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PID ALGORITHM
and
TUNING METHODS

 

John A. Shaw
Process Control Solutions

Rochester, New York

585-234-5864

The PID control algorithm is used for the control of almost all loops in the process industries, and is also the basis for many advanced control algorithms and strategies. In order for control loops to work properly, the PID loop must be properly tuned. Standard methods for tuning loops and criteria for judging the loop tuning have been used for many years, but should be reevaluated for use on modern digital control systems.

While the basic algorithm has been unchanged for many years and is used in all distributed control systems, the actual digital implementation of the algorithm has changed and differs from one system to another and from commercial equipment to academia.

We will discuss controller tuning methods and criteria. Also discussed will be the digital PID control algorithm, how it works, the various implementation methods and options, and how these affect the operation and tuning of the controller.

 



 

Table of Contents

1. The Control Loop

Basic feedback control
Valve Linearity
Valve Linearity:
Fail Open Valves

2. 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

3. The PID algorithm

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

 

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www.learncontrol.com/tutorial WWW

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Buy and download my eBook: The PID Control Algorithm: How It Works, How To Tune It, and How to Use It. 2nd ed. 62pp Adobe .pdf file                      only $15

Most major credit cards accepted. Get more details, buy, and download

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