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#1
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Re: pic: DiscoBots Nano-killough
do you recall the gears before and after? I think earlier in the post mentioned 10.7:1. We have been using 12.7:1 with 6" wheels and that seems plenty fast. Were you going for higher top speed, more acceleration or more control?
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#2
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Re: pic: DiscoBots Nano-killough
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At 12.7:1, unless my math is wrong, your actual speed is going to be under 10 feet per second. That's not fast by FRC standards. It really depends on the game and what you want your robot to do. |
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#3
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Re: pic: DiscoBots Nano-killough
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#4
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Re: pic: DiscoBots Nano-killough
We are using the 12.7:1 gearboxes mainly because we did not want to spend money on an off season experiment. We need to better understand the limitations of Killough drive.
Two things we are doing that modify the standard calculations. 1) We plan to use MiniCIMs which have a free speed of 6200RPM (16.7% faster than CIM). Given that we don't intend to get into pushing matches, closer match to Jaguar current capabilities and the reduced weight, this seems like a trade we thought we should try. 2) According to Ether's kinematic calculations, by toe-in the wheels we lose cos(30) in torque but gain 1/cos(30) in top speed. This is another 15%. At face value 10fps * 1.16 * 1.15 = ~13fps. Our crude estimator spreadsheet predicts 12.5fps front/back and >18fps lateral (1/cos(60)). We are planning to instrument and measure the step response on Thursday (students tell me the code and electrical are finally ready). I will report what we achieved by end of week. Assuming we hit >12fps, do you think that would be competitive? If we are able to control it, and the game warrants Holonomic we can always change the gears in the TBnano to a lower ratio in the production robot. |
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#5
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Re: pic: DiscoBots Nano-killough
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A 4-wheel Holonomic, as 2587 did in 2011 has similar issues encountered by mecanum when it comes to maintaining contact and similar weight distribution between all 4 wheels. Without equal contact throughout a movement, motion will become unpredictable - this can be mitigated with a decent closed-loop control system using a gyro sensor or other feedback, but you are just trying to overcome an inherent mechanical issue. The 3 basic factors are the floor flatness, frame stiffness, and (optionally) the use of suspension. obviously 4-wheel mecanum drivetrains are much more common than 4-wheel holonomic, so it is easier to look there for successful platforms. Essentially, most successful mecanum bases have either a flexible frame (like a kitbot C-Base), or they have a stiff frame and some sort of suspension. On very flat fields without any 1/4" lexan to drive on some teams may have mild success with a stiff frame and no suspension, but this is not ideal. An interesting early 4-wheel holonomic used in FIRST competition is 116's 2005 robot: http://team116.org/our-team/robots/2005-robot/ They did a few unique things, including 82-degree cambered omni-wheels (this accomplished 2 primary things - pushed their contact point right to the edge of the frame for stability, and put more rollers in contact with the ground at a time for a much smoother ride on the older-style AM trick wheels). With dual roller omni's available today the second benefit isn't important, but the 1st one is still pretty cool. They had a pretty stiff welded frame, and I can't tell if they had suspension - but it was a very flat field and I think they at least had an adjustment to make sure all 4 wheels were planar. Last year we exercised the flexible frame perimiter rules to our advantage, chose to do an omni-directional drive, but side-stepped many of these potential issues by going with a 3-wheel holonomic (sometimes called a kiwi drive). Because you only have 3 points of contact, they are all always touching the ground (remember, 3 points make a plane, and a 3-legged stool will never teter). It was the first time we had built something like that (had experimented with mecanum before), but it was a huge success for us. We attended 2 regionals and championships, ranked 4th in our division and made it to semi-finals (our best CMP showing in our 10-year history). In 44 matches throughout the season, plus another ~25 in off-season events we never once changed wheels, or performed any transmission maintenance - the drive was completely trouble-free. We definitely didn't play defense much, but with a skilled driver we were pretty good at quickly maneuvering around the field and outrunning/avoiding defense. We used robot-relative control (not field-relative, but our driver had RC experience and a fair amount of drive practice). We only used one gyro, which greatly improved our rotational control, and was pretty simple to implement. If the frame perimeter remains flexible, we have a flat field, and we value maneuverability over pushing power I would say we might likely re-use this drivetrain, but in other situations it might not be ideal (fitting it in 28"x38" may be a pretty big stability sacrifice). ![]() ![]() |
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#6
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Re: pic: DiscoBots Nano-killough
Wow... that's a very cool design. I presume it was for a 30pt climber. How did it work out?
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