RC15OmniWheel

This page is a sub-section of the RoboCup 2015 Mechanical page. Specifically, the Omni Wheels allow the robots to move in any direction at anytime.

Background

The omni wheel serves the purpose of allowing the robot to move in any direction at any time. This is important for having highly dynamic robots that can execute their plays as fast as possible.

The 2008 fleet of robots designed by the team utilized a 

The 2011 fleet of robots utilized rollers supported by individual pins which proved to be easier to assemble despite the increased part count. Additionally, the omni wheel had an internal ring gear mounted on the rear to engage the spur gear mounted on the 30W motors. This design proved to be compact, allowing for more internal space on the robot. 

Both the 2008 and 2011 fleet of robots use wheels that are mounted on 1 radial bearing. While this design works, it allows for a lot of out of plane motion of the wheel, which has resulted in significant rubbing and reduced performance. Both designs use simple o-rings as contacts for the ground. The 2011 robots utilized an outsourced aluminum core to seat the o-ring.

Requirements

1. In-house manufacturable (including ease of manufacturability)
2. Provide more grip/contact than previous designs
3. Must be able to mount the internal ring gear for a more compact design
4. Ground clearance of 0.XX inches
5. Must include more rollers for smoother driving performance.

Potential Design Variants

Body/Housing

1. Injection molded design
   1. Pros:
      1. Highly geared towards mass manufacturing. Saves development time
      2. Significantly more freedom with design parameters than a purely machined design
      3. Potentially lighter if designed and analyzed correctly
   2. Cons:
      1. Relatively unknown realm of manufacturing
      2. Generally weaker material than most metals
      3. Development of the mold is time consuming and not easy
2. Typical aluminum machined design
   1. Pros:
   2. Cons:
3. Thinned and heat treated steel machined design
   1. Pros:
   2. Cons:

Roller Rubber

1. Round o-rings
   1. Pros:
      1. Simple
      2. The most inexpensive option
      3. Same as what we've done in the past, so it the easiest to re-imple
   2. Cons:
      1. Smallest contact area ("point" contact)
      2. Most effectively seated via a semi-circular groove on the roller - this was an outsourced job in the past
2. Double seal o-rings
   1. Pros:
      1. More contact area = more grip
      2. Relatively cheap
   2. Cons:
      1. Slightly more expensive than round rings
3. Custom rubber rings
   1. Pros:
      1. Easily designed to have high contact area
   2. Cons:
      1. Not a commercial-of-the-shelf product (requires our team to manufacture it)
      2. Potential inconsistencies with our in-house manfacturing process (the waterjet cutter may leave a slight flap on the edge at the "lead-in/lead-out" locations.
      3. Requires more individual components

Materials

Item	Material	Fleet Quantity	Stock Material	Cost per part	Total Cost	Vendor	Part Number	Manufacturing Method	Notes
Body	Aluminum 7075	32				McMaster		CNC Mill
Cap	Hardened Steel	32				McMaster		Waterjet
Pin	Stainless Steel	576				McMaster		(Stock component)
Roller Core	Carbon Steel	576				McMaster		Waterjet	Hardened after cut
Roller Rubber	Rubber	576				McMaster		Waterjet

Assembly Structure

Parts List

1. 15 - Rollers (RC-2008-01-00)
   1. 1 - O-Ring (RC-2008-01-01)
   2. 1 - Roller Hub (RC-2008-01-02)
   1. 1 - Wheel Body (RC-2008-02-01)
   2. 1 - Plate (RC-2008-02-02)
   3. 1 - Ring / Axel (RC-2008-02-03)
   4. 3 - 6-32 1/4" Long (RC-2008-07-04)

   Instructions

   1. Place 15 rollers on wire axle.
   2. Bend to circular shape.
   3. Drop in omni body.
   4. Place omni plate on omni body.
   5. Fasten using RC-2008-07-04

   Cost Estimates

   1. Rollers - $0.42 x 15 = $6.30
   2. Body - $4.00
   3. O-rings - $0.03 x 15 = $0.45
   4. Plate -
   5. Fasteners - $0.30 x 3 = $0.90

   Total - $11.65 (missing plate)

   Note - Shipping is included in per unit cost.

   Action Log

   1. 12/04/2007 - RC-2008-PO-02 - Purchase Order for Omni Roller Hub Samples (Option A & B).
   2. 12/14/2007 - RC-2008-PO-01 - Purchase Order for Omni Body Prototypes.
   3. 12/28/2007 - Fabrication of Omni Plates completed.
   4. 12/28/2007 - Full omni prototype completed.

      Notes - Very hard to get axle wire in to omni bodies must find new material.

   5. 12/28/2007 - Roller Option A revised and selected for prototype set.

      Notes - Option B (square) is cheaper and requires a costs analysis change in dimension and different roller material may be required.

   6. 12/29/2007 - RC-2008-PO-03 - Purchase Order for Omni Roller Hub (Rev 2).
   7. 12/31/2007 - Looking at 8867K25, 9495K92 for alternate wire.
   8. 01/29/2007 - Keeping original wire.
   9. 03/05/2007 - Omni rollers finalized.
   10. 03/08/2008 - Omni rollers sent out. (20 cents each)
   11. 03/17/2008 - Investigating the use dowel pins to cut down on machining costs.
   12. 03/30/2008 - Designs for body are finalized and sent out to supplier.

       Notes - Dowel / locating pins, relaxing tolerances, a new supplier, and hex broaching our selves have brought down the machining cost to around $4 per body (prototype $53).

   13. 04/13/2008 - Fleet rollers arrive and are fitted with o-rings.
   14. 05/14/2008 - Fleet bodies arrive, are hex'd, and have locating pins pressed in.

       Notes - A few were damaged in process a fitting for press was made, and replacements ordered.