2 This manual may contain outdated information. For documentation of the most recent features
4 http://wiki.flightgear.org/index.php/Howto:_Design_an_autopilot
5 http://wiki.flightgear.org/index.php/Autopilot_Configuration_Reference
8 ==============================================================================
10 Currently four types of digital filter implementations are supported. They all serve an
11 individual purpose or are individual implementations of a specific filter type.
12 Each filter implementation uses the same set of basic configuration tags and individual
13 configuration elements. These individual elements are described in the section of the
17 ==============================================================================
18 Each filter has several driving values, like the input value itself, sometimes a reference
19 value, a gain value and others. Most of these input values can bei either a constant value
20 or the value of a property. They all use the same syntax and will be referred to as InputValue
21 in the remaining document.
23 The complete XML syntax for a InputValue is
27 <!-- any condition as defined in README.conditions -->
29 <property>/some/property/name</property>
42 The enclosing element <some-element> is the element defined in each filter, like <input>, <u_min>,
43 <reference> etc. These elements will be described later.
44 The value of the input is calculated based on the given value, scale and offset as
45 value * scale + offset
46 and the result is clipped to min/max, if given.
47 With the full set of given elements, the InputValue will initialize the named property to the value
48 given, reduced by the given offset and reverse scaled by the given scale.
52 <property>/controls/flight/rudder</property>
58 Will use the property /controls/flight/rudder as the input of the filter. The property will be initialized
59 at a value of zero and since the property usually is in the range [-1..+1], the the value of <input> will
60 be in the range (-1)*0.5+0.5 to (+1)*0.5+0.5 which is [0..1].
62 The default values for elements not given are:
73 <property>/position/altitude-ft</property>
76 Gives the altitude in meters. No initialization of the property is performed, no offset applied.
81 A constant reference of zero.
83 A abbreviated method of defining values exist for using a just constant or a property. The above
84 example may be written as
85 <reference>0.0</reference>
86 Or if the reference is defined in a property
87 <reference>/some/property/name</reference>
88 No initialization, scaling or offsetting is performed here.
89 The logic behind this is: If the text node in the element (the text between the opening and closing tag)
90 can be converted to a double value, it will be interpreted as a double value. Otherwise the text will
91 be interpreted as a property name.
93 <reference>3.1415927</reference> - The constant of PI (roughly)
94 <reference>/position/altitude-ft</reference> - The property /position/altitude-ft
95 <reference>3kings</reference> - The constant 3. The word kings is ignored
96 <reference>food4less</reference> - The property food4less
98 The <property> element may also be written as <prop> for backward compatibility.
100 There may be one or more InputValues for the same input of a filter which may be bound to conditions.
101 Each InputValue will have its condition checked in the order of InputValues given in the configuration
102 file. The first InputValue that returns true for its condition will be evaluated. Chaining a number
103 of InputValues with conditions and an unconditioned InputValue works like the C language equivalent
105 // compute value of first element
106 } else if( condition2 ) {
107 // compute value of second element
108 } else if( condition3 ) {
109 // compute value of third element
111 // compute value of last element
114 Example: Set the gain to 3.0 if /autopilot/locks/heading equals dg-heading-hold or 2.0 otherwise.
119 <property>/autopilot/locks/heading</property>
120 <value>dg-heading-hold</value>
125 <!-- Hint: omit a condition here as a fallthru else condition -->
131 If the element <abs> is used and set to the value "true", only the absolute value of the input
132 (the positive part) is used for further computations. The abs function is applied after all
133 other computations are completed.
136 ==============================================================================
137 Each filter drives one to many output properties. No scaling or offsetting is implemented
138 for the output value, these should be done in the filter itself.
139 The output properties are defined in the <output/> element by adding <property/> elements
140 within the <output/> element. For just a single output property, the <property/> element
141 may be ommited. For backward compatibility, <property/> may be replaced by <prop/>.
142 Nonexisting properties will be created with type double.
144 Example: (Multiple output properties)
146 <property>/some/output/property</property>
147 <property>/some/other/output/property</property>
148 <property>/and/another/output/property</property>
151 Example: a single output property
152 <output>/just/a/single/property</output>
154 Other Common Settings
155 ==============================================================================
156 <name> String The name of the filter. Used for debug purpose.
158 <name>pressure rate filter</name>
160 <debug> Boolean If true, this filter puts out debug information when updated.
164 <input> InputValue The input property driving the filter.
165 Refer to InputValue for details.
167 <reference> InputValue The reference property for filter that need one.
168 Refer to InputValue for details.
170 <output> Complex Each filter can drive one to many output properties.
171 Refer to OutputValue for details.
173 <u_min> InputValue This defines the optional minimum and maximum value the output
174 <u_max> is clamped to. If neither <u_min> nor <u_max> exists, the output
175 is only limited by the internal limit of double precision float computation.
176 If either <u_min> or <u_max> is given, clamping is activated. A missing
177 min or max value defaults to 0 (zero).
178 Note: <u_min> and <u_max> may also occour within a <config> element.
179 <min> and <max> may be used as a substitude for the corresponding u_xxx element.
180 <period> Complex Define a periodical input or output value. The phase width is defined by the
181 child elements <min> and <max> which are of type InputValue
183 Example: Limit the pilot's body temperature to a constant minimum of 36 and a maximum defined in
184 /pilots/max-body-temperature-degc, initialized to 40.0
186 <prop>/pilots/max-body-temperature-degc</prop>
193 Implicit definition of the minimum value of 0 (zero) and defining a maximum of 100.0
198 This defines the input or output as a periodic value with a phase width of 360, like
199 the compass rose. Any value reaching the filter's input or leaving the filter at the
200 output will be transformed to fit into the given range by adding or substracting one phase
201 width of 360. Values of -270, 90 or 450 applied to this periodical element will allways
202 result in +90. A value of 630, 270 or -90 will be normalized to -90 in the given example.
209 <enable> Complex Define a condition to enable or disable the filter. For disabled
210 filters, no output computations are performed. Only enabled
211 filters fill the output properties. The default for undefined
212 conditions is enabled.
213 Several way exist to define a condition. The most simple case
214 is checking a boolean property. For this, just a <prop> element
215 naming this boolean property is needed. The boolean value of the
216 named property defines the enabled state of the filter.
217 To compare the value of a property with a constant, a <prop> and
218 a <value> element define the property name and the value to be
219 compared. The filter is enabled, if the value of the property
220 equals the given value. A case sensitive string compare is
222 To define more complex conditions, a <condition> element may be
223 used to define any condition described in README.conditions.
224 If a <condition> element is present and if it contains a valid
225 condition, this conditions has precedence over a given <prop>/
227 The child element <honor-passive>, a boolean flag, may be present
228 within the <enable> element. If this element is true, the property
229 /autopilot/locks/passive-mode is checked and if it is true, the
230 filter output is computed, but the output properties are not set.
231 The default for honor-passive is false
232 Example: Check a boolean property, only compute this filter if gear-down is true and
233 /autopilot/locks/passive-mode is false
235 <prop>/gear/gear-down</prop>
236 <honor-passive>true</honor-passive>
239 Check a property for equality, only compute this filter if the autopilot is locked in heading mode.
241 <prop>/autopilot/locks/heading</prop>
242 <value>dg-heading-hold</value>
245 Use a complex condition, only compute this filter if the autopilot is serviceable and the lock
246 is either dg-heading-hold or nav1-heading-hold
249 <property>/autopilo/serviceable</property>
252 <property>/autopilot/locks/heading</property>
253 <value>dg-heading-hold</value>
256 <property>/autopilot/locks/heading</property>
257 <value>nav1-heading-hold</value>
263 INDIVIDUAL FILTER CONFIGURATION
264 ==============================================================================
268 Six different types of digital filter can be configured inside the autopilot
269 configuration file. There are four low-pass filter types and two gain filter
272 The low-pass filter types are:
279 The gain filter types are:
284 To add a digital filter, place a <filter> element under the root element. Next to
285 the global configuration elements described above, the following elements configure
287 <filter-time> InputValue This tag is only applicable for the exponential and
288 double-exponential filter types. It controls the bandwidth
289 of the filter. The bandwidth in Hz of the filter is:
290 1/filter-time. So a low-pass filter with a bandwidth of
291 10Hz would have a filter time of 1/10 = 0.1
293 <samples> InputValue This tag only makes sense for the moving-average filter.
294 It says how many past samples to average.
297 InputValue This tag is applicable for the noise-spike filter.
298 It says how much the value is allowed to change per second.
300 <gain> InputValue This is only applicable to the gain and reciprocal filter types.
301 The output for gain filter is computed as input*gain while
302 the reciprocal filter computes output as gain/input for input
303 values != 0 (zero). Gain may be a constant, a property name
304 defined by a <prop> element within the <gain> element or a
305 property name initialized to a value by using a <prop> and
308 Example: a pressure-rate-filter implemented as a double exponential low pass filter
309 with a bandwith of 10Hz
312 <name>pressure-rate-filter</name>
314 <type>double-exponential</type>
316 <prop>/autopilot/locks/pressure-rate-filter</prop>
319 <input>/autopilot/internal/pressure-rate</input>
320 <output>/autopilot/internal/filtered-pressure-rate</output>
321 <filter-time>0.1</filter-time>
324 This will filter the pressure-rate property. The output will be to a new
325 property called filtered-pressure-rate. You can select any numerical property
326 from the property tree. The input property will not be affected by the filter,
327 it will stay the same as it would if no filter was configured.
332 <name>airspeed elevator-trim gain reciprocal filter</name>
335 <prop>/autopilot/locks/airspeed-elevator-trim-gain</prop>
338 <type>reciprocal</type>
340 <prop>/autopilot/settings/elevator-trim-airspeed-reciprocal-gain</prop>
343 <input>/velocities/airspeed-kt</input>
344 <output>/autopilot/internal/elevator-trim-gain</output>
349 This will use the /velocities/airspeed-kt property to produce a gain factor
350 that reduces as airspeed increases. At airspeeds up to 350kt the gain will
351 be clamped to 0.02, at 700kt the gain will be 0.01 and at 1400kt the gain will
352 be 0.005. The gain will be clamped to 0.005 for airspeeds > 1400kt.
354 The output from this filter could then be used to control the gain in a PID
358 <name>Pitch hold</name>
361 <prop>/autopilot/locks/pitch</prop>
365 <prop>/orientation/pitch-deg</prop>
368 <prop>/autopilot/settings/target-pitch-deg</prop>
371 <prop>/autopilot/internal/target-elevator-trim-norm</prop>
376 <prop>/autopilot/internal/elevator-trim-gain</prop>
389 IMPORTANT NOTE: The <Kp> tag in PID controllers has been revised to operate in
390 the same way as the <gain> elements in filters. However, the original format
391 of <Kp> will continue to function as before i.e. <Kp>0.02</Kp> will specify a
392 fixed and unalterable gain factor, but a warning message will be output.
394 The gain type filter is similar to the reciprocal filter except that the gain
395 is applied as a simple factor to the input.
396 -------------------------------------------------------------------------------
399 <name> The name of the filter. Give it a sensible name!
401 <debug> If this tag is set to true debugging info will be printed on the
404 <enable> Encloses the <prop> and <value> tags which are used to enable or
405 disable the filter. Instead of the <prop> and <value> tags, a <condition>
406 tag may be used to define a condition. Check README.conditions for more
407 details about conditions. Defaults to enabled if unspecified.
409 <type> The type of filter. This can be exponential, double-exponential,
410 moving-average, noise-spike, gain or reciprocal.
412 <input> The input property to be filtered. This should of course be a
413 numerical property, filtering a text string or a boolean value does not make
416 <output> The filtered value. You can make up any new property.
418 <u_min> The minimum output value from the filter. Defaults to -infinity.
420 <u_max> The maximum output value from the filter. Defaults to +infinity.
422 These are the tags that are applicable to all filter types. The following tags
425 <filter-time> This tag is only applicable for the exponential and
426 double-exponential filter types. It controls the bandwidth of the filter. The
427 bandwidth in Hz of the filter is: 1/filter-time. So a low-pass filter with a
428 bandwidth of 10Hz would have a filter time of 1/10 = 0.1
430 <samples> This tag only makes sense for the moving-average filter. It says how
431 many past samples to average.
433 <max-rate-of-change> This tag is applicable for the noise-spike filter. Is
434 says how much the value is allowed to change per second.
436 <gain> This, and it's enclosed <prop> and <value> tags, are only applicable to
437 the gain and reciprocal filter types. The <prop> tag specifies a property node
438 to hold the gain value and the <value> tag specifies an initial default value.
439 The gain defaults to 1.0 if unspecified.
441 The output from the gain filter type is: input * gain.
442 The output from the reciprocal filter type is: gain / input.
444 The gain can be changed during run-time by updating the value in the property