Quote:
Originally posted by Jon Lawton
Then, I decide where I want to run my motors on the Torque-Speed Curve:
Drill Motor (Maximum Power occurs at > 40A):
Speed at 40A = (40-127)(19270)/(4.5-127)=13969.7 RPM
Torque at 40A = -0.87/19670 * 13969.7 + 0.87 = .252123 N-m
Fisher Price Motor (Maximum Power at 29.05A):
Speed: 15000/2 = 7500 RPM
Torque: 0.38/2 = 0.19 N-m
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This is probably the most effective way to match the ratios, by using these two speeds instead of free speed matching... good job!
Quote:
Originally posted by Jon Lawton
Then I made up a table of successive 2:1 reductions and listed the effective torque and speed at each. I noticed that after 5 stages, the Fisher Price motor would spin at 234.375 RPM when loaded at 6.08 N-m, and after 6 stages the Drill motor would spin at 218.277 RPM when loaded at 16.128.
...
Where nStages is the number of stages of gearing to achieve each overall ratio. 90% per stage is a conservative estimate to account for all of the various losses.
My math also says my wheel will be spinning at 224.789 RPM under this load condition (exerting 130 pounds of force). My wheel has an 18.84 inch circumference, so that comes to 5.88 feet per second.
So once I account for losses, I'd wager I'm somewhere between 4.5 and 5 feet per second pushing with 130 pounds of force.
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I think that you are underestimating the speed and power of your system. There are two things that you can change to give yourself a dramatic increase in your calculated speed and power.
1. Don't settle for a series of 5 or 6 stages of 2:1 reductions. Go for a couple of 4:1 reductions and reduce the # of stages.
2. A 90% efficiency loss at each stage is VERY conservative. As long as you use ball bearings to hold the shafts and get +/- 0.002 or so with the accurracy of your bearing hole locations, you can get up to 97% efficency. Take your time in doing this, and your 'bot will go faster and have more power.
Figuring efficiency loss with 0.9x6 is much less than 0.97x3.
At least, that is the way I see it. Unfortunately, our team doesn't do enough post-season evaluation of output power to see if these thoughts are accurrate. I'm very interested to see others opinions on this also.
Jon... excellent job. I especially like that your system will probably be more fast and powerful than you are estimating... which will make your design skills look even better.
Andy B.