http://www.manufacturing.net/magazine/ce/archives/1998/ctl0301.98/03a305.htm
http://www.expertune.com/tutor.html
http://www.expertune.com/PIDspec.htm
http://www.manufacturing.net/magazine/ce/archives/1998/ctl0801.98/08abas.htm
http://members.tripod.com/aabi/index.htm
http://members.tripod.com/aabi/optimumpid/optimum_menu.htm
http://www.ctc-control.com/tutorials/pid.htm
http://www.newport.com/Motion_Control/Tutorial/Servo_Tuning_Principles
/Servo_Tuning_Principles/description.php
PID tuning: Lieslehto, J., Tampere University
of Technology, Tampere, Finland, (used by Kees ("Case")
Stolk)
This last one is a good web reference for PID tuning, with Java applets.