BridgeBot
Nov.2021 // A robot designed to build a bridge for ENGS 75: Machine Engineering. The bot was created in collaboration with Clara Hahn ‘22, Jason Carpio ‘22, Josh De La Cruz ‘22, and Oluwafemi Kola-Jebutu ‘22.
The Challenge
A SolidWorks assembly of the play-area given in the design challenge. Two teams build simultaneously, one from each side.
Changing term-to-term, this iteration of ENGS 76: Machine Engineering tasked the class with creating a bot capable of building a bridge using a set of 2" tall foam blocks, 1" tall PLA blocks and 1/4" aluminum decking. Each team's score was a combination of bot weight, percent of bridge completion, build speed, and material usage.
Design Overview
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We elected to utilize a rotating base design, relying on both vertically and horizontally placed custom roller bearings in order to counter the extreme bending moment exerted by our tower design at maximum extension.
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We chose a simple box-section tower made from laser-cut 3mm plywood, as it allowed us to iterate quickly, was relatively light weight, and had excellent stiffness according to our analyses and testing.
The Carriage that runs up and down the tower uses a roller system inspired by 3D-printer designs and is raised and lowered using a 3-stage gearbox with a spool and string.
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The Arm of our bot is a simple rack and pinion design with a two laser-cut acrylic plates and press-fit cartridge bearings to keep the gears and rollers running smoothly.
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Since the given materials were entirely rectilinear, we chose a vise-gripper design as opposed to a more complex parallel linkage. We also made sure our design could accommodate lifting stacks of multiple blocks since that was critical to our overall strategy for the challenge.
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A last minute addition, but ultimately crucial to our performance, the electromagnet mounted opposite the gripper allowed our team to leverage the fact that both the PLA and aluminum materials were embedded with high-strength magnets. In competition, our dual-ended arm design allowed us to retrieve materials with both the gripper and the magnet simultaneously, proving to be a significant advantage.
Bridge design strategy // SolidWorks assembly + physical test
An annotated view of each subassemblies model involved in our final bot.
Design Process
Iterative Prototyping: Gripper
We started by doing some early prototyping of the gripper since the ability to lift the building materials was a critical function of the bot. This process, included the parallel linkage gripper shown, driven by a 3D printed lead-screw and nut since such components were not included in the kit materials allotted.
Gripper V2 // Sketch
Gripper V2 // Mockup
Gripper V1.2 // CAD Mockup
Gripper V1 // Prototype 3D-printed lead screw drawn from scratch to satisfy necessary size, loading and tolerance specs.
Gripper V3 // Mockup
Gripper V4.2 // CAD Subassembly
Gripper V4.1 // CAD Subassembly
Base module sketches
Mechanical Analysis: Column
Throughout the bot design process, we validated initial concepts using a number of analysis methods including FEA and hand calculations to ensure that we were designing within a healthy factor of safety.
Column V1 // From testing, we learned that the height would not satisfy the starting position requirements
Column module sketching
Column V2 // Rudimentary FEA model to get rough numbers on deflection at given heights for a 10-pound load
Column V2 // Buckling analysis hand calculation
Column V2 // Slimmed down material use + weight
Competition
Final Bot
In our final report, we discuss the iteration of the modules in our bot, show our CAD models and technical drawings for each module, as well as our bill of materials required to construct our bot.