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-   -   Crab Steering on Regolith (http://www.chiefdelphi.com/forums/showthread.php?t=71150)

Dan2081 04-01-2009 13:35
Crab Steering on Regolith
 
In Lunacy this year, will a crab steer be allowed in the form of a brake?
I know the rules specify that all wheels must be in orientaion with the robot, but with crab steer, you could possibly turn your wheels to stop you from gliding more efficiently before you change directions.

Is this a legit idea or do the rules not permit it?

nitsua60 04-01-2009 13:42
Re: Crab Steering on Regolith
 
Quote:
Originally Posted by Dan2081 (Post 791025)
...you could possibly turn your wheels to stop you from gliding ....
<R06> Seems to create a problem with any type of steered wheel this year: if the axis of rotation of the wheel has to be parallel to the ground, then you can't rotate the wheel about a vertical diameter, as you typically would while steering. However, according to the rule you could rotate the wheel about a horizontal diameter and make use of the higher transverse coefficient of friction do propel the bot. I hope we see some clarification of <R06>.

EHaskins 04-01-2009 13:46
Re: Crab Steering on Regolith
 
The only rule I'm aware of which could limit it is R06, and as I read it I don't see any problems with your idea.

However, you won't get more stopping force from that since the friction with the ground would still be equal. My guess would be that you'd have better luck either running the wheels in reverse, or at least equal results by using encoders and some simple programming to prevent the wheels from moving when you want to stop.

Quote:
Originally Posted by <R06>
ROBOTs must use ROVER WHEELS (as supplied in the 2009 Kit Of Parts and/or their equivalent as provided by the supplying vendor) to provide traction between the ROBOT and the ARENA. Any number of ROVER WHEELS may be used. The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub). No other forms of traction devices (wheels, tracks, legs, or other devices intended to provide traction) are permitted. The surface tread of the ROVER WHEELS may not be modified except through normal wear-and-tear. Specifically, the addition of cleats, studs, carved treads, alterations to the wheel profile, high-traction surface treatments, adhesive coatings, abrasive materials, and/or other attachments are prohibited. The intent of this rule is that the ROVER WHEELS be used in as close to their “out of the box” condition as possible, to provide the intended low-friction dynamic performance during the game

qwertyuiop[]\ 04-01-2009 13:50
Re: Crab Steering on Regolith
 
horizontal axis of rotation just means that you have to put the wheels flat on the ground. the horizontal axis is your axle so you can spin your axle on a horizontal plane .

EHaskins 04-01-2009 13:55
Re: Crab Steering on Regolith
 
Quote:
Originally Posted by nitsua60 (Post 791031)
<R06> Seems to create a problem with any type of steered wheel this year: if the axis of rotation of the wheel has to be parallel to the ground, then you can't rotate the wheel about a vertical diameter, as you typically would while steering. However, according to the rule you could rotate the wheel about a horizontal diameter and make use of the higher transverse coefficient of friction do propel the bot. I hope we see some clarification of <R06>.
As i interpret this rule the key part is

Quote:
Originally Posted by <R06>
The ROVER WHEELS must be used in a “normal” orientation (i.e. with the tread of the wheel in contact with the ground, with the axis of rotation parallel to the ground and penetrating the wheel hub).
The axis i believe they're referring to is the axle. So then I interpret it as saying the wheel must remain vertical, relative to the ground, but doesn't necessarily need to maintain its orientation relative to the robot chassis.

I could be misinterpreting this, so if it a serious issue for your design it would be best to contact FIRST for clarification.

cj.reeves 04-01-2009 14:13
Re: Crab Steering on Regolith
 
why not try using a massive flywheel and rev it up to a couple thousand rpm, next turn the flywheel with your windsheild wiper motor in the direction you want to go, see how the robot reacts to this inertia, if it does not turn on a dime in zero friction enviornment why would nasa use it on their shuttles and other vehicles?

DonRotolo 04-01-2009 14:24
Re: Crab Steering on Regolith
 
Quote:
Originally Posted by cj.reeves (Post 791064)
why not try using a massive flywheel and rev it up to a couple thousand rpm, next turn the flywheel with your windsheild wiper motor in the direction you want to go, see how the robot reacts to this inertia, if it does not turn on a dime in zero friction enviornment why would nasa use it on their shuttles and other vehicles?
This is known as a "reaction wheel". You should research if it will provide more turning capability than what you can already do with a standard wheeled drivetrain. If it will, then use it, but if it won't....

EDIT: Oh, typically you would use a reaction wheel horizontally, speeding it or slowing it rapidly to transfer the angular momentum to the robot chassis. What you propose is a simple gyroscope, which could also be effective.

XXShadowXX 04-01-2009 14:25
Re: Crab Steering on Regolith
 
I was wrong... about the flywheel, still think it's risky


Secondly, the crap drive concept seams legal, since the wheel is turning about a horizontal that would be parallel to the playing surface, the wheel is able to turn becuase it whole drive assembly/input power shaft can turn with it. Those rules make it so teams don't lay wheels on their side (axis of rotation is perpendicular with the plane of play), or thats what I think.

cj.reeves 04-01-2009 14:30
Re: Crab Steering on Regolith
 
I am not using it to acelerate but to turn in tight conditions, the primative skid (coded) tank steering or omni steering is old news and the wheels would skid out of control. this steering works in SOLIDWORKS and it can cause the robot to turn about an axis without interacting with the ground but under no circumstances will it acellerate the robot.

SuperJake 04-01-2009 14:58
Re: Crab Steering on Regolith
 
It hasn't seemed to have been mentioned here, so I'll point it out.

You may wish to consider that especially on this playing surface, the orientation of the wheel doesn't really matter when it comes to putting on the breaks against a pushing robot. Any of the motors generally chosen for the drive train of a robot are more than capable of holding the wheels locked (not rotating) when pushed on this surface - the friction between the wheel and playing field is much less than the force required to overcome the stall torque of the motor. That said, the orientation of the wheel to the direction of undesired motion doesn't really matter.

Additionally once you are sliding, it will be very difficult to regain control of the robot by slamming on the gas in the direction you want to go. Your already sliding wheels would still have very little traction, so it would take a while to regain control of your robot.

sdcantrell56 04-01-2009 15:43
Re: Crab Steering on Regolith
 
So this hasn't really been debated yet but would a crab drive even be particularly effective on this surface?

It seems to me that the lack of friction would make it so that turning the wheels would not really result in a change of direction quickly. It could actually be worse since the wheels would be skidding and thus kinetic friction would apply.

dtengineering 04-01-2009 15:43
Re: Crab Steering on Regolith
 
Note that the published values for the wheel's transverse coefficient of friction are almost twice that for the inline coefficients.

In other words, yeah... turning the wheel 90 degrees to your direction of travel will bring you to a stop more quickly than just stopping the wheel.

The downside is that you will be in a skid at that point and will have sacrificed directional control.

Jason

sdcantrell56 04-01-2009 15:52
Re: Crab Steering on Regolith
 
So based off of the transverse frictional value a crab drive could actually be effective. It seems like the programming might be even more difficult than a normal crab drive though to get full performance from the system.

Otaku 04-01-2009 15:59
Re: Crab Steering on Regolith
 
How would the direction of the wheels dictate the friction any more than normal? Technically your "contact patch " doesn't grow or shrink any, and the wheels have no lip (read: they're rounded), so you don't have any edge to dig in to.

sdcantrell56 04-01-2009 16:01
Re: Crab Steering on Regolith
 
That is something I dont understand either. The wheels are molded from a uniform material and there is no tread pattern so I dont see how friction would be different. Maybe it is due to the plastics grain or something like that.


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