How can you make turning smoother?

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

What kind of drive train are you running?

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

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.

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

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

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

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.

Um… yes it does.

Where are you getting your information?

To interpret this a different way, if you’re talking about the robot turning really suddenly and being hard to control, make sure it weighs the full amount. Light weight robots are often squirrely. Also running on carpet vesus tile makes a huge difference.

If the robot IS having the opposite problem ( it’s difficult to turn) and you are running a 6/8 wheel drive you can do a couple of things.

  1. Change the outer wheels on one/both sides to a lower coefficient wheel. In general FRC terms Omni wheels have the least CoF (aside from Lunacy wheels [ew]) and pneumatic wheels have the most. The old gray rubber wheels were a happy medium between the two. Wedgetop is also a pretty good inbetween after it’s been run on for a while.

  2. Increase the drop in your chassis. If it is impossible to actually lower the middle wheels with your current design. turning down your outer wheels could work (although less desirable because it changes the surface speed of those wheels relative to the middle.)

  3. CoG and tortional stiffness. It’s probably too late to change these but your frame stiffness and keeping the robot’s Center of Gravity close to the ground are large factors in a high performing chassis.

Regards, Bryan

Instead of the omni wheels from andymark I suggest you build your own, we learned the omni wheels they sell are kinda week if you get another robot pushing on you and your trying to maneuver out or away, My team has built our own omni and mecanum wheels for a few years now and they work just as good if not better and are a lot better on the teams wallet. Good luck, and hope to see you at worlds

The omni wheels from AndyMark work brilliantly and are used by hundreds of FRC teams… but I’m glad there are teams out there who choose to build their own, even if only for the fun of doing so.

But there is a pretty good chance your turning problems can be solved with a quick visit to Andy’s website if you don’t feel like machining a new part, from scratch, with a week left to go. Just order the correct diameter omni wheel that matches your current hub and pay for the expedited delivery and chances are you’ll be turning smoothly by Wednesday.

Jason

robiticxs,
What you describe is classic for four wheel drive systems as described above. What you don’t see is the current drawn in this configuration. Current skyrockets as the drive motors consume near stall currents turning. Often the jerking occurs when the wheels finally break free of the carpet for a moment and then grab again. Omni wheels do correct this when added to the front or the back, not both, please. There are other solutions. Teams have put down a turning wheel that is at right angles to the drive wheels. When they want to turn, the additional wheel descends using pneumatics or some other power and lifts two of the wheels off the ground. A simple pneumatic cylinder with a caster wheel or nylon tip works almost as well.

Al just a question when you use a drop wheel to aid in turning.

Do your bumpers have to stay within the bumper zone?

As I read and try to understand the rules, I would say Yes you do, but could be wrong.

Not trying to be Al, but yes your bumpers have to stay in the bumper zone.

You do not have to lift much to get the desired effect.

The bumpers must stay within the bumper zone when the robot is flat on the floor.
4.1.6.4 R25
BUMPERS must be located entirely within the BUMPER ZONE, which is between 2 and 10 in. from the floor, in reference to the ROBOT standing normally on a flat floor.
Any robot using the six wheel (or more) dropped center, will rock and so will the bumpers. When climbing,pushing or interacting with the pyramid, the robot is expected to change attitude ans so too the bumpers. Also bumpers do not need to be parallel to the floor if that helps in your design.
Does that help?

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.

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?

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.