Andrew Warren wrote:

> There are LOTS of books on PID... Look under "Process Control".
>
> PID is an acronym for "Proprtional-Integral-Derivative".  Like all
> process-control algorithms, it takes an input (from whatever process
>......

The proportional term uses the error directly to generate the output, so
that if any output is needed to maintain the set point, there has to be
some error.

The integral term provides an adaptive offset to counteract the error
needed to generate an output. This allows the system to sit at the set
point.

The derivative term speeds up or slows down the changing system in such
a way as to try to get the system to settle as quickly as possible
without overshoot.

When Andy says "proportional to" he literally means that each term is
assigned a number that determines the proportion it contributes to the
final output, which is just a sum of the individual contributions. these
are the numbers that are tweaked when a controller is tuned.

There are several refinements to the basic algorithm that help with it's
behavior when the output isn't yet stable, as when the process is
started or the setpoint changed. Also, there are a bunch of historically
derived names for each term & proportionality constant that should be
learned you want to, say, have your controller tuned by the plants'
resident 'magic fingers' technician.

-- Tom