paper: DIY 'encoder stage' for VersaPlanetary

[AllenGregoryIV]

Originally Posted by Nate Laverdure

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

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

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.

[DampRobot]

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.

[GeeTwo]

these modifications can be done by any moderately-capable FRC team in a reasonable amount of time and without use of external machining resources

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.)

[AdamHeard]

Originally Posted by GeeTwo

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.)

Why wouldn't you let students touch?

[GeeTwo]

Originally Posted by AdamHeard

Why wouldn't you let students touch?

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.

[Nate Laverdure]

Originally Posted by GeeTwo

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.

For production you would probably choose to replace the entire bronze input coupler with a ferrous version that includes the external teeth.

Originally Posted by GeeTwo

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.)

All lathe operations were done on a machine that I picked up off Craigslist for $200.

[Michael Hill]

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.

[asid61]

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.

[Monochron]

Originally Posted by Nate Laverdure

All lathe operations were done on a machine that I picked up off Craigslist for $200.

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 :/

[Nate Laverdure]

Originally Posted by Monochron

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 :/

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.

[asid61]

Originally Posted by Nate Laverdure

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.

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.

[AllenGregoryIV]

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.

[Ryan Dognaux]

Originally Posted by AllenGregoryIV

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

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

[AllenGregoryIV]

Originally Posted by Ryan Dognaux

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

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.

[Nate Laverdure]

Originally Posted by AllenGregoryIV

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

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.