Some comments.
While I am glad to have more folks knowledgeable about motor selection and, in particular, how to pick the right motor and gear box combination for various tasks with respect to FIRST, there was a lot that I was not so happy about.
Motors:
Why all the discussion about van door motors? Are team using the van door motors that much often? They have not been included in the Kit for years. I know they are legal but I just don’t see them being used very often since they are no longer shipped with the KOP crates. I love these motors but I just don’t see them very often any more and in fact, with all the other motors in the kit, I am not sure they buy their way on board any more. They are harder to mount, harder to interface to, they are much more backdriveable than window motors. I love 'em, but I think there are almost always better options for FIRST teams with today’s rules.
Fisher Price? I love these motors too but, they were not allowed last year and I don’t suppose they’ll be in this year either.
Also, Ken seemed to have a bias against CIMs in favor of open frame, fanned motors (500 and 700 series motors). In my view, he misses the point. First, full marks for his discussion about ratios and picking them well so that motors don’t get that hot. BUT the biggest difference between the CIM and the 500 & 700 series motors when you are pushing them to their limit, the fan helps you very little because motor speeds are likely very low (i.e. near stall) AND heat only kill motors via TEMPERATURE. Size and thermal mass help CIMs survive when a 500 series motor would be on fire. Size is the much bigger factor. The rise in temperature for a given amount of heat needed to dissipate is proportional to area. Those CIMs have much more area and so run cooler.
Chain.
120 degree wrap. This is good advice for chain without tensioners but with tight chain you can get away with much less. That said, it is generally good advice if you can manage it.
As to his comment about larger than 70 some odd teeth, I have made sprockets with 100s of teeth that worked very well. I just have never had a problem with sprockets being too large. Have I been lucky or what?
Gearboxes:
I am a gear expert. I love gears. I design gears for a living. There is basically no reason for me to build my own gearboxes in for FIRST any more. There are many off the shelf options that are just too good to and too easy. If you want to build your own and you feel it is inspiring to your team, Feel Free. But honestly, I think you are better off spending your time designing & building the rest of your robot than you are rolling your own gearbox.
Worm gears:
There is almost NEVER a good reason to use a worm gear (other than inside an existing motor/gearbox like a window motor). If you think you need it, think again. There is almost always a better way. Really. Worm gears are hard to get right.
Powered Anti-backdrive:
While you can get into trouble doing this (and a lot of teams do), it is my considered opinion that he’s just wrong. You need to design the gearbox and ratio for this condition but I have build many many robots that used very backdriveable motor/gearboxes to hold very large arms in a set position. You should think very hard about ways to get as much counter balance as you can in the systems. In addition if you cannot provide enough counter balance you are going to need to design the motor to be running WAY to the left on the speed-torque curve so that it will take only a small torque at the motor armature to resist backdriving, but saying that you must use a brake or a one way clutch is just too strong of a limitation.
FYI, the main arm pivot on this robot (CD5 - ah, I loved that robot) http://www.chiefdelphi.com/pics/bin/047robot05.jpg had almost no counter balance, a HUGE long arm and was easily backdriveable and used the motor to hold it in place. YMMV but it definitely can be done.
By the way, I am not talking about holding winches like last year in those cases, by all means, get the load off the motor.
Multi Motor Drives:
This is a much more complex situation that his answer suggested. There are acceleration concerns and there are current draw concerns (circuit breakers & battery amperes limits). For most teams 4 CIMs (2 on a side) are generally the right answer but there are other solutions that can make sense. My suggestion is that you know what you are doing before you consider other options. There is a high bar to get over before you go to another answer.
Belt Vs. Chain:
Good answer. I really like chain and can’t see a good reason go to belt. For all the reason they said. Preserving the option to change designs late in the process is a huge advantage.
Final note:
I would have liked to have some discussion around what the motors curves do when we run at different voltages. This is important because some times the motor specs are given at 18V or 10V or some other voltage but it also help people think about what happens when the motors are put into PID loops or other situations where less than 12V are applied to the motors (i.e. almost every FIRST application).
So… …I like the show in general. Still, I can wish for better.
Joe J.
