Sunday, 30 June 2013

Air speed from two MS5611

Some time ago on this blog I promised an experimenters version of the BlueFlyVario. Earlier today I posted details of it on the blog. My first project with this version was to add an additional two MS5611 pressure sensors so I could rig up a custom airspeed sensor. It seems to be working ok, even though I have not tested it in flight. 

The setup

I got some little boards made up that use that pick up the VDD and GND on the bottom, link to the SPI MOSI, MISO and SCLK lines, and just use two additional IO pins for cable select. The headers are designed to line up with the headers on the experimenters board.

The picture below shows a spare board in the middle, and the made up one slightly lower left. Over the top of the MS5611 I put some neoprene like my standard v6. The small pcb tangs that contain the sensors are designed so that 7mm inner diameter vinyl tubing fits over the top. The header ends of these tubes are sealed with a bit of hot melt glue. I was quite amazed how well hot melt sticks to vinyl tubing. I had thought it would just peal off, but no; it's really sticky. 

The 7mm id vinyl tubing then has some 5mm id vinyl tubing to extend to the custom pitot tube. I made this up from some brass tubing from a hobby shop, with a few holes drilled in the right place and some hot melt to seal stuff up. There is actually a thin brass tube through the middle of the main one, with appropriate connectors to pitot and static sensors. Wikipedia shows a little theory of pitot-static tube design.

Determining the air speed

The micro controller code was altered to measure the pressure from each of the three sensors. It outputs PRS N1 N2 N3, where N1 to N3 is the pressure in pascals in hexidecimal of the form XXXXX. N2 and N3 are connected to the pressure sensors on the tangs. This gives us a dynamic and static pressure measurement, 50 times a second. To translate this to m/s is as simple as

Velocity (m/s) = sqrt(2 * (N2- N3) / airdensity).

I just used the standard air density in kg/m3 in my simple testing. This gives Indicated Air Speed. The conversion to True Air Speed should be easy enough given our slow speeds and some simple assumptions about atmospheric density based on pressure. Also, it will be important to put the pitot tube in really clear airflow.

I did a few calculations on the theoretical error. If we assume the error in the differential pressure is about the same as double the error in the sensors (RMS of 20 pascals), then the error in airspeed is very low (less than 1%) for speeds over 3 km/hr. In practice there are many other errors, but a quick 'blow on pitot' test seems to be able to produce reliable results given a 'constant' amount of blowing.

My next steps will involve more extensive testing and calibration. Then maybe a total energy compensated vario.