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H-Drive Strafing issues
Hey CD
So we are experiencing traction trouble with our H-Drive chassis. When we attempt to strafe, sometimes the wheel just turns and doesn't catch on the ground. We were doing it on polished cement, so that may be an issue, but what to do you all think, what should we do? |
Re: H-Drive Strafing issues
When we did this in 2015, we found that on carpet, the robot sinks significantly into the carpet for the wheel to catch. You should probably try this out on carpet and see if this is the case. If it is, great! If it still doesn't catch, you may need to lower your center wheel.
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Re: H-Drive Strafing issues
Carpet will most certainly help, what wheel is your strafing drive?
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Is the strafing wheel at a set position? Is there any way you can increase the amount of normal force on that middle wheel? Carpet will almost certainly help. But it probably won't fix the problem.
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The center wheel has no applied force on it, but we do have the majority of our heavy weight items (battery, etc.) toward the center wheel. It is just an Omni wheel with rollers. We should be taking it up to the practice field soon
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In 2015 our strafe wheel was in a spring powered suspension setup to keep it in contact with the ground. I don't know what your setup looks like, but it's very difficult to keep weight on a fixed center wheel without lifting up other wheels. One of the best methods I've seen is what 148 did in 2014 with their "rocker" module. Perhaps someone from their team can elaborate on it further.
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We tried strafe with H-drive in 2015, using a hard chassis mount. No matter how high or low we put it, it never worked right. Off season, we used a pneumatic cylinder to apply a steady force to it, and it worked first try. Springs can also work, I understand. The force on that wheel should be roughly between 20% and 35% of the weight of your robot, depending on how much strafe force you want and how much forward drive force you're willing to give up while you strafe.
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Re: H-Drive Strafing issues
Is the center wheel rigidly mounted to the frame, or is it articulated? An articulated strafe wheel will do a much better job contacting the ground. One COTS way to do this with VersaFrame would be to use a drop drive module with only one wheel in it (I've never tried this, it just seems like it would work).
If your frame uses VersaBlocks, be sure you don't accidentally have the drop center in the wrong direction. A picture would really help here. |
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When we did our H-drive in 2011 (which worked very well), our center wheel was on a simple suspension system. We put the center wheel on a swing arm and used a gas strut from McMaster Carr to provide the spring and damping. The trick was to size the gas strut to provide 1/5 of the robot's weight to the center wheel.
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We also used a pneumatic cylinder as a "spring" on our H-drive setup in 2015. We put the cylinder on its own air circuit and tuned the pressure to the point where we got the max down-force without lifting the "normal" wheels too much. ("too much" being a technical term meaning the point where things don't work correctly :) )
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What we did in 2015 is have 2 pneumatic pistons on either side of axle, to push down when needed with a single trigger pull.
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We used a copy of 148's design in 2015. https://www.youtube.com/watch?v=uL-NnuZzOkU It is the easiest way to accomplish H drive... - no springs, no pneumatics, no extra controls, and it accommodates changes in floor height seamlessly. I wish I had invented it.:)
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At this point, though, if you want to delay it further, then continue to deliberate. But you've only got a couple weeks left, and if you want this thing driving well by the end of build, you're going to need to either order the parts you need very quickly and get it done, or just switch to a basic 2+2 or 6-wheel tank drive. |
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https://content.vexrobotics.com/vexp...-top-trans.png And andymark has a H drive kit?? |
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Anyways, you'll probably want to use that Robowranglers method of actuating the strafing wheels. Or switch to a tank drive. Given that you have your wheels in the corners, a 2+2 (2 traction in front, 2 omni in back) would probably work alright. |
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It is possible to make a ridged mounted H-Drive work, but not work well.
In 2015 we gave H-Drive a try without realizing the need for the center wheel to be articulated, by the time we were done we had offset the center wheel 1/16"-1/8" below all the other wheels on the robot, this resulted in some very odd rocking in the chassis and the strafe only worked about 75% of the time (because the carpet floors are never perfectly flat). It wasn't accurate enough to use during autonomous, but driving it manually it worked ok. If I had to build an H-Drive again though I would definitely put it on a suspension. |
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Getting the strafe wheel at the same height as the other four is NOT what you need. Even when we had about 1/16" of a drop on ours relative to the corners, it did not provide enough traction to move the robot well. By the time we got the drop deep enough to produce consistent strafing, we had problems driving in forward and reverse. There just is no right height for a rigid mount. |
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All in all I would definitely take a look at 148's robots both their 2010 and 2014 strafe wheel assemblies. |
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Re: H-Drive Strafing issues
A picture of the robot in question would help us out a lot. I have built the exact same H-drive from the Vex Drive in a Day kit. It could barely even drive on linoleum or another hard surface. Driving on carpet was much more successful, but there's a large chance you'll still run into consistency issues. Now, you have a couple options open to you.
Those options are listed in rough order of difficulty. You and your team will have to decide which option is best for you. |
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My team ran an H-drive in 2015, and had this issue for a while. Our solution was to use a spring to pull the module down while simultaneously acting as a suspension allowing for the robot to not get completely shaken off-balance upon hitting bumps.
I agree that a picture or model of the drivetrain would be extremely helpful for us to see where a solution could be implemented. Simply put, if I can't see it, I can't solve it. |
Re: H-Drive Strafing issues
Our team is also going with a H drive this year. We have found that it isn't difficult to put a sideways module on a pair of articulated arms. The plates we're using are cnc'd but you can cut them out of an old AM14U if you need to. They just have to have the Toughbox's hole pattern. Here's a picture: https://scontent-syd2-1.xx.fbcdn.net...f5&oe=589F53C9
Hope this helps. |
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