The Problem of the Dancing robot

[SpaceOsc]

ummn our drives finally on its wheels and running around and how well it does run around indeed I'm so proud of it

not here the big but the the problem

since we used pneumatic 9 inch high traction wheels not it doesn't seem to turn... if it trys to turn it dances like crazy... we are using a tank drive


Have any team gone threw this and have any idea on how to make it turn well?



-Oscar- help!!

[Chris Hibner]

Here is what I would suggest (none of these options are easy, so I would get to work ASAP to make the changes):

1) Go to a much lower gear ratio. This still might not work, though, and your robot will still most likely bounce a lot when you turn.

2) Change the wheel footprint. Take a look at where the wheels contact the ground. I'd be willing to bet that your track width is smaller than your wheel base (track width is the left-to-right measurement between the wheels; wheel base is the front-to-back measurement of where the wheels touch the ground). You can do a quick static analysis to show that if your track width is smaller than your wheel base, you will have a lot of trouble turning.

3) Change the center of gravity. It is possible to make your robot turn by moving the center of gravity to either the fore or aft limit. This has an undesired side effect of making your robot easier to be tipped over in one direction.

4) Change to a different type of wheel that allows side slippage (like holonomic wheels).

5) Remove one set of wheel and replace with casters. This is probably the easiest thing to do, but just realize that you will probably lose about half of your pushing force.

If you need anything (like to know why changing the wheel-base to track-width ratio makes it easier to turn), either PM me or send me an e-mail.

-Chris

[Skabana159]

Our team (159) had this problem last year with the pneumatic wheels. The problem was not easily solved. First, we went to the judges and asked if we could use duct tape as a "Traction reduction device" rather than as a fastener, and they actually allowed us to. Then, we replaced the two front wheels with non-pneumatics, so they could slip. The problem is simply that pneumatic wheels have too much traction on carpet for tank drive to work well. We actually managed to trip our breakers with the drill motors on a regular basis trying to turn in place. Good luck solving this problem!

[Mercutio]

Our robot also uses a four-wheel drive, and we also had turning problems... we could barely even make wide turns on a tile floor! Our solution was to completely remove the tread from our front wheels and cut down the tread width on the back wheels to decrease traction. We left tread on the back wheels instead of the front because our bot will be heavier in back, so we should end up with a big difference in traction between the front and back wheels.

Now we just gotta figure out how to water-cool our motors. Then we'll be in business! ^_^

[Adam Y.]

The problem is simply that pneumatic wheels have too much traction on carpet for tank drive to work well. We actually managed to trip our breakers with the drill motors on a regular basis trying to turn in place. Good luck solving this problem!

Im not sure but that cheap knockoff duct tape works really well at reducing traction of wheels. It helped solve our traction problem lasat year.

[Solace]

just a question, but just how many motors is your robot using in its drive train? If you're using only two, then I can understand how it completely stops the robot from turning - but if you're using four than you must have some EXTREMELY grippy wheels.

[Cory]

Im guessing your tires have a very large surface area. There is so much friction that your motors do not have enough torque to overcome. This happened to us when in high gear last year. if your tires do not have a large surface area, im confused as to why this is happening. Last year we used very very grippy pneumatic tires, and we have pretty much a zero turning radius.

I think changing your gear ratios is the best path to take. if you are using sprockets and chains, with the drill motors, we have found that 4:1 works very well. Lots of power in low gear. If using sprockets, you can attain roughly a 4:1 ratio with an 11 tooth sprocket on whatever your output shaft may be and a 45 tooth sprocket (I think)mounted to the wheel or shaft that the wheel is on. I don't know if this will overcome your friction problems, but it might.

Cory

[Al Skierkiewicz]

You didn't give a detail on your design, so I am guessing you have a four 9" wheel design. If that is the case, when turning with a tank drive, the only way to turn is to loose side traction on the drive wheels. With high traction wheels, the only way to loose traction is to come off the carpet, in your case dance. If you haven't checked motor current yet, I will predict it is extremely high during turns and is likely tripping circuit breakers which in turn adds to the dance.

[JTH]

We had this problem last year. My solution was simple: I hit the driver and yelled at him every time he tried to do a spin-in-place. Well, I didn't hit him, but I did yell at him. He had to adapt to the hopping effect by taking wider turns.