20-01-2004 11:31

Skabana159

Re: pic: Swerve^2 Prototype

Are you using potentiometers or digital encoders to track the orientation of your wheels?
Also, can I get a close-up shot of your drill's gearbox? It looks neat.

20-01-2004 15:29

Stormhammer

Re: pic: Swerve^2 Prototype

k, now THAT is interesting... though... hm

20-01-2004 16:10

Greg Needel

Re: pic: Swerve^2 Prototype

very nice system....have you had any problems with drawing too much current or stalling the drill motor since you are only using 1 to power all 4 wheels?

20-01-2004 22:00

WakeZero

Re: pic: Swerve^2 Prototype

We use two pots on both steering chains to determine position, so four total. As for the one drill motor issues, we have noticed that problem to a small extent... but they will go away once we install the second drill

21-01-2004 00:00

WakeZero

Re: pic: Swerve^2 Prototype

We lovingly refer to this little contraption as the X-Box, since it's chains form an X that go out to drive each wheel. What it allows is for two drill motors to be mounted horizontally on the robot to drive the wheels. It also has a nice feature in that it makes the drill motors spin in the same direction (Clockwise and Clockwise, or CCW and CCW)

We may be taking out the four shafts around the middle one and just putting four or two sprockets on the top of the middle shaft... we would only do this if we thought it was too inefficient, or we were hurting for weight

21-01-2004 00:24

Madison

Re: pic: Swerve^2 Prototype

It's clear that it can translate along the ground -- but how does it turn? With two motors powering a single gearbox that drives all four wheels, is differential steering possible?

21-01-2004 00:29

edomus

Re: pic: Swerve^2 Prototype

looks like they also are using crab drive with different motors powering the turning

21-01-2004 00:38

WakeZero

Re: pic: Swerve^2 Prototype

Here is what we use to turn ONE set of wheels. We have one of these on the back two wheels, and one on the front. You can't see them in the picture because they are mounted upward, and hidden by the frame

I explained it in the picture description, but that doesn't show up in the discussion. So here it is again

21-01-2004 01:08

Travis Covington

Re: pic: Swerve^2 Prototype

From what I can tell, it looks like there is no support on the other side of the globe motor shaft. It may just be the picture you showed, but in that configuration there is a significant side load that (typically) results in the globe motor releasing its magic smoke

If I am incorrect, sorry about that... Just thought I would note that... its an easy fix.

Otherwise it is VERY original! Looks good!

21-01-2004 01:43

WizardOfAz

Re: pic: Swerve^2 Prototype

Well, you're right about that. I've worried about it some, but since steering is such light duty cycle, I don't think we'll have a problem. Also, the side load is very close to the motor - it would be much worse with a longer shaft. I'd guess in a typical game we'll turn the wheels an aggregate of a few revolutions, maybe a few tens at the upper bound. In our setup, the globe motors rotate 2.8x as fast as you steer the wheels, so maybe we'll get on the order of 100 total turns of rotation in a game. I'm thinking the motor won't suffer much with that much use. Does this disagree with your intutition or experience?

Bill

21-01-2004 04:56

Travis Covington

Re: pic: Swerve^2 Prototype

Your assumption would be correct for almost all motors in the kit except the globe motor (in my honest opinion).

For some reason (maybe someone knows specifically) the globe motor is VERY sensitive to side loads. In past years when my team used these motors, if the shaft was not supported on both ends we would ruin the motor, even if it was used rarely.

If it is possible (it looks failry easy) you might want to try and put a bearing supporting the shaft on the other end... even though you might not really need it... it seems like it serves an important enough function to deem the extra work beneficial... even if it only gives you peace of mind.

Hope that helps

21-01-2004 10:19

WakeZero

Re: pic: Swerve^2 Prototype

Thanks Travis, we will surely be looking into this now

21-01-2004 12:02

Paul Copioli

Re: pic: Swerve^2 Prototype

I am going to have to back up Travis on this one. We used the globe motors for swerve steering in 2002 and we hade to add a bearing to support the load. We ruined 3 globe motors on our prototype unitl we figured it out. Once we added the bearing, we did not fail any globe motors.


-Paul

21-01-2004 12:44

WizardOfAz

Re: pic: Swerve^2 Prototype

OK, I hear the growing concensus. Our original design had a top plate and a bearing. Just "simplified" it to a single plate to the the prototype going quick, and it worked pretty well, so I was hoping to leave it that way. The prototype in the photo will be getting some road time in the next few days, so we'll see how that goes. What are the first symptoms of impending failure? Noise? Smoke? Just locks up?

21-01-2004 13:13

Paul Copioli

Re: pic: Swerve^2 Prototype

The first indications on our robot were that the wheels were not getting to the position we commanded. At first, we thought it was the potentiometer going bad, but it was actually the motor not having enough torque at the lower gains (as it gets closer to the commanded position). The motor slowely loses torque capacity as you fail the front bushing. The planetary gears inside the globe transmission start to wear and bind up, thus losing torque capacity. The motor will eventually lock up, but the swerve steering will suffer long before lock up.

-Paul

21-01-2004 21:16

Joe Johnson

1 bearing is not enough.

The main reason the Globes need support is that they have only one bearing if you don't. The second bearing becomes the planetary gears on the last stage if you don't give it an external one. This is very bad for efficiency of the gearbox.

Joe J.

22-01-2004 12:31

ajlapp

just going over the steps?

i noticed you have clearance for the steps....any thoughts about going up the sides of the platform. there isn't much room around that goal once you go up the steps.

22-01-2004 14:31

WizardOfAz

Re: just going over the steps?

Yes, we're working on that. Essentially raising the body bottom for more clearance and putting front and mid-body skids. Will post another picture soon.

Bill

22-01-2004 16:56

Paul H

Re: pic: Swerve^2 Prototype

How much does that prototype weigh?

23-01-2004 00:04

WizardOfAz

Re: pic: Swerve^2 Prototype

It's about 100 pounds, but about half of that is the wood base which will be aluminum for the competition guy. But it is heavy. Lot of hardware in each wheel. We're a tad concerned about it...

Bill

23-01-2004 02:12

FotoPlasma

Re: pic: Swerve^2 Prototype

Do you think you might be able to post a CAD or two of the wheel modules? I am very interested in how you implemented the coaxial swerve system.

23-01-2004 11:42

WizardOfAz

Re: pic: Swerve^2 Prototype

Here's a photo of a pre-season prototype wheel, #0 as it were. Though the new ones have minor improvements (e.g. better spacers), they are essentially like this one.

http://crush1011.wookus.net/pix/2003...lPrototype.jpg

The motive drive comes down the center shaft, through the miter gear pair, then by chain to the wheel. The ratio is 1-1 all the way from the S-B motor up to the last sprocket, where it gets geared down by 1-2.8 (10 to 28 tooth sprocket). The vertical shaft is spinning in and supported by a half inch bore in the large teflon bushing.

The steering drive comes in the big sprocket on top. That sprocket is bolted through the large teflon bushing to the top plate of the yoke. Steering is accomplished by turning the teflon bushing in the top plate. The top plate mounts to the robot frame. The steering drive is also a 1-2.8 geardown, using the globe motor.

For size reference, the top plate is 6"x6". The large teflon bushing is 3" OD with a 3/8" groove where it rides in the top plate. The teflon splits in two pieces within this groove in order to make assembly possible.

We have some CAD drawings, but a few changes were made to the design post-CAD. Hopefully this picture makes it all reasonably clear. If not, please ask.

This system is not without its challenges, but we're working through them. First, it's a lot of gears and chain, and resulting end-to-end losses. Second, there is some coupling from the motive drive to the steering drive, which makes good steering tracking code more important.

It's very fun working on it though [;-)

Bill

23-01-2004 11:59

bjammin64

Re: pic: Swerve^2 Prototype

hey, i was just wondering how exactly the driver will control all of this. would it mbe one person, or would both drivers have to work together to achieve topmost maneuveurability

23-01-2004 12:11

WizardOfAz

Re: pic: Swerve^2 Prototype

We have a few different driver control options in the works, but all are single driver, at least for the "driving" part. Another operator will probably manage other gadgets on the 'bot.

We think the "preferred" driving mode will be a single joystick, where the joystick displacement from center is speed, and joystick angle is the drive direction. All four wheels steer in tandem to the same angle and always roll the same direction (never have to run the drill motor backwards).

We'll also be able to lock the back wheels and steer just the front, which makes it drive like a car more or less. This is one way we could rotate the body of the 'bot. Notice that in the "preferred" mode above there's no way for the driver to rotate the body. In "car" mode we'll enable the motors to go backwards for more maneuverability.

Another variation will be to steer the wheels in opposite directions and do a turn-in-place (a tank turn), then return to swerve for moving around the field. This gives a quick way to turn the body to a needed orientation.

Bill