Pneumatic PI controller

  • In this case, an extra bellows with a variable restriction is added to the proportional system. Suppose the input pressure shows a sudden increase.
  • This drives the flapper toward the nozzle, increasing output pressure until the proportional bellows balances the input as in the previous case.
  • The integral bellows is still at the original output pressure, because the restriction prevents pressure changes from being transmitted immediately.
  • As the increased pressure on the output bleeds through the restriction, the integral bellows slowly moves the flapper closer to the nozzle, thereby causing a steady increase in output pressure (as dictated by the integral mode).
  • The variable restriction allows for variation of the leakage rate, and hence the integration time.