pic: 3CIM Ball Shifter

[AdamHeard]

Originally Posted by Chris is me

While this discussion has quickly gone on a tangent, I do have a question: Other than lowering current draw for pushing situations, what's the point of a low gear? Unless you're traveling quite fast, your acceleration won't be notably better. For short distances it seems a low gear would be helpful, but because the distances traveled are so short, you're not saving that much time anyway.

I guess what I'm wondering is when someone would want to use a 7-9 FPS low gear with a 14-16 FPS high gear. It seems like to me the only situation where low gear would be more useful than high gear for those speeds would be when you have to push or make really fine adjustments, and both of those are accomplished well with a ~5.5 FPS, 40A traction limited low gear.

I don't have any experience here; I've only ever worked with single speeds, so I'd like to learn.

We use our low gear in more of a West Coast Style. It seems like 254/968 (when I was growing up) rarely used their low gear, even for short distances. It was moreso for the few times you were forced to push through defense.

We use it in the same way, and then also use it for thins like balancing on the bridge last year.

For how our drivers operate, I view low gear as insurance that lets you gear as fast as you desire for other objectives, but still have a drive mode where you won't get destroyed by defense (tripping breakers, etc...)

Because of this, we love a real slow low gear.

[apalrd]

Our driving theory around low (this was not always the case, but several of us on 33 have converged on this point independently) is that accelerating from a standstil is better in low gear, usually to around 60-80% of the low gear speed, than in high gear. So any time where we never reach peak speed in low, it's faster to drive in low. These are marginal gains in time. BUT, since high gear is in the 'bad side' of the motor power curve for the entire time (with 2:1 or more spread), the current draw from high gear will be WAY higher than in low. For the same or worse dynamic performance. This becomes MUCH worse on a dieing battery, which can frequently be seen near the end of a match.

We've sometimes run auto-upshift software to automate this, it begins the shift around 60% of low gear speed, but the shift takes time to execute under load (not sure exactly what speed it's at by the time the dog disengages).

We also run auton in low gear usually, for precision/control reasons, so a slightly high low gear is good for this. But we could run in high, we just don't, so it's not a huge objective.


I guess this differs from the 'west coast' opinion that 'high gear is where we operate, and low gear is just in case we need to push'. We operate in both gears.

[AdamHeard]

Originally Posted by apalrd

Our driving theory around low (this was not always the case, but several of us on 33 have converged on this point independently) is that accelerating from a standstil is better in low gear, usually to around 60-80% of the low gear speed, than in high gear. So any time where we never reach peak speed in low, it's faster to drive in low. These are marginal gains in time. BUT, since high gear is in the 'bad side' of the motor power curve for the entire time (with 2:1 or more spread), the current draw from high gear will be WAY higher than in low. For the same or worse dynamic performance. This becomes MUCH worse on a dieing battery, which can frequently be seen near the end of a match.

We've sometimes run auto-upshift software to automate this, it begins the shift around 60% of low gear speed, but the shift takes time to execute under load (not sure exactly what speed it's at by the time the dog disengages).

We also run auton in low gear usually, for precision/control reasons, so a slightly high low gear is good for this. But we could run in high, we just don't, so it's not a huge objective.


I guess this differs from the 'west coast' opinion that 'high gear is where we operate, and low gear is just in case we need to push'. We operate in both gears.

I don't disagree with your points at all, I like the level of automation you've built in.

I don't know that anyone has run a decent autoshift out here.

I can certainly appreciate the lesser current draw, a dirty secret of some teams out here is we buy a new set of batteries every year, and if we didn't we'd like show worse performance on the field.

Part of this is we practice A LOT, so we use the last 1-2 seasons of batteries for practice to keep the new seasons worth nice.

[Andrew Schreiber]

Originally Posted by AdamHeard

I don't know that anyone has run a decent autoshift out here.

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.

[AdamHeard]

Originally Posted by Andrew Schreiber

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.

They're not out here

[apalrd]

Originally Posted by Andrew Schreiber

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.

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.

[magnets]

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.

[apalrd]

Originally Posted by magnets

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.

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?

[magnets]

Originally Posted by apalrd

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?

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.

[Ian Curtis]

Originally Posted by apalrd

I guess this differs from the 'west coast' opinion that 'high gear is where we operate, and low gear is just in case we need to push'. We operate in both gears.

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.

[DampRobot]

Originally Posted by AdamHeard

We use it in the same way, and then also use it for thins like balancing on the bridge last year.

For how our drivers operate, I view low gear as insurance that lets you gear as fast as you desire for other objectives, but still have a drive mode where you won't get destroyed by defense (tripping breakers, etc...)

Because of this, we love a real slow low gear.

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.