How Do mecanum wheels handle the bumps?

[Joe Ross]

Originally Posted by Ether

Are you sure it's not 71.03% ?

All seriousness aside, where did you get the 71% number?

sqrt(2)/2

The derivation is left as an exercise to the reader. Think of the angle of the rollers.

[Ether]

Originally Posted by Joe Ross

sqrt(2)/2

The derivation is left as an exercise to the reader. Think of the angle of the rollers.

Thanks Joe. I know that 71% is half the square root of 2 :-)

The question is, does this number have anything to do with the claim that "Only ~71% of the output torque is transmitted in the direction of travel", when the bot is being commanded straight forward or backward. I say no. I say all the torque being applied to the wheel gets reacted by the carpet. I am challenging what appears to be the conventional wisdom on this topic. Think about it.



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[Chris is me]

Originally Posted by Ether

The question is, does this number have anything to do with the claim that "Only ~71% of the output torque is transmitted in the direction of travel", when the bot is being commanded straight forward or backward. I say no. I say all the torque being applied to the wheel gets reacted by the carpet. I am challenging what appears to be the conventional wisdom on this topic. Think about it.

My post assumes the rollers of a mecanum wheel spin. If they don't, the number is greater than 71%, but then you don't have a mecanum wheel anymore, at least one as efficient in any other direction.

190's an exception to the rule. That's not exactly an easy to design feature.

[Ether]

Originally Posted by Chris is me

My post assumes the rollers of a mecanum wheel spin.

If they don't, the number is greater than 71%, but then you don't have a mecanum wheel anymore, at least one as efficient in any other direction.

Hi Chris,

My reply to your post also assumes we are talking about mecanum wheels with rollers which are free to spin. Whether or not they actually spin in a given scenario is a different question.

For example, consider the following thought experiment:

There are two identical robots, RobotA and RobotB, which are absolutely identical in every detail except that RobotA has mecanum wheels and RobotB has standard wheels. The tread on RobotB's standard wheels is the same material as the rollers on RobotA's mecanum wheels. The standard wheels have the same effective diameter as the mecanum wheels.

RobotA and RobotB are each facing a brick wall, and there is a load cell on the front of each robot to measure how hard it is pushing on the wall. Each robot is given a slowly increasing identical forward command.

I claim the following happens:

- Up to a certain point, both robots push with exactly the same force. There is no 71% factor involved.

- If the motors are sufficiently powerful, a point will be reached where the wheels start to slip. RobotA, with the mecanum wheels, will reach this point before RobotB, with the standard wheels.



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[Chris is me]

Originally Posted by Ether

Hi Chris,

My reply to your post also assumes we are talking about mecanum wheels with rollers which are free to spin. Whether or not they actually spin in a given scenario is a different question.

Mecanum wheel rollers should spin in all circumstances when the robot is attempting to move "forward". I don't think your illustration of traction limited pushing is quite correct, but I'll let someone smarter than me clarify independent of mecanum wheel rollers always spinning.

[JesseK]

Originally Posted by Ether

I claim the following happens:

- Up to a certain point, both robots push with exactly the same force. There is no 71% factor involved.

- If the motors are sufficiently powerful, a point will be reached where the wheels start to slip. RobotA, with the mecanum wheels, will reach this point before RobotB, with the standard wheels.



~

Again, the assertions assume the rollers pit all of the force forward. The reality is that the rollers, even when not spinning, will put part of the force inward toward the robot. The force vector calculations easily show this. The resulting forward force is based on the fact that one component of both force vectors faces the other side of the robot; these two components cancel each other out. The forward component of the force vector is what pushes on the hypothetical wall.

It is the same logic as the situation where a box sits on an inclined plane. Gravity pull the box straight down, yet only a component of the gravitational force creates the force normal to the ramp that keeps the box on the ramp. the other component is parallel to the ramp. Same logic, different situation.

Interestingly enough, the speed of a Mecanum drive train is the same as standard skid steer, and perhaps that's the confusion?

[efoote868]

Originally Posted by Ether

I claim the following happens:

- Up to a certain point, both robots push with exactly the same force. There is no 71% factor involved.

- If the motors are sufficiently powerful, a point will be reached where the wheels start to slip. RobotA, with the mecanum wheels, will reach this point before RobotB, with the standard wheels.

Consider a wheel with rollers free to spin on the same axis as the main axle. It doesn't matter how grippy the material is for the rollers, these wheels can't apply a force no matter how hard they drive or how fast they spin.

Consider a wheel with rollers free to spin on an axis perpendicular to the main axle (traditional trick/omni wheels). It doesn't matter how hard they drive, they'll have the same coefficient of friction as wheels without the roller but made with the same material.

Mecanum wheels are in the middle - they spin on a 45.


If you take a mecanum wheel and spin it, you'll find that it pushes you on a 45. Its hard to describe the feel until you actually do it.
No matter which way the wheels spin, they're fighting each other.

[NyCityKId]

Originally Posted by alicen

mecanum wheels are absolutely wonderful for maneuverability, as for going over the bump, they don't really have a problem as long as you're going forward and not diagonal or sideways. The only other thing is that mecanums work best when all four wheels are touching the ground at all times, so an active suspension is fairly important

hope that helps

what do you mean by active suspension. i thought we weren't allowed to have a suspension on this years robot...

[Ether]

Originally Posted by NyCityKId

what do you mean by active suspension

I think he just meant a working suspension.

An "active" suspension would be a pretty neat trick for a FIRST robot.

http://en.wikipedia.org/wiki/Active_suspension


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[Kibaspirit]

Our mechanum wheels worked wonders over the bump. We lowered our center of gravity as far as possible though, to avoid tipping, and to help maneuverability over the bump.

We had no problems. [:

[Peter Matteson]

Originally Posted by NyCityKId

what do you mean by active suspension. i thought we weren't allowed to have a suspension on this years robot...

See this link for a perfectly legal active suspension, fully controlled by sensors on the robot. Follow the link to the youtube video.

http://www.chiefdelphi.com/forums/sh...ad.php?t=84187

As for mechanums, there is always a question as to the loss of power vs manueverability issue. You have to trade it carefully based on how you think the game will play out. There is no straightforward answer. You need to decide what the strategy your team wants to execute is and whether or not that system fits into your strategy. This is why in the first part of build season should set criteria your robot needs before you build anything figure out how to play the game to win and develop a strategy before you build your robot and these decisions become easy now that you can buy mechanum and swerve as COTS.

[NyCityKId]

I've been looking into this locking mecanum design as seen here and i would like to know if anyone has found a way to make this or something similar work (in reference to the lockable rollers not the rotatable rollers). I would like to apply something like that our robot next year and I'm starting research early. anyway if anyone can help me understand this system your help would be greatly appreciated.