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Originally Posted by phrontist
How do you optimize wheel diameter for a given set of motors? What variables effect optimal wheel diameter?
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1) Decide how fast you want or need to go.
2) Decide how much pushing force you need.
3) Decide which of the above is more important
4) Decide how much power you have available.
5) Remember that Power = Torque X Rotational Speed
6) Remember that your motors are happiest running about 75% of free speed.
7) Think about what factors other than flat ground robot performance influence your choice? (stair climbing etc.)
8) Now you can pick a wheel size.
8) Design your drive train so that your motors are running about 75% of free speed at the speed or torque output you desire. (If it winds up going too fast you can always limit the output in software, but you can't add power that way)
Note that you are at step 8 before you even think about wheel size.
Define the operational requirements FIRST. Check the power availability before you start designing. If you need 2000W to accomplish a task in a certain time and you only have 1000W of motor available, it just ain't going to happen. You need to either double the time or find more power.
To illustrate some different paths, let us look at two very different but highly successful robots from this year, 330 and 254/60.
The 254/60 robots were designed to be very fast and manouverable and dominate the floor. They used small wheels (4" dia as I recall). Since they weren't planning to climb the platform, they could use small wheels. The small wheels mean they have to turn faster to achieve the same robot speed as a larger wheel. But they also mean less gear reduction which results in more power to the ground, because every stage of gear reduction costs you power. The smaller wheels also give a larger wheel base which gives more stability.
330 decided to play the game differently. Our goal was to dominate the bar. We felt that this strategy fit better with our team's personality and abilities than all out domination of the floor. To do that we wanted to be on the platform at the end of autonomous. While 15 seconds is not a long period of time, it was more than adequate for the task. So we designed our robot to go about 7 ft/sec, turn and climb the platform. The wheel size was chosen to facilitate that. We used 6in wheels with a ramp in the front to get us on the lip of the platform and 12" pnuematic tires in the back to finish the job.
330 used a simple tank style drive train that just used two drill motors with the stock tranny's and mounts and a simple pnuematic shifting system. 254/60 used four motors (chips and drills) with custom gear boxes.
All the robots discussed above made it to their respective division elemination rounds in Atlanta on their own merits. I was not present in the design discussions for teams 254 and 60, but I was for 330. But I do know both 254 and 60 pretty well. We have been friends for years and often sit and chew the fat at competitions and other events. WE ALL USE THE ABOVE METHODS for figuring out drive trains. I can't say it enough. Figure out you requirements first, then figure out how to accomplish them.
In this post:
http://www.chiefdelphi.com/forums/sh...34&postcount=4 Dave Lavery says that 254 has figured out something important. This is part of it. 60 has it figured out too, they just don't get as much credit.
ChrisH