Hello forum! With our first competition approaching, I’ve decided to reveal my team’s robot and see what constructive criticism I can get from the community.
Drivetrain: 1x3 C-Channel, 4 393 (torque right now but might change them to speed) motor Mecanum drive, 1 single Bogie wheel assembly (see tank tread kit for details) between each pair of Mecanums (elevated off ground, meant simply to make contact with the bump)
Lift: 6-bar linkage powered by 4 393 (torque) motors geared 5:1 for torque
Intake: 2 side rollers (sprocket and flaps) at parallel angle to metal tray (slightly less than 45º from ground), 2 wider apart rollers on the same motors (2 269s) for large ball intake
Two batteries are used to accommodate the 8 393 motors.
The starting height of the robot is 11.5 in. At max height, the intake sits at 26in and the top of the lift is about 36.5in. We are still deciding exactly how we want to hang, but we do intend to eventually.
In the attached images, the intake has not yet been attached, they only show the drivetrain and lift (one side of which is not set up properly and will be corrected tomorrow). The images also only show one of the 2 batteries. I trust that you can use your imaginations for those little details.
Any constructive criticism would be greatly appreciated!
It looks very promising, one minor thing that you could change is the orientation of your two front motors. Rotating them 180 degrees around the axle would give you a little more clearance for your intake.
I do have a question about your elastics though. What purpose do they currently serve? I ask this because from what I gather from the images is that they are oriented in such a way that they do not give you any advantage while lifting.
I will definitely do that with the motors, thanks!
As for the elastics, they serve no purpose as they currently are placed, I had put them where they are in those pictures simply so they would be with the robot in the closet, I will post some slightly more accurate pictures tomorrow once we mount the intake.
Looks very nice. I assume it is fairly easy to tip though? Laying off the weight is causing tipping problems with our robot. You may want to consider a passive tip bar of some sort.
Hm, we haven’t had any issues with tipping just yet… we don’t plan to drive with the lift at max height very often, so I doubt that it will be too big an issue if any, but it’s definitely something too look out for just in case. Maybe some counterweight up front or something like that. Thanks
You need to brace that tower in the back with triangles. It’s going to start pulling forward and backwards if you don’t, and eventually become unstable. You may also want to add some braces in the middle of the drivetrain between where your Mecanum Wheels are. They will help keep everything square.
You should also gear that drive up. I had Mechs at 2:1 earlier in the year without a problem. You don’t look like you’ll have much more weight than I did, so it would probably work. At the very least, I would go 1.6:1. See if you can move the motors on the front wheels back using chain or something. They’re going to get hit when you try to drive into the Cylinder.
Sink the bars supporting your Cortex into the frame. It looks better, and will lower it a bit which prevents your intake from hitting the VEXnet key (if that’s a problem).
I only see 2 393’s on your lift, but the OP says 4. Where are the other two going? I would just stick them on the axle you’re already driving, personally.
Just a few ideas. You also don’t seem to have any sensors on that robot. Any thoughts on what you were going to use?
I will definitely brace the back and base a bit more, thanks for that bit.
We were planning to reconfigure the drive motors to 1.6:1 but we simply haven’t gotten to it yet. The issue with using chain to move the front motors back is that due to the width of the mechs, there is very little space between the wheels and the sides of the base as it is, so chain would likely not fit without hitting the wheels.
Our fear with sinking the bars supporting the cortex was that we wouldn’t have a place for the batteries, since the one that is already connected to the robot is strapped to the underside of that central plate. We don’t want them to be dragging on the ground. Not to mention that having those bars where they currently are placed acts as a sort of a stop for the lift to make sure it does not smash into the ground or anything like that.
There are going to be 2 more motors on the lift, probably directly below the 2 that are there already although putting them on the driven axles may not be such a bad idea. We thought about only using 2 but since we want to hang, 4 is a better idea.
Thanks for all your ideas!
EDIT: Forgot to address the bit about what sensors we’re using. We are going to have quadrature encoders on each wheel and potentiometers on the lift. Nothing too complicated, and I personally don’t know what they’re going to be used for in the code. I leave all that to my co-captain/our programmer.
Looks like the bottom of the c-channel of your chassis has less than 2 inches clearance from the ground? I would think this may cause you difficulty in getting across the bump? If so, lower your wheels and drive motors relative to your chassis.
1.6 drive ratio is fine using mecanums as long as you do not plan to do much strafing. Test and see, but we could not reliably complete a match using strafe with mecanums at 1.6 ratio, without overheating our drive motors and stalling them. Mecanums are still a good choice, even if you do not use strafing, because they go over the bump well and, unlike omni wheels, they give you resistance to being pushed sideways.
If you don’t have a bump to test with just make something out of wood or even cardboard. It will help answer a whole lot of questions about your robot’s ability to traverse the bump. One thing I’d be worried about is the C-channel actually hitting the bump before the wheel starts lifting the robot over. Seems awfully close to me and if you weren’t perpendicular to the bump this will be even worse. Any chance of pushing the wheels out to the ends of the C-channel?
Looks like a good start to the season and no doubt plenty of work still to be done!
Hi Paul, the robot has gotten caught on the bump a few times. We plan to fix this by adding a small high traction wheel in the middle of each pair of mechs. We tried using something even smaller that we found from an old tank tread kit, but it didn’t work well. If the small wheels on each side don’t work, we will be lowering the wheels as you suggested.
How much strafing was your robot able to do at 1.6? We are considering using strafing as part of our autonomous to knock both large balls off the barrier.
We do have a field to test on, and we have found that we get stuck on the bump more often than not, perpendicular or not. We’re fixing this by adding a small high traction wheel (not powered) in the middle of each side simply to make contact with the bump and give it that extra push it needs. The wheel placement is otherwise perfectly fine.
I’ve already tried the high traction wheels in the middle of the chassis. You can see pictures and our results here. They get stuck on the bump rather than helping. Instead, use pillow blocks to lower the wheels further beneath the chassis. This will get you over the bump.
Strafing works as well as Mecanums do at 1.6:1. It’s going to be inferior to an X-Drive, but it works. It’ll be faster than at 1:1, and have less power. That’s exactly what you would expect, though.
EDIT: Are you really getting caught on the bump? We didn’t have any troubles with our base going into it forward/backwards. Strafing we couldn’t cross with Mecanums, but I think that was due to the loss of power. You might consider cantilevering your wheels if you want to try strafing over.
Wow thanks for saving me the trouble of testing that! I guess pillow blocks are the way to go then, we just have to wait for them to arrive. At least I’m not the only one who had that idea.
Just wondering, how did the high traction wheels get caught? Would a pair of small omni wheels in the same place be any better? Just as something to try before the pillow blocks come.
It caused us to bottom out, rather than tipping either forward or backward so the wheels could grip the ground. If you have a couple of 2.75" Omni Wheels, it might be worth trying. They only takes 10 minutes to put in.
The motor stalling was bad enough for the boys to decide that a 4 motor, 1.6 ratio, strafing mecanum drive was not a workable configuration. I believe that the motor stalling is exacerbated because a strafing mecanum drive does not allow you to chain the front and back drive motors together (because the wheels need to counter-rotate when strafing). Strafing places more load on the motors, possibly due to friction between the mecanum rollers and the foam tiles, which is evidenced by the slower speed achieved when strafing sideways compared to direct forward or reverse driving.
I think you should still try it. If your robot is light enough and your drive build has low friction, it may well be workable. If you strafe only in auton, you will probably be fine. If it doesn’t work to your satisfaction, go for plan B!
I have a 1/1.6 (internal gear speed-up) direct driven lightweight robot (10 or 11 lbs. or so) and wholeheartedly agree with this post.
I mainly use strafing to aid in adjustment/alignment and seceded in going though my first tournament without staling. though the sound the motors make is less than pleasant :rolleyes:
just make sure that the weight is evenly distributed over the wheels and whatever you do don’t go diagonal. (only makes use of two motors)
also be carful about strafing in aton. my experience has been that when the bot first gets under way, it tends to turn to the right or to the left. just something to keep in mind.
Hi everybody! Thanks for all of your suggestions. After some work on the robot and driving tests today, we have reached some conclusions about the robot:
We added a pair of 2.75" high friction wheels in the middle of each pair of mechs and found that the robot can sometimes get over the hump depending on where exactly it tries to go over. This is just due to dirt on the field.
We added a 17.5" steel c-channel across the width of the robot for stability, as it was starting to cave in in the middle.
We realized, as previously suggested, that we have to move our front wheels slightly forward so that they make the first contact with the bump, rather than the drivetrain hitting it as it currently is.
We also have to rotate our front drive motors 180° in order to get a bit closer to the stash.
We also have to attach our intake side rollers. That will be tomorrow.
I took video of our driving today and will post a link to that in a little while.
looks very nice ! my worry is mecanums on bump … probs need testing as i know 5119 had some problems with them, (i didn’t really look into it though) so might be fine.
