The in-wheel swerve shifter!

[Jared]

This looks really cool. It looks like it could work well on a 2013 climber, where you'd want to deploy quickly, but raise the robot with lots of torque.

What happens if you're going slowly and somebody pushes your robot from behind? Instead of the wheels driving the motor to go faster, the dog will back out, but it won't engage with the other side because it's going the wrong way, so the dog will be bouncing off the two sides until you speed up the motor or reverse direction.

[Chris is me]

Originally Posted by asid61

Why can this not be shifted on the fly? Am I missing something here? Keep in mind that this can ony go forward. There is no reverse because I'm driving sun-ring, not sun-carrier in the planetary gearset. This means that the reverse of direction by the cim will only work against the inertia of the drive shaft, not the whole robot.

It can't shift on the fly because even if the output direction doesn't change, reversing the CIM will still slow the drive down to zero and then go back up to speed. The motor has to fight the inertia of the drive system when reversing input direction, and this takes time at which point you aren't moving.

[asid61]

Originally Posted by Chris is me

It can't shift on the fly because even if the output direction doesn't change, reversing the CIM will still slow the drive down to zero and then go back up to speed. The motor has to fight the inertia of the drive system when reversing input direction, and this takes time at which point you aren't moving.

Like I said, the motor only fights against the drive shaft. That's just a couple miter gears, two 3" shafts, 2 sprockets and a gear. The CIM will instantly reverse unloaded, so even with the above it's not a lot of inertia.
That would be something that would be debugged and found in testing.

Originally Posted by Jared

This looks really cool. It looks like it could work well on a 2013 climber, where you'd want to deploy quickly, but raise the robot with lots of torque.

What happens if you're going slowly and somebody pushes your robot from behind? Instead of the wheels driving the motor to go faster, the dog will back out, but it won't engage with the other side because it's going the wrong way, so the dog will be bouncing off the two sides until you speed up the motor or reverse direction.

Interesting example. If a fast robot pushed you, the dog would instantly lock into the other gear, be it low or high. Then as you said, it would probably bounce around. I would want to test this too to see for myself the ramifications.
You would have to have code to debug this situation, probably with current sensors to see if the current gets too low, signifying a robot is pushing you. Then the cim would reverse direction.
In any case, then you could just shut down the cims if it continues (it would be a pretty obvious strategy) and you're going in the direction you want to anyway, but faster. So that situation would be useless for the other robot.

[jbsmithtx]

Would you then have to put some sort of brake system on your roobt? or would you simply have to spin the wheels the other direction to get them to slow down?

[Chris is me]

Originally Posted by asid61

Like I said, the motor only fights against the drive shaft. That's just a couple miter gears, two 3" shafts, 2 sprockets and a gear. The CIM will instantly reverse unloaded, so even with the above it's not a lot of inertia.

Why are you ignoring the load of the robot and the drivetrain itself? Even with your shifting coupler, it will only disengage as the motor attempts to turn the shaft the opposite direction. Until this is disengaged, as the motor slows the drivetrain slows, and the load of the drivetrain is placed on the motor.

CIMs do not "instantly" switch direction under zero load. Quickly, maybe, not instantly.

[DonRotolo]

I share the concerns about the forces seen by the dog teeth; steel or not, the shock loads can easily exceed several tens of LbFt, translating to hundreds of LbF on the tooth.

But that can easily be compensated in the final design. Overall a very ingenious solution!

As for needing a 4-axis CNC: Naah. We got metal files and hordes of Freshmen...

[Jared Russell]

I want to see an in-wheel CVT/IVT using two motors and a planetary transmission.

[Greg Woelki]

If I'm visualizing this correctly, if you are driving and then stop, the engaged gear will push past the dog gear because of the inertia of the robot and the dog gear will be pushed back and forth as the robot comes to a stop.

[Seth Mallory]

If you took a Sturmey Archer S3X fixed gear 3 speed hub you could make a 3 speed swerve drive. It would most likely be to wide but it would work for testing and as a base for a interesting design. Remove the flanges and mount some tread to the hub and you have a 3 speed wheel. You would want to cut the axils down and a few other things but it would be great for testing concepts and to work from.

[Greg Woelki]

Originally Posted by Seth Mallory

If you took a Sturmey Archer S3X fixed gear 3 speed hub you could make a 3 speed swerve drive. It would most likely be to wide but it would work for testing and as a base for a interesting design. Remove the flanges and mount some tread to the hub and you have a 3 speed wheel. You would want to cut the axils down and a few other things but it would be great for testing concepts and to work from.

You could certainly test the concept with them like you said, but as far as a competition robot goes they aren't just, wide, they're also two pounds each!

[Seth Mallory]

That is why I said they would be good for testing. Also the design is worth looking at as a start of a possible gear box design. I am not saying use the parts but some of the ideas. Any way this was posted for giving ideas to think about.

[asid61]

Originally Posted by Chris is me

Why are you ignoring the load of the robot and the drivetrain itself? Even with your shifting coupler, it will only disengage as the motor attempts to turn the shaft the opposite direction. Until this is disengaged, as the motor slows the drivetrain slows, and the load of the drivetrain is placed on the motor.

CIMs do not "instantly" switch direction under zero load. Quickly, maybe, not instantly.

Okay, consider this:
As soon as the cim begins to slow, the mated hub/gear will continue spinning at the previous speed, as it is not mated to the shaft except by the dog (which is slowing down). The obtusely angled faces on the dog and hub/gear will contact each other and push the dog into a mate with the other hub/gear, which is spinning in the opposite direction as the original mated gear/hub.
Keep in mind the cim is accelerating to match the speed of the second gear/hub, so its opposite spin, combined with the cim itself accelerating in the opposite direction, will make the shift time extremely low. The inertia of the robot aids the shift.

I said "instantly" as a hyperbole. It was undetectable by the human eye. The only way I knew it has reversed was because I was holding the cim and I felt it shake.

Originally Posted by DonRotolo

I share the concerns about the forces seen by the dog teeth; steel or not, the shock loads can easily exceed several tens of LbFt, translating to hundreds of LbF on the tooth.

But that can easily be compensated in the final design. Overall a very ingenious solution!

As for needing a 4-axis CNC: Naah. We got metal files and hordes of Freshmen...

I still want to test this. Like I said, the inertia aids the shift, so I want to test under load to see if it shifts properly.

Freshmen doing this would be interesting.

Originally Posted by Seth Mallory

If you took a Sturmey Archer S3X fixed gear 3 speed hub you could make a 3 speed swerve drive. It would most likely be to wide but it would work for testing and as a base for a interesting design. Remove the flanges and mount some tread to the hub and you have a 3 speed wheel. You would want to cut the axils down and a few other things but it would be great for testing concepts and to work from.

Funny story, I was just looking into this. They seem really cool, but I'm worried about it shifting when the motors slow down due to the uncontrolled autoshifting on weird turns. I also am not sure what drives the shift. Love those bicycle gear hubs though.

Originally Posted by Jared Russell

I want to see an in-wheel CVT/IVT using two motors and a planetary transmission.

I cadded something like 5 of these for a few weeks before I realized there was no torque advantage to using a planetary IVT. And the only other CVTs I saw were cone CVTs and the like. So I moved onto planetary shifters and swerves.
None of my designs were in-wheel though. That's like... weird...

Originally Posted by Greg Woelki

You could certainly test the concept with them like you said, but as far as a competition robot goes they aren't just, wide, they're also two pounds each!

Well, if you can spare the weight. Swerve chassis that don't shift normally can be extremely light (<30lbs with motors).

[asid61]

Originally Posted by jbsmithtx

Would you then have to put some sort of brake system on your roobt? or would you simply have to spin the wheels the other direction to get them to slow down?

Interesting question; I hadn't considered that.
To slow down...
hm...
when you stopped down the cim, the shifter would shift into the other gear and then stop. Merely slowing it down would cause the angled faces on both sides to push the dog around I think, until the robot decelerated to the speed of the dog? Or does the motor actively force a deceleration by contacting the other mating gear/hub? I need to visualize this.
I need to think on this one. Very insightful of you.

[Greg Woelki]

Funny story, I was just looking into this. They seem really cool, but I'm worried about it shifting when the motors slow down due to the uncontrolled autoshifting on weird turns. I also am not sure what drives the shift. Love those bicycle gear hubs though.

You can see shifting at just past 2:01.
http://www.youtube.com/watch?v=7LTZPrksqU0

I don't really see a simple way to shift these without limiting the rotation of each wheel, though.

[asid61]

Originally Posted by Greg Woelki

You can see shifting at just past 2:01.
http://www.youtube.com/watch?v=7LTZPrksqU0

I don't really see a simple way to shift these without limiting the rotation of each wheel, though.

So they need human input then? I swear I saw autoshifting versions.
I'm opening up the CAD so I can see what happens when you slow down.

EDIT: yup, the dog bounces back and forth like crazy while rubbing against the shifting elements when you slow down, which would completely destroy the teeth on the mating elements. I'm going to find a solution to that.
DOUBLEEDIT: So what would happen is this:
1. The dog slows.
2. The dog is pushed into the other gear by the obtusely angled faces.
3. The obtusely angled face there pushed the dog back.
4. Repeat steps 2-3 until the dog contacts the flat of the teeth and cannot shift out of its current gear.
5. The gear that it is in is forced to slow down slightly until the dog bounced back into the other gear.
Repeat until slowed. There would be a lot of bouncing, but nothing too high load except for the rubbing at the end. Another thing to test under load, although this is a much more serious issue.

Of course, that doesn't matter if you are doing something at a constant 100% speed like loading a spring or climbing, but for drive this seems like a problem.