We are planning on using a 2 stage gearbox (the VexPro 3CIM Ball Shifter) for the first time next year. We used the JVN calculator to do some calculations on what gear ratios to use, and we came up with this:
We tried to match the real-life speed in high gear to be a little faster than our robot last year, and the low gear to be what seemed to be a reasonable amount slower. We wanted to require minimal changes if we have to use pneumatic wheels, so with this configuration, only the third stage has to change to maintain a reasonable speed if we use 6in wheels. The only real issue we see here is that the pushing current draw in high gear seems too high, but programming team thinks they can programmatically downshift when pushing in high gear to make that a non-issue. Is there anything else wrong with this gearing for our plan? We’d like to order parts ASAP so that we can spend some time in the offseason practicing putting together the gearbox and a drivetrain.
It’s a little difficult to purchase a drive gearbox before the season starts as you normally choose your speeds according to your strategy. Personally, assuming similar games to past years, I would go faster than 10 ft/s adjusted. As reference, this year, 254 had the high gear speed of 19.6 ft/s and we were actually a bit faster than that (definitely not something I would recommend for all teams and for all years). With that in mind, choose something you can control.
All things considered, I would bring your ratios closer together. If maintaining flexibility with wheel size and final ratios are a primary goal right now, you can’t go wrong with the ball shifter.
Gearing for a pushing match has largely been a wasted effort since 2012, when safe zones started becoming more prevalent. In 2014, robots who geared for defense pushing found out that they couldn’t even catch the offensive robots. The robots who geared to push through defense that year also found they wasted a lot of time getting around faster defensive robots, so their strategies may have been better-served by a faster low gear. Even in 2017, which had major pinch points, robots rarely pushed with success - they instead found a way around.
Current draw while momentarily pushing in high gear will brown out your robot. However, with the CTREPDP and VRM, neither your bridge nor your RIO will cut out. You probably won’t even realize it happened. This means that monitoring current draw becomes less of a protection and more of a means to save energy.
Note that it’s current draw “per gearbox”. With two motors per gearbox, each motor will draw half of that amount of current, so 45 Amps per motor in high gear and 20 Amps in low gear. Note that this is only when you are pushing against a wall. If the robot is driving around freely, it won’t draw that much current for an extended period.
The numbers you posted are definitely in a safe range. I would suggest getting another gear ratio for the output stage so you can play with a faster drivetrain, too!
+1 to the idea of not thinking about a specific gear ratio (or pair of ratios) until after you’ve done strategic analysis of the game. Of course, working out a suite of ratios and how you would achieve them is much more likely to be useful. One of the truths I’ve learned working for the US Navy which seems to translate well into FRC is that:
Planning is more important than the Plan.
As an example, in late 2015, we developed several dozen drivetrain plans for an AM14U2 to use in 2016. We did not wind up using ANY of these plans, but the breadth of plans and the thinking that went into them significantly facilitated development of the ten-wheels-in-four-planes drive we actually used. Planning is more important than the Plan.
