pic: FRC488's Octocanum Ver 2.0

[Ether]

Originally Posted by Madison

Ask questions and I'll answer them.

What mecanum wheels are you using, why did you select them, and what (if any) issues have you had with them?

Originally Posted by Madison

We implemented mecanum drive better than most of what I've seen on the field

What makes your mecanum implementation better?

[Madison]

Originally Posted by AllenGregoryIV

This looks really great I have a couple questions

1. Why did you choose to pivot the traction wheels down instead of moving the mecanum wheels?

2. What are the final gear ratios to each set of wheels?

3. Why did you choose to build under the kit frame instead of on top of it?What's the sub-frame made out of?

We pivot the traction wheels down for a few reasons. First, by putting them nearer to the center of the machine, it makes turning with four high-traction wheels a bit easier to accomplish. We also gear down further to that wheel set, so it makes sense for it to be at the end of the gear train.

The gear ratios are 8.45:1 at the mecanum wheels -- that's one of the options for the Toughbox Nano -- and about 25:1 at the traction wheels. The traction wheel ratio makes things pretty slow, but it's easy to swap in sprockets to speed things up as required.

The kit frame is 1.25" thick and the output of Toughbox is 1.5" from its edge, so when using a 6" mecanum wheel, we'd end up with just .25" of ground clearance if we built things the other way around.

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.

Originally Posted by Ether

What mecanum wheels are you using, why did you select them, and what (if any) issues have you had with them?



What makes your mecanum implementation better?

This is based on AndyMark 6" mecanum wheels -- http://www.andymark.com/product-p/am-0136.htm

We used this last season and I'm happy with the build quality and performance. They're heavy, but they're not as heavy as the 8" set we originally played with years ago and we're willing to deal with the weight and cost in place of building our own.

Our mecanum implementation worked. I didn't program it, so I can't speak too much to what made it work, but our programming team did a fantastic job there. We had accurate, fast, field-oriented drive that allowed us to fully use the movement capabilities of the mecanum wheel set. In my experience, most teams fail to achieve the level of control we managed.

[AllenGregoryIV]

Thanks that cleared some things up.

Are the wheel assemblies and bearing blocks the only machined parts?

How do you avoid compressing the C-channel when you bolt the tough-boxes through it?

[Madison]

The wheel assemblies are 1/4" ABS with .5x.5x.125" angle added for rigidity. We can laser cut the ABS parts in a few minutes; then it's simply a matter of cutting the angle to length and match drilling it to the ABS.

The bearing blocks are of similar construction, but are really ugly right now. I'm still working on those.

If possible, we'll bolt the transmissions through just one wall of the C-channel. Otherwise, we'll laser cut 1/2" ABS inserts to stick inside the C-channel to prevent it from buckling where it's bolted through.

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?

[Ether]

Originally Posted by Madison

Our mecanum implementation worked. I didn't program it, so I can't speak too much to what made it work, but our programming team did a fantastic job there. We had accurate, fast, field-oriented drive that allowed us to fully use the movement capabilities of the mecanum wheel set. In my experience, most teams fail to achieve the level of control we managed.

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

^ Those help too.

[JohnGilb]

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

Granted, the 488's 2011 machine was built quite well, but there were still all sorts of mechanical inconsistencies. The center of gravity moved around significantly as the arm moved, generally there was more weight in the back (and slightly more on the left side, I believe), not to mention occasionally getting rammed around or crashing into objects.

The base aspect of converting a desired vector/rotation into wheel speeds is pretty easy (it's even included in the WPI libraries), but we did a lot of additional work so the robot would _actually_ move the way you intended. There were many PID operations that more or less worked in concert to allow smooth robot control. It was essentially a solved problem from a theory perspective, but still required a lot of code in order to operate well.

[Ether]

Originally Posted by JohnGilb

There were many PID operations that more or less worked in concert to allow smooth robot control.

Other than closed-loop speed control of the 4 wheels (did you do that?), what other closed loop controllers did you use for controlling the drivetrain?

[JohnGilb]

Initially, we had 3 closed-loop systems:

Rotational - used a gyro to keep the robot on target heading
Translational - used encoders on "follow wheels" (unpowered wheels) to gauge ground speed and keep the robot translating on a target vector
Wheel Speed - used encoders on the drive wheels themselves to achieve precise wheel speed control

After a while, we actually disabled the wheel speed, as we didn't appear to get much performance improvement from it and we were looking to save on some CPU cycles.

[Ether]

Originally Posted by JohnGilb

Rotational - used a gyro to keep the robot on target heading
Translational - used encoders on "follow wheels" (unpowered wheels) to gauge ground speed and keep the robot translating on a target vector

Were these controls presented to the driver as the interface? i.e., did the driver interface allow the driver to command for example "Translate in direction A with the vehicle pointing in Direction B" ?

[JohnGilb]

The driver had two joysticks:

1st: X/Y joystick movement controlled X/Y direction & magnitude of the robot relative to the field
2nd: X joystick movement controlled rotational rate relative to the robot

So, not exactly what you described, but fairly close.

[Ether]

Originally Posted by JohnGilb

The driver had two joysticks:
1st: X/Y joystick movement controlled X/Y direction & magnitude of the robot relative to the field
2nd: X joystick movement controlled rotational rate relative to the robot

OK, so let me read back to you what I hear you saying:

- You do closed-loop control of the vehicle speed as commanded by the radius¹ of joystick #1

- You do closed-loop control of vehicle heading as commanded by the angle² of joystick #1

- You do open-loop command of rotation rate as commanded by joystick #2 (X, Y, or Z axis?)

Am I understanding you correctly?

How many follower wheels did you use, and how are they mounted?


¹ Do you use sqrt(X^2+Y^2) or max(abs(X),abs(Y))?

² Do you use atan2(X,-Y) to calculate angle, or something else?

[JohnGilb]

Sorry, I think something got lost in my description.

-We perform closed loop control of X translation as commanded by the X axis of Joystick1

-We perform closed loop control of Y translation as commanded by the Y axis of Joystick1

-We perform open loop control of Robot rotation rate as commanded by the X axis of Joystick2 (however, when desired rotation rate is 0, we perform closed-loop control of robot angle as commanded by the heading we were at when we stopped rotating)

This was accomplished with 3 follow wheels and a gyro. Two of the follow wheels were used to track the robot moving forward/backward, and one was placed directly under the center of rotation and used to track the robot strafing.

[pfreivald]

An interesting design! We've used mecanum for the past two years and were very satisfied with it in many respects, but know that it needs some push to really up our competition level. Thus, we've been working on our own drive (though I didn't know it had a cool name like 'octocanum') which is similar in idea to this.

Thanks for sharing! (And great questions everyone. This is a good thread to follow for anyone considering such shenanigans like we are!)