This past week we built this mount and might try and use it on the drive train. We know there are concerns about wear and tear on the VP when used in a drive train, so we are looking for suggestions either with the mount or the programming that would help give it the best chance of lasting for 2 events. On the programming side we have talked about using voltage ramping to control the acceleration.
The power tools we have are a band saw, a drill and a grinder, so any suggestions would need to take that into consideration.
Last year we ran basically a single VP each of the 4 sides of our drive train that had omni wheels on each side, so we are comfortable with using them and we have worked this offseason in using the VP encoders to do some precise autonomous driving. In anticipation of moving to a tank style drive train we started looking at options for using the VP again with two on each side.
We know there are other options for a 2 CIM gearbox and the ability to add an encoder to them, but we haven’t actually tried that yet.
Given your tools that you have at your disposal I can see the draw to want to use a VP gearbox since you will not have to machine the parts. I think that a off the shelf gearbox with 20dp gears might be a better way to go however. The drive-train is something that you never want to break, it is vital to your success and a pain to work on so I would suggest sticking with what you know will work.
With that said here are a few points that will make the VP last longer.
Support the other end on the hex shaft, this will reduce the load on the gearbox.
put your final reduction in your pulleys. Currently it looks like you are close to a 1:1 ratio between your pulleys and your wheels. If you change that to a 2:1 or better than that ratio (and make up for that difference in the VP) you will reduce the load on the VP by whatever your ratio is. IE a 2:1 ratio will reduce the stress on your VP by half of what a 1:1 ratio will produce.
Again though I would stick with a more traditional gearbox, it will last longer and you won’t have to worry about it in every push match that you get into.
The COF on your Omni wheels was very low and that probably saved you a lot of problems with the VPs. You can use Vex Encoders on their gearboxes to avoid having to start from scratch on programming.
Nice thinking outside of the box. The shaft ends need to have another support though. With belts as shown, there is significant side loading that will increase the friction inside the transmissions leading to high motor currents and failure of the transmission.
We did talk about supporting the other end of the shaft, but decided to mount an additional bearing closer to the base to give some additional support.
Changing the pulley ratio is definitely something we will consider.
Just to be clear, there is an upgrade kit available for the kitbot that allows you to use mecanum wheels to create a holonomic drivetrain. If you factor in the fact that the kitbot and CIM motors are free, its actually cheaper than the drivetrain you have pictured above.
This year we aren’t doing the holonomic again, but we did like using versaframe for the drive train last year since it seemed easier for making modifications, so we aren’t doing kitbot this year either.
If you opt out of the kitbot you get a $450 voucher that you can use for some of those parts and for other items on your robot, which I don’t think was figured into the price comparison.
Am I correct in assuming this Dual VP drive would be for a 4WD, either as a tank setup or a holonomic setup? AKA: is this thing utilizing 8 VPs/8CIMs or 4 VPs/4CIMs?
One thing to consider if you did go to a tank setup is that the drive dynamics and how they affect the gearboxes are going to be different. You’ll have less DoF therefore the potential for those gearboxes to see greater peak loads.
VPs are amazing gearboxes and one of the best innovations we’ve seen in FRC products, maybe ever. I’m sure I’m not the only experienced FIRSTer who looks at that application of them and sees images of gear dust flashing before their eyes.
Assuming you go tank (and the game says thats a good choice) - I have to ask why even bother with the VP setup then? You could go with the kitbot and be driving a couple hours after the kickoff animation airs.
Take the extra time and go all in on driver practice. Take the VPs and go all in on mechanism design.
Couple of reasons. First, I haven’t figured out a Vex gearbox and encoder combination that would work for us based on the tools we have available and also based on what we have been working on this fall. This fall we have been running VP encoders into Talon SRX, which is a really simple setup. The couple of Vex gearboxes I have looked at adding encoders onto either require us to use a tool we don’t have or it didn’t seem like the encoder would easily connect into the Talon SRX. Maybe I just haven’t found the right combination. I would love to be able to just stick the SRX Mag Encoder onto this gearbox, but the documentation only talks about attaching a Grayhill encoder, which I’m not familiar with.
Second reason: I’d like to make this a learning opportunity for the students. The plan is to present the issues caused by using the VPs and what things could be done to minimize the issues and whether they for instance are willing to avoid pushing matches or have multiple replacement VPs sitting around when something does happen.
This is what we did last year before we decided to go ahead with the holonomic omni-wheel drive train. I went through CD and found all the issues people had brought up with this type of drive train and we talked about ways to minimize the issues. Now it was a great experience, but we would probably do mecanum the next time we needed holonomic.
Connecting ANY incremental encoder[1] to a Talon is as simple as using one of the encoder breakout boards. I’ve done it on a test bed with the encoders baked into the Rev HDHex gear motors without any issue.
Intentionally adding work in a subsystem that everything else is reliant on doesn’t seem like the best idea nor does it feel like teaching proper engineering techniques. If a solution causes problems just for the purposes of “learning” my boss would fly down here, break into my apartment, and beat me with my laptop. [2]
[1] Addendum - any sane one. I’m sure there’s some funky ones out there just no.
[2] Probably not, but he’d be VERY angry and use some adult words.
Please see pp 15-19 of the CTRE Mag Encoder User Guide. Happy to consult on how you may use your hacksaw and hand drill in lieu of the recommended lathe-- just shoot me a PM.
Couple of reasons. First, I haven’t figured out a Vex gearbox and encoder combination that would work for us based on the tools we have available and also based on what we have been working on this fall. This fall we have been running VP encoders into Talon SRX, which is a really simple setup. The couple of Vex gearboxes I have looked at adding encoders onto either require us to use a tool we don’t have or it didn’t seem like the encoder would easily connect into the Talon SRX. Maybe I just haven’t found the right combination. I would love to be able to just stick the SRX Mag Encoder onto this gearbox, but the documentation only talks about attaching a Grayhill encoder, which I’m not familiar with.
If you do decide to use the kitbot (and assuming that they stick with ToughBox Minis again this year), Andymark has tons of options for replacing the output shaft with one that already has the required magnet bored in. Just make sure you get one for ToughBox and not EVO. This should be the easiest way of doing this for a team without a lathe. If you are really, really set on the Vex SSDR, then you may want to consider putting an encoder on the shaft that drives one of the wheels (assuming live axle).