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 K_c
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