Question

Q1. We have a controller, and every time the Operator changes the Set Point, the controller responds, and gets the process sorted out. Now, somewhere, we have read that load disturbances needed to be considered in processes, as well. How will this controller react, if there is a load disturbance, and not a Set Point change?                                                                                                                       

Q3. The Process Variable jumps around, quite a bit. Which of the PID settings should we not be using, to improve controllability, and extend the life of our Final Control Element?                                                                                                                                                              

Answer 1

The universe of single input single output control loops can be separated into two classifications; self-regulating and non self-regulating. Self regulating - These loops are such that if the controller is placed in manual, the process variable will go to some stable state, assuming the interacting variables are held constant. Examples of these are flow, heat exchanger temperature control, and even pH. This is how a self regulating process behaves. If while the controller is in manual and the process is stable, if the output is change, the process variable or signal will also change, but it will come to a stable point. If the output is moved again, the process variable will come to a different point. The system is stable at an infinite number of points. Non Self regulating - These loops behave such that if the controller is placed in manual, the process variable will go to some saturated state. Examples of these are gas pressure and tank level with a constant input or output. Imagine a gas volume with a fixed orifice in the outlet, to a "constant" backpressure. Assume that a control valve drops a gas pressure to this tank from a much higher pressure. If the loop is placed in manual and the valve moved to some point, the pressure, in time, will either build to a point almost equal to the supply pressure or drop to a point almost equal to the back pressure. The system is stable at only one point. Non self-regulating loops have "built in" reset action and therefore can be tuned without reset. Sometimes, depending on the controller's method of gain, you may experience some offset, but offsetting the set point or adding bias can compensate for this. One can see the differences in these loops by experiments done at home. Try the level experiment on your bathroom sink drain; higher level will occur for higher water flow rates. This is an example of a self regulating process. Try not to fall in the trap of assuming every control system for a particular type of physical or chemical process is one or the other. As you see with the sink level, an integrating process, it becomes self regulating because as the head increases, the flow increases.