I. INTRODUCTION Electrical valve positioner is one of the key accessories of pneumatic control valve. Its function is to turn the electrical signal output by the regulating device into the gas signal for actuating the regulating valve. It has the valve positioning function, not only to overcome the stem friction, but also to overcome the pressure imbalance caused by the media force, which enables the valve to quickly follow, and the regulator output control signals corresponding to the rapid adjustment valve position , Enhance its adjustment quality. Second, the intelligent electric valve positioner working principle and characteristics 2.1 Intelligent electric valve positioner on the output air pressure regulator works 1) The output pressure adjustment using PID pulse width modulation (PWM) technology, quickly and accurately. As the CPU on the piezoelectric valve control using a five-step switching process to control, you can accurately and quickly control the output pressure increase or decrease. The control algorithm generally adopts the digital PID adjustment mode. The CPU calculates the PID according to the size and direction of the deviation between the input signal and the valve position, and outputs a PWM pulse width modulation pulse signal to control the piezoelectric valve to open and close. Since the width of the pulse corresponds to the increment of the positioner output air pressure, the air pressure output P1 can be quickly and accurately changed. When the deviation is large, the positioner outputs a continuous signal, which is continuous in a quick and continuous manner, greatly changing the size of P1. When the deviation is small, the positioner outputs a pulse signal with a smaller pulse width, intermittently and slightly changes the size of P1, When the deviation is small (into the dead zone), then no pulse output, the valve position stable work. 2) The new piezoelectric valve components used to ensure that the control of high-precision. The dominant component of a piezo valve is a piezo-flexible switch valve, also known as a silicon micro-control valve, which, due to its small mass and very low switching inertia, can perform very high switching frequencies and therefore acts as a high frequency pulsing valve for Output pressure P1 to control the road, driving the implementing agencies, you can achieve high valve positioning accuracy. 3) valve position feedback component positioning accuracy, long life. Valve position feedback element is a simple structure, high precision, high reliability of conductive plastic potentiometer, the actuator linear or rotational displacement into a resistance signal, which can accurately detect the valve position and the valve can be easily zero , Fullness and valve flow characteristics of the positioning curve. Due to the use of new control elements such as conductive plastic and piezo-electric valves, the valve can be positioned with high precision. Due to the use of micro-processing, there is a great improvement in the alignment of the positioner as well as the application range. The main features are: 1) easy to install; can be automatically adjusted. Configuration is simple, flexible, can be very convenient to set the valve positive and negative effects, flow characteristics, stroke limit or split operation and other functions. 2) Locator air consumption is minimal. Conventional positioner nozzle, baffle system is a continuous gas-consuming components. The intelligent positioner uses pulse piezoelectric valve instead of the traditional positioner nozzle, baffle system, and five-step pulse piezoelectric valve control valve can achieve fast and accurate positioning. The intelligent positioner vent exhausts only when the output pressure is reduced, so it is non-air-consuming most of the time, negligible compared to conventional positioners. 3) With intelligent communication and on-site display function, it is convenient for maintenance personnel to check and repair the positioner's working condition. 4) has a wealth of self-diagnostic capabilities. Not only can the positioner itself work fault self-diagnosis, but also on the performance of regulating valve and actuator quantitative measurement and diagnosis. Such as valve stroke change detection, the valve limit position changes in the measurement can diagnose the valve wear; measurement of the valve positioning time can diagnose the positioning cycle is appropriate, whether it will cause shocks; also on the pneumatic actuator seals, etc. For diagnosis, thus providing a scientific basis for the maintenance of the valve. 5) can be very convenient for safety testing and testing. Third, the actual use should pay attention to the problem Although the intelligent locator is simple to use, powerful, but in engineering applications or should pay attention to some issues, to make it reliable work, play a better control role to extend its service life. 3.1 Locator 2/4 wire connection mode selection Intelligent valve positioner input impedance is high, and with the input current increases. Therefore, the selection of intelligent electric valve positioner must check the controller output control signal with load capacity, should be greater than 500 ohms, in order to ensure the opening of the locator when the normal work. In view of this, for the regulator output control signal with insufficient load capacity, the following solutions should be considered: 1) Set the signal isolation device in the input signal loop to increase the load capacity of the control signal. That is, the use of high load isolation with intermediate isolation device, the device with a load capacity should be greater than 500 ohms. If the site is an explosion-proof place, you can use isolated safety barrier with high load capacity, such as Shenzhen Wanxun MSB series isolation barrier, 2) 4-wire connection to reduce the input impedance of intelligent locator input signal loop. 3.2 Intelligent Locator Selection of flow characteristics Intelligent locators have the flow characteristics of choice to set the function. However, in actual use, according to the valve with the flow characteristics of the process and the specific requirements to be reasonable. General locator with the flow characteristics of the valve is determined by the processing characteristics of the spool. The spool has a straight line, such as percentage, fast open flow characteristics, if the process requirements consistent with this, the intelligent output characteristics of the locator should be selected as a linear output, so as to ensure that the overall valve flow characteristics consistent with the original design requirements. When the actual use of the valve core flow characteristics can not meet the process requirements, you can change the output characteristics of the valve positioner to change the overall flow characteristics of the valve. Such as: you can modify the intelligent positioner for the same percentage of the output, the linear valve with a valve into equal percentage flow characteristics of the valve to use; or modify the intelligent positioner for the reverse percentage output, the valve can be equal percentage valve Adjust to a linear flow characteristics of the valve to use. 3.3 The application of explosion-proof environment In the explosion-proof environment should be used intrinsically safe or flameproof smart locator. Also, be aware of the smart locator input impedance matching with the load-carrying capability of safety-related equipment when wiring. If the intrinsically safe circuit is constructed, it is better to use an output type isolation barrier with a carrying capacity of more than 500 ohms. If a Zener barrier is used, the 4-wire connection method should be adopted to reduce the input impedance of the positioner signal loop. Such as the use of flameproof smart locator, the site installation methods should be consistent with the installation of flameproof electrical equipment specifications. Fourth, the summary As the CPU and the introduction of new devices, intelligent electric valve positioner performance compared with the traditional valve positioner has a big leap forward. Intelligent electric valve positioner positioning accuracy, a wider range of applications, and the use of more simple and reliable. However, in the specific application, the type of locator should be selected reasonably according to safety requirements, better control effect, matching with regulating loop, adapting to special environmental requirements and prolonging service life, etc., and setting and adjusting its function parameters school.
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