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Apalrd’s Choo-Choo Analysis Spreadsheet
Basic design spreadsheet to analyze the pull-in process of a choo-choo type mechanism, for determining the optimal gear ratio and geometry given a constantly-increasing spring load during pull-in
I wrote this spreadsheet to assist with the Killer Bees 2014 robot design. This spreadsheet is a slightly cleaned-up version.
With this simulator, you can look at the motor speed and torque during pull-in and optimize the gearbox and choo-choo design accordingly. The spring forces you are using (both fully extended and fully retracted) are used to calculate the torque on the motor through the choo-choo mechanism.
It is notable that the length of the choo-choo fling length can be used to adjust the motor torque during the ‘first hump’ and ‘second hump’ of choo-choo pull in separately, so the motor load remains more constant. As the spring force increases with pull-in, a shorter fling length will cause the torque arm length to decrease on the second hump, so the motor will see less torque, and more length will be pulled in during the first hump vs the second hump. You can play with all of these lengths to optimize the design based on your spring setup.
The top graph shows motor torque and reference torque (set at a % of motor stall torque). Try to keep the two humps under the reference. I used 50% stall torque for this, since the choo-choo is not pulled in very many times in a match and I felt that the CIM could handle 2 seconds of heating. The second graph shows motor RPM during the pull in process. The simulation will repeat over and over for a fixed number of iterations, so the graphs will repeat also.
On 33’s 2014 robot there was a lead-screw based spring adjuster, so I ran the numbers in each of the positions it operated at to determine the worst-case choo-choo design and the others operated with a safe margin.
Let me know if there are any issues with the file, I saved it from OpenOffice.
choo_choo_math.xls (760 KB)