Prototype transmission for potential use this year

[AdamHeard]

Regardless of gearing method, the motor will shift up and down it's curve as the applied load changes (being pushed is an applied load).

If your statement were true, the gear train would magically be creating and dissipating energy.

Originally Posted by sanelss

Well just think about it. Any wheel drive where you can move the motor by turning the wheel you will have drifting wheel speeds depending on what else is going on. for example another robot is trying to push you around or you want to slow down but the inertia carries your robot further. With this setup where the wheel can't move the motor. If we tell the motor to go at 100rpm, then it doesn't matter what else is going on that motor is going to draw up to 100A or apply extreme braking to make that motor turn at 100 rpm, not any slower and not any faster. That means if u want the robot to stop, it's going to come to a complete dead stop instantaneously. Also if we meet some resistance(another robot) then the PID loop will keep increasing force to the absolute limits to try to make that motor turn at the set speed. Also if another robot tries to push us around, even if we don't have enough power to overpower them, simply not trying to do anything will provide tons of force and keep them from pushing us around, well as much resistance as the friction of the omni wheels provide. With this design we don't need the force to over power other robots, we just need enough traction and that will prevent anyone from pushing us around.

[sanelss]

Originally Posted by AdamHeard

Regardless of gearing method, the motor will shift up and down it's curve as the applied load changes (being pushed is an applied load).

If your statement were true, the gear train would magically be creating and dissipating energy.

indeed it will. I never said it wouldn't however with this system is such that it makes the job much easier on the cim. For example if we were moving forward at a certain speed. A robot behind us is attempting to push us forward faster. It's not going to happen. All him trying to do so would REDUCE the load on our drive motors and the extra force would be directed into our chasis. If we wanted to remain stationary, and someone tries to push us, provided we have enough traction our robot will move absolutely nowhere regardless of how hard they push. or we can advance forward at the rate we want to advance at and not any faster than we want, due to this type of setup. Now granted if we faced head on against another robot, if we don't have more force than they do we won't be able to push them back, but at the same time they wouldn't be able to push us back either and all the cims simply have to do is not move and all the force is transferred into the transmission structure

[Madison]

Originally Posted by sanelss

indeed it will. I never said it wouldn't however with this system is such that it makes the job much easier on the cim. For example if we were moving forward at a certain speed. A robot behind us is attempting to push us forward faster. It's not going to happen. All him trying to do so would REDUCE the load on our drive motors and the extra force would be directed into our chasis. If we wanted to remain stationary, and someone tries to push us, provided we have enough traction our robot will move absolutely nowhere regardless of how hard they push. or we can advance forward at the rate we want to advance at and not any faster than we want, due to this type of setup. Now granted if we faced head on against another robot, if we don't have more force than they do we won't be able to push them back, but at the same time they wouldn't be able to push us back either and all the cims simply have to do is not move and all the force is transferred into the transmission structure

Forgive me if I missed this, but what kind of feedback are you receiving that allows you to measure actual velocity versus desired velocity?

With an encoder on the worm, you're only able to measure the CIM's speed; you can't empirically determine whether or not the rotation of the worm is actually being transformed into linear motion along the ground. The presence of (or lack of) backlash between the wheel / worm gear / worm doesn't change that.

[sanelss]

Originally Posted by Madison

Forgive me if I missed this, but what kind of feedback are you receiving that allows you to measure actual velocity versus desired velocity?

With an encoder on the worm, you're only able to measure the CIM's speed; you can't empirically determine whether or not the rotation of the worm is actually being transformed into linear motion along the ground. The presence of (or lack of) backlash between the wheel / worm gear / worm doesn't change that.

Yes you are right. I was talking from a more theoretical viewpoint rather than a practical one where assumptions like wheels not slipping aren't necessarily accurate. Although thinking about it, since we know how much and in what direction force is applied to the motors, we can have a feedback loop with the accelerators which would indeed tell us actual acceleration vs attempted. so if wheels are moving but robot isn't well we know we arn't actually going anywhere or if the proportions are off what they should be.

[yash101]

Saneless,
This is a really great design. Is your robot an omni-drive robot? I showed this to my team captain and he seems quite interested! Also, I do not know if this has been discussed yet, but since the worm can turn the gear but the gear can't turn the worm, wouldn't the momentum of the robot cause the gears to strip? I do not see the point of worm gears because one sudden stop could possibly strip the gear!

I think that the internal braking in motors should be enough. With that high of a gear ratio, that momentum is converted into electricity and discarded as heat, casuing no damage to any physical component!

[MichaelBick]

Originally Posted by yash101

Saneless,
but since the worm can turn the gear but the gear can't turn the worm, wouldn't the momentum of the robot cause the gears to strip?

192 ran worm gears and did not have any problems from what I heard. They have the nice benefit of giving you more space in the center of your robot.

[yash101]

Yes! That is one nice reward from using worms! They are very compact but give a great gear ratio! They are also lightweight!

[DampRobot]

Originally Posted by MichaelBick

192 ran worm gears and did not have any problems from what I heard. They have the nice benefit of giving you more space in the center of your robot.

They also used 4 lead worms for increased efficiency, which made them backdrivable. This let them coast to stops, and even theoretically get pushed

The other thing to keep in mind about worm gears is that most of their non-back drivability (even for 1 lead worms) comes from when they're stopped. When they're stopped, you've got a ton of force pushing the worm gear's teeth into the worm (if the output's under load), and because of the angle of the contact between the teeth, it usually can't slip. However, when the tooth surfaces are turning relative to each other, you're dealing with kinetic sliding friction, which is a lot less. You'll actually see the output pushing the input a bit when some worm gearboxes aren't under power, but moving. Because they no longer have to deal with the huge static coeficient of friction, they become essentially temporarily backdrivable. This means that worm gearboxes can coast for a little while, especially when they've got a fair amount of force on the output.

[yash101]

That seems brilliant! Do you know how that would work? It seems quite tricky!

[MichaelBick]

Originally Posted by DampRobot

They also used 4 lead worms for increased efficiency, which made them backdrivable. This let them coast to stops, and even theoretically get pushed

From what I've heard they weren't backdriveable until the gears wore in, and they had no problems.

[sanelss]

Originally Posted by yash101

Saneless,
This is a really great design. Is your robot an omni-drive robot? I showed this to my team captain and he seems quite interested! Also, I do not know if this has been discussed yet, but since the worm can turn the gear but the gear can't turn the worm, wouldn't the momentum of the robot cause the gears to strip? I do not see the point of worm gears because one sudden stop could possibly strip the gear!

I think that the internal braking in motors should be enough. With that high of a gear ratio, that momentum is converted into electricity and discarded as heat, casuing no damage to any physical component!

What do you mean by omni-drive? I think the answer is yes. The sudden stop I don't really think will be an issue, yes the wheels will lock up but worse case the energy will go into the robot frame and might tip the robot slightly or make the wheels just skip over carpeting before anything bad happens. It takes considerable force to strip these teeth but we will see how much of an issue it becomes. We already have several contingency plans in case it becomes an issue(such as a rubber interface as a clutch system that would slip on high force changes).