Class 29 pi, pd and pid pneumatic controllers

PI, PD and PID pneumatic controllers

1. ICE401: PROCESS INSTRUMENTATION 
AND CONTROL
Class 29
PI, PD and PID Pneumatic Controllers
Dr. S. Meenatchisundaram
Email: [email protected]
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015

2. PI Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015

3. PI Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•In this case, an extra bellows with a variable restriction is added to
theproportionalsystem.
•Suppose the input pressure shows a sudden increase. This dri ves
the flapper toward the nozzle, increasing output pressure u ntil
theproportionalbellowsbalancestheinputasinthepreviouscase.
•The integral bellows is still at the original output pressur e, because
the restriction prevents pressure changes from being trans mitted
immediately.
•As the increased pressure on the output bleeds through the
restriction, the integral bellows slowly moves the flapper closer to
thenozzle,therebycausingasteadyincreaseinoutputpressure.
•The variable restriction allows for variation of the leakag e rate, and
hencetheintegrationtime.

4. PD Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015

5. PD Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•A variable restriction is placed on the line leading to the ba lance
bellows.
•Thus, as the input pressure increases, the flapper is moved t oward
the nozzle with no impedance, because the restrictions prev ent an
immediateresponseofthebalancebellows.
•Thus, the output pressure rises very fast and then, as the inc reased
pressure leaks into the balance bellows, decreases as the ba lance
bellowsmovestheflapperbackawayfromthenozzle.
•Adjustment of the variable restriction allows for changing the
derivativetimeconstraint.

6. PID Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015

7. PD Controllers:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•The three-mode controller is actually the most common type
produced, because it can be used to accomplish any of the prev ious
modesbysettingofrestrictions.
•This device is shown in Figure, and, as can be seen, it is
simply acombinationofthethreesystemspresented.
•By opening or closing restrictions, the three-mode control ler can be
usedtoimplementtheothercompositemodes.
•Proportional gain, reset time, and rate are set by adjustmen t of
bellowsseparationandrestrictionsize.

8. References:
• Process Control Instrumentation Technology, by Curtis D.
Johnson, Eighth Edition, Pearson Education Limited.
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015