Radii V2

Radii V2 - Second iteration of Radii, the 3lb drum spinner battlebot (October 2017)

Carbon fiber madness. This placed 7th out of 34 competitors with win loss ratio of 4:2

 Radii V2 follows the design of Radii which competed in Motorama 2017. This is the improved version that takes care of the weakness of the previous Radii. Many thanks to Eunchang for being a good team member for this project, working together from CAD design to fabrication. Also, huge thanks to Robojackets and its members for funding this project and providing design reviews.

Goal of this project was to:

• fix all problems that previous Radii had

• utilize 3D printed armor

• compete in SparkFun AVC 2017 competition

Parts I used for this build are:

All aluminum parts	6061 aluminum
Top & Bottom plates	2mm thick carbon fiber
Drive Motors	9.7:1 Metal Gearmotor 25Dx48L
Drive Motor Controllers	Scorpion Mini
Weapon Motor	Turnigy 2836 Brushless 450-Size Heli Motor 3200kv
Weapon Motor Controllers	Racerstar MS Series 35A opto
Receiver	Hobby King 2.4Ghz Receiver 6Ch V2
Remote Control	Hobby King 2.4Ghz 6Ch Tx and Rx V2
Battery	Turnigy nano-tech 850mAh 3S 45~90C Lipo Pack

CAD overall

Internal CAD view

  All the heavy weighing electronics (weapon motor, battery) are now shifted behind the wheel so that the wheels are closer to center gravity. Mistake from the previous Radii was that the CG was too far away from the wheels and it was extremely difficult to drive. This design partially solved the driving issue. 

Section view of the weapon

  The drum is smaller in length but has slightly thicker wall inside. This is to maximize moment of inertia and minimize the deformation of aluminum that holds the screws upon impact. However, the weapon is lighter in the new version. The weight saved from down sizing the weapon is used to strengthen the side armors and adding the front plate to hold the side plates from shearing.

Playing around with Inventor FEA stress analysis

  Did some FEA stress analysis for reference. The part above was designed to be used as armor (3D printed) and would be one of the primary places to take hits from the opponents. Regular FEA analysis would not be the best method since the 3D printed parts are not isotropic. 3D printed parts are generally strong along the filament (tensile strength) but weak when the force is trying to pull apart the layers (delamination).

Internals

  Since the weapon motor got shifted behind, the belt has to run over one of the drive motors and some wires. This looks sketchy but it works. Due to long belt, I suspected the loss of belt tension from stretch would be a problem. I designed the weapon motor mount so that I can shift the motor more towards the back to adjust belt tension.

Fabricated & comparison with the first Radii

  Side, front and back aluminum plates are cut using waterjet and then pocketed with CNC mill for fitting in motors and extra weight savings. Top and bottom carbon fiber plates are waterjetted too. The aluminum part of the drum and the shafts are machined using lathe and then CNC milled for screw holes. External side plates that covers the wheels and aluminum side plates are 3D printed using invention studio's Markforged 3D printer with nylon and carbon fiber filaments. Infill density was set to 50% and pattern was set to triangular shapes (honeycomb shapes for maximum impact resistance. 6 regular triangles = 1 regular hexagon = honeycomb).

Damage taken from project darkness pic 1

  Most of the damage taken from 2017 Sparkfun AVC was from project darkness, a 3lb vertical spinner with front wedge.

Damage taken from project darkness pic 2

  Steel screws have significant amount of chunks taken out from hits. Too many of this would ultimately cause the weapon to be not balanced and this will increase vibration when spinning.

  Although Radii V2 improved a lot of faults from the old Radii, it wasn't a great success as it brought new faults from changes in design.

  First of all, the drive system wasn't reliable. The wheels were closer to the center of gravity so it drove well. However, when the robot got hit, went airborne, and then landed on the wheels, the shock which the drive motors took from the weight of the robot was greater than the previous Radii and easily broke the gearbox. Hard mounting the drive motors on the rigid aluminum plate with screws and using not-so-soft banebots wheels were also the cause since they would easily transfer load to the gearbox and the shaft whenever those take impacts.

  The weapon was an issue as well. The weapon motor wouldn't spin up to its maximum speed and I think the belt tension was the issue. The belt I received was too small in length so I had to constantly stretch it before the competition. Apparently, that wasn't enough and I believe the belt put too much load on the motor and prevented it from spinning freely.

  Using carbon fiber plate as top and bottom plate was a mistake. Carbon fiber is definitely light weight and rigid enough to hold internal electronics in place but it was too brittle. It shattered immediately after taking hits from vertical spinner. 

  Furthermore, the experience from Motorama 2017 and SparkFun AVC 2017 made me realize the limitation of the drum spinner design. It can take and give out vertical hits well but it is weak against horizontal hits. Unless designed to accommodate different types of opponents (ex: vertical spinner, horizontal spinner, wedges, etc), it would be difficult to reach high placing ranks in the competition. Thus, I end the iteration of Radii here.

Improvements and changes for the future design would be:

• interchangeable weapon system

• hub motor design. no more belts

• vertical weapon made out of steel. no more steel bolts as weapon teeth

• chassis made out of 3D printed materials (carbon fiber/ kevlar/ fiberglass combined with nylon/ onyx)

• 6 cell lipo battery as a main power source

• aluminum or plastic for top and bottom plates

• brushless drive system

• soft foam wheels instead of rigid banebots wheels

• soft mount drive motors instead of hard mounting them with screws

• use of metric screws. no more imperial bs

On a side note...

  I was taking the course at Georgia Tech called ME 3210 Design, Materials, and Manufacture at the same semester and the extra credit was to make a video about manufacturing stuffs. My group decided to just make a video about the manufacturing process of Radii V2 which was already built. Luckily, I had recorded several manufacturing processes (ex: lathe, waterjet, 3D printing, etc) so I sent those to my group members and they helped making an informative video clip about the build summary (Thanks Jeremy, Eddie, Dallas). 

extra credit video for ME 3210