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Unread 07-22-2018, 06:33 PM
justin.chau justin.chau is offline
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pic: Slip Ring Swerve

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Unread 07-22-2018, 11:43 PM
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Re: pic: Slip Ring Swerve

Hey man, that's a pretty sweet design!! What's the black ...thing... on top of the wheel sticking vertically up? I can't see an obvious purpose for it.
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Unread 07-23-2018, 12:03 AM
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Re: pic: Slip Ring Swerve

That's the slip ring, not showing the wires.

I agree it is a pretty cool design. Very low ground clearance, so maybe not for every game.

I also think it is geared aggressively. 17.33 from the 775pro to a three inch Colson puts the slip current (on FRC carpet) a little over 60 Ampere, so you are traction limited on the "wrong" side of peak power. No real issue as long as you stay out of shoving matches, with other robots or with a barrier.
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Unread 07-23-2018, 01:22 AM
Marcus Q Marcus Q is offline
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Re: pic: Slip Ring Swerve

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Originally Posted by Richard Wallace View Post
That's the slip ring, not showing the wires.

I agree it is a pretty cool design. Very low ground clearance, so maybe not for every game.

I also think it is geared aggressively. 17.33 from the 775pro to a three inch Colson puts the slip current (on FRC carpet) a little over 60 Ampere, so you are traction limited on the "wrong" side of peak power. No real issue as long as you stay out of shoving matches, with other robots or with a barrier.
Would you mind expanding on being traction limited in the "wrong" side of peak power?
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Unread 07-23-2018, 09:18 AM
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Re: pic: Slip Ring Swerve

Couple of questions:

Why the 775 for steering? With the drive motor on the turret, there will not be any reaction torque back into the steering motor from the drive torque. So it seems like you could easily reduce the size, weight and cost of the module by switching to a smaller motor like an RS550.

What slip ring are you using? How many wires and what rating per wire? What are your wire allocations?

The most appropriate slip ring we have been able to find is a 6 wire with 30 amps per wire rating. But this slip ring would not allow you to send full power to the drive motor without using 2 wires to feed power and 2 wires for power ground return. This leaves only 2 wires for a wheel speed encoder. However, if you limit the motor power to 30 amps in the motor controller, then you would still have 4 wires available for the encoder.
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Unread 07-23-2018, 09:35 AM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by wgorgen View Post
Couple of questions:

Why the 775 for steering? With the drive motor on the turret, there will not be any reaction torque back into the steering motor from the drive torque. So it seems like you could easily reduce the size, weight and cost of the module by switching to a smaller motor like an RS550.

What slip ring are you using? How many wires and what rating per wire? What are your wire allocations?

The most appropriate slip ring we have been able to find is a 6 wire with 30 amps per wire rating. But this slip ring would not allow you to send full power to the drive motor without using 2 wires to feed power and 2 wires for power ground return. This leaves only 2 wires for a wheel speed encoder. However, if you limit the motor power to 30 amps in the motor controller, then you would still have 4 wires available for the encoder.
I think that's a Talon downstream of the slip ring. Looks like the plan is to pass CAN through the slip ring. 2 for power, 2 for ground, 2 for CAN.

RS550 would be a better choice for azimuth IMO.
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Unread 07-23-2018, 11:05 AM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by Mark Wasserman View Post
I think that's a Talon downstream of the slip ring. Looks like the plan is to pass CAN through the slip ring. 2 for power, 2 for ground, 2 for CAN.
Has anyone (i.e. Bomb Squad) done this and had success? I would be worried about noise in the slip ring contactors. But maybe at typical swerve rotational speeds the slip ring is pretty noise-free.
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Unread 07-23-2018, 11:07 AM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by wgorgen View Post
Has anyone (i.e. Bomb Squad) done this and had success? I would be worried about noise in the slip ring contactors. But maybe at typical swerve rotational speeds the slip ring is pretty noise-free.
At least one team has successfully run CAN through a slip ring, although I forget who. I was quite impressed with it.
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Unread 07-23-2018, 11:31 AM
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Re: pic: Slip Ring Swerve

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Originally Posted by asid61 View Post
At least one team has successfully run CAN through a slip ring, although I forget who. I was quite impressed with it.
323 and I believe 16 as well are doing this.
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Unread 07-23-2018, 11:35 AM
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Re: pic: Slip Ring Swerve

323 did (as has 16), we had no major issues pertaining to data transmission. I don't recall seeing CAN errors caused by it. (Logs showed CAN errors caused by some issues on the PDP). Quantifying this data is on my TODO list but it may be OBE by other developments, will depend on prioritization of resources and goals.

It used 2x 775Pros and thus 2x Talon SRXs on each module, they were on 30A breakers each. Encoder feedback could be retrieved via CAN bus but was local to the SRXs for motion profiling purposes.
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Unread 07-23-2018, 11:35 AM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by Mark Wasserman View Post
I think that's a Talon downstream of the slip ring. Looks like the plan is to pass CAN through the slip ring. 2 for power, 2 for ground, 2 for CAN.

RS550 would be a better choice for azimuth IMO.
Wouldn't you need 4 wires for CAN, as it's run in series, as in 2 in and 2 out?
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Unread 07-23-2018, 11:44 AM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by mman1506 View Post
Would you mind expanding on being traction limited in the "wrong" side of peak power?
For reference, the power curve for the 775 pro is the green line in the first graph on this page.

The "right" side of peak power is faster and lower torque, which is on the right side of the graph. If the load increases, the motor slows down, the torque increases, and the power rises to meet the demand. This side of the power peak also has significantly greater efficiency (dotted red line), meaning less of your battery power turning into heat in the motor.

The "wrong" side is slower and higher torque than the power peak. Same statements, in reverse.
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Unread 07-23-2018, 12:18 PM
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Re: pic: Slip Ring Swerve

I have never seen heaven and hell in one picture. Great design.
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Unread 07-23-2018, 12:26 PM
justin.chau justin.chau is offline
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by Ollystorm View Post
Hey man, that's a pretty sweet design!! What's the black ...thing... on top of the wheel sticking vertically up? I can't see an obvious purpose for it.
Thank you! And yup, that's the slip ring.

Quote:
Originally Posted by wgorgen View Post
Couple of questions:

Why the 775 for steering? With the drive motor on the turret, there will not be any reaction torque back into the steering motor from the drive torque. So it seems like you could easily reduce the size, weight and cost of the module by switching to a smaller motor like an RS550.

What slip ring are you using? How many wires and what rating per wire? What are your wire allocations?

The most appropriate slip ring we have been able to find is a 6 wire with 30 amps per wire rating. But this slip ring would not allow you to send full power to the drive motor without using 2 wires to feed power and 2 wires for power ground return. This leaves only 2 wires for a wheel speed encoder. However, if you limit the motor power to 30 amps in the motor controller, then you would still have 4 wires available for the encoder.
The 775pro is used for steering mostly due to my unfamiliarity with swerve. I know the 775pro is overkill for this application though. If this is built, I plan to run the 775pro below 12v. It should help reduce thermal buildup compared to an RS550. It is a VP though, so it's easily changeable to an RS550 if wanted.

The slip ring I found is also a 6 wire rated at 30 amps per wire. I suspect it's the same one you found. There is a Talon SRX on the module though, so CAN can be used to read the encoder data. This leaves 4 wires for the motor.

Quote:
Originally Posted by kyle_hamblett View Post
Wouldn't you need 4 wires for CAN, as it's run in series, as in 2 in and 2 out?
The CAN topology is relatively flexible. I believe that 16's setup was similar in that they ran CAN through their slip ring. It is possible to use only 2 wires for can using a star topology, for example.
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Unread 07-23-2018, 12:34 PM
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Re: pic: Slip Ring Swerve

Quote:
Originally Posted by kyle_hamblett View Post
Wouldn't you need 4 wires for CAN, as it's run in series, as in 2 in and 2 out?
The 2 yellow wires and 2 green wires are electrically the same. Inside the Talon they literally twist them together and solder them to the PCB. That's why there is no labeled "innie" or "outie" on the CAN wires. I don't see why you can't make the junction on the stationary side of the swerve. I would dissect the slip ring to try to figure out which 2 slip rings that are farthest away from the others for noise purposes.
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