Integrating Encoder Into Gearbox

[Owen Busler]

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!

[AustinSchuh]

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.

[asid61]

Originally Posted by AustinSchuh

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.

+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.

[AustinSchuh]

Originally Posted by asid61

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.

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.

[mman1506]

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.

[Bruce Newendorp]

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

[Oblarg]

Originally Posted by asid61

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.

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.

[InFlight]

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.

[AustinSchuh]

Originally Posted by mman1506

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.

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.

[The Swaggy P]

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

[cbale2000]

Originally Posted by Owen Busler

Good evening CD,

...

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

...

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

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


Thanks so much!

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.

[roboruler]

Originally Posted by cbale2000

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.

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

[marshall]

Originally Posted by AustinSchuh

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.

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.

[tr6scott]

Originally Posted by AustinSchuh

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.

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.

[InFlight]

Originally Posted by The Swaggy P

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