Hi,
While creating the designs for our drivetrain/chassis, we can’t decide between using a holonomic x drive with Omni wheels or mecanum drive. We were considering using an x drive but the building part was easy but the coding part looks pretty complicated. For the mecanum drive, we haven’t really researched a lot about it but we heard it is easier to build and code.
Plz share your ideas about what drivetrain is best for spin up.
mecanum drive is probably easier to build, but as far as I’m aware the code is exactly the same as with an x drive.
One thing you should ask yourself (which you may have already done) before making a choice between x and mecanum drive is if you are confident that you will be able to make good use of a holonomic drive. I see a lot of teams make x or mecanum drives because in theory they are better (why wouldn’t you want to move in all directions?) but in practice the added complexity and tradeoffs make it only better than tank drives when used well. That said, the best way to find out if you can make good use of a holonomic drive is to try one, and don’t let me discourage you from doing that if your intent is to learn and experiment.
If you’re still stuck on the decision between x and mecanum drives, see this topic, there is some good discussion about the pros and cons of each:
If you did a six motor drive and a two motor flywheel with a transmission/PTO to run your intake, should you build it to switch two motors to the intake, leaving a four motor drive, or just have it so that it draws power from the 6m drive?
I feel like at 350 rpm, the 4m drive transmission would burn out very quickly, but the added strain of the intake to the 6m drive would cause it to overheat more quickly. Or maybe just have a 4m drive to use those other motors for other things. What do you think?
I personally wouldn’t do a transmission between drive and intake, simply because you will end up running it so often it’s going to be easier to kust separate them. A pto between expansion and drive would be good if you can make it work though.
but if you can do a pto, why not? :)) it will be helpful in the endgame and in certain cases throughout the match. you can swap it instantly back and forth whenever so its not like you are wasting any time
i am a fan of this idea😉
4m drive at 360rpm on 3.25” omnis doesn’t burn that fast on 4 motor drive with under 15lb robots. it will last a couple intense matches most but its really all you need. plus you have 6m drive assisting it whenever you want
why would an expansion mech need to be active? i would just use pneumatics to deploy whatever i’m doing
Is it a good idea to go even 300rpm on 2.75’’ or 3.25’’ wheels? I have heard that with 4m, it is not a good idea to go above 260rpm, or is that only with mechanum wheels and holonomic drive?
257 on 4 " wheels with 4 motors is good depending on your strategy and weight of your bot.
this was probably from tipping point, as it required alot of drive power and the bots were heavy
you can probably do 360 on 3.25" wheels on 4 motors, or 450 on 2.75 in wheels (basically the same speed). On 4 motors, 400 rpm on 3.5" or 480 on 2.75 could also possibly work this year. On 6 motors, you could do 600 on 2.75’s.
And I’m building mine at 400…
pto between intake and drive will, I expect, be very nice later in the season when goals will probably fill up very quickly. If a goal is full in 30-45 seconds or so, which I fully expect to happen in good matches by winter, then suddenly both the intake and flywheel serve little purpose on the bot. If you’re using 4 motors on intake and flywheel, this essentially reduces your useful motors down to just the 4 on your drive, plus whatever roller mechanism you have.
but if you can pto 2 intake motors between intake/roller mech to drive, you’ll have a power advantage over 4m drive teams when it comes to fighting over the rollers, and I expect that fighting to get pretty intense, as once goals fill up, rollers are going to be very important and valuable.
What speed do you then recommend? I feel like 350 on 2.75’’ omnis is a good balance between speed and torque.
350 is a decently tricky ratio to get, would be a 7:4 ratio off of 200 rpm cartridges. this would require offsetting the driven 74t gears above because they are much larger in diameter then 2.75" omnis. Just seems like a bad layout to have in general, and you can actually push speeds more than that, even with 4m.
I think on 4m, good speeds are anywhere between 257 on 4" wheels, and 360 on 3.25" wheels.
For a 6m setup, I think 257 could still be a good ratio for really nice snappy acceleration and torque, but you could push 6m a lot faster than 4m if you wanted to. If you build light, could probably get all the way to 600 on 2.75, but personally I wouldn’t go more than 400 on 3.25".
We are looking to fill up the high goal, but then fight for the rollers. We would need to be fast but also have enough torque to push other bots off the rollers. So would a good balance be 257 rpm or more around 300 or 360 on 3.25’’ omnis?
360 on 3.25" is roughly 1.14 times faster than 257 on 4". So not a huge difference.
But the choices here are for you to make, you have to decide for yourself what balance of speed and torque you want, based off of the rest of your design strategy. there is no one best ratio.
Thanks for your help.
Okay, that makes sense. I had been thinking about it the way that many Tipping Point teams used PTOs between drive and lift, where they had code that would automatically switch it to the lift when it was run. My thought was that it would use up a lot of air switching, but it doesn’t need to switch every time. I’ve never used a pto before, so I guess I’m not really qualified to talk about them.
Of course, PTOs aren’t always an option for teams who are low on pneumatics.
Realistically only about 20 discs fit in the high goal. Factoring in the 2 preloads you get at the beginning of the match, you’d need to intake maybe only 6 sets of 3 discs at a time and shoot, meaning that you’d have to shift maybe 12 times at most, which should be a perfectly reasonable amount. If you really think about it though, if you’re focused on shooting discs at the time, you can probably cycle through multiple sets of 3 without shifting into 6m drive. I would expect one to realistically shift maybe 8-10 times in a match. Factoring in the fact as well that you can make a PTO with 1 cylinder, a well-optimized pneumatic system can easily handle 50+ individual piston actuations. Plus, you shouldn’t be extending your cylinder all the way out for a PTO anyway.
Not discrediting the idea, but here’s my feedback after playing with robots on a real field:
However this is not to say that the design is invalid. Just that you might have to fire off a few more shots that your calculations show due to real world things.
Yeah and depending on how you build it, they can actually use extremely low amounts of air. they can be regulated to a very low psi and they only have to use a fraction of the cylinder’s stroke length.
You’re right, however the overall point I was trying to make is that you really wouldn’t spend much air at all in a full match.
600rpm 2" mecanum sounds pretty good