17-06-2014 19:21

Rylet1086

Re: pic: Plywood Butterfly Drive Render

Is there any reason why you cant shift the colsons closer together and use two pistons between the two sides instead of the four to reduce complexity? Otherwise it looks good although you may find you want to gear it to be faster.

17-06-2014 20:37

AllenGregoryIV

Re: pic: Plywood Butterfly Drive Render

I would highly suggest you have the traction wheels on the outside and the omni wheels on the inside. We did it like your current setup this year, against the advice of teams like 148, and we regret it. When you shift into traction and want to push you'll tip slightly and end up partially on your omni wheels.

17-06-2014 20:43

ekapalka

Re: pic: Plywood Butterfly Drive Render

So butterfly drive is like octocanum with omni wheels instead of mecanum wheels?

17-06-2014 20:55

s_forbes

Re: pic: Plywood Butterfly Drive Render

Looks good, I want to see the real life prototype!

I expect this to depend a lot on the wheel base of the robot while in traction mode (distance from center of front traction wheel to center of back traction wheel). It probably depends on the torque the piston provides on the wheel modules as well. What was the wheel base for your design? Did the tipping result in any lifting of the opposing robot? Curious on your results.... for science.

My biggest pet peeve of this type of drivetrain approach will always be that the two CIMs on the front don't contribute much power in a pushing situation. If you are geared to be traction limited, I don't think this is an issue.

17-06-2014 21:37

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

What about arranging the power transmission components like this? (Warning: MS Paint Art):
That would allow all 2 (or 3) CIMs on each side to drive both wheels and move the omni wheels to the inside.

17-06-2014 23:27

sanddrag

Re: pic: Plywood Butterfly Drive Render

I like it. I have to ask, does your team have a laser capable of making this? If so, what kind?

17-06-2014 23:44

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

We work out of a local hackerspace with a Trotec Speedy 300. Some of the parts are longer than the laser cutter's bed but can be cut in two operations by turning the wood sheet around.

17-06-2014 23:45

Re: pic: Plywood Butterfly Drive Render

Precisely.

17-06-2014 23:52

AllenGregoryIV

Re: pic: Plywood Butterfly Drive Render

This is pretty close to what we will be prototyping in the fall. Though we will have spots for at least 4 CIM motor mounts per side.

I have been thinking of a way to have the CIMs still face towards the outside of the rail yet recess into that space between the modules and distribute power like your drawing. This year we used the space on one side to hold the cRIO while on the other it was just wasted space. The reason I would want the CIMs to face the outside is it would allow the overall rail thickness to be reduced since it wouldn't have to contain the entire CIM body.

18-06-2014 02:01

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

Here's a 6-CIM version, with the wheels belt-driven from a central gearbox. I did not switch the omni and Colson wheels; the robot has to tilt to an extreme angle for the raised omni wheels to touch the ground. It now has adjusted speeds of 13.8 and 6.9 fps.

18-06-2014 02:42

Re: pic: Plywood Butterfly Drive Render

It seems like you can literally flip your modules around to put the omnis on the inside and the traction on the outside. Just move the shaft with the pulley on it a bit closer in, flip the modules, and you're gold.

18-06-2014 08:09

Joe G.

Re: pic: Plywood Butterfly Drive Render

Looks good! Always love to see laser cut wood construction in FRC. Really like the 6 CIM setup with the reversed gearbox. Lot more open in the middle than a lot of octocanum and butterfly designs. With the 6 CIM layout especially, I would recommend flipping your wheels so that it pivots about the traction wheel rather than the omni. Doing this prevents the module from being side loaded when pushed sideways in traction mode.

Have you considered using pancake cylinders for module actuation to save some space and weight?

18-06-2014 08:13

pfreivald

Re: pic: Plywood Butterfly Drive Render

Looks great! In addition to the above (swapping wheel placement), I think you could get a more compact design using four fat pancake cylinders instead of four longer cylinders with a long lever arm on the butterfly. You're losing a lot of pod-turning torque based on that angle anyway, so why not mount a pancake cylinder so it can push the pod straight down?

(We've been iterating octocanum for quite a few years now, and that's how we intend to do it this year if we keep octocanum this year...which we may not.)

18-06-2014 11:54

AllenGregoryIV

Re: pic: Plywood Butterfly Drive Render

The biggest issue with this is it makes it harder to do your reductions. If the omni wheel is floating, it's the module that normally is powered first before the traction wheel. In many designs this means the CIMs need to float with the modules, 1477 did it this way this year. Floating the traction wheels allow for much easier gearing. We didn't see any bending in our modules this year that had the traction wheels floating but we used steel side plates. I've heard of teams putting delrin blocks between the modules and the frame to prevent the modules from taking all the side loads when the traction wheels are pushed down.

18-06-2014 12:18

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

It's not too hard with the central gearbox design since you can do a second reduction with the belts to the modules. From JVN's calculator, a 12:60 reduction in the gearbox and a 24:42 reduction with the belts gives adjusted speeds of 6.4fps on the Colson wheels and 12.9 fps on the omni wheels.

I'll look into doing that.

I noticed a few problems with my design that I also need to fix in the next iteration: the belts to the modules run into the bellypan, the bolt that serves as an axle for the gearbox is not adequately supported and the bottom two CIMs in each gearbox can't be installed or removed once the chassis is assembled.

18-06-2014 13:10

AllenGregoryIV

Re: pic: Plywood Butterfly Drive Render

The other reason we prefer to float the traction wheels is that if we lose air, or if we lose a belt in a module we default to the omni wheels which is where we spend around 90% of our time anyway.

18-06-2014 13:40

Chris is me

Re: pic: Plywood Butterfly Drive Render

If it simplifies the design, you could consider the option of using shifters for the central gearboxes rather than a pulley reduction between the traction and omni wheel. That way, you can shift from high speed to high torque independently of which wheels are on the ground and pivoting about the traction wheel isn't a big hassle. However, this is added complexity.

20-06-2014 00:14

fb39ca4

Re: pic: Plywood Butterfly Drive Render

It's great to see another team going with this method of construction. Mine has been doing laser cut plywood chassis in the past two years, and we love it.

20-06-2014 00:21

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

Is there anywhere I could find more information on your designs and construction method? What thickness and type of wood did you use? How did it hold up in competition?

It's hard to find high-quality information about wood chassis in FRC since so few teams have used them.

20-06-2014 02:27

RKIyer

Re: pic: Plywood Butterfly Drive Render

How are the cut plywood pieces of your frame and wheel assemblies attached together? Are you using a glue? Or are there tiny screws?

20-06-2014 08:45

Electronica1

Re: pic: Plywood Butterfly Drive Render

You can go on http://frcdesigns.com/ and look at 1771's cad from 2011 and 2012. They had some very good wood robots.

20-06-2014 11:13

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

It's designed to be assembled using wood glue.

27-06-2014 07:47

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

Here's my progress on the next iteration of the drivetrain. The chassis will be sturdier when complete, but the module and powertrain are just about done. Currently, it weighs 30.5 lbs (the same as the original 4-CIM version).

Improvements in this version include:
-Simpler and lighter powertrain due to direct-drive gearbox
-Stiffer module with less wasted space
-Smaller, lighter pancake cylinders for switching between wheels
-Narrower side rails and more open bellypan

The outside plates will be attached with T-nuts (http://goo.gl/XBWxdN) so they are removable for easy maintenance.

27-06-2014 17:57

z_beeblebrox

Re: pic: Plywood Butterfly Drive Render

Here's the more-or-less complete version of the drivetrain, except for the finger joints, which are time-consuming and no fun to draw. Sturdier structure, lightening holes and encoder mounts are added. The pneumatic cylinder mounting is improved.

The two aluminum tubes on top are an example of how a hinged superstructure can be mounted, similar to our 2014 robot (http://www.chiefdelphi.com/media/photos/39338).

I'd like to integrate bumper mounts into the wood frame, but am uncertain of how best to do so in a way that is simple, robust and allows for quick swaps. Anyone have ideas?



Unless anyone has ideas for improvement, next steps are probably cutting some test pieces on the laser to learn about best practices for finger joints, large parts and bearing fits before manufacturing a full drivetrain.