Joint Strike Minisumo

[Rick Brooks]

Stig,

At 6 volts the stall current is 1220 ma. Using Resistance = Volts / Amps, the resistance is about 5 ohms. I measured 4 ohms on one of my motors.

Using Amps = Volts / Resistance at 11.1 volts gives about 2.2 amps at 5 ohms, or 2.8 at 4 ohms.

So, I'd figure between 2 and 3 amps stall per motor.

And, yes, you can stall these motors with good tires.

Rick

[sap1975]

Thanks Rick
That’s great input and should get us close enough.
Just out of curiosity… isn’t there something about the resistance changing while under load. i have no idea if it qualifies for motors but i do know it happens in speakers?

[Rick Brooks]

Just out of curiosity… isn’t there something about the resistance changing while under load. i have no idea if it qualifies for motors but i do know it happens in speakers?

Stig,

The stall current is the worst case for current draw. The battery is simply pushing current through the wires of the armature.

I slept through my motors class many years ago, but I seem to remember that a permanent magnet DC motor acts like a generator all the time. The voltage that it generates is called Back EMF or something and is proportional to the rotational speed of the armature. The Back EMF opposes the battery driving the motor, so there is less of a voltage drop across the armature wire resistance as speed increases.

So, when you load the motor down, the rpms drop and it draws more current. The armature resistance doesn't change with load.

There may be other things that change the armature resistance, like heat.

If anyone remembers this stuff better that I, please correct me.

Rick

[sap1975]

Sorry about the late reply. I have some non robotic things to tend to.

So, when you load the motor down, the rpms drop and it draws more current. The armature resistance doesn't change with load.

That sounds very plausible and since no one has corrected you i think i will adopt it as Rick’s Law

I did the “new” Base plate last weekend and i haven’t been able to put in any work since then but i did get the wheels from Bob.
Now I’m no machinist but to me they look nothing short of beautiful work.

More Pictures.

[sap1975]

Come to think of it building robots/machining is an odd hobby.
Not including getting the design thought through i just spent the better part of 6 hours
making a drawing, doing the cam part, machining and polishing a piece of aluminium and what do i end up with?
Hmm… a small piece of pipe with a “block” still attached to the side. It’s a good thing it’s a clamp and i need it because otherwise it would be just downright daft

On the positive side it should be the last of the “complex” machining parts.

Still need to cut a slot, prepare the horizontal pipe and do a few holes and threads.

[sap1975]

I got the clamp done this ”morning” and it looks like it will work just fine.
Jon’s design is better no doubt but the clamp and pipe thing seems to do the trick just fine.

Based on the way Rick has mounted his motors i’m not that concerned with not using the intended mounting technique.
.

[bob.cook]

Wow that is a nice motor mount. I opted for something a bit more traditional. I've uploaded some pictures of my base plate and integrated motor mount block here:

http://bobandeileen.com/gallery2/main.p ... &g2_page=3

I've still got a long way to go but it is starting to look more like a minisumo every day.

[PeterW]

The standard of engineering seems to be going up in leaps and bounds in the mini sumo world.

[sap1975]

Thanks Peter

Looking at the way Colin does ”business” sometimes i can’t help but think that maybe we're over engineering this thing I bet that with the time and money put into JSM Colin would have built three fully functional MS’s.

I guess well have to wait and see if it pays off.

Besides that this being a hobby fortunately we don’t have to be cost effective.

[slurp]

I got the clamp done this ”morning” and it looks like it will work just fine.

That looks interesting, was just thinking "not much of the motor in the clamp" when I realised you've got them mounted inside a split tube... it would be good to have the screw mounting to the chassis spaced as far apart as possible to give better resistance to the torque. Have you given consideration to the twist (horizontal plain) when you reverse the motor direction?

The standard of engineering seems to be going up in leaps and bounds in the mini sumo world.

I think it's always the case that hobbyists improve their skills, I get the impression that most people don't stick around minisumo that long, they head to more interesting projects rather than put the effort into the competition - a passing interest, that's mostly filled by off the shelf kits

Looking at the way Colin does ”business” sometimes i can’t help but think that maybe we're over engineering this thing I bet that with the time and money put into JSM Colin would have built three fully functional MS’s.

first of all I'd need to put more time in in terms of cost I hope so!

I'd like to keep with the simple and accessible (cost and complexity of build) but there's room for everyone. The trouble will be competing with speed with a low budget robot, entry against the established will always be a little daunting for the beginner - we need to get past that!.

regards,
Colin

[sap1975]

That looks interesting, was just thinking "not much of the motor in the clamp" when I realised you've got them mounted inside a split tube... it would be good to have the screw mounting to the chassis spaced as far apart as possible to give better resistance to the torque. Have you given consideration to the twist (horizontal plain) when you reverse the motor direction?

I was planning to place the clamp in a recess in the base plate which should take care of the clamp moving about. And before i had the motor mount in my hand i was a bit worried that the split tube would deform or shift in the clamp but once i had a look at the physical part the worries completely disappeared.

I think it's always the case that hobbyists improve their skills, I get the impression that most people don't stick around minisumo that long, they head to more interesting projects rather than put the effort into the competition - a passing interest, that's mostly filled by off the shelf kits

I believe you’re right. It seems to be the trend. It’s a shame though since there’s plenty of complexity to go around. From what i can tell it’s one of the few robotic “sports” where you’re running in a dynamic environment in real-time against a fellow robot builder.
Not to mention the design restrictions you have to comply to.
To me at least it doesn’t get much better that that

[sap1975]

As you properly guessed by now the wheel casting didn’t quite turn out the way i planned it. It’s a release agent problem. Again…
I have some purpose made PU release agent coming my way and once it arrives I’ll give it another go. You wouldn’t believe how hard the stuff is to come by in Denmark.

Just to let you know i started working on Red Devil again since i need to get it competition ready for Robotchallenge in Austria. This in turn means that i won’t be doing much work on JSM until after the tournament.

[JonHylands]

Its a lot simpler to get the wheels out of the mold if the mold can come apart.

http://www.huv.com/miniSumo/seeker3/Tire-Mold-1.jpg



But even with that, you really want the proper release agent. Having done it with and without, I can say having the right stuff makes it way more likely to work properly...

- Jon

[slurp]

Ok, maybe a bit ahead here but I thought I'd add a few links back from the Mini-Sumo Gallery - I know the job isn't finished. I'm just starting a wiki page too but I'll need to make the picture uploading easier for everyone else.

regards,
Colin

[sap1975]

Sorry for not replying earlier but things have been a bit hetic. not much time for robotics

Jon- i had actually had a look at your mold earlier and it looks like a really nice piece of work. Unfortuanatly my circular pocketing is not up to standard yet so i had to come up with another way of doing it.

I am planning on giving my version another shot once the proper release agent turns up and if that doesn't work out i figured i would split the tube on the mill. (hmm. i really need to get a hold of a lathe)

Colin- thanks for putting JSM in the gallery and starting the wiki if there's anything i can do to help out. Pictures 3D models or just about anything else please let me know.

I was planning on putting the STL files of the parts in here somewhere too when we get closer to finishing the project. Just in case anyone else would like to have a go at it.