hello Bruce,

I would like to implement a PID controller, but I feel that I need to understand the theory behind it first.

The problem is, I know how to use bode plots to design an efficient and stable controller. But its simply an exercise. I dont really understand HOW the controller controls the "plant".

Thus far, i have been tought in this thread that, filters are also used for differentiation and intergration, I know that I sound slow, but I cant see it! Why are filters placed at different corner frequencies? what do they do to the input
(difference between the reference and feedback signal) of the controller to control the plant? and what exactly does stability mean? Its these basics that I like to understand. the books that have read dont go down to the basics, they just jump to the controller design.

My example, a simple ac or dc motor. as the load on the shaft varies, the speed varies. how dose a controller, using the difference between the ref and feedback signals, and implementing filters enable to drive the motor at a contaant speed?

I sorry if I sound beyond help, but i am really tryng to understand this stuff.

Thank you for your time.
sun