Over the last few months I have continued my experiments with airspeed measurements using two MS5611 pressure sensors. I built on some of the work described in this earlier post. I was initially really confident, but there are a few issues which are difficult to overcome. So now I have a new plan with a differential pressure sensor. Through this journey I learnt a lot more about how our pressure sensors work. In this blog post I will describe what I have learnt and the way ahead.
My main motivation in continuing with the twin MS5611 setup was to make a vario that had both super accurate static pressure changes (like the current BlueFlyVario), and could very accurately measure airspeed. I built on the design and theory outlined in this earlier post. The BlueFlyVario_Twin was designed for sailplane pilots. Bluetooth transmission of vario information from the total energy probes near the tail fin will overcome some of the accuracy issues associated with the volume of air in the tubes which normally run to the cockpit. I imagined that it would also be useful for hang glider pilots, but I am still not really confident that there is a way to mount a pitot tube for practical paraglider use.
The BlueFlyVario_Twin prototype design (see the images below) built on the basic BlueFlyVario and added some extra features:
- The extra MS5611 pressure sensor.
- Some more power configuration options which allow for 12V in (instead of the LiPo battery).
- I exposed the second UART on the mirocontroller. This will allow for custom firmware to add a GPS or export the data vial TTL serial.
- I planned to use the more capable PIC 24F32KA301. I was running out of space on the current micro-controller.
An assembled prototype is shown below. This was the test setup.
What went wrong?
In short, zero drift. Each MS5611 has a bunch of calibration coefficients. On start up these coefficients are read and then used to calculate the temperature compensated pressure. There are some remaining errors. The error from one measurement to the next is small (less than 10cm in pressure altitude terms) which means an individual sensor is perfect for use as a vario. However, the error in the absolute pressure is up to 100 times the relative pressure (about 10m in pressure altitude terms).
I thought that the error in absolute pressure could be compensated for by taking an initial measurement from each sensor at start up (when both sensors were subject to the same pressure), then applying that difference as a calibration coefficient. However, the difference drifts over time, not quickly, but over ten minutes or more it can drift far enough to mean that relative pressure measurements between the two sensors start to become inaccurate. The image below shows how the difference drifts over about two hours. The x axis is in seconds, and the y axis is the difference in pressure in pascals. To get good airspeed accuracy requires measuring differential pressure at accuracies of less than 5 pa. A drift of over 40pa would screw that up.
There is still some hope with this setup as we care more about the rate of change of differential pressure rather than the absolute differential pressure. In the data above we can see the base rate of change of differential pressure is no more than +/-0.01 pa/s. In a typical sailplane scenario on entering a thermal you might slow from 70 kts to 50 kts in 30s. The pitot pressure change would be about 300 pa (depending on air density), a differential pressure rate of about 10 Pa/s. This is a thousand fold difference from the rate of change from zero drift. This would allow for a roundabout way to calculate a total energy compensated vario measurement without the calculation of airspeed. However, I want airspeed as well so I needed another plan.
What is causing the zero drift?
I spent some time understanding how piezo resistive pressure sensors work. This document provides a wealth of great information that I will not repeat here. In summary, I suspect that each sensor is drifting based on subtle temperature dependent characteristics of each sensor. As each sensor is subject to different resistive heating the absolute pressure drifts, which is magnified for two sensors.
The way ahead
I will continue with the BlueFlyVario_Twin in a different configuration. I now plan to use a differential pressure sensor instead of the second MS5611. The first MS5611 will still be there to give accurate static vario, but the differential pressure sensor will be used for airspeed measurements. I am going to test the MEAS MS4525HRD in 0-1psi /differential configuration (http://meas-spec.com/downloads/MS4525HRD.pdf). It is designed for airspeed measurement so I suspect the zero drift will be low enough (the datasheet does not quote zero drift) . With a busy summer and some time waiting for components it will probably not be until March until I know if my new design will be good enough to progress to a prototype I will be happy for others to use. I am optimistic once again!