Difference between revisions of "Radio Control"

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== Example ==
== Example ==


Here an example of an radio file, in this case openuas_mx22_regular.xml
Below an example of an radio file, in this case openuas_mx22.xml
 
<?xml version="1.0"?>
<!DOCTYPE radio SYSTEM "../radio.dtd">
 
<!-- $Id: openuas_mx22.xml,v 1.1 2010/07/05 21:46:00 mmm Exp $
--
-- No copyright whatsoever, free to use, 2010 OpenUAS.org
--
-- This file can be used in combination with the paparazzi autopilot.
-->
 
<!--
-- Attributes of root (Radio) tag :
-- name: name of RC transmitter, e.g. Graupner MX22
-- data_min: min width of a pulse to be considered as a data pulse
-- data_max: max width of a pulse to be considered as a data pulse
-- sync_min: min width of a pulse to be considered as a synchro pulse
-- sync_max: max width of a pulse to be considered as a synchro pulse
-- min, max and sync are expressed in micro-seconds
-->
 
<!--
-- Attributes of channel tag :
-- ctl: name of the command on the transmitter - only for displaying
-- no: order in the PPM frame
-- function: logical command
-- averaged: channel filtered through several frames (for discrete commands)
-- min: minimum pulse length (micro-seconds)
-- max: maximum pulse length (micro-seconds)
-- neutral: neutral pulse length (micro-seconds)
-- Note: a command may be reversed by exchanging min and max values
-->
 
<!--
  <x channel xctl="CONTROL5"            xfunction="GAIN1"    xmin="1100" xneutral="1500" xmax="1900" xaverage="0"/>
  <x channel xctl="CONTROL6"            xfunction="CALIB"    xmin="1100" xneutral="1500" xmax="1900" xaverage="0"/>
-->
 
<!--
The order of the list below is of importance if you do not define a "no=" (order in the PPM frame) parameter.
If you do not define this then the order of the PPM is the one of the order of the functon in the list
-->
 
<radio name="OpenUAS MX22" data_min="1000" data_max="2000" sync_min="5000" sync_max="15000" pulse_type="NEGATIVE">
  <channel ctl="LeftStickVertical"    function="THROTTLE" min="1100" neutral="1120" max="1900" average="0"/>
  <channel ctl="RightStickHorizontal" function="ROLL"    min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="RightStickVertical"  function="PITCH"    min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="LeftStickHorizontal"  function="YAW"      min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="CONTROL8"            function="MODE"    min="1100" neutral="1500" max="1900" average="1"/>
  <channel ctl="CONTROL9"            function="CAMBER"  min="1100" neutral="1500" max="1999" average="0"/>
  <channel ctl="CONTROL10"            function="AIRBRAKE"  min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="SW8"                  function="MANUALRELEASE" min="1001" neutral="1999" max="1999" average="0"/>
</radio>
 


== Measuring the PPM time values ==
== Measuring the PPM time values ==

Revision as of 14:40, 2 August 2010

Radio

This XML file, usually located in the conf/radios directory, contains a description of the radio control transmitter PPM signal. It should follow the grammar described in radio.dtd.

The contents are an ordered sequence of elements describing each channel with its name and its range:

<!DOCTYPE radio SYSTEM "radio.dtd">
 <radio name="cockpitMM" data_min="900" data_max="2100" sync_min ="5000" sync_max ="15000" pulse_type="POSITIVE">
 <channel ctl="D" function="ROLL"     min="2000" neutral="1498" max="1000" average="0"/>
 ...
 <channel ctl="E" function="MODE"     min="2000" neutral="1500" max="1000" average="1"/>
 ...
 </radio>

The order of the channels must be the order of the pulses in the PPM signal.

Among the top attributes, we find

  • name: used only in debug traces.
  • data_min (resp. _max): the minimum (resp. max) width (in microseconds) used to code one channel of the PPM signal.
  • sync_min (resp. _max): the minimum (resp. max) width (in microseconds) between two impulses set of the PPM signal.
  • pulse_type: the polarity of the PPM pulse. Can be either POSITIVE (FUTABA, Hitec, Multiplex etc) or NEGATIVE (JR, Graupner etc.).

Each channel is described with its transmitter name (ctl), its function name, its range in microseconds and its neutral value in microseconds. These values are used by the autopilot to compute a normalized input from the PPM signal (this file is preprocessed and the produced code is included in the airborne code). Note that the min and max attributes can be exchanged to reverse the direction of the command.

Wrong attributes of the "radio" element will prevent the decoder to recognize any PPM frame; same for a wrong number or channels.

Averaging

The average attribute must be set to 1..x for discrete channels for which a trivial averaging filter will be applied. The neutral of a discrete channel need to be halfway through the min and max values. Otherwise it won't be mapped properly by the filter. Note that averaging a channel gives the output an additional small delay.

Directions

The following signs have been used in the radio configuration files distributed in conf/radios:

  • ROLL: The min attribute value stands for the right position of the stick (turning right)
  • PITCH: The min attribute value stands for the down position of the stick (pitching down)

Example

Below an example of an radio file, in this case openuas_mx22.xml

<?xml version="1.0"?>
<!DOCTYPE radio SYSTEM "../radio.dtd">




<radio name="OpenUAS MX22" data_min="1000" data_max="2000" sync_min="5000" sync_max="15000" pulse_type="NEGATIVE">
  <channel ctl="LeftStickVertical"    function="THROTTLE" 	min="1100" neutral="1120" max="1900" average="0"/>
  <channel ctl="RightStickHorizontal" function="ROLL"     	min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="RightStickVertical"   function="PITCH"    	min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="LeftStickHorizontal"  function="YAW"      	min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="CONTROL8"             function="MODE"     	min="1100" neutral="1500" max="1900" average="1"/>
  <channel ctl="CONTROL9"             function="CAMBER"   	min="1100" neutral="1500" max="1999" average="0"/>
  <channel ctl="CONTROL10"            function="AIRBRAKE"   	min="1100" neutral="1500" max="1900" average="0"/>
  <channel ctl="SW8"                  function="MANUALRELEASE"	min="1001" neutral="1999" max="1999" average="0"/>
</radio>


Measuring the PPM time values

R/C receiver timing diagram

There are two common ways to measure the time characteristics of the PPM signal:

  1. Using an oscilloscope: easy to achieve with a high level digital scope with capture and measure facilities.
  2. Using the telemetry of the autopilot: the PPM message (defined in conf/messages.xml) contains the sequence of a (recently) received PPM signal.

With the default telemetry configuration file (conf/telemetry/default.xml), this message is not sent in the default mode (numbered 0). This mode can be changed to debug (numbered 1) in the airframe file by setting the TELEMETRY_MODE_FBW constant:

 <define name="TELEMETRY_MODE_FBW" value="1"/>.  

Or it can be set within the makefile section by adding:

 ap.CFLAGS += -DTELEMETRY_MODE_FBW=1 

The time unit used in this PPM message is hardware dependent:

  • On the obsolete AVR hardware, 1 microsecond = 16 units (because the crystal is running at 16MHz)
  • on the LPC hardware, 1 microsecond = 15 units (because the cristal is running at 12MHz)
    (conf/autopilot/tiny.h), the CPU clock is 5 times more, the peripheral bus is 4 times less, and the timer is not prescaled (sw/airborne/arm7/sys_time_hw.h) !!!)

Tips

If one has a good servo tester the output values at the receiver side can be measured