What is P in PID? a (too) simple answer: it is the “Proportional” component of Proportional-Integral-Derivative control.
Well, you’ve got to start somewhere. So this paper assumes (rightly or wrongly) that the reader is already familiar with the very basic idea that PID (Proportional-Integral-Derivative) is a mathematical function (an equation) which may be used to automatically control systems such as temperatures, flow rates, speeds, pressures, levels, etc.
Allen Bradley PLCs (Programmable Logic Controllers).
The subject of this paper is the Integral component of PID control as used in manydifferent types of Allen-Bradley PLCs (Programmable Logic Controllers). Please keep in mind while reading this that there are exceptions to every rule. This material is intended to be a beginner’s level introduction to a very complex subject. In order to keep things as straightforward as possible, it has been necessary to leave out quite a few topics which I wish I had time and space to cover but this is at least a starting point at answering the basic question:
What is P in PID?
I’ve had some pretty good success using the following explanation in the hands-on “technician level” PID classes that I teach. Most of the students in those classes need to work around PID-controlled systems, but they don’t really have very strong math skills. Let’s see if this approach helps you too.
First things first. There are MANY different ways of setting up PID control. But so that we’ll have a specific brand and model of controller for our examples, I’ll be using the Allen-Bradley PID for this discussion specifically, I’ll be using the “Dependent Gains” equation (or as it’s also known) the “ISA” equation of a PLC-5 processor. Note: in addition to the most-common “Dependent Gains/ISA” equation, the PLC-5 and ControlLogix platforms also support the “Independent Gains” or (as it’s also known) the “AB” equation. Fortunately for the topic of this discussion, both of these equations perform the Proportional action in exactly the same manner. So as far as the Proportional action is concerned, the biggest differences between the various Allen-Bradley platforms are in the ranges of the analog input and output signals used by each controller. Where possible, I’ll use “percentages” for these units rather than the raw data values. Students using the SLC-500 and MicroLogix controller platforms should be able to follow the basic ideas of this text by mathematically converting between the different data ranges involved.
And so for the basic question: “What is P in PID?”. One simple answer is that “P” represents the Proportional action and that it reacts to the amount of Error – the difference between the SP (Setpoint) and the PV (Process Variable). Well, if you’re anything like I am, answers like that one leave much to be desired. They seem to be written by people who fully understand the subject – and intended for other people who already fully understand the subject that doesn’t help me out a great deal and so the material that I’m posting here is intended for people who do NOT already understand the subject . Going one step further, as everyone is certainly aware, all students are not “created equal”. The same explanations that can be effectively to teach one, might not work at all while trying to teach another.Throughout this thread I’ll be using some of the most effective methods of explaining the topic that I’ve used over the years in my week-long PID classes.
Keep in mind that these classes are not intended for engineers who will be setting up a control strategy for a new system – but rather for maintenance technicians who will be working on and around existing systems and specifically on Allen-Bradley PLC systems basically that means that the “elegant” math (calculus, differential equations, etc.) that most people expect (and many people dread) will not be included here ... instead I’ve always had good success from using a more “common sense” approach to explain the ideas involved and I promise that the math included here will never get any more complicated than the simple arithmetic used to balance a checkbook or calculate a proper tip for a good waiter. Moving right along before we can really understand how the PID’s. Proportional action functions, we need to have a basic idea of how a typical process reacts when placed under any type of automatic control .We’ll use a simple gas oven as an example .
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