5036 - 2019 Offseason WCD CAD

This past off season we designed a WCD for a low ground clearance (a-la 2017) game:





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“Specs:”

  • 21" x 26" Frame Perimeter
  • Single speed, 3 Mini Cims/Side: 5.64:1, 4" wheels, ~18ft/s free speed
  • 15mm HTD Belts
  • 42T HTD Pulleys
  • “Double sided” electronics board
  • Fixed C-C belts
  • 1/16" drop center, outer omnis, middle blue-nitrile on AM performance wheel

This is the product of lessons we’ve learned over the years with drive trains. After having gone back to the kitbot last year (due to graduating students) it was important we did not lose knowledge with graduating students.

We’re happy to hear any suggestions! You can find the STEP here.

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Why did you put center drop? You have omni wheels on front and back side of the robot .

This was done to ensure the traction wheel in the middle keeps good contact with the carpet. FRC Fields are not perfectly flat, and with wear on the center wheel tread, we could well have a situation where the drive sporadically handles as a 4WD Omni if there was no drop.

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Thanks

Love it! One word of caution - having built a 20" wide frame this summer, make sure your center of gravity isn’t too high, or the robot will tend to tip over in high speed turns. Ask me how I know :slight_smile:

I like the idea of offset belts. Very clean! But make sure the bit of structure and belly pan that sticks through the belt is attached with accessible bolts, rather than rivets, so that you can remove the belt if ever necessary

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Not a huge fan of having the second belt on the outside of the drive rail. Beyond the cantilever issues, this pushes your entire drive rail in resulting in the entire frame’s lost space rather than just the belt length’s. You could make the case if you were sticking to a strict 1" or 2" hole pattern, but it seems like you’ve accepted the .5" so might as well put the belt inside and get some space back. Side by side, you aren’t saving much of anything

I am guessing that the outermost tubes on each side are your bumper mounts. You should consider using short pieces of tube stacked on top of the tubes across the ends instead of using the standoffs to support these. Side impacts will likely bend where the standoffs are attached to those tubes and where the standoffs are attached to the tubes across the ends.

With your control system on the bottom, it would be best to ensure that the workmanship of the installation of the components and the wiring is top notch. Otherwise, you will be flipping the robot over a lot.

What is the path for the wiring between the panel with the Roborio and panel with the PDP and motor controllers?

What is the path for the cables from the battery and main breaker to the PDP? Are the battery and the PDP on opposite ends of the robot?

Where will your main breaker be located?

The cables between the battery and main breaker and the PDP are pretty stiff and thick and it will be difficult to make sharp bends with them.

You are probably better off if you locate the battery and the PDP at the end of the robot with the belts on the outside and turn both so that the big cables are nearer the outside of your robot (nearer to the bumper). This will shorten the big cables and give you better access for disconnecting the battery. It will also make it easier make your main breaker readily accessible. You will still have to allow space for your cables to come up from the PDP. It may be best to have them run to one side of the battery so they don’t impede removal of the battery.

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What led you to go with belts over chain?

Also, have you done underside electrical before? If you’ve done it with no problems, go ahead, but otherwise I’d be worried that in the event of an issue it’d be a nightmare to work on, having to go back and forth between the top and bottom of the robot.

Depending on what you have above the belly pan, it could be just the opposite. It’s way easier to tip the robot and have access to everything, rather than digging under mechanisms. I’m speaking from experience.

That’s fair- how did your layout compare to theirs?

Point noted. For our drive trains, we have a requirement that the gearbox be COTS. The WCP SS used here can only accept one 15mm pulley without custom length spacers. Having the belts on the outside for half of the drivetrain allows us to fit motor controllers close to the drive rails without worrying about wires getting caught in belts - although its mostly the former.

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What is the path for the wiring between the panel with the Roborio and panel with the PDP and motor controllers?

Cables go between the two gearboxes, across to the battery - likely going to add a thin piece between the two belly pans to secure those wires to.

What is the path for the cables from the battery and main breaker to the PDP? Are the battery and the PDP on opposite ends of the robot?

Yes, the battery and the PDP are opposite sides. These cables would follow the same route mentioned above.

Where will your main breaker be located?

Somewhere opposite the manifold. It’s not modeled here since we don’t know what the superstructure is. This is somewhat inline with the path between the battery and the PDP.

You are probably better off if you locate the battery and the PDP at the end of the robot with the belts on the outside and turn both so that the big cables are nearer the outside of your robot (nearer to the bumper).

Definitely thought about this, but for motor controller packaging, and not wanting a tight bend radius on those 6AWG cables, we went the route we did.

We have a lot more experience with belts than chain, and we wanted a zero maintenance drive system. Some teams have definitely gotten away with fixed C-C chain with some adder, we just haven’t established that adder for our team’s build tolerances yet. If we went chain, we would definitely have some sort of tensioner (versablocks). Belts are also marginally more efficient. In short, in order to minimize part count, minimize maintenance and some bonus efficiency - we went with belts.

We haven’t done an upside down electrical panel before, but have routinely found our electronics hard to access due to mechanisms. Having uninterrupted access to most of the electronics on the robot is very attractive.

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It sounds like the loop of cable from the battery to the breaker to the PDP and back to the battery is pretty long. This will increase the voltage drop when you have heavy current draws. Depending on your mechanisms, the cable routing could be difficult when that loop spans so much of the chassis. It is in your best interest to keep the length of this loop as small as possible.

If you push the PDP away from the end tube and bumper, you will have sufficient bend radius for the big cables.

You may want to mock this up with different arrangements using cardboard and pieces of wood to check out the wiring.

As stated before, your execution of the electronics must be top notch or troubleshooting will be miserable. How reliable has your electronics been in the past?

Have you considered installing the electronics on a vertical panel on the outside of your mechanisms? Of course, you would install a protective cover over the electronics.

It is in your best interest to keep the length of this loop as small as possible.

Depending on the super structure, the breaker may go adjacent to the compressor, that would make the wire run a lot shorter than putting it all the way near the bumper rail. This chassis is pretty small as it is, as all of our battery cables are more or less the same (fairly short) length.

As stated before, your execution of the electronics must be top notch or troubleshooting will be miserable. How reliable has your electronics been in the past?

We haven’t died in a match in the last two years (knock on wood). I disagree this will make trouble shooting miserable. I think it’s far more miserable to try and diagnose a problem when all of the electronics are buried under the superstructure.

Have you considered installing the electronics on a vertical panel on the outside of your mechanisms? Of course, you would install a protective cover over the electronics.

Momentarily, a-la 1241 2017, but likely won’t do this unless it’s critical to game piece manipulation.

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If you install your breaker next to your compressor and your robot catches fire, the refs and field reset people are going to let it burn rather than reach into it to shut it off.

Having to flip the robot over repeatedly and burying your electronics under major mechanisms will both make your life miserable. With your current design, you might end up with both forms of misery. The best designs make the control system easily accessible for troubleshooting, modifications and repair.

In every season, you will find many examples of electronic panels mounted vertically and easily accessible.

You could move the battery to the other side, and fit the compressor beside it. That would open up space to place the Rio on the top if it happens to be more accessible. I’ve sketched the idea, and included some options for the main breaker placement too.

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Appreciate the sketch! I’m not comfortable putting the battery on the belly pan without some structure underneath. (Belly pan is only 1/16" thick) I like the compressor being off to the side; it lowers the profile of the belly pan along the middle for game piece manipulation! I’m also a fan of the left-most breaker placement! Having that “empty channel” through that part of the robot is attractive.

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