William K.
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software tool rejects process disturbances.
William K.   8/10/2011 10:10:25 AM
It does not take much skill or effort to produce an unstable process control system, the hard part is producing an optimized one that has very little room for improvement. That is a fact. Optimizing a system is seldom a trivial effort. But I don't think that rejecting disturbances is a good approach, at least not for those systems that are designed to hold something constant in spite of disturbances. In fact, most of the process control systems that I have designed were intended to remove the effect of disturbances.

Of course, in many systems there is more than the control algorithm that influences the system stability, which is why anybody attempting to make the system stable needs to understand the whole system, not just the controller. System time constants that produce delays produce an effective phase-shift, which will usually tend to cause oscillation, or just overshoot. System elasticity will also cause instability, and it must either be compensated for or eliminated. Another characteristic is nonlinearity, where the same amount of correction has a different response , depending on some external condition.

Then there is non-process noise that randomly enters the system, and has very little to do with the process being controlled. Typiaclly that sort of noise needs to be kept out of the system, or at least compensated for, typically by filtering. IT seems like this may be what the software tool takes care of. Unfortunately it is often very difficult to reject the noise once it enters the system, leaving noise rejection through adequate shielding as the far superior choice. The term "shielding" here refers to all kinds of noise shielding, not only to electrical shielding.

Beth Stackpole
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Beth Stackpole   8/9/2011 8:58:06 AM
Given the huge advances in software development and the key role software plays today even in mainstream appliances, it's surprising to me that it's taken this long to develop a tool to monitor control system operations and act as an early warning system. Seems like a no-brainer to me. Any sense why this isn't more prevelant, or perhaps, it is?

Douglas Smock
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Douglas Smock   8/9/2011 8:10:32 AM
PIDs certainly have done yeoman's duty for the past 100 years, and can certainly be improved where people want to make the effort and investment. Melt flow indexes for material used in a hose extrusion line can vary dramatically, requiring many steps to enhance productivity and product quality. Certainly an improved control algorithm is a good step. In the example cited, the PID is apparently driving heater band performance. Other areas for improvement could be the heating mechanism for the barrel as well as the screw design.

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