Wednesday 12 June 2013

Some answers

Over the last few months I have responded to many emails, forum posts and some blog comments about the BlueFlyVario. Much of this discussion has similar themes. In this post I hope to address some issues so I can point future queries towards this page. This is kind of like a FAQ.

"It would be great to see the BlueFlyVario with a wired connection to my device"

I have spent way too many hours thinking how the BlueFlyVario could connect in a wired way. There are very many options such as;

  • a 3.3V serial (UART) connection via the micro USB pin
  • add a chip like the FT232RL that provides a USB device that acts as a virtual COM port.
  • change the PIC microprocessor to one that supports USB OTG 
  • output the data using some kind of analog encoding for input via a device microphone port. 

All of these options have issues. The main problem that I have faced is that each of these solutions would be specific to a small range of devices. Choosing Bluetooth SPP means that pretty much every device with bluetooth (except iOS stuff) can be interfaced with the BlueFlyVario. Perhaps I am influenced by the fact that every device I own or have access to that I might want to use as a flight instrument can interface to via bluetooth; however, among the six android devices (four phones, two tablets) I would struggle to create one type of wired connection that would be successful with all of them. 

"I have a pressure sensor built into my Android device. I would like to use your software with it"

I have not implemented this in the BlueFlyVario Android software as it is pretty device/Android version specific. The BlueFlyVario Android software is open and you are free to modify it to meet your needs. 

Additionally, I am not aware of an Android device with a high resolution pressure sensor. Some pilots have been happy with a vario based on BMP085 based sensor (30cm resolution at less than 50Hz), which is the kind of sensor built into those phones that have one, but most pilots feel it is a step back from their current vario. If Android devices get high resolution sensors (10cm at 50Hz), and this information can be accessed from the OS, then there will be no need for the BlueFlyVario. 

"I would like to see other sensors on board the BlueFlyVario, like GPS or multiple pressure sensors "

I have focused on just a pressure sensor because it is the key thing missing to turn an Android device into a vario. 

The GPS on Android devices is good enough for 95% of pilots. My software uses it for windspeed calculations

Some pilots, particularly glider pilots, want multiple pressure sensors arranged in such a way that pitot and static pressures can be measured. This would allow for better wind speed calculations and provide the ability for total energy compensated vario measurements. 

The Experimenters Version of the BlueFlyVario will allow those with serious tinkering skill to incorporate other sensors. I will be working on releasing that over the next few months. 

"The BlueFlyVario should cost less"

I agree. The component costs for me at batch volumes of around 50 represent about half of the device cost. The remaining 50% comes from consumables used in production, apportioned overheads used in keeping my little workshop going, packing and shipping costs, and paypal fees. At this stage it would really only be possible to get the costs down if production increased to volumes of many hundreds. I am not trying to profit from this hobby.

"Can you get the BlueFlyVario to work with other software?"

Yes. It is already integrated into XCSoar. Work is underway to integrate it in XCTrack. If you are a software developer and can access a Bluetooth SPP stream then I encourage you to look at the Android BlueFlyVario app software on GitHub for some inspiration. If you need a BlueFlyVario device for testing contact me and I will help you out. 

"How does the BlueFlyVario perform compared to other varios?

Performance is a complex question. To address some of the issues that affect responsiveness, accuracy, precision and latency I will summarize what I have experienced in BlueFlyVario development. My aim has been to get the best performance possible out of the best sensor available. 

Performance starts with the sensor. I am using the MS5611 for the reasons I describe here. Essentially, there is a relationship between the number of readings possible per second, and the RMS error in each reading. The MS5611 can theoretically provide an RMS error of around 10cm (in pressure altitude terms) at a rate of 50 samples per second (at an oversampling rate of 4096). In practice, to get this good, requires very careful circuit design to filter the power supply. I have been able to get to an RMS error of 12cm at a rate of 50 samples per second. 

Next, we need a filter on the sequence of noisy pressure measurements to turn it in to a precise estimate of the rate of change of altitude. There are many approaches to this. I started with a linear regression and IIR filter. I have tested many approaches, including an AB filter, but in the end I have settled on a Kalman filter for its performance and theoretical underpinnings. I have found that the filter choice does not matter anywhere near as much as the sensor selection and power supply design. 

The time between the filtered lift rate being above the threshold and the commencement of the beep matters quite a bit. This depends on the way code is designed on the vario and supporting software. For my new hardware audio, it is less than 5ms until the beep commences. Transmission of the pressure data over bluetooth adds around 150ms. Based on feedback from other pilots it sounds like this delay does not matter much. 

The type of audio also matters, perhaps subconsciously based on what most pilots are used to. Variable frequency and cadence based on lift rate is anecdotally important. 

The most important thing from the above list is pressure sensor selection and the power filtering on the pressure sensor supply. I have managed to get the BlueFlyVario to the point where I can lift it from chest to head height and get ultra responsive beeping. I know most pilots don't need a vario that is this sensitive, but it is a kind of addictive obsession to pursue performance. 

Most pilots who have used it say the BlueFlyVario is as good as the best varios on the market.