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Old April 22nd, 2005, 11:51 AM   #3
Ron Beaufort
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Figure 3 shows a more realistic application for a pressure controller ... here the outlet valve has been opened for a setting of 2% ... and now we see one of the major drawbacks to a controller which uses only Proportional action ...








notice that in this example, the pressure never gets quite all the way up to the setpoint ... instead it settles out at some slightly lower pressure and then continues along that path ... this is the same type of “steady state” situation that we had with the oven example in my earlier thread ...



we could go through the math involved here if we wanted to ... but some simple common sense should be enough to convince us of what’s going on at the end of this “steady state” test ... suppose that we could somehow (magically?) just nudge the pressure up a little bit more and have it reach the target value of 60 psi ... in that case, the error (the deviation from the target) would become zero ... and in that case, the controller’s output would also be zero ... and now ... what do you suppose would eventually happen to the pressure in tank (T) if the controller gave us a zero signal and closed the inlet valve completely? ... simple answer: “the pressure would eventually drop back BELOW the setpoint” ... the pressure would have to drop BECAUSE OF THE LOAD (the leakage) through the open outlet valve ...



and so to our friend Ayman, I would say this: the difference between the oven example in my earlier thread – and the pressure tank example in Figure 2 - is that the oven can NEVER become a “sealed system” ... specifically, there will always be SOME amount of heat leakage from the oven system ... heat will ALWAYS be able to leave the oven either through conduction, or through convection, or through radiation ... BUT! ... IF (big IF) we COULD somehow design a PERFECTLY insulated and “sealed” oven system, then (and only then) would a controller using Proportional-only action be able to achieve the desired setpoint ...



hopefully the examples of Figure 2 and Figure 3 will help answer most of your questions with regard to “pressure” as a Process Variable ...
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