>-- Original Message --
>Date: Mon, 12 Jul 2004 12:09:04 +1200
>From: Birger Brunswiek
>To: FlightGear developers discussions <[EMAIL PROTECTED]>
>Subject: Re: [Flightgear-devel] Autopilot
>Reply-To: FlightGear developers discussions
>ver these books which I'm going to get from the library: (name and ISBN)

PID controllers / by Karl J. ?str?m and Tore H?gglund. 1556175167
Adaptive Control (2nd Edition) 0201558661
Control Theory : Multivaria
le & Nonlinear Methods 0748408789
>Control Handbook 0849385709
Handbook of PI and PID controller tuning rules / Aidan O'Dwyer. 1860943500
Digital Control of Dynamic Systems (3rd Edition) 0201820544
Advanced Control Unleashed: Plant Performance
Management for Optimum Benefit

Control Systems Engineering 0471445770

I'll add one of the textbooks that I used in my education:
Process dynamics and control 0471863890

Isn't it more apropriate to have a multidimentional PID c
ntroller rather
than multipl
> 1 dimentional PID controllers? Or is that just the same?

I'm not sure what you mean by multidimentional PID controller, but I believe
that that it would be the same as multiple one dimentional PID controllers.

we look at multivariable control, where we have multiple inputs and multiple
outputs, we still use one PID controller for for each input-output pair.
Consider an autopilot that is supposed to control the ailerons and the rudder.
We know from experie
ce (and probably from theory) that an aileron deflection
will result in a roll, and that a rudder deflection result in a yaw, but
also in a roll. Aileron deflection also result in a yaw, but I guess not
as much as rudder deflection.

So we have tw
 inputs: aileron and rudder, and two outputs: roll and yaw.
Theese variables are not independent, the aileron does not only control
roll, and the rudder does not only control yaw. They both interact. Of course
the aileron controls roll much more tha
 it controls yaw and the rudder
controls yaw more than it controls roll. So our two PID controllers would
be: one with roll as input and aileron as output, and one with yaw as input
and rudder as output. This might seem obvious, but still one should
the theory behind and note that it also makes sense in theory.

When it comes to tuning the two PID controllers, one has to keep in mind
that they interact. In general more conservative controller settings must
be used, but I guess you'll fin
 more on multivariable controller tuning
in one of the books :-)

>What is the benefit of cascading PID controllers?

Lets look at an autopilot that is supposed to steer the aircraft towards
the heading bug. Suppose that the only thing we can man
pulate is the aileron.
Obviously we need a PID controller with aileron as its output. We choose
roll angle as input. Now we can set the reference to say 20 degrees, and
a properly tuned PID controller will hold a 20 degree roll angle.

But how do 
e get it to steer towards the heading but? One solution is for
the pilot to set the reference roll angle apropriately so that the aircraft
steers toward the heading bug. Another is to let another PID controller
set the reference roll angle. The outp
t of this controller is then connected
to the reference of the first controller. This is what we call a cascade
configuration. For this example the benefit is that, well without cascading
we wouldn't be able to do what we wanted (steer towards the h
ading bug).
For situations where cascading is not required, it can still be used and
might give a more robust or smoother control loop.

Roy Vegard Ovesen

Roy Vegard Ovesen

Flightgear-devel mailing list

Reply via email to