Pneumatic PD controller

  • Here a variable restriction is placed on the line leading to the balance bellows.
  • Thus, as the input pressure increases, the flapper is moved toward the nozzle with no impedance, because the restrictions prevent an immediate response of the balance bellows.
  • Hence, the output pressure rises very fast and then, as the increased pressure leaks into the balance bellows, decreases as the balance bellows moves the flapper back away from the nozzle
  • Adjustment of the variable restriction allows for changing the derivative time constraint.