How can you make turning smoother?

[roboticsx]

does anyone know how to make turning smoother? our robot movement is too jerky...

[darkember]

What kind of drive train are you running?

[EricDrost]

Run a 6 or 8 wheel tank drive with the center wheel(s) dropped 1/8".

[FrankJ]

Run mechanum wheels?

Any kind of tank drive with grippy wheels or tracks are going to have issues turning smoothly. It is because you have to slide the wheels side way. There are compensations: short wheel base, six wheel drive with a dropped center wheel, etc. Having enough torque is also critical. Either by two cims per side or low enough gearing.

[Akash Rastogi]

Originally Posted by roboticsx

does anyone know how to make turning smoother? our robot movement is too jerky...

Please post a picture of your drivetrain so we can help you better.

[Alan Anderson]

Chris Hibner's white paper should be required reading for anyone worried about smooth turning.

[stveception]

Originally Posted by roboticsx

does anyone know how to make turning smoother? our robot movement is too jerky...

Square the inputs from your joystick into your robot drive. This will make the joystick less sensitive.

[dtengineering]

Hey... one of the new Calgary teams! Welcome aboard, and welcome to Chief Delphi. I'm hoping to get out to Calgary this year, and look forward to seeing your machine.

We had this problem with our first robot, a four wheel drive machine... it worked great on our cement and linoleum floors at school, but when we finally got to Toronto and put it on carpet... we couldn't turn worth a darn.

The problem is inherent to skid steering... the front and rear of the robot needs to skid sideways, in order for the robot to turn. Since we design the robots to have high traction wheels (which were great for pushing) they were awful for turning. We had to reduce the coefficient of friction of one set of wheels so that they could slide sideways.

With a six or eight wheel drive (which we eventually switched to) the trick is to lower the centre wheels... this reduces the "normal force" (hey, physics in action) on the front and rear wheels, making it easier for them to skid sideways.

If you are locked into a four wheel drive design, you might want to consider some omni wheels... http://www.andymark.com/Omni-s/51.htm they also work great with six and eight wheel drive machines.

Good luck! Hope you get the machine running well soon.

Jason

[Jibri Wright]

Originally Posted by dtengineering

Hey... one of the new Calgary teams! Welcome aboard, and welcome to Chief Delphi. I'm hoping to get out to Calgary this year, and look forward to seeing your machine.

We had this problem with our first robot, a four wheel drive machine... it worked great on our cement and linoleum floors at school, but when we finally got to Toronto and put it on carpet... we couldn't turn worth a darn.

The problem is inherent to skid steering... the front and rear of the robot needs to skid sideways, in order for the robot to turn. Since we design the robots to have high traction wheels (which were great for pushing) they were awful for turning. We had to reduce the coefficient of friction of one set of wheels so that they could slide sideways.

With a six or eight wheel drive (which we eventually switched to) the trick is to lower the centre wheels... this reduces the "normal force" (hey, physics in action) on the front and rear wheels, making it easier for them to skid sideways.

If you are locked into a four wheel drive design, you might want to consider some omni wheels... http://www.andymark.com/Omni-s/51.htm they also work great with six and eight wheel drive machines.
Good luck! Hope you get the machine running well soon.

Jason

It doesnt cause the normal force to decrease. What happens is that the centripetal force when pivoting on the dropped wheels causes the outer wheels to lift and cause them to be in a state of balance with each other. While pivoting the outer wheels are applying no force to the ground or in other words not touching.

[Ether]

Originally Posted by dtengineering

With a six or eight wheel drive (which we eventually switched to) the trick is to lower the centre wheels... this reduces the "normal force" (hey, physics in action) on the front and rear wheels, making it easier for them to skid sideways.

Originally Posted by Jibri Wright

It doesnt cause the normal force to decrease.

Um... yes it does.

Originally Posted by Jibri Wright

What happens is that the centripetal force when pivoting on the dropped wheels causes the outer wheels to lift

Where are you getting your information?

[Jibri Wright]

Originally Posted by Ether

Um... yes it does.



Where are you getting your information?

It doesn't cause normal force to technically decrease. When driving forward or backwards, there are at least four wheels on the ground. During this time, the amount of normal force on the dropped wheels as well as the normal force on the set of outer wheels is evenly distributed and the same. When the robot is pivoting on the dropped wheels, the normal force pushes in tangential to the place of pivot, this case being the dropped wheels. Inertia therefore causes the outer wheels to lift and balance which in turn means the dropped wheels are the only wheels touching the floor at this moment. I got this information from A.P. Physics B where I am currently getting a high B in. I learned of centripetal force last year and asked my mentor who also happens to be my A.P. Physics teacher is this what caused a dropped 6wd to turn smoothly. He said yes.
Study centripetal force and you'll know what im talking about.

[Ether]

Originally Posted by Jibri Wright

It doesn't cause normal force to technically decrease.

You need to go back and read Jason's post more carefully:

this reduces the "normal force" (hey, physics in action) on the front and rear wheels

Dropping the center wheels has exactly the effect described above by Jason, and this is the main reason why dropping the center wheels slightly makes the robot easier to turn.

I learned of centripetal force last year and asked my mentor who also happens to be my A.P. Physics teacher is this what caused a dropped 6wd to turn smoothly. He said yes.
Study centripetal force and you'll know what im talking about.

Could you please politely ask him to send me a PM?

[Jibri Wright]

Originally Posted by Ether

You need to go back and read Jason's post more carefully:



Dropping the center wheels has exactly the effect described above by Jason, and this is the main reason why dropping the center wheels slightly makes the robot easier to turn.




Could you please politely ask him to send me a PM?

Sure but he isn't on this website. Can you PM me your email? That way he can get in touch with you. He's the one that founded our robotics team and not really an engineer, but helps us determine how things should theoretically behave.

[TheOtherGuy]

Originally Posted by Jibri Wright

Sure but he isn't on this website. Can you PM me your email? That way he can get in touch with you. He's the one that founded our robotics team and not really an engineer, but helps us determine how things should theoretically behave.

Jibri, what Ether is saying is that when you have weight centered on the robot and you add two dropped wheels, the weight is more distributed over the center wheels than the outer wheels, causing a higher normal force on the center wheels than either the front or back set of wheels. Your point about the centripetal force is still valid though.

[Jibri Wright]

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!