Chief Delphi - pic: Small CIM in wheel swerve

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Chief Delphi (http://www.chiefdelphi.com/forums/index.php)

- Extra Discussion (http://www.chiefdelphi.com/forums/forumdisplay.php?f=68)

- - pic: Small CIM in wheel swerve (http://www.chiefdelphi.com/forums/showthread.php?t=137177)

Ether

10-05-2015 14:49

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Ether (Post 1481224)

6) incorrect programming (resetting the counts rather than letting the FPGA accumulator run freely).

Quote:

Originally Posted by Bryce2471 (Post 1481321)

We used the WPI lib encoder software, so I'll hope it wasn't #6.

Using the WPILib doesn't rule out #6. #6 refers to using the reset() function repeatedly in your software in the roboRIO.

Code:

/**

 * Reset the Encoder distance to zero. Resets the current count to zero on

 * the encoder.

 */

public void reset() {

        if (m_counter != null) {

                m_counter.reset();

        } else {

                ByteBuffer status = ByteBuffer.allocateDirect(4);

                // set the byte order

                status.order(ByteOrder.LITTLE_ENDIAN);

                EncoderJNI.resetEncoder(m_encoder, status.asIntBuffer());

                HALUtil.checkStatus(status.asIntBuffer());

        }

}

I've seen code where the counts are read and then reset() every cycle. You can miss counts that way. I'm not saying that's what your code is doing... but if it is, that might explain your "drift".

bobcroucher

10-05-2015 15:28

Re: pic: Small CIM in wheel swerve

Deleted

Bryce2471

10-05-2015 15:35

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Ether (Post 1481338)

I've seen code where the counts are read and then reset() every cycle. You can miss counts that way. I'm not saying that's what your code is doing... but if it is, that might explain your "drift".

Oh, I see what you mean now. Although I have never heard of that issue before, we were not reseting the counts each cycle. We only reset them when necessary.

Chinske4296

10-05-2015 22:19

Never trust inventors weight simulation. We tried it for our robot with correct materials and it was off by about 20 lbs

Navid Shafa

11-05-2015 00:00

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Chinske4296 (Post 1481393)

Never trust inventors weight simulation. We tried it for our robot with correct materials and it was off by about 20 lbs

Look for individual parts or sub-assemblies that you may have missed. Make sure Electronics, KOP-items, COTS and downloaded models are all correct. I've never had any problems, but it is pain-staking work to get all the details right.

asid61

11-05-2015 00:08

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Chinske4296 (Post 1481393)

Never trust inventors weight simulation. We tried it for our robot with correct materials and it was off by about 20 lbs

Off above or off below?
We were about 10 lbs under the real weight in the CAD. Next year I'm planning for 100lb bot, to leave a wide margin.

Dunngeon

11-05-2015 00:11

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by InFlight (Post 1481291)

If you could work out the incremental encoder setup, it would free you up to put a slip ring module at the top. That trade off would solve the motor wiring issue, and not restrict you to partial rotations.

Which slip ring would you use? Mercotac's are banned.

GeeTwo

11-05-2015 00:30

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Dunngeon (Post 1481417)

Which slip ring would you use? Mercotac's are banned.

I was thinking of a custom solution when I suggested a module no taller than the wheel diameter. A quadrature encoder or a CAN controller wired in the traditional method would require 6 wires. Is it possible to make a CAN talon work with only two control wires that branch a bit away from the talon? If so, you could reduce the connection to four wires, if you could figure out a place to park the talon on the far side of the sliprings with the motor, gearbox, encoder, and wheel.

InFlight

11-05-2015 01:24

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Dunngeon (Post 1481417)

Which slip ring would you use? Mercotac's are banned.

There are quite a few Gold on Gold, or Silver on Silver contact designs that don't contain Mercury. The Wind turbine industry has lots of low cost high current options.

http://www.amazon.com/Sunwin-Wires-2...ords=Slip+ring

Company Specs
http://www.moflon.com/mw.php?a=1

Just one option, lots of other low cost (no-mercury) models. Moog would be nice, but really expensive.

Rachel Lim

11-05-2015 01:26

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Chinske4296 (Post 1481393)

Never trust inventors weight simulation. We tried it for our robot with correct materials and it was off by about 20 lbs

I've found the weight estimates in SolidWorks to be pretty good (and I'd be surprised if Inventor is significantly worse). What I have realized is all the "small" parts we don't add in really add up. Doing a weight calculation for our intake arms this year, I realized that just the fact that we don't add bolts/nuts to assemblies made my estimate under by ~0.6-0.8lbs.

My preferred method (if we're cutting it close), since I think we're unlikely to get complete enough assemblies to just use the CAD model, is to create a spreadsheet and list out every part and its weight. For custom parts I get the weights off the CAD parts, and for COTs ones I just look at the supplier's website. This way I can ensure I'm not missing a part that ends up to weigh a ton.

In all, even if we have the proper weights for all COTs parts that we add into CAD, and the correct materials for all custom parts, I wouldn't be surprised if the CAD was several pounds under because of the bolts / nuts / wires / pneumatic tubing / chain / belts / all other parts we don't add in.

Dunngeon

11-05-2015 11:37

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by InFlight (Post 1481426)

That slip ring you linked is only rated for 30A per the website, FRC rules require that it be rated for 40A if it's on the 40A breaker. I wouldn't want to put it on the 20A, for obvious power reasons. Which is why I've been looking for a low cost 40A Slip Ring. Unfortunately, I haven't been able to find any legal ones.

Andrew Schreiber

11-05-2015 11:53

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Dunngeon (Post 1481478)

There's a few out there, I don't have any links handy right now though. I'll see if I can scrounge up a part number when I get home.

Out of curiosity, why would you ignore the fact that 30A breakers are totally a thing and also require your drive to be on a 40A breaker?

Bryce2471

11-05-2015 15:14

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by Rachel Lim (Post 1481427)

I think that Inventor gets pretty good estimates too. Many of the designs I posted on Delphi have never been built, but my team did use the manually machinable swerve that I posted here before the season. The CAD for that said that It would weigh 6.85 lbs, and the real thing weighed in at 7 lbs even (and the real module had long motor wires on it when we weighed it). So I wouldn't say it's perfect, but it can be pretty close.

InFlight

11-05-2015 15:40

Re: pic: Small CIM in wheel swerve

You may be able to find 40 amp slip rings, but all the ones I've seen are large and very expensive.

Both the M1330 and the MW1440 (4 wires, 30 amp per contact) both use 12 gage wire. Thus the wire size meets the R38 requirement for 40 amps.

R 37 states that you may use up to a 40 amp breaker for motor controllers, thus a 30 amp breaker to match the contacts is thus also permitted and fully compliant.

For a 40 amp rating; It seems like it could be permissible (R39) to splice two contacts in parallel (w/ MW1440) to achieve the 40+ amp load rating. Would be a good area for a rule interpretation, as it would make slip rings a more available and useable device.

[U]R39 Branch circuits may include intermediate elements such as COTS connectors, splices, COTS flexible/rolling/sliding contacts,
and COTS slip rings, as long as the entire electrical pathway is via appropriately gauged/rated elements.[/u]

Probably enough hijacking of this Swerve Drive thread on encoders and slip rings.

Andrew Schreiber

11-05-2015 18:25

Re: pic: Small CIM in wheel swerve

Quote:

Originally Posted by InFlight (Post 1481523)

The last sentence in R39 is what makes these illegal despite being the appropriate size wire. They are not RATED for 40A. (I've been trying to find a way around this for a while). I'd be especially hesitant with the splicing.

As promised: http://www.ebay.com/itm/200661012580

As with most things: Light, Cheap, Legal. Pick 2.

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