Thursday 7 June 2012

The MS5611 - a resounding success

I got a few MS5611 samples through a friend in Europe to integrate onto the BlueFlyVario prototype just before I went away a year ago. My initial attempts at soldering on some legs and making dodgy breakout boards failed. I ruined at least one of the samples by melting the protective plastic case. During my time away more people started to use the sensor and I managed to get a few breakout boards from ebay.

To solder the MS5611 onto the PCB breakout board I first put a few very small dabs of solder on the PCB SMD pads. This is easily achieved by first using liquid flux on the pads, then loading up the tip of a standard soldering iron with a lump of molten solder, then just finely touching the molten lump to the pad. The idea is to get a small bit of solder on each pad (with the same amount on each pad!) Next I cleaned the contacts on the MS5611 with flux and wiped it all off - the idea is to remove the oxidation layer. Then I took the PCB and put in on an electric frying pan (skillet). As soon as the pan warmed enough to see the solder melt I put the MS5611 into place with a pair of tweezers. The surface tension from the molten solder held the component in place with a little wiggle of the board. As soon as it was cool I found that I had a perfect SMD solder joint.

I put the associated other components on the breakout board, really only a capacitor and headers, then tested them with the prototype. I had to remove the BMP085 and all of the associated circuitry, then put the breakout board in place. I used the SPI interface functionality rather than I2C as the datasheet said I could expect better noise performance (hence, lower error). It took me a few hours to get everything up and working again. I had to remember how to properly configure the PIC programmer and desktop prototype app. Modifying the PIC 24F code to do SPI ended up being pretty easy, there were a few examples on the net. The first time I properly compiled the program and ran it I got a huge shock, it was working with good, not too noisy, pressure data coming out. I lifted up the prototype vario and my computer went beep.

Some fiddling with the desktop app allowed me to see the altitude graphically and I added in a RMS error to debug the noise. Initially I was getting about 25cm RMS noise with about 50 samples per second. The 50 samples per second actually measures the both the pressure and temperature 50 times a second, then outputs the pressure only. It should be possible to measure the temperature only about 10 times per second, using each temp measurement for about 9 pressure measurements. That would give us about 90Hz for the pressure measurements. I am not sure we need that much...

The prototype was pretty sensitive and it was defiantly an improvement over the BMP085, but I was not happy yet. I added a RF Choke on the V IN line of the MS5611 and the noise went down to about 15cm, although sometimes it floats up if I touch the wrong component on the prototype. I am not sure what the value of the RF Choke is, it was just one that I grabbed from the workbench. The source of the noise could be the linear voltage regulator, the Bluetooth transmitter, or the switching of the PIC microcontroller. It does not really matter, what does matter is that the the noise suppression works really well. I have ordered some ferrite beads for the next prototype. I should get even better noise performance if I filter out noise from the ground plane as well. I don't have an oscilloscope so I can't actually measure what frequencies I am trying to filter and perfectly match the ferrite bead to it. If only I was rich.

Below is a short video of the vario in hand with the prototype desktop app overlayed on it. Note the white altitude display with the vario data (damped) on top of it.

The settings are:
altDamp=0.05 (i.e. 5% of each new measurement is added to the 'current' measurement - see the previous post about the IIR filter to understand what this means)
varioWindow=50 (about 1 second at the 50 Hz I am measuring at)
vario2Window=100 (about 2 seconds)

Next steps:
- Order components for the next round of prototypes (done). I am going to make a couple, with pretty much all SMD components.
- Order PCB prototyping stuff.
- Prepare Eagle circuit diagram and PCB artwork.
- Develop prototype Android app.

Stay tuned, I am back on track. I aim to have a few working SMD prototypes and the prototype Android app by the end of July. Circuit diagrams and code are coming.