Quote:
Originally Posted by apalrd
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.
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I fully agree that whatever drive train and gearing is chosen, it should be chosen as a solution to a set of functional objectives. Ideally these objectives are based on proper strategy, physical analysis, and experience. (This sounds an awful lot like engineering)
The two main functional objectives I would aim to achieve with most games and a 6 CIM gearbox are (over simplification):
-Traction Limited at 160 Amps Total for the drive train (best met in Low Gear)
-Time to travel a particular distance, based on strategy (best met in High Gear)
Usually the time to travel reasonable distances in FRC is optimized by gearing which corresponds to a top speed somewhere between 13-18 fps, accounting for voltage drop and inefficiency. This doesn't change terribly much between 4 and 6 CIMs. Our functional objectives and corresponding gearing seems to agree here.
Now looking at our other objective, we have to do some math to figure out what gearing we want. I arrive at the 160 Amp total current draw at the traction limit because 40 amps per motor has been used as a low gear standard by many teams for many years on 4 CIM drive trains largely without issue. Then last year, we hear about many 6 CIM drives tripping the main breaker. To be sure that our robot won't trip the main breaker, it would be nice to design it such that it won't, which we can achieve with proper gearing. These claims are also supported by the
main breaker spec sheet. A properly functioning breaker will be at risk for tripping after 10 seconds at 240 Amps, whereas it will go for over half of the match at 160 Amps. This is also neglecting any other sources of current draw on the robot.
To do that math, we will need some assumptions:
-145 pounds for battery + bumpers + robot
-6 CIMs in drive train
-4 inch wheels
-CoF=1.2
-Efficiency = 80%
Our gearbox will need to be designed to overcome a tractive force of 174 pounds. If I want this traction limit to occur at 160 amps of total current draw, that will require that the drive train achieve this traction limit at 26.7 amps per motor. Based on motor curves, this corresponds to 63.2 oz. in. of torque output from each motor. This is equal to 3.95 in. lbs. Which means that the drive train as a whole will input 23.7 in. lbs. before taking efficiency into account.
Our desired output torque is 348 in. lbs. This leads to a gearing of 16.5:1, using the square root of 0.8 to account for efficiency (making the best guess assumption that efficiency losses apply evenly to speed and torque).
This gearing corresponds to a top speed of 4.8 fps using a speed derating of 85%. (5.6 fps theoretical)
This shows that a low gear speed of 5 fps for a 6 CIM drive train is perfectly reasonable given this particular design objective.
The gearing spreads for a gearbox to complete both functional objectives I mention would be between 2.6:1 at the lower speed end and 3.6:1 at the higher speed end.
Commenting on another person's experience because they speak of drive train specs in terms of top speed is undue. By that logic, teams like Simbotics and The Cheesy Poofs who have published on their website their robots' top speeds in each gear would really be in need of experience. This actually shows a wealth of experience because it takes tremendous experience to be able to look at a design, hear a number in fps, and then immediately know what this means for the performance of the drive system. Based on a number in fps or gear ratios, I can look at a teams robot design and understand what the implications are for their drive performance. If something seems off or unreasonable about gearing decisions, I will ask a team about their thought process. At this point, I usually learn something new or realize the team doesn't know what they're doing. It is the industry standard to speak in terms of top speed; we wouldn't answer the question "What's the top speed of your robot?" with "We travel 10 feet from a dead stop in 0.95 seconds."