New Featherweight Robot (Beginner)

Hi all,

Having watched Robot Wars a lot as a kid, and being inspired by the resurrection of the series on TV, I decided to have a go at building my first featherweight robot. I have a background in Electronic Engineering, so will probably have less of a problem with the electrical parts and more of a problem with integrating into a machine! Here's what I picked up so far:

- Plastic is a good starting point, will use this for at least the prototype robot and will purchase thicker armour should the concept work
- Ordered a radio controller, batteries, receiver, ESC and motors, are there any other parts I will need? For example, additional wheel drive components and any failsafe/current limiting parts.
- Are there any good batteries that you have used in your machines?
- What works best with machining and drilling plastic?

My idea is to create a relatively low wedge shape with a lifter powered by a geared down motor and a wide base, either 4 or 6 wheels (2 driven). I've had a look at build threads similar to this design.

I'm renting in Hertfordshire (work in Stevenage), so if there is anyone close by I wouldn't mind meeting up to have a look at existing machines and how they have been built. I've found it's the best way to learn :)

Cheers,
-Rob

Ok, first things first, what plastic are you using? Folks around here tend to suggest HDPE, it's super resistant to impacts and can be machined with basic woodworking tools (that is, saws and bits intended for wood). What batteries and motors did you order? Could use knowing what you've already got before commenting. For me personally, finding adequate wheels to fit to the motor shafts was a problem, you'll want to give that some thought.

I'm planning on using HDPE as I picked that up from other build threads. For now I have ordered 4x 3.7V LiPo batteries and the motors are 2x Philips DC Geared Motor, Brushed, 12V DC from RS (330rpm). I worked out with some maths that would give the robot reasonably steady speed with 5cm radius wheels. For the first iteration I'm learning as I go so will decide on different motors if those don't work out. So far I've gone for cheap to mid range options.

I don't think those motors are suitable. The standard for featherweights would be something that completes with http://ranglebox.com/product/saturn-...etary-gearbox/ mated with this http://ranglebox.com/product/775-18v-motor/. The Saturn 16 & RS-775 combo has approx 1375rpm at 8.5Nm of stall torque. The motor you've mentioned is 330rpm at 0.025Nm stall torque.

Typical batteries are 5s (18.5V nominal, 21V fully charged) lithium polymer packs from hobbyking or OptiPower.

My normal recommendation is to come along to an event and check out what other people have done. Unfortunately there will be a bit of silence at the moment as everyone collectively panics about RobotWars. Your best bet is to read through the build diaries.

Awesome. I am doing exactly the same thing, and made pretty much the same decisions as you - flat wedge, mainly plastic. I am in two minds about doing a lifter for my first bot - might just use all 13 kilos on a really solid pusher. We should totally have a battle when we are both done.

I think I'm a couple of weeks ahead of you so let me know if you have any silly newbie problems that I might have already come across.

Since i'm in a quiet similar build status, i'll just jump on here, okay?

Would Polystyrol also be possible for building?
It's way cheaper (or i only found expensive sources for HDPE) and at least here more available.
Or would that shatter or get ripped to pieces on first contact?

And on the Batteries, for the combination of motor/gearbox from ranglebox suggested in this thread, how much capacity should the Battery have?
5s come in a lot different sizes/shapes/capacities, so... how much would you need, suggested you only use it for driving and possibly some minor stuff like a servo or such?

I'm not sure where you've been shopping, but HDPE is really cheap on Ebay in comparison to other armour materials. If you're really struggling, you can even melt down plastic milk bottles and make your own HDPE sheet. I don't think I've ever seen polystyrol as a sheet, but my best guess is that it probably isn't good enough because I've never heard of it being used before.

As for batteries, mine is a 5s 3300mAh battery, rated for 65-130C discharge. My build is still in progress, but it's seemed adequate so far.

yeah, seems like ebay really is a lot cheaper than the 2-3 shops i looked before. didn't really search much, since when you see prices of more than 300€/m² on multiple shops, looking for alternatives seems more useful than comparing prices^^

No idea why those are that expensive...

Is polystyrol the clear sheet they sell in Wickes as glazing? I'm new to robots but OK at science and I think it would shatter too easily when hit. It might be OK behind something else for extra bulk, but not as standalone armour or anywhere it could take a point impact.

thx lowndsy, that was exactly what i hoped to hear. We don't have Wickes around here, but yeah, it's often used for windows in hobby and model building.
Since i'm gonna build a chassis with extra armor around (much like on full body spinners), looks like i could use it for that.
just from my feeling i bent equal weight of aluminium, Steel (simple stainless steel, since it happened to be in reach) and Polystyrol, and the last one was the hardest to bend, and didn't stay in a bend shape. so... could work if used just for the chassis, maybe with a hardox bottom plate (won't put extra armor there).

Any chance we could have a photo of this polystyrol? I'm rather intrigued.

I'd check with smarter roboteers than me, but For the chassis it might be too brittle? I am doing much the same by layering different materials for their specific strengths, but using cheapo chopping board as backfill - nothing structural, just to fill voids and stop things flexing.

I googled Polystyrol, and all I got back was polystyrene sheets. - http://modelshop.co.uk/Shop/Raw-Mate...-board/ITM4883 - There is no way that is suitable for any class of robot except maybe a flea weight I'm afraid.The same goes for the clear sheets from Wickes, which is acrylic sheet for windows and the like, very unsuitable.

https://www.directplastics.co.uk/hdpe-sheet - Are one of the main sources of HDPE for the entire community. Depending on the design you want at least 10mm on every side; this varies depending on the style of robot but to keep costs down I'd just buy one sheet and use it for everything rather than buying multiple thicknesses.

PS is used for jewel cases for CD's. Far too brittle and not good for armour or frame.

So, an update! (mainly maths)

I got 2x 250x250x8mm HDPE to use for spare material and gauge how heavy a bot made out of HDPE weighs. I came up with these dimensions and weights: Warning lots of numbers ;)

Robot basic wedge shape 500x600x70mm with cutoff for wedge at the front
Base armour: 300x600x16mm 2742g
Ceiling armour: 300x400x8mm 914g
Side armour: 2x 70x500x16mm 1066g (wedge shape averages out 70x600 and 70x400)
Inner panels for central motors/electronics: 2x 70x500x8mm 533g
Front wedge flat panel 212x500x8mm 807g (trigonometry base 200mm height 70mm)
Rear armour: 70x500x16mm 533g
HDPE armour total 6595g (almost 1/2 the weight!)

I won't bore you all with the rest, but I have the motors and Saturn 16 gearbox weight, then I estimated the rest. Without a weapon, the robot will weigh ~9kg with the above armour. With a lifter, thats another motor, scoop, and gearbox/gears which will likely add 1 to 2kg since it's a small scoop at the front. Looks like plenty of margin for reinforcing armour or adding more inner panels.

I will add some pictures later of the parts I have so far! The radio controller looks awesome and it's the first time I have held one in the flesh.

Just don't forget the screws, wheels and stuff like that.
Really, a handfull screws is heavier than you might think.

500mm x 600mm is an absolutely massive footprint for a featherweight! There are some heavyweights out there not much bigger than that. You could save a chunk of weight by dropping the size down to something more reasonable, such as 300mm x 400mm and cutting the baseplate thickness down from 16mm to 8mm, which will be perfectly adequate for a featherweight.

I support Jamie. Hannibalito 3 is BIG compared to most featherweights, and that 's 440mmx395mmx170mm.

Thanks for the feedback, will reduce the size down and up the height a bit to make room for the electronics.

i have 10mm hdpe on my bot and it took direct hits from the mini terrorhertz at full power with no issues

Quick update: Built a prototype out of scrap wood and whatever else I could find, and got the robot running with the Ranglebox Saturns/ESC/motor combo! Videos/pictures are on my phone so will upload when I'm feeling less lazy :P

So far its invertable and has a low profile, I cut the plastic base plate out and have just fitted the gearboxes in the proper place. Next up is cutting out holes for the wheels and starting to build up on the real thing. Hopefully the weight of the final chassis is low enough that adding a lifter/flipper is not an issue.