use of electrical sliprings
 

Hm. i wasnt specific the last time, so let me reformulate my question:

through the use of this years game's rules, could you use an electronic slipring (something similar to what's found here: http://www.polysci.com/SlipRings/slipring.html) for control of an omnidirectional drive system?

i know the current cant be conducted through the frame of the robot, and there are simpler ways to pull off wiring an omni (ive done it already) but i want unlimited rotation if i were to place a motor ON the wheelbase.
i know there are chain and mechanical configurations that negate the need for motors on individual wheels, but i want to further experiment with the idea.

Re: use of electrical sliprings
 

I cant see why it would be against the rules if the purchased part or whatever you built followed all the part rules and the flow chart. I don't think they are that "Exotic" and as long as your following the electrical rules, you should be OK. The only question i could come up with is if they could handle the current and gauge requirements.

Honestly i think they are a great idea. I would love to see them used on a robot, Maybe ill make my team use one next season if they are required :D

Re: use of electrical sliprings
 

well, we spiffed up our robot from first frenzy that had mounted motors on each wheelbase, and the 10 or so gauge wire started to aggrivate me, so i inquired to oceaneering people about it..not too sure.

but hey, the only reason to do it this way is if you have 4 (+ or - depending on the shape) of the same motors and want a TRUE omni with each wheel independently controlled. there are better ways to achieve crab driving if you just configure the chain so that it runs 4 wheels horizontally for rotation on the vertical axis, and then use miter gears going through the wheelbase to connect the chain that will spin the wheels. ive CADded that design some time ago now, and also remember seeing 118 coming close last year. they streamlined it this year by eliminated the tank function and a few other things.

Re: use of electrical sliprings
 

If its for non FIRST sanctioned competition. You can do pretty much anything you want, as long as its safe. The FIRST rules only apply to FIRST competitions, its your robot, you can add whatever crazy stuff you want. Just keep in mind if you want to participate in any off-season events you must make sure that it is still FIRST legal or legal within the events rules.

Re: use of electrical sliprings
 

OK, Ryan,
Here is some answers for you. Electrical slip rings are legal if you can follow the flow chart, (commercial ones can) if they can handle the current (you need to prove it to an inspector), they meet the max cost limit, and if they do not conduct current to the frame (most do not). I have seen a team who pulled apart a light reel (the kind you see in garages that hang from the ceiling) and used the innards. You need to account for the cost of the reel in BOM, I think it is about $100.
On to your other issues. You will find anyone who does crab effectively (we have done it for several years but not this year) will not use bevel gears for drive. There are two problems, they are inefficient and they need substantial bearing surfaces to keep them in alignment. Remember that as the wheels turn, the engagement also moves. If you are not dead nuts on in alignment, your frictional losses change as you turn. Those losses translate into speed changes that make things very hard for the driver.
Crab drives require some software massaging to make things easy for the driver, that interpret where you are and where the driver wants to go. Although it looks good on paper, continuous rotation brings some other problems to bear like feedback from the steering. If you use a continuous pot, you have to compensate for when the pot goes from max resistance to min resistance in a fraction of a revolution. If you use a rotary encoder, you need to know where your steering is when the robot wakes up or set it to some fixed point during initialization. (You can't know where you are going till you know where you have been) All of these obstacles are surmountable though and many teams have come up with their own twist. Check out Beatty or HOT at nationals for two teams off the top of my head that are using crab.
The last thing to consider, crab drive, although very flexible (sort of like point and shoot cameras) cannot turn to change attitude. This is a must in a game like this year where the goals are angled to your direction of travel. You can drop into a tank style drive in software, but run into the high friction those kinds of turns demand. Or what some teams do, is drop a low friction foot, pneumatically, to raise two wheels off the floor, set the robot into a tank style turn and then retract the foot when the desired attitude is achieved. HOT uses a three wheel crab where the third wheel appears to steer independently. I didn't get a chance to look closely at WMR but it looked good on the field and I could see two Chalupas on the back drives.

Re: use of electrical sliprings
 

well yeah, but i also have the intention to make a pure omni for a first competition. with that in mind, and taking into consideration previous year's rules having to do with similar issues, could it be used legally?

Re: use of electrical sliprings
 

Ah, didnt see that new post at the same time--wonderful information, thank you.

Frankly, fixing a few bugs in the design itself, weve had few problems placing a tank drive operation in our robot. the friction it encounters didnt seem too bad; however, we also used larger wheels--part of the reason the design ultimately wasn'y streamlined enough.
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so, what do you suggest from a personal standpoint instead of using bevel gears? we made a prototype, and there didnt seem to be too many issues. we were originally concerned with the amount of play in the gears, but it didn't seem too bad. then again, i havent made a fully functional robot with those kind of wheelbases.

Re: use of electrical sliprings
 

I would contact some teams that have used bevel gears and see what they think. Chief Delphi I think used a four wheel drive with bevel gears way before we did. You might have to reach way back for some advice there. Martus, Eaubry or Johnson may remember, it was long ago, maybe '98.

If you are going to nationals, take a camera and a notepad and go around the pits asking and shooting. Soap may be able to give you a list from the their database of who might have crab this year. We did not do crab this year, I think that was a good decision the way the field and game are layed out. Next year who knows. Our 2003 Championship robot used crab and a descending foot and it was just right for that game. What I consider the best way to get from here to there in a straight line.
For a bit of trivia, crab steering is what is used for studio camera pedastels in television. They have known for many years, that it is the best way to get a shot when you aren't handheld.

Re: use of electrical sliprings
 

I guess the alternative is really just using spur gears. As a disclaimer, I have never built a crab drive, but I've seen a lot of them, and they typically just have the motor swinging around, attached to the drive modual in line with a spur gear reduction set... or something like my fancy pants picture below will show:



Whew.. that's too much artwork for me. I need a nap.

Matt

Re: use of electrical sliprings
 

well, just as a note, we had the motor and all the gear reduction done on the wheel base for last years robot(exactly like in that picture!). the biggest concern then lied in the fact that different gear reductions were used on two of the wheels (cause there were two chalupas and 2 drill motors) and thus were different sizes and all that good junk. the gear reduction now, however, should be outside the wheelbase altogether and mounted on the base with a chain going to the wheels. the only thing that then changes would be the use of bevel gears on the inside of the wheelwell

Re: use of electrical sliprings
 

I like Matt's idea because we used it several times. We used drills and FP because they are fairly compact and just about the right length for a wheel assy width. I don't do this very often but I have a hint instead of a suggestion. (OK so today was a long day and so is tomorrow so I am a little punchy) Take a close look at the drawing and visualize the forces encountered with a robot traveling at say 10 ft./sec and then it gets hit from the side by another robot. Think, think, is there anything wrong with the picture. Is there anything missing that could really make this design better?

Re: use of electrical sliprings
 

I suppose if you weren't designing the crab module in microsoft paint and in about 15 seconds you'd want to put the spur gears inside of the crab module. :p

And... (in no particular order)

• Fasteners. They're cool to use.
• You'd want that pivot a much larger diameter than what I drew to add stiffness.
• You could also substitute on of the gear sets for big sprockets reduction, having the larger sprocket be a little less than the size of the wheel diameter.

Definitely time for a nap now.

Matt

Re: use of electrical sliprings
 

of course! we noticed that the susceptibility to crashes was high and we build a cage around the base though, extending off from the frame. there WAS some protection. an i agree, the spur gears should completely be inside the wheelwell. but paint does quite a nice job of conveying the point!
like i said earlier though, its harder to do gear reduction in a small space when a wheel has a CIM motor directly connected to it.....last year there was the limit of only two of a motor, remember?

Re: use of electrical sliprings
 

To use Dave's baseball terminology, that was high and outside, batter swung anyway, strike one. Visualize what takes place when it drives and when the robot gets hit. Three dimensional thinking caps now.

Re: use of electrical sliprings
 

Well we have done 4 wheel steering also in the past, based on the picture I would either:

A. Make the pivot point on the top MUCH larger in diameter and make sure that it is beefy enough to support the side forces . The shaft that is holding the wheel needs to be captured on the outside of the vertical plates or it will most likely spread out on a side impact and your wheel will fall out. (Not that I have seen that happen or anything :rolleyes: , we ended up with a brace accross the gearbox at the bottom to hold it firmly together) Also in the pic if the vertical side plate starts to bend it would take off the engagement of the teeth on the CIM motor or it would push them together harder and start to bind

B. Mount a second ring just above the floor that spins in a holder connected to the chassis so that any lateral movement of the drive module is immediately transferred to the chassis instead of the rotating point of the drive module.

Is this what you are looking for Al?

Re: use of electrical sliprings
 

And it's going, going, it's out of here! Yes that is the answer I was looking for. If you think about the forces involved, a fast acceleration or a hit form any side transfers the forces through the fulcrum created by the gearbox directly to the upper joint and there is virtually no way to compensate without a prohibitively large bearing surface. A ring at the bottom of the assembly that mates with a stationary assembly on the robot frame, transfers all lateral movement directly to the frame and very little to the the upper bearing. Side movement is reduced to a fraction of an inch, an nothing in the gearbox is sacrificed. Good practice also dictates your first statement. Without cross bracing, (I assumed it was left out for clarification) the gearbox pulls easily out of alignment, shooting frictional losses in the bearings through the roof. An enclosed gearbox or one with bracing prevents bearing and gear misalignment and keeps the drivers happy.

Re: use of electrical sliprings
 

Ah, now THAT makes more sense! Sorry, running on no sleep and hating baseball doesnt seem to help my problem solving and analytical skills too much.

we pretty much did that already though...best of my knowledge. I'd have to pull the robot out of jesse's....garage? I don't even remember where it is anymore. Oh dear.

Re: use of electrical sliprings
 

geeze Matt, you're making me look bad! :p Last time I ever try to wander into the electrical forums... even if it's about something mechanical!

Now I need to try to redeem myself with another fancy pants work of art...

Matt

Re: use of electrical sliprings
 

A little late but anyways. We've used bevel gears the last two years in our crab drive. And now this year in our turret. In the wheel boxes for our crab drive we locked them in tight and haven't had any negative effects like slipping or coming out of place. The wheel boxes use 1:1, and our turret uses 2:1 this year, and i must say they are grand for our purposes.