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Old April 22nd, 2005, 11:49 AM   #2
Ron Beaufort
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Figure 2 shows a Proportional-only controller (indicated by the diamond) being used to control a “sealed system” ... here the pressure reading from sensor (T) is being fed back to the controller ... in an Allen-Bradley system, this feedback signal is called the “Process Variable” and is abbreviated “PV” ...








in this example, the setting of the inlet valve could range anywhere from 0% to 100% of fully open ... we’ve given the controller a desired target value of 60% of full pressure ... in an Allen-Bradley system, we call the target value the “setpoint” and abbreviate it as “SP” ... as the test begins, we’ve got our controller turned completely off by setting our Proportional action for a gain of 0.00 ... this means that the inlet valve is now closed completely ... so the pressure in tank (T) is at 0 psi as we start the test ... then shortly into the test, we set our Proportional action for a gain of 1.00 ... the controller sees an error (abbreviated E) of 60% (E = SP – PV) ... it multiplies the error by the Proportional setting (1.00) and calculates an output of 60% ... in an Allen-Bradley system, the controller’s output is called the “Control Variable” and is abbreviated as “CV” ... this causes the red trace to instantly jump from 0 up to 60% ...



and just like in our first example, the curve shows that the pressure in the tank starts to rise rapidly at first ... but then it begins to rise less rapidly as the test continues ... until finally it is rising VERY slowly as the pressure makes its final approach to the setpoint ...



and notice that as the pressure gets closer and closer to the setpoint, the controller keeps steadily decreasing the amount of opening for the inlet valve ... this action was covered in my original “What is P?” thread so we won’t go into all of the details again here ... but be sure to notice that just as soon as the pressure reaches the target, then the controller finally closes the inlet valve completely ...



and now we’ve come to a major point of Ayman’s question ... and the point is that SOME systems WILL be able to reach the setpoint using only Proportional action ... this is one example which seems to defy the textbook’s “rule” that a Proportional-only controller cannot reach or maintain a setpoint ...



just be sure to keep in mind that as long as the inlet valve is opened even a tiny pinhole of a crack, then air will continue to flow from the higher pressure of the reservoir (C) into the lower pressure of tank (T) ... and eventually the pressure WILL reach the setpoint ... AS LONG AS (and here’s the tricky part) as long as THERE IS NO LEAKAGE FROM THE TANK ... and in this example, the outlet valve is being kept completely closed ...



the big question now becomes: “how realistic is this example?” ... and the sad answer is: “it’s not very realistic at all for processes in the real world” ... in other words, pressure controllers aren’t often used on simple “sealed” tanks ... specifically, there’s usually something flowing out of the tank whose pressure we’re trying to control ...
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