4607 Butterslide Drivetrain

[Ginger Power]

Here is 4607's Butterslide Drivetrain.

Butterfly Drive + Slide Drive = Butterslide Drive

Specs:
65 lbs including pneumatics, and all electrical components.
Everything can be built with simple power tools plus a mill.
10 Wheels including six 6" Duraomni wheels and four 4" Stealth Wheels.
In omni mode: the drivetrain has a 12:84 gear ratio which equates to 16 ft/s Adjusted Speed accoding to the JVN Calculator.
In traction mode: There is an additional 20:30 reduction which, along with the difference in wheel size equates to 7ft/s Adjusted Speed. When in traction mode, the strafe wheels are lifted off the ground.

With a strafe wheel at each end of the robot, there would seem to be many possible maneuvers this drivetrain would be capable of.

Now that I've fallen in love with the design, I need somebody to rip it apart and tell me what's wrong with it!

[thatprogrammer]

The stealth wheels are not super grippy from what I have seen. Maybe you should consider using other wheels such as the VEX Versa Wheels or colsons?

[JoaquinC]

Great Design I like the idea of having two strafe wheels at either end of the robot for more control and freedom of movement. Most slide drives I've seen (and the one's I've built) only have one strafe wheel in the center. The only case I can think of in which going back to one center wheel would be more beneficial is if you needed to intake something through an open bumper.

[Jon Stratis]

Can yo post a more detailed rendering of the butterfly modules? I don't think I've ever seen them set up that way... I would potentially be concerned about stress on the traction portion, the way it's cantilevered off the main rail, but it's hard to tell how things are attached and if that would be a problem. I do like it all being direct drive with no belts or chains, though!

[Chris is me]

"Butterslide" is a cool name. This drive has existed in various iterations before (The "nonadrive" / "decadrive" 148 sometimes uses that the butterfly drive emerged from is a different version of this), but this is still cool. The design has some neat tricks in it and is a great start

The first challenge you'll encounter with this style of drive is that in Omni mode, you'll have six wheels touching the ground at the same time. These probably won't end up coplanar, and you may encounter weird variations in strafing ability based on CG, chassis rigidity, etc. Lots of teams spring-load at least the sideways Omni wheels (or sometimes, all of the Omni wheels!) in order to deal with variation in chassis rigidity, etc. With good code, you may be able to compensate for unexpected chassis maneuvers though. Not sure how it'll work out.

The second challenge is going to be how the traction wheels are loaded. You are essentially cantilevering a wheel off of another cantilevered shaft, and this multiplies the forces and torques at play to an extent. Minimizing the length of that second lever arm (which it seems you've done) is a great first step to mitigating this issue, but only testing will show if this setup is robust enough. I would look into how the Vex Drop Drive handles this issue - these modules actually transmit some of the forces through the 2x1 tube with physical constraints in a really clever way.

Some teams (148 most notably) have found the easiest way to make a robust setup is to have the TRACTION wheel permanently mounted to the chassis, and make the OMNI wheel pivot. Since the Omni wheel obviously deals with less side load than the traction wheel, this setup is more robust. However this does present a challenge since generally the traction wheel runs at a lower gear ratio than the Omni (which is a good choice by the way) and thus it's hard to gear it right without also putting the CIM on the pivot. Other teams (3928, the Drop Drive) deal with this issue differently.

Great start! Keep doing stuff like this. Make sure you have enough clearance to actually insert your gears into the tubing / whatever as well.

[Knufire]

I would highly recommend switching to a different traction wheel. The AM Stealth Wheels were not designed to hold the weight of an FRC robot. Will they anyway? Maybe, but it's definitely not guaranteed.

Given the way you've mounted and powered the wheel, it should be easy to swap to a HiGrip wheel with a hex bore hub (as long as that the HiGrip assembly isn't too wide).

[Ginger Power]

Originally Posted by thatprogrammer

The stealth wheels are not super grippy from what I have seen. Maybe you should consider using other wheels such as the VEX Versa Wheels or colsons?

This is great to know, thanks! Honestly I chose the stealth wheels without much thought. It'll take 5 minutes to switch out the wheels on the CAD, and nothing has been ordered yet!

Originally Posted by JoaquinC

The only case I can think of in which going back to one center wheel would be more beneficial is if you needed to intake something through an open bumper.

Of course this isn't designed with any specific manipulator in mind, if I were to do an open bumper manipulator, the strafe wheel would definitely be moved!

Originally Posted by Jon Stratis

Can yo post a more detailed rendering of the butterfly modules? I don't think I've ever seen them set up that way... I would potentially be concerned about stress on the traction portion, the way it's cantilevered off the main rail, but it's hard to tell how things are attached and if that would be a problem. I do like it all being direct drive with no belts or chains, though!

I'll do my best to do a quality rendering of the modules! My laptop was burning up last night after 12 hours of working on it. Plus I'm terrible at rendering... I've thought a lot about the stresses on the actuated member that supports the traction wheel. I plan to do both statics and dynamics problems later today to see what I find. We'll see how that goes!

Originally Posted by Chris is me

"Butterslide" is a cool name. This drive has existed in various iterations before (The "nonadrive" / "decadrive" 148 sometimes uses that the butterfly drive emerged from is a different version of this), but this is still cool. The design has some neat tricks in it and is a great start

I've done a lot of research on different drivetrains, especially 148. I was lucky enough to see 3928's 2013 drivetrain in person at my first FRC Regional. I haven't seen anything exactly like what I've designed before, so at least it's an interesting thought experiment. I do plan on building the drivetrain during a robotics summer camp with 4607.

The first challenge you'll encounter with this style of drive is that in Omni mode, you'll have six wheels touching the ground at the same time. These probably won't end up coplanar, and you may encounter weird variations in strafing ability based on CG, chassis rigidity, etc. Lots of teams spring-load at least the sideways Omni wheels (or sometimes, all of the Omni wheels!) in order to deal with variation in chassis rigidity, etc. With good code, you may be able to compensate for unexpected chassis maneuvers though. Not sure how it'll work out.

This is obviously a concern! I don't see an easy way to add suspension without increasing the weight of the drivetrain significantly. I know 2175 used a tennis ball for suspension on their 2011 slide drive, but that doesn't make sense for the way I have things set up. What other methods of adding suspension have teams used? Crazy, probably ineffective idea: Would vertically slotting out the bearing holes and using a spring or functionally equivalent item to push down on the axles work?

The second challenge is going to be how the traction wheels are loaded. You are essentially cantilevering a wheel off of another cantilevered shaft, and this multiplies the forces and torques at play to an extent. Minimizing the length of that second lever arm (which it seems you've done) is a great first step to mitigating this issue, but only testing will show if this setup is robust enough. I would look into how the Vex Drop Drive handles this issue - these modules actually transmit some of the forces through the 2x1 tube with physical constraints in a really clever way.

I'll check out the Versadrop a little more closely! After I do my statics and dynamics analysis on the members involved that'll give me a good idea of whether or not I need to reinforce anything. I have indeed tried to minimize the length of the lever arm on the traction wheel, while maximizing the gear ratio within the lever arm (20:30).

Some teams (148 most notably) have found the easiest way to make a robust setup is to have the TRACTION wheel permanently mounted to the chassis, and make the OMNI wheel pivot. Since the Omni wheel obviously deals with less side load than the traction wheel, this setup is more robust. However this does present a challenge since generally the traction wheel runs at a lower gear ratio than the Omni (which is a good choice by the way) and thus it's hard to gear it right without also putting the CIM on the pivot. Other teams (3928, the Drop Drive) deal with this issue differently.

I thought to do this, and tried for a solid 2 hours before giving up and going in a different direction.

Great start! Keep doing stuff like this. Make sure you have enough clearance to actually insert your gears into the tubing / whatever as well.

I'm not sure I slotted all of the tubing enough for the 84t gears, but that's a very easy change! Thanks for the advice! I'll most definitely take it into consideration when I'm improving the drivetrain!

[D.Allred]

Originally Posted by Ginger Power

This is obviously a concern! I don't see an easy way to add suspension without increasing the weight of the drivetrain significantly. I know 2175 used a tennis ball for suspension on their 2011 slide drive, but that doesn't make sense for the way I have things set up. What other methods of adding suspension have teams used? Crazy, probably ineffective idea: Would vertically slotting out the bearing holes and using a spring or functionally equivalent item to push down on the axles work?

Most suspension systems I've seen are pivoting pods, not slotted bearings, that can be pressed down with springs or cylinders.

Take a look at the VexPro clamping gearbox as a motor mounting method, or come up with some custom equivalent. Currently your "gearbox" is two separate pieces of metal. It would be better practice to have the CIM centerline and main wheel bearing be one unit.

David

[Chris is me]

Originally Posted by D.Allred

Most suspension systems I've seen are pivoting pods, not slotted bearings, that can be pressed down with springs or cylinders.

The main reason for this is, that a pivoting pod that pivots about the same axle as the power transmission (gear, sprocket, etc) will hold the same center to center distance between the output and pivot axles. A floating slotted bearing block will not, increasing complexity and potential issues with misalignment. Pivot style suspensions are an easy solution to this problem.

[Ginger Power]

Here are some higher quality images.

Originally Posted by D.Allred

Most suspension systems I've seen are pivoting pods, not slotted bearings, that can be pressed down with springs or cylinders.

Take a look at the VexPro clamping gearbox as a motor mounting method, or come up with some custom equivalent. Currently your "gearbox" is two separate pieces of metal. It would be better practice to have the CIM centerline and main wheel bearing be one unit.

David

I'll definitely look into integrating the motor directly onto the module assembly. I had attempted to do this, but it proved to be difficult given the geometry of the module. In order to do any kind of suspension I'll need to solve the motor mounting issue first...

The question now begs, is suspension worth it? I feel like it would be fairly trivial to implement a pivoting suspension system on the strafe wheels, but not on the modules. I'm not sure how flexible a 2"x1"x1/8" aluminum frame is, but I feel as though it is more feasible to do the precision machining to ensure all 4 modules are coplanar than it is to add suspension to the modules.

Thanks for the awesome advice!

[Chris is me]

Originally Posted by Ginger Power

The question now begs, is suspension worth it? I feel like it would be fairly trivial to implement a pivoting suspension system on the strafe wheels, but not on the modules. I'm not sure how flexible a 2"x1"x1/8" aluminum frame is, but I feel as though it is more feasible to do the precision machining to ensure all 4 modules are coplanar than it is to add suspension to the modules.

I would do the suspension on the strafe wheels and probably skip it on the drive rails. Drive rails are four points of contact but you can be pretty sure each pair will always be coplanar with each other, so you shouldn't need a suspension there.

I'll respond more to your other points in a bit when I get the chance.

[DaveL]

That's an interesting concept.

I too agree that the stress on the pivoting drive shaft will be high and that might cause a significant amount of flex.

Some ideas to reduce the possible bending on the shaft:
1. Use ThunderHex rod for the shaft.

2. Support the traction wheel on both sides using a wider aluminum extrusion and cutting a slot for the wheel. Locate the wheel closer to the outside (gears on the inside).

3. Like others have mentioned, have the omni wheels do the pivoting.

Dave