Team Update #11

[PaW]

Originally Posted by** ratdude747**
4.4.1 Robot Wireless Control
 Robots may be operated via wireless control only on the competition fields and the
practice field with the FIRST supplied radio; and
 Teams are not allowed to set up their own 802.11a/b/g/n (2.4GHz or 5GHz) wireless
communication (access points or ad-hoc networks) in the venue

sounds like scouting may be more of a challenge this year…

@Akash Rastogi

This has been part of the rules since 09…

This is a good opportunity to remind students, mentors, parents, guests, etc. to disable the “hotspot” capability (and battery sucking feature) of the current generation of smartphones.

Each venue is different of course, so please make sure to check with your local director or coordinator. Some venues may have ‘free wifi’ in their concourse areas, and your scouting members might be able to set-up in those areas.

[Racer26]

The wifi (no team-setup 802.11a/b/g/n networks allowed) rule has always perplexed me. It actually has existed for as long as I can remember in FRC (yes, it PREDATES the cRIO and robot control systems running on 802.11n.)

The old IFI controls ran on 900MHz radio modems (they run an RS-422 Serial link over a radio connection).

The current cRIO+wifi controls run using 802.11n in the 5GHz band, everywhere except Israel, where 5GHz is a restricted military frequency, so they use [email protected].

The rule has always been in place with the intent of preventing robot interference, and it USED to outlaw ALL wireless communication devices in ANY band, except for cell phones IIRC. (this meant teams couldn’t use walkie talkies or similar devices)

I’ve always found it silly for several reasons.

Cell phones regularly make use of the 900MHz frequency band, especially historically during the era of the IFI controls. If interference was such a problem, the literally hundreds of cell phones at an FRC event should have caused some robot weirdness. None occured, to my knowledge. Fast forward to today, and smartphones are now equipped with Wifi abilities, simultaneously operating in several frequency bands. You could never outlaw bringing your cellphone to an FRC event. It would be impossible to police.

Event venues, and indeed, even FRC events themselves, frequently have Wifi running at, or within range of FRC events. Additionally, the 2.4GHz, and 5GHz bands are unlicensed bands, usable by anyone for anything, anywhere in North America. Many FRC events have residential, commercial, industrial, or institutional lands within range. Interference in this band comes from all over the place, whether or not teams are wilfully adding to it or not.

The 802.11 standard (in all flavors) has proven itself to be fairly resilient to interference, otherwise you and all your neighbors couldn’t run your wifi routers in such close proximity to one another without causing problems. Furthermore, since FRC is using the comparitively unused 802.11n @ 5GHz, any interference in the 2.4GHz or 900MHz bands would cause no problems at all.

If interference were a true problem for FRC bots (its not), FRC could apply to the FCC and CRTC to use the licensed 3GHz band. I know of at least one company that makes 802.11 compliant devices using that band. This solution would surely eliminate any interference concerns, however, it would potentially be problematic in that each team might have to apply for a license, unless FIRST could convince both CRTC and FCC to grant the program a blanket license.

[Bill_B]

@Teched3

It seems the GDC has come up short by allowing us to repair burned out components on the Tetrix motors. How many times can you bend those retaining tabs before they break off? How will they be able to determine if the replacement inductor/capacitor was the exact replacement? What they should’ve included, IMO, was to allow us to put minifuses on the motor lead(s) to protect the motors. It is just poor safety and engineering practice not to have those motors properly protected. Sort of closing the barn door after the horse has gotten out. Anyone else out there agree?:yikes: :yikes:

It is quite possible, likely even, that GDC had no idea how these motors would react to the devil-may-care “design” work done by our enthusiastic high-schoolers. Can you give us more details about the minifuses? I would put them into my test machinery regardless of competition legality, then remove them for competition.

As far as I can tell the biggest oversight was the failure to include motor leads at all in the first choice parts kit. They ran down the list of needed motor stuff to add to the resource kit but fell short of even the standard connection method. That connector is at least designed for the machine. No visible strain relief in it and no way to secure it to the motor either. I get the idea that it’s an afterthought from Tetrix.

[ratdude747]

@1075guy

The wifi (no team-setup 802.11a/b/g/n networks allowed) rule has always perplexed me. It actually has existed for as long as I can remember in FRC (yes, it PREDATES the cRIO and robot control systems running on 802.11n.)

The old IFI controls ran on 900MHz radio modems (they run an RS-422 Serial link over a radio connection).

The current cRIO+wifi controls run using 802.11n in the 5GHz band, everywhere except Israel, where 5GHz is a restricted military frequency, so they use [email protected].

The rule has always been in place with the intent of preventing robot interference, and it USED to outlaw ALL wireless communication devices in ANY band, except for cell phones IIRC. (this meant teams couldn’t use walkie talkies or similar devices)

I’ve always found it silly for several reasons.

Cell phones regularly make use of the 900MHz frequency band, especially historically during the era of the IFI controls. If interference was such a problem, the literally hundreds of cell phones at an FRC event should have caused some robot weirdness. None occured, to my knowledge. Fast forward to today, and smartphones are now equipped with Wifi abilities, simultaneously operating in several frequency bands. You could never outlaw bringing your cellphone to an FRC event. It would be impossible to police.

Event venues, and indeed, even FRC events themselves, frequently have Wifi running at, or within range of FRC events. Additionally, the 2.4GHz, and 5GHz bands are unlicensed bands, usable by anyone for anything, anywhere in North America. Many FRC events have residential, commercial, industrial, or institutional lands within range. Interference in this band comes from all over the place, whether or not teams are wilfully adding to it or not.

The 802.11 standard (in all flavors) has proven itself to be fairly resilient to interference, otherwise you and all your neighbors couldn’t run your wifi routers in such close proximity to one another without causing problems. Furthermore, since FRC is using the comparitively unused 802.11n @ 5GHz, any interference in the 2.4GHz or 900MHz bands would cause no problems at all.

If interference were a true problem for FRC bots (its not), FRC could apply to the FCC and CRTC to use the licensed 3GHz band. I know of at least one company that makes 802.11 compliant devices using that band. This solution would surely eliminate any interference concerns, however, it would potentially be problematic in that each team might have to apply for a license, unless FIRST could convince both CRTC and FCC to grant the program a blanket license.

thats funny, i can always seem to remember teams having wireless networks set up for scouting… if the rule has existed that long, has it really been this poorly enforced?

[Teched3]

@Chris is me

What good will that do a week before ship?

You can withhold the minibot and insert fuses in the feed lines from the motor at your liesure. It would take a few dollars for the parts, and about 10 minutes. Hopefully you’re prototyping using fuses. Certainly a lot less time and expense than taking a motor apart and replacing an inductor, or the complete motor assembly.

[Teched3]

@Bill_B

It is quite possible, likely even, that GDC had no idea how these motors would react to the devil-may-care “design” work done by our enthusiastic high-schoolers. Can you give us more details about the minifuses? I would put them into my test machinery regardless of competition legality, then remove them for competition.

As far as I can tell the biggest oversight was the failure to include motor leads at all in the first choice parts kit. They ran down the list of needed motor stuff to add to the resource kit but fell short of even the standard connection method. That connector is at least designed for the machine. No visible strain relief in it and no way to secure it to the motor either. I get the idea that it’s an afterthought from Tetrix.

You can pick up minifuse holders and fuses at any car stereo shop or auto parts store. I would suggest you start out with 2 - 2.5 amp fuses, and have spares on hand. They originally had the less expensive motor leads listed, then added the thermal protection leads (at about 4x’s the price), which do not react fast enough to protect the motors. Remember, these components are used in robots that run across the floor. That is a whole lot different than using them to climb a 10 ft. vertical pole. In addition, these components are"new" to even experienced FTC teams for the same reason.

[Justin_Montois]

@martin417

After talking with an EE, I have been informed that there COULD be significant gains by replacing the inductor with a wire. Possibly as much as a 10% power boost. has anyone tried this? An easy test for this is a resistance check across the motor windings. The motor winding resistance is normally quite low, so a quick check could show that someone has “mistakenly” replaced the inductor with a non-complying part.

We found around 20ohms of resistant across the windings with a stock motor and 6 ohms with just a wire in place of the inductor. Pretty big difference.

Does anyone know if the inductor I listed earlier in this thread an identical part?

[Racer26]

@ratdude747

thats funny, i can always seem to remember teams having wireless networks set up for scouting… if the rule has existed that long, has it really been this poorly enforced?

Prior to 2009 when the cRIO+wifi became the method of doing things, and there was 6 Wifi networks required to just run a match, it was just that poorly enforced.

2009-present, the FTAs have had spectrum analyzers and stuff hooked up, and can tell what Wifi is going on in the area. If teams set up their robot on wifi in the pits, the FTA will know about it.

[Al_Skierkiewicz]

Guys,
Although the Bourns page is similar it is not nearly the identical part. When I first started research on this a few weeks ago, I found an identical sized inductor with the same conformal package. I believe they were more like 900ma max DC current. The series resistance is much lower than the Bourns above. AT 900 ma used with a motor that stalls at 7.5 amps, you can see why the motors constantly burn open.

BTW, I started the discussion to allow opening Tetrix motors to replace the inductor. My recommendation was based on the purchase cost factor of replacing a motor that had failed simply because an under rated part had failed. I had hoped that teams could cut their losses by replacing the inductor. FIRST and the GDC was overly generous by allowing teams to open and repair other items besides the Tetrix motor and delineating “repairs” from the “modification” rules.

[Bill_B]

@340x4xLife

We found around 20ohms of resistant across the windings with a stock motor and 6 ohms with just a wire in place of the inductor. Pretty big difference.

Does anyone know if the inductor I listed earlier in this thread an identical part?

That’s a big help in telling whether a motor is repairable. My armature coils measured much less than 20 ohms. That is further support for my presumption that there is invisible damage to my coils internally.

I’m confused about your 6 ohm measure though. Why would shorting the inductor cause less than the coil winding resistance?

So, my advice now about trying to repair a motor is to measure your coils first, across all three pairs of commutator contacts. If you find a coil pair that is significantly less than 20 (or 6?) ohms, don’t bother trying to replace the inductor. Even if you were to get an operating motor going, its work and power characteristics will be different than a stock motor. That would make the repaired motor questionable to use for prototyping. It might still be useful as an arm mover on an FTC robot though.

[jskene]

This may be a closer match to the stock inductor:

http://www.mouser.com/ProductDetail/Bourns/5800-390-RC/?qs=sGAEpiMZZMueR%2blcXtRMwcdgH6oJRmDtqeZBsTfTG7Q%3D

It has a maximum current rating of 0.8 amps, still well below what I would have expected the design to require.

There is another Bourns inductor of the same inductance but with a 4 amp capability:

http://www.mouser.com/Search/ProductDetail.aspx?R=5900-390-RCvirtualkey54200000virtualkey542-5900-390-RC

I believe this is physically larger than the stock one, although I haven’t measured the stock one yet.

[GaryVoshol]

Has anyone seen the rules files with the Team Update 11 changes in them? The said they would be published on Wednesday; it’s now Friday and a new Update should be appearing.

[Al_Skierkiewicz]

Jerry,
You might want to check out the data sheet. That 4 amp Mouser inductor is almost 1/2" in diameter.

[vamfun]

@Bill_B

That’s a big help in telling whether a motor is repairable. My armature coils measured much less than 20 ohms. That is further support for my presumption that there is invisible damage to my coils internally.

I’m confused about your 6 ohm measure though. Why would shorting the inductor cause less than the coil winding resistance?

So, my advice now about trying to repair a motor is to measure your coils first, across all three pairs of commutator contacts. If you find a coil pair that is significantly less than 20 (or 6?) ohms, don’t bother trying to replace the inductor. Even if you were to get an operating motor going, its work and power characteristics will be different than a stock motor. That would make the repaired motor questionable to use for prototyping. It might still be useful as an arm mover on an FTC robot though.

Thats a good idea… I was wondering about the possible failure modes of the choke. If it fails open, the motor should be a brick which I suspect most people see that can revive the motor by replacing that. If the choke can have a partial failure mode and still allow current to flow then we should see a partial power motor… like all three of the ones we smoked. But if the choke can typically only fail open, then likely the windings are shorted and should be tested like you propose.

What confuses me is the motor R should be 12v/Imax = 12/7.5 = 8/5 ohm.
So I suspect that the shunt capacitor may be interacting with the ohm meters to corrupt the reading.

[Al_Skierkiewicz]

Chris,
The stall current should occur when two windings are in contact with the brush at the same time. If you happen to measure when only one winding is contacted, you will see a higher reading. The failure mode on the inductor is open if stall occurs often or for an extended period, accompanied with a little smoke and possible debris. Partial short with smoke if running extended over several amps. The cap is so small that you likely can’t measure any resistance even if it was outside and not connected. The choke is so small that high heat causes the coating to crack and the in some cases the core cracks as well.

[Justin_Montois]

So does anyone have a distributor for the stock inductor?

[pfreivald]

Justin, one of my people found the exact inductors today – but I don’t know the URL to order them. I texted him and will let you know where to get them. If you don’t hear from me by, say, 11am tomorrow, have Rees call me!

[Justin_Montois]

@pfreivald

Justin, one of my people found the exact inductors today – but I don’t know the URL to order them. I texted him and will let you know where to get them. If you don’t hear from me by, say, 11am tomorrow, have Rees call me!

Sounds good! Thanks!

[pfreivald]

Sorry so late:

http://www.mouser.com/Search/ProductDetail.aspx?R=MICC/N-3R9J-00virtualkey60130000virtualkey434-22-3R9

[diviney]

All:

Based on everybody’s work here, and my own experience, I would suggest the following:

If you still have any good motors, protect them with the “fuse protected” cable (really a thermal switch) sold by Pitsco: http://shop.pitsco.com/store/detail.aspx?ID=6122. These should be acceptable for use in competition, but minimally use them during test to prevent motor damage. I have not tried them, but they automatically reset and presumably can protect the motor (will they trip under “normal” load conditions though)?

If you have motors that are already burned out, it is indeed possible to repair them. You can carefully open them up by un-crimping the small tabs on the back cover. The hardest part of this is re-assembly because the brushes spring closed and you can’t safely get them to go back over the commutator when trying to install the lid after repair. I found a good way to do this is to drill two small holes through the back plate 180 degrees apart next to the brushes (See attached diagram, thanks to Colin for the photo):

These holes allow you to insert small pins (safety pins for example) from the outside, holding the brushes in the open position while you re-install the cover plate. Once the cover is in position, then remove the pins and presto, the brushes are back in contact with the armature. I used a 0.036" PC board drill. You could use a more common 1/16" drill too.

You can replace the inductor with an equivalent (The best one I have seen was recommended by Patrick Freivald (Thanks Patrick)) –> Here:
http://www.mouser.com/Search/Product...lkey434-22-3R9

** Important ** – Since these inductors burn out so easily, I would recommend repairing the motor by removing the inductor and replacing it with a jumper wire. That is what is shown on the attached diagram. Then install the new inductor EXTERNAL to the motor. You can put it in-line with the wire and cover it with heat shrink tubing. That way when it fails again, you don’t have to open the motor to fix it. I cannot imagine any sane person disputing that this is an exactly equivalent repair. Deleting the inductor would not be legal (although perfectly functional for prototyping), but if it is replaced externally, that is exactly equivalent.

-Tom