pic: FRC488's Octocanum Ver 2.0

[Ether]

Imagine a robot going around a circular track at constant speed such that it completes one lap every minute. The robot has a gyro mounted on it. The gyro rate will be 1 rpm, the same as if the robot were spinning in-place at 1 rpm.

I'll have to check the math, but I believe that with 3 followers mounted as shown, you can extract the vehicle's rotation rate (the same rate measure by a gyro) for any vehicle motion.

[AdamHeard]

Originally Posted by Ether

Imagine a robot going around a circular track at constant speed such that it completes one lap every minute. The robot has a gyro mounted on it. The gyro rate will be 1 rpm, the same as if the robot were spinning in-place at 1 rpm.

I'll have to check the math, but I believe that with 3 followers mounted as shown, you can extract the vehicle's rotation rate (the same rate measure by a gyro) for any vehicle motion.

Yes, but that's with 3 follower wheels. 488 only has two follower wheels measuring the floor, so I was clarifying as our swerve has two and there is no way we could infer rate from that (in fact we have to factor in the gyro rotation rate to even to get an accurate linear velocity).

EDIT: 488 actually has three follower wheels in that diagram, I was mistaken. Trivial calculation in that case.

[Ed Law]

Originally Posted by Ether

Mecanum is not hard to program. It is very straightforward. The same is true of field-oriented drive.

If your robot had superior control, I suspect the reasons are probably:

1) superior craftsmanship and attention to detail of the mechanical design (wheel and frame alignment, leveling, weight distribution, minimal and consistent drivetrain friction, carefully assembled and adjusted mecanum wheels)

2) attention to detail of the electrical design (proper wiring to motors and gyro, selection of gyro with minimal drift, etc)

3) well-designed driver interface (match the driver interface to the driver)

4) skilled drivers with lots of practice

Ether,

We tried Mecanum wheels and tried field oriented drive. It was a good exercise for our programmer who did it all on his own. It would work for 10 to 15 seconds and then the headings will be off. I think it is because of gyro drift. We use the gyro that comes in the KOP. Is there a better one that we should use? We tried a compass sensor but there is too much electro magnetic interference from the motors. Any help would be appreciated.

[Ether]

Originally Posted by Ed Law

We use the gyro that comes in the KOP. Is there a better one that we should use?

We're still waiting for JohnGilb to tell us what sensor his team used.

How about it, John? :-)

[AllenGregoryIV]

The Discobots used the kit gyro last year to do field oriented drive on our omni-drive. We didn't have to much issues with drift over the duration of the match. We did have it setup so that the driver could zero the gyro him self at any of the cardinal directions but he only needed to use it in a few matches.

Our biggest problem with the kit gyro was the initial calibration routine we were dead in the water during a few elimination matches at Lone Star because we didn't let the robot sit still when we turned it on.

[JohnGilb]

I've sent an email to the electrical lead on our team, I'll let you know once he digs up the model number of our gyro. =]

[Dad1279]

Originally Posted by theprgramerdude

What's the maximum speed at which you can switch between Mecanums and high-traction wheels? Plus, about many switches can be performed before there becomes a serious air deficiency such that it is unreasonable to switch at all, assuming you have about 3-4 tanks at 120 PSI at the start.

YMMV, but we used 1 tank, (2) 1.5" bore, 2" stroke cylinders to switch between traction & mecanums, (2) .75" (6 or 8" stroke) to lift claw, and 2 small cylinders to deploy the minibot, and never had a want for more air.

[Dad1279]

Originally Posted by Jonathan Norris

....
I was wondering if you pivoted around the traction wheels instead of the Mecanum wheels in the octocanum, if you would see any advantages because the pistons could act as a suspension system. I've been told that mecanum drive systems perform better with a suspension system, I would be interested in hearing from teams that have used suspension in their mecanum drives. But for me I would need to see a big improvement in acceleration/deceleration to advocate for using mecanum again.

That is what we did. Not necessarily for the suspension, but we wanted the traction wheels to be the most outward wheels for stability, and on the the fixed axles for strength. Also, if there was an pneumatic failure, we wanted the default wheels to be the traction wheels.

Not shown in the sketch below, the pistons were vertical, one on each side, and they pushed down where the front and rear pivot arms met in the center of the robot. We used the 8" Andymark wheels, not by preference, but budget. We had a new, unused set on the shelf as spares from the prior year. Also not shown below, the transmissions and outer axle bearing were rigidly mounted to chassis.

Yes, 8 motors on drivetrain, 1 CIM and 1 CIM-U-LATOR/775 per wheel.



While I personally preferred the field-orientated driving, unfortunately (for me) mentors don't drive. Our driver preferred traditional controls, so that his orientation didn't change when swapping between traction and mecanums.

[Joe Ross]

Originally Posted by Dad1279

While I personally preferred the field-orientated driving, unfortunately (for me) mentors don't drive. Our driver preferred traditional controls, so that his orientation didn't change when swapping between traction and mecanums.

You could do something like 451's SOAD - Semi Omni Arcade Drive.

[JohnGilb]

Originally Posted by Ether

We're still waiting for JohnGilb to tell us what sensor his team used.

How about it, John? :-)

So, our electrical mentor says we used http://www.sparkfun.com/products/9410, and it looks pretty familiar.

[AdamHeard]

Originally Posted by JohnGilb

So, our electrical mentor says we used http://www.sparkfun.com/products/9410, and it looks pretty familiar.

Did you take advantage of both output rates? As in, only referencing the faster output when the slower output was saturated (or some more intelligent way of mixing them)?

[JohnGilb]

We did that for a time, but it turned out the finer resolution was overkill and a bit noisy. The lower resolution (500 deg/sec max output) was accurate enough.

Actually, I thought the gyro was broken, because sometimes it wouldn't drift at all for long periods of time. Then I'd kick the robot and see it respond. The gyro combined with the default gyro libraries from WPI worked pretty well.

[Jared Russell]

Originally Posted by JohnGilb

We did that for a time, but it turned out the finer resolution was overkill and a bit noisy. The lower resolution (500 deg/sec max output) was accurate enough.

Actually, I thought the gyro was broken, because sometimes it wouldn't drift at all for long periods of time. Then I'd kick the robot and see it respond. The gyro combined with the default gyro libraries from WPI worked pretty well.

This is probably due to two features of your gyro:

1. The bandwidth of the gyro as implemented on that evaluation board.
2. The "auto zero" function, which apparently uses Vref to provide a dynamic estimate of what the true bias is (if you used this feature)

[Madison]

Sorry for taking so long to reply -- I've been pretending to be very busy. Let me try to address some of the non-programming-related questions.

Originally Posted by SuperNerd256

Woah.....

I have seen many amazing drive trains in my time (and I mean MANY), but this is just AMAZING! I love the idea! Are you going to implement it next year?

Just some questions:

1. How many KoP parts does it use?
2. How many other parts?
2.5 Where can these parts be obtained?
3. How easy is this to make (during build season, with a team of builders)
4. Is it open source?

We'll implement it if it's right for the game. A design like this obviously favors a flat, open field; though it can navigate ramps pretty safely. Platforms, stairs and varying surfaces may make things more challenging.

1. It uses a KoP frame as its main frame, but nothing else is really from the KoP.

2.

• The gearboxes are AndyMark ToughBox Nanos with the long hex output shaft option and gearing of 8.46:1.
• The wheels are AndyMark 6" mecanum wheels and 4" Plaction wheels. Sprockets are AM 16 tooth hex broached sprockets on the gearbox output and 36 tooth hubless sprockets on the traction wheels.
• The subframe is 1x1x.125" aluminum angle surrounding McMaster-Carr #92985T25.
• The mecanum wheels are driven by an AM hex broached hub. The plaction wheels ride on AM hex bearings. The wheel modules pivot on hex bearings.
• The plaction wheel axle is a 1/2" hex standoff -- McMaster-Carr #92230A335
• The wheel module sides are laser cut 1/4" ABS with .5x.5.x.125" aluminum angle for rigidity
• 1.5" bore pneumatic actuators with a 3" stroke to actuate the drive. I want to use spring-return actuators here but don't know if they'll be legal.

3. This should be very easy to assemble, generally speaking. The most difficult parts to deal with are the wheel module sides. Last season, we used 2x1x.125" channel there, but because the gearbox was driving the mecanum wheels via chain, everything was a dead axle. Here, the mecanum is driven via live axle and thus requires bearings in the wheel module sides so that the plaction wheels can be rotated down efficiently. The manufacture it the way I've shown, you'd need some ability to CNC plastic/aluminum or otherwise to mill accurate bearing pockets.

4. Maybe. There are still details absent from the model. If I ever get those in place, I'll share the CAD models.

Originally Posted by Jonathan Norris

Any thought into making the Mecanum wheels the pivoting ones??

I was wondering if you pivoted around the traction wheels instead of the Mecanum wheels in the octocanum, if you would see any advantages because the pistons could act as a suspension system. I've been told that mecanum drive systems perform better with a suspension system, I would be interested in hearing from teams that have used suspension in their mecanum drives. But for me I would need to see a big improvement in acceleration/deceleration to advocate for using mecanum again.

I aimed for simplicity in the design, so because we want the mecanum wheels to be the fast travel wheels, they end up higher in the gearing. They're geared at 8.46:1 and the plaction wheels are geared at 19:1 (in addition to being smaller). There aren't any reasonable, commercially available gearboxes that have a final ratio of 19:1, so to get we want out of the traction wheels, we'd run into a bunch of complications in our gearbox selection and then we'd have to gear up again to get the mecanum wheels going at the desired speed.

It's possible to drive the pivoting wheels first, I'm sure, either by allowing the entire gearbox to move when the wheelsets actuate or by devising a system that allows the chain length to vary as the distance from gearbox output to wheel changes. I've done the latter (about ten years ago) and would rather avoid doing it again. The former is probably achievable, but I feel that it'll complicate the frame design more than I'd like and we didn't have discernable trouble last season with rigidly mounted mecanum wheels, so I'm happy to do it again.

Originally Posted by theprgramerdude

What's the maximum speed at which you can switch between Mecanums and high-traction wheels? Plus, about many switches can be performed before there becomes a serious air deficiency such that it is unreasonable to switch at all, assuming you have about 3-4 tanks at 120 PSI at the start.

The switch can happen at full speed and takes a fraction of a second. We had a small compressor on-board last season and a single Parker accumulator, both feeding the actuation cylinders -- two 1.5" bore, 3" stroke -- and a single gripped actuator (about 1.5" bore, 9" stroke). We never had problems having enough air in reserve.

[lcoreyl]

First off--very nice design, and very accessible. We're not very capable in terms of build complexity, but might prototype something like this next season.

One of our main concerns is the side loading.

Originally Posted by Madison

The subframe, right now, is 1x1x.125" angle bolted to the perimeter of a .125" perforated PVC sheet and hung from the Toughbox Nano housings. It is intended to hold electronics and would need to be enhanced if it were going to see more substantial loading.

Is this also intended to handle some of the sideways forces that the wheel assemblies would encounter if a robot hit from the side while in traction mode? It appears that the subframe and wheel assembly are flush, so I assumed that was an additional purpose?

If I recall, from octocanum v1.0 I saw video of you testing side loading by deploying traction while strafing. Did you run into any problems with this during last year’s competition?