20-11-2014 09:26

evand4567

Re: pic: GBX-114 swerve with PTO

Why did you choose those gear ratios? Most swerve is ~ 10fps and your low gear is 12fps. I haven't done the math, but based on our experience, you'll have problems accelerating in high gear. A 5-8 fps low and 10-15 fps high would be better IMO. Also, what's the sprocket at the top of the wheel module for?

20-11-2014 10:15

asid61

Re: pic: GBX-114 swerve with PTO

This is a 4-cim, 4-minicim drive.
This year, we ran a free speed of around 17fps with 4 cims only and had no problems accelerating at all. 1678's 6-cim drive was geared for around 22fps. This is an even more powerful drive, so I bumped it up to 24fps. The math works out; for anything more than a couple feet it is easy to use high gear (with < 0.05s of lag behind low gear).
The sprocket is the output of the PTO. It is not mounted to the module, but shifts on the shifting shaft.

20-11-2014 11:23

Ty Tremblay

Re: pic: GBX-114 swerve with PTO

Have you done the calculations to estimate current draw on this? 1678 had to take extra steps to reduce the chance of popping their main breaker. Same goes with 254 and many other teams. The only reason 6-CIM drivetrains are even possible is because we know the breaker can hold more than 40A for longer than the spec sheet says.

You're adding 230W of power AND reducing torque. It's not a matter of acceleration, it's a matter of keeping your robot powered while you accelerate.

20-11-2014 11:25

75vs1885

Re: pic: GBX-114 swerve with PTO

Looks sweet, but I am concerned about the turning gears, they're so close to the ground that if anything bounced up, it could potentially break or chip a gear tooth.

20-11-2014 11:47

Nemo

Re: pic: GBX-114 swerve with PTO

I don't think the math will work out on those speeds when you take into account the voltage drop that happens when you run all twelve motors on at the same time.

20-11-2014 19:45

Arpan

Re: pic: GBX-114 swerve with PTO

The current drop from the turning motors is a significant factor here, I think.

I would be very surprised if a swerve like this didn't blow the main breaker in a pushing match, even in low gear. IIRC FRC 2451 had current-related problems pushing an immovable object (read = our robot) last year with just 4 CIMS at a lower speed.

20-11-2014 20:40

asid61

Re: pic: GBX-114 swerve with PTO

This will blow the breaker in a pushing match due to the massive power behind it. However, as with any high-geared 6+ cim drive, there must be good autoshifting code to prevent disasters. IIRC 1678 had a main breaker blow in the finals at SVR due to their autoshifting code having problems.

With swerve drives, if we get into a pushing match, we have three
options:
1. Translate away. Good for offensive tactics, especially with this blistering speed.
2. Push back. Useful only against lightweight robots or if we are stuck somehow. Or to slow other's robots down.
3. Cross the wheels. If the wheels are in an X-formation, then we can't be pushed unless they lift us (cue opponent's breaker blowing). Esentially, the driver has a trigger to lock the wheels, and when the opponent tries to go another direction our driver releases the trigger and moves. This is an extremely effective defense (in theory) as long as our driver can move to block them when they try to get past. I'm confident our speed is high enough for this.

The purpose of low gear is to allow for precise positioning and very short drives. That's why it's geared so high here.

The 40a breakers are on each motor. The main breaker is 120a, which is usually the problem.

Thank you for the input. I hadn't considered that. However, this only supports slightly more power than 6 cims, so it might not be that bad.

There is around 0.25" clearance to the ground. Not much we can do about that, but at least they're vex gears so they're easy to replace. Many swerve drives use gears so close to the ground, and I haven't heard of so many problems.

EDIT: One of our mentors mentioned that a bigger problem would be the current spike killing us on startup. Some things we have to solve with software; this kind of drive won't be used for at least a year (2016 game) so we have plenty of time to do tests.

20-11-2014 21:19

Mike Marandola

Re: pic: GBX-114 swerve with PTO

How did you come to the conclusion that you can't be moved when the wheels are in an X?

20-11-2014 23:42

asid61

Re: pic: GBX-114 swerve with PTO

In order to move the robot, the pushing robot would be acting against the traction wheels. Any direction that they are pushing from they would need to scrub the tires.
You could turn the wheels 90* from their push too.

21-11-2014 00:05

asid61

Re: pic: GBX-114 swerve with PTO

D'oh. The low gear is actually 9.8fps in the photo. I accidentally marked it down as 36 tooth instead of 44 tooth in the calculator.
Then the robot is definitely traction limited. It draws 240 amps at maximum (lifting) torque. I was worried about increasing the pulley to get the free speed under 12fps, but it turns out that won't be an issue.

So the speeds are 24.0fps/9.8fps.

21-11-2014 01:19

nathannfm

Re: pic: GBX-114 swerve with PTO

Assuming your contact patch is square an X with 45° wheels is the most effective (I know that a 1" wide, 4" dia wheel with Blue Nitrile is approximately square) because you are applying force along the longest line you can draw in that shape. If you measure your contact patch and find it is a rectangle you can calculate the most effective angle to put your wheels at when you make this maneuver. (Not sure how much difference that would make tho...)

21-11-2014 01:31

asid61

Re: pic: GBX-114 swerve with PTO

It would increase contact area, but I want to be able to lock the wheels without having to spend power on the motors. I guess putting them in brake mode would do the trick.

21-11-2014 02:16

Mike Marandola

Re: pic: GBX-114 swerve with PTO

Since the contact area and amount of force would be the same regardless of the wheel angle, there might not be any difference. This is not taking the tread pattern into consideration though. Which is why the X formation might be beneficial. I think the X pattern will both reduce the wheel's ability to rotate when pushed and utilize the effects of the tread "fingers".

The contact area would remain the same.

Nathan, I know you said that 365 did testing on diagonal vs. normal tread. Did you guys happen to test the tread sliding sideways as well?

21-11-2014 02:41

asid61

Re: pic: GBX-114 swerve with PTO

Oh wait, you're right. I was thinking of another tread with diagonal tread in it...
Regardless, it shouldn't matter. An x-configuration would allow the motors to just sit in brake mode I think.

21-11-2014 02:59

nathannfm

Re: pic: GBX-114 swerve with PTO

Yea, I figured whenever you sent them to an X configuration you would want to be breaking.

Yes, I wrote up a procedure in that thread. Thats actually what started us doing the testing, we were getting pushed sideways more than we liked.

21-11-2014 03:06

Mike Marandola

Re: pic: GBX-114 swerve with PTO

Awesome. I will check that out.

21-11-2014 03:06

asid61

Re: pic: GBX-114 swerve with PTO

Pushed sideways while in an X? That woule be very hard to do if you set the turning motors in brake as well; they are geared down 1:26 with a secondary of 30:84.

21-11-2014 03:10

nathannfm

Re: pic: GBX-114 swerve with PTO

Sorry, we did the tests last year on our 8 wheel WCD. With a swerve that has straight cut tread it should be just as hard to do as being pushed backwards. (I will have to think about how best to "break" with diagonal tread)

21-11-2014 03:12

asid61

Re: pic: GBX-114 swerve with PTO

Thank goodness. I was thinking, "Oh god, did my numbers come out totally wrong!?"
They did, as a matter of fact. This is only geared for 20fps, sadly. I messed up the numbers in the calculator (again). Fortunately, the speed is easy to increase, but hard to decrease.
EDIT: I added a 3-position cylinder to the CAD, but for some reason these cylinders from Bimba are around 6" tall. So you can technically put on one here, but I would probably learn to live with this version.

21-11-2014 03:24

nathannfm

Re: pic: GBX-114 swerve with PTO

I'm confused, what's that have to do with being pushed while in X mode?

21-11-2014 03:35

asid61

Re: pic: GBX-114 swerve with PTO

If you get pushed from the side while in x mode, then because the contact patch with the floor (ideally) is a line or thin rectangle, the pusher will torque the wheel sideways a bit like a caster. It seems like it would take a ton of force to shove those wheels back into a straight position, but it is doable. I'm sure that the robot will lift before that though.

21-11-2014 03:59

nathannfm

Re: pic: GBX-114 swerve with PTO

Theoretically it's a line, if your wheels and the floor were both hard and solid, ideally it's as large a rectangle as you can make it because with tread on carpet the physics class friction model goes out the window and surface area is quite important. But 45° wheels pushing from the side should be identical to pushing from the front. You are right though, it's quite hard to turn a (for us) 100:1 braked planetary gearbox. As for lifting, unless your bumpers are higher than your opponents there is no force that is going to lift your robot. Infact, if your bumpers are lower they will be pushing you into the floor a small bit, increasing your normal force and your friction.

21-11-2014 19:00

Bryce2471

Re: pic: GBX-114 swerve with PTO

Out of curiosity, how are the side plates of your caster box attached to the turning gear that the robot is resting on?

21-11-2014 21:31

asid61

Re: pic: GBX-114 swerve with PTO

Ah, caster box. That's a good name for it.
The turning gear is a modified vexpro 84 tooth gear. A big hole is in the center for clearance, and about 1/8" is cut out of the face of the gear (deburring would have to be done carefully on the teeth). Of the 1/8", there is a small pocket that sticks up which the caster box side plates drop in to.
The pocket is wallled on three sides and has screws on the bottom to connect it properly. The fourth side of the pocket is the big hole in the center of the gear.