pic: Inverted CIM 2-Speed Gearbox

[KohKohPuffs]

loading photo ...

[asid61]

Looks good. You mounting of the idlers is interesting.
That current draw seems a little low for 17.5fps. What CoF are you using to calculate that?
What is your weight at?

[KohKohPuffs]

Originally Posted by asid61

Looks good. You mounting of the idlers is interesting.
That current draw seems a little low for 17.5fps. What CoF are you using to calculate that?
What is your weight at?

CoF is 1.3
Would it be higher if I'm using the rubber treads that go on the performance wheels?
And the weight is under the specs: 7.87lbs.

[Chris is me]

I'd lower your low gear quite a bit. For 2 CIM gearboxes, you wanna pull around 40 amps each pushing in low gear, so you don't trip the 40A breakers very easily. Between voltage drop and the breaker safety margin, you can afford to draw a bit more theoretical current, but no real need to push it. You're not driving in low gear to go fast. Around 6 FPS (81% speed loss constant) is roughly where you want to be, not 10.

[KrazyCarl92]

Originally Posted by Chris is me

You're not driving in low gear to go fast. Around 6 FPS (81% speed loss constant) is roughly where you want to be, not 10.

This is dependent on your design objectives. 2 speed shifting gearboxes are desirable because they allow for design criteria which are often conflicting with a single speed gearbox. There are kind of 2 "schools of thought" as far as the design criteria each gear is intended for.

School A:
-Low gear is intended for pushing matches, and in some cases fine movement (e.g. bridge balancing in 2012). You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs around or below 40 A per motor. With 6 CIMs, you might need to begin to consider the Main Breaker 120 A limit, but be sure to check out the spec sheet and understand how long your breaker will last at max current for the drive train.
-High gear is what you use at all times when you're doing anything other than pushing an opponent or doing fine motion which is less effective at a higher speed. The aim should be to minimize the travel time for a given sprint distance that aligns with your style of game play. It's nice if this is traction limited, but don't lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.

School B:
-Low gear is intended for completing game objectives with a short sprint distance, and again some fine movement tasks. The gearing should be chosen to optimize that sprint distance. You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs at some value which will allow you to be in a pushing match for some amount of time. You will also use this gear when in pushing matches.
-High gear is is intended for completing game objectives with a farther sprint distance. The aim should again be to minimize the travel time for a given sprint distance that aligns with your style of game play. Similarly to School A, it's nice if this is traction limited, but don't lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.

The merit to School A is that it's often easier on the drivers to have a simple dichotomy of which gear to use when. When you're mindset is "Pushing match = low gear, Every other situation = high gear", it's hard to go wrong. It is also good peace of mind to never worry about losing power during a pushing match, especially in years like 2014 (well...almost never ). Chris is spot on with his 6 ft/s suggestion if your design objectives align with School A. Depending on your efficiency and your CoF, 5-7 ft/s is a general range that gets you down to 40 A per motor at your traction limit for a full weight robot with battery and bumpers.

If my memory serves me correctly, one example of a School B design is the Killer Bees' robot in 2013. Their robot was a floor pick up machine and would often times pick up discs from the floor during teleop if the opportunity presented itself. However, there weren't always discs on the floor. In that case they would drive to the opposite end of the field to get discs from the feeder station. Playing the floor pickup role was a short sprint distance objective, and playing the feeder station role was a longer distance objective. They couldn't necessarily know going into each match what role they would play, and sometimes it would change throughout the match, so having a separate drive train gear ratio for each style of play was an elegant solution. It seemed to work well, they were World Finalists after all.

In general, drive train gearing is a trade-off between how long it takes you to go from point A to point B and how much current you're pushing through your breakers. When going above about 15 ft/s, you start to reduce your pushing force and initial acceleration in high gear at the expense of additional top speed (assuming a full weight robot plus battery and bumpers).

[Knufire]

I thought 33 2013's robot used shifitng for improved acceleration with their autoshift code.

[JesseK]

I'm not crazy about the tiny idler gear - seems like it'd wear very quickly and as a result become a detriment to efficiency as the season went on. Is that the largest gear you can get for that initial stage?

Originally Posted by KrazyCarl92

School B:
-Low gear is intended for completing game objectives with a short sprint distance, and again some fine movement tasks.

More and more I find myself learning from the masters in School B for certain games. 2011/12 would more have a School A thought - the field was congested or blocked and objectives needed slow/meticulous movements. 2013/14 required much less precision in control (so long as the intake made up for margin of error) so School B would have dominated.

As a result
- School A needs the best high-traction treads available to maximise performance in the situations the design is made for.
- School B needs tread that WILL slip under enough torque so as to not trip the breakers. Colsons fit that spec nicely, imo.

[asid61]

Originally Posted by JesseK

As a result
- School A needs the best high-traction treads available to maximise performance in the situations the design is made for.
- School B needs tread that WILL slip under enough torque so as to not trip the breakers. Colsons fit that spec nicely, imo.

I've found colsons tend to be rather grippy if you're aiming for slippage under high traction. Something like sky wheels or the white KOP wheels would work better IMO.

[KohKohPuffs]

I'm not crazy about the tiny idler gear - seems like it'd wear very quickly and as a result become a detriment to efficiency as the season went on. Is that the largest gear you can get for that initial stage?

So the main purpose of the idler gear is to get the gear space in between the CIMs because the way the gearbox is designed. If we do end up doing idler gears, then we could just replace them with new ones if they ever wear down. On the other hand, the big gear is a 72T gear, so I might be able to replace it with an 84T and remove the idlers. However, I remember on another gearbox I was working with another person, we had trouble with the gear sizes with an 84T.

I plan on making another gearbox of a similar design, and perhaps take a look at Andymark dog gears, which come in smaller diameters, so hopefully that will eliminate the need for dog gears.

I've found colsons tend to be rather grippy if you're aiming for slippage under high traction. Something like sky wheels or the white KOP wheels would work better IMO.

I was thinking this gearbox to be one of those that goes decently fast on high gear, and slow for pushing matches on low (I guess this would be School A?). With that being said, I was considering the nitrile rubber treads that go on Andymark performance wheels.

Hopefully this is the right way to go if I'm going for a maneuverable and defensive bot?

[MichaelBick]

Originally Posted by KohKohPuffs

So the main purpose of the idler gear is to get the gear space in between the CIMs because the way the gearbox is designed. If we do end up doing idler gears, then we could just replace them with new ones if they ever wear down. On the other hand, the big gear is a 72T gear, so I might be able to replace it with an 84T and remove the idlers. However, I remember on another gearbox I was working with another person, we had trouble with the gear sizes with an 84T.

Switching to belts might be a good solution here. Plus, you'd get a nice efficiency boost.

[Ari423]

Originally Posted by asid61

I've found colsons tend to be rather grippy if you're aiming for slippage under high traction. Something like sky wheels or the white KOP wheels would work better IMO.

You know which wheels aren't too grippy and work really well for slippage? Lunacy wheels

[GeeTwo]

Originally Posted by Ari423

You know which wheels aren't too grippy and work really well for slippage? Lunacy wheels

If it weren't for the emoji, I'd have to report you for trolling. My right eye is twitching, even though I never heard of FRC until April or May of 2011.

[KrazyCarl92]

Originally Posted by Knufire

I thought 33 2013's robot used shifitng for improved acceleration with their autoshift code.

You may be right. You likely had more interaction with them that year than I did. The information from my post was what I remember from a conversation I had with one of their members at Championship when I asked how they arrived at their drive train gear ratios (they seemed odd to me, and I wanted to learn more).

I would not be surprised at all to hear that they incorporated autoshifting in 2013, given that they developed a 4-speed autoshifting drive train and code as far back as 2004. However, the logic for gear ratio selection for autoshifting would still probably be consistent with the thought of having good sprint distance for both floor pick up and cycling. Maybe someone from Killer Bees could fill us in with more details?

Originally Posted by JesseK

2013/14 required much less precision in control (so long as the intake made up for margin of error) so School B would have dominated.

Interesting...I would have classified 2014 into School A given the prevalence of defense and the fact that just about every team would be playing D at one point or another. That is to say, sustaining a pushing match for 15+ seconds would be more valuable than shaving a few tenths of a second off of a 10 foot sprint. But like we were saying before, depends on which criteria you're aiming to meet.

[JesseK]

Originally Posted by KrazyCarl92

Interesting...I would have classified 2014 into School A given the prevalence of defense and the fact that just about every team would be playing D at one point or another. That is to say, sustaining a pushing match for 15+ seconds would be more valuable than shaving a few tenths of a second off of a 10 foot sprint. But like we were saying before, depends on which criteria you're aiming to meet.

(Apologies, just noticed this comment)
It seemed to me that the better defense for 2014 was a 'pillaring' technique. Pillaring is a tank warfare term, where the tank drives back & forth perpendicular to the cannon's aim. It requires planning & setup, but it makes the tank much harder to hit while making it relatively easy for it to maintain sighting on a target. This is prevalent in the Battlefield series of games.

This same concept works for defense on the FRC field. Sprint into position, then pillar back/forth and force the other team to either push you sideways or drive fast enough around you to get to their goal. The likelyhood of them pushing you is high - yet it's time consuming and usually not as effective as one would thing since it still doesn't solve the problem of them getting to their desired spot for an open shot.

Faster low gear speeds on an open field also give more opportunities to clip/turn a corner of a shooting bot - much more effective than raw pushing.

[EricH]

Originally Posted by GeeTwo

If it weren't for the emoji, I'd have to report you for trolling. My right eye is twitching, even though I never heard of FRC until April or May of 2011.

Actually, I'm not entirely sure what the CoF of a Lunacy wheel is on carpet. I'm pretty sure it's more than on that pesky Glassliner FRP (AKA regolith), though.


For those that don't get the reference... 2009 (Lunacy) was played on a field of Glassliner FRP with about a foot of carpet on all sides next to the rail. If you want to know what that looks/feels like, there's probably something similar in your nearest school/park restroom (as an anti-graffiti/easy-clean sort of measure). All robots were required to use certain wheels for their floor-contacting propulsion--the CoF between said wheels and the floor was something just under 1 as I recall, while your typical nitrile wheels are 1.something-or-other. Unlimited quantity... but that was the ONLY type allowed! Low-traction game, low-speed, low-friction...And then there were the trailers, but I'll end there.