Integrating Encoder Into Gearbox
 

Good evening CD,

Next year my team is looking to create our own custom gearboxes. We are quite confident in our shop bot and we have made a few test gearboxes so far and everything is looking great. Now we come to the question of how to mount our encoders into our robot. We have about 10 of these encoders:

http://www.usdigital.com/products/en...otary/shaft/H1

and although they are bulky we really like how they perform so we would like to keep them. In the past we have had them on our drive axles with 3d printed mounts and it never came out nicely because the mounts didnt hold up. Next year we were looking into a more robust system. One plan of attack was to lathe a 1/4 inch hole into a shaft in the gearbox and hold the encoder in place with a set screw. Here are links to two different angles. My biggest concern with this design is losing a lot of integrity in that hex shaft. I was hoping to get thoughts on whether or not this would hold up?

https://drive.google.com/open?id=0B6...VFSaVFpTDRobk0

https://drive.google.com/open?id=0B6...WhZeGVUVFZESGM


Thanks so much!

Re: Integrating Encoder Into Gearbox
 

You are being too concerned with space in your design. Poke the shaft 3/8 - 1/2" past the gearbox plate, and put the set screw on the other side. That's what we do.

When you *turn* the hole into the shaft, finish it with a reamer. You'll get a nicer fit. Probably a .2505 reamer.

For mounting, we've been bending a lexan z out of 1/32" lexan and using that to constrain the encoder torsionally. It seems to be working fine. There should be some pictures on our picasa site of how we've solved this exact problem before.

Re: Integrating Encoder Into Gearbox
 

+1 on the Z-shaped lexan/polycarb mounting. We used 1/16" and it's fine as well. also +1 on sticking the shaft out of the gearbox farther, it helps with making mounting easier.
We have used set screws in the past and it's worked fine. We used the plastic/nylon tipped ones from McMaster to avoid killing the encoder shaft.
Some teams have used surgical tubing for a connection between shaft and encoder, but personally we've experienced lots of slippage both in absolute and incremental situations. That being said, it definitely helped avoid breakage due to the flexible coupling.

Re: Integrating Encoder Into Gearbox
 

We tried that for years, and eventually figured out that the rubber connection was actually acting as a filter, and was causing controls problems. We were kind of shocked.

You can also make a male end to your shaft rather than female, and use a shaft coupler. There are flexible shaft couplers. Unfortunately, that's a bit more pricy of a solution.

Re: Integrating Encoder Into Gearbox
 

We just ream a .25 hole and use some retaining compound to permanently attach our encoders to the hex shaft. The wires were then zip tied down without any slop preventing the encoder from rotating. I'm fairly sure 254 does the same thing. Never had any issues with structural integrity.

Re: Integrating Encoder Into Gearbox
 

Aluminum flashing found in the roofing department at your local Home Depot or Lowes also works well to make the Z bracket.

Re: Integrating Encoder Into Gearbox
 

Really? We use surgical tubing that really has to be stretched to fit on the shafts, and secure it with zipties. It's rather difficult to get it to slip even if you're trying to do so, and there's very little loading on the encoder shaft.

Re: Integrating Encoder Into Gearbox
 

We have used these same encoders with Andy Mark gearboxes. While direct mounting might be o.k, there could be issues. Any Axial shaft movement or bearing eccentricity will put loads into the resolver.

We have had very good luck installing these resolvers on a 3D printed standoff bracket, and using a small section of surgical tubing to connect the resolver shaft with the 1/4 shaft on the AM gearboxes. We zip tie both sides of the surgical tube for insurance.

There are multiple sizes of surgical tube. I believe we used 3/16 ID and 5/16 OD, for a very tight fit on 1/4 shafts. If you use 1/4 ID tube I could see it slipping.

The resolver and standoff bracket combined would be shorter than the mounted CIM motor length, so you really don't save much space by direct mounting.

Re: Integrating Encoder Into Gearbox
 

254 makes it a slight press fit, and secures with the wires.

I spent a long time tuning a PID loop for one of 254's bots, and was only able to fix the issue by securing the pot by a bracket instead of by the wires. Sure, it works most of the time, but I'm no longer willing to take most of the time as an answer.

Re: Integrating Encoder Into Gearbox
 

The shaft should easily hold up to the stress, you should focus more on reducing the profile of the encoder outside the gearbox.

Re: Integrating Encoder Into Gearbox
 

While it wouldn't help you use the encoders you already have, that shaft design looks like it would be a perfect fit for a CTRE magnetic encoder. Just press fit in the appropriate magnet, mount the encoder to the back, and you're in business.

Re: Integrating Encoder Into Gearbox
 

One of the mag encoders was included in the KOP it is in a reasonably small black rectangular box with a green stripe.

Re: Integrating Encoder Into Gearbox
 

I had a feeling that this was an issue. I just don't see how you can avoid slip if you are securing an encoder with its wires.

We used magnetic encoders from CTRE this year and have fallen madly in love with them. I suspect we'll continue to use them where they make sense. We liked that they are contactless. I'm hoping to create some more 3d printable housings for them though.

Re: Integrating Encoder Into Gearbox
 

We saw the same issue last year on our elevator, as we pushed the pid to get more speed out of the elevator, we had trouble tuning the system at higher speeds. The issue was not surgical tubing slipping, it was some allowed twisting when elevator changed directions when stacking at high speeds.

We replaced the surgical tubing with 1000psi 1/4" fuel line from autozone, and was able to tune system just fine at high velocity change rates.

I did not believe it that a ball bearing encoder could flex the surgical tube, but the system controls was significantly less stable than with the fuel line.

Re: Integrating Encoder Into Gearbox
 

The the encoder is adjacent two 2.5 Inch OD CIM Motors that protrude 4.5 inches out the rear of the gearbox. Minimizing the encoder profile outside the gearbox provides no advantage, and is counter to having a more reliable gearbox system.

The mechanical specs for this encoder are on the website. Please take note of the hole tolerances. CAD is one thing; student repeatable machining of center holes to a 0.0003 inch tolerance on center is quite another.

Re: Integrating Encoder Into Gearbox
 

According to the product page (link's broken) the maximum TIR allowed is 0.006", so there's a bit more breathing room than that. Still not much though. I think the 0.0003" is the encoder shaft relative to the encoder, otherwise anything less than an ABEC-5 bearing could be out of tolerances.

Re: Integrating Encoder Into Gearbox
 

971 Encoder Mount Example Here

It's a pretty simple operation in the lathe - just drill undersize and then ream (as suggested by Austin). The doc you linked just establishes that the encoder shafts are made 2 to 6 tenths undersize, just means that you can probably get away with a 0.2500" reamer if that's all you have on hand.

You want a mount that prevents the encoder from rotating, and doesn't create any angular or parallel misalignment. The bent lexan design will allow the encoder to "float" a bit, which solves the parallel/angular problem.

Re: Integrating Encoder Into Gearbox
 

There seems to be an assumption that a basic lathe comes from the factory, and continuously maintains perfect alignment between the tail stock ram axis and the spindle axis; to allow perfectly on-center holes to be drilled.

The reality is it takes a somewhat experienced machinist with a test bar and a dial indicator to adjust the tail stock to be aligned perfectly laterally and vertically. The average FRC lathe is use is likely only accurate to 0.01 inch on-center.

Drilling center holes to the stated -0.0000/+0.0006 on center tolerance would be a non-trivial challenge for many FRC teams. To do it consistently you would need to use a collet holder in the spindle.

Re: Integrating Encoder Into Gearbox
 

Did the same thing on all my test robots.
Works adequately and allows me to use a wider range of encoders.
Less picky than encoders with the encoding wheel on the assembly shaft as well.

Re: Integrating Encoder Into Gearbox
 

You're confusing the shaft diameter tolerance with TIR. The H1 encoder has a maximum TIR of 0.006", and the shaft itself is between 2 and 6 tenths undersize (-0.0002 to -0.0006).

So the shaft diameter has a tight tolerance, but the encoder might have up to 6 thou of runout, an order of a magnitude larger than the shaft diameter tolerance.

In any case - using a reamer would give you a nice clearance fit, and then use a set screw/retaining compound/whatever to hold.

If your lathe is poorly setup, and doesn't drill holes on center:

1) Fix that
2) It doesn't matter - since the 1/32" piece of lexan holding the encoder will happily flex/float if the encoder has any axial/parallel misalignment. Same idea as using a helical beam coupler/flexible coupling that can deal with axial/parallel/radial misalignment.

Re: Integrating Encoder Into Gearbox
 

Thank you everyone for your replies! I really like the robustness of this design in addition to to how it doesn't require much sacrifice in terms of material in the load bearing part of the shaft. I used some 1 inch leg .75 inch tall 1/16" thick Z 6061 to support it and I think this will work very well thanks again!

https://drive.google.com/open?id=0B6...VpFZ05IY0lNLUE

Re: Integrating Encoder Into Gearbox
 

Many on the forum may think we are crazy, but for 2 years now we have been using the CUI AMT 10X series encoders set to max cpr. We have mounted them to Andymark, vex pro and custom gear boxes. The kit comes with every thing needed to mount them to several different shaft diameters. yes, the bottom housing is fragile if you want to disassemble the encoder. Digikey sells replacements. I was in a rush and had to replace 2. They have been rock solid.

Re: Integrating Encoder Into Gearbox
 

I would go for plastic instead due to the flexibility and ease of fabrication. 6061 doesn't bend well either, so you'll need to have a large radius on it or risk cracking.

Re: Integrating Encoder Into Gearbox
 

To add to that, we like lexan/plastic because you avoid putting much stress on the encoder.

Re: Integrating Encoder Into Gearbox
 

He was referring to the extrusion profile that you can buy, not bending your own sheet. You can use really thin aluminum to get the same flex as the plastic but the plastic is so much easier to work with.

Re: Integrating Encoder Into Gearbox
 

Adding onto that, we sheared the shafts off two encoders last year because we used 1/8" aluminum mounts bolted on with 10-32 screws. When push came to shove, the encoders lost. :o

I didn't know you could buy aluminum like that, that's pretty neat!

Re: Integrating Encoder Into Gearbox
 

If you measured the current runout of your own lathe you might be surprised. Most will be over the 0.006 value which was my point.

If your drilled off center; your applying a bending load into the resolver shaft every rotation with direct mounting. At 100 rpm your going to be challenging a floating mount, and getting higher loads in the resolver bearings.

This is the reason many of us prefer alternate non-direct mounting. For close mounting a no contact magnetic rotation sensor would make far more sense.

Re: Integrating Encoder Into Gearbox
 

We have two lathes, one that is so terrible it cannot make straight cuts of any depth (and I mean any depth) and another smaller one that belongs to a teacher. The teacher one was just bought this year and has runout of well under 0.001" with the stock 3-jaw, which I bored out slightly for accuracy. The larger one, for all it's junk, has only .003" TIR on the 3-jaw. I've gotten the tailstock inline with it once before to within 0.001", but the machine didn't hold up very well over time. I've never experienced a failure with plastic encoder mounts regardless of mounting, just because they are so flexible. I'm all for magnetic encoders, but as far as the COTS options go they are pricey compared to AMTs or similar.
If our lathe can do it, any lathe can do it. :P Seriously though, getting stuff within 0.006" is not difficult if you check it every once in a while. Grinding the chuck jaws and using a good indicator help immensely.

Re: Integrating Encoder Into Gearbox
 

Assuming you're talking about tailstock alignment now? This is one of those "fixing the wrong problem" situations. Just align your tailstock, it's really not that hard. Alternately, get a tiny boring bar and solve the problem in a different way.

Sounds like it works perfectly well for 971. I've always used helical beam couplers, or through bore encoders, no issue with either.

Re: Integrating Encoder Into Gearbox
 

If you mount everything tighyly, and try to make the assembly as simple as possible it should work just fine.

Are you trying to mount the encoders inside of the gearbox or on the outside?

Re: Integrating Encoder Into Gearbox
 

For an option mentioned earlier, Automation Direct has some nice shaft couplers (Product Page)

We used a couple of these on our drivetrain gearboxes and were very happy with them. Obviously our setup is a little different than your gearbox.

Re: Integrating Encoder Into Gearbox
 

I like that encoder mount, looks like it would be really simple to take on and off for maintenance, not to mention light weight. Are there issues with the mount shaking while it runs, or does it all fit pretty snug?

Any chance you could post the CAD for it?

Re: Integrating Encoder Into Gearbox
 

On the 3 jaw chuck, please make sure the jaws are installed in order 1-2-3. (Number Stamped) If someone installed them in a mixed order you will have a really hard time.

A procedure to setup lathes.
First you need to use a decent level (preferably a precision level) to make sure the entire bed is level. Adjust if it's not.

Lubricate and adjust the Gibs on the sliding and compound rests and the saddle to remove any free play. ** Edit - Mini Mills can take a real beating machining a lot of Hex Stock. We trashed the saddle gib on our first small mill. Needed to completely take it apart to repair. Do check these. **

You will need:
1) A Dead Center with the correct Morse Taper for your Lathe. Typically MT2 for small lathes and MT3 for larger.
2) A lathe dog for (1/2?) round shaft.
3) A micrometer
4) A large piece of unhardened steel or aluminum bar the same size as your lathe dog.

Procedure:
1)Cut off 5 inch piece of bar and set aside.
2) Cut a Piece of bar that will fit easily between centers. Face Each side and Center Drill & Counter Bore.
3) Install the dead Center in the tail stock.
4) Take the 5 inch section of bar and clamp in your 3 Jaw. Move the adjustable rest and tooling so as to make a 60 degree point at the end of this bar. Don't remove it.
5) Install the lathe dog on the large bar, and install the bar between the two centers by moving the tail stock in.
6) Machine at least .01 off the OD of the bar for the entire length.
7) If the Diameter of the Bar is wider at the tail stock than the spindle, adjust the tailstock 1/2 the difference toward the tool post. If it's narrower adjust the tail stock away from the tool post.
8) Repeat the machining the diameter and adjusted the tail stock steps, until the error is acceptable.

You can adjust the cutting tool quite high, and use the similar method to adjust the tailstock height if adjustable.

Re: Integrating Encoder Into Gearbox
 

Mounts lasted through 4 competitions with zero issues. The set screws on the coupler retained the encoder/mount from sliding in or out. I designed the mount to have just a bit of spring against the CIMs to make sure it would have a tight fit.

CAD file: Link

Re: Integrating Encoder Into Gearbox
 

Similar to the lexan Z mounts, we have seen success in the past with small aluminum mounts (these fit really nicely in the lightening holes of other parts that are cut on a water jet, which makes for efficient layouts). These can then be secured to the frame with a rubber mount which can be purchased from McMaster here.

As for the shaft of the encoder we simply drill the appropriate hole in the shaft (haven't always reamed the hole) and secure it with some loctite 680 (which has a diametrical clearance max of 0.015" so your hole doesn't need to be all that precise). Takes a heat gun to remove it, but luckily none of the defenses this year involved a heat gun, so the encoders stayed put.

We made this our standard method of mounting encoders and potentiometers in 2015. The rubber mounts allow for some give, allowing some misalignment, but are quite rigid which I would imagine eliminates the issues seen with latex tubing used as a coupler.

~DK

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