Heat Manager Pro User Manual V1.5 Page 65
APPENDIX
B:
PID
CONTROLLERS
The default settings for the PID controllers in the Heat Manager have been
determined by performing a lot of tests in actual real-life process situations.
When you keep the ratio between heating power and work piece mass the same
you can use the same settings for most process conditions.
PID control can be considered as the
intelligence or “brain” of the controller. The same is when
you are driving a car on the highway and wish to control the speed at 100 km/hr. Because of
varying road conditions (different tarmac, driving up- or downhill) it is necessary to increase and
decrease throttle to hold the desired speed. You will also notice that your foot automatically
does this.
Try to concentrate more on what you are actually doing, and you will notice that it is actually
quite
impressive on how the brain executes this apparently easy task. Your brain is now acting as
some kind of very high level adaptive PID controller.
B.1
A
BIT OF HISTORY
In the past century, when automation became more and more important in industry,
mathematicians and engineers tried to find a way to integrate this knowledge into an automated
system.
PID controllers date back to the 1890s governor design. PID controllers were subsequently
developed in automatic ship steering. One of the earliest examples of a PID-type controller was
developed by Elmer Sperry in 1911, while the first published theoretical analysis of a PID
controller was by Russian American engineer Nicolas Minorsky in 1922. Minorsky was
designing automatic steering systems for the US Navy, and based his analysis on observations
of a helmsman.
Observing that the helmsman controlled the ship not only based on the current error, but also on
past error and current rate of change. This was then made mathematical by Minorsky. His goal
was stability, not general control, which significantly simplified the problem. While proportional
control provides stability against small disturbances, it was insufficient for dealing with a steady
disturbance, notably a stiff gale (due to droop), which required adding the integral term. Finally,
the derivative term was added to improve control.
Trials were carried out on the USS New Mexico, with the controller controlling the angular
velocity (not angle) of the rudder. PI control yielded sustained yaw (angular error) of ±2°, while
adding D yielded yaw of ±1/6°, better than most helmsmen could achieve.
The Navy ultimately did not adopt the system, due to resistance by personnel. Similar work was
carried out and published by several others in the 1930s.
