Drive Module Design Which Allows Wheel Flexibility Tom Ore FRC 525

A Random Thought… After we got back from Champs I started thinking about various discussions around wheel configurations In general, the process is you make your best guess about how the game will play and then select your wheels The rest of the robot is designed around the wheels (okay, that may be overstating it a bit)

A Random Thought… Sometimes the game doesn’t play out as expected and your wheel choice may not be the best one It’s often difficult to do anything about it because the robot is designed around the wheels I wondered what the drive base would look like if you designed it for maximum wheel flexibility

A Random Thought… So here is my thought: – Design a drive base for maximum wheel flexibility – Wait until deep in the build season before deciding which type of wheel to use – As competition weeks go by, it may be handy to change your wheel choice

A Random Thought… What if the wheel configuration could be changed in a few minutes? – You could even change wheel configurations before each match This assumes driver training and software issues could be handled

How to do it… With our suitcase bot, we could use either mecanum wheels or traction wheels – It was just a 4 wheel setup and the traction wheels were not geared/chained together What I wanted for this exercise was an 8 wheel drive setup which could use traction wheels, mecanum wheels, or omni wheels

What Options to Include I decided to stay with a single speed gearbox I thought about two speed, but it didn’t fit nicely into what I wanted to do – maybe this is an exercise for another time Many teams use combination mecanum/traction or omni/traction and I wanted these to be possible

What Speeds? We used a 12T / 60T single reduction with 4 inch wheels in 2014 and were pretty happy with the speed I can see going a bit faster or slower VexPro makes 4 CIM gears: 11T, 12T, 13T and 14T The 11T and 12T work on the same center distance as do the 13T and 14T

What Speeds? GearingCIMMini CIM 11T / 60T13.8 ft/sec16.1 ft/sec 12T / 60T15.0 ft/sec17.5 ft/sec 13T / 60T16.3 ft/sec19.0 ft/sec 14T / 60T17.5 ft/sec20.5 ft/sec

The gears in a module… The two outer gears are the 60T output gears The center gear is a 60T idler on a dead shaft The CIM gear on the left can be either an 11T or 12T The CIM gear on the right can be either a 13T or 14T Therefore, you can change speed by swapping CIM gears and/or the position of the CIM

A section through the middle of the shafts

A section through CIM / output shaft

A section through the idler shaft and pivot

All three wheels side-by-side (All VexPro wheels)

Assembly Sequence The following slides show the assembly sequence of the module

Press bearings into outer plate

Press bearings into inner plate

Press bearings into center gear

Press bushings into pivot

Slide gear and washers into pivot

Slide shaft through and put this assembly aside

Add piston pin and spacers to outer plate and hand tighten screws (with threadlocker)

Slide shafts through bearings

Add washers to the shafts

Slide gears onto shafts

Add washers

Put pivot assembly into place

Put inner plate into place

Add screws (with threadlocker) and torque screws on both sides

Add washers and screws to center pivot shaft (with threadlocker), torque

Install retaining rings

Put CIM washers on inner plate

Put CIM into place

Install CIM screws and torque

Add washers, CIM gear, and retainer clip

Repeat sequence to build two right hand and two left hand assemblies

The module is attached to the frame rail with ¼” button head screws

A complete side to the drive base I’ve ignore bumper brackets and how this rail would attach to the rest of the frame I haven’t done any structural analysis

Fixed Modules For the following configurations, the modules are attached directly to the frame with the desired amount of drop in the center wheels

1) 8 Traction wheels, the 2 center wheels dropped (center wheels chained together)

2) 4 traction / 4 omni, 2 center wheels dropped (2 center wheels chained together)

3) 4 mecanum wheels

4) 4 omni wheels

5) 8 omni wheels

Floating Modules For the following configurations, the modules are allowed to float. Maybe use when the floor is uneven.

6) 4 traction, 4 omni (probably shouldn’t use chain with floating modules)

7) 8 omni wheels

8) 8 mecanum wheels (this would look cool at least)

Powered Modules The following configurations would use a pneumatic cylinder to force the wheels up or down

9) 4 traction / 4 mecanum

10) 4 traction / 4 omni

11) 8 traction (maybe use when you don’t want to be turned easily)

12) 4 omni / 4 mecanum (can’t be pinned by side force, can still strafe)

13) Mecanum / traction switched on one side (maybe if CG isn’t near center of robot)

14) Omni / mecanum switched on one side