For the Caltech ME 72 Design Competition, I worked in a team of seven over a twenty week period to design, fabricate, and compete with three different robots in a Sumo competition. The goal of the Sumo competition is the push other robots off of a 1.5 meter "Dohyo". Robots must fit within a 20cm by 20cm size constraint and weigh less than 3kg. For this competition, we built three distinct robots: An RC-controlled robot, an autonomous robot, and an RC-controlled Strandbeest robot (Walking robots based off of the Jansen mechanism). For this competition, I focused on the design and fabrication of the robot drivetrain and transmission, including the selection of motors, gearing, and other components.
My primary work for the Sumo Robots started with the design of the robot mobility systems. In the research phase, I considered a variety of mobility systems including tread-based tank locomotion, holonomic drive systems, and differential wheeled drivetrains. In doing so, I considered the drivetrains in light of multiple criteria:
Treads, while providing much greater traction that alternative drive trains, would not perform well under the shock load of colliding robots. This shock loading would possibly lead to tread tooth loss, and would require continual check and replacement. The tread tank-drive system would also be slower to turn and locomote, thus removing our ability to get up to speed and track the opponent's robot.
Holonomic "Swerve" drive systems would provide a strong competitive advantage for the autonomous robot, as it would allow the robot to strafe around the opponent, dodging attacks and hitting the opponent from the side where they are weakest. However, its complexity and cost were huge liabilities for usage in the competition, and it would be a huge risk to go with this method.
Differential wheeled drive was the best solution as it was reliable, low-cost, and had a small form factor. It was also easily-programmable and easily maintainable as compared to tread or a holonomic drive system.
Transmission Analysis and Motor Selection
After picking the drive train type, my focus then shifted to analysis for selection of a motor and drive transmission. Performing a dynamic analysis of necessary torque and speed, I found that we need 50W of power for each motor to achieve a desired top speed of 3 m/s. Considering the competition includes battling other robots, we would require 100W for each motor. A factor of safety FOS = 2 would yield our final desired motor power of 180W max output for shock loading.
These specifications, as well as criteria such as max current draw and voltage, weight, and size, led to the selection of two different possible motors: The Maxon Re35 and the Vex 775pro. The Maxon motor, being of higher quality, would be the perfect choice for our robot. However, it was very expensive. On the other hand, the 775pro motor was cheap and would fit the bill for our desired specifications, however due to its high stall current it may burn out during the competition.
Since the Maxon Re35 motors were out of our budget, but would be the best and most competitive option, I decided to spend some time and contact Maxon for chance of reduced prices through a sponsorship. Through my efforts in procuring a sponsorship deal, I was able to get the price of the Maxon Re35 reduced from $415 to $115, a discount of 72%. This discount allowed our team to afford the Re35 motors, and utilize them on our robot.
To determine the desired gear ratio I considered both the desired max speed and torque needed, but also the acceleration of the robot including rotational inertia of transmission components. For different strategies, we would like to choose different gear ratios. A lower gear ratio would provide a faster max speed and greatest momentum when impacting another robot, however it would take longer to get up to max speed. A higher gear ratio would provide a lower max speed, but give a short run up. Somewhere in between would give the robot a good performance both in max speed and acceleration to the max speed, considering the size of the Dohyo ring.
Thus, we aimed for a gear ratio of 6:1, balancing maximum speed and acceleration.