After a few months of hiatus, Big Dave 3 is back! So…. what happened? Well, we at Team ARC had a little disagreement with some metal and some bloke called Issac. The original Big Dave 3 had a huge drum, held in place with 15mm plates. It stuck out of the robot body far as we needed it to be without hitting the inside of the robot, was driven with big 80mm brushless motors and had some pretty rudimentary reduction to drive some much-hated and hard fought Go Kart tyres. We got a long, long way with it – many driving tests, which we’ve posted on here before, and even a 20% speed weapon test – which is below.

 

But. Gravity was afoot. The community and Chris told us that it wouldn’t drive with the drum on the front. So we tried it. Lots of wheel spin, but no drive. We tried bolting extra grip to the tyres – it worked, somewhat. However, a lot of wheel spin, and the math told us that with the drum at 50%, it would start to lift on one side – dramatically. So, Rob spent a good couple of weeks designing the new Big Dave 3 – same drum, but a lower centre of gravity (a little less lifting & more drivability), a bit smaller (W:700 L:630 H:121), more reduction ratio with more easily available motors, the venerable Turnigy SK3 6374. It also meant that we could SimonK the ESCs, but I’m sure that Chris will be along soon to explain that.

So… the redesign cometh.

The redesign was based around 3 core principles:

1. Reliability
2. Modularity
3. Keeping the Drum

Immediately, we all decided that having brushless for the drive was still one of the things we still wanted to do due to the immense power density that they offer. After some quick Googling, we settled on the 192Kv version of the Turnigy SK3 6374. Easily available from HobbyKing, relatively quick to ship (unlike the Alien Power motors, which took over 6 FREAKING WEEKS to arrive), and only an 80A peak current draw…which meant that ESCs were more easily available.

Secondly, the modularity of the robot – we’ve designed it around the ability to remove and replace sections as quickly as possible. It should be possible to strip and rebuild the entire robot within the 2 hour allotment that Robot Wars gives us. With only a few bolts holding the 8mm polycarbonate sheet in place, we can gain access to the interior of the robot within seconds.

If you’re wondering about those few bolts – why not bolt in as many places as possible? Well, in order to absorb energy (in our case, shock loads), polycarbonate needs space to flex. With only a few fixing points, that allows the material to bend and flex and not shatter. Having loads of mounting points creates weak sections in the polycarbonate, which might cause it to break.

However, back to the modularity. There are 3 main sections to the updated Big Dave 3 – the Drive Pods, the Weapon Assembly and the Electronics Assembly. As this is just a basic overview of the update, I’ll gloss over many of the points and focus on the big changes. The first big change, of course, is the drive system. We’ve got 4 independently controlled drive systems, all of which talk to each other to deliver 21HP to the ground, allowing us to go to 16MPH with close to 130 newton meters of torque/pushing power on the ground – about the same as a base model Ford Fiesta. It’s a 2-stage system, all driven by tensioned HTD belts. The first stage is all HTD3M 9mm belt – which, of course has the lowest breaking force of anything in the drive system, meaning we hopefully won’t screw up the motors. The second stage is all HTD8M, with each belt rated to transfer 10Kw of power – plenty of safety margin for us. The entire system is an 8.75:1 reduction.

Hopefully, by having 4 independent systems, that will give us some semblance of reliability in the drive system.

The wheels are, quite honestly, very simple pieces of rubber and metal. We’re going to pour 50A durometer rubber around the wheel (in a mould being made by Gary of PP3D/Crossfire/Typhoon fame) and then bolt that directly to the final driven stage. It should give us enough give in the wheels for a decent amount of grip and should allow Ali to push other robots around. The bearings that everything rides on are held in with bearing locker loctite and have a few millimetres of play so that, on impact, we’ve got some wiggle room. A great part about having belts (as opposed to chains) is that they’re somewhat self-centering and are more resistant to off centre rotation – so even if our drive pod is more battered than a cod at a fish & chip place, it should still work.

Weapon reliability is made up of 2 parts – mechanical and electrical. I’ll leave it to Chris to explain the nuances of 1200A+ start up amperage as I’m not too sure how it all works, so I’ll stick to the mechanical side of things. The weapon motor is the same – a 12HP brushless inrunner with a 40mm V Belt pulley in it, driving the V belt groove in the drum with a slightly-too-small cogged V belt. This will give us some proper oomph when we’re spinning up (see the video above) to full speed. The keen eyed reader might notice 2 V belt grooves in the drum – those were left over when I was going to put 2 motors on the drum, but Chris has talked me out of that. It’s now a redundancy feature – if we screw up one side, we’ve got the other. I’ve changed the drum mounts though – they’re now 38.1mm thick (read 1.5″) and are mounted with some hefty M20 screws. They should be able to withstand pretty much anything we meet. Unless Sir Killalot is in a bad mood, or Carbide is upside down.

The drum itself hasn’t changed a dot. It’s still got huge taper roller bearings riding on a 100mm axle. It’s still made out of EN24 steel and weighs a ridiculous 40Kg when everything is attached to it. With nearly 100Kj of impact energy, the drum is deadly – but now, it’s controllable. It’s got a lower centre of gravity (behind the front wheels), a section cut out of the baseplate so it’s nice and close to the ground. A new thing though – our impactors are now Grade 5 Titanium.

Why?

Glorious white sparks!

And it should be bloody devastating. 

Until Next Time
– Rob