H Drive

[Marcow]

Originally Posted by cbale2000

It's probably worth noting that to make an effective H-Drive you really need to have the center wheel on some kind of suspension otherwise its very hard to get traction.
We found that out the hard way this year (our first time building an H-Drive), even after dropping the center wheel 1/16" we still only had enough traction to strafe about 75% of the time.

I'm not entirely sure how 'correct' our method is, but we ran an H drive this season and had to somewhat scramble to put it together since we were so far behind. Our solution to this was to put a very small drop on our center wheels which we adjusted by turning a few bolts until we were happy

[cbale2000]

Originally Posted by Marcow

I'm not entirely sure how 'correct' our method is, but we ran an H drive this season and had to somewhat scramble to put it together since we were so far behind. Our solution to this was to put a very small drop on our center wheels which we adjusted by turning a few bolts until we were happy

Sounds similar to what we did. Unfortunately the way we designed the support structure for our center wheel didn't allow a lot of variability in the placement, we basically had to slot some mounting holes and add a shim to get the offset we needed. Lesson learned.

[BenGuy]

Originally Posted by GeeTwo

Thanks for the renderings! I like that the swing is clutched to the cluster axle, which causes the wheel on the side that you're accelerating away from to touch the carpet, which should improve traction. I do have a few questions about some of the details:

• Did you have trouble overshooting your strafes? There does not seem to be any way to properly brake in the strafe direction.
• The gearing looks rather fast for strafing in Recycle Rush; counting of gear diameters, I get about 8:1 or 9:1. Did you actually use strafing as a means of travel, or just to line up for pickups and scoring? Unless you needed the top speed, you could have made the output cluster gear smaller, pulled the wheels pulled in to match, and probably done as well with one CIM.
• How do you control the coupling between the seesaw and the cluster shaft, to get the proper wheel loading? Is there a friction clutch between the output cluster gear and the seesaw, or is it just in the bearings, or what?
• Is there a spring, or does the seesaw return to the horizontal when not driven just based on gravity? Given the amount of friction needed in that bearing, the short offset of the CoG from that shaft, and the large moment of inertia of the seesaw, gravity doesn't appear to be enough.

1st Answer: No, the robot was heavy enough that as soon as we dropped power to our system, the robot stopped, if we were going too fast that it might have slid, we just slammed the opposite direction for a split second which brought a dead stop.

2: We used it mainly as a lining up tool, but occasionally to actually move far distances across the field, and yes, it was very fast. The system was better used as a quick way to move into perfect lineup with the human station. And about the one CIM thing... I didn't say it my last post, but in order to gain more weight for other things, we did take a CIM off, it does work, however, pretty much the exact same with one CIM or two. The only difference is in speed, and that's not much.

3. There is no friction clutch in the system, it's just the bearings. I attached different pictures to illustrate.

4. The bearings allow the system to be balanced only on gravity, as the bearing are just a pivotal point. Most of the weight is under the pivot, so gravity is the force used to balance the seesaw.


Thanks for the questions - keeping me on my game during the off season I'm happy to answer any more questions you have too.

[GeeTwo]

Originally Posted by BenGuy

1st Answer: No, the robot was heavy enough that as soon as we dropped power to our system, the robot stopped, if we were going too fast that it might have slid, we just slammed the opposite direction for a split second which brought a dead stop.

2: We used it mainly as a lining up tool, but occasionally to actually move far distances across the field, and yes, it was very fast...

Thanks for the answers! By the way, the first and third renderings answered questions 3 and 4 better than your text. I had pictured the module rotating about 20 to 30 degrees to make contact with the floor; the actuality is more like 70 or 75. This greatly reduces the torque which must be applied to the module to achieve a given contact (and therefore traction) force.

Also, to put a new subthread about H-drives, or more particularly about omni wheels: The renderings shown here depict classical omnis with ~18 nearly cylindrical rollers per duallie wheel, which are available from both AndyMark and Vex, and probably others. AndyMark came out with a "Duraomni" wheel this year, which has eight longer, more (american) football shaped rollers for an integrated dualie. 3946 used two non-driven 6" idle omnis for Aerial Assist, and five 4" Duraomnis for Recycle Rush, but we didn't do anything that could serve as an "apples to apples" comparison. Did anyone do any comparison tests between the different types and is willing to share results?