Shifting Gearboxes

[AlecS]

Originally Posted by Andrew Lawrence

From JVN's Acceleration calculator, I've found a speed of around 16 ft/s gains the largest time loss to travel a set distance when using 6 motors instead of 4 (Shaves off about a half a second on average). As you get further from 16 ft/s in either direction, the time difference to travel a set distance between 6 motor drives and 4 motor drives gets closer and closer to 0.

While this may be true in certain situations, there are many factors at work here. When you factor in battery and total circuit resistance, friction (static and kinetic), rolling resistance those numbers start to vary dramatically.

Using my own calculator, which incorporates math based on Ether's Voltage Drop model, I could shift the "ideal" speed you're talking about with relatively small changes. 5lbs difference in rolling resistance or a less than perfect internal battery resistance each shifted this number by +-2ft/s.

TL;DR the 16ft/s number could be correct in some situations, but there are simply too many variables at work here to conclude that it would be correct in all situations.

[markmcgary]

Originally Posted by AdamHeard

Why not use singles? Doubles waste an extra relay output.

We have yet to graduate to two-speed gearboxes. But, we have used pneumatics in each of our two years. When you speak of 'single solenoids' and 'double solenoids', I assume these are matched up with appropriate cylinders.

'Single Solenoid' working with a 'Single Acting Cylinder' (Spring-loaded).
'Double Solenoid' working with a 'Double Acting Cylinder'.

I can see a Single Solenoid, feeding two Single Acting Cylinders working for shifters.

Do I have this right?

[Pault]

Originally Posted by pmangels17

We always use a double solenoid, and we also use plumb both shifters to one valve so they throw at the same time. Please remember to USE PNEUMATICS TO SHIFT. Servo shifting is pure garbage and wont work under the the slightest pressure against the side of a dog, such as when pushing.

We usually start in low gear, and use high gear when we want to go really fast.

We have never tried using ball shifters, but since we have familiarity with the dog shifting style used in AndyMark gearboxes, and it has proved reliable so far as a shifting method, that probably won't change. Dog shifting is proven and tested, but ball shifters have been generally well reviewed as well, though they may be harder to implement in a custom gearbox.

What would you say to the team that does not plan to use pneumatics on their robot, but wants shifting? Do they add the weight, space, and complexity for an entire pneumatics system? Or do they just decide that they can't shift?

Honestly, if you already have pneumatics on your robot, then there really is no good reason to not use it for shifting. But if you don't, it's not the end of the world. Servo shifting is better than no shifting at all. In fact, I think that it actually is not that much worse than pneumatics.

[DampRobot]

Originally Posted by Pault

What would you say to the team that does not plan to use pneumatics on their robot, but wants shifting? Do they add the weight, space, and complexity for an entire pneumatics system? Or do they just decide that they can't shift?

I'd say that if they wanted to shift, that they should have planned for pneumatics (at least a stored system with a small plastic tank). There's not a whole lot of point to buying an expensive, heavy shifter and then crippling it by not letting it shift on the fly.

If you really can't find the weight for pneumatics, calculate the strategically optimal speed, and just go single speed. You honestly don't need to shift that much anyway, and I'm sure you'll be able to improve your manipulator with the 2 lbs you just bought yourself.

[AlecS]

Originally Posted by markmcgary

We have yet to graduate to two-speed gearboxes. But, we have used pneumatics in each of our two years. When you speak of 'single solenoids' and 'double solenoids', I assume these are matched up with appropriate cylinders.

'Single Solenoid' working with a 'Single Acting Cylinder' (Spring-loaded).
'Double Solenoid' working with a 'Double Acting Cylinder'.

I can see a Single Solenoid, feeding two Single Acting Cylinders working for shifters.

Do I have this right?

The terminology here refers to the number valves/ports the solenoid valve has.

A single solenoid has one valve, and two ports. (Port A and Exhaust A.) When you apply voltage to the solenoid it switches between EA and A, or vise versa. This means you can control two lines of air, by plumbing them to A and EA, as long as the robot is enabled. When it is disabled, no voltage can be applied and it will default into one port. In practice this means you can control a double acting cylinder, as long as you are fine with it defaulting into one position all the time, as the default can only be changed by plumbing.

A double solenoid has two valves are 4 ports. (Port A, Port B, Exhaust A, Exhaust B). The solenoid has two control inputs, and can control both A,EA and B,EB. It typically controls two lines of air by plumbing them to A and B, and venting/plugging EA and EB. In practice this means you can control which way a cylinder defaults when disabled, without having to re-plumb the cylinder, as you have two valves to control. You can also use double solenoids to provide a "neutral" position, where no air is supplied to the cylinder.

[smistthegreat]

Originally Posted by DampRobot

I'd say that if they wanted to shift, that they should have planned for pneumatics (at least a stored system with a small plastic tank). There's not a whole lot of point to buying an expensive, heavy shifter and then crippling it by not letting it shift on the fly.

If you really can't find the weight for pneumatics, calculate the strategically optimal speed, and just go single speed. You honestly don't need to shift that much anyway, and I'm sure you'll be able to improve your manipulator with the 2 lbs you just bought yourself.

Team 67 would like to disagree with you. This is taken from their 2012 Engineering Design:
"Since we are not using pneumatics on our robot but we want higher power shifting than servos will provide,
a Window motor shifter was designed to switch between high and low gear. This design uses an opposing
cam design to push or pull the dog gear in the transmission from high to low gear. Springs were designed
into the arms to allow for each side to shift at different times without destroying the other side. We were
able to package this design so that it fits right between our gearboxes and take up no more room than a
pneumatic setup."
The entire paper can be viewed here: http://hotteam67.org/hot_Engineering...Tech_Notes.pdf

[BBray_T1296]

Originally Posted by smistthegreat

Team 67 would like to disagree with you. This is taken from their 2012 Engineering Design:
"Since we are not using pneumatics on our robot but we want higher power shifting than servos will provide,
a Window motor shifter was designed to switch between high and low gear. This design uses an opposing
cam design to push or pull the dog gear in the transmission from high to low gear. Springs were designed
into the arms to allow for each side to shift at different times without destroying the other side. We were
able to package this design so that it fits right between our gearboxes and take up no more room than a
pneumatic setup."

You are right. But I think the main point he was trying to bring is not to use servos, which 67 clearly states they did not and why.

[T^2]

Originally Posted by pmangels17

ball shifters have been generally well reviewed as well, though they may be harder to implement in a custom gearbox.

Would you care to back up the second part of this statement?

Originally Posted by BBray_T1296

You are right. But I think the main point he was trying to bring is not to use servos, which 67 clearly states they did not and why.

In addition, 67 has stated that they wished to use pneumatics in 2013, but couldn't find the weight for it.


My $.02: If your driver is incapable of easily navigating the field with a single-speed drivetrain (looking at you, lots of rookie teams), how would a two-speed gearbox possibly improve matters? Your time would be better spent on driver training. (This is true for much of time spent on FRC bots, in general.)

[Isaac501]

I don't see double solenoids as required.. We use single solenoids with springs and it works like a champ. Even with no air pressure and medium load, the gearboxes still go into gear without issue on startup. This also allows for a "limp home" mode if your pneumatic system fails catastrophically (or a low noise, slow-speed demo)

To echo this again, do not use servos. They are weak and slow.