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I have another question. Why bother with a 6-CIM, 2-speed drivetrain at all? It seems like a lot of resources to invest in only marginal gains over either a 4-CIM, 2-speed or 6-CIM single speed.

Re: pic: 3CIM Ball Shifter
 

33 Ran an autoshift in '04 with their 4 speed. The the whitepaper is http://www.chiefdelphi.com/media/papers/1580 but it doesn't go into much detail on the shift algorithm. I'm sure Jim has/would describe it somewhere.

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Something very important that also needs to be considered is what is the opportunity cost of those two extra CIMs. The concept of opportunity cost is invaluable in making effective strategic designs decisions, especially when it comes to resource allocation.

Here's a Kahn Academy lesson that does a great job of explaining opportunity cost. A bit more detailed than is needed for these purposes, especially in terms of the PPF, but hey at worst you're going to learn some extra stuff.

https://www.khanacademy.org/science/...?v=pkEiHZAtoro

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They're not out here ;)

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For us it came down to the fact that we wanted to utilize our drive motors to hang (we set our initial goal at less than 10s) and we also wanted to maintain the responsiveness and acceleration of our 2011 robot, except 30 lbs heavier. To do that we needed the extra CIMs.

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PTO's are an entire different ballgame, and negate some (most?) of the issues Karthik was trying to bring to light with looking at the opportunity cost.

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These are design philosophy issues. Sometimes your design philosophy leaves every day performance on the table. Airplanes could be a lot lighter (and more efficient) if they weren't good for a very very very very bad day.

Something else I think is important for many teams to consider is that there is a driver in the loop. Even if your slow gear is Killer Bees fast, if you're driver isn't up to snuff you will spend lots of time monkeying around when you could have been scoring. I still think one of the best things FIRST could do to improve game play is make the default speed of the kitbot slower. You see lots of operators that really just don't have the practice to be efficient at top speed.

Re: pic: 3CIM Ball Shifter
 

Our 11 code has the full implementation of it (for a 2-speed), but the code is a bit messy and I would implement it differently now. It's drivetrain/autotrans.vi. Upshift was enabled (and the driver feedback was all good), both downshifts were disabled due to driver feedback (Specifically he would rely on coasting into the peg with a tube, and the coast down would throw off his maneuvering as he was slowing down). Better calibration could have likely improved this.

The logic is fairly simple. We do three types of shifts:
Upshifts
Coast down shifts
Kick down shifts

Acceleration should be multiplied by the sign of velocity (or the abs of velocity should be used to calculate acceleration) to normalize for changes in direction.

Upshifts are based on thresholds for speed (greater than), abs of avg of throttles (for skid steer - this is pre-halo and culver drives) (greater than), vehicle acceleration (greater than), abs of diff of throttles (less than) (not turning)

Coast down shifts are based on absolute low speed (~2fps).

Kick downs are based on speed (we used 8 fps, which is higher than redline in low) (less than), abs of avg of throttles (greater than), not turning, vehicle acceleration negative and less than calibration (large negative number).

Upshift handles normal upshift driving.
Coast down shifts back when the vehicle speed is close to zero so it can upshift again at he next launch.
Kick down shifts down when you hit something and need to push.

This algorithm worked well enough for FRC. The corner cases (when turning) are simply ignored by the autoshifter, which was 'good enough'. We also impose a minimum time between shifts to prevent gear hunting, most automatic shift implementations see this and our solution is a 500ms inhibit timer.

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How did you stop kick down shifting from happening when you ran into stuff? I tried a similar thing, but it would wear the shifting dogs out pretty quickly when we would hit something that couldn't be pushed.

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To generalize what Aren is saying, always try to put your mechanical advantage as close to where you need it as possible. (That is, pass the big forces or big torques through as few components as possible.)

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I agree that 6 CIMs are overkill if you aren't using a PTO. 4 CIMs + 2 550s seems like a good compromise and additionally makes it harder to blow the main breaker(our 6 motor drive was geared for 19.2 FPS @ 100% efficiency and never blew the main breaker). I agree with karthik though: if you can't find an easy way to add the extra motors, it isn't really worth the effort of completely repackaging the gearbox.

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Agreed. Maybe this is just a west coast perspective, but in my mind a slow gear gives you the ability to have a super fast top speed (15+ fps as some west coast teams like to have). A low gear is only used when you're in a pushing match, or positioning accurately such as in autonomous, which isn't all that often. I personally don't see much point at having a low gear being lower than traction limited, which is about 6fps for a 4 CIM drive.

Part of this may (and I'm speaking with a complete lack of emperical evidence here) be because of a slight efficiency advantage of WCDs over "east coast drives." Most non-WCDs I've seen have more places where power can be lost (gears, more chain reductions, 35 chain vs 25 chain, etc). West coast drives have as little as two reductions down to a wheel, and typically have either 25 chain or belts to transmit power to outside wheels. Non-WCDs on the other hand are usually based off AM shifters, which require at least three reductions for the power to release the wheels to reach the wheels. Perhaps this small added efficiency lets west coast accelerate slightly more quickly in high gear.

I know I just threw out a bunch of stuff which I really can't prove and don't 100% believe is true, stuff that I would usually not post. But, given the apparent difference between east coast and west coast perspectives here, I wanted to propose a theory that might explain some of the differences.

In terms of 6 CIM vs 4 CIM drives, I'd say 6 CIMs is nice but not really necessary. You'll accelerate better, be faster and more powerful, but there are real drawbacks. One is current draw, which detracts from other mechanisms and can trip the main breaker. The other, as Karthik pointed out, is that it takes away from your ability to have CIMs power other mechanisms.

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The whole purpose of the kick down case is to shift when you hit stuff.

The dog is a fairly robust part, have you actually worn one (of the AM stainless dogs) out?

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Yup. We managed to break one in half with our auto shifting code several years ago. We had made our own transmission with two CIMs + another motor that was based off of the AM shifting transmission. The problem was that when our driver would hit the walls in the room we were testing, it would shift into the slower gear violently. Now that I think back to it, the problem probably occurred because we were testing with a large diameter cylinder because our little ones didn't arrive yet, so we increased the pressure to 120 psi to get a faster response when we shifted.

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The problem with having such an insanely high gear is that the robot will be very hard to control. Another issue with such a huge difference in gearing is momentum. This year, my team used AM supershifters at 5/12fps. If we we were in high gear then switched to low at full speed, we would tip over 75-80% of the time. Imagine how bad it would be going between 17 and 5.

I'm pretty sure 111 did a 3-speed this year. Also, how could 4 speeds possibly be beneficial, even as an autoshift?