Looking for slip rings and distributors
 

Our team is attempting to do swerve drive this year and we are looking for some slip rings but having troubles finding appropriate ones. we need 2 40A lines for a cim and 3 signal lines.

What slip rings does your team use(if any) and where do you get them from?

Re: Looking for slip rings and distributors
 

I have used these on non-FRC related projects with great suceess: http://www.mercotac.com/html/430.html

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Adafruit has 2 amp ones that you can run PWM cable through, that might be helpful if you can modify your design a little

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the only unit they have that meets our needs is the model 830. Why do they not list prices???? I imagine they will be 100+ each but if they are too much more than that may be an issue.

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for the servo no problem, but we still need 2 40 amp channels for the cim and if we are to use a secondary slip ring the cim slip rings needs to be a through hole style.

Re: Looking for slip rings and distributors
 

As with most suppliers you have to contact them for a price:
http://www.mercotac.com/html/purchasing.html
Contact info at the bottom, they are local to me so I just drive over and pick stuff up.

These are expensive, have you thought about designing your swerve so it is independent like 1717/others?

Re: Looking for slip rings and distributors
 

I believe Bomb Squid :cool: (Team 16) uses slip rings in their swerve. You might try contacting one of them.

Re: Looking for slip rings and distributors
 

Even through CIM motors are usually on 40A breakers, they can draw significantly more current for brief periods of time. The breakers do not trip instantly at 40.00001A, they trip after X seconds depending on the ambient temperature and current being drawn. Thus, your slip rings should be rated to handle the stall current of a CIM motor.

Re: Looking for slip rings and distributors
 

I don't think you need slip rings that can handle stall current of a CIM those would be pretty insane 180amps? .... Just like the breakers I am sure the slip rings aren't going to melt at 40.00001 amps. I would contact the manufacture for a more detailed answer on what they can handle.

Bomb squad if I remember correctly uses one of the Mercotac models...

Re: Looking for slip rings and distributors
 

The 12 gauge wiring required by FRC rules for circuits on 40A breakers is also only rated for ~40A. The breaker is designed to trip in a way that reflects how the wiring is heating up. I imagine the conductors in a slip ring have similar behavior and can be similarly rated and protected by the breaker. Basically, if the wiring only needs a 40A rating (130A would require 4 gauge wire...), then a slip ring should be fine with that as well. Please correct me if this reasoning doesn't work.

Can someone chime in on any differences between these parts of the circuit and their current ratings?

Re: Looking for slip rings and distributors
 

The 2014 robot rules allow slip rings as part of branch circuits "as long as the entire electrical pathway is via appropriately gauged/rated elements." Since a branch circuit is rated for 40 amps and 12/10 AWG wire, the slip ring should be rated accordingly.

Re: Looking for slip rings and distributors
 

Link to source? I have been looking for legality stuff on slip rings for a while, but I couldn't find anything on the Q&A or in the official rules, other than that custom circuits are prohibited.

Also, bomb squad uses the Mercotac Model 230 slip ring according to a PM I got a while back. They're ~$100 each online it looks like.

Re: Looking for slip rings and distributors
 

independent? not sure what you mean here. can you elaborate?

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we can't even find slip rings rated for 40A much less the hundreds of amps stall current. those things would be the size of a truck. and i imagine slip rings can also handle higher peaks as well anyway, i'm just trying to get as close numbers based on continuous ratings as I can.

Re: Looking for slip rings and distributors
 

Like, able to rotate freely without the cim. Essentially, outsource all wires to areas outside the rotating module.

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aren't those illegal then?. the 230 is only 2 channels at 30 amps per channel and doesn't meet the proper rated requirements of 40 amps?

Re: Looking for slip rings and distributors
 

well our design not only has a cim rotating with it but also a servo(we are doing a 2 speed shifting swerve, or at least attempting to) so we need some kind of slip ring with 5 channels 2 of which needs to be 40A for the cim. i mean we could just stick wires through the center as people have pointed out but we don't want to have to deal with all the issues related to untangling the wires.

Re: Looking for slip rings and distributors
 

You can also find through-bore slip rings that would allow you to combine multiple different slip rings with a wide variety of specs. For example these guys.

Some suppliers can also build custom slip rings for you, but you'd have to be careful about the legality of these during the season.

Re: Looking for slip rings and distributors
 

If you look at Pwnage's CIM in wheel swerve ( I think), they have a piece of code that, depending on how far the swerve module has rotated (with three complete rotations in one direction being the max IIRC) automatically turns the wheel the opposite direction to compensate for rotation of the wires. It's a fairly simple fix, and they said it had a minimal impact on reaction speed of the robot.

This would remove the need for slip-rings, which would bring the overall cost of the modules down quite a lot while simplifying construction.

Also, if you don't mind me asking, exactly how are you planning on packaging this swerve module?

Re: Looking for slip rings and distributors
 

thanks. i've found a few potential suppliers but none of them have prices and who knows what kind of lead time is required. sourcing these $@#$@#$@#$@# things is no easy task =\

Re: Looking for slip rings and distributors
 

We've done a system like that before and we know it is doable we would just rather avoid it hence why we are trying this approach. it certainly is going to cost more but we want to try this during the off seasons to see if we can perfect it by competition time or go with another system(like the one you mentioned)

not sure what you mean in regards to packaging it, elaborate? As in the over all structure of it or???

Re: Looking for slip rings and distributors
 

Everyone,
The Mercotac slip rings are not slip rings in the true sense of the word. They are mercury filled contacts and are not legal on FRC robots. If you read the application notes on these contacts, you will see warnings about mercury leakage when mechanically stressed or when max current is exceeded. I contacted the manufacturer and was told that those events are rare but do occur. Mercury spills on an FRC field will close down the event.

Re: Looking for slip rings and distributors
 

2014 R48. Obligatory rules may change next year disclaimer.

Re: Looking for slip rings and distributors
 

Ummmm...

Re: Looking for slip rings and distributors
 

Which rule is this illegal based on? Looks like <R8>?
What is the definition of hazardous?
Is there a list of hazardous materials? Mercury isn't mentioned in the blue box next to this rule.

If there is no list, does that mean it is up to the individual inspector to judge the hazardous nature of the material?

As such... isn't the legality of this sort of component determined by the lead-inspector at an event?

Re: Looking for slip rings and distributors
 

John,
Not sure what to answer here...
Does mercury need to be on a hazardous materials list?

Re: Looking for slip rings and distributors
 

Per Q/A, they were legal in 2006, 2009, and 2011, and illegal in 2010. The reason for them being illegal in 2010 was not because of mercury, but because any type of slip ring was not legal in 2010. There have been no relevant Q/As since 2011.

Re: Looking for slip rings and distributors
 

I'm now slightly confused. Were these slip rings legal or illegal in 2014?

Re: Looking for slip rings and distributors
 

Search "2451 PWNAGE Off-season 2012" for a custom slip ring. This design is not currently legal due to the COTS rule unless the brush assembly is considered COTS since it is the slip ring itself. We have asked First to consider allowing custom "mechanical" slip rings. Let the kids explore the possibilities. Our prototype uses the brush assembly out of a CIM motor and we double up the brushes for each motor wire so this will be more than adequate for powering a CIM. Bag motor brush assemblies could be added to allow for sensors, etc. or possibly used instead of CIM units to bring the size down. We have done testing using these slip rings in the off season with great results. We have been playing around with the idea of rapid prototyping the housings and sharing the designs with the First community if they would be allowed for future seasons. We have also shared our design with Andy Baker and spoken with individuals at Vex.

Re: Looking for slip rings and distributors
 

Joe,
As far as I know, mercury contacts have never been legal on FRC robots even when slip rings were allowed due the the hazards.

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These type of connectors were explicitly allowed in 2006

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Perhaps you may want to consider changing your mechanical design then. You can still have 2-speed swerve modules without having the CIM or the servo rotate with the module, and thus avoid the slip ring problem all together (along with other benefits). Look at how many previous teams have implemented co-axial swerve modules (1717, 148 in 2008, 973's "emperor swerve", 118 in 2007, etc.).

Re: Looking for slip rings and distributors
 

Regarding Mercotac "slip rings":

They passed inspection in 2008 without any concerns being communicated to our team by any inspectors, and even contributed to the TechnoKats' being awarded the Industrial Design Award at the Boilermaker Regional.

I rarely disagree with Al, but in this case I think his objection is too strong. In my opinion, the hazard of contained mercury-wetted contacts is akin to the hazard of sealed lead-acid batteries. Of course, it's his opinion that counts when determining legality of components, not mine.

Re: Looking for slip rings and distributors
 

Kevin,
I think the GDC changed their mind on that issue later in the season. Alan, I am not aware of the Technokats using those.

Re: Looking for slip rings and distributors
 

We used the Mercotac slip rings all season without issue, including long practice sessions with CIM motors getting extremely hot. We first saw them on Bombsquad's 2013 bot. I saw them on Bombsquad's 2014 bot at the St Louis offseason event, and I saw them on Pwnage's bot at the Rockford offseason event.

We included a mercury cleanup kit in our safety kit. But I see practically no risk of it being released, even in the event of severe mechanical damage.

I was under the impression that mercury was not disallowed by the FRC rules. FTC rules however have specifically disallowed mercury for a number of years (in the same rule as lead is disallowed).

Re: Looking for slip rings and distributors
 

For reference, we did not use the Mercotac slip rings during the season but did test them post season. I also contacted Mercotac years ago and came to the conclusion that we would be running them above their specifications. Mercotac has multichannel units and we could have run multiple circuits in parallel but those get big, heavy and expensive. Ultimately we read the rules to say mercury was not legal and didn't want to hinge a design on that risk. Once we found that Bomb Squad ran the 230's all season with no issues we decided to test them ourselves post season. If First specifically disallows the Mercotac slip rings and lifted the COTS slip ring rule we feel that we have a good alternative in the two designs we've posted on CD. These could be 3d printed by the teams and assembled or if Andy has his ears on... There are also windmill slip rings on Ebay that we have evaluated and would handle the current no problem but they are big and heavy.

Re: Looking for slip rings and distributors
 

I had a look at the documentation for the Mercotac slip rings, and judging by the external design and description, I suspect they have very little mercury inside. Given how catastrophically mercury corrodes aluminum (like the housings), they're presumably designed to resist that failure mode—and the simplest way to do that would be to use just enough mercury to wet two closely spaced parallel plates with concentric traces, and then fill the housing with an insulating oil. (The aluminum housings are anodized, which helps, but likely not enough.) The amount of mercury needed to go around the circumferential contact would likely be on the order of microlitres. Judging by voltage ratings on the larger ones (maybe the small ones don't have oil?), that seems like a plausible method of construction.

If that's the case (and contact with their applications engineers ought to confirm/repudiate it), then we're probably looking at a level of hazard comparable to lead in batteries and soldered components, or hexavalent chromium and cadmium on plated components. While not ideal, there hasn't been a blanket prohibition on other components that contain metals that exhibit toxicity when ingested, and with form factors that permit accidental exposure to the metal.

As for the implications of a spill, I think that would depend a lot on how much information was immediately available about the quantity and toxicity of the substances released, and the stringency of the applicable regulations (as interpreted by the people in charge). If cleanup is necessary, it could be a fairly onerous process requiring external assistance.


As for the contact ratings, check with Mercotac's applications engineers if they are willing to publicly provide duration vs. current ratings, because typically, the failure mode is due to temperature rise. If they compare favourably to the trip performance of the circuit breakers, they may be usable for brief periods in excess of their constant current rating. Similarly, if they can provide ratings that presuppose a reduction in the expected life of the device, but won't cause catastrophic failure, perhaps you could get away with more current. In any of the above circumstances, you'd want an engineer's input to have documentation for the inspectors as to why the figures in the published datasheet are not applicable.

Re: Looking for slip rings and distributors
 

Tristan,
You must have missed my earlier post. I did check with Mercotac engineering, earlier this year. Their response was that mercury spills were a reality when the maximum current or the mechanical specifications (specifically misalignment of the two rotating bodies and excessive vibration) are exceeded. There is a difference between "mercury wetted contacts" and using mercury as a contact. These fit into the latter category. For more info please refer to their literature... http://www.mercotac.com/html/literature.html
All of these documents discuss the need to correctly dispose (must be recycled) of these contacts due to the mercury they contain. In almost every paragraph of the two application sheets, connector failure is mentioned as a danger. In the case of a mercury spills on an FRC field does the amount or cleanup procedure really matter? Any is too much!

Re: Looking for slip rings and distributors
 

Al,

I think the underlying theme of this thread is that the FRC rules have been unclear, at best. It looks like your opinion is clear so please help reduce an obvious Q & A question and use your influence to get the 2015 FRC rules to be clear on this issue.

Paul

Re: Looking for slip rings and distributors
 

In today's regulator environment, Mercury should be avoided period. All of the products that I install have had the mercury removed for a couple years now. My company would not purchase and install any product containing mercury. We go to great lengths to identify products that are removed and replaced that contain mercury. After, removal they are sent to a approved recycler. One must be "compliant". After, the above statement I can't help thinking about the amalgam fillings in my mouth and I made tuna fish salad this week. Ate 4 sandwiches. How can I still be alive?

Re: Looking for slip rings and distributors
 

I emailed the EH&S gurus at my place of business, UTAS, a division of UTC, and they sent me these two links: http://www.epa.gov/mercury/exposure.htm
http://www.epa.gov/mercury/schools.htm

We go through great pains to make sure mercury is NOT in any of the equipment we make for NASA and being used for life support on ISS.

I did take a look at them, I didn't read everything, but ANY product containing mercury being used in FIRST should not be deemed legal. IMHO.

Re: Looking for slip rings and distributors
 

Agreed about the ambiguity. Here's what I've gathered from this thread.

1. The GDC never ruled about these slip rings in 2014, either via Q&A or manual.
2. Some LRI's, who have final authority at the events, have come out and said these slip rings are illegal in this thread.
3. Other LRI's (or RI's) have ruled these slip rings to be legal, as seen by the teams who were allowed to use them throughout the 2014 season.

This is not a good combination, especially when there are safety concerns about a part. There needs to be a uniform ruling on this going forward. We can't have a situation where a key robot part's legality is determined by which Robot Inspector sees it. This leads to a situation where teams who err on the side of safety end and don't use a risky part, end up giving up a competitive advantage to the team who takes the risk and decides to use the part.

Re: Looking for slip rings and distributors
 

Part of the issue is that teams may not necessarily disclose the use of them to inspectors.

Re: Looking for slip rings and distributors
 

Then they are cheating (if they are knowingly breaking the rules)?

Like a lot of things, we already use the honor system on so many things like Bag and Tag, I'm not really sure what you could propose to enforce "disclosure" besides what they write on the BOM and tell the inspector about.

Re: Looking for slip rings and distributors
 

I think "cheating" is a bit of a strong term, in terms of how you apply it. Since these were specifically allowed in previous years, they may not even know they aren't allowed. This is compounded when an LRI acknowledges a team's use of it and passed them.

Re: Looking for slip rings and distributors
 

That's why I qualified it with knowingly. The tense you used I thought meant to imply that teams may do it in the future after parts are (possibly) declared illegal.

I also will give you that cheating might be a strong word to use there. I play a lot of Magic the Gathering, and at competitive levels doing something knowingly illegal is classified officially as Cheating :)

Re: Looking for slip rings and distributors
 

I know of at least one team that used them and DID disclose their use to inspectors.

Re: Looking for slip rings and distributors
 

Message received.

I do not think a team that used these in the past was cheating. They read the rules one way or didn't consider the interpretation I am making in this thread. This is quite common with parts that are not used on a lot of robots. Inspectors who don't know an item on sight alone may or may not ask for identification or description. FRC teams make some great robots so it follows that those teams will try and find an advantage in using a COTS part that someone hasn't thought of using. Even inspectors with a fair amount of industrial experience may not have seen some of these items. Inspectors with little experience may not recognize some of the things LRIs are trained to look for.
For an interesting story...A few years ago at an LRI training weekend in NH, we asked teams to supply a few robots for our training purposes. One of the group activities was to walk through a typical inspection process with me. I showed them how I interact with the team, how i use the Inspection Checklist to work through the robot and how I attack individual systems separately so that I get a look inside the robot for several different items (mechanical, electrical and pneumatic). As I was "working the list" I looked down and noticed an illegal item on the robot. I asked if anyone saw a problem and only one of the LRIs in training caught it. It was a good training day for that reason.

Re: Looking for slip rings and distributors
 

I didn't say you couldn't spill mercury by breaking these. I said that there probably wasn't much to spill. (In other words, it's a question of the magnitude of the hazard, rather than the existence of the hazard.)

I'd previously referred to that documentation, and I don't believe it explained that difference. Can you elaborate on why these are in one category, but not the other?

If a fluorescent light above the field were to shatter, surely the amount of mercury released and its state would be considered when choosing corrective actions. Why is this different?

In terms of assessing risks, even if a prohibition exists, inspections won't catch everything, and force majeure is still a possibility, so event staff may still be in the position to decide whether a chemical hazard justifies (for example) suspending the event. It's fine to set zero mercury contamination as a target, but it's unreasonable to suppose that contamination is inherently unacceptable, irrespective of degree.


I think the best action is definitely to communicate it to FIRST, so that they can decide what risks are acceptable and codify prohibitions and procedures accordingly. It's crucial to do so in a way that educates FIRST about the hazards without sensationalizing them, because there are important considerations other than safety which must be weighed realistically. If we expected to be completely safe, we probably wouldn't attend competitions where robots fling balls around—so perhaps the thresholds of risk due to chemical exposure should be evaluated against that sort of benchmark.

Re: Looking for slip rings and distributors
 

If this has already been mentioned in this thread, please forgive me, I didn't see it.

Funny timing here. Sparkfun just announced yesterday that it was introducing slip rings into it's product offerings. They are offering 3 versions: 12 wire (2 A), 6 wire (2 A), and 3 wire (15 A).

While none of these carry enough current for drive-train motors, they do provide serious potential for some very interesting mechanisms.

Re: Looking for slip rings and distributors
 

Tristan,
Mercury wetted contacts have mercury droplets in them that pull together when the contact is closed. They do not form the only conducting surface in the contact but they do make the resistance of the contact low and are especially useful in very low current applications where surface contaminants prevent good contact. These often are in the form of a relay and require specific mounting attitudes to work. These were common in low power microwave applications and in video switching prior to reliable electronic switching.
In mercury contacts, the mercury is used as the contact. Mercury bulbs in thermostats are this type. In these applications, all current flows through the mercury. Such is the case with the rotating device under discussion. The temperature of the mercury is proportional to the current flowing through, much the same as in wire. Arcing within the medium also raises the temperature (significantly) which is true of the switched load used with PWM speed controllers feeding brushed motors.
In comparing the amounts of mercury in these contacts to that in a fluorescent bulb, I believe there is no comparison. The contact will have far more. In the case of the bulb, mostly in a gaseous form.

Bilbo,
The transformers you link are the type that have been used for years on video recorders to link the record and playback heads on rotating disks to the electronics. A good application for these would be sensors and perhaps PWM signals. Please be advised that these carry specific RPM limitations and a finite life. Exceed the RPM and the contacts become intermittent. The brush assemblies are often multiple (3-5) pieces of fine wire in contact with the rotating mechanism. As the RPM increases, the wires become air born and cease to conduct. Low RPM applications are ideal. The lifespan is dependent on the wear of the wires. VTR manufacturers made these assemblies a repairable/replaceable item.

Re: Looking for slip rings and distributors
 

For 2015, it's in the manual, and illegal.

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I'm very appreciative that FIRST and the GDC eliminated this ambiguity. This is a great example of them listening to community feedback.

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I couldn't agree more. Reading through the manual this year, it looks like they really listened to the community about problems (like this ambiguity), but also opening up motors to unlimited quantities, and overall giving more flexibility. I don't read any of the rules and ask "but why?!?" I think they're all very reasonable rules.

Re: Looking for slip rings and distributors
 

A piece of equipment I service used to use the Mercotac connectors, had one blow during a fault. The plant's Hazmat team got to put their mercury cleanup procedure to use.

Had that been at an event somebody would probably have gotten a bill for cleanup.