Deathly Hallows - FW - Alligrater

[Deathly Hallows]

I think I have cracked it. - Alignment.

There was a wobble in the wheels, where the sprocket was bolted to a spacer onto the hub. Also the drive sprocket and the wheel sprocket were not aligned precisely. Now cumulatively this can't have been that much a few mm. But I ran it full speed forward and back a few cycles no problem. Going from max power to stop does generate a clunk but that can be the chain going from teapnsionnone direction to the other.

Again I'll add photos soon at the momentum need to get back and repeat process on the other side!

[Deathly Hallows]

I think it's fixed. Tests at full speed on both drives, and rotations. All tests on the stand but I am fairly confident it's fixed.

Here's a video of it at low speeds. Didn't want to be standing in front when this thing spins up, just in case!

I am not as happy with the right hand side as the left, ( as you look at it) but time is of the essence and I don't have the hours left to be pedantic.

After this video I laid out the electronics again. Finalizing the positions, I am using the filter foam under the battery and around most of the sensitive stuff. The battery will need some form of retainer but I'll address that once I have the front on and the lineac hooked in properly. That's my main task for tomorrow.

[Deathly Hallows]

Disaster!

Still throwing the chain. I am getting desperate now.
The chain is so tight I struggle to fit it, yet once on it flexes enough to derail.
Clearly after testing on the stand and it worked, then simply fitting everything else back in and failing meanwhile too sensitive for combat. One knock and it would fail.

I think the only thing I can try now is to move the motors them selves.
Fit the chain and then bolt in the motors so there is no give at all.
With a job it's hard to imagine I can achieve this plus the open topics in time for the weekend now. Grantham is looking doubtful

[Roboteernat]

did you calculate the correct centre spacing for the axles when choosing chain length? The best method would be to fit the chain then the motors down.
not the one im looking for but this will help:
http://www.botlanta.org/converters/d.../sprocket.html

[Deathly Hallows]

did you calculate the correct centre spacing for the axles when choosing chain length? The best method would be to fit the chain then the motors down.
not the one im looking for but this will help:
http://www.botlanta.org/converters/d.../sprocket.html

Thanks for the input. I am such an amateur, I'd originally considered having the drives mounted so they could slide back and forth to take up tension before fixing and this would have been a lot better.

Trying to continue now at this stage is more effort, but I will try.

I tested all the electrics again and it's all good. Still need to build the ramp have around 600g left for that which seems short but we will see.

[RogueTwoRobots]

For what it's worth, featherweights at heavyweight events aren't really weighed. There's a good deal of leeway as the meleés are just for fun so no-one is going to be nitpicking if it's a few grams overweight (or even a few hundred grams, unless there's a competition run). Hopefully this will free up some time for you to put towards the drive issues.

Is there any side-to-side wobble on the wheel sprockets or motor sprockets? This will be a contributor to throwing chains, with the other one being the sprockets mounted eccentrically (not concentric to the shaft) so that the tension will increase and decrease as the wheel rotates. It's a difficult thing to get right without decent tooling. The side to side wobble can sometimes be sorted with strategically placed washers, but eccentric mounting is harder to rectify. Having chains really tight isn't good, you want a little slack in them but obviously this is exacerbating the problem too. Not much to do except keep plugging away to find the right positioning that gets it working

[Deathly Hallows]

Not much to do except keep plugging away to find the right positioning that gets it working

Thanks Jamie - I really appreciate all the support and encouragement from everyone. Been a tough few days felt like giving in a few times and starting again.

But finally it seems I have it sorted ( but I thought I had it sorted before). Took her for a spin around the patio, controllable and fast. This was with the electronics just laid inside as a quic test.

Assembled fully again and now the ramp is rough fitted to the linear actuator. Sorry no pictures again but they'll come.

I'd hoped to add some fun design elements but not looking like I'll have time. That said this time yesterday I thought I'd never be ready.

Building robots is an emotional rollercoaster.

[Ocracoke]

Building robots is an emotional rollercoaster.

Just wait until it is in the arena. I find myself getting really nervous beforehand and doesn't really pass until about a minute after 3...2...1.

[Deathly Hallows]

Today is not a good day

Alligrater was fully assembled and being tested when the chain came off yet again! Repeated hard testing on the bench seems not to be the same as driving it on the pavers.

It doesn't stop there however, as the self righting mechanism also fails to deliver. The motor was rated at 17.5N @12V which I imagined would lift the 13.6kg but it's no where near the power needed I failed to consider the lever point reducing the effectiveness. Even though I am running 18V.

Finally the weight (though this was not critical for the event) was 14.12kg so 500g over.

I won't be going to Grantham. Living in Scotland it's too far for what is most likely to be 5 seconds of combat with this build.

That said I am not giving up. But it's back to the drawing board.

Here are pictures of the current design:

Front view ramp open to vertical
Aluminium C bar
IMG_20180621_174514.jpg
The polycarbonate sheet has a little give to allow it to scrape the floor
Behind that is some grade 5 titanium
The idea was to be able to replace the polycarbonate or have different designs depending on opponent
IMG_20180621_174526.jpg
At the back you see the alive light
Had to drill the holes for weight saving
The kill switch/jumper is an anti-spark XT90
IMG_20180621_174601.jpg
Wedge fully open at I estimate more than 120° from the rest position

Trying to cheer myself up. Looking for positives. If I go quiet next few weeks it's not me quitting but I have three work trips and then a summer holiday so could be August before I can be very active again

[Ocracoke]

Oh mate, sorry to hear. It's looking really good as well.

[Wkdintent]

It's a shame about the chain and self right, but at least the rest of the build looks great.
For the LinAc, I have been using a free trial program called Mechanical Expressions. It's 2D, and the modelling is done with simple nodes, lines and coordinates. Add a load of 13.6kg, and it'll tell you the force on the LINAC.

If you decide to give it a try and struggle, I can try and lend a hand.

[Deathly Hallows]

Back from my holidays and back to the drawing board and time for some reflection.

Lessons:
The scooter motors are too heavy for anything but a push in this class (as I was advised)
Got the electronics working woohoo!
Realised chain drives are a lot tougher to get right than I expected
All the nuts, bolts and washers add up to weigh a lot more than I expected

Due to the number of failings I am going to park this concept and try something new