PID CONTROLLER

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PID CONTROLLER
Process Control Instrumentation

• MAPUA INSTITUTE OF TECHNOLOGY
• School of Chemical Engineering Chemistry

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OUTLINE

• Important Concepts
• Process Time Lags
• PID Control Algorithms
• Selection Of Control Action

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PROCESS CONTROL

• Definition
• the physical regulation of a process to maintain
  a particular process variable as close as
  possible to a desired value.

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4 BASIC COMPONENTS IN A FEEDBACK CONTROL LOOP
Operator
Set Point (SV)
3. Controller
Controller Output
Process Variable (PV)
2. Measuring Element
4. Final Control Element
Controlled Variable (CV)
Manipulated Variable (MV)
1. Process
Load Variable
Refining Process (Plant)
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FEEDBACK CONTROL ALGORITHM

• Feedback Control PID Control
• where
• OUT(t) controller output, 0 - 100
• OUTdesign design (steady state) controller
  output
• ?OUT control adjustment

OUT(t) OUTdesign ?OUT
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PID CONTROL ALGORITHM

• P Proportional Control Action
• I Integral Control Action
• D Derivative Control Action

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Proportional Control Action

• control adjustment is proportional to the
  magnitude of the error
• where
• ?OUT control adjustment,
• Kc Proportionality Constant (Gain or
  Sensitivity)
• e ERROR Set Point (SP) - Process Variable (PV)

?OUT (Kc)(e)
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Integral Control Action

• control adjustment is proportional to the time
  integral of the error
• where
• ?OUT control adjustment,
• ?I Integral Time Constant, time

t
?OUT (Kc)/(?I) (e)dt
0
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Derivative Control Action

• control adjustment is proportional to the rate of
  change of the error
• where
• ?OUT control adjustment,
• ?D Derivative Time Constant, time

?OUT (Kc)(?D)(de/dt)
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Types of PID Controller

• P - Proportional Controller
• PI - Proportional Integral Controller
• PD - Proportional Derivative Controller
• PID - Proportional Integral Derivative Controller

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P - Proportional Controller
OUT(t) OUTdesign (Kc)(e)

• only one tuning parameter ( Kc )
• there is always an offset steady state error or
  permanent deviation between the Set Point and the
  Process Variable

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PI - Proportional Integral Controller
t
OUT(t) OUTdesign (Kc)(e) (Kc)/(?I) (e)dt
0

• eliminates offset
• more unstable compared to P Controller
• two tuning parameters ( Kc , ?I )

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PD - Proportional Derivative Controller
OUT(t) OUTdesign (Kc)(e) (Kc)(?D)(de/dt)

• faster response
• does not eliminates offset
• susceptible to noise
• two tuning parameters ( Kc , ?D)

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PID - Proportional Integral Derivative Controller
t
OUT(t) OUTdesign (Kc)(e) (Kc)/(?I) (e)dt
(Kc)(?D)(de/dt)
0

• faster response
• eliminates offset
• susceptible to noise
• three tuning parameters ( Kc , ?I , ?D)

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Selection of Control Action