Characterizers for control loops Shinskey F.G.
Shinskey F.G.
Год: 1999
Язык: Английский
Формат: CHM / RAR
Формат файла: RAR
Commercial controllers such as the PID series (proportional, integral, derivative, and their combinations) are linear devices within their normal operating range, i.e., within set limits of input and output signals. Yet most fluid processes which they are assigned to control are nonlinear to some degree, which means that their gain in response to control action is subject to change. In this case the controller cannot be optimally tuned except at one specific operating condition. When the process load or controller set point change, the tuning parameters may no longer provide the optimum recovery from disturbances. In some cases, the process gain can even vary with the magnitude of the disturbance or the deviation of the controlled variable from set point.
If the variation in process gain is less than about 50 percent over the full operating range, the response penalty will not be severe. However, the controller should be tuned at the operating point where the gain is highest, so that the loop will be stable at all times. For some loops, the gain variation is much greater than this, in which case the controller gain is much lower than optimum at normal production conditions, substantially compromising its performance. Two examples are given here where the gain variation was sufficient to cause control problems, and was corrected by applying characterization.
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