Axe support. Made up a couple of blocks to bolt either side of the axe shaft to
support it better. Rotates around a fixed 20 mm dia. silver steel shaft. I made
up a new 3 mm wall thickness tubular plain bearing for it out of Molybdenum
Disulphide impregnated Nylon.
NiCads. With the introduction of the high performace Sanyo 3Ah 'C' cell, NiCads have become a real alternative to the usual lead acid battery. Had planned to use 36V this year. Using three sets of 3Ah NiCads in parallel, we would still save 2 kg over the two Hawker Genesis 13 Ah Lead Acid batteries.
Bought some more Sanyo N-3000CR NiCad 'C' cells from Lesro
Models update - now available from technobots.co.uk and made up some packs to supplement the packs we used on beta. See
pic on left. Made up with Deans bars and card separators bonded in with contact
adhesive. Once they were stuck together and soldered up, they were covered with
two layers of 3.5" heat shrink tubing.
We had problems running the Vantec speed controller at 36 volts however, so decided to play it safe and stick with 24 V, using the four 24 V packs we use on beta in parallel. This saved more weight and left the possibility of removing one of the packs if we needed yet more weight for adaptations.
As a performance comparison, a 3Ah NiCad can deliver 30 Amps for around six minutes according to tests, so three sets is parallel can deliver 90 Amps for six minutes. A 16 Ah Hawker Genesis Lead acid battery can deliver around 90 Amps for 5 minutes according to the manufacturers figures. As you can see, NiCads are much more efficent at these sort of discharge rates, still pretty near to 100%, whereas the best lead acids are only able to deliver 40% to 50% of their quoted capacity. The situation is even worse for lead acids at Battlebots, with the matches only lasting three minutes.
Two Genesis batteries (24V) weigh 12 kg. Three sets of 24V NiCad packs weigh 5.5 kg. Three sets in parallel seems to be sufficient for powering just two drive motors like Boschs. If you are also powering a weapon or are using skid steering, you may need more sets so that you are not drawing too much current from them. Around 100 Amps seems to be a sensible current limit for short periods. Even higher currents can be achieved if the cells are actively cooled.
Note that while in RC Car racing it is essential to have well matched cells to get the best performance, this is not required for robots. With NiCads in robots, you generally are not limited by capacity, but by current handling ability, so you generally do not get close to fully discharging the cells, which is where matched cells become important. With unmatched packs, you should not discharge the pack below around 0.9 Volts per cell, or around 18 Volts for a 24V pack, to avoid overdischarging any cell in the pack. Also note that the vast majority of other sizes and makes of NiCads do not perform nearly as well as the 3 Ah and 3.6 Ah Sanyo 'C' cells. These cells have a low internal resistance (3.4 mOhm for the 3Ah) that means you can draw high currents from them without them overheating. This is not true of many other cells. Make sure you compare the internal resistances if you are considering using any other cells.
With the weight gained from the switch to NiCads, we added some anti-Fluffy
guards, bent up from 4 mm Stainless Steel. The side skirts are still 6 mm
stainless. Hopefully we'll change these to 4 mm Titanium next year and save a
bunch of weight.
Wheelie bars. To prevent the nose lifting when the axe fired, I welded some
stainless bars onto the back 'nose'. Made a big difference. Last year we were
only running 90 psi. This year we were safely running at 140 psi for most of the