
18-01-2017, 10:14
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LabVIEW aficionado and robot addict
AKA: The guy with the yellow hat
 FRC #5987 (Galaxia)
Team Role: Mentor
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Join Date: Mar 2015
Rookie Year: 2012
Location: Haifa, Israel
Posts: 658
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Re: 2017 Drive Train
Quote:
Originally Posted by mypie4050
I know that thread has been open for awhile now, but I'll just throw in my two sense. Our team has officially decided to go with a custom-build mecanum drivetrain. I know most teams have decided by this point, but our reasoning was as follows:
1. Agility/maneuverability will be key for auto/gear placement
2. Defensive situations can be avoided by "running away" - it is harder to trap a mecanum robot
3. Possibility of a "ground-engage" mechanism for staying in one spot (perhaps for shooting)
4. Sacrifice torc for speed - i.e. with a mecanum drivetrain we have little reason for torc, so can therefore throw torc to the wind, and go for as much speed as possible in our drivetrain.
I know some have said that mecanum is not the way to go this year, but we think there's plenty of use to be had there.
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While most of your points are true in a general sense, be careful about over-extrapolating. - Maneuverability will be important for placing gears, but in order to take advantage of a mecanum drive's maneuverability, the driver needs a lot of practice with the robot in different situations. This means that you need to either build a practice robot or have your robot finished by week 5 at the latest so your drivers can get a lot of practice before they start competing. Often drivers end up driving mecanum as if it were a tank drive (ignoring strafing). This is especially true if you are not using field-centric driving.
- While mecanum drives have the ability to be more maneuverable and can "run away" from a pushing match, often drivers get caught up in pushing matches anyway. If the pushing match occurs in a tight space or a place on the field where visibility is limited, drivers tend to try to push through the defender instead of running away. Large amounts of driver practice can correct these tendencies, but you will need practice 2-on-1 defense in very limited visibility areas in order to fully practice getting out of these situations. Also, running away usually means you will have to take a longer route, which takes more time.
- Just make sure that your "ground engage" mechanism keeps the robot balanced even when it is being pushed. A single plate in the middle of the robot leaves the robot susceptable to tippping if a defending robot tries to push it. Also, if you get knocked out of alignment, you will need to retract the grounding mechanism, re-align, and redeploy it, which takes a lot of time. And make sure to cover your mechanism in tread to avoid breaking R05. At this point, you're getting pretty close to an octocanum drive.
- Assuming you mean torque, the mechanical influence, not torc, a necklace worn by the ancient Gauls, you are correct to a degree. Mecanum wheels have a forward CoF of about 0.7, compared to around 1 for traction wheels. This means you can gear your robot faster and not draw as much current. This does not, however, mean that you can completely disregard torque. You still need to make sure you have enough torque to accelerate your robot from a standstill and change directions quickly.
I'm not expecting to change your design; you made an engineering decision and you should follow through that. I just want to set the record straight for other teams who may be looking to this thread for ideas.
My team, to my chagrin, has run mecanum drives 4 of the 5 years I was a member (not including this year). I was a driver for 2 of those 4 years. So I have a lot of experience with the benefits and drawbacks of mecanum drives. In general, I think there are very few cases (practically none) where a decent mecanum drive is better than a good WCD. Even in 2015, I still would have preferred that my team would have stuck with a WCD.
__________________
2017-present: Mentor FRC 5987
2017-present: CSA for FIRST in Israel
2012-2016: Member FRC 423
2013: Programmer
2014: Head Programmer, Wiring
2015: Head Programmer, Wiring
2016: Captain, Head Programmer, Wiring, Manipulator, Chassis, CAD, Business, Outreach (basically everything)

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