NOTE:
This manual may contain outdated information. For documentation of the most recent features
refer to
http://wiki.flightgear.org/index.php/Howto:_Design_an_autopilot
http://wiki.flightgear.org/index.php/Autopilot_Configuration_Reference

COMMON SETTINGS
==============================================================================

Currently four types of digital filter implementations are supported. They all serve an 
individual purpose or are individual implementations of a specific filter type.
Each filter implementation uses the same set of basic configuration tags and individual
configuration elements. These individual elements are described in the section of the
filter. 

The InputValue
==============================================================================
Each filter has several driving values, like the input value itself, sometimes a reference 
value, a gain value and others. Most of these input values can bei either a constant value
or the value of a property. They all use the same syntax and will be referred to as InputValue
in the remaining document.

The complete XML syntax for a InputValue is

<some-element>
  <condition>
    <!-- any condition as defined in README.conditions -->
  </condition>
  <property>/some/property/name</property>
  <value>0.0</value>
  <scale>1.0</value>
  <offset>0.0</offset>
  <max>infinity</max>
  <min>-infinity<min>
  <abs>false</abs>
  <period>
    <min>-180.0</min>
    <max>-180.0</max>
  </period>
</some-element>

The enclosing element <some-element> is the element defined in each filter, like <input>, <u_min>, 
<reference> etc. These elements will be described later.
The value of the input is calculated based on the given value, scale and offset as
value * scale + offset 
and the result is clipped to min/max, if given.
With the full set of given elements, the InputValue will initialize the named property to the value
given, reduced by the given offset and reverse scaled by the given scale.

Example:
<input>
  <property>/controls/flight/rudder</property>
  <value>0.0</value>
  <scale>0.5</scale>
  <offset>0.5</offset>
</input>

Will use the property /controls/flight/rudder as the input of the filter. The property will be initialized
at a value of zero and since the property usually is in the range [-1..+1], the the value of <input> will
be in the range (-1)*0.5+0.5 to (+1)*0.5+0.5 which is [0..1].

The default values for elements not given are:
<value/> : 0.0
<scale/> : 1.0
<offset/>: 0.0
<property/> : none
<min/>   : unclipped
<max/>   : unclipped
<abs/>   : false

Some examples:
<input>
  <property>/position/altitude-ft</property>
  <scale>0.3048</scale>
</input>
Gives the altitude in meters. No initialization of the property is performed, no offset applied.

<reference>
  <value>0.0</value>
</reference>
A constant reference of zero.

A abbreviated method of defining values exist for using a just constant or a property. The above
example may be written as
<reference>0.0</reference>
Or if the reference is defined in a property
<reference>/some/property/name</reference>
No initialization, scaling or offsetting is performed here.
The logic behind this is: If the text node in the element (the text between the opening and closing tag)
can be converted to a double value, it will be interpreted as a double value. Otherwise the text will
be interpreted as a property name.
Examples:
<reference>3.1415927</reference>             - The constant of PI (roughly)
<reference>/position/altitude-ft</reference> - The property /position/altitude-ft
<reference>3kings</reference>                - The constant 3. The word kings is ignored
<reference>food4less</reference>             - The property food4less

The <property> element may also be written as <prop> for backward compatibility.

There may be one or more InputValues for the same input of a filter which may be bound to conditions.
Each InputValue will have its condition checked in the order of InputValues given in the configuration
file. The first InputValue that returns true for its condition will be evaluated. Chaining a number
of InputValues with conditions and an unconditioned InputValue works like the C language equivalent
if( condition ) {
  // compute value of first element
} else if( condition2 ) {
  // compute value of second element
} else if( condition3 ) {
  // compute value of third element
} else {
  // compute value of last element
}

Example: Set the gain to 3.0 if /autopilot/locks/heading equals dg-heading-hold or 2.0 otherwise.
<digital-filter>
  <gain>
    <condition>
      <equals>
        <property>/autopilot/locks/heading</property>
        <value>dg-heading-hold</value>
      </equals>
    </condition>
    <value>3.0</value>
  <gain>
  <!-- Hint: omit a condition here as a fallthru else condition -->
  </gain>
    <value>2.0</value>
  <gain>
<digital-filter>

If the element <abs> is used and set to the value "true", only the absolute value of the input
(the positive part) is used for further computations. The abs function is applied after all 
other computations are completed.
    
OutputValue           
==============================================================================
Each filter drives one to many output properties. No scaling or offsetting is implemented
for the output value, these should be done in the filter itself.
The output properties are defined in the <output/> element by adding <property/> elements
within the <output/> element. For just a single output property, the <property/> element
may be ommited. For backward compatibility, <property/> may be replaced by <prop/>.
Nonexisting properties will be created with type double.

Example: (Multiple output properties)
<output>
  <property>/some/output/property</property>
  <property>/some/other/output/property</property>
  <property>/and/another/output/property</property>
</output>

Example: a single output property
<output>/just/a/single/property</output>

Other Common Settings
==============================================================================
<name>        String      The name of the filter. Used for debug purpose.
Example:
<name>pressure rate filter</name>

<debug>       Boolean     If true, this filter puts out debug information when updated.
Example: 
<debug>false</debug>

<input>       InputValue  The input property driving the filter. 
                          Refer to InputValue for details.

<reference>   InputValue  The reference property for filter that need one. 
                          Refer to InputValue for details.

<output>      Complex     Each filter can drive one to many output properties. 
                          Refer to OutputValue for details.

<u_min>       InputValue  This defines the optional minimum and maximum value the output
<u_max>                   is clamped to. If neither <u_min> nor <u_max> exists, the output
                          is only limited by the internal limit of double precision float computation.
                          If either <u_min> or <u_max> is given, clamping is activated. A missing
                          min or max value defaults to 0 (zero).
                          Note: <u_min> and <u_max> may also occour within a <config> element.
                          <min> and <max> may be used as a substitude for the corresponding u_xxx element.
<period>      Complex     Define a periodical input or output value. The phase width is defined by the 
                          child elements <min> and <max> which are of type InputValue

Example: Limit the pilot's body temperature to a constant minimum of 36 and a maximum defined in
         /pilots/max-body-temperature-degc, initialized to 40.0
<u_max>
  <prop>/pilots/max-body-temperature-degc</prop>
  <value>40.0</
</u_max>
<min>
  <value>36.0</value>
</min

Implicit definition of the minimum value of 0 (zero) and defining a maximum of 100.0
<config>
  <u_max>100.0</u_max>
</config>

This defines the input or output as a periodic value with a phase width of 360, like 
the compass rose.  Any value reaching the filter's input or leaving the filter at the 
output will be transformed to fit into the given range by adding or substracting one phase 
width of 360. Values of -270, 90 or 450 applied to this periodical element will allways 
result in +90. A value of 630, 270 or -90 will be normalized to -90 in the given example.
<period>
  <min>-180.0</min>
  <max>180.0</max>
</period>


<enable>      Complex     Define a condition to enable or disable the filter. For disabled
                          filters, no output computations are performed. Only enabled
                          filters fill the output properties. The default for undefined
                          conditions is enabled.
                          Several way exist to define a condition. The most simple case
                          is checking a boolean property. For this, just a <prop> element
                          naming this boolean property is needed. The boolean value of the 
                          named property defines the enabled state of the filter.
                          To compare the value of a property with a constant, a <prop> and
                          a <value> element define the property name and the value to be
                          compared. The filter is enabled, if the value of the property
                          equals the given value. A case sensitive string compare is 
                          performed here.
                          To define more complex conditions, a <condition> element may be
                          used to define any condition described in README.conditions.
                          If a <condition> element is present and if it contains a valid 
                          condition, this conditions has precedence over a given <prop>/
                          <value> condition.
                          The child element <honor-passive>, a boolean flag, may be present
                          within the <enable> element. If this element is true, the property
                          /autopilot/locks/passive-mode is checked and if it is true, the 
                          filter output is computed, but the output properties are not set.
                          The default for honor-passive is false
Example: Check a boolean property, only compute this filter if gear-down is true and 
         /autopilot/locks/passive-mode is false
<enable>
  <prop>/gear/gear-down</prop>
  <honor-passive>true</honor-passive>
</enable>

Check a property for equality, only compute this filter if the autopilot is locked in heading mode.
<enable>
  <prop>/autopilot/locks/heading</prop>
  <value>dg-heading-hold</value>
</enable>

Use a complex condition, only compute this filter if the autopilot is serviceable and the lock
is either dg-heading-hold or nav1-heading-hold
<enable>
  <condition>
    <property>/autopilo/serviceable</property>
    <or>
      <equals>
        <property>/autopilot/locks/heading</property>
        <value>dg-heading-hold</value>
      </equals>
      <equals>
        <property>/autopilot/locks/heading</property>
        <value>nav1-heading-hold</value>
      </equals>
    </or>
  </condition>
</enable>

INDIVIDUAL FILTER CONFIGURATION
==============================================================================

Digital Filter                          

Six different types of digital filter can be configured inside the autopilot
configuration file. There are four low-pass filter types and two gain filter
types.

The low-pass filter types are:

* Exponential
* Double exponential
* Moving average
* Noise spike filter

The gain filter types are:

* gain
* reciprocal

To add a digital filter, place a <filter> element under the root element. Next to 
the global configuration elements described above, the following elements configure
the digital filter:
<filter-time> InputValue  This tag is only applicable for the exponential and
                          double-exponential filter types. It controls the bandwidth 
                          of the filter. The bandwidth in Hz of the filter is: 
                          1/filter-time. So a low-pass filter with a bandwidth of 
                          10Hz would have a filter time of 1/10 = 0.1

<samples>     InputValue  This tag only makes sense for the moving-average filter. 
                          It says how many past samples to average.

<max-rate-of-change> 
              InputValue  This tag is applicable for the noise-spike filter. 
                          It says how much the value is allowed to change per second.

<gain>        InputValue  This is only applicable to the gain and reciprocal filter types. 
                          The output for gain filter is computed as input*gain while 
                          the reciprocal filter computes output as gain/input for input
                          values != 0 (zero). Gain may be a constant, a property name
                          defined by a <prop> element within the <gain> element or a 
                          property name initialized to a value by using a <prop> and 
                          <value> element.
          
Example: a pressure-rate-filter implemented as a double exponential low pass filter
         with a bandwith of 10Hz

  <filter>
    <name>pressure-rate-filter</name>
    <debug>false</debug>
    <type>double-exponential</type>
    <enable>
      <prop>/autopilot/locks/pressure-rate-filter</prop>
      <value>true</value>
    </enable>
    <input>/autopilot/internal/pressure-rate</input>
    <output>/autopilot/internal/filtered-pressure-rate</output>
    <filter-time>0.1</filter-time>
  </filter>

This will filter the pressure-rate property. The output will be to a new
property called filtered-pressure-rate. You can select any numerical property
from the property tree. The input property will not be affected by the filter,
it will stay the same as it would if no filter was configured.

Example 2:

  <filter>
    <name>airspeed elevator-trim gain reciprocal filter</name>
    <debug>false</debug>
    <enable>
      <prop>/autopilot/locks/airspeed-elevator-trim-gain</prop>
      <value>true</value>
    </enable>
    <type>reciprocal</type>
    <gain>
      <prop>/autopilot/settings/elevator-trim-airspeed-reciprocal-gain</prop>
      <value>7</value>
    </gain>
    <input>/velocities/airspeed-kt</input>
    <output>/autopilot/internal/elevator-trim-gain</output>
    <u_min>0.005</u_min>
    <u_max>0.02</u_max>
  </filter>

This will use the /velocities/airspeed-kt property to produce a gain factor
that reduces as airspeed increases.  At airspeeds up to 350kt the gain will
be clamped to 0.02, at 700kt the gain will be 0.01 and at 1400kt the gain will
be 0.005.  The gain will be clamped to 0.005 for airspeeds > 1400kt.

The output from this filter could then be used to control the gain in a PID
controller:

  <pid-controller>
    <name>Pitch hold</name>
    <debug>false</debug>
    <enable>
      <prop>/autopilot/locks/pitch</prop>
      <value>true</value>
    </enable>
    <input>
      <prop>/orientation/pitch-deg</prop>
    </input>
    <reference>
      <prop>/autopilot/settings/target-pitch-deg</prop>
    </reference>
    <output>
      <prop>/autopilot/internal/target-elevator-trim-norm</prop>
    </output>
    <config>
      <Ts>0.05</Ts>
      <Kp>
        <prop>/autopilot/internal/elevator-trim-gain</prop>
        <value>0.02</value>
      </Kp>
      <beta>1.0</beta>
      <alpha>0.1</alpha>
      <gamma>0.0</gamma>
      <Ti>2.0</Ti>
      <Td>0.2</Td>
      <u_min>-1.0</u_min>
      <u_max>1.0</u_max>
    </config>
  </pid-controller>

IMPORTANT NOTE: The <Kp> tag in PID controllers has been revised to operate in
the same way as the <gain> elements in filters.  However, the original format
of <Kp> will continue to function as before i.e. <Kp>0.02</Kp> will specify a
fixed and unalterable gain factor, but a warning message will be output.

The gain type filter is similar to the reciprocal filter except that the gain
is applied as a simple factor to the input.
-------------------------------------------------------------------------------
Parameters

<name> The name of the filter. Give it a sensible name!

<debug> If this tag is set to true debugging info will be printed on the
console.

<enable> Encloses the <prop> and <value> tags which are used to enable or
disable the filter. Instead of the <prop> and <value> tags, a <condition>
tag may be used to define a condition. Check README.conditions for more
details about conditions.  Defaults to enabled if unspecified.

<type> The type of filter. This can be exponential, double-exponential,
moving-average, noise-spike, gain or reciprocal.

<input> The input property to be filtered. This should of course be a
numerical property, filtering a text string or a boolean value does not make
sense.

<output> The filtered value. You can make up any new property.

<u_min> The minimum output value from the filter.  Defaults to -infinity.

<u_max> The maximum output value from the filter.  Defaults to +infinity.

These are the tags that are applicable to all filter types. The following tags
are filter specific.

<filter-time> This tag is only applicable for the exponential and
double-exponential filter types. It controls the bandwidth of the filter. The
bandwidth in Hz of the filter is: 1/filter-time. So a low-pass filter with a
bandwidth of 10Hz would have a filter time of 1/10 = 0.1

<samples> This tag only makes sense for the moving-average filter. It says how
many past samples to average.

<max-rate-of-change> This tag is applicable for the noise-spike filter. Is
says how much the value is allowed to change per second.

<gain>  This, and it's enclosed <prop> and <value> tags, are only applicable to
the gain and reciprocal filter types.  The <prop> tag specifies a property node
to hold the gain value and the <value> tag specifies an initial default value.
The gain defaults to 1.0 if unspecified.

The output from the gain filter type is: input * gain.
The output from the reciprocal filter type is: gain / input.

The gain can be changed during run-time by updating the value in the property
node.

<startup-current> If true, internal state is initialised with the current error
value, which can reduce startup oscillations. Otherwise if false (the default),
internal state is initialised to zero.