Introduction:
On behalf of FRC Team 2168, The Aluminum Falcons from Groton, Connecticut we are pleased to share with the FRC community a FRC project that we have been working on in the 2019-2020 offseason as a training and learning opportunity. This post is intended as a public release of our design data prior to the 2020 kickoff with links to the CAD files below.
Background:
Each year the game brings a playing field with varying obstacles and terrains ranging from a flat carpeted surface to regolith to moats and barriers. These elements require every team to strategize and design their robot and more specifically the drive train in the most optimal manner to play the game. As part of the design process, most teams begin with the chassis to determine its overall size and use sketches to see how well the chassis size and wheel configuration(s) can overcome any obstacle with the most ease. This leads to the next step where teams figure out how to power the drivetrain on their robot, what kind of motors to use and what the gearing ratio should be to most efficiently maneuver around the field. All of the possible configurations, such as: wheel size, chassis dimensions, cutout/no cutout, ground clearance, motors quantity, and gearing lead teams to have a different drivetrain design from year to year. This leads to 3-5 day CAD effort for our students during the first week of build season and takes key resources focus away from designing for the robot sub-system required to play the other aspects of the given game.
Purpose:
The goal of this project was to design a set of west coast drivetrain configurations in the 2019 off season, that mixed self-adjusting chassis dimensions, link adjusting bumper configurations, wheel configurations from 4” to 8” diameter leverage COTS traction, pneumatic and omni wheels, and gearboxes configurations allowing for hardware swaps to cover targeted speeds for 4”-8” wheels and supporting single speed, single speed with PTO, and two speed gear boxes. In summary, a drive train CAD model that within 1-2 hours of effort can be configured in all aspects including live updates to drawings to match the design requirements the team establishes early Week 1. Fundamentally helping add schedule and design efficiency to our build season design effort.
Discussion:
The chassis was designed with a few objectives in mind: 1) enable interchangeability to allow various attachments, 2) maximize sheet metal construction to reduce cost for 2168, 3) simplicity of custom in house parts to reduce manufacturing time and 4) robustness to reduce downtime in practice and on the field.
Chassis & Drive Train:
• Configurations for 6WD, 8WD, & Omni/Traction WCD
• 2x1x0.125 6061-T6 Side Rails
• 2x1x0.0625 6061-T6 Front and Rear Rails
• #35 C-C + 0.017 Chain drive w/12T Sprockets
• Chain is #35 Pre-Stretched, Shot Peened, Cold Rolled Tsubaki Chain
• Wheel Configurations for 4”, 6”, and 8” AM, WCP, VEX AL Traction, Pneumatic and Omni Wheels
Model Configurations:
At first glance this is a run of the mill WCD. As stated in purpose paragraph the intent of this project was not to put yet another spin on the WCD design but rather to create a model that allows automatic adjusting through configuration settings and adjustment parameters. As you glean through the model tree you will find a rather staggering amount of configurations and a core set of parameter equations that allow for a number of configurations tied to 6WD and 8WD and the wheel sizes to match the requirements of the game.
Gearbox:
While there are many COTs gearboxes that teams can buy, they come at a high cost and are not easily adaptable, i.e. single speed versus double speed.The gearbox design enables us to choose between a sheet metal single speed, two speed, or single speed PTO configuration without having to create unique plates for each gear box type. It also allows a combination of gearing ratios deemed ideal for each standard wheel size without having to change the gearbox plates. We can gear up to increase speed or gear down to increase torque by swapping out different configurations with standard gear spacing. Shown in the picture above is a two speed configuration designed for direct drive of a 4 inch wheel which is rear mounted. The picture shows both the Falcon 500 and the NEO motors to demonstrate interchangeability.
The profile was designed as flat to enable the 60T idler to be switched out to a dog gear for engagement of the PTO drive. Also this enables the output shaft to be placed on either side of the gearbox to drive a rear wheel or in the center to directly drive a center wheel. The external spacer plates (shown in black) allow the gearbox to come off the robot as one unit and it keeps the chain in place throughout removal.
To ease maintenance a few features were added, including:
- Cutouts to enable motors to be removed through the belly pan cutout without taking out the gearbox.
- Hollow standoffs with push in threaded inserts were added to enable quick assembly/disassembly of the gearbox, to increase alignment between the plate and add overall stiffness without a compromise for weight. Credit to Citrus Circuits for the idea!
- The sheetmetal design allows the gearbox to have cutouts on both the inner and outer sides of the plates without needing separate machine operations. Slots were added which capture the nuts and standoffs to prevent them from rotation when the fasteners are being removed or applied, you only need an allen wrench.
Fundamentally, this project created a great challenge for our 2nd year CAD students and in theory provided our team with a tool to increase our design efficiency during build season. We wish all teams the best of luck in the 2020 season and hope to see many of our friends again in Detroit this year.
Project CAD Files: