One speed vs Two speed gearboxes

[Abhishek R]

Originally Posted by Jared

I've seen you say this a few times. How would you set up an auto-shift system? When do you shift up and shift down?

Maybe you could use encoders to determine the speed the wheels are running, and go from there setting a threshold above which to use 2nd gear and then below that value it goes back down to first gear. We used auto-shifting this year, and it worked pretty well. We also had a manual override in case we were in a pushing match or other scenario where auto-shifting may not be entirely reliable. However, all in all, it definitely worked well for us. One less thing for the driver to worry about, although we are looking into single-speed gearboxes over the offseason.

We also had a timer that did a 1 second lockout to prevent rapid repetitive shifting between 1st and 2nd gear when reading the values.

[AdamHeard]

Originally Posted by asid61

Autoshifting is done by either current sensing or heat sensing on the main breaker. When the current spikes (or the heat) for an extended period of time, you shift down. You can find a curve fit for the graph of the main breaker current limit versus time, or just have hard limits and times. This prevents breaker blows.
Generally you only need to utilize this in a pushing match, and the driver gets distracted. You could potentially also use collision detection, where the driver is pushing on the joystick but the robot is not moving. Then the program shifts down after a couple seconds. Shifting up is manual.
There is a number of ways to accomplish autoshifting.

I know no team is autoshifting off heat currently.

I'm unsure any teams are using current (as you technically could with the jaguars).

Teams that autoshift now are primarily shifting based on encoder feedback, once they hit a certain rpm in low gear they shift up (for a full voltage output). Once they hit below a certain rpm in high gear (for full voltage output) they shift down. There is sometimes a timer on it to prevent gear hunting (rapid back and forth).

Easy current monitoring will make the down/up shift easier next year.

By checking rpm and assuming full voltage output, the above method was essentially shifting based on current.

[Caleb Sykes]

Originally Posted by rick.oliver

I also value the ability to maintain a desired field position once achieved.

Originally Posted by asid61

More pushing power, so you can force them away to stop them from slipping past.

These seem like very different goals to me. Maintaining a desired field position seems to imply an offensive strategy. For example, this year it might have been beneficial for a robot to hold its shooting position for a few seconds while getting ready to shoot. Forcing away opponents to stop them from slipping past seems to imply a defensive strategy.

[EricH]

Originally Posted by inkling16

These seem like very different goals to me. Maintaining a desired field position seems to imply an offensive strategy. For example, this year it might have been beneficial for a robot to hold its shooting position for a few seconds while getting ready to shoot. Forcing away opponents to stop them from slipping past seems to imply a defensive strategy.

They are not necessarily different. We didn't see this last year, but in 2013, a robot that could maintain a desired field position could be VERY desirable for shutting down a FCS-type robot. I mean hanging out in their line of fire with a maximum-height blocker and STAYING there--but not necessarily keeping them from moving, just keeping them from moving YOU. In other games, a similar form of defense could have been played. I believe it's known as "plugging the gaps" in some sports.

Your reasons for holding a desired field position are your own. Your method had better have a lot of traction, though... Which is why you want a low gear. But getting to that position so that you can hold it may need a high gear. Depends on the game whether the low gear you need and the high gear you need are close enough to do it single-speed or far enough a part to use a shifter.

[Jared]

Originally Posted by AdamHeard

I know no team is autoshifting off heat currently.

I'm unsure any teams are using current (as you technically could with the jaguars).

Using current alone doesn't work.
The initial current spike when beginning a pushing match (where you want to shift) looks the same as the initial current spike during acceleration.

Autoshifting is really, tricky.
There are so many instances where the robot can't possible know that shifting would be an issue at the current point in time. When you're turning, drifting to a stop, or grabbing a game piece, you don't want to shift.

Yes, it's possible, but I think it may be more efficient for the driver to just remember where the shift button on the joystick is, and the coach can instruct the driver on what gear to be in (if the driver is too busy to make the decision himself.)

[Lil' Lavery]

Originally Posted by EricH

Your method had better have a lot of traction, though... Which is why you want a low gear.

How does having a low gear increase your traction?

[EricH]

Originally Posted by Lil' Lavery

How does having a low gear increase your traction?

It doesn't increase your traction, but it increases your use of traction (until your wheels slip, that is). If you're being pushed backwards, and your wheels aren't slipping, you aren't using enough.

[AdamHeard]

Originally Posted by Jared

Using current alone doesn't work.
The initial current spike when beginning a pushing match (where you want to shift) looks the same as the initial current spike during acceleration.

Autoshifting is really, tricky.
There are so many instances where the robot can't possible know that shifting would be an issue at the current point in time. When you're turning, drifting to a stop, or grabbing a game piece, you don't want to shift.

Yes, it's possible, but I think it may be more efficient for the driver to just remember where the shift button on the joystick is, and the coach can instruct the driver on what gear to be in (if the driver is too busy to make the decision himself.)

I should've been more thorough; You don't just do a greater/less than check on current, you need to look at it over time (how it's changing), along with applied voltage to make intelligent decisions.

[Lil' Lavery]

Originally Posted by EricH

It doesn't increase your traction, but it increases your use of traction (until your wheels slip, that is). If you're being pushed backwards, and your wheels aren't slipping, you aren't using enough.

If your wheels are slipping, you're reducing your traction.

[asid61]

Originally Posted by AdamHeard

I should've been more thorough; You don't just do a greater/less than check on current, you need to look at it over time (how it's changing), along with applied voltage to make intelligent decisions.

+1.
On heat checking: That's the best way to do it IMO, as I believe the breaker trips based on heat, but it also something that hasn't been done before. You would need to gen an IR gheat sensor and direct it at the breaker.

[Mike Marandola]

Originally Posted by asid61

+1.
On heat checking: That's the best way to do it IMO, as I believe the breaker trips based on heat, but it also something that hasn't been done before. You would need to gen an IR gheat sensor and direct it at the breaker.

It doesn't cool down very rapidly at room temp though.

[asid61]

Originally Posted by Mike Marandola

It doesn't cool down very rapidly at room temp though.

True. So only an emergency measure then.

[Jared Russell]

I don't think auto-shifting makes a lot of sense in the world of FRC. The "correct gear" to be in at any time is a function of two things:

1) What am I doing now?
2) What am I going to be doing in the near future?

Only the driver knows #2.

[Rauhul Varma]

Originally Posted by tindleroot

Other than the actual cost, shifting gearboxes require more space on the robot, more weight, and more air.

I would beg to differ on all accounts. You can definitely make light and small 2 speed gearboxes. Additionally the amount of air required to shift is extremely small, for example the volume of air required for a single shift on 192's 2014 gearbox was (9/16)^2 * pi * 3/8 = 0.373 in^3 of air, which is nothing as many tanks hold around 35 in^3 of air.

[asid61]

Originally Posted by Jared Russell

I don't think auto-shifting makes a lot of sense in the world of FRC. The "correct gear" to be in at any time is a function of two things:

1) What am I doing now?
2) What am I going to be doing in the near future?

Only the driver knows #2.

You don't need full autoshifting. Just enough such that your driver doesn't blow the breaker.

Originally Posted by Rauhul Varma

I would beg to differ on all accounts. You can definitely make light and small 2 speed gearboxes. Additionally the amount of air required to shift is extremely small, for example the volume of air required for a single shift on 192's 2014 gearbox was (9/16)^2 * pi * 3/8 = 0.373 in^3 of air, which is nothing as many tanks hold around 35 in^3 of air.

+1, but some caveats. COTS shifters weigh a lot and take up a lot of space. A custom shifter is much easier to work in like 192's design this past year and hang over the wheels, taking up less "valuable space" than a single speed, depending on the chassis.