Mechanum drive-strafing issue
 

Our team has had issues with the mechanum drive system we have employed. We appear to not have enough torque to "strafe" and move against the natural direction of the wheels. I have verified that it is not a programming error because the motors move how they are supposed to when the robot is suspended on blocks.

We use 2.5" CIM motors and CIMple boxes for transmissions. (one CIM per transmission)

Any other teams that use mechanum drive, please respond and tell me which motors and transmissions are being used.

P.S. I am new to the forum, so please be forgiving.

Re: Mechanum drive-strafing issue
 

Are you using the CIMple boxes directly attached to the wheels, or do you have a reduction between the gearboxes and wheels? A CIMple box directly attached to a wheel will not have enough torque to turn the wheels. You need some other sort of reduction. Also what size wheels?

Re: Mechanum drive-strafing issue
 

There is a pulley that yields an approximate mechanical advantage of (3/2).

The mechanum wheels are 6".

My question is: What mechanical advantage or modification is necessary to make the mechanum wheels strafe?

Re: Mechanum drive-strafing issue
 

So a 2:3 reduction off of a CIMple box is a 7:1 total reduction, which is not enough. With 6 inch wheels, you need somewhere between a 10:1 and 12:1 reduction to have enough torque to strafe. You can get this with either a larger reduction coming off the CIMple box, or replace the CIMple box with a toughbox.

Re: Mechanum drive-strafing issue
 

Thanks for the help on this.
By the way, how much would adding additional CIMS to each transmission help?
(I know that only 6 are allowed, for post-competition purposes)

Re: Mechanum drive-strafing issue
 

Have you ensured that your wheel rollers form an "X" when viewed from above, or an "O" when viewed from below?

Re: Mechanum drive-strafing issue
 

When you say "can't strafe" does that mean it struggles to move very fast or literally no movement? If the latter, have you tried giving it a push to get it started? Is your acceleration in the forward/backward direction normal?

You might double check that your rollers spin freely. When I spin our (3 year old) AM mecanum wheel rollers, they spin for several seconds before friction stops them, or they are cleaned, adjusted or swapped.

In general, the amount of power in your drivetrain is decided by the number of motors. More motors, more power. Power is torque x angular speed
Deciding on a gear ratio for your drivetrain is deciding how to distribute power between speed and torque. If you change the ratio, one goes up, and another goes down. You can't just pick any top speed you want, however. Your wheel translates torque to a linear force and pushes the floor, and Newton says the floor pushes back the same amount. This is the force that accelerates your robot. acceleration tells you how long it will take to get to your top speed. So, if you choose a gear ratio with a crazy high top speed, you won't have the acceleration to get to your top speed within a typical FRC distance. Or, you might not have enough torque to overcome the friction in your gearbox and wheel bearings to move at all...
soooooo, if you're still awake...:rolleyes: If you keep the gear ratio as is, but add an extra CIM to each gearbox, you are keeping the theoretical top speed the same but basically doubling your torque. It would be legal to add an extra mini-CIM to each gearbox for this year's competition (assuming you haven't used them elsewhere)

OK, if you've made it this far, then one other thing to consider is the current draw of your motors at stall. Have you seen JVN's calculator? This will show you speed and current based on your design choices.


FYI - the spelling is mecanum.

Re: Mechanum drive-strafing issue
 

This will not affect the ability to strafe. If the programming assumes worm's eye view "O", but mechanically it is "X", then strafing will be opposite of the commanded direction. It would make rotation difficult,if not impossible, however.

Re: Mechanum drive-strafing issue
 

Thank you for the help. I have arranged the mecanum wheels in the "X" fashion to allow it to move freely. The weight distribution is also fairly even, and the robot is slightly underweight. The wheels also have a lot of friction to them, and some of the rollers were locked up when I found them. (vise grips time)

I appreciate all of the math stuff. I attempted to show the difficulties of it at the beginning of the year, but I had little luck.

I think the primary problem here is that there isn't a high enough mechanical advantage. Since my last response, the team has decided to use ToughBox transmissions that we found in the shop. I looked at the gear ratios, and that should be the solution to the problem.

Re: Mechanum drive-strafing issue
 

Is that an X where it touches the floor, or an X when you look down at it from above?

If the rollers in contact with the floor form the X, you have a problem. With that arrangement, the robot can be turned easily by an outside force, and it will have a difficult time turning itself.

That is definitely a problem. It'll actually help the robot rotate in place despite the misarranged wheels, but strafing will be a chore. Get all the rollers rolling, and make sure the wheels are installed in the proper corners. Then the torque required for strafing will be lower, and your drivetrain might be able to handle it.

Re: Mechanum drive-strafing issue
 

Your rollers may be tight due to the bushings having urethane leaking into them during the molding process. It can be cleared out following the procedure shown in the following video we made several years ago. The process is different from the one Andy Baker shows in his video. We did clear it with him before posting.

http://www.youtube.com/watch?v=1mrNH0Czq0I

The other reason the rollers may be tight is if the aluminum spiders are "pinching" the rollers. We removed the pinched roller and used large pliers or channel-locks to bend the fingers for that roller apart slightly.

Hope this helps.

Re: Mechanum drive-strafing issue
 

2013 used mechanum back in breakaway and that was the last time they used that system due to the limitations and problems they had. If I remember correctly, they were running about 16:1 reduction off CIM motors with 8" custom built wheels. http://www.youtube.com/watch?v=bxw4sGeKakE

Ever since though:

Re: Mechanum drive-strafing issue
 

Think about it this way: If the rollers are locked up, then it's not a mecanum wheel. So you cannot strafe.