Hello PIC List,

This is a follow up to my previous post about two months ago regarding a
PID algorithm for an AUV to control it's depth and direction.  At that
time, I was in a mad rush to have some sort of control algorithm that
simply worked (even if the overshoot and settling time were large) as
the competition date that the AUV was designed to enter was looming.  In
the end, the state of the AUV never made it to a point where we needed
even a simple control algorithm.  Now, however, we
(www.engr.uvic.ca/~auv) have another year to design and are at a point
where we need a control algorithm.

We now have a simulator for our AUV that we are using to do first order
testing of control and mission planning algorithms.  The simulator is
not accurate enough to allow us to tune an algorithm on the simulator
and expect it to work on the actual AUV, but it gives as a convenient
way to learn how to tune the algorithm and do initial testing on
different algorithms.

My question: how do I tune a PID algorithm?  There were some positive
response to this question last time I asked and some people that were
willing to help me learn the 'art' of PID algorithms.

I have been reading about PID in depth from textbooks and on the web,
but am starting to feel more and more like tuning it is an art and not a
science.  In addition, there is some fundamental questions I have that I
can not find answers for.

Right now I am trying to tune a PID algorithm to allow our AUV to dive
and keep a constant operating depth.  My first problem is that I want to
have an operating depth of X meters and I know my current depth.  To
move the AUV I have to set the voltage of the vertical thruster.  How do
I relate the input to the plant (the desired depth) to the output of the
plant (the motor voltage)?  All examples I have found assume there is a
magical sensor that changes the output of the plant to the same units as
the input.

This is probably sufficient to show how lost I am and what sort of help
I need.  Thanks for any and all help.  The simulator code is in MATLAB
and I would be happy to send it to people that wish to help me out.  I
can also generate plots of the step response, impulse response, etc...
of the AUV.

Thanks,
Donovan

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