I. INTRODUCTION The electric valve positioner is one of the key accessories of the pneumatic regulating valve. Its role is to turn the electric signal output from the regulating device into the gas signal that drives the regulating valve. It has a valve positioning function, which overcomes the friction of the valve stem and overcomes the unbalanced force caused by the change of the medium pressure, enabling the valve to follow quickly and responding to the control signal output by the regulator, realizing the quick positioning of the regulating valve. , improve its adjustment quality. IV. Summary Due to the adoption of CPUs and new devices, the performance of smart electric valve positioners has made a big leap compared to traditional valve positioners. The intelligent electric valve positioner has higher positioning accuracy, wider application range, and more convenient and reliable use. However, in the specific application, the type of the positioner should be reasonably selected in terms of meeting safety requirements, better control effects, matching with the control loops, adapting to special environmental requirements, prolonging the service life, etc., and setting and adjusting its function parameters. school.
Second, the principle and characteristics of the intelligent electric valve positioner 2.1 The working principle of the intelligent electric valve positioner on the pressure regulation of the output air source 1) The output pressure adjustment adopts the PID pulse width modulation (PWM) technology, which is rapid and accurate. Because the control of the piezoelectric valve by the CPU is controlled by a five-step switching procedure, the pressure of the output air source can be accurately and quickly controlled. The control algorithm generally adopts digital PID adjustment method. The CPU performs PID calculation according to the magnitude and direction of deviation between the input signal and the valve position, and outputs a PWM pulse width modulation pulse signal to control the opening and closing of the piezoelectric valve. Because the width of the pulse corresponds to the increment of the air pressure at the output of the positioner, the pressure output P1 of the air source can be quickly and accurately changed. When the deviation is large, the positioner outputs a continuous signal, rapidly and continuously changes the size of P1. When the deviation is small, the positioner outputs a pulse signal with a smaller pulse width, which changes the size of P1 intermittently and slightly. When the deviation is very small (into the deadband), there is no pulse output and the valve position works stably.
2) The adoption of new piezoelectric valve devices ensures high-precision control. The main element of a piezo valve is a piezo-flexible on-off valve, also known as a silicon micro-control valve. Because of its small mass, the inertia of the switch is very small and it can perform very high switching frequencies, thus acting as a high-frequency pulse valve. Output air pressure P1 is controlled to drive the actuator and high valve positioning accuracy can be achieved.
3) The valve position feedback element has high positioning accuracy and long life. The valve position feedback element is a conductive plastic potentiometer with simple structure, high precision and high reliability. It converts the linear or angular displacement of the actuator into a resistance signal, so that the valve position can be accurately detected and the valve can be easily zeroed. , Full and valve flow characteristics of the positioning of the curve.
Due to the use of new types of control elements such as conductive plastics and piezoelectric valves, the positioning of the valve can be achieved with high precision. Due to the use of micro-processing, the positioner can be adjusted and the application range can be greatly improved. The main features are:
1) It is easy to install; it can be adjusted automatically. The configuration is simple and flexible. It is very convenient to set the positive and negative action of the valve, flow characteristics, stroke limitation or split-range operation.
2) The gas consumption of the positioner is minimal. The nozzles and baffle systems of conventional positioners are continuous air-consuming components. Because the intelligent positioner uses a pulsed piezoelectric valve instead of the nozzle and baffle system of the traditional positioner, and the five-step pulsed piezoelectric valve control method can achieve fast and accurate positioning of the valve. The intelligent positioner only exhausts outside when reducing the output pressure, so it is in a non-consumed state for most of the time and is negligible compared to the conventional positioner.
3) It has intelligent communication and on-site display functions to facilitate maintenance personnel to check and repair the positioner's working condition.
4) Rich self-diagnosis function. Not only can the malfunction of the positioner itself be self-diagnosed, but also the performance of the regulating valve and the actuator can be quantitatively measured and diagnosed. Such as the valve stroke change detection, the measurement of the valve's extreme position changes, can diagnose the valve's wear condition; The measurement of the valve's positioning time can diagnose whether the positioning period is appropriate, whether it will cause oscillation; It can also seal the pneumatic actuator, etc. Diagnose it to provide a scientific basis for valve maintenance.
5) It is very convenient to conduct safety test and action.
Third, the actual use of the problem should be noted Although the use of smart locator simple, powerful, but in engineering applications, or should pay attention to some problems in order to make it reliable work, play a better control role, to extend its life.
3.1 Positioner 2/4 wire connection method selection
Due to the high input impedance of the smart valve positioner, it increases with the input current. Therefore, in the selection of intelligent electric valve positioner must check the regulator output control signal with load capacity, should be greater than 500 ohms, in order to ensure the normal operation of the positioner when the large opening. In view of this, the following solution should be considered when the regulator output control signal has insufficient load capacity:
1) Set signal isolation devices in the input signal loop to increase the load capacity of the control signal. That is, the intermediate isolation driving device with high load capacity is selected, and the device carrying capacity should be greater than 500 ohms. If the site is an explosion-proof place, an isolated barrier with high load capacity can be used, such as Shenzhen Wanxun MSB series isolated safety barrier.
2) Use 4-wire connection to reduce the input impedance of the smart positioner input signal loop.
3.2 Smart Locator Flow Characteristics Selection Smart Locators all have flow characteristics selection settings. However, in actual use, it is necessary to reasonably determine the flow characteristics and process requirements of the valve with which it is fitted. The flow characteristics of valves used in general positioners are determined by the processing characteristics of the valve plug. The spool has linear, equal percentage, quick opening and other flow characteristics. If the process requirements are consistent with this, the output characteristics of the smart positioner should be selected as a linear output, so that the overall valve flow characteristics are in line with the original design requirements. When in actual use, if the flow rate characteristic of the spool cannot meet the process requirements, the overall flow characteristics of the valve can be changed by changing the output characteristics of the valve positioner. Such as: can modify the intelligent positioner to equal percentage output, the valve with a linear spool into an equal percentage of flow characteristics of the valve to use; or modify the intelligent positioner to reverse the same percentage of output, can equal percentage of the valve spool Adjust the valve for linear flow characteristics to use.
3.3 Application in explosion-proof environment In the explosion-proof environment, the intrinsically safe or explosion-proof intelligent positioner should be selected. In addition, the matching between the input impedance of the smart positioner and the load capacity of the safety-related equipment should be taken into account when wiring. If an intrinsic safety loop is constructed, it is best to use an output barrier with an output capability greater than 500 ohms. If a Zener barrier is used, a 4-wire connection should be used to reduce the input impedance of the positioner signal loop. If a flameproof intelligent positioner is selected, the on-site installation method shall meet the requirements for the installation of explosion-proof electrical equipment.