RC15OmniWheel

As part of the RoboCup 2015 Mechanical, 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

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:

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

   Parts & Components

   Amounts are based on per robot. Materials are in sizes for prototyping not lowest cost per amount.

   Stock

   Plate - Wheel Plate

   Vendor - McMaster-Carr

   Part No. - 8885K13

   Cost - $21.63

   Description - Alloy 7075 Aluminum Sheet .063" Thick, 12" X 12"

   Plate - Wheel Body

   Vendor - McMaster-Carr

   Part No. - 9037K21

   Cost - $158.77

   Description - Alloy 7075 Aluminum Sheet .500" Thick, 12" X 12"

   Note: will look at thinner and optimize sheet size to a per robot amount

   Shaft - Rollers

   Vendor - McMaster-Carr

   Part No. - 9063K243

   Cost - $17.48

   Description - Alloy 7075 Aluminum Precision Ground Rod 1/8" Diameter, 3' Length

   Note: Usually ships within 2 weeks.

   Fasteners

   6-32 Low Head Socket Cap Screw

   Vendor - McMaster-Carr

   Part No. - 92220A141

   Cost - $7.42 (packs of 25)

   Description - Alloy Steel Low Head Socket Cap Screw 6-32 Thread, 1/4" Length

   3/16" Dowel Pin

   Vendor - MSC

   Part No. - 88231758

   Cost - $43.08 (per 100)

   Description - Precision Ground Dowel Pins - Stainless Steel Type: Dowel Pin Diameter: 3/16 Overall Length: 1/4 Material: Stainless Steel - 416 SS Finish/Coating: Bright

   Other

   O-Ring (option 1)

   Vendor - McMaster-Carr

   Part No. - 94115K012

   Cost - $2.87 (Pks of 100)

   Quantity -

   Description - Neoprene O-Ring AS568A Dash Number 012

   O-Ring (option 2)

   Vendor - McMaster-Carr

   Part No. - 90025K137

   Cost - $9.70 (Pks 100)

   Quantity -

   Description - Double Seal Buna-N O-Ring AS568A Dash Number 012

   Axel / Wire

   Vendor - McMaster-Carr

   Part No. - 9666K69

   Cost - $2.93

   Description - 1080-1090 Carbon Steel Wire .059" Diameter, 1/4-lb Coil, 27' Coil

   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.