151 lines
5 KiB
Text
151 lines
5 KiB
Text
Six different types of digital filter can be configured inside the autopilot
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configuration file. There are four low-pass filter types and two gain filter
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types.
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The low-pass filter types are:
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* Exponential
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* Double exponential
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* Moving average
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* Noise spike filter
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The gain filter types are:
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* gain
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* reciprocal
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Example 1:
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<filter>
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<name>pressure-rate-filter</name>
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<debug>false</debug>
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<type>double-exponential</type>
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<enable>
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<prop>/autopilot/locks/pressure-rate-filter</prop>
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<value>true</value>
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</enable>
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<input>/autopilot/internal/pressure-rate</input>
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<output>/autopilot/internal/filtered-pressure-rate</output>
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<filter-time>0.1</filter-time>
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</filter>
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This will filter the pressure-rate property. The output will be to a new
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property called filtered-pressure-rate. You can select any numerical property
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from the property tree. The input property will not be affected by the filter,
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it will stay the same as it would if no filter was configured.
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Example 2:
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<filter>
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<name>airspeed elevator-trim gain reciprocal filter</name>
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<debug>false</debug>
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<enable>
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<prop>/autopilot/locks/airspeed-elevator-trim-gain</prop>
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<value>true</value>
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</enable>
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<type>reciprocal</type>
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<gain>
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<prop>/autopilot/settings/elevator-trim-airspeed-reciprocal-gain</prop>
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<value>7</value>
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</gain>
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<input>/velocities/airspeed-kt</input>
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<output>/autopilot/internal/elevator-trim-gain</output>
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<u_min>0.005</u_min>
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<u_max>0.02</u_max>
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</filter>
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This will use the /velocities/airspeed-kt property to produce a gain factor
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that reduces as airspeed increases. At airspeeds up to 350kt the gain will
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be clamped to 0.02, at 700kt the gain will be 0.01 and at 1400kt the gain will
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be 0.005. The gain will be clamped to 0.005 for airspeeds > 1400kt.
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The output from this filter could then be used to control the gain in a PID
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controller:
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<pid-controller>
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<name>Pitch hold</name>
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<debug>false</debug>
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<enable>
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<prop>/autopilot/locks/pitch</prop>
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<value>true</value>
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</enable>
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<input>
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<prop>/orientation/pitch-deg</prop>
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</input>
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<reference>
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<prop>/autopilot/settings/target-pitch-deg</prop>
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</reference>
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<output>
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<prop>/autopilot/internal/target-elevator-trim-norm</prop>
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</output>
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<config>
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<Ts>0.05</Ts>
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<Kp>
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<prop>/autopilot/internal/elevator-trim-gain</prop>
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<value>0.02</value>
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</Kp>
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<beta>1.0</beta>
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<alpha>0.1</alpha>
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<gamma>0.0</gamma>
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<Ti>2.0</Ti>
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<Td>0.2</Td>
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<u_min>-1.0</u_min>
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<u_max>1.0</u_max>
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</config>
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</pid-controller>
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IMPORTANT NOTE: The <Kp> tag in PID controllers has been revised to operate in
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the same way as the <gain> elements in filters. However, the original format
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of <Kp> will continue to function as before i.e. <Kp>0.02</Kp> will specify a
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fixed and unalterable gain factor, but a warning message will be output.
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The gain type filter is similar to the reciprocal filter except that the gain
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is applied as a simple factor to the input.
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-------------------------------------------------------------------------------
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Parameters
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<name> The name of the filter. Give it a sensible name!
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<debug> If this tag is set to true debugging info will be printed on the
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console.
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<enable> Encloses the <prop> and <value> tags which are used to enable or
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disable the filter. Defaults to enabled if unspecified.
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<type> The type of filter. This can be exponential, double-exponential,
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moving-average, noise-spike, gain or reciprocal.
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<input> The input property to be filtered. This should of course be a
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numerical property, filtering a text string or a boolean value does not make
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sense.
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<output> The filtered value. You can make up any new property.
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<u_min> The minimum output value from the filter. Defaults to -infinity.
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<u_max> The maximum output value from the filter. Defaults to +infinity.
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These are the tags that are applicable to all filter types. The following tags
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are filter specific.
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<filter-time> This tag is only applicable for the exponential and
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double-exponential filter types. It controls the bandwidth of the filter. The
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bandwidth in Hz of the filter is: 1/filter-time. So a low-pass filter with a
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bandwidth of 10Hz would have a filter time of 1/10 = 0.1
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<samples> This tag only makes sense for the moving-average filter. It says how
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many past samples to average.
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<max-rate-of-change> This tag is applicable for the noise-spike filter. Is
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says how much the value is allowed to change per second.
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<gain> This, and it's enclosed <prop> and <value> tags, are only applicable to
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the gain and reciprocal filter types. The <prop> tag specifies a property node
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to hold the gain value and the <value> tag specifies an initial default value.
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The gain defaults to 1.0 if unspecified.
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The output from the gain filter type is: input * gain.
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The output from the reciprocal filter type is: gain / input.
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The gain can be changed during run-time by updating the value in the property
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node.
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