I started this week by retrieving the heat-shrink tube I left in Lake Matoaka last week. I brought back to the lab, dried it off, and cut it open. Inside everything was still dry and the seals looked good, so the test was successful. The next step is to repeat the test with a real Sharkduino inside.
Next I assembled a new Sharkduino. I put on the new accelerometer so this constitutes Sharduino V2.1. The new accelerometer is theoretically identical to the old one electronically but has a few extra features like a FIFO buffer, which will allow us to store data on the chip. This means that I was able to put the new accelerometer on a PCB made for the old one. I still have not figured out if this actually worked though, and I’ll update you all on it next week.
The next thing I did this week was general housekeeping work. I made sure the naming conventions for Sharkduinos are well defined, and began putting the Sharkduino names into the non-volatile memory on the Arduinos. I also did some maintenance and housekeeping work on the git repositories for the project.
Finally this week I updated my Sharkduino poster, and presented it at the William and Mary Undergraduate Research Symposium. This was a lot of fun, and you can find a copy of the updated poster here.
I spent this week mostly working on the final push for the proposal I’ve mentioned over the past few posts. I did however get some more interesting work done to write about after the proposal was due.
First I looked at the LiPo charging circuitry of the newest Sharkduino V2 that I assembled. I found that it is appearing to output a constant voltage, but that voltage is 0.3V below what is expected. I do not know what is causing this, and will look into it soon. Overall this charging circuitry has been giving me problems throughout the project. It seems every time I try to put it together it either doesn’t work at all, or breaks in a strange way. I do not know why this is, it’s a pretty simple circuit taken from the datasheet in conjunction with a trusted source. I does seem however that I am going have to invest a fair amount of time into it to really iron out the kinks.
Also this week the flat iron came in, and I worked on developing a procedure to seal heat shrink with it. I did this through a lot of trial and error. First I tried just putting the heat shrink in the flat iron at differing temperatures for differing amounts of time and then taking it out. This did not work well as I kept getting air bubbles in the seal. I eventually realised that the air bubbles were getting in while the tubing was still hot, so I needed to let the tubing cool while still clamped flat. To do this I put a clamp on the flat iron, heated the tubing for a while in it, and then unplugged the flat iron and let the whole thing cool while still clamped together. A picture of this can be seen below.
Flat iron clamped on heat shrink tubing
This lead to a very good seal, but takes a long time to do. This is because flat irons can heat up really fast, but take forever to cool, especially if they are being held shut. I started trying to think of ways to speed up the process and decided to try to move the tubing straight from the flat iron to an ice bath in hopes of cooling everything down before air bubbles could get in. I tried this for a while and found that you always get some air bubbles while taking the tubing out of the flat iron, but if you dunk it directly into the ice water things cool down enough that they do not have time to spread and it’s not a big deal. Overall I think that letting the tubing cool while clamped in the straight iron is the best, using the ice bath is a lot faster, and works well enough. Pictures of the seals from the different methods can be seen below.
Top: First sealing attempts
Middle: Ice water seal
Bottom: Clamped seal
Finally as a field test of the sealing techniques, I sealed up some paper towels and rocks in a tube using the ice bath procedure and tied them to a dock on Lake Matoaka. I will go back on Monday and tuesday to retrieve the experiment and make sure everything is still dry inside the tube (assuming it’s all still there).
Sealed tube hanging in the lake
I began this week by trying to fix the sharkduino V2 with the broken gyroscope that I made last week. I did this by using a hot air reflow station, which is like a heat gun with precision temperature controls. I used this to heat the solder up in the area surrounding the chip, and once the solder melted I tried to slide the gyro into place. This was not very easy though and I ended up sliding the gyro all over the board before getting it back in place. Once it was in place and the soldered solidified I found that the board was shorted between VCC and ground. This is not surprising but did make me give up on trying to save the board and move on.
Next I went back to trying to solve the heat shrink sealing problem. What I need is something to get hot and clamp flat to both sides of the tubing to fuse the tube together. I decided that the best tool for the job is a flat iron. It’s not the most scientific tool, but it has two flat plates that heat up and push together, and it’s cheap so I am going to try it out. I have ordered one and hopefully will be able to test it next week.
I also ordered new accelerometers to test out. They are a variation of our current gyroscopes so they do not need new hardware, but they have a FIFO buffer which will allow us to store data on it and write to the SD less often, and thus save power. While we have reached our primary battery lower power is still better because it will allow longer deployments, or smaller batteries. Smaller batteries would be good because that would allow us to attach the device to a wider range of animals.
This week I also assembled another Sharkduino V2, it went together well and seems to be fully functional.
Finally I am still working on a funding application and that is taking a good chunk of my time.
Hi all, this is the first weekly update of the new semester. The semester started a little more than a week before this, but I spent that time do administrative type work for the project that is not very exciting to read about. I also spent majority of this week doing that sort of stuff, writing proposals, going to meetings, bring new people into the project. In addition to that I also did a little bit of more technical work that I like to write about on this blog.
I started by trying to assemble a new sharkduino V2. The emphasis is on trying, I was a little careless and did not push the gyroscope into the solder paste firmly so it shifted during reflow. The results can be seen in the picture below. I doubt that this is recoverable, but I am going to give it a try before I just start assembling another one. I did gain some good experience with the solder stencils, so even if I can not fix it this build would not be a total loss.
Messed up Sharkduino V2
I also did a battery test of the new Sharkduino with new code and found that it can run for approximately 5days 10hr on a 500mAh battery while sampling at 25Hz. This is better then we expected, and a 2.4x improvement in battery life over the Sharkduino V1s with the older code. We are still checking that everything worked and the data if good, but assuming everything checks out we have basically hit our battery life goal. We want the tag to last 7 days in deployment, and we have been deploying with 800mAh batteries. The extra 300mAh in the larger batteries should more then get us the extra day and a half of life that we want.
Finally we got impulse sealer for sealing the heat shrink tubing we are using for waterproofing. I have only worked with it for about half an hour but it is giving me some trouble and not exactly doing what I want. It auto turns off after a few seconds and that is not long enough to fully melt the thick heat shrink tube we are using. This means that I have to continually lift and lower the arm of the machine to keep it turned on and even doing that I am not getting seals that I am happy with. I am going to keep playing with this thing and seeing if I can develop a procedure that works reliably. I am also thinking about getting a different tool. I might try a different impulse sealer, or I might get more creative and try using a straightening iron to do what I need. A picture of the sealer and some failed seals can be seen below.
I have decided to open source our single chip breakout boards. We made these boards to test new chips before integrating them into the Sharkduino system. We have made three of these , one for the L3GD20H gyro, the FXAS21002 gyro, and the DS1339B real time clock. These boards can be found on my github page, or indivdualy at the following links: