What Is an Actuator Positioner?

Actuators are an essential part of automatic control systems. Its role is to accept the control signal sent by the controller and change the size of the controlled medium, so as to maintain the controlled variable at the required value or within a certain range. Actuators can be divided into three categories according to their energy forms: pneumatic, hydraulic and electric. Pneumatic actuators use compressed air as the energy source, which is characterized by simple structure, reliable operation, smooth, large output thrust, convenient maintenance, fire and explosion protection, and low price. Therefore, it is widely used in chemical, papermaking, oil refining and other production processes. It can be easily used with passive instruments. Even when using an electric instrument or computer control, as long as the electrical signal is converted to a standard air pressure signal of 20-100kPa through an electric-to-gas converter or an electric-to-gas valve positioner, a pneumatic actuator can still be used. Electric actuators have easy access to energy and fast signal transmission, but have complex structures and poor explosion-proof performance. Hydraulic actuator is basically not used in the production process of chemical industry, oil refining, etc. It is characterized by large output thrust. [1]

Actuator

Actuators are an essential part of automatic control systems. Its role is to accept the control signal sent by the controller and change the size of the controlled medium, so as to maintain the controlled variable at the required value or within a certain range. Actuators can be divided into three categories according to their energy forms: pneumatic, hydraulic and electric.
The actuator is a combination of an actuator and a control valve in an automatic control system. Its role in the automation control system is to receive signals from regulators or computers (DCS, PLC, etc.), and adjust the flow of process media with its position and characteristics in the process pipeline, so that the controlled automation instruments are produced in production. Within the required range of the process.
(1) Actuators are divided into pneumatic, electric and hydraulic actuators according to the driving energy used.
(2) According to the form of output displacement, the actuator has two types: corner type and linear type.
(3) Actuators can be divided into three types: switch type, integral type and proportional type.
(4) According to the input control signal, the actuator is divided into air pressure signal, DC current signal, electrical contact on-off signal,
In the gear stage, the engine speed can be transmitted to the output lever through two sets of gears. The main reducer is completed by a planetary gear, and the auxiliary reducer is realized by a worm gear. It is fixed in the center by a set of tight springs. In the case of overload, that is, when the output lever exceeds the set torque of the spring, the central worm gear will be axially displaced, and the switches and signaling devices will be fine-tuned to provide protection for the system. Affected by the coupling operated by the external change control lever, the output lever is coupled with the worm wheel when the engine is operating, and coupled with the hand wheel during manual operation. Can be easily disconnected when the engine is not working
The selection of double acting actuators is based on DA series pneumatic actuators. The output torque of the rack and pinion actuator is obtained by multiplying the piston pressure (supplied by the air source pressure) by the pitch circle radius (force arm), as shown in Figure 4. And the friction resistance is small and the efficiency is high. As shown in Fig. 5, the output torque is linear when rotating clockwise and counterclockwise. Under normal operating conditions, the recommended safety factor for double acting actuators is 25-50%
Selection of single-acting actuator Take SR series pneumatic actuator as an example. In the application of spring return, the output torque is obtained during two different operations. According to the stroke position, two different torque values are generated for each operation. The output torque of the spring return actuator is obtained by multiplying the force (air pressure or spring force) by the force arm. The output torque is obtained after the air pressure enters the middle cavity compression spring, which is called "air stroke output torque". In this case, the pressure of the air source forces the piston to turn from 0 degrees to 90 degrees. Due to the reaction force generated by the compression of the spring, the torque gradually decreases from the maximum at the starting point to the second condition: the output torque is the spring recovery when the cavity is out of air The force acting on the piston is called "spring stroke output torque". In this case, due to the extension of the spring, the output torque gradually decreases from 90 degrees to 0 degrees. As described above, the single-acting actuator is based on the two It is designed on the basis of a balanced moment under these conditions.
1.Straight-through single seat valve
The so-called single seat means that there is only one valve core and one valve seat in the valve body. Its characteristics are simple structure, small leakage (even can be completely cut off) and small allowable pressure difference. Therefore, it is suitable for occasions requiring clean medium with small leakage and small working pressure difference. Special attention should be paid to the allowable pressure difference in the application to prevent the valve from closing.
2.Straight-through double seat valve
Through two-seater
1,
Actuator enterprise through application
The main purpose of the control screen is to distribute the electrical energy output by the generator to the user's load or electrical equipment. It is also used to indicate the operation of the diesel generator and to keep the generator voltage stable under load changes. The control screen panel is generally equipped with voltmeters, frequency meters, ammeters, power meters, three-phase current transfer switches, three-phase voltage transfer switches, voltage setting knobs and various indicators. For oil pressure gauges, oil temperature gauges, battery charging ammeters, water temperature gauges, start buttons and start electric locks, some are directly installed on the control screen panel according to design requirements, and some are installed on the diesel instrument panel. The components installed inside the control panel are mainly related to the excitation method used by the generator and the automatic control of the diesel engine. The internal structure of the diesel generator control screen.
Simple control screens are generally equipped with voltage regulators, silicon rectifier diodes, rheostats, automatic air switches, and current transformers. More complex control screens also have overload and short-circuit protection devices, electronic speed regulators, and Electrical equipment such as thyristors, relays and various fuses and small transformers.
First, the history of the development of generator controllers. The automatic control of generator sets has gone through relay control, which has greatly simplified the control circuit. The reliability has also been improved. More humane? There are also some generator manufacturers producing small gasoline engine all-in-one machines. ? Practical and convenient.
Second, the structure of diesel generator set Diesel generator set generally consists of three parts: engine, generator and controller. Among them, the engine provides power? The generator converts the mechanical energy of the engine into electrical energy? The controller is the brain part of the generator set? It not only provides the engine start-up, shutdown, data measurement, data display and fault protection functions, but also provides the power measurement of the generator , Power display and power protection.
Third, (i.e. only provide startup, shutdown and high water temperature, low oil pressure, overspeed protection? No power display is provided? Water temperature gauge, oil pressure gauge, hour meter, battery voltmeter, AC voltmeter, AC are required to be installed on the control panel Meters and other instruments)? Standard type (provides power display, provides protection functions? Without communication interface)? High-end type (in addition to providing standard functions? Can also provide communication interfaces? Can perform remote control, telemetry, remote signaling functions? Some It also has a network interface, can be monitored through the Internet, and some have a text message function, which is more suitable for unattended computer rooms in remote areas).
Fourth, the inspection steps of the test set of diesel generator set When the diesel generator set is purchased? The customer is most concerned about the price of the generator set? There are many types of generator sets according to the brand? And the price of generator sets equipped with different diesel engine power is different. ? Diesel generator set manufacturers will tell you how to compare the price of diesel generator sets.
One question: Ask the purchase time of the old diesel engine, ask about the past repairs? What major components have been replaced? Ask what are the problems in use? In order to have a more comprehensive understanding of the old diesel engine.
Second look: look at the signs and appearance of old diesel engines. See which factory produced it? When did it leave? How long is it from now? See if the paint on the exterior is peeling off? Is the department defective? Whether the model is eliminated. Judging the old and new of the old diesel engine from the signs and appearance.
Three trials: Test run. Check the old diesel engine carefully by cranking the crankshaft, turning the flywheel, starting the machine, etc. Testing is a very important step? The specific inspection steps are as follows:
1. Shake the crankshaft to supply oil to the nozzle. If the fuel injection nozzle makes a clear sound? It means that the performance of the fuel injector and the plunger pair is good. If there is no abnormal noise such as "whistle" in the wheel room, then the gear wear is not serious.
2. Fly up and down. If there is no sound, it means that the gap between the main shaft of the crankshaft and the bearing pad is not large.
3. Turn the flywheel so that the piston approaches the bottom dead center? Then shake the flywheel left and right? If there is no "click", it means that the wear between the connecting rod shaft and the tile is not serious.
4. Reduce the pressure of the cylinder and crank the crankshaft. When the decompression is removed, if the rebound force of the piston is large, the flywheel rotates rapidly, indicating that the degree of wear of the cylinder, piston and piston ring is small. When cranking the crankshaft, the pointer of the oil pressure gauge should not be lower than 1 or the red mark of the oil approval can be raised quickly, and the manual pressure should be laborious.
5. Starting the machine, if it is easy to start? The exhaust is colorless or light gray. The speed is stable and there is no noise. This shows that the technical condition of the old diesel engine is good. Remarks: During the test process of diesel generator set, it is best to operate in the diesel generator controller and track and monitor the changes of data such as air pressure and oil pressure. Therefore, the requirements for the stability and quality of the diesel generator controller are very high. The market for this controller is very high.
V. How to choose a diesel generator set What should you pay attention to when choosing a diesel generator set? Diesel generator sets are widely used in telecommunications, finance and finance departments, hospitals, schools, commerce and other departments, industrial and mining enterprises, etc. power supply. Enterprises must have their own standards when purchasing diesel generator sets.
1. The performance and quality of the purchased diesel generator set must meet the requirements of relevant standards. When purchasing a diesel generator set for communication, it must meet the requirements of G3 or G4 level in GB2820-1997? At the same time The 24 performance index requirements stipulated in the Implementation Rules for Quality Certification and Testing? At the same time, they must pass the strict inspection by the Quality Supervision and Inspection Center for Communication Power Equipment set up by the Chinese industry authority. When choosing military communications diesel generator sets, they must meet the relevant GB2820-1997, GJB related standards, and the relevant departments of the army to formulate the "Quality Test Standards for Telecommunication Power Equipment" and pass the strict inspection of equipment quality by organizational departments.
2. The main factors to be considered when choosing a diesel generator set The factors to consider when choosing a diesel generator set are mainly mechanical and electrical performance, the purpose of the unit, the capacity and change range of the load, and automation functions.
1. Uses of diesel generators. Because diesel generator sets can be used in three situations: common, standby and emergency. Therefore, the requirements for diesel generator sets are different for different uses.
2. Load capacity of diesel generator set. The load capacity and load variation range should be selected according to different uses. Determine the stand-alone capacity of the diesel generating set and the capacity of the standby diesel generating set.
3, the use of environmental conditions for diesel generators. Mainly refers to altitude and climatic conditions.
4, the choice of diesel generators
5. Pay attention to generators and excitation methods when choosing diesel generator sets
6.Selection of the automation function of diesel generating set
Electric actuators are mainly applied in the following three areas 1. Typical applications of power plants are:
The thermal power industry applies blower damper baffle, primary air damper, air preheating, damper recirculation, bypass damper, secondary damper, main damper, damper, damper, burner, adjustment lever, burner, swing drive, hydraulic Push rod drive, turbine speed control, gas control valve, steam control valve, ball valve and butterfly valve to control sliding gates. Valve actuators in other power industries. Ball valve dust control, water jet turbine speed control, large hydraulic valve, gas control valve, burner ignition start. Steam control valve condensate water recirculation, deaerator, boiler feed water, superheat controller, reheating thermostat controller, and other related valve applications 2. Process control is used in the production of chemical, petrochemical, mold, food, pharmaceutical, packaging and other industries Process control, precise positioning, starting, stopping, opening and closing, turning of valves, tools, pipes, baffles, chute, platforms, etc. according to established logic instructions or computer programs, using the temperature, pressure, flow rate, Size, radiation, brightness, chroma, roughness, density and other real-time parameters adjust the system to Achieve control of batch, continuous and cyclic processes.
3. Industrial automation is used in a wide range of aviation, aerospace, military, machinery, metallurgy, mining, transportation, building materials, etc., and performs various forms of adjustment and control on the movement points (moving parts) of various types of automation equipment and systems.
Examples of main applications in process control and industrial automation are as follows:
Application in sulfur mine production Water injection flow control ball valve and dish valve control Application of potassium carbonate pipeline valve actuator Sliding door diverter gate ball valve and butterfly valve ball control valve Application of water flow valve actuator liquid flow control Pressure valve pressure control Acid solution flow control Application of limestone / cement plant valve actuator Ball or butterfly valve control to deal with dry cement, gypsum, or liquid supply and induced draft fan adjustment type damper damper bypass damper damper Environmental pollution control and dust removal Sliding door of the device for logistics control of the raw materials in the hopper and storage. The gate controls the flow of raw materials at the inlet. The gas control valve regulates the burner air intake on the converter. The steam control valve controls the steam required in the production process. Application of gate shunt valve distributor Material unloader / heater dedusting isolation baffle Airflow control (logistics drying) Ball and butterfly valve control Application of steel plant damper baffle and valve actuator Ball or butterfly valve control to control cooling water, waste water, Or other cooling medium-regulated damper baffle air supply and induced draft fan bypass damper baffle gate environmental pollution control And dust removal device. Sliding door controls the flow of raw materials at the inlet. Logistics control of raw materials in the hopper and storage. Gas control valve. Steam control valve adjusts the amount of burner air on the converter. Controls the steam required in the production process. Application of plate and valve actuator Blower damper primary air inlet damper air preheating damper flue gas recirculation bypass damper secondary inlet damper damper main damper damper burner adjustment rod burner Swing drive, hydraulic push rod drive, impeller speed control, flue gas control valve, steam control valve, ball valve and butterfly valve, sliding door gate, process control baffle, air supplement, exhaust fan bypass, hot / cold air mixing, and emergency shutdown. In the petroleum industry Application of oil injection process flow control, gas lift pipeline main valve pressure control, water injection process flow control, oil well sampling test / production valve, application in natural gas production and transportation industry, gas lift air flow control, gas pipeline main valve pressure control, compressor surge Natural gas pressure control Natural gas pipeline main valve pressure control Emergency shutdown natural gas pressure regulator control Pressure control compressor surge control traffic control
Comparison of pneumatic, electric and hydraulic actuators
The actuators used for the regulating valve are nothing more than pneumatic, electric, and hydraulic (electro-hydraulic). Their use performance has their own advantages and disadvantages, as described below.
1. Pneumatic actuators: The actuators used in most industrial control applications today are pneumatic actuators, because they are powered by air, compared with electric and hydraulic, they are more economical and economical, and have a simple structure that is easier to grasp and maintain. From a maintenance point of view, pneumatic actuators are easier to operate and calibrate than other types of actuators, and it is also easy to achieve positive and negative interchange at the scene. Its biggest advantage is safety. When using a positioner, it is ideal for flammable and explosive environments, and if the signal is not explosion-proof or intrinsically safe, there is a potential danger of fire caused by fire. So now the application range of electric control valve is getting wider and wider. However, in the chemical industry, pneumatic control valves still occupy an absolute market advantage.
The main disadvantages of pneumatic actuators are: slow response, poor control accuracy, and poor resistance to deviations. This is due to the compressibility of the gas, especially when using a large pneumatic actuator, the air needs to fill the cylinder and empty. time. But this should not be a problem, because in many operating conditions, high control accuracy and extremely fast response and resistance to deviation are not required.
2. Electric actuators: Electric actuators are mainly used in power plants or nuclear power plants, because a smooth, stable and slow process is required in high-pressure water systems. The main advantages of electric actuators are high stability and constant thrust that can be applied by the user. The maximum actuator can generate thrust of up to 225000kgf. Only hydraulic actuators can achieve such a large thrust, but the cost of hydraulic actuators is higher than electric actuators Much higher. The anti-deviation ability of the electric actuator is very good. The output thrust or torque is basically constant. It can overcome the unbalanced force of the medium and achieve accurate control of the process parameters. Therefore, the control accuracy is better than that of the pneumatic actuator. high. If equipped with a servo amplifier, it is easy to realize the exchange of positive and negative effects, and it is also easy to set the break signal valve position state (hold / full open / full close), and when it fails, it must stay in place. This is Pneumatic actuators can't do it. Pneumatic actuators must be secured by means of a combined protection system.
The disadvantages of electric actuators are: the structure is more complicated, and it is more prone to failure. Due to its complexity, the technical requirements for on-site maintenance personnel are relatively high; the motor must generate heat if it is adjusted too frequently, which is likely to cause The motor overheats, generates thermal protection, and also increases the wear on the reduction gear. In addition, it runs slowly. It takes a long time from the output of a signal from the regulator to the corresponding position of the regulating valve. It is not as good as pneumatic or hydraulic actuators.
3. Hydraulic actuators: When abnormal anti-deviation ability, high thrust and fast forming speed are required, we often choose hydraulic or electro-hydraulic actuators. Because of the incompressibility of the liquid, the advantage of using a hydraulic actuator is better resistance to deviation, which is very important for adjusting the operating conditions, because the throttle operating conditions are unstable when the regulating element is close to the valve seat. The larger the pressure difference, the worse this situation becomes. In addition, the hydraulic actuator runs very smoothly and responds quickly, so it can achieve high-precision control. The electro-hydraulic actuator is an integrated motor, oil pump, and electro-hydraulic servo valve. It can work as long as the power supply and control signals are connected. The hydraulic actuator is similar to the cylinder, but it can withstand higher pressure than the cylinder. Work requires an external hydraulic system, and the factory needs to be equipped with hydraulic stations and oil pipelines. In contrast, electro-hydraulic actuators are more convenient.
The main disadvantages of hydraulic actuators are that they are expensive to build, huge and heavy physical examinations, are particularly complex and require special engineering, so most of them are used in some special occasions such as power plants and petrochemicals.

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