Simple Feedback

Simple Feedback Systems:

The close control of pool size, as required for many of life's processes, is no accident. Instead it depends on active and continual adjustments on the part of the body. This process cannot rely simply on static inputs and outputs. As we have seen, even simple equilibria don't have the needed precision. Special systems must be developed to accomplish this aim.

The process of actively maintaining constancy, even in the face of changing input and output, is called homeostasis (the term itself comes from Claude Bernard, a pioneer physiologist). A homeostatic system needs a special set of elements. These include:

	The pool itself.
	A system for measuring the size of the pool.
	A foreknowledge of the desired level for the pool (the setpoint).
	A system for comparing the pool size with the setpoint.
	A system for adjusting the pool should it differ from the setpoint.

These elements make a loop -- changes in the pool are sensed, and an adjustment is made to that pool to bring it back to the set point. Such a loop is called a feedback loop. In particular it is an example of "negative feedback", since pool changes are corrected in the opposite direction.

Lets look at these elements in action in a simple feedback system, as shown below. Here strip chart recorders represent each of the major elements described above. The arrows show the connections between each system (with the color and width of the arrow showing relative intensity). The system is modeled to represent a heating system, which is an everyday example of a negative feedback loop.

You're missing a great java applet demonstrating balance

The simulation above represents the feedback loop of a climate control system. The strip chart at the bottom is the room temperature. The left chart is the thermometer temperature. The top chart is the difference between the thermometer and the set point. The chart at right is the climate control output.

When running (you can start and stop the simulation with the button at bottom), the system uses negative feedback to keep the room temperature close to the set point. Use the pull-down menu centered at the top to change the set point.

Also, use the pull-down menu centered at the bottom to change the climate conditions of the room: Add positive heat (light a fire in the woodstove), or negative heat (open a window) to alter the work the system must do.

This feedback loop models a climate control system, but with little change it could also be showing physiological systems for control of body water and salt, for controlling blood pressure, or for maintaining steady levels of blood sugar.

However, this feedback system is quite simple. In particular, the thermometer responds instantaneously to changes in room temperature, and the heating/cooling system is very powerful and quick. Often systems are much more complex than this. Look at the next page for a simulation of a complex feedback system.

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Copyright 1998, Joe Patlak, Department of Physiology, University of Vermont.
For problems or questions regarding this web contact Joe Patlak.
Last updated: October 29, 1998.