What Is a Mechanical Actuator?

The actuator uses liquid, gas, electricity or other energy sources and converts it into a driving action through a motor, cylinder or other device. The basic actuator is used to drive the valve to the fully open or fully closed position. The actuator used to control the valve can accurately move the valve to any position. Although most actuators are used to open and close valves, today's actuators are designed to go far beyond simple switching functions. They include position sensing devices, torque sensing devices, electrode protection devices, logic control devices, and digital communication modules. And PID control modules, and these devices are all installed in a compact housing.

Chinese name: Executive agency
Types of: mechanism

Affiliation: mechanical
Definition: Drives that provide linear or rotary motion

Definition of executive agency

The broadest definition of an actuator is: a driving device that can provide linear or rotary motion, which uses a certain driving energy and works under a certain control signal.

Because more and more factories adopt automatic control and manual operations are replaced by machinery or automation equipment, people require the actuator to play the role of the interface between the control system and the mechanical movement of the valve, and they also require the actuator to enhance the work safety performance and Environmental protection performance. In some dangerous situations, the automatic actuator device can reduce the injury of personnel. Some special valves require emergency opening or closing in special situations. Valve actuators can prevent further spread of danger while minimizing plant losses. For some high-pressure large-caliber valves, the required output torque of the actuator is very large. At this time, the required actuator must improve the mechanical efficiency and use a high-output motor to operate the large-caliber valve smoothly.

Classification of implementing agencies

Powered by electric energy, compressed air, or pressure oil, it outputs a rotation angle or linear displacement corresponding to the control signal, and drives the adjustment mechanism with a certain torque or thrust to complete the parameters required for the production process. It is also called an actuator. Depending on the power source used, there are usually electric actuators, pneumatic actuators, hydraulic actuators, etc. The execution mechanism is an important part of the automatic control system. It accepts control signals from control instruments or manual given, amplifies its power, and then converts it into the corresponding angular displacement or linear displacement of the output shaft, which is used to promote various regulating mechanisms, such as regulating valves, damper flaps, etc. Change the flow rate of the media being adjusted to complete automatic control or manual control of various process parameters. The action law of the actuator is usually linear, and there are also equal percentages. The control signals include continuous current signals and intermittent voltage or pulse signals. ^[1]

At present, the most commonly used actuators are electric signal pneumatic long stroke actuators and electric actuators.

Electric actuator pneumatic long stroke actuator

Pneumatic actuators that use compressed air as power and can directly receive standard current control signals have the characteristics of smooth action, large thrust, high precision, intrinsically explosion-proof, and easy to achieve the required control law. Most of its varieties have the function of "three-off" self-locking retaining function with power-off, gas-off, and power-off signals, and have high safety. ^[1]

Figure 1 is a functional block diagram of a pneumatic actuator. The pneumatic actuator is composed of two parts: an automatic working system and various auxiliary devices. The former includes electro-pneumatic converters, positioners, cylinders, connecting rods and other components, while the latter includes manual operating mechanisms, "three-break" self-locking devices, valve position transmitters, and travel switches. The electro-pneumatic converter changes the current control signal into a pressure control signal. The function of an automatic working system composed of a positioner that works according to the principle of force balance and a cylinder and a connecting rod as power components is to make the output angular displacement of the actuator correspond to the input control signal. Manual operation mechanism is used for device adjustment and local emergency operation. The "three-break" self-locking device is composed of a monitoring circuit for the air source, power supply, and electrical signals, and a locking device when the air source is disconnected. When one of the above-mentioned faults occurs, the output of the actuator is maintained to ensure the safety of the equipment and operation. The valve position transmitter converts the output angular displacement of the actuator into a corresponding current signal, and the position switch is used to send a switch signal of the limit position.

Figure 1 Principle block diagram of pneumatic actuator

The fully functional pneumatic actuator with "three breaks" protection is widely used in various industrial sectors. However, its structure is complicated and the maintenance workload is large. Common pneumatic actuators include pneumatic actuators with only air supply protection, pneumatic diaphragm regulating valves, and pulsed electric signal pneumatic long-stroke actuators. The former two are simple in structure and the latter adopts pulse control, which has good safety.

Electric actuator

Angle travel, straight travel two categories. According to the signal system and the full travel time, it is divided into basic types and multiple derived types. In the automatic control system, they are used with different types of electric manipulators to achieve automatic control of process parameters, manual / automatic two-way disturbance-free switching of the control system, midway limit and remote manual operation. ^[1]

The electric actuator is composed of two parts: servo amplifier and servo mechanism. Figure 2 is a functional block diagram of an electric actuator. It is an automatic position control system. The deviation between the control signal from the control instrument and the valve position feedback signal returned by the position transmitter is amplified by the power of the servo amplifier, and then the servo motor is driven to cause the reducer to push the adjustment mechanism to reduce the deviation, and the output shaft finally stabilizes at At the corner position corresponding to the control signal, the role of the electric operator is to perform manual / automatic switching of the control system and remote manual operation.

Figure 2 Functional block diagram of electric actuator

1servo amplifier; 2electric operator; 3servo mechanism; 4servo motor; 5reducer; 6position transmitter

Actuator Valves and Automation

In order to successfully realize process automation, the most important thing is to ensure that the valve itself can meet the special requirements of the process and the medium in the pipeline. Generally, the production process and process medium can determine the type of valve, the type of valve core, and the structure and material of valve trim and valve.

After the valve is selected, the next step is to consider the requirements of automation, that is, the selection of the actuator. Actuators can be considered in terms of two basic types of valve operation.

1. Rotary valve (single swing valve)

Such valves include: plug valves, ball valves, butterfly valves, and dampers or flappers. This type of valve requires an actuator with a required torque for 90-degree rotation operation

2.Multi-turn valve

These valves can be non-rotating poppet stems or rotating non-lift poppets, or they require multiple turns to drive the valve to an open or closed position. This type of valve includes: globe valve (stop valve), gate valve, knife gate valve, etc. As an option, a pneumatic or hydraulic cylinder or membrane actuator with linear output is also opened to drive the valve.

There are currently four types of actuators that can use different drive energy sources and can operate various types of valves.

1. Electric multi-turn actuator

Electric actuator

Electric-driven multi-turn actuators are one of the most commonly used and most reliable actuator types. Use a single-phase or three-phase motor to drive gears or worm gears to drive the stem nut last. The stem nut moves the stem to open or close the valve.

Multi-turn electric actuators can quickly drive large size valves. In order to protect the valve from damage, the limit switch installed at the end of the valve stroke will cut off the power of the motor. At the same time, when the safe torque is exceeded, the torque sensing device will also cut off the power of the motor. The clutch-equipped handwheel mechanism can manually operate the valve in the event of a power failure.

The main advantage of this type of actuator is that all components are housed in a single housing. All basic and advanced functions are integrated in this waterproof, dustproof and explosion-proof enclosure. The main disadvantage is that when the power fails, the valve can only be kept in place. Only by using a backup power system can the valve achieve a fail-safe position (fault on or fault off)

2. Electric single-turn actuator

This type of actuator is similar to an electric multi-turn actuator, the main difference is that the final output of the actuator is a movement of 90 degrees at 1/4 revolution. The new generation of electric single-turn actuators combines most of the complex functions of multi-turn actuators, such as using a non-entering user-friendly interface to set parameters and diagnose functions.

Single-turn actuators have a compact structure and can be installed on small-sized valves, usually with an output torque of 800 kgm. In addition, they should have a small power supply. They can be installed with batteries to achieve fail-safe operation.

3. Fluid driven multi-turn or linear output actuator

This type of actuator is often used to operate globe valves (stop valves) and gate valves, which use pneumatic or hydraulic operation. Simple structure, reliable work, easy to implement fail-safe operation mode.

Normally, people use electric multi-turn actuators to drive gate valves and shut-off valves. Hydraulic or pneumatic actuators are only considered when there is no power supply.

4.Fluid-driven single-turn actuator

Pneumatic and hydraulic single-turn actuators are very versatile, they do not require a power source, are simple in structure, and have reliable performance. They are used in a wide range of fields. Usually output from a few kilograms to tens of thousands of kilograms. They use cylinders and transmissions to convert linear motion into right-angle outputs. Transmissions usually include: forks, gear racks, and levers. The rack and pinion output the same torque over the full stroke range. They are very suitable for small size valves. The fork has a high efficiency. It has a high torque output at the beginning of the stroke. Pneumatic actuators are generally equipped with accessories such as solenoid valves, positioners or position switches to control and monitor the valves.

This type of actuator can easily implement fail-safe operating modes.

Elements for implementing agency selection

The following factors must be considered when selecting a suitable valve actuator type and size:

Actuator- driven energy

The most commonly used driving energy is a power source or a fluid source. If a power source is selected as the driving energy, a three-phase power supply is generally used for large-size valves, and a single-phase power supply is available for small-size valves. General electric actuators can have multiple power types to choose from. Sometimes a DC power supply is also available. In this case, you can install a battery to achieve power fail-safe operation.

There are many types of fluid sources. First, they can be different media such as compressed air, nitrogen, natural gas, hydraulic fluid, etc. Second, they can have various pressures. The third actuator has various sizes to provide output force and live torque.

Actuator valve type

When selecting an actuator for a valve, you must know the type of valve so that you can choose the correct type of actuator. Some valves require multi-turn drives, some require single-turn drives, and some require reciprocating drives, which affect the choice of actuator type.

Multi-turn pneumatic actuators are generally more expensive than electric multi-turn actuators, but reciprocating straight-stroke output pneumatic actuators are cheaper than electric multi-turn actuators.

Actuator torque

For 90-degree rotating valves such as ball valves, disc valves, plug valves, it is best to obtain the corresponding valve torque from the valve manufacturer. Most valve manufacturers test the operating torque required by the valve at the rated pressure. Torque is provided to the customer. The situation for multi-turn valves is different. These valves can be divided into: reciprocating (lifting) motion-valve stem does not rotate, reciprocating motion-valve stem rotation, non-reciprocating-valve stem rotation, the valve stem must be measured Diameter, stem connection thread size has determined actuator specifications.

Selection of executive agencies

Once the type of actuator and the required drive torque of the valve are determined, the data sheet or selection software provided by the actuator manufacturer can be used for selection. Sometimes the speed and frequency of valve operation also need to be considered.

Fluid-driven actuators can adjust the stroke speed, but electric actuators with three-phase power have only a fixed stroke time.

Some small-sized DC electric single-turn actuators can adjust the stroke speed.

Actuator switch control

The biggest advantage of the automatic control valve is that it can operate the valve from a long distance, which means that the operator can sit in the control room to control the production process without the need to visit the scene to manually open and close the valve. People only need to lay some pipelines to connect the control room and the actuator. The driving energy directly excites the electric or pneumatic actuator through the pipeline. The 4-20mA signal is usually used to feedback the position of the valve.

Continuous control of the actuator

If the actuator is required to control parameters such as the level, flow or pressure of the process system, which is a task that requires frequent action of the actuator, an analog signal of 4-20 mA or 0 to 10 V can be used as the control signal, however this signal may Will change as frequently as the process. If an actuator with very high frequency is required, the only choice is a special regulating actuator that can frequently start and stop. When multiple actuators are required in a process, the various actuators can be connected by using a digital communication system, which can greatly reduce the installation cost. Digital communication loop can transfer instructions and collect information quickly and efficiently. At present, there are multiple communication methods such as: FOUNDATION FIELDBUS, PROFIBUS, DEVICENET, HART, and PAKSCAN designed for valve actuators. Not only can digital communication systems reduce investment costs, they can also collect a large amount of valve information, which is valuable for predictive maintenance procedures for valves.

Implementing predictive maintenance

The operator can use the built-in data memory to record the data measured by the torque sensing device each time the valve is actuated. These data can be used to monitor the valve's operating status, can prompt the valve whether it needs maintenance, and can use these data to diagnose the valve.

The following data can be diagnosed for the valve:

1. Valve sealing or packing friction

2.Friction moment of valve stem and valve bearing

3. Valve seat friction

4. Friction during valve operation

5. Dynamic force on the spool

6. Valve stem thread friction

7.Stem position

Most of the above data exist in all types of valves, but the emphasis is different. For example, for butterfly valves, the frictional force during valve operation can be ignored, but for plug valves, this force value is very large.

Different valves have different torque operating curves. For example, for wedge brake rice, the opening and closing torques are very large. During other strokes, only the packing friction and thread friction are used. When closed, the hydrostatic pressure acts on the brake plate and increases. Because of the friction of the valve seat, the final wedging effect rapidly increases the torque until it is closed in place. Therefore, according to the change of the torque curve, the faults that will occur can be predicted, and it can provide valuable information for predictive maintenance.

Intelligent frequency conversion control of actuator

During the working process of the actuator, due to the frequent startup of the motor, the rated frequency changes at work. The frequency can reach the rated value through intelligent frequency conversion control.

For example: due to resistance or external force, the starting speed of the motor becomes slower, resulting in an error in the stroke control of the actuator. Using intelligent frequency conversion control, the input speed can be changed, thereby making the work of the actuator more reliable and stable.

Executive agency

Li Xuerong first proposed the concept of a generalized actuator in 1988. There are two main starting points: 1) In order to develop machines with better performance, the purely mechanical mechanism must be expanded to develop a new structure that extends the functions of the traditional mechanism. Generalized executive mechanism; 2) generalize the components that constitute the mechanism. On this basis, Zou Huijun set out a more precise definition of the generalized actuator from the development trend of modern machinery: the generalized actuator is a controllable actuator composed of a drive unit and an execution unit, and it is used to realize the mechanical energy conversion and motion generation of the machine The performers of functions such as transformation and transformation are the core of mechatronics systems. ^[2]

Basic characteristics of the generalized actuator

(1) Controllability: Generalized actuators integrate electromechanics through sensing technology, electronic technology, control technology, etc. According to changes in functional requirements, as long as the drive components are programmable, complex and variable output can be achieved. Movement makes the original "rigid" output develop into "flexible" output, realizing the diversity of output movement.

(2) Intelligent: By using some intelligent driving components, such as shape memory alloy, the output movement of the mechanism is intelligent, and the intelligent control of the machine is realized.

(3) Miniaturization: Micro-motors, piezoelectric crystals, etc. can be used to generate micrometer-scale working strokes to achieve miniaturization of the mechanism.

(4) Integration: With the development of modern mechanism technology, "movement integration blocks", such as linear displacement units, can be manufactured by design to realize various motion outputs.

(5) High performance: The motion output of the generalized actuator is related to the characteristics of the driving element and the type of mechanism, rather than simply depending on the type of mechanism. Modern drive elements include various types of motors, hydraulic and pneumatic cylinders, piezoelectric actuators, electromagnetic switches, shape memory alloys, and other forms. Their drive characteristics are different from traditional single power sources. Extending the connotation of the mechanism to the integration of the driving element and the mechanism makes the designer's design space change from a one-dimensional design space to a two-dimensional design space that simultaneously designs the driving element parameters and the mechanism structure parameters and considers the integration of the two. The designer has more The design parameters are used to improve the motion and dynamic performance of the mechanism and expand the function of the mechanism.

Application of actuator

(1) Sewing equipment

For more than a century, people have continuously improved and innovated sewing machines, from ordinary pedal-type household sewing machines to multi-functional industrial sewing machines. After entering the 1960s, the development of sewing machines began to use electronic technology, and mechatronic sewing machines appeared. This new type of sewing machine uses a controllable motor and microprocessor to control the sewing process and improve the flexibility of the sewing machine. At present, the development of sewing equipment in the direction of electromechanical integration has been the consensus of the industry. Many people think that it is necessary to make the actions of the four major mechanisms, the horizontal needle mechanism, and the thread trimming mechanism "whatever they want," such as changing the behavior of the needle bar, the trajectory of the thread picking hole, and the movement of the horizontal needle. There is nothing we can do, we must turn to "electromechanical motion technology" to improve the performance of sewing equipment.

(2) Camera focusing system

The camera is composed of a lens, a shutter, an aperture, a focusing device, a viewfinder, a film winding mechanism, and a case. Due to the widespread application of mechatronics technology in this field, a large number of complex mechanisms have been replaced by integrated circuits, drive motors and electromagnetic actuators. The camera has developed from a traditional product combining precision machinery and optics to an automated system that integrates precision machinery, optics and microelectronics.

(3) CNC milling machine

In the CNC milling machine, the servo motor, gear reducer and screw mechanism are mainly used as the prime mover, the transmission mechanism and the actuator, respectively, by using the servo control system to control the output motion of the servo motor to compensate the motion error of the screw mechanism. The application of the generalized actuator makes the input motion of the screw mechanism a non-linear function, which can effectively improve the three-dimensional accuracy of the output motion, thereby improving the overall performance of the machine tool.

(4) Metal forming press

In the design of the metal forming press, a two-degree-of-freedom planar seven-bar mechanism was driven by a combination of a high-power constant-speed motor and a low-power servo motor. This hybrid drive press has very superior performance and can realize low cost numerical control of the press. In addition, the Japanese Muratec company uses a servo drive system to improve the working performance of the press. This system uses a servo motor to control the working mode of the punch, which achieves a punching speed 150% higher than similar machinery, while suppressing vibration and noise. ^[3]