Thursday, 19 November 2009

GDP Week 6

Been a little busy with GDP so I thought I'd take a moment to explain what my group's Project is and how it is going. At the moment the military use two AA Alkaline Batteries to power troops personal role radios (PRRs). The PRR provides short range (500m) communications between soldiers and is a replacement to hand signals. The manufacturer, Selex Communications, worked hard to get the battery life up to 24 hours, which is impressive, considering most missions are approximately 3 hours long.



The problem with using disposable batteries is that they present a logistical nightmare. Surplus batteries must be carried for each mission and used batteries must be collected at the end. Standard procedure is to always replace the batteries in the PRR before every mission, regardless of previous use. The military have expressed a strong dissatisfaction with lithium-ion batteries, on the grounds that they are slow to charge and their state of charge is unpredictable/unreliable.

Our brief was to research and present an alternative technology which could be retrofit into the PRR. Ideally it would have competing capacity and be of comparable size. Importantly it must overcome the failings of present Li-Ion technology. Bring on the Ultracapacitor:



The beasts we are using are Philips 50F 2.3V capacitors from Farnell. We also have some MAXIM 100F 2.5V Capacitors to play with. They are pretty chunky, and present some problems in themselves, namely:

  • Interfacing the PRR (xV to 3V)

  • Fast Charging circuit (T ~ 60s)

  • Cell Monitoring



The last point is interesting and of most importance. The caps are damaged if taken above their working voltage of 2.3V or 2.5V. Because of their low energy density, many ultracapacitors, connected in series and parallel will be required to replace the AA Batteries. This makes it easy for unbalanced cells to exceed their working voltage during normal use. Over Voltage Protection (OVP) circuitry is required, and that forms my part of the project. I have just finished the final design of my circuit which is capable of detecting the OVP condition, isolating the capacitor and bypassing the charging current. I hope to blog more about it soon.

Thursday, 5 November 2009

Tuesday, 3 November 2009

Automata & Useless Machines

Back in design and technology class during GCSE I made an automata of a flying pig. It had angle wings that flapped and floated around a little, whilst the clouds moved in the background. The word 'shoddy' would aptly describe it. My woodworking skills were, and I can only assume still are, poor. Whilst always backed up with plenty of enthusiasm, my creations generally looked better in my mind than in reality. I think it has something to do with a lack of patience.

I completely forgot about all of this until about fifteen minutes ago, when I stumbled upon this construction on the hack-a-day website. This then lead me on to a Youtube binge of useless machines and inspired automata. I think I'm envious of just about every one of these machines. They are clearly a labour of love. Here were my personal highlights. For those cautious of entering the same Youtube binge, its too late.

Automata



Simple but brilliant - Flipping Pancakes

Adorable - Marching Penguins

Sad - An Afternoon in the park

Bizarre - Drifting Apart

Useless machines


Art? - Unplugger

Hot? - Banana Fan

Spinning - Its the things you can't change that change you