View Single Post
  #23   Spotlight this post!  
Unread 12-02-2013, 14:58
Jeffy's Avatar
Jeffy Jeffy is offline
Retired, for now
AKA: Jeff Gier
FRC #2410 (Metal Mustang Robotics) #159 (Alpine Robotics)
Team Role: College Student
 
Join Date: Mar 2008
Rookie Year: 2008
Location: Fort Collins
Posts: 523
Jeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant futureJeffy has a brilliant future
Re: How can you make turning smoother?

Quote:
Originally Posted by Jibri Wright View Post
I just talked to him. He said that the centripetal force does cause the outer two wheels to lift. Its the same force that NASA is currently studying to create "artificial gravity" in space stations. Centripetal force (in the form of static frictional force in this case) pulls inward and the outer wheels are pulled out parallel two the floor. As the robot begins to pivot, the centripetal force is out of balance. If you have ever done work using vectors, you can tell that one of the outer wheels on the ground and one wheel floating is what causes this imbalance. The centripetal force in turn forces a balance by making the forces on the two outer wheels equal a.k.a. set their vectors parallel to each other.

Think of it like a spinning top. When the top isn't spun, it is laying sideways on the table right? But when you spin it, it balances on a single point that touches the table, almost as if defying gravity. The force that is causing the top to balance is the same force that is causing the outer two wheels to lift.

My mentor also said that this observation is not as observable with an FRC 6wd because the dropped wheel is not dropped enough to notice, the FRC field carpet is too high to notice, FRC robot weight normally isn't evenly distributed on both sides of the robot, etc. so its hard to notice and may not come fully off the ground, but the wheels do lift.

He also said that this only occurs when the robot is pivoting. When the robot is driving forwards or backwards, four wheels are normally touching the ground and the weight (or as you said normal force) is even on all four. One set of wheels will be off of the ground and none of the weight will be on those wheels.

To put it plainly, since the wheels are probably touching the carpet when pivoting, normal force is decreased on the outer wheels when pivoting but not because of the dropped wheel. It is because of the centripetal force acting on the robot by tangential friction. Because a lot of Newtonian mechanics use a lot of the same measurements like Newtons, seconds, meters, etc.., it isn't always easy to figure out exactly what causes what. Even so in this case, it is centripetal force. The dead giveaway is the circular motion.

Btw Ether, love the Einstein picture. I love how he's the father of quantum mechanics yet dedicated his life to proving it wrong!
While centripetal force exists, I'm not sure it does a lot for helping you turn on an FRC robot.
Also a test to show centripetal force is causing the better turning (or isn't) would be to decrease velocity of the turns and see if you notice more "scrubbing" due to a lower centripetal acceleration.

Also, I don't see a way that centripetal force/acceleration could be shown to be the reason an eight wheel drive with four dropped center wheels works.

Do a little search for "wheel base" on here and I'm sure you can find the reason drop center 6wd/8wd works the way it does.
__________________
Metal Mustang Robotics 2410 (2008-2011)
2008 STL Rookie All-Star
2010 GKC Finalists
2010 OKC Champions
Alpine Robotics 159 (2012-)
2012 CO Finalists

700 miles from home, 2 miles from FRC. Life is good.