Note: It is best to try this demonstration after you have learned to do the tracking task in the Nature of Control demonstration.
Start the demonstration by pressing the "Run" button. Keep the lower line (the cursor) directly under the upper line (the target) by moving the mouse to compensate for the disturbances to the cursor. Once you have done this for a while the effect of the mouse on the cursor will suddenly reverse. Try to regain control as quickly as possible.
This tracking task shows that control systems operate in the context of other control systems. In this case, we see that the cursor control system works in the context of another control system that might be called a "quality" control system -- one that controls the quality of the controlling done by the cursor control system. When the quality of cursor control becomes poor (due to the sudden change in the polarity of the connection between mouse and cursor movements), the quality control system acts to return the quality of cursor control back to what it was before the polarity change. The quality control system does this by changing the way the cursor control system deals with error (the deviation of intended from actual cursor position). If, prior to the polarity shift, the cursor control system had moved the mouse in a direction that is opposite to the sign of the error, it now moves the mouse in a direction that is the same as the sign of the error. Similarly, if the cursor control system had moved the mouse in a direction that was the same as the sign of the error, it now moves the mouse in a direction opposite to the sign of the error.
A graph of the operation of these control systems at the time of the polarity reversal is displayed automatically shortly after the reversal. The graph will remain on the screen until you start the tracking task again by pressing the "Run" button.
First, notice that it is virtually impossible to avoid the brief "runaway" period that occurs when the polarity of the connection between mouse and cursor changes. During this runaway period you are actually pushing the cursor away from the target in an exponentially accelerated manner. This happens because the change in polarity makes the sign of the cursor control loop positive. The runaway is positive feedback in action.
You can't avoid the runaway because the quality control loop cannot act quickly enough to prevent it. It takes about 1/2 second for the quality control loop to change the way the cursor control system deals with error. During this 1/2 second, the cursor control loop is in a positive feedback relationship with respect to the variable it is controlling (cursor position). This can be seen in the graph of your behavior. The horizontal line in this graph is the location of the target; the vertical line indicates the point at which the polarity reversal occurs. The open squares are a trace of your cursor movements just before and for a period after the polarity reversal. The filled squares are a trace of the cursor movements made by a simple control model during the same period.
Notice that both you and the model accelerate the cursor away from the target as soon as the polarity reversal occurs. Your cursor control system works just like a simple control system for about 1/2 second, at which point the model's cursor continues off the graph while your cursor return to the target position (usually after a brief overshoot in a direction opposite to the initial runaway). Your cursor returns to the target due to the operation of another control system -- the quality control system -- that operates on the cursor control system. The quality control system must have been monitoring the performance of the cursor control system all along; in this sense, the quality control system is at a higher-level that the cursor control system. When the quality control system sees that the cursor control system is in trouble, it "fixes" the problem.
Even if you are well practiced at this tracking task from the Nature of Control demonstration, it will probably take a while to learn how to regain control of the cursor when the mouse - cursor polarity is reversed. The results of this task are clearest when you are well practiced and can skillfully regain control after the polarity reversal.
Last Modified: March 15, 2014