Big Dave 3 Design & Build

After several months of trawling forums, the RioBotz handbook and google, myself and two of my friends are ready to start building Big Dave 3, our first Heavyweight robot.

Where we're at:
We have the basics of a design in Autodesk Inventor. We're intending to get each of the plates (there are 8 main plates) cut out of 7075-0 aluminium. We have yet to decide on a material for the drum, which we're intending to get cut in sections and then bolt together to save cost & machine time.

We've selected drive & weapon motors. Alien 8085's and Turnigy T203T's respectively.

Drive: 5:1 via spur gears. Wheels are 11x7.10-5 GoKart tyres, driven with a keyway'd 25mm axle. We're hoping to get axles made up from a local lawnmower racer.
Weapon: 6:1 via toothed pulley. 95% of full speed is just a hair under 11000RPM.
Batteries: 8 x 8400MaH LiFePO4 batteries.

Things we're slightly worried about:

• Weight! Autodesk puts the framework (seen below) at 89Kg. Once we add in the bolts and electronics, I would image that would put us about 95Kg, without armour. The obvious solution to this is to mill pockets in the side and back plates, which could save us a good 10-15Kg. However, the machine time for that would be crazy expensive.
• Weapon Drive - we're intending to use a toothed belt that's slightly overtensioned (.5mm), which might tear when the drum hits something.
• Clearance, Clarence. There isn't that much room between the edges of the tyres and the side plates, nor is there much room between the drive motor and the gears. When running inverted, there's a 5mm ground clearance.
• Bearings. None of us have the slightest clue how to design bearing housings in the side plates in a way that doesn't need machine time.

https://lh3.googleusercontent.com/cX...c=w871-h548-no

Unfortunately, it's just a screen grab with none of the gearing shown, because trying to render something with that much detail screws up my laptop!

Next steps:
Buy everything once we're 100% happy with the design.
Find a waterjet/laser cutting service, as well as a local machine shop/school/university that will allow us to cut pockets in our plates.
Design & Print the battery housing.
Start assembling!
Drive Test & Tune.
Weapon Test & Tune.
Armour.

Trivia: Back in 2004/5, we built/helped to build several autonomous robots for the "MicroMouse" competition: Lil' Dave, Dave, Big Dave & Big Dave 2. Thought we may as well continue the naming tradition.

Great detail but brushless drive in heavyweight robots is still relatively un proven technology but best of luck.

Good luck, looking forward to following the build

The drive has a WAY to low gearratio.
Better using higher KV motors and a gearratio of 20-1 or so.

Even Bosch 750's struggled with a 5-1 gearratio to 250mm wheels.

And if you think speed is a problem.
Bullfrog uses a 20-1 gearratio and 250mm Full foam wheels, with 170 KV 120mm diameter motors on 10S.

Quote:

---

Originally Posted by maddox10

The drive has a WAY to low gearratio.
Better using higher KV motors and a gearratio of 20-1 or so.

Even Bosch 750's struggled with a 5-1 gearratio to 250mm wheels.

And if you think speed is a problem.
Bullfrog uses a 20-1 gearratio and 250mm Full foam wheels, with 170 KV 120mm diameter motors on 10S.

---

We can put a 15:1 ratio in without much bother, but even with higher KV motors (250, 80mm) I'd be worried about how slow we'd go - with luck, that might maybe touch 10mph. With using higher ratios, I'd also be a bit worried about trashing the gears at a higher ratio.

What sorts of speeds should I be looking to realistically achieve?

All three of my heavies have been geared from 12-18mph some go higher. Starting amps would be your issue if you gear for high speeds

Gravity 3 was geared for 15km/h.
Tough as Nails for a smidgeon faster, 18km/h
Bullfrog can get up to 14km/h.


Lets take a hypothetical setup.

250KV, 37V 250mm wheels.

With a 5-1 ratio you end up with ludricous speed.
0.25m x pi x (280x37)x60/1000= 97km/h

With 130KV motors, it's still more than 40km/h

With 130KV motors and a 15-1 gearratio you're back in the normal range. Between 12 and 20km/h.

Some machines are build to be faster. But getting the power to the floor is a problem.

I cant see the pictures?

Quote:

---

Originally Posted by maddox10

With 130KV motors and a 15-1 gearratio you're back in the normal range. Between 12 and 20km/h.

---

I get 11.2km/h with the 15:1 ratio with the 130KV motors, 16.4km/h with the 250Kv motors.

Quote:

---

Originally Posted by adamclark

I cant see the pictures?

---

Are you on a Mac? I've tried it on PC/Android/iOS and it displays, although it doesn't work on my work Mac.


Has anyone ordered from Alien Power before? Almost everything shows as out of stock.

Then the 250 KV motors would get my preference.

I ordered in the past with Alien Power, and know of others.

I ordered a lot more with Hobbyking.

Ah, I'm having almost nothing but problems with Hobbyking it seems! Finding out the mounting pattern for the weapon motors is a lost cause - I'm ordering the motors before the mid plates just to make sure I've guesstimated right!

Quote:

---

Originally Posted by emisnug

Are you on a Mac? I've tried it on PC/Android/iOS and it displays, although it doesn't work on my work Mac.

---

Tried on all of the above, gives a no-entry sign as its some protected google doc/image

How strange! Try an Imgur link: http://imgur.com/StqVqh3

For those wanting to keep up-to-date with this build, you can find all the latest on our blog at https://www.arc-robotics.org.uk/category/big-dave-3/

We'll try and remember to post here as and when updates arrive.

Ali - our driver - has just posted a (fairly lengthy) commentary on what driving our rolling chassis is like.

https://www.arc-robotics.org.uk/2016...ve-big-dave-3/

Over the next few weeks (until we get back into the workshop in early January), we'll be explaining some of the logic behind what we're building, and how we're keeping everything on track to be ready in time for the next series.

Given how little time we can actually spend in the workshop, we're quite keen to make sure that we maximise the use of that time. That means that while I'm preparing electronics in Southampton, Rob's getting parts made in Norwich, and Ali's working on actually giving the team that professional edge in Leeds. As long as Rob's done his numbers right..... it'll all just about fit.

So, we've ignored advice from this fine community for at least a second time by trying out a HiFei speed controller rather than dropping £500 on an MGM.

So far, so good - I appreciate we haven't put much load on it yet (~15A) compared to what will be a considerably higher current when we have the weapons motor hooked up to the drum. You can view my testing at my flat in Southampton on our blog: https://www.arc-robotics.org.uk/2016...weapons-motor/

Next workshop weekend for us is 7th/8th January, so we'll see how far we can get. There's been a slight hiccup in the drum department - but I think Rob (emisnug) is on that to get it sorted asap. All being well, we'll be spinning everything up soon....

Thanks to all you wonderful people on here with advice over the last few weeks, Rob's just been frantically redesigning the front end of Big Dave 3.

Here's what he had to say on the matter: https://www.arc-robotics.org.uk/2017...bout-bearings/

How are you preloading the taper roller bearings in the design?

SKF are just over 300 meters from my workshop, if you want any advice on applications i can have a word for you.

They are very approachable in the factory.

Quote:

---

Originally Posted by cmalton

Thanks to all you wonderful people on here with advice over the last few weeks, Rob's just been frantically redesigning the front end of Big Dave 3.

Here's what he had to say on the matter: https://www.arc-robotics.org.uk/2017...bout-bearings/

---

Bearing preload wise, I'm leaning towards a beville washer running an ever-so-slight preload. However, preloads are something I have never, ever done before, and if you've got any experience with them, I'm all ears. Currently, they seems pretty straightforward, which normally means I'm missing something huge.

Katch, I may well take you up on that....

Attachment 6388

Taper rollers arrived today. They're scary levels of large.

Needs a banana for scale

Quote:

---

Originally Posted by shakesc

Needs a banana for scale

---

Aye, but I ate it

Big ol' bit of steel that will become our drum arrived today. Still a fair amount of material to take off and fun things to machine in (anyone know a good lathe person?), but it's big and heavy and deadly even in it's current state of being my footrest. Again, no banana for scale, because I needed a snack.

Attachment 6399

We did some more things last weekend, it's all over on the ARC Robotics blog:

https://www.arc-robotics.org.uk/2017...uary-update-1/

I can see you have HiFei’s Fulcrum Pro 300A Controller for the weapon but what did you end up with for the drive?

Quote:

---

Originally Posted by shakesc

I can see you have HiFei’s Fulcrum Pro 300A Controller for the weapon but what did you end up with for the drive?

---

Chris chose the 180A Turnigy Marine ESCs I believe. As our motors draw 150A at stall, we're running an OK safety margin on them, but Chris is the guy for details on that (cmalton on here, I believe)

Chris - we ended up on Turnigy's (supposedly) water-cooled 180A ESCs for the drive at the moment.

One of them doesn't like how much power we're pumping through it in extended tests - but we think that's actually a mechanical issue as the drive is much more reluctant to turn on one side.

Hopefully we'll fix that fairly quickly....

Was in the middle of replying - Rob beat me to it because I'm currently in the middle of a field running paintball/airsoft/laser tag events.....

After a fairly short Saturday in the workshop, we now have an idea what this creation is going to look like...

The word "Yikes" sprung to mind when we managed to get things into a test fit for the first time.

Attachment 6426

There's a (little) bit of milling work that needs to be done on the drum before we can mount that properly, but we're probably going to send that to one of our new-found machine shops (because they're proving infinitely more reliable than our previous choices).

We sent them the files for some parts on Sunday last week..... and I picked up the finished parts on Friday.

Attachment 6427

The price wasn't bad either for a 1-week turnaround.

We are so painfully close to the point where it's no longer "safe" for us to continue testing...... and when we get to that point.... we know we're onto something.

Time for an update.

Things have changed. A lot. After a third attempt at getting on Robot Wars and some drivability problems with Big Dave 3, we sat down and had a little (almost complete) redesign with input from all three of us to design something that we're both proud of and is still the same levels of destructiveness...

Presenting Big Dave 3 (MK3)

https://scontent-lhr3-1.xx.fbcdn.net...3d&oe=5AA965A0


And even then, it's somewhat changed from the CAD above - now with some more armour sections and a different tread pattern that I wasn't able to render because my computer is now 9 years out of date.

So - the drum, the taper roller bearings and the 100mm axle are all the same. Everything else is modular (because modular) - two drive pods and an electronics enclosure. The drive pods are exactly that - everything we need for the drive sections - two motors, 4 sets of gears, 4 belts and a couple of wheels and many bearings. Changes are as follows:

• We've changed from the Alien Power 8085 motors - we're now running quadruple Turnigy SK3 6374-192Kv motors and coupled them to an 8.75:1 gearing arrangement, allowing us to run at 16MPH with 16Kw/21Hp on the ground.
• The wheels are now cast with 50A Durometer rubber
• HTD Belts! (Because of an inhernent dislike for chains)
• Slightly longer impactors (249.31MPH mechanical tip speed)
• 44.4V (12s) throughout
• There's a wedge
• Polycarbonate top panel with flexinessâ„¢ to absorb hits
• We've decided on an Orange/Black colour scheme
• Marine anodising is happening for extra hardness
• Titanium impactors! We're fully aware that Ti isn't an ideal material for this, but white sparks!)

Our good friends over at GSD Tech in Southampton have started machining parts for us - we should hopefully have a fully working robot by the end of the year.

https://scontent-lhr3-1.xx.fbcdn.net...66&oe=5A029E45

Well, Robot Wars has not been renewed on the BBC. Sad news for sure, but that doesn't stop us! Progress continues.

Big updates: we have a chassis, weapon, batteries and a renewed sense of purpose and drive to "win" the live circuit. The chassis is relatively simplistic but mostly together - our friends over at GSD in Southampton have been sense checking us left and right to make sure we're not being silly. Of course, after several screw ups and far too many hours on weight distribution, we're deliriously thankful for their help. The weight distribution is now 60/40 front/rear - to counter act the increased weight at the front, those wheels will be a harder durometer rubber. We're also pretty sure our 15mm ground clearance opens us up to flippers!

Attachment 7884Attachment 7885