAN. In fact, most intelligent electric actuators abroad now come with fieldbus interfaces, and my country has also developed some intelligent actuators with fieldbus interfaces.
2. Digitalization and intelligentization
Intelligentization is the current trend in all industrial control equipment. Inexpensive microcontrollers and new high-speed microprocessors will completely replace the control units of electric actuators, which are primarily based on analog electronic devices, thus realizing a fully digital control system. This full digitalization transforms hardware control into software control, allowing the application of advanced algorithms from modern control theory (such as optimal control, artificial intelligence, fuzzy control, and neural networks) to improve control performance in electric actuators. Traditional electric actuators are generally considered linear systems, consisting of amplification and integrator components. However, in reality, most actuator parameters change significantly during operation. Applying parameter scheduling and model identification adaptive control will greatly improve the control performance of electric actuators. Compared to pneumatic and hydraulic actuators, intelligent electric actuators, with their simple wiring, powerful functions, and reliable operation, will continue to expand their application range.
3. Miniaturization and mechatronics
The high integration of power electronics, the use of microcontrollers, and the integration of powerful modules have led to increasingly smaller and lighter electric actuators. Currently, intelligent electric actuators typically integrate the entire control circuit into a single field instrument, combining the servo motor and field instrument controller into one unit. This integration simplifies installation and commissioning; consolidating the entire control circuit into a single field instrument also reduces the impact of signal leakage and interference on the system, thus improving system reliability.
Internationally, electric actuators are rapidly developing towards miniaturization, integration, digitalization, intelligence, bus-based systems, and networking. Domestically produced electric actuators still lag significantly behind in terms of product variety, control precision, manufacturing processes, reliability, intelligence, and networking capabilities. High-performance electric actuators with independent intellectual property rights are extremely scarce. However, we are certain that the gap between us and the world will continue to narrow.
