Hi,
I have just commited a new feature for the XMLAutopilot.
Each filter supports periodical values at it's inputs and the output.
This is best described by an example:
Image you want to compute your heading offset as a delta between your magnetic
heading and your autpilot's target heading with the following simple-pid-
controller:
<pi-simple-controller>
<name>AP:Heading Offset Computer</name>
<debug>false</debug>
<input>
<property>orientation/heading-magnetic-deg</property>
</input>
<reference>
<property>autopilot/settings/target-heading-deg</property>
</reference>
<output>autopilot/internal/heading-offset-deg</output>
<config> <!-- just build the delta, no integration -->
<Kp>1.0</Kp>
<Ki>0.0</Ki>
</config>
<period> <!-- this is funky new stuff -->
<min>-180</min>
<max>180</max>
</period>
</pi-simple-controller>
which computes
output = reference - input
or
heading-offset-deg = target-heading-deg - heading-magnetic-deg
with possible results of -359..+359 which is more or less useless.
For the following stages of the autopilot it would be best to have the output
in the range of -180..+180, telling you to steer left or right (on the
shortest path) to your new heading.
This can now be achieved by using the <period> tag. It tells the digital
filter to shift either the input or output value into the given period by
adding/substracting multiples of one period (360 in the given example).
The above examples writes 10 to heading-offset-deg if target-heading-deg is
zero and heading-magnetic-deg is 350 (-350 without the <period> element).
You may choose to normalize the value at the input, so all internal
computations use the normalized value or process the unaltered input value and
than normalize the output value (like in the given example).
This should make several Nasal based helper functions for autopilots obsolete.
I tried my best to document everything in README.digitalfilters.
BTW: the <min>/<max> elements are complex types like any other input element.
You may use properties, scaling, offsetting and conditions if you really want
something complicated like:
<period>
<min>
<condition>
<equals>
<property>/settings/angular-unit</property>
<value>radians</value>
<equals>
</condition>
<property>/constants/a-property-containing-the-value-of-pi</property>
<condition>
<equals>
<property>/settings/angular-unit</property>
<value>degrees</value>
<equals>
</condition>
<value>-90</value>
<factor>2.0</factor>
</min>
<max>
[..]skipped due to laziness[..]
</max>
No intentional changes for existing autopilot implementations come with this
patch, the <period> tag is optional.
Torsten
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