Michael Brown Control Engineering CC
Practical Process Control Training & Loop Optimisation
LOOP SIGNATURE 9
DIGITAL CONTROLLERS – PART 1
INTRODUCTION TO THE “SIMPLE” PID CONTROLLER
There is a commonly held belief in control circles, that all PID controllers are similar, and relatively simple. This is a very dangerous fallacy. Our experience has shown that the vast majority of people in plants (even very experienced I&C personnel), have little real understanding of the controllers they use.
Incorrect controller usage can, and has cost plants very dearly in terms of time and money. For example, a plant in the UK recently lost nearly £2,000,000 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 the controls in your plant to obtain the most effective control, it is really essential that you really gain a full understanding of your controllers.
The first and very important point to realise is that there is no such thing as a standard PID controller. Each manufacturer produce 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 proper internal standards, and every time a new version of the controller software is released things may differ quite substantially from earlier versions of that software. This is probably due to changes in staff amongst the programmers who write the controller software. Also it a sad thing, but one finds that even amongst some very reputable manufacturers, the people who write the controller software, and also those who produce the technical manuals on the controllers, often display an alarming lack of knowledge of practical control themselves, so as a result they do some really stupid things. This will be become clearer as this series of loop signatures on controllers progresses.
It also amazes me that most manufacturers generally do not provide any form of training so that their clients can understand and operate the 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 which 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 the modern digital controller generally has a host of complex features and options available which are important to understand, if one is to use the controller to the best advantage.
It is my intention to provide a fairly detailed and in-depth explanation of controller operation and of the possible features and options that may be available in them, in the following series of Loop Signature articles. The subjects to be covered will 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 at the current stage of development, DCS systems usually offer many more features and options than most PLC’s, which (with a very few exceptions) are normally very limited in the analogue regulatory control features they offer. In a recent article I did mention why I personally find that DCS systems are far better for regulatory control than PLC’s, and will not repeat these points here. However in retrospect, I think it fair to point out that the people building PLC’s on the whole have extremely limited experience in regulatory control systems, as opposed to DCS manufacturers who have in many cases been supplying such control equipment for many decades.
The remarks above do come down pretty hard on the manufacturers, and it is only fair to point out that the users themselves, on their side, often are also lacking badly, as they generally display little interest in the PID controllers they purchase. In fact, frequently in specifications put out by prospective purchasers of control systems, which often contain copious and detailed requirements of things like man-machine interfaces, redundancy and 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 that 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, and little known fact, that the majority of PID controllers in the world are configured in such a way that it is virtually impossible to practically tune derivative action into them. Even though D action is probably only really effective in one in several hundred control loops, the D term can increase speed of response by up to a factor of 4, 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 would not allow me to properly use the D term in it.
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.