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[Paparazzi-devel] Fwd: DIY airspeed indicator.

From: Helge Walle
Subject: [Paparazzi-devel] Fwd: DIY airspeed indicator.
Date: Mon, 9 Mar 2015 13:26:19 +0100


Hopefully this is of interest to someone.

After no success trying to find a pattern in the temperature drift of my ETS airspeed sensor, I wanted to try another alternative. I connected two ms5611 barometers, and put a header over each of them made from heatshrink and 2mm aluminium tube. The heatshrink is formed to fit over the sensor bodies and sealed on the outside with epoxy, see photos. Two GY-63 boards (approx $20 for the pair on aliexpress) is used.

I created a new module to enter into the airframe file:
<load name="baro_ms5611_i2c_twin.xml">
  <define name="MS5611_I2C_DEV_1" value="i2c0" />
  <define name="MS5611_I2C_DEV_2" value="i2c0" />
  <define name="MS5611_SLAVE_ADDR_1=0xEE"/>
  <define name="MS5611_SLAVE_ADDR_2=0xEC"/>
  <define name="MS5611_DIFF_OFFSET=-176."/><!-- adjust to zero diff on ground -->
  <!-- <define name="MS5611_DIFF_ALWAYS_POSITIVE"/> -->
  <define name="SENSOR_SYNC_SEND"/>

The code is here:

The two ms5611 Messages in messages.xml have been modified to take values from the two baros.
This is just an extension of the Paparazzi code for one ms5611. There might be timing issues, as a couple of times I had to re-power the Twog at startup to get the i2c protocol running. But this is something I have experienced with other equipment. Any input is very welcome.

I tested with a Twog and Paparazzi v5.4. Bench testing seems promising so far (in my opinion anyway), and I plan to test the sensors in flight when spring comes to Norway.
I moved the sensors between in-house temperature of 24,5C and outside temperature of 9C to monitor the temperature drift. I estimate the error in the airspeed reading, within a 15C temperature span, to approx +/-0,6m/s at 10m/s and +/-0.4 to 0.45m/s at 15m/s. The noise error gets less significant as speed increases.

There is no automatic offset calibration at the moment, so this needs to be entered manually. I may get to that later.


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