Editor's Choice

Loop Signatures 9: Digital controllers – Part 1: Introduction to the simple PID controller

October 2021 Editor's Choice

There is a commonly held belief in control circles that all PID controllers are similar and relatively simple. This is a dangerous fallacy. Our experience has shown that most people in plants, even very experienced instrumentation and control personnel, have little real understanding of the controllers they use.

Incorrect controller usage can and often has, cost plants dearly in terms of time and money. For example, a plant in the UK recently lost nearly £2 million in under-production, simply because they did not choose a more suitable option, which was available as standard in their DCS! Operators often make changes on control systems manually rather than in automatic because the controllers have not been configured correctly for the application, even though they may have been reasonably well tuned.

If you wish to optimise your plant to obtain the most effective control, it is essential that you gain a full understanding of your controllers.

Lack of basic understanding and training

The first and very important point to realise is that there is no such thing as a standard PID controller. Each manufacturer produces their controllers entirely the way they wish and as a result, their controllers are different to those of any other manufacturer. Not only that, but I have found that some controller manufacturers do not even have their own internal standards, so every time a new version of the controller software is released things may differ quite substantially from earlier versions. This is probably due to changes in staff amongst the programmers who write the controller software. Also, it is a sad thing that one finds, even amongst some very reputable manufacturers, the people who write the controller software and those who produce the technical manuals on them, often display an alarming lack of knowledge of practical control themselves, which results in them doing some odd things. This will be become clearer as this series of Loop Signature articles on controllers progresses.

It also amazes me that, in general, manufacturers do not provide any form of training so that their clients can understand and operate their controllers correctly and be able to choose and apply the options available. As mentioned above, I believe it essential that people do acquire an in-depth understanding of the operation of their controllers if they are to successfully optimise their plants, but many manufacturers appear to share the common belief that their controllers are a standard simple thing and that there is no need to provide training, or produce decent technical manuals on these products. I was once told by the chief training officer of one of the biggest companies making DCS control systems, that their job was not to train people on how to use PID controllers that any instrument and control person should already understand and he was quite offended when I said to him that their controllers were different to any other make of controller and that in over 50 plants employing their DCS system, I had never yet met any of their users who really understood the controllers.

One of the reasons for the lack of understanding of the basic PID controller is that modern digital controllers generally have a host of complex features and options, which are important to understand if one is to use the controller to best advantage.

Subjects to be covered in future article

It is my intention to provide a detailed and in-depth explanation of controller operation and the possible features and options that may be available in them, in the series of Loop Signature articles to be published in future issues. The subjects to be covered include:

• Testing controller operation.

• The controller algorithm (equation) used to provide the PID action.

• Digital implementation of the PID algorithm.

• The operation of each of the three controller actions, P, I and D.

• Reset windup and its prevention (both internal and external).

• Controller scan rate.

• The derivative filter.

• Controller responses to errors arising from setpoint changes, as opposed to load changes.

Some controllers offer more features than others. In general, DCS systems offer many more features and options than most PLCs, which, with a very few exceptions, are limited in the analog regulatory control features they offer. In a recent article, I mentioned why I personally find that DCS systems are far better for regulatory control than PLCs and will not repeat those points here. However, in retrospect, I think it fair to point out that PLC suppliers mostly have only limited experience in regulatory control systems, as opposed to DCS manufacturers who been supplying such control equipment for decades.

The remarks above come down somewhat hard on the manufacturers, so it is only fair to point out that understanding is also lacking on the user side, with little interest displayed in the PID controllers that are purchased. In fact, frequently in specifications put out by prospective purchasers of control systems, which often contain detailed requirements of things like HMIs, redundancy, hot back-up and many other important things, one usually finds very little about the PID controllers themselves. I believe it is essential that users of control systems should acquaint themselves with the pros and cons of the equipment they are buying to run the regulatory controls in their plants. To illustrate this point, I will mention later in this series of articles, the hard to believe fact, that the majority of PID controllers in the world are configured in such a way that it is practically impossible to tune derivative action into them. Even though D action is probably only effective in one in several hundred control loops, the D term can increase speed of response by up to a factor of four if the right process dynamics are present. If I was a control engineer in a plant purchasing control equipment, I wouldn’t dream of buying a controller that did not allow me to properly use the D term!

Lack of knowledge about your controllers can cost you dearly in terms of poor control and productivity. The following articles in this Loop Signature series will explain why.

About Michael Brown

Michael Brown.

Michael Brown is a specialist in control loop optimisation with many years of experience in process control instrumentation. His main activities are consulting and teaching practical control loop analysis and optimisation. He gives training courses which can be held in clients’ plants, where students can have the added benefit of practising on live loops. His work takes him to plants all over South Africa and also to other countries. He can be contacted at Michael Brown Control Engineering cc, +27 82 440 7790, michael.brown@mweb.co.za, www.controlloop.co.za


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