In the previous article we introduced the Vortex ring and derived the governing equations. We found that the voltage developed on the sense coil could be described by the following equation:
We also defined a rough process for designing the ring.
The first step is to select the material and dimensions of the ring itself. For a proof of concept, it is unnecessary and expensive to create a custom ring and so instead I looked online for existing solutions. Luckily, toroidal ferrite cores are a common component for creating inductors and transformers. These are effectively just a ring of a high permeability material which is exactly what I need – although they aren’t generally made to fit on a finger. The best solution I found was the TDK Electronics B64290L0647X038 which has an inner diameter of 17.7mm, an outer diameter of 31.0mm and a length of 16.1mm. The magnetic permeability is about 10000 which is pretty good. This is definitely a chunky ring and barely fits on my finger, but it was the best off-the-shelf compromise I could find for now. The effective area and effective magnetic length can be read from the data sheet as 76.98mm2 and 73.78mm.
Now that I have finally wrapped up EOGee/EOGlass I can focus on my next project. I decided to do something with magnetics because I wanted to build a little more applied intuition and understanding of magnetic sensors and actuators. My original plan was to investigate building my own electro-permanent magnets, because I can think of some interesting applications for a switchable magnet. However in the process of my research I came up with a new idea for a wearable communication technology.
Human Body Communication (HBC) is a catch-all term for communication methods that use the human body as a communication channel. This means that the signals are contained within the human body and do not travel through a wire or through the air. This is potentially useful in wearable devices that may need to communicate with each other, for example a smart-watch can communicate with a heart rate monitor on the user’s chest, or an accelerometer on their leg, or even a medical device like pacemaker or cochlea implant. In such a network, all devices can communicate with a central hub (e.g. smart-watch) that then communicates to the user’s phone or other device to deliver the data to the user. Advantages of this approach potentially include reduced power consumption (due to the body’s higher channel gain compared to air), reduced network congestion (as each human will constitute a separate network rather than all sharing the air), as well as increased security and/or privacy (since the signals are constrained to the user’s body).