2017 Drive Train

[Zebra_Fact_Man]

Originally Posted by pHolmgren

Our team decided to go with a 6 wheel tank drive this year, with a sort of modified west coast style. We have a frame CAD model attached and would love to hear what you think.https://drive.google.com/file/d/0BzJ...ew?usp=sharing

Please make sure that the gaps in you bumper supports are indeed less than 7", or that you run a second support bar across the side of the frame. Otherwise you will be in violation of R31.

Otherwise, great work. My team is also doing 6wd DS WCD. So serviceable and utilitarian.

[Michael 4499]

Originally Posted by Zebra_Fact_Man

Please make sure that the gaps in you bumper supports are indeed less than 7", or that you run a second support bar across the side of the frame. Otherwise you will be in violation of R31.

Otherwise, great work. My team is also doing 6wd DS WCD. So serviceable and utilitarian.

Is this correct? From my understanding of R31 it sounds like it is more the definition of a supported bumper. In the chassis displayed, they aren't trying to count that as a supported area but instead just not putting bumpers there. As long as they have 6 in. of support on each side, they should be fine.

Please correct me if I have a misunderstanding of this.

[Cothron Theiss]

Originally Posted by Michael 4499

Is this correct? From my understanding of R31 it sounds like it is more the definition of a supported bumper. In the chassis displayed, they aren't trying to count that as a supported area but instead just not putting bumpers there. As long as they have 6 in. of support on each side, they should be fine.

Please correct me if I have a misunderstanding of this.

Solomon isn't referring to the cutout in the front, he's referring to the bumper mounts on the sides of the robot between the drive wheels. That gap around the middle wheel looks like it's close to being in violation.

[Cothron Theiss]

Originally Posted by pHolmgren

Our team decided to go with a 6 wheel tank drive this year, with a sort of modified west coast style. We have a frame CAD model attached and would love to hear what you think.https://drive.google.com/file/d/0BzJ...ew?usp=sharing

Looks good! Any reason you are going with the AndyMark DuraOmnis over the Vex Omnis? And what is the purpose of the center rail running from the back of the cutout to the front of the back rail?

[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.

[JesseK]

Originally Posted by pHolmgren

Our team decided to go with a 6 wheel tank drive this year, with a sort of modified west coast style. We have a frame CAD model attached and would love to hear what you think.https://drive.google.com/file/d/0BzJ...ew?usp=sharing

Consider what happens if you need to take a wheel off in this setup. Do the plates in the corners prevent it? If so, you should adjust the plating to make sure you can do any maintenance you need to do on this drive train.

[Ari423]

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.

While most of your points are true in a general sense, be careful about over-extrapolating.

1. 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.
2. 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.
3. 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.
4. 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.

[Ether]

Originally Posted by Ari423

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.

Would you please provide a more detailed explanation what you meant by this? i.e. How do you get from the first sentence to the second one?

[Kevin Sevcik]

Originally Posted by Ether

Would you please provide a more detailed explanation what you meant by this? i.e. How do you get from the first sentence to the second one?

You're traction limited at a lower torque, so if you're trying to spin wheels at 40-50A (so as not to blow a breaker) you can have a higher gear ratio on a mecanum vs a 6WD.

[Ari423]

Originally Posted by Ether

Would you please provide a more detailed explanation what you meant by this? i.e. How do you get from the first sentence to the second one?

The way I understand it, if you keep the gear ratio constant and lower the wheel's CoF, you lower the friction load placed on the motor (when pushing) and therefore lower the current the motor draws. If you are limiting your speed to keep the pushing current at or around 40A, gearing that would bring a traction robot to 40A should only bring a mecanum robot to around 28A. That means that you can gear the robot to go faster (i.e. draw more current for the same load) by about 140% to bring the current back to around 40A.

[Ether]

Originally Posted by Kevin Sevcik

You're traction limited at a lower torque, so if you're trying to spin wheels at 40-50A (so as not to blow a breaker) you can have a higher gear ratio on a mecanum vs a 6WD.

That's much clearer.

Better to ask than to assume.

[pfreivald]

Originally Posted by Ari423

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.

I have never found this to be true. A mecanum drive controls *exactly* like a first-person shooter on any given game console--once told that, I have yet to have a student who can't drive it pretty well the moment they're handed the controls, and it doesn't take much time to get really good at it.

Originally Posted by Ari423

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.

This is the reason that I, coach of a team that's used mecanum four times and octocanum another four, agree that mecanum is definitely the wrong drivetrain for this year. My prediction is that monstrous defense (especially in the tournament) with criss-crossing robots and the restricted driving space around the airships is going to make most mecanum teams rue their choice.

Originally Posted by Ari423

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.

Specifically, too many teams gear their drivetrain so fast that they lose too much acceleration on the low end, and ultimately traverse the field more slowly than if they'd have geared lower.

Originally Posted by Ari423

In general, I think there are very few cases (practically none) where a decent mecanum drive is better than a good WCD.

I agree, though I'm a huge proponent of octocanum. It's highly competitive, impresses sponsors, and is psychotically easier (and cheaper) to implement than swerve. I highly recommend any team looking to branch out into other drive systems to give octocanum an off-season try.

[ZMarks]

When people say "Tank Drive" I get all warm and fuzzy thinking back to my first FIRST experience.
FRC 971's 2004 Half Track


COTS Snowblower tracks, mounted at an angle, with the previous year robot's spare wheels

[Robosparks2926]

Are the CIM bag motor (217-3351) and the vex mini CIM (217-3371) considered CIM motors as described in rule R32?

[jnicho15]

Originally Posted by Robosparks2926

Are the CIM bag motor (217-3351) and the vex mini CIM (217-3371) considered CIM motors as described in rule R32?

Only (full size) CIMs are CIMs. Other things with CIM in the name or that look like CIMs are NOT CIMs. MiniCIMs are MiniCIMs and so on. Also, the numbers you list are explicitly mentioned next to their respective names with unlimited quantity.
https://frc-qa.firstinspires.org/manual/R32