Mecanum Advice Request

[Eigenvector]

We (1002) ran 8" mecanums last year off standard Toughboxes. They're surprisingly quick and we were very pleased with how well they worked. I'd recommend avoiding chain if you can, the added strains from the different types of motion didn't agree with it. Also, once you've got them assembled, check that all the rollers spin freely.

Good luck!

[pfreivald]

If I may ask, where did you mount your encoders on the p80?

[jblay]

Originally Posted by BigJ

"beast mode"

love that

Originally Posted by Creator Mat

However if you use an extended shaft you'll just kill the gear box and the shaft if there is no support on the other end of the shaft. My team used mechanum last year and used the P80 gear boxes with an extended shaft. It was extremely efficient, had good mount points for the encoder, and we had no problems. Hope this helps

I don't see why the longer shaft would destroy the gearbox unless using that long shaft meant mounting your wheel far away from the gearbox which creates problems. Also wouldnt a third mounting point create more problems because a third mounting point gives the shaft a place to bend.

[BigJ]

Originally Posted by Jogo

Can you further explain this? Why would you only get 71% when going forward?

Not sure how much you know overall about Mecanum so I'll lay it all out.

Mecanum wheels produce a force at a 45% angle when driven. The x and y components respectively equate to roughly 71~% of the total vector. (Actually thinking about it my sqrt(2) comment makes no sense. it's actually 1/sqrt(2) for each side. 1/1.414 ~= .71)

In my head, without looking at our algorithm it isn't making a ton of sense (since the forward vectors would be 71% too), but I know that when we're running "normally", which is translating at the same speed in every direction, our motors run at only that 71% when going forwards.

EDIT2: It is because the motor outputs in our algorithm are limited by a trigonometric function and motor offsets. It just so happens that our motor outputs when going forwards or backwards are +/- sqrt(2)/2, which is the same ~71%.

We give the drivers the option to drive our motors at 100% forward/backward.

[Ether]

Originally Posted by BigJ

If you have an algorithm that translates in every direction the same, you will get about 71% ( sqrt(2)) of full power from your motors when going forward with no other modifications. 1675 usually includes a "beast mode" for going stright forward/back with 100% power.

Originally Posted by Jogo

Can you further explain this? Why would you only get 71% when going forward?

If you use the provided library functions (such as LabVIEW Holonomic vi), you don't have to worry about this and there is no need for a "beast mode", because the normalization is performed on the wheel speeds after the inverse kinematic calculations are performed, not on the joystick commands. You get the full range of motor command to the wheels in all directions.


Now, don't confuse this with the pushing force of a mecanum, which is a separate issue. In a traction-limited situation, mecanum has less pushing force than a standard-wheel vehicle using the same tread material, because the floor reaction force (for a given forward force) is higher on a mecanum wheel so it loses traction more easily.

[Trevor1743]

Originally Posted by BigJ

We give the drivers the option to drive our motors at 100% forward/backward.

When you say 100%, do you mean the equivalent of ~71% of regular wheels or do you mean 100% of regular wheels?

[BigJ]

Originally Posted by Trevor1743

When you say 100%, do you mean the equivalent of ~71% of regular wheels or do you mean 100% of regular wheels?

Still 71% force of regular wheels, 100% motor output

[Trevor1743]

Is it possible to move in any direction with Mecanums, like you can with omni wheels?

[Ether]

Originally Posted by BigJ

Still 71% force of regular wheels, 100% motor output

The forward force Ff provided by a mecanum wheel is the same as the forward force that would be produced by a “standard” wheel of the same diameter being driven with the same torque, but in the case of the mecanum wheel the reaction force of the floor on the wheel (roller) is greater by a factor sqrt(2), because this reaction force is at 45 degrees to the forward direction.

It is this greater reaction force which causes the mecanum wheel to break traction with the floor and slip at lower forward-force levels than a standard wheel. This explains why a mecanum vehicle has less traction than a standard-wheel vehicle, even if they both use the same wheel material.

http://www.chiefdelphi.com/media/papers/download/2739

[EricH]

Originally Posted by Trevor1743

Is it possible to move in any direction with Mecanums, like you can with omni wheels?

Yes.

[Ether]

Originally Posted by Trevor1743

Is it possible to move in any direction with Mecanums, like you can with omni wheels?

Yes, both omni and mecanum are holonomic. They can simultaneously execute all 3 degrees of freedom: fore/aft, strafe, & rotate.

[jblay]

In what different ways can you control mecanum? The ideas I know of are:

using the joystick that twists.
have a tankdriive setup with one stick that controls strafing.
one stick controls your movement in every direction and the other spins the bot to the direction you point it.

[pfreivald]

Originally Posted by jblay

In what different ways can you control mecanum? The ideas I know of are:

using the joystick that twists.
have a tankdriive setup with one stick that controls strafing.
one stick controls your movement in every direction and the other spins the bot to the direction you point it.

The joy of a holonomic drive system is that it handles exactly like a first person shooter... So if you set up your controls to mimic a FPS, most drivers will already have an intuitive feel for the setup.

...that's what we did last year, anyway, and it worked great!

[Ben27Lacrosse]

Okay, Here are a few things to look out for and suggestions that my team (1296) uses/does.
First of we choose our gearbox, toughbox nano with the long key. The reason we do this is for extra support. We use mounting plates one both sides of the wheel, for extra support.
Secondly, throw together the KOP base and put mecanum wheels on there to give your progamming team time to flush out and software problems.
Finally, when assembling your final bot be sure to distribute the weight equally do that your wheels function properly.

[JonA]

More details of what BigJ is talking about can be found in this paper about the control strategy used by FRC Team 1675. Basically the output is limited based on the inverse kinematic calculations to allow for maximum speed output of the robot to be the same in all directions with overhead for the yaw rate to be added allowing the same yaw rate control regardless of the heading. These calculations result in the reduction of total overall maximum speed (not using the full range when traveling in the forward/reverse direction) in favor of overall normalization of speed and yaw rate. "Beast Mode" basically allows the driver to override this control decision.

The considerations we have mistaken before include accidentally mounting the mecanum wheels in X and not O configuration and balancing and maintaining the weight/downward force on each of the wheels.

The typical 1675 mecanum setup looks like this:

• Four CIM motors
• Four BaneBot P80 gearboxes
• Four AndyMark 8" Mecanum Wheels

We have also experienced issues with the AndyMark 6" Mecanum wheels (GEN 1 and GEN 2) in that the hubs did not hold up to the impulse loads incurred by 120lb robots in competition. The new AndyMark 6" Mecanum HD wheels should be more than adequate based on the testing document they have published.

We have used two different encoder mounting strategies as sometimes we have used chain and sprockets off of the P80s.

• Vex Quadrature Encoder on an idler sprocket
• Vex Quadrature Encoder (Modified output shaft) or USDigital Encoder
• Considered but did not use the austriamicrosystems magnetic rotary encoder which just requires affixing a magnet to the end of the shaft