Module/Aeroprobe OTF
Overview
The Aeroprobe On-The-Fly! air data computer measures air pressure from a 5-hole pitot tube and can write resulting data to an SD card or transmit it through an UART. It outputs speed, angle of attack, angle of sideslip and altitude. The air data computer input "1" goes to the middle tube of the probe, the longest one. The next shorter one goes to "2", then "3" and so on.
| Module name | sensors/airspeed_otf |
| Sensor type | airspeed, flow angle |
| Range (speed) | 8m/s .. 63m/s |
| Range (angle) | ±20° |
| Resolution (speed) | 0.02m/s |
| Resolution (angle) | 0.01° |
| Refresh rate | 100Hz |
Hardware
The air data computer needs to be configured by the manufacturer to output data at the desired speed. The serial interface does have original RS-232 levels, that is not compatible with the autopilots 3.3V interface. You need a RS-232 level shifter.
Air data computer/pitot tube supplier: Aeroprobe
RS232 level shifter supplier: Sparkfun
OTF connector supplier: Farnell
Wiring
| Umarim UART1 pin | Umarim UART1 | Level shifter |
| 1 | GND | GND |
| 2 | ||
| 3 | +3.3V | VCC |
| 4 | RX1 | TX-O |
| 5 | TX1 | RX-I |
| Level shifter Sub-D pin | Level shifter Sub-D | OTF | OTF pin |
| 1 | |||
| 2 | RXD | RX (data in) | 4 |
| 3 | TXD | TX (data out) | 3 |
| 4 | |||
| 5 | GND | GND | 5 |
| 6 | |||
| 7 | |||
| 8 | |||
| 9 |
| BEC Power supply | OTF | OTF pin |
| + | +VCC | 1 |
| - | GND | 2 |
Ground usage
This is for the lab only, not for flight. The modem serial port of an Umarim will be occupied by the OTF! compter. Therefore the data is transferred through USB.
| File: conf/airframes/myplane.xml |
<firmware name="fixedwing">
...
<subsystem name="telemetry" type="transparent_usb"/>
...
|
To use it load the airspeed_otf module:
| File: conf/airframes/myplane.xml |
<modules>
...
<load name="airspeed_otf.xml"/>
</modules>
|
The used UART has to be set globally:
| File: conf/airframes/myplane.xml |
<target name="ap" board="<my_board_type>">
...
<configure name="OTF_UART" value="UART1"/>
|
Result message
The output is airspeed (velocity), angle of attack (a_attack), angle of sideslip (a_sidesl) and altitude. The data is secured by a packet counter and a checksum.
| File: conf/messages.xml |
<message name="WIND_AP_OTF" id="179">
<field name="counter" type="uint32"/>
<field name="velocity" type="int16" unit="cm/s" alt_unit="m/s"/>
<field name="a_attack" type="int16" unit="centideg" alt_unit="deg"/>
<field name="a_sidesl" type="int16" unit="centideg" alt_unit="deg"/>
<field name="altitude" type="int32" unit="cm" alt_unit="m"/>
<field name="checksum" type="uint8"/>
</message>
|
Sample log file lines
11.522 123 FLOW_AP_OTF 1076 1480 628 1636 -100 2 11.535 123 FLOW_AP_OTF 1077 1457 735 1707 900 29 11.548 123 FLOW_AP_OTF 1078 1451 661 1867 800 19 11.555 123 FLOW_AP_OTF 1079 1422 697 1685 -300 11