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Old March 7th, 2005, 02:03 PM   #1
Ron Beaufort
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What is P in PID? ...

Greetings to all,



this thread is intended to complete the series of topics which sort of “came to be” in response to the questions “What is I in PID?” which I attempted to answer in this thread ... and “What is D in PID?” which I covered in this thread ...



disclaimer: there are MANY ways of setting up PID control ... the information below focuses on just ONE of the most common methods used for the Allen-Bradley PLC-5 family of processors ... specifically, I will be using the “ISA” equation (also known as the “Dependent Gains” equation) for the examples in this thread ... readers who are interested in PID control as used in the SLC-500 and MicroLogix and ControlLogix systems should also be able to follow this material without too much trouble ...



and so for the basic question: “What is P in PID?” ... the simplest answer that I can offer 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 come up with over the years in my week-long PID classes ... these are the same types of hands-on experiments that I use to give the students something of a “feel” for how the PID is supposed to act when it’s “right” ... and how to recognize and correct problems when it’s “wrong” ... just keep in mind that these classes are not intended for engineers who will be developing a control strategy for a brand new system – but rather for maintenance technicians who will be working on and around existing systems ... and specifically on Allen-Bradley PLC systems using PID control ... 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 ...



on the other hand ... I sincerely hope that my distinguished colleague Peter Nachtwey will take a look at this and offer his constructive criticism to the discussion ... I’m always interested in comparing the “elegant math” methods that he uses for analysis and tuning with the (shall we say?) “less-than-elegant” methods that I rely on ... and of course anyone else out there who wants to take a crack at this is more than welcome to jump in ...



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 control ... we’ll use a simple gas oven as an example ...







in Figure 1 the gas to the oven has been turned off for a very long time ... the temperature of the room air surrounding the oven is 75 degrees F ... the oven has cooled down so that its temperature is also exactly 75 degrees ... in other words, the system has reached a “steady state” in which it will neither heat up, nor cool down, until something is changed ... as noted in the figure, there is no heat entering the oven ... and no heat leaving the oven ...
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