Stacy DeRuiter's How To Make Almost Anything Blog - Week Four

Octber 11, 2005

This week's assignment is to work on planning for our final projects. I am hoping to make an acoustic and position-sensing archival tag for harbor porpoises. You can go back to see the little video I made of this device in action on my Week One page. What I know so far is that the tag will have to have various components:

• a hydrophone that is sensitive in the 100-200 kHz frequency range
• an accelerometer
• sensors that can record pitch/roll and heading
• electronics that can record the data (the acoustic data will have to be recorded at or above above 400 kHz if I want to actually record porpoise sounds faithfully) and do some processing, such as high-pass filtering to cut down flow noise...maybe data processing...maybe some compression? I have a lot of work to do here figuring out what can be done and how to do it.
• some system for tag recovery - usually for these types of tags, that means a radio beacon or some sort
• a battery to power it all
• a housing to make it waterproof
• some way to attach the tag to the animal, including a release mechanism (preferably one that can be programmed or at least depended upon to release a certain amount of time after deployment)
• flotation material so that the tag is a little bit buoyant (and will float when it releases from the animal)

Octber 17, 2005

Here is a reworked and expanded version of the list of desirable tag components I made a week ago:

• a hydrophone that is sensitive in the 100-200 kHz frequency range. I might be able to make this myself out of piezoceramic or some other material (some form of a capacitative microphone that could work underwater/in an epoxy casing?), or otherwise buy something that is already designed to work as a hydrophone (and possibly even calibrated. This component is potentially tricky and/or expensive to get right!
• an accelerometer. I might be able to build some sort of accelerometer, or I could buy a 3-axis one or a set of several less-than-3 axis ones for a reasonable price. One choice would be an instrument from the Analog Devices AD xl family. DigiKey also has a few 3-axis models available. I found a few other 3-axis devices online, billed as among the "world's smallest," but have not been able to get pricing information yet (and they probably cost more than I want to spend) -- e.g. Hitachi Metals The advantage of using a ready-made accelerometer would be that it would probably be smaller than one I would build, and one of the goals in designing this tag is to make it as small and light as humanly possible.
• sensors that can record pitch/roll (some sort of gyroscope?) and heading (magnetometer?). I still need to do more research on just how these will work and whether I will build/buy them and how I would install them.
• a pressure sensor to record the tagged animal's depth
• a microcontroller and a flash memory chip for controlling the sensors and storing the data that comes out of them. I have to learn how to set up circuits so that the controller, the sensors, and the memory chip all 'talk' to one another, and I have to figure out how I will recover the data from the tag after a deployment - if the tag is potted in epoxy for waterproofing, recovering data could be more tricky. One solution used by Mark Johnson is a infrared serial port that allows both data offload and control of his tag's programmable components. I don't even know enough about how this works to say whether it would be a good solution for my prototype tag as well.
• some system for tag recovery - usually for these types of tags, that means a VHF radio beacon that can transmit for a few days after tag release from the animal (transmitting during tag-on is ok too, but will not help you AT ALL to find the tag - you will only be able to detect the VHF signal when the tag/radio antenna are above water, since the signal is very rapidly attenuated underwater).
• a battery to power it all - this might be a traditional battery or, as Neil Gershenfeld suggested, a supercapacitor (which could be charged without contact - even if the tag is potted) and would probably provide enough power for a short tag deployment.
• a housing (and/or potting the whole tag in epoxy or some other material) to make it waterproof
• some way to attach the tag to the animal, including a release mechanism (preferably one that can be programmed or at least depended upon to release a certain amount of time after deployment).
• flotation material so that the tag is a little bit buoyant (and will float when it releases from the animal)

• Is waterproof- once I have the components talking to each other, I will pot it in some resin or epoxy so that it is waterproof and can withstand the pressure of being underwater. It should also have attached flotation or a housing containing flotation, so it will float, and a way to attach it to a porpoise!
• Is powered by a supercapacitor, allowing non-contact charging even if it's potted in epoxy or some other material.
• Senses 3 axes of accelerometer data, controlled by a microcontroller and stored on a flash memory chip.

1. gyroscopes
2. pressure sensor
3. VHF radio beacon
4. audio capability (hydrophone)
5. dsp and signal processing capability
6. all the rest