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How to Select the Right Type of Controller

Distinguishing Characteristics

 P-only

  • Accelerates the response of controlled process
  • Produces offset except for integrating (1/s) processes

PI

  • Eliminates offset
  • Usually higher maximum deviations than P-only
  • Sluggish, long oscillating responses
  • Increased gain may lead to larger oscillations and instability

PID

  • Introduces stabilizing effect on closed-loop response
  • Exacerbates noise
  • May cause additional wear on valves, etc.

General Guidelines

  1. If possible, use P-only
  2. Use if offset acceptable for moderate values of Kc
  3. Use with integrating processes
  4. Use PI when P is unacceptable
  5. Systems that respond fast
  6. Use PID to increase the speed of closed-loop response if PI is sluggish

Examples

1. Liquid -level control

P-only

  • integrating process
  • maintain level within certain range – offset acceptable

2.Gas pressure control

P-only

  • maintain pressure within certain range – offset acceptable

3.Vapor pressure control

PI

  • direct control such as in flash tank where vapor valves is controlled to directly control vapor pressure – fast response

PID

  • indirect control such as partial condenser on distillation column – control valve adjusts cooling water to change condensation rate – response may be too sluggish with PI

4.Flow Control

PI

  • flow control is fast and direct – PI eliminates offset but D not necessary

5.Temperature control

PID

  • heating and cooling jackets involving heats of reaction and slow mass transfer rates – slow, sluggish responses will usually require D action

6.Composition control

PID

  • slow mass transfer rates usually require D action

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