Sportsman Mechanical Sub-team
For Cornell Combat Robotics, I am on the Sportsman team. This is an interdisciplinary team, where we work towards developing a remote control robot that falls under the 12lb class. There is a Sportsman Firmware sub-team that works on the coding for our robot, however I am involved in the Sportsman Mechanical sub-team, as I am more interested in the material, geometric, design aspects of the robot. Our battle bot is called Lance, because of its long flipper design resembling Lancelot from Arthurian Legend. Here is a photo of the full Sportsman Mechanical Sub-team.
The Design Process
We began the design process with brainstorming. Each member brainstormed eight ideas on their own and shared their top two at our sub-team meeting. We then grouped the ideas into a few categories: flipper, rammer, passive weapon, and clamper. We then eliminated unfeasible ideas (expensive, too complicated, or too much weight). For example, designs including pneumatic parts (run on compressed air) were immediately nixed because pneumatic parts are overly expensive. Below are some of my ideas:
We then created a Pugh Decision Matrix featuring the remaining brainstorm ideas listed in the ”Options” columns. Design criteria were listed in the ”Criteria” column while the remaining brainstorm ideas were listed under the ”Options” rows. Each of these criteria was given a weight based on its importance in our decision. Since we had yet to build a functioning robot and had heavy budget concerns, affordability and design simplicity were key. Therefore, we assigned ”Interest in Design” a low weight and ”Affordability” a high weight. In the future, after we have created several functioning robots and have dependable funding, the weights of these criteria will change.
Preparing and Presenting in the Preliminary Design Review
After choosing our final design, we spent the next few meetings sketching our designs in preparation for the Preliminary Design Review (PDR). The PDR consisted of an in-depth 30 minute presentation of each subteam’s design and a 30 minute question period. The presentation was broken into the specific, technical reasons behind each design decision to reinforce the team’s shift to more research-driven design decisions. The question period opened the floor to the entire team as well as our guests (including Todd Hayes, a Georgia Tech combat robotics member). These questions pointed out design flaws, offered suggestions, and clarified design decisions. These questions and suggestions were by far the most important part of the PDR. These questions led to important design changes such as changing the flipper’s material from Lexan, a specific brand of polycarbonate, to aluminum. Our takeaway was that any design decision that cannot be reasonably justified when questioned should be researched more and/or modified to be justifiable
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In preparation for the PDR, our initial sketches were meant to accommodate both a 3 or 12 lb version of our design in case we were forced to create a 3 lb bot due to budget concerns. Our initial sketches included several of the core design elements that feature in our final design such as the high-density polyethylene (HDPE) chassis and indirect drive (motors are not directly connected to wheels). It also included aluminum wedges (eliminated due to weight concerns) and a square chassis (widened to fit horizontal drive motors). Visualizing our design helped create the foundations of Lance’s most important design features. At this point, our subteam split off into 3 subsystems: Powertrain, Weapon, and Chassis. In preparation for our initial CAD and the PDR, we created a PowerPoint-Aided Design (PAD), primarily for the base configuration of the drivetrain. Below is a diagram of the powertrain design we had in mind while presenting in the PDR.
Developing the CAD
After the PDR, we chose to use puzzle fits for the chassis because it makes it easier to attach screws through the x and y axis and can better withstand forces from both directions, making it a more secure fit. We added polycarbonate trusses to reinforce it, since it is lightweight and can support the components inside the chassis. However, we rejected the trusses in the end, because the CAD would be very complicated and it would be hard to attach it to the chassis. Furthermore, after discussing it with the other subsystems, we decided that we would need to add protection on the sides of the chassis. This came in the form of foam bumpers. We chose this instead of building some barrier with aluminium, like aluminium side wedges, because they would not provide as much support and protection, and could bend easily. Foam bumpers enclosing the chassis of the robot would absorb the shock much more effectively and reduce the weight unlike aluminium. It would be easy to manufacture, attach with zipties, and replace since it is cheap. After the design decisions were finalized, we completed the CAD. I worked on the chassis side plates for Lance, and other members worked on inserting McMaster-Carr components into the Fusion360. Here are some renders of the CAD we had for Lance:
Machining
Once all the components on Fusion360 were assembled and completed, we had to finish our part drawings for all the chassis walls, as we had to machine them ourselves. In each part drawing, we covered the relevant dimensions and tools (drills, mill bits) required for the part. Furthermore, we also included a manufacturing plan for each part that we were assigned to. In the manufacturing plan, we explained how the mill should be used step by step; which drill bit should be used, and then what passes should be made. And also cautionary warnings like when setting up the origin on the mill (you need to offset the wiggler by 0.1'), otherwise the entire chassis part's dimensions will all be off and incorrect along that axis! Here is one of the part drawings I have completed for my team:
As for machining, I have used the mill to make the chassis walls. At first, we were a bit worried whether HDPE is suitable for the mill to machine. However, after consulting the shop director, Joe Sullivan, we have found out that HDPE is strong enough for the mill. And I have been using the mill to tap holes and make passes on the side wall. Also, I have threaded the holes for the side wall to enable the screws to go into the puzzle fit joints. Here are photos of the machining process and the outcomes:
Assembly
The final part is the assembly of the chassis. This was personally the most difficult part of the process. Given that the parts all required many precise dimensions, there were puzzle fits that didn't fit at all. We had to file down the HDPE parts, and many parts were milled over again with new stock. But with more practice, our team managed to get all the walls to fit together and the chassis was screwed together successfully. Nonetheless, there were also many problems in the drivetrain. The pulleys were ordered with the incorrect size and the circuit wiring was a little too long for the lid to close, so we had to cut the wires and solder the wires to make them shorter such that they can all fit in the box. Moreover, we had overlooked a part in the bottom plate. We didn't provide countersinks for the bottom plate, so the screw heads were sticking out of the bottom plate. Luckily it was easy to use the mill and a drill bit to create a countersink on the bottom plate. Here is a photo of Lance before all the final adjustments are made:
TO BE CONTINUED
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We are still working on Lance and the assembly; more progress will be updated on the portfolio when it has been done.
Lance - Combat Robotics Cornell
