Intro2u.com - PID
When must a PID be tuned,  why and How is a PID tuned?

PID Controller

Tuning a PID (Three-Mode) Controller

The Control Loop - Basic feedback control, Valve Linearity, Valve Linearity, Fail Open Valves

Process responses - Steady state: effect of controller output, Steady state: effect of disturbances, process Dynamics: Simple lag, Process Dynamics: Dead time, Measurement of dynamics
Disturbances

The PID algorithm - http://www.jashaw.com/pid/tutorial/
Action
Auto/Manual
Key concepts
Proportional
Proportional—units
Proportional—Output vs. Measurement
Proportional—Offset
Proportional—Reducing offset with manual reset
Adding automatic reset
Reset or integral mode
Derivative
Complete PID response

4. Additional PID concepts

Interactive or Noninteractive algorithm
Converting between interactive and non-interactive
External feedback
Saturation Properties

5. Other controller features

Gain on process rather than error
Derivative on process rather than error

6. Loop tuning

Tuning Criteria
Mathematical criteria
On-line trial tuning
Ziegler Nichols tuning method: open loop reaction rate
Ziegler Nichols tuning method: open loop point of inflection
Ziegler Nichols tuning method: open loop process gain
Ziegler Nichols tuning method: closed loop
Controllability of processes
Flow loops

Links