Re: [Flightgear-devel] Autopilot Tuning

[Torsten D. <To...@t3...>]

> On Monday 25 Jan 2010, Pete Morgan wrote:
> > I'm having a successful time tuning my first autopilot on the
> > 787.
> >
> > I've managed to get rid of the "jerks" and wobbles, mainly by
> > using a "noise-spike" filter with <max-rate-of-change>. That
> > thanks to Syd Adams for that wonderful tip.
> >
> > All flight level changes, and NAV, heading can be tuned very
> > well, at the current tuning speed of 250 knots.
> >
> > However, at lower and higher speeds, the behaviour changes,
> > sometimes significantly.
> >
> > So it it possible to have two different PID/configs's operating
> > at different airspeeds..
> >
> > eg in psuedo code
> >
> > <config>
> >    {if getprop("airspeed") < 150 }
> >     <Kp>0.07</Kp><beta>0.5</beta>
> >    {else}
> >     <Kp>0.06</Kp><beta>0.3</beta>
> >     {if}
> > </config>
> >
> > Hope it makes sense..
> > pete
> 
> You can vary the proportional gain of the pid controllers by
> specifying a property and default value instead of just sticking a
> fixed value between the <Kp> tags e.g.
> 
> <Kp>
>   <prop>/autopilot/FCS/settings/example-gain</prop>
>   <value>-0.2</value>
> </Kp>
If you run the CVS version (or wait a couple of days for the new release), you 
can make use of even more flexible input parameters:
- specify a constant for a constant value
- specify a property name to read the value from a property
- add a <scale> and/or <offset> for linear modifications of the value
- add a <condition> to enable this value and choose from more than one input

Trivial example, constant
<Kp>0.2</Kp>

Trivial example, property
<Kp>/some/property</Kp>

More complex: 
if( /foo == 123 )
  Kp = /some/property * /some/other/property + 3
else
  Kp = 4711

<!-- case f00 == 123 -->
<Kp>
  <condition>
    <equals>
      <property/foo</property>
     <value>123</value>
    </equals>
  </condition>
  <property>/some/property</property>
  <scale>/some/other/property</scale>
  <offset>3.0</offset>
</Kp>
<!-- default, or else branch -->
<Kp>4711</Kp>

Recursion is supported, so something like this works:
<Kp>
  <property>/some/property</property>
  <offset>
    <property>/some/offset/property</property>
    <scale>
      <condition>
        some funky condition here
      </condition>
      <value>23.0</value>
      <condition>
         some other condition
      </condition>
      I'm loosing my track here, but you might see the idea behind.

you can model polynoms like a0+a1*x+a2*x^2(+...) that way.

More examples is in Docs/README.digitalfilters or the HansaJet autopilot (work 
in progress).

HTH
  Torsten