Quote:
Originally Posted by Andrew Lawrence
I love the 3-CIM shifter, but the ratio is terrible. 17 ft/s is a great high gear for 3 CIMs, and 5 ft/s is a great low gear for 3 CIMs, but speeds of 9-12 fps low or high gear don't benefit much from the extra CIM. In order to see a noticeable increase in power/acceleration with the extra CIM, your high gear needs to be pretty high (I'd say 17 ft/s is a good starting point), and your low gear needs to be pretty low (4-5 ft/s is what I'd personally do with 3 CIMs).
tl;dr - the extra CIM isn't going to help unless your high gear is really high, and your low gear is really low.
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I couldn't disagree more. I think 5 ft/s is a terrible low gear speed.
Think about what your functional objectives are for the drivetrain in general. Set some hard numerical goals. Make sure you include all reasonable aspects, including weight, power input (motor selection or options), and dynamic performance goals. Top speed is actually a terrible metric of drivetrain performance. Acceleration and dynamic performance are huge, especially when driving fast, but actual top speed should not be a functional objective. Most objectives in FRC are time to x, where x is game dependent. We like to use sprint distances, the time it takes to travel x feet from a standstill, where x is dependent on the game. Sometimes there are several x's, depending on the game. 15'-20' long distance analysis is usually good for mostly open field games, while 10' is usually good for short distance pick and place maneuvers, and you can prioritize based on your game strategy.
THEN, start turning those functional objectives into machine attributes, with numbers and analysis to back them up. Do some math, using one of the many spreadsheets available on chief delphi if needed, to see what the minimum hardware possible to achieve your functional objectives is. Can you meet all of your dynamic goals with a single speed gearbox for your motor input? Good. The fun stops here then. You should then use hard test data, usually by building it and testing it fully weighted, to see if your correct.
As you get better at this, and design, build, and test more drivetrains, you will get much better at setting the functional objectives correctly to start. I also suggest you benchmark other robots and teams, to see what kind of performance they can get with what power input they have, to set your functional objectives.
IF you decide you can't meet your functional objectives with a single speed gearbox, then you can evaluate a multispeed gearbox. Since it's easiest to start with COTS, you can evaluate what ratio spreads you can get from the commercial suppliers. AM sells 4:1 and 2.56:1 spread, while Vex sells 2.2:1 spread, but it's possible to make something different by changing the mating gears. With the dog shifter, you also have to be careful your gears don't crash into the dog.
The ratio spread is the most important attribute about a shifting gearbox. Modifying the final drive ratio (final chain or gear reduction) is super easy given the vast assortment of COTS 20dp spur gears, but the shifted gears are much harder to change. Initially, do your analysis with a COTS shift spread and modify the final drive ratios to get your desired performance in either or both gears (depending on what your desired performance is).
You will find that some of your performance metrics are easier to meet in low gear, and some are easier to meet in high gear, and some aren't that great in either gear. This is where, if possible, you would ideally like to optimize the shift ratio spread to put your two gears where they best meet your targets. This is also where you learn the importance of setting your functional objectives properly, and simulating properly. All of this comes with experience, which I don't expect you to have yet, but you can start from the vast knowledge of the Chief Delphi community.
My ideal vehicle speeds (useful for comparison, not terribly useful for analysis) of a 6-CIM 2 speed drivetrain are around 8.5 and 15 fps for low and high respectively (90% eff speeds), based on several years of simulation, test data, model improvement, and more simulation. But I don't have enough data for 6-CIM drivetrains, I need to build, instrument, and test one to see how I like it, and improve the model accordingly to design the next one.
Is your functional objective to be traction-limited at 40a/motor (240a for the entire gearbox, note) in low gear? Are you actually going to push a solid wall and hope the main breaker (120a) doesn't trip?
@krazycarl92, IMHO the simulation and data I have suggests a lower ratio spread is better, and that the VP ball shifter has too much spread. It's all about what you really want, and really need, and how you want to use your gears.