paper: DIY 'encoder stage' for VersaPlanetary
 

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DIY 'encoder stage' for VersaPlanetary by Nate Laverdure

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

This is pretty great. A little mad I never came up with this one.

Using the space where the bearing used to be is a great choice. The bearing was removed in the 2nd year because it was causing some misalignment issues since that bearing would have been the third bearing in the VP assembly including the two in output section.

Added
Any reason you couldn't use the 30 tooth gears in the VP dual input stage adapter? They have an OD of right at 1 in and spin freely in that space. The teeth are a bit smaller but I don't see anything in the sensor spec sheet to prevent it from working. You would also get a bit more resolution by going up to 30 teeth.
I think you could make 3 of them form this $10 kit.

Also were you running the sensor at 12v or 5v?

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

OMG awesome sauce!!!!

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

This sounds like a good idea, I hadn't considered it.

We ran the sensor off regulated 12V from the VRM.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

The spec sheet lists the min supply voltage at 4v; so you may be able to get away with running it directly off the DIO port or the data port on the Talon SRX at 5v.


As I'm still looking at this, does anyone know the max encoder rate for the roboRIO and Talon SRX? With 21 or 30 counts/rev at 20000+ RPM we may be coming pretty close to or surpassing that spec. For the cRIO I think we are over the spec. For lower speeds it would be fine and we could also possibly remove some teeth, but I'm not sure how easy that would be.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

I don't post here a lot any more, but I wanted to say congratulations. This is a really awesome design, and brings together so many of the principles I love in good FRC designs: low parts counts, using COTS parts, compatibility with existing systems, etc. A VP planetary with an encoder really has been a holy grail in my mind for a while, and you guys did it (well, did it and then posted it).

I worked on a couple of designs for stuff like this, like an encoder in a hex bearing package, but never got anything as elegant as this. I wish I'd thought of this or that this design had been around in my 100 days.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Well, it's certainly clear that our team is not yet moderately-capable! (I'm not entirely sure that our mentors are, but I'd give us a 50/50 shot at it, after spending a few more thousand dollars on tools that we never let a student touch.)

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Why wouldn't you let students touch?

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

I think this can all be done with a hacksaw and a drill press. They did it very well with a lathe but I don't think any part of this needs a lathe to be functional.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Wow, very cool way to add an encoder. I hope Vex seriously considers adding something like this as an option for VersaPlanetarys in the future.

The only concern I have (aside from the potential issues others have already pointed out regarding high RPM applications) is that, though this setup does track rotations at the motor, it isn't fault tolerant if some other part of the gearbox breaks.

One of the nice things about most gearbox-mounted encoders is that they are usually connected directly to the output shaft and show its actual rotation (short of a shaft failure, I suppose). With this, if you strip (or partially strip) a gear, the sensor has no way of detecting or compensating for it. Granted, I have never experienced this failure mode on a VersaPlanetary (actually, the only failure I've ever had with one was the input coupler itself), but it seems like it could be plausible.

Or I could just be paranoid. :rolleyes:

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

That didn't come across quite like I meant it. We would never buy a tool that we would not let the students touch - it makes much more sense to outsource on so many levels, including inspiration. I meant that a couple of our mentors might be able to do the job with tools that our students had never touched; we have yet to have a band saw or drill press or soldering iron see more than a few dozen hours of student use without becoming a less precise tool than it was designed to be. Until we can teach students not to damage the tools, there's no use in spending thousands of dollars on a tool. It is sort of a catch-22, but so far we're stuck in it. I recognize that at the root this is more a mentor problem than a student problem - we only have one mentor who uses cutting tools on a daily basis (and those hand tools, not shop tools), and he can't be everywhere at once.

Also, back to the original project - it might be more efficient (especially if you were to put this into production) to start with 3/16" steel plate (or even better a thick washer) and cut the teeth. The teeth could also then have a more rectangular encoder-like shape.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

For production you would probably choose to replace the entire bronze input coupler with a ferrous version that includes the external teeth.
All lathe operations were done on a machine that I picked up off Craigslist for $200.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

This is fantastic. I had an idea very similar a couple of years ago, but it never amounted to more than thoughts in my head. We've got enough Versaplanetary spare parts, we should be able to try this out. Also, I loved the nice touch of using the potting compound.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

AWESOME! This is an amazing modification! A VP encoder stage without adding extra space is amazing! It seems a bit tricky to do right now, but not to the point that it can't be done. I'll have to try this on our mill.

EDIT: The Talon SRX has 80,000,000 / CPR maximum RPM. So 80,000,000/ 30 = 2,666,666 max RPM. I think we'll be fine as far as counting ticks goes.
Does the sensor sense direction? It appears to have a quadrature detection scheme, but I was not sure.
EDIT2: For the CRIO, Ether mentioned that the max polling speed for a quadrature encoder is 2,129 RPM for a 360 CPR encoder in the worst-case scenario. For a 30CPR encoder like we have here, that's 25,548 RPM max. That should be compatible with your typical FRC motor.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Oops, I have a correction from one of our electrical & controls mentors:

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

The RoboRIO samples faster and has much higher bandwidth then the cRIO. 30 cpr at 20,000 rpm is still well within the capabilities of the roboRIO.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

What is the limit? Would 10000 cpr at 5krpm work?

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Great work. Inspired to look at this sensor for our swerve module. Question. How were the pull up resistor and caps added?

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

What brand/model of lathe is that? We have been looking for a good one second hand but size is our limiting factor right now :/

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

It's a Chinese 7x10 similar to Harbor Freight 93212. I've since rebuilt it as a 7x16 with a number of other modifications, but stock it would have been able to do this job just fine. They sell new for $550.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

You can find Harbor Freight 20% off coupons online, and it's usually only $500 there, so it ends up being $400 for a 7x12. The Seig style chinese lathes are all made in the same place, so you still get the same thing as if you bought from Grizzly, with minor customizations.
mini-lathe.com is a good resource.
The 7x10 is actually only a 7x8 (7" diameter part, 8" max length to be simple) and the 7x12 is an actual 7x12, so it's a good deal to invest in the latter IMO.
Craigslist can sometime have good deals, but you should know what you're looking for to do that. Measuring the accuracy is critical.

EDIT: In general I would use a mill because of the versatility for FRC, so if it comes down to buying a tool a mill would be my first choice.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

We just posted our version of this modification to our blog.

We haven't got to play around with it to much yet but we did hook it up and check that the system was getting good distance values from the encoder.

We made several substitutions to reduce the overall cost of this setup. Each additional gearbox modification for us should only be around $10.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Getting a lot of broken images on the blog post. The read through part looks awesome though, can't wait to see all the pictures :)

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Thanks for the heads up, I tired a new system for putting the pictures up from google photos but apparently it doesn't actually make them public. Should be fixed now.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Excellent work!

I'd love to see someone (VexPro?!) sell one of these... it'd be really great to not need to make a part year-after-year just because it's not sold when it'd be so widely useful.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

GREAT improvements! Thanks for the excellent documentation.

As far as epoxy vs RTV silicone:
We were going to use RTV at first, but after a short debate we chose a potting epoxy that my professional colleagues use. The internal magnet(s?) within the Allegro package are quite strong, and when the steel gear is nearby there is a significant attractive force between them. It's probably a dynamic force as the gear rotates. Had we suspended the sensor in a flexible potting compound like RTV, we were worried about the effects of the induced vibration.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Sorry I haven't had a chance to read the blog, but that would be a great point to mention in the blog. Many teams probably aren't familiar with the differences in those compounds and if that is an issue, it would be good for them to see it coming and avoid it.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

Sorry I haven't had a chance to read the blog, but that would be a great point to mention in the blog. Many teams probably aren't familiar with the differences in those compounds and if that is an issue, it would be good for them to see it coming and avoid it.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

That makes sense, we'll test and see how it goes. We ran it for a while with out any ill effects.

Something like this may be a viable solution if that problem does arise.

Re: paper: DIY 'encoder stage' for VersaPlanetary
 

This is great. I'm adding it to our projects to try in the off-season. We use VersaPlanetary gearboxes everywhere that we can for their small form factor, ease of use and ability to modify gear ratios as-needed. Integrating sensor feedback into that package would be absolutely huge for a team like ours.