PID Control

PID Control (proportional-integral-derivative) is by far the widest type of automatic control used in industry. Even though it has a relatively simple algorithm/structure, there are many subtle variations in how it is applied in industry. This has probably resulted in confusion for plant engineers and operators, who are happy to leave control loops in a sub-optimal configuration.

Introduction to PID Control

PID Controller Tuning

Alternative Forms of PID

PID Control Trainers (Software and Hardware Systems)

ACTC Case Study Reports

PID Control Resources

These sites give a brief introduction to PID Control, it's features and application to control systems.

• Instrument Service & Equipment Inc.: PID for Process Control Tutorial
• Thomas Bullock: Adjusting PID Gains Tutorial
• Thomas Bullock: PID and Servos Tutorial
• Expertune: What is PID - Tutorial
• www.engin.umich.edu/group/ctm/PID/PID.html - the excellent Controls Tutorial for Matlab from Carnegie Mellon and University of Michigan. This particular section provides an introduction to PID and it's tuning.
• http://members.aol.com/JohnShaw/pid/index.html - a simple introduction to industrial PID control, including code for a simple PID algorithm.

There any many companies and software tools that provide PID Controller Tuning, here are a few. Be sure to know your own controller's configuration before re-tuning.

• Contek Systems's PID Tuning Package (http://www.contek-systems.co.uk/Products.htm) - Low priced process identification and controller tuning software. Free technical information and process control consultancy services.
• RaPID - Robust Advanced PID Control integrating data acquisition, process modeling and optimal PID control design.
• EZYtune - a product from Matrikon which calculates PID gains for common low-order process models based on IMC tuning rules. A free 30 day evaluation period is provided in the software.
• ProTuner - a control loop tuning and analysis tools widely used in industry
• Expertune Corp - specialises in sofware, training and services for controller tuning. Top Control also provide training on loop tuning using Expertune, their web-site has some very useful papers available.
• Rockwell Software RSTune - Windows® based software that helps you optimize your PID control loops.
• InTune - a controller by from ControlSoft Inc which has had some good reports
• Bestune PID Tuning Software - Matlab based PID loop tuner and evaluation package.
• Universal Dynamics Technologies - produces a pacakge called Brainwave, an advanced closed loop controller that works in conjunction with your DCS.

There are many subtly different forms of PID controller, and the structure you are working with needs to be taken into account when tuning and designing the control system. Here are some alternatives:

• http://www.expertune.com/artCE87.html - Comparison of PID Control Algorithms (All Controllers Are Not Created Equal) by John Gerry (Expertune)
• http://members.aol.com/pidcontrol/pid_algorithm.html - this article focusses on the fact that there are differences in PID algorithms and to describe them
• Instrument Engineers Handbook - describes in detail all of the different types of PID configuration, including the various specific implementations found in PLCs and DCS. It also describes the tuning of PID controllers.
• PID time-proportional - this commonly used term is very well explained by the following email from Vladimir Garner of GE (Vladimir.Garner@SUPPLY.GE.COM)

Basically, PID time-proportional is a way to control the CV of your PID loop using a discrete output.  If you are familiar with Pulse Width Modulation, it is an idea similar to that.  You take the calculated "analog" output of your PID and convert that to an ON/OFF ratio for a discrete output. (e.g. if you need 50% output, the on time = off time over a defined time period).

I myself have only seen this used on heat/furnace control. I am not sure what PLC mfgs do the calculation as an option in you PID block, but you can also do the conversion within ladder.

These sites are resources for software/hardware tools for training in the tuning and application of PID controllers. These packages are targetted at either educational and industrial training needs. Some are commercially available, others are available free of charge.

• Control Station Process Control Laboratory by Prof. Doug Cooper (University of Connecticut) for both process control training and controller design and tuning.
• Review of "PC-ControLAB for Windows"
• David StClair: Software for "PID CONTROLLER TUNING, A PRIMER"
• Rigdon Emprise: PID Tutorial and Process Control Training
• PIDSIM : MSDOS PID Simulator
• Process Control Simulator: PID Tuning Process Simulator

The ACTC Case Studies often look at the more advanced control techniques, however, many do look at a Classical control solution, using PI or PID, as the basis for comparison. The Case Study Reports that include such designs are:

• British Aerospace: Classical versus Modern Control Design Methods for Safety Critical Control Engineering Practice (CS08/1995) - comparison of Classical and Robust (LQG, H-inf) control for safety critical systems, including a pumped water supply system and flight control system

• http://www.isa.org/mcweb/contpid/0,1917,0,00.html - ISA list of PID resources

• http://www.pidlab.html - Internet PID Controller Laboratory Offering Powerful Java Applets for PID Tuning and Loop Analysis