The origin and principle of intelligent positioner

    The regulating valve is the terminal of the control system. Once it fails, it will directly affect the safe operation of the device, which has a great impact on the production process. The intelligent valve positioner can improve the flow characteristics and performance of the regulating valve. It can improve the production control ability of the enterprise through digital information communication withDCS or bus equipment, and provide guarantee for the safe and stable production of the device.

1 . The conventional locator has the following disadvantages: 1) The conventional locator is mostly the principle of mechanical force balance. It adopts the nozzle baffle mechanism, and there are many movable parts, which are susceptible to temperature fluctuations.　 
　　

The influence of external vibration and other disturbances is poor in environmental resistance; the spring coefficient of the spring can change under harsh conditions, which will result in

The non-linearity of the regulating valve leads to a decrease in control quality; the external vibration is transmitted to the force balancing mechanism, which is likely to cause component wear and

Zero point and stroke drift also make the positioner difficult to work; 2) Due to the characteristics of the nozzle itself, the actuator also consumes a large amount of compressed air when it is in a steady state, if the number of actuators is used
　　

More, more energy consumption; and the nozzle itself is a potential source of failure, easy to be blocked by dust or dirt particles, so that positioning

The device does not work normally; 3) It is impossible to use special equipment when the conventional positioner is manually adjusted, and it is impossible to isolate the control circuit, and the zero point and the stroke 
　　

The adjustments affect each other, and must be set repeatedly, time-consuming and laborious. When the nonlinearity is serious, it is more difficult to adjust. 
2 . The composition and principle of the intelligent valve positioner 2.1 The composition of the 　　　intelligent valve positioner The intelligent valve positioner is a valve positioner with HART communication protocol, which consists of three parts: microprocessor electronics
　　

Controlled modules, including HART communication modules and local user interface switches;piezoelectric valves for electro / pneumatic converter modules;

Valve position sensor. 2.2 Working principle of intelligent valve positioner The 　　　whole control circuit is controlled by two lines and 4 to 20 mA signals. HART module send and receive superimposed on 4 ~ 20mA signal
　　

Digital information that enables two-way digital communication with the microprocessor. The analog 4 to 20 mA signal is transmitted to the microprocessor.

Compared with the feedback of the position sensor, the microprocessor performs control calculation according to the magnitude and direction of the deviation (first level control)

System), an electric control command is issued to the piezoelectric valve to open and close. The piezoelectric valve is based on the width of the control command pulse

The output pressure of the pneumatic amplifier should be increased, and the output of the pneumatic amplifier is fed back to the internal control loop again.

Compare with the operation result of the microprocessor (secondary control), and output the signal to the actuator through two levels of control.

The change in air pressure within the actuator controls the valve stroke. When the control deviation is large, the piezoelectric valve emits a wide pulse

The signal causes the positioner to output a continuous signal, which greatly changes to the actuator's signal pressure to drive the valve quickly

Action; as the valve approaches the required position, the difference between the commanded position and the measured position becomes smaller, the piezo valve output

A pulse signal with a small pulse width, intermittently and slightly changed to the signal pressure of the actuator, so that the actuator is connected

The action of the new command position is gentle. When the valve reaches the required position (into the dead zone), the piezo valve has no pulse output.

The positioner output remains at zero, keeping the valve stable at a certain position. 3 .Calibration of the intelligent positioner 　　　The gain, positive and negative action, positioner characteristics and permission of the positioner can be completed by setting the switch on the local user interface.
　　 

Basic settings such as automatic adjustment; automatic or manual calibration of the positioner without adding tools;

Local user interface and may be manually controlled by a button , to manually control the control valve. 4 . Other features of the intelligent valve positioner 1) Indicate the operating status or warning condition through various combinations, with diagnostic and monitoring functions;　　 2) Very low air consumption , only 0.12NM3/h in the steady state of 0.6 MPa , less than normal 8 % of the positioner ;
　　
　　
　　

The change of gas source pressure is not sensitive; 3) It can be used for both straight stroke and angular stroke with the same model; control double can be realized by matching double-acting modules
　　

Acting piston cylinder actuator; 4) Two-way communication with the positioner using HART communication protocol; 5 . The problem that should be paid attention to in actual use 5.1 has higher requirements 　　　on the load capacity of the adjustment signal. In actual use, the input impedance of the intelligent positioner is higher. When the input signal is 20 mA , the supply voltage is Zui.　　　
　　
　　
　　

The small requirement value is 12VDC , and the load capacity is not less than 600Ω . Otherwise, the positioner cannot work normally; the small input current of Zui is not less than 3.6mA to ensure its performance.

5.2 Reasonable setting of the action dead zone of the 　　　positioner The smaller the positioner dead zone setting, the higher the positioning accuracy, which causes a misunderstanding to the people, thinking that the dead zone is as small as possible.　　　

However, this will make the movement of the moving parts such as the piezoelectric valve and the feedback rod more frequent, sometimes causing the valve to oscillate, affecting the service life of the positioner and the valve, so the dead zone setting of the positioner is not easy to be too small; after the positioner setting is changed, Must be recalibrated before it can take effect;

5.3 Installation of the 　　　positioner An important principle of the installation of the positioner is that the positioner, the valve stem and the feedback lever form a closed-loop negative feedback.

It can be checked during installation: after the positioner is installed, the valve stem and the feedback lever are not connected, and the feedback lever is rotated by hand, if the valve stem

The direction of action is opposite to the direction of action of the feedback lever, indicating that closed-loop negative feedback is formed; at this time, the position of the regulating valve is to be

50 %, and the feedback lever is in the horizontal position, then the feedback lever and the valve stem are fixed, which ensures that the positioner works in the good linear section. The positioner is not properly installed and will increase its linearity deviation.

5.4 Selection of flow characteristics of the positioner The flow characteristic of the 　　　regulating valve is determined by the processing characteristics of the valve core. If the process requirements are consistent with it, the positioner

The output characteristics should be linear output; in actual use, if the valve core characteristics do not match the process requirements, the overall flow characteristics of the valve can be changed by the setting of the positioner output characteristics, such as the valve that can linearize the spool. By setting the positioner output characteristic to an equal percentage characteristic, a valve with a linear spool can be used as a valve with equal percentage flow characteristics.

5.5 Maintenance of the 　　　positioner When the different function modules of the positioner are damaged, causing the positioner to be unusable, if the whole is replaced, the cost is high;

The positioner can be reassembled with a fault-free module, but it must be reassembled according to different regulators after assembly

Setting, because the regulator valve (stroke, etc.) using the positioner has changed, the use of automatic adjustment may not meet the requirements for use.

At this time, you can manually adjust the stroke to determine the stroke, and then use the automatic calibration calibration. This allows the adjustment valve to be positioned accurately,

With the right response speed to meet process control requirements, you can also save a lot of money.