How does an intelligent valve positioner work?

2023-03-21

The entire control loop is regulated by a two-wire, 4-20mA signal. The HART module transmits and receives digital information superimposed on the 4-20mA signal, enabling bidirectional digital communication with the microprocessor. The analog 4-20mA signal is transmitted to the microprocessor, where it is compared with feedback from the valve position sensor. The microprocessor performs control calculations (primary control) based on the magnitude and direction of the deviation, issuing electrical control commands to the piezoelectric valve to initiate opening or closing actions.

The piezoelectric valve adjusts the output pressure increment of the pneumatic amplifier according to the pulse width of the control command, while the output of the pneumatic amplifier is fed back to the internal control loop. This feedback is again compared and processed with the microprocessor's computational results (secondary control), generating a two-stage control output signal to the actuator. Changes in air pressure within the actuator regulate the valve stroke. When the control deviation is large, the piezoelectric valve emits a wide pulse signal, causing the positioner to output a continuous signal that significantly alters the signal pressure to the actuator, driving rapid valve movement. As the valve approaches the desired position, the difference between the commanded position and the measured position decreases, prompting the piezoelectric valve to output narrower pulse signals. These intermittent, minor adjustments to the actuator's signal pressure allow the actuator to approach the new commanded position smoothly. When the valve reaches the target position (entering the dead zone), the piezoelectric valve ceases pulse output, and the positioner's output remains zero, stabilizing the valve in place.