- What is Omni Base (H Configuration)?
- What is Omni Base (X Configuration) and it strengths and weaknesses?
Maybe this article can help provide some information to your questions:
Hope this helps!
Drivetrains that are able to move freely in any direction, are generally known as holonomic drivetrains. These drivetrains allow your robot to move sideways without turning. Holonomic drivetrains are a diverse group of drivetrains, and can be generally broken down into three groups: all-omni drivetrains, swerve/crab drivetrains, and mecanum drivetrains. Your questions mainly deal with the first type of drivetrain so we’ll focus on those.
I personally love swerve/crab drivetrains, and they have many great applications, but unless you are aiming solely for judged awards/gaining new experience with something that will probably not be competitively viable, I would not build swerve/crab (for fun or experience is still a good reason to try to build one though).
H-Drives are constructed similar to a 4 or 6 tank drive with all omni wheels (sometimes referred to as 2+2 or 3+3 drivetrains, respectively), with an extra wheel(s) perpendicular to the main wheels in the center of the drivetrain. The main 4/6 wheels allow the robot to function like a tank drive. The robot can move forward, backwards, and turn. The middle wheel allows the robot to strafe (move sideways). H-Drives were among the earlier holonomic drivetrains (to the best of my knowledge), because of their simplicity- just add a wheel in the middle of the robot.
X-Drives seem to be more popular (especially on this forum) because of the competitive advantage they have in terms of speed (there is some fun vector math here, google it/search the robotics forums to find it- personal opinion is that the “advantage” is present, but reduced because of friction/other losses by omni wheel roller not spinning ideally). As you can see from the wiki linked in the above post, the wheels are located at the corners of the robot. This allows the robot to turn, and move in any direction (to go forward, the wheels all spin towards forwards). The challenges of X-drives are their mechanical and programming complexity compared to H-drives. While H-drives function very closely to a tank drive in terms of mechanics and programming, X-drives are more complex, and some trigonometry may be necessary, especially for autos (although not strictly necessary).
Benefits and Drawbacks
The benefits of these all-omni drivetrains are that they allow you to move in any direction. This increases the maneuverability of the robot, since it is easier to drive sideways than to back in and out to line up with a target (say the climbing post in this year’s game). Something of note is that since your robot is using only omni-wheels, you may have less pushing power (more noticeable in other robotics competitions, much less in VRC).
The glaring drawback of an all-omni drivetrain is the complexity. If you have not built several drivetrains before, and with good confidence, I would reconsider whether the complexity of a drivetrain will help your robot. A poorly executed drivetrain is worse than a well-constructed simpler drivetrain. Simple and efficient often wins the day. These drivetrains take up more space which could be valuable if used for other mechanisms. Alliance partners (probably) not choose you because you can drive sideways. They will choose you for having a robot that does well consistently. If you can pull off driving sideways such that it is an improvement over not driving sideways, it is worth it to go sideways. Otherwise, build a reliable “tank”(or west coast) style drivetrain that doesn’t break and that you get a lot of practice on and you will be all set.
There are a lot of terms in this post that are specific to the robotics community. Some of the names of the drivetrain mechanisms that I used are referred to less frequently or are antiquated (2+2 drivetrain is still used in some documents e.g. NASA RAP guide, but isn’t used much elsewhere). Other names for drivetrains in this post include Slide Drive (for H Drive, I never use this) and bendy drive (for swerve drive, again I disagree with this naming and have never used it).
Another popular all-omni drivetrain not mentioned in this post is the kiwi drive. With the current VRC rules, I only see very specific use-cases of this drivetrain, which could really limit what your robot could do so I didn’t include it in the main post. A kiwi drive can be described as an x-drive with only three wheels. Once again, would not recommend because of the inherent smaller wheelbase that gives less space to easily put mechanism(s) on your robot. Someone can prove my assertion that kiwi is not viable this season, but I hold that in almost all cases, a team that could implement a kiwi drive well in this game could/would do better with a different drivetrain.
H drive is really good at maintaining the forward push for say you do getting into a pushing match(likely). It could be hard to code it could be easy just depends. X drives are really good for going any direction also if you get stuck and only have one wheel on the ground you can get out of a tight spot. ALso good for mounting subsystems at different angles. The cons are it can get pushed really easy even though it is a 4 wheel drive it is not the strongest because of the angled wheels. Also my teamates found it have to code autonamous but it was also our second year.