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?
<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>.
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.
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 .
As i interpret this rule the key part is
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
you need to use momentum sustained in an independent body from the robot and then transfer the momentum to the robot from the independent body.
You need to hit a wall or another robot? 
Yeah, I was playing with a wheel yesterday and the only non uniform part i noticed was the grain of the plastic, to me it seemed like there was a substantial difference between the wheel sliding inline with itself and it sliding along its axis
Well, now that I think about it, would just running your wheels backwards give u better stopping than turning your wheels sideways???
I haven’t got my hands on a wheel yet, but those who have tell me that it is slightly rounded to give a “high centre”.
This could cause the change in friction published in the manual section 10.2.4.1
Jason
Running the wheel backwards (dynamic inline friction… 0.5) would be worse than just turning the wheel sideways (dynamic transverse friction 0.10) according to the published values.
Do you mean .05? or 0.5? for the dynamic inline friction?
I’m no physics master, but I’ll take a guess. :yikes:
Could this have anything to do with the coefficient of static friction being greater than that of kinetic? That is, when the wheel is in the forward direction it is rolling and therefore applies static friction, while the wheel is turned perpendicular to the direction of travel it slips and applies kinetic friction. Feel free to correct me if I’m wrong.
I don’t know if this theory works because the wheel may also be slipping while the robot goes forward as either way there is barely any friction.