Joint Strike Minisumo

[bob.cook]

I was thinking… anyone up for making this a joint venture?
Joint Strike Minisumo

Sounds good to me, I'm definitely in for a joint venture! After the mechanics are done we'll need some electronics and I'd be happy to put together a custom board for it if there is interest. Although I'm quite fond of the Atmel processor line, not sure if that works for you.

Last but not least. If you don’t mind I’ll send you a mail with the specs on the wheels. Can you read STL files or would you prefer something like eDrawing???

Uh, sounds very technical. :^) How about a drawing with measurements? I'm a low-tech machinist. On the other hand, if you send me files I will try to find a way to read them!

- Bob

[sap1975]

Colin- Thanks for the word on my English. I’ll have to see if i can live up to that

Yes please. Having read Bob’s latest post i think a public area for the “JSM” project would be great.

Bob- that sounds just perfect. Now all we need to figure out is if there’s an interest besides the two of us.
I don’t have any experience with the Atmel processors. I use Picmicro mainly because it’s the first and so far only microprocessor i actually know anything about. But if it can be “re-programmed” using a serial interface and it has onboard UART’s, ADC’s and perhaps PWM ports that would be fine by me
Ohh. one more thing, i will normally program the Pic’s using Basic. Do you know of a Basic compiler for the Atmel?

Regarding the wheels. I was originally thinking of the Maxon motors off the banebots site http://banebots.com/c/MP-MISC but if you or anyone else for that matter have any alternatives I’d be glad to hear about them. We kinda need to know what the shaft looks like before we start any work on the wheels.

[bob.cook]

I don’t have any experience with the Atmel processors. I use Picmicro mainly because it’s the first and so far only microprocessor i actually know anything about. But if it can be “re-programmed” using a serial interface and it has onboard UART’s, ADC’s and perhaps PWM ports that would be fine by me
Ohh. one more thing, i will normally program the Pic’s using Basic. Do you know of a Basic compiler for the Atmel?

The AVR chips support all that built-in h/w and more. As for Basic, yep some folks I know use BASCOM-AVR, it seems to work very well and there is a free version available (limits the amount of code it will compile, iirc). Buying the full version is quite reasonable as well I believe.

Regarding the wheels. I was originally thinking of the Maxon motors off the banebots site http://banebots.com/c/MP-MISC but if you or anyone else for that matter have any alternatives I’d be glad to hear about them. We kinda need to know what the shaft looks like before we start any work on the wheels.

I tried to order some of those motors tonight but their website is giving me an error when I tried to create an account. Hopefully that will get sorted out soon and I'll have them in my hands in a week or so.

- Bob

[slurp]

Yes please. Having read Bob’s latest post i think a public area for the “JSM” project would be great.

I'm disappearing for a few days, I'll sort it on my return - Forum space is easy but I think I'm going to install a wiki, that might be more suited to documenting the project along side the discussion but it might be a better way of handling the rest of the website in general.

I'm happy to help with the project where I can but I think with my other sumo on the go I've got enough on my plate - motor at $39 are a bit more than I want to spend at this time.

regards,
colin

[sap1975]

Thanks Colin.

I know we went over the cost of the motors earlier but we need to have some umph for this to work.

Well… I guess the next step for me is to order a couple off motors too and send my input on the wheels to you Bob. And then it’s back to the mill. So far we only have the one part

[sap1975]

I have an idea for securing the rim to the shaft using a grove in the motor shaft and a circlip. But that requires making a D-shaft adapter/mirror in the rim as I’m trying to show in the picture. I hope you get the idea if not please let me know and i'll try to elaborate on it.

[PeterW]

This is a quick sketch of how I have fixed a similar wheel onto a motor in one of my antweights Militant. The M3 grub screw has a 1.5mm Allen socket to enable it to be tightened onto the shaft flat and I dont stick the tyre down around hole to enable it to be removed.
http://i109.photobucket.com/albums/n66/ ... /Wheel.jpg
It was made out of Delrin plastic and stood up to full spinner combat quite well. In aluminium it would be even stronger.
This is the actual robot
http://i109.photobucket.com/albums/n66/ ... CF0302.jpg

[bob.cook]

I was definitely thinking along the lines of what Peter's illustration shows. Using a set screw makes it quite easy to put on or take off, and it can be quite secure when fastened well.

I notice all the diagrams of wheels always bore holes in the face. I've always assumed this would be to save weight but in the case of a minisumo is this really a good idea? Don't we want to increase the weight of the wheels?

Which leads me to the question of how heavy can or should the wheels be? I can fabricate them out of Delrin or aluminum or brass, and by varying the thickness of the wheel wall we can get something that is quite heavy or something that is quite light.

- Bob

[sap1975]

Since i don’t have much experience with this stuff and you guy’s do let’s go with the set screw. I can defenently see the advantages of that over the version i suggested.

Regarding the holes in the wheel face i think its mainly for show and i think you’re right Bob getting as much weight over the wheels as possible would be preferable. I believe that means going with brass and a thicker wheel wall. Which in turn leads us to motor mounting.
I currently don’t have a clear idea for that one but it might be time to start thinking seriously about it

Status: The second side is coming along nicely. I did however discover i made an error in the CAM part of the first one so I might have to re-do that one.

[sap1975]

Ohh… by the way.
-Bob i assume you would prefer a set of aluminium sides instead of plastic ones. So i’ll try to get a set done for you (or me) as soon as i’m done with this set.

[slurp]

Which leads me to the question of how heavy can or should the wheels be? I can fabricate them out of Delrin or aluminum or brass, and by varying the thickness of the wheel wall we can get something that is quite heavy or something that is quite light.

The answer to this is based on the weight distribution on the rest of the minisumo... heavy wheels may give a good center of gravity with respect to increasing the available push for any given co-efficent of friction but where the moment of inertia, when starting the minisumo, can not be ballanced by the rest of the chassis then you'll see the front pick up as you drive or worst case the 'bot flips over.

Mass close to the wheels is good but we don't want it all in the wheels - that question of ballance once more. The second part of the ballance might be a little of a sticking plaster fix, speed control to restrict the rate of accelleration but I'd take this as detrimental to over all performance.

regards,
colin

[PeterW]

I agree you don't want all the weight in the wheels but on LowBlow the wheels which are 20mm dia and 20mm wide solid brass only weigh in at 55gms each so there is plenty left even when the whole robot in this case is only 260 gms.

[slurp]

I'd guess you've got a CoG about 30-40mm out from the axle where as the wheel has it's mass within millimetres... the ratio here is good in terms of the accelleration that can be applied before the overturning moments are ballanced.

Your point I guess is that even with brass the mass may not be significant in the over all weight... particularly as the motor is only 13mm diameter and the wheels may be a similar scale to LowBlow's!

I'll shuffel off with my theory toughts that are more significant on larger minisumo robot applications....

best regards,
colin

[sap1975]

Hi Guy’s

We have been doing a bit of mail exchange lately so not much has been posted.
But we agreed that we should post the contents.

It’s in reverse but here it goes.

Hi,

I thought about it some more and think its ok if the motors are held
only by the clamp on the back end of the motors. The motors are 41mm
long and if the wheel extends back ~30mm from the shaft end that leaves
plenty of space to "grip" the motors and hold everything securely. I
also figured out a way to make the clamp myself but it would have to be
square... :^)

I figure I can bore out ~23mm to fit over the motors, leaving 7mm at the
end to secure to the motor shaft. I make that at 5,553 cubic
millimetres. That would give 14.9g for aluminum and 46.5g for brass. So
quite a bit of difference and it might be that the brass is too heavy.
What do you think?

No problem posting to the forum. I'll pick up on the forum if you post
this stuff there.

- Bob

Stig Ankjær Pedersen wrote:
Well. i think i can get close enough to make it work. Maybe we will
need an additional cross member (is that right) on top of the motors to
make sure they stay level? what do you think?
If the "cross member" is to be installed im thinking a hight of about
1.5mm which would then leave a wheel inner diameter of about 16mm if
we have a 1.5mm clearance. Provided that the motors are actually 13mm
diameter as stated.
I had originally targeted the entire wheel to be 25mm with a 2.5mm
"rubber" coating so if we go for that we shold get something like a
4mm wall.

My math isn't really good enough to come up with an estimate on the
weight off that.

I was also thinking about doing the bottom plate out of mild steel
instead of aluminium to add weight plus it would keep a low centre off
gravity.

Just for kicks i did a fast calculation on the weight if we where to
do the bottom plate in Tungsten.
270 something grams

Last but not least. would you mind if i posted our mails on the forum?
it's been a while now.

Regards

Stig.


----- Original Message ----- From: "Bob Cook" <bob@bobandeileen.com
To: "Stig Ankjær Pedersen" <sap@apsys.dk
Sent: Sunday, November 11, 2007 11:13 PM
Subject: Re: Joint Strike Rims


Opps, I hit send before typing anything!

I spent a couple of hours on the lathe today. I ended up scrapping the
piece I was working on. I did however learn a few things, and I have
concluded that your clamp is a much better way to mount the motors, as
there won't be enough clearance for the T style bracket I had
envisioned. The only problem is that I can't fabricate it! Can you?

I eventually got a Banebots account set up and placed my order. My
motors should arrive in the next week or so.

- Bob





Stig Ankjær Pedersen wrote:

Hi Bob



Funny thing… i replied to the forum before i saw your mail.

In the forum i wrote i didn’t really have a plan which is not entirely
accurate. I just think we should get as much input as possible. (you
know… two minds are usually better that one). I’m having trouble
visualizing the T frame so if you could explain it a bit more detailed
i would really appreciate it.



Anyway here’s what i had in mind. Just securing it to the bottom plate
should do.









Regards. Stig.

[sap1975]

Colin- If you ask me any input is good input. It may not all be correct or relevant but usually two or more minds work better that one.

Bob- the reason i thought up the cross member is that in the original design the wheels where 40mm long. Hence leaving only about 16mm for the clamp (1.5mm clearance).
From my limited experience the wider the wheel the better the traction. And i don’t care much for theory on this one. There’s way too many variables in the real world

To me the weight sounds just right. Personally i’d much rather have the majority off the “ballast” weight as close to the wheels as possible. And if they turn out to be to heavy we can either lighten something else or the wheels. To me it sound like keeping our options open.

I haven’t gotten much work done on the mill but i hope i get the time this weekend