I spent most of this week doing research for the project. Currently we want to look into the feasibility of removing all ports from the device. This would make waterproofing the tag significantly easier, and cut down the chance of the tag being destroyed on deployment. Currently there are only two ports that are needed once code is uploaded to the device. The first is the microSD port and the second is the USB charging port. What this means is that we need to develop ways to wirelessly charge the device and get data off of it. The power problem is the easer on to solve, we do it with inductive charging. This technology is common in all sorts of technologies from electric toothbrushes to high end smartphones and smartwatches. Inductive charging works by placing two coils of wire near each other and allowing the electromagnetic field produced to transfer energy. The problem with this technology is that it can be slow, especially if the charging coils being used are small. However for the lower power usage and small batteries we are working with we think that this should not be a problem even with the small footprint, and thus small coil for the tag.
For the wireless data transfer these is less of a clear cut solution. We all transfer data wirelessly all the time every day through Wifi, bluetooth, cellular networks, etc. The problem with these methods is that they are power intensive, and complex. We do not want to add a lot of hardware to the device that will draw a lot of power, even if the power is only drawn while the device is charging we might still not be able to power it. Lucky people have already developed solutions to this problem. The first is called bluetooth low energy (BLE) which is in fairly common use today. It was released as part of bluetooth 4.0 and is similar to bluetooth but uses far less power. It does this by having a slower data rate, working over a shorter range (~50m), having fewer channels to advertise over, and a series of other differences to cut power usage. The goal of this standard is run bluetooth off a watch battery. Bluetooth lower energy devices are pretty common, things like step trackers and other wearables, along with bluetooth mice and keyboards use this standard. One problem with this standard though it pairing the devices.
The other technology we are currently looking at is peer-to-peer near field communication (NFC). NFC is another common technology in modern cell phones, and is used for things like touch pay. It is also used whenever you touch your phone to something to get it to react. This technology is not in super common use in phones outside of payment systems though because Apple does not give outside developers access to the iPhones NFC capabilities. For NFC in electronics the most common use is not peer-to-peer, often on one device is powered and it is reading data off some sort of passive tag. Peer-to-peer NFC has however been used for data transfer by android in the mostly failed android bump feature. This feature transfer photos, music, or other information between android phone by touching the back of the phones together. The benefit of NFC over BLE is that NFC does not have pairing problems, and I think it may have a little less hardware overhead, but I’m not sure. The problem is that it has a relatively slow data rate, this may be okay though because we are working with relatively small files and have time to transfer them while the device is charging. Also NFC has a very short range of communication, maybe a couple centimeters at most.
Overall for the data transfer problem I have not done nearly enough research or experimentation to come to any type of conclusion at all.
In addition to all the research I spent some time this week working on assembling Prototype 5. This prototype is going to replace the destroyed prototype 4, and use one of the gyroscope chips I made. Unlike prototype 4 however the gyroscope is not stacked on to of the accelerometer, but placed next to it. This makes the device longer but narrower. Currently the prototype is assembled, but only partially working. I need to do a little trouble shooting on it. A picture of it can be seen below.
Finally on Monday I submitted my final report from the REU program. If you want to read it, you can find it here.