Module/Airspeed ETS
Overview
The EagleTree Airspeed Sensor is a low cost module and comes with a good pitot tube (Prandtl style, pitot-static tube) that includes static and dynamic ports. It has an I²C interface that connects directly to the Autopilot I²C port. The paparazzi autopilot code is able to regulate the throttle in order to keep the airspeed constant (and a minimum ground speed).
| Module name | sensors/airspeed_ets |
| Sensor type | air speed |
| Range | 4m/s .. 150m/s |
| Resolution | 0.3m/s |
| Refresh rate | 10Hz |
| I2C address | 0xEA |
Hardware
The sensor directly interfaces to the 3.3V I2C port of the autopilots and is supplied by +5V. When you buy the airspeed sensor it is set to operate in the default mode. Make sure you did not set it somehow to 3rd party mode.
Wiring
| Autopilot I2C pin | Autopilot I2C | ETS Airspeed wire colour |
| 1 | GND | white |
| 2 | +5V | red |
| 3 | ||
| 4 | SDA | yellow |
| 5 | SCL | brown |
Usage
To use it load the airspeed_ets module:
| File: conf/airframes/myplane.xml |
<modules>
...
<load name="airspeed_ets.xml"/>
</modules>
|
and enable I2C0 if you are not using it already:
| File: conf/airframes/myplane.xml |
<firmware>
...
<define name="USE_I2C0"/>
</firmware>
|
Configuration
You can also set some optional parameters to change the default configuration. For example to use i2c1 instead of i2c0, a scale of 2 (default is 1.8) and offset of 50 (default is 0):
| File: conf/airframes/myplane.xml |
<modules>
<load name="airspeed_ets.xml">
<define name="AIRSPEED_ETS_SCALE" value="2"/>
<define name="AIRSPEED_ETS_OFFSET" value="50"/>
<define name="AIRSPEED_ETS_I2C_DEV" value="i2c1"/>
</load>
</modules>
|
Usage as sensor for speed control
To use the sensor to control the speed of your aircraft add the aggressive climb flag, define which I2C device you are enabling and enable airspeed control:
| File: conf/airframes/myplane.xml |
<target name="ap" board="twog_1.0">
<define name="AGR_CLIMB"/>
<define name="USE_I2C0"/>
<define name="USE_AIRSPEED"/>
</target>
|
Airframe configuration
Now to use real airspeed values for adjusting your aircraft autopilot behavior, add the following to the end of the "VERTICAL CONTROL" section of your airframe file:
| File: conf/airframes/myplane.xml |
<section name="VERTICAL_CONTROL" prefix="V_CTL_">
....
<!-- auto airspeed and altitude inner loop (for airspeed sensor) -->
<define name="AUTO_AIRSPEED_SETPOINT" value="13.0" unit="m/s"/>
<define name="AUTO_AIRSPEED_PGAIN" value="0.060"/>
<define name="AUTO_AIRSPEED_IGAIN" value="0.050"/>
<define name="AUTO_GROUNDSPEED_SETPOINT" value="7.0" unit="m/s"/>
<define name="AUTO_GROUNDSPEED_PGAIN" value="0.75"/>
<define name="AUTO_GROUNDSPEED_IGAIN" value="0.25"/>
</section>
|
Note that the SETPOINT values may need to be adjusted to suit your aircraft.
Note that depending on whether you set the AIRSPEED setpoint or the GROUNDSPEED setpoint higher, either constant airspeed or constant groundspeed, respectively, will be the goal of the controller.
See paparazzi/conf/airframes/easystar_ets_example.xml for an example airframe configuration.
Seeing the speed values
To debug or log the raw values from the ETS airspeed sensor define SENSOR_SYNC_SEND in your airframe file to send the message AIRSPEED_ETS on every sensor reading. Note that defining this sends the AIRSPEED_ETS message at the sensor read rate as defined in conf/modules/airspeed_ets.xml. This does not have any bearing on the AIRSPEED message (if both SENSOR_SYNC_SEND and USE_AIRSPEED are defined, then both messages are sent).
| File: conf/airframes/myplane.xml |
<modules>
<load name="airspeed_ets.xml">
<define name="SENSOR_SYNC_SEND"/>
</load>
</modules>
|
The general airspeed can be displayed in the Messages tool by adding the AIRSPEED message to the telemetry file as follows:
| File: conf/telemetry/default.xml |
<process name="Ap">
<mode name="default">
<message name="AIRSPEED" period="1.0"/>
...
|
The AIRSPEED_ETS message does NOT need to be added to the telemetry configuration since it is sent directly by the module if SENSOR_SYNC_SEND is defined.
NOTES
- In the GCS, the strip displays ground speed and not airspeed by default. In order to display airspeed, drag-and-drop the airspeed message field from the Messages tool onto the 2D map of the GCS.
- The telemetry name AIRSPEED" should actually be called SPEED and contains Groundspeed and airspeed return values.
Measurement only
To use the sensor for airspeed measurement is also possible. This Measurement only mode does thus not imput to the control loops to have effect to the aircraft behaviour.
To see the sensors data in the log file set the SENSOR_SYNC_SEND in your airframe file. Every time new data is available it will be sent directly.
| File: conf/airframes/myplane.xml |
<modules>
<load name="airspeed_ets.xml">
<define name="SENSOR_SYNC_SEND"/>
</load>
</modules>
|
Result message
The raw data (adc), estimated offset at init time (offset) and the converted result (scaled) is written to the log file.
| File: conf/messages.xml |
<message name="AIRSPEED_ETS" id="57">
<field name="adc" type="uint16"/>
<field name="offset" type="uint16"/>
<field name="scaled" type="float"/>
</message>
|
Sample log file lines
149.529 123 AIRSPEED_ETS 1626 1606 8.024844 149.633 123 AIRSPEED_ETS 1626 1606 8.024844 149.730 123 AIRSPEED_ETS 1627 1606 7.942226 149.841 123 AIRSPEED_ETS 1628 1606 7.942226