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
Is it possible to move in any direction with Mecanums, like you can with omni wheels?
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
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Yes.
Yes, both omni and mecanum are holonomic. They can simultaneously execute all 3 degrees of freedom: fore/aft, strafe, & rotate.
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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!
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.
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
As Ether noted this year the math in the provided holonomic drive VI’s corrects for this already and no optimizations are required to achieve optimal drive output, though this was an issue last year.
This is something that we had trouble with the first time we used holonomic wheels also. Be careful that viewed from below your robot the rollers on your wheels form an O and not an X.
Let me just try to clear this up for me and others…the rollers should form an “X” from top-view and an “O” when looking at it from below?
No, the rollers should form an O either way.
I’m not looking at a holonomic robot right now, but I’m pretty sure that the rollers should form an O from below and an X from above.
As described here:
http://www.chiefdelphi.com/forums/showthread.php?t=80962
EDIT: With mecanum wheels at least, mounted parallel with the drive chassis. Omniwheels are another story.
Peter - Last year, BaneBots sold a kit, EN-G0561-KT, that consisted of an optical digital encoder that could be mounted directly (glued) to the output shaft of the gearbox, with a neat plastic cover and mounting kit. It doesn’t appear that they offer these any longer on their website, though. I believe they were US Digital encoders, though, so the BaneBots folks may still be able to help you locate some sets.
Thanks for that… Given BB’s inability to ship anything even vaguely on time (including non-P60 stuff that wasn’t notified as having any kind of delay), I think we’ll figure out something for ourselves!
I am looking at one, and you are correct.
Bird eye view = X , Worm eye view= O