Omni-directional drive

Technical Technical Discussion
1

My team’s been addled with an argument concerning omni-directional drive. Some of the team-members believe that it’s possible to create an omni-diretional drive system (meaning, it’s capable of strafing) using omniwheels (not mech wheels) in a normal tank-drive configurations (all the wheel’s turned straight). We’ve heard that omnidirectional driving can be done with omniwheels by turning them at an angle, but some team-mates don’t think that this is necessary. We’ve had trouble finding internet resources that would give us an explicit yes or no answer (using straight omniwheels in a normal tank drive configuration to strafe; can or cannot be done), so we were wondering if anybody on this forum could settle the matter for us.

For a final bit of clarification, our omni-wheels would look like this = =, and we want to strafe. Sorry if I’ve repeated myself, I just want to communicate clearly. Please include any references so that we may refer to those as well.

2

What you’re describing is not capable of omnidirectional motion. You could, however, place a wheel or wheels perpendicular to what you’ve shown and achieve strafing – though perhaps not true omnidirectional motion.

3

That configuration will not strafe. It will go forward and back and turn on a dime though. If you want true omni directional movement from omni wheels you need to place them like this <> in a square. Or as said above do some wort of H-drive |-| with the vertical lines being 2 omni wheels each and the center being the strafing wheel.

4

Actually, using all omni wheels would allow for omnidirectional movement, it’s just that moving left and right wouldn’t be up to you, it would be up to whatever hit you from the side.

Any omni, mecanum, or holonomic style drive works because of vectors. If you don’t have at least one vector pointing sideways, you won’t be able to drive sideways.

With 4 omnis all oriented the same way, all the driven vectors point front/back. The rollers don’t really have vectors of their own, as they aren’t driven.

I really hope that makes sense.

5

You’ll need at least 3 omni-wheels each at an angle so that the axles point toward he center of the robot in order to strafe in any direction. My team (1501) did this last year with 3 wheels and had good success, although the code to make it work is a little tricky.

6

Would a drive with omnis that was placed like my crude pic below work. Does anyone have any experience with this? We discussed it last night and are thinking about it as an option. The frame would be an octagon. Any help would be great

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7

Yes, it would work (if by “work” you mean “would it be holonomic”). What help specifically are you requesting?

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8

Yes that is what I meant (holonomic). I was wondering if other teams have tried this and what are the pros and cons of this design. Thank you

9

Options for achieving strafing using Omni wheels, as seen used on past FRC bots:

3 Omni’s 120deg opposed (Kiwi Drive)
4 Omni’s 90deg opposed (the traditional method, placing them at 45deg angles in the corners)
5 Omni’s 4 Traction wheels, the Nonadrive, as dubbed by (and used on) 148 and 217’s twins for the 2010 FRC season.

EDIT: There might be other valid configurations to achieve strafing with omni wheels, these are just the ones I can think of right away.

10

The general answer to these questions is determined by drawing an arrow at each wheel position, appropriate for the type of wheel. For example, an omni has a two headed arrow parallel to the roller pins, indicating how the wheel can enact a force on the robot. Mecanum wheels would have a two headed arrow at ~45 degrees, parallel to the roller pin, etc.

The wheel speed direction determine the vector lengths, and once you have these, you combine the vectors to determine the robot direction. In many cases, the vectors are in a single heading, so they simply sum to produce an overall robot force vector. If the vectors are in at different angles, pay attention to the moment from a common point such as the center of the robot. For a qualitative evaluation, you will soon be able to do this in your head, and it will be “common sense”, but it is good to be able to analyze new systems you have never seen, and when you want to start predicting numbers, the vectors will do that as well.

Greg McKaskle

11

The wheel pattern you have shown is similar to a standard Omni wheel pattern (with each Omni wheel angled at 45degrees).

You mentioned that your vehicle chassis will be an octagon. If it is a regular octagon, then your wheels are at the corners of a square, and your wheel pattern is identical to the wheel pattern of a standard Omni on a square chassis, and you can use existing Omni code just by rotating your command vector by 45 degrees (assuming the “front” of your vehicle is the top of your diagram).

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12

Team 1111 used a holonomic drive with 4 omnis, one in each corner at 45 degree angles. We used this for the 2008 game “Overdrive.” Fairly simple mechanically, but took a bit of tme to program.

As stated before, the direction of motion is based on the sum of force vectors. Make sure your inputs to the drive motors are balanced, otherwise you may start fishtailing. Ways of overcoming this is by having a good driver or a gyro sensor to compensate.

Because of the force balancing act needed for omni, I would think a mechanum drive would be easier, but still provide a good strafing capability.

13

Mecanum (no “h”) has the same problem. If you don’t get all the wheel speeds right, it won’t do what you want.

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14

You might like to take a look at Youtube for the varying setups and capability of each. Some are helpful ones may be

Airtraxs sidewinder uses the meccanum wheel

Eric Allards set up uses fixed centre wheels and combination of omni-directional wheels.
Boththe above mount wheels in-line
The typical omni-directional wheel solutions use a either a quad set up or triple holonomic set-up, however for muilti-directional driving the wheels need to be mounted at an angle (offest) to each other for driving. You will find many videos on Youtube of examples of this.
Good starting point in seach by on YouTube by “rotacaster” and or other keywords such as holonomic, Robotics, omni-wheels etc.
http://www.youtube.com/results?search_query=rotacaster&aq=f
Good Luck:

15

Hey there

I have been using the Rotacaster wheels for a while now for my large Lego robots and I can’t praise them enough. They cure the sidewards drag issue with large tracks and move very easily under large amounts of weight.

I have used the 48mm Lego compatible wheels for 1 project and the 125mm wheels for another project. If anyone has any questions concerning these wheels. I would love to answer them.