Effectiveness of Casters

I broused through a few web sites, if you search yahoo on

PID control feedback

you will get many links

this one gives a good overall explaintion without going into the math too much:

http://www.netrino.com/Publications/Glossary/PID.html

I found others that use fequency domain equations, if you can get past that, they are very informative - ie, skip over the math you dont understand and look at the graphs and you will get some insite.