2 ==============================================================================
4 Currently four types of digital filter implementations are supported. They all serve an
5 individual purpose or are individual implementations of a specific filter type.
6 Each filter implementation uses the same set of basic configuration tags and individual
7 configuration elements. These individual elements are described in the section of the
11 ==============================================================================
12 Each filter has several driving values, like the input value itself, sometimes a reference
13 value, a gain value and others. Most of these input values can bei either a constant value
14 or the value of a property. They all use the same syntax and will be referred to as InputValue
15 in the remaining document.
17 The complete XML syntax for a InputValue is
21 <!-- any condition as defined in README.conditions -->
23 <property>/some/property/name</property>
36 The enclosing element <some-element> is the element defined in each filter, like <input>, <u_min>,
37 <reference> etc. These elements will be described later.
38 The value of the input is calculated based on the given value, scale and offset as
39 value * scale + offset
40 and the result is clipped to min/max, if given.
41 With the full set of given elements, the InputValue will initialize the named property to the value
42 given, reduced by the given offset and reverse scaled by the given scale.
46 <property>/controls/flight/rudder</property>
52 Will use the property /controls/flight/rudder as the input of the filter. The property will be initialized
53 at a value of zero and since the property usually is in the range [-1..+1], the the value of <input> will
54 be in the range (-1)*0.5+0.5 to (+1)*0.5+0.5 which is [0..1].
56 The default values for elements not given are:
67 <property>/position/altitude-ft</property>
70 Gives the altitude in meters. No initialization of the property is performed, no offset applied.
75 A constant reference of zero.
77 A abbreviated method of defining values exist for using a just constant or a property. The above
78 example may be written as
79 <reference>0.0</reference>
80 Or if the reference is defined in a property
81 <reference>/some/property/name</reference>
82 No initialization, scaling or offsetting is performed here.
83 The logic behind this is: If the text node in the element (the text between the opening and closing tag)
84 can be converted to a double value, it will be interpreted as a double value. Otherwise the text will
85 be interpreted as a property name.
87 <reference>3.1415927</reference> - The constant of PI (roughly)
88 <reference>/position/altitude-ft</reference> - The property /position/altitude-ft
89 <reference>3kings</reference> - The constant 3. The word kings is ignored
90 <reference>food4less</reference> - The property food4less
92 The <property> element may also be written as <prop> for backward compatibility.
94 There may be one or more InputValues for the same input of a filter which may be bound to conditions.
95 Each InputValue will have its condition checked in the order of InputValues given in the configuration
96 file. The first InputValue that returns true for its condition will be evaluated. Chaining a number
97 of InputValues with conditions and an unconditioned InputValue works like the C language equivalent
99 // compute value of first element
100 } else if( condition2 ) {
101 // compute value of second element
102 } else if( condition3 ) {
103 // compute value of third element
105 // compute value of last element
108 Example: Set the gain to 3.0 if /autopilot/locks/heading equals dg-heading-hold or 2.0 otherwise.
113 <property>/autopilot/locks/heading</property>
114 <value>dg-heading-hold</value>
119 <!-- Hint: omit a condition here as a fallthru else condition -->
125 If the element <abs> is used and set to the value "true", only the absolute value of the input
126 (the positive part) is used for further computations. The abs function is applied after all
127 other computations are completed.
130 ==============================================================================
131 Each filter drives one to many output properties. No scaling or offsetting is implemented
132 for the output value, these should be done in the filter itself.
133 The output properties are defined in the <output/> element by adding <property/> elements
134 within the <output/> element. For just a single output property, the <property/> element
135 may be ommited. For backward compatibility, <property/> may be replaced by <prop/>.
136 Nonexisting properties will be created with type double.
138 Example: (Multiple output properties)
140 <property>/some/output/property</property>
141 <property>/some/other/output/property</property>
142 <property>/and/another/output/property</property>
145 Example: a single output property
146 <output>/just/a/single/property</output>
148 Other Common Settings
149 ==============================================================================
150 <name> String The name of the filter. Used for debug purpose.
152 <name>pressure rate filter</name>
154 <debug> Boolean If true, this filter puts out debug information when updated.
158 <input> InputValue The input property driving the filter.
159 Refer to InputValue for details.
161 <reference> InputValue The reference property for filter that need one.
162 Refer to InputValue for details.
164 <output> Complex Each filter can drive one to many output properties.
165 Refer to OutputValue for details.
167 <u_min> InputValue This defines the optional minimum and maximum value the output
168 <u_max> is clamped to. If neither <u_min> nor <u_max> exists, the output
169 is only limited by the internal limit of double precision float computation.
170 If either <u_min> or <u_max> is given, clamping is activated. A missing
171 min or max value defaults to 0 (zero).
172 Note: <u_min> and <u_max> may also occour within a <config> element.
173 <min> and <max> may be used as a substitude for the corresponding u_xxx element.
174 <period> Complex Define a periodical input or output value. The phase width is defined by the
175 child elements <min> and <max> which are of type InputValue
177 Example: Limit the pilot's body temperature to a constant minimum of 36 and a maximum defined in
178 /pilots/max-body-temperature-degc, initialized to 40.0
180 <prop>/pilots/max-body-temperature-degc</prop>
187 Implicit definition of the minimum value of 0 (zero) and defining a maximum of 100.0
192 This defines the input or output as a periodic value with a phase width of 360, like
193 the compass rose. Any value reaching the filter's input or leaving the filter at the
194 output will be transformed to fit into the given range by adding or substracting one phase
195 width of 360. Values of -270, 90 or 450 applied to this periodical element will allways
196 result in +90. A value of 630, 270 or -90 will be normalized to -90 in the given example.
203 <enable> Complex Define a condition to enable or disable the filter. For disabled
204 filters, no output computations are performed. Only enabled
205 filters fill the output properties. The default for undefined
206 conditions is enabled.
207 Several way exist to define a condition. The most simple case
208 is checking a boolean property. For this, just a <prop> element
209 naming this boolean property is needed. The boolean value of the
210 named property defines the enabled state of the filter.
211 To compare the value of a property with a constant, a <prop> and
212 a <value> element define the property name and the value to be
213 compared. The filter is enabled, if the value of the property
214 equals the given value. A case sensitive string compare is
216 To define more complex conditions, a <condition> element may be
217 used to define any condition described in README.conditions.
218 If a <condition> element is present and if it contains a valid
219 condition, this conditions has precedence over a given <prop>/
221 The child element <honor-passive>, a boolean flag, may be present
222 within the <enable> element. If this element is true, the property
223 /autopilot/locks/passive-mode is checked and if it is true, the
224 filter output is computed, but the output properties are not set.
225 The default for honor-passive is false
226 Example: Check a boolean property, only compute this filter if gear-down is true and
227 /autopilot/locks/passive-mode is false
229 <prop>/gear/gear-down</prop>
230 <honor-passive>true</honor-passive>
233 Check a property for equality, only compute this filter if the autopilot is locked in heading mode.
235 <prop>/autopilot/locks/heading</prop>
236 <value>dg-heading-hold</value>
239 Use a complex condition, only compute this filter if the autopilot is serviceable and the lock
240 is either dg-heading-hold or nav1-heading-hold
243 <property>/autopilo/serviceable</property>
246 <property>/autopilot/locks/heading</property>
247 <value>dg-heading-hold</value>
250 <property>/autopilot/locks/heading</property>
251 <value>nav1-heading-hold</value>
257 INDIVIDUAL FILTER CONFIGURATION
258 ==============================================================================
262 Six different types of digital filter can be configured inside the autopilot
263 configuration file. There are four low-pass filter types and two gain filter
266 The low-pass filter types are:
273 The gain filter types are:
278 To add a digital filter, place a <filter> element under the root element. Next to
279 the global configuration elements described above, the following elements configure
281 <filter-time> InputValue This tag is only applicable for the exponential and
282 double-exponential filter types. It controls the bandwidth
283 of the filter. The bandwidth in Hz of the filter is:
284 1/filter-time. So a low-pass filter with a bandwidth of
285 10Hz would have a filter time of 1/10 = 0.1
287 <samples> InputValue This tag only makes sense for the moving-average filter.
288 It says how many past samples to average.
291 InputValue This tag is applicable for the noise-spike filter.
292 It says how much the value is allowed to change per second.
294 <gain> InputValue This is only applicable to the gain and reciprocal filter types.
295 The output for gain filter is computed as input*gain while
296 the reciprocal filter computes output as gain/input for input
297 values != 0 (zero). Gain may be a constant, a property name
298 defined by a <prop> element within the <gain> element or a
299 property name initialized to a value by using a <prop> and
302 Example: a pressure-rate-filter implemented as a double exponential low pass filter
303 with a bandwith of 10Hz
306 <name>pressure-rate-filter</name>
308 <type>double-exponential</type>
310 <prop>/autopilot/locks/pressure-rate-filter</prop>
313 <input>/autopilot/internal/pressure-rate</input>
314 <output>/autopilot/internal/filtered-pressure-rate</output>
315 <filter-time>0.1</filter-time>
318 This will filter the pressure-rate property. The output will be to a new
319 property called filtered-pressure-rate. You can select any numerical property
320 from the property tree. The input property will not be affected by the filter,
321 it will stay the same as it would if no filter was configured.
326 <name>airspeed elevator-trim gain reciprocal filter</name>
329 <prop>/autopilot/locks/airspeed-elevator-trim-gain</prop>
332 <type>reciprocal</type>
334 <prop>/autopilot/settings/elevator-trim-airspeed-reciprocal-gain</prop>
337 <input>/velocities/airspeed-kt</input>
338 <output>/autopilot/internal/elevator-trim-gain</output>
343 This will use the /velocities/airspeed-kt property to produce a gain factor
344 that reduces as airspeed increases. At airspeeds up to 350kt the gain will
345 be clamped to 0.02, at 700kt the gain will be 0.01 and at 1400kt the gain will
346 be 0.005. The gain will be clamped to 0.005 for airspeeds > 1400kt.
348 The output from this filter could then be used to control the gain in a PID
352 <name>Pitch hold</name>
355 <prop>/autopilot/locks/pitch</prop>
359 <prop>/orientation/pitch-deg</prop>
362 <prop>/autopilot/settings/target-pitch-deg</prop>
365 <prop>/autopilot/internal/target-elevator-trim-norm</prop>
370 <prop>/autopilot/internal/elevator-trim-gain</prop>
383 IMPORTANT NOTE: The <Kp> tag in PID controllers has been revised to operate in
384 the same way as the <gain> elements in filters. However, the original format
385 of <Kp> will continue to function as before i.e. <Kp>0.02</Kp> will specify a
386 fixed and unalterable gain factor, but a warning message will be output.
388 The gain type filter is similar to the reciprocal filter except that the gain
389 is applied as a simple factor to the input.
390 -------------------------------------------------------------------------------
393 <name> The name of the filter. Give it a sensible name!
395 <debug> If this tag is set to true debugging info will be printed on the
398 <enable> Encloses the <prop> and <value> tags which are used to enable or
399 disable the filter. Instead of the <prop> and <value> tags, a <condition>
400 tag may be used to define a condition. Check README.conditions for more
401 details about conditions. Defaults to enabled if unspecified.
403 <type> The type of filter. This can be exponential, double-exponential,
404 moving-average, noise-spike, gain or reciprocal.
406 <input> The input property to be filtered. This should of course be a
407 numerical property, filtering a text string or a boolean value does not make
410 <output> The filtered value. You can make up any new property.
412 <u_min> The minimum output value from the filter. Defaults to -infinity.
414 <u_max> The maximum output value from the filter. Defaults to +infinity.
416 These are the tags that are applicable to all filter types. The following tags
419 <filter-time> This tag is only applicable for the exponential and
420 double-exponential filter types. It controls the bandwidth of the filter. The
421 bandwidth in Hz of the filter is: 1/filter-time. So a low-pass filter with a
422 bandwidth of 10Hz would have a filter time of 1/10 = 0.1
424 <samples> This tag only makes sense for the moving-average filter. It says how
425 many past samples to average.
427 <max-rate-of-change> This tag is applicable for the noise-spike filter. Is
428 says how much the value is allowed to change per second.
430 <gain> This, and it's enclosed <prop> and <value> tags, are only applicable to
431 the gain and reciprocal filter types. The <prop> tag specifies a property node
432 to hold the gain value and the <value> tag specifies an initial default value.
433 The gain defaults to 1.0 if unspecified.
435 The output from the gain filter type is: input * gain.
436 The output from the reciprocal filter type is: gain / input.
438 The gain can be changed during run-time by updating the value in the property