Mike Sunners <mikes@CAA.ORG.AU> wrote:
> Andy, can you tell us what PID is, or give a text reference?

PID is a standard type of closed loop control system.  Let's take as an
example a mechanical positioning system.  You have a desired position
and a sensor that provides feedback of the current poition.  You need to
generate a control signal to the positioner to control its velocity
(including its direction).

The difference between the current position and the desired position is
called the error.  The control signal is computed as the weighted sum of
three terms derived from the error:

    P:  Proportional to the error
    I:  Integral of the error
    D:  Derivative of the error

Perhaps someone more knowlegeable than I will be able to explain the
specific reasons for each term.  The simple answer is that if only the
proportional term is used, the system will experience overshoot, oscillation,
and other nasty effects.

Cheers,
Eric