Quote:
Originally Posted by Abhishek R
So we've been thinking about auto-shifting some more, and how to increase our efficiency. I've also heard that 1678 achieved speeds of 22 ft/s on the field and wondered how they did that, especially if they started in first gear then shifted or if they were in 2nd the entire time (assuming they used shifters). We were happy with our auto-shifting system last year and would like to improve it further. This led to a few questions:
How do we figure out the correct gearing for the low gear supposing we know we want to have a high gear of 17 ft/s? This needs to maximize acceleration as well as battery life.
What exactly does it mean to be traction-limited and what are the physics equations and calculations surrounding that?
We are planning on using either 4 CIMs, 4 CIMs and 2 MiniCIMs, or straight up 6 CIMs, so any advice on those configurations would help as well. I didn't want to start a new thread since there has been so much discussion on the topic, but since using auto-shifting as a method of avoiding blowing the main breaker was discussed here, I figured it was apt. We didn't have to worry about that last year since we only had 4 CIMs on the main drivetrain.
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Figuring out the ideal High / Low gear split requires a bit of thinking, a bit of math, and honestly becomes a bit of a preference issue in the end. If you're planning on auto-shifting, especially if you're trying to optimize acceleration, you want to look at the acceleration of both options, and pair them together in a way that is going to give you the most smooth accel. If your split between high and low gear is too extreme, you'll notice a "dip" in acceleration, between the top end of low gear, and it's shift into high, where the robot appears to struggle a bit. Anecdotally, anything beyond a 2.5 split will stair having a significant "dip" between high and low gears, but it's been a while since I've looked at the issue in detail.
With that being said, and as Michael mentioned above, you can get away without shifting from low gear to high gear even when gearing aggressively - if your driver / team knows what they're doing. If you get used to driving at speed, and or optimizing paths so that you're not changing directions a lot, you're minimizing the amount of time spent getting up to speed.
As far as being "Traction Limited" is concerned - it's a function of you're drivetrains traction, relative to the "thrust" of the drive. Essentially if your drivetrain can produce enough torque (thrust) to spin the wheels before your breaker would trip, that'd mean you're traction limited. To calculate this, you need to first find the force of friction (Ff = Fn x CoF - is a good start) and then find the effective "Trust" (wheel torque) of the Drivetrain. (Input Torque x Gear Ratio / Wheel Radius - as a starting point) From there, you look at the current draw for that torque, and then compare that to the performance of your breakers.
One of the biggest things to be aware of is the current per motor, and the number of motors in the system. In a 4 CIM drivetrain, designing to be traction limited usually means designing around the 40A breakers in the PDB, so that you're not drawing enough current to trip them - But in a 6 CIM drivetrain, it's better to design around the 120A main breaker, specifically not exceeding a total current draw of about 150% (180A) which works out to about 30A or so a motor - doing this ensures that you shouldn't ever (or at least rarely) trip the main breaker...