Omni-directional drive

[Ether]

Originally Posted by 1493kd

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

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

[1493kd]

Originally Posted by Ether

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

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

[Racer26]

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.

[Ether]

Originally Posted by 1493kd

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

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).

[Chipawa]

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.

[Ether]

Originally Posted by Chipawa

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.

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

[Greg McKaskle]

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