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Complex Feedback Systems:The simple feedback simulation that was shown on the previous page really works too well! It quickly responds to changes in its set point, and the new room temperature settles in smoothly to whatever value is set. Even if it were challenged with complex changes to the room's temperature, it would give the same old boring responses. Too bad the climate control systems in our homes and workplaces don't work this well. Feedback systems can be quite sensitive. Of particular importance is the relative speed of action for the thermometer, for the climate control unit, and for the temperature change itself. The simulation below adds the possibility for adjusting these parameters to see how feedback performance degrades when the system is not adjusted "just right". The simulation shown is an extension of the simple feedback system. It uses the same four basic elements, but now permits three new adjustments. First, the speed of response of the thermometer can be adjusted. Second, the speed and inertia of the climate control system can be determined. Finally, fluctuations (noise) in the room temperature can be added or not.
This simulation doesn't just represent the behavior of poorly designed heating systems. When elements of the body's feedback systems are degraded by disease, those systems too can become unstable. For example, in diabetes mellitus, the body can no longer produce insulin to control blood sugar. Instead the patient has to measure it, and determine an amount of insulin to inject. With care this manual feedback can be made to work reasonably well, but in some patients the delays in measurement and in the insulin action lead to the same kind of instabilities shown above. The normally brisk and accurate responses of the body's feedback systems are a monument to the careful "design" that has evolved in these systems. |
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Copyright 1998, Joe Patlak, Department of Physiology, University
of Vermont. |
