[Pneumatic valve positioner] May I ask the working principle of intelligent valve positioner?

The whole control loop is controlled by two lines, 4 ~ 20mA signal. HART module sends and receives digital information superposition on 4 ~ 20mA signal, realizing bidirectional digital communication with microprocessor. The 4-20mA signal of the analog quantity is transmitted to the microprocessor, and compared with the feedback of the valve position sensor, the microprocessor carries out control calculation according to the size and direction of the deviation (primary control), and sends electronic control instructions to the piezoelectric valve to make it open and close.

Piezoelectric valve according to the control instruction corresponds to the width of pulse pneumatic amplifier output pressure increment, pneumatic amplifier output and is at the same time feedback to the control circuit, comparing with the result of the microprocessor operation again (secondary control), through two levels of control output signal to the actuator, the change of air pressure in the actuator controls the valve stroke. When the control deviation is very large, the piezoelectric valve sends out a wide pulse signal, so that the positioner outputs a continuous signal, which greatly changes the signal pressure to the actuator to drive the valve to act quickly; As the valve approaches the required position, the difference between the command required position and the measured position becomes smaller, the piezoelectric valve outputs a pulse signal with a small pulse width, which changes intermittently and slightly to the signal pressure of the actuator, making the actuator's action near the new command position gentle. When the valve reaches the required position (entering the dead zone), the piezoelectric valve has no pulse output and the positioner output is kept at zero to stabilize the valve in a certain position.