Through my time in VEX, I have seen a variety of different speeds of drivetrains. I’ve seen Avery thing from a 66 rpm drive to a 300 rpm drive (both with 4in wheels). Personally, I think about 240 rpm, which puts a robot at a slightly offesnsive advantage to others. What do you feel is the optimal robot drive speed?
Well honestly you have to think about what the game itself is about and what you will be putting on your base. I think right now it is more around the normal green motor speed or a bit higher. This is because you get speed but still have some torque left to ensure you dont just get pushed around the field, and also some torque for the weight and things you putting on your drive train. But everything can change depending on what you need and what you are doing.
That is kinda what I originally thought. But I figure that in most cases, while being pinned, your drive’s torque usually is irrelevant, but a higher speed would allow a team to not as easily be pinned. The higher speed also helps to play “passive defense” by not directly pushing an opponent while still preventing them from scoring. I do agree with you on how speed definitely is dependent on the game and individual robot characteristics.
As others have said, it really depends on the game. For Tower Takeover, being able to speed around the field at crazy speeds wasn’t as important as it was in Turning Point. For this game, it was more important to have a base that can get you to places quickly but still have enough power for large loads from cubes, so we ran a 200 RPM direct drive with 4 inch wheels.
Turning Point, on the other hand, required a much higher speed to stay competitive. If you couldn’t collect balls and shoot flags faster than your opponent, you had no chance. There was a ton of defense in that game, but we decided to sacrifice a bit of torque for extra speed, running a 266 RPM drive with 4 inch wheels. So it really depends on the game itself.
I use 360rpm, and I find that, while it burns out after about 5 minutes, it is able to outspeed a lot of things for the first few minutes, and thats all thst really matters.
I think you should always have some extra speed. For me, I was able to get 220-230 rpm on 200 rpm cartridges just through setting the voltage of the motors to max. This is because I wasn’t using velocity control like most people do so I didn’t have to have a gear ratio. However, I wasn’t able to reach that speed with high loads.
All this to say: yes, you should probably have a little more than 200 rpm as almost everyone uses 200 rpm. It’s just a standard. Speed is almost always an advantage. The only game I can say speed wasn’t an advantage is for a game like star struck (and vex really hasn’t made a split field game except for star struck). Ideally, 250-260 rpm is suitable for any game. A well made chassis with minimal friction losses will be able to withstand defense and tolerate relatively large loads.
At the end of the day, however, it comes down to driver preference. One thing is your driver not being used to high speed drives (they should try it in a match, not just in practice), and another thing is the driver simply disliking it.
to start, It really depends on the game and your playstyle:
for tower takeover 200 rpm 4 inch wheels is plenty since you should only really move as fast as you intake
for turning point 200 rpm 4 inch is fine I guess but 240-280 is good for fielding
for itz 200rpm is probably good since intaking is comparatively slow since you intake one by one
but to get into it, the relationship between high speed and slower drive trains is for any speed you gain, you lose torque, acceleration, accuracy, simplicity, and reliability proportionally.
you need to torque to play defense and resist defense. high-speed drive trains don’t have amazing pushing power so get pushed around a lot.
I think it’s important to not say speed not as an rpm that you are always going at but more of a “top speed”. It takes more time to get moving since high-speed drive trains lose the torque it requires to get to the top speed. being able to turn change directions and move forwards and backward and generally being more agile can sometimes end up being more important than being overall faster.
accuracy is decreased with a high-speed drive train both in driver since you have less time to react to what your doing since your moving faster. and in autonomous you can’t get direct motor encoder values since your using gears/sprockets to move faster and they aren’t 100% efficient and have slack so they wont give 100% accurate encoder values.
high-speed drives do more work to move faster therefore they must use more power. if they use more power they the motors generate more heat. basically they die more often and quicker than slower drives.
and being to just directly attach your wheels to your motors is a building blessing. it removes so many points of failure and friction and leaves you with a very compact drive compared to high speed.
But overall, the optimal drive speed is probably just the basic 200rpm drive with whatever variety of wheels you like (mechanum, 4inch, 3.25inch). being able to move faster in most situations (besides skills, moving faster always op) doesn’t give you a huge advantage. with a 240rpm drive, you are only THEORETICALLY moving 20% faster. I think it’s important to bring up approximate efficiency and actual efficiency. With vex parts and the lack of actual bearings and tight restrictions of lubricants, It can be really hard to get a drive as efficient as a 200rpm drive. with a “240 rpm” drive depending on how well its built with friction and other stuff in consideration, might only be going 220 rpm or etc.
Having a 200 rpm drive is nice because you don’t really have to worry about any of the downsides of going even a little bit faster. a 200 rpm drive is painfully easy to make so this allows more time to work on the ACTUAL things that make your robot better like autonomous, driver skill, intake speed, deposit speed and most importantly consistency.
I think that for most games 200 rpm is fine. Some games, like tt and itz, a 200 rpm drive is better than something like a 260 rpm, because your driving consists of shorter rapid movements, where acceleration speed is more important than your max speed.
I think drive speed doesn’t depend on how fast you intake, but rather how much scarcity plays a roll in the current game. For instance, itz never really had serious scarcity. There were always cones on the field or at least preloads to stack, so 200 worked (let’s pretend it was v5).
However for tp, balls would make their ways to the back of the field opposite of flags and because there were few enough that the straggler balls couldn’t be left behind, we saw a lot more drives that were geared for speed.
For high level tt games, scarcity was again an issue. Both teams capable of placing 3 11-12’s with only 66 cubes meant any cube that your partner gathered at the end of the match while you stacked your alliances 3rd was a cube that the opponent couldn’t stack themselves, and so speed during those scarcity last 30 seconds (or even last 60 sometimes this game needed another end game!) was a big deal.
Intake speed is the bottle neck when scarcity isn’t an issue. Drive speed is the bottle neck when scarcity significantly changes the game.
However, as mentioned above, geared drive trains are actually quite hard to make efficiently. If your build quality isn’t yet good enough, I highly recommend sticking to 200rpm for future games until you can work up to 200 geared 3:5 or even 600 geared 5:3 or 7:3. Geared drives have inherent slop that direct drives just don’t have to deal with. A blanket statement is if you’ve done vex for more than 3 years, you can probably build a decent geared drive. Just remember simplicity is key. Faster drives have an advantage but at a cost of reliability that can only be overcome by high level building.
Direct drive 200rpm will always be fast enough to compete, it just might not be optimal. Also rpm changes with wheel size so 200rpm 3.25” for instance is 3.25/4 * 200rpm which I think is too slow to compete.
The last point is you should always have a 4m drive. Don’t limit yourself to 4… if there’s a way to get 6 and do all other functions with the 2 remaining motors maybe with a transmission or ratchets, those robots are insane (and very complex) but always have at least 4. 2m drives are not viable, 3m drives w booster wheel or with h are not viable. In my experience the minimum speed to properly compete is 200rpm on 4” wheels. Mechs vector like an x-drive and are actually about 200 * root(2) rpm so they’re definitely viable.
mech’s don’t actually vector, they’re just the speed of normal omni tank drive but with less traction. X drives are finally viable now with v5 and if you direct drive X drives with 4 inch omni’s and 200rpm cartridges, they should go as fast as a 280 rpm mecanum drive.
In conclusion: Holonomic drives are OP and you should definitely use them if your build quality and coding skills are up to spec
just wanted to clear this up since our team spent an entire week arguing about this and wouldn’t want this info to waste
I feel like strafing is kinda overrated. In terms of actually using a mechanum or x-drive some teams can truly use them to their full potential such as 7k the past two years, using mechs to line up for shots/towers/stacks. On most teams the need to strafe just makes it easier for an inexperienced driver to do well. However, an advanced driver should be able to complete te the same tasks by just driving more precisely. Overall, strafing is usually good, but can easily be compensated for with good driving.
As for drive speed, I feel like it should usually be determined by torque. IMO by deciding torque and then speed U will usually get a robot with a strong enough pushing power, but with great speed.
On the point on maximum robot speed, at high levels of competition, robots should never be going full speed the entire time, but be using small movements over short distances and only really use their full speed for longer distances.
I believe this is called “resolution”. For torque reductions, you actually increase the resolution of the IME data, but for speed reductions, you lose resolution, though there are workarounds for that (namely tracking wheels are the simplest most efficient solution)
Slight wording issue here, don’t wanna confuse anyone. The linear displacement of the robot changes for one rotation of the wheel when the circumference of the wheel increases/decreases. This is because the robot is displaced approximately the same distance of the magnitude of the circumference (for one rotation). Intuitively, this makes sense. The angular velocity is typically measured in RPM, which for this case will remain constant.
What are the multiple ways that u can increase the drive speed?
Sprocket and chain
Motor cartridge (or if 393, internal gears)
Increase wheel circumference
Make the robot lighter
More motors on drive (increases acceleration, not speed)