Team Update #11
 

http://usfirst.org/uploadedFiles/Rob..._Update_11.pdf

Updated manual to follow tomorrow. Burned out Tetrix motor inductors may be replaced, and parts may be repaired (keep the performance the same, though)--though teams are encouraged to use caution when doing to, and assume voided warranties.

Re: Team Update #11
 

Does anyone know what the value of the inductor is? I've only taken them apart after they burned up.

If you have taken apart a good motor and know the color bands please let us know, so I can put up a part number and a vendor so we can have spares.

We're up to 5 additional motors now for the one mini-bot.

Re: Team Update #11
 

Thanks for posting this =D

Re: Team Update #11
 

Does anyone have a supplier for the inductors?

This opens up a situation where gracious professionalism is going to be paramount. Even in an instance where a team makes an honest mistake an thinks they have replaced the inductor with an identical part, the performance difference can be huge.

Re: Team Update #11
 

Thank you for posting this. How does FIRST know that today I ordered both mini-bot and pneumatic parts?

Re: Team Update #11
 

I would hope that FIRST publishes the recommended inductive component part number so that teams with limited technical resources can take advantage of this change.

Re: Team Update #11
 

This post has the only mention of inductance value that I remember seeing. It's a tiny part, so I expect a small current rating would apply based on its size. The thermal protector is rated at 2.5A. I'm off to search for an inductor with those specs.

Let's carry on further discussion in the topic above, OK?

Oh, and thanks to the GDC for injecting some rationality into the meeting of component fragility with somewhat freewheeling experimentation.

Re: Team Update #11
 

I understand your desire to keep the discussion in that thread, but due to the fact that this Update specifically says that you can replace the inductor, I feel more people will access the information in this thread.

Below are links that may be an identical part to the inductor that comes stock on the Tetrix motor.

Picture of the Inductor on the motor:
http://www.chiefdelphi.com/forums/sh...62&postcount=1

A PDF listing the specs, also in the upper left hand corner a picture that appears to show a similar inductor as the stock component:
http://www.bourns.com/data/global/pdfs/9310_series.pdf

And a link where you can but the above product:
http://www.mouser.com/ProductDetail/...7kb9bpww%3d%3d

At $0.86, if this is an identical part it sure beats the heck out of having to buy a brand new motor.

Can anyone confirm/deny this part as being identical?

Re: Team Update #11
 

My Electric Circuits theory is a bit rusty, but I am pretty sure that an inductor appears the same as a wire to a DC current. I don't think there can be any advantage gained by using different components in place of the inductor. The purpose of the inductor is to work with the shunt capacitor to make the motor less electrically "noisy". Unless the inductor has a resistive component as well (more than an equivalent length of wire) the only effect of substituting the component will be more electrical noise in your system.

___pause to check the specs____

I just looked at the specs for the 3.9 microhenry inductor listed, and it shows a 2.3 ohm DC resistance. It also lists a max current of 280 mA. at the max listed current, the inductor would drop 0.65 volts. I doubt that .65 volt drop would account for a "significant" difference in pole climb time, but it could make a small difference.

Re: Team Update #11
 

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...

Re: Team Update #11
 

My first ever Q&A question was used for an update!

The 2011 Kit of Parts Checklist, Rev E has been updated to include VUVG-L10-B52-T-M7 as an alternate part number for the FESTO valves delivered in the Kit of Parts. There were two part numbers shipped for use in kitting and this revision allows teams to use either or both valves on the ROBOT.

Re: Team Update #11
 

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.

Re: Team Update #11
 

Your circuit theory cannot be much rustier than mine, however, the current to a brushed DC motor, direct as it is, still varies as the commutator redirects the current to the armature coils. This motor has a three coil armature, so two coils are powered at a time. The coils are inductors themselves, although they are not usually rated that way. One way to think of an inductor is a current sustainer. That is, it tries to maintain the same current flow across voltage fluctuation. (contrast to a capacitor that tries to maintain voltage in current fluctuations) A collapsing field in a 3.9 micro henry inductor will not be able to generate much current flow for this motor. The capacitor will be equally inadequate to maintain much voltage. The result is called a low-pass filter, because only the very high frequency parts of the direct voltage variation will be absorbed by this LC filter. Well into the radio range I'd guess without doing any math. :D

This brings me to the current rating of the inductor. Based on various other observations, we can be pretty sure that more than 280mA is being drawn by this motor in minibot operation. Probably more than that in FTC robot operation as well. So the mystery becomes, how can such a relatively low-rated component expect to survive these over-current excursions? by duty cycle? by conservative rating spec.? Another spec. I read mentioned a 40 degree C rise for their inductor at rated current. This setup won't allow for very much heat dissipation, especially being enclosed and in as close proximity to a plastic mounting as it is.

The one thing I do know is that replacement by a wire in one case allowed way too much current for the fuse I was using. There may be something else wrong with that motor, but it wasn't obvious visually. Meanwhile, I have replacement motors to use for testing using the thermal limiter device. I will postpone further repair attempts until time allows or more learned commentary here convinces me to have another shot at it.

Thanks for your consideration.

Re: Team Update #11
 

Does anyone know why they changed the rules about the floor protector for the tower? They removed the sentence about it only having a 1/4in ridge in the field but did not replace it with anything else. Does anyone know if this means the bump will be larger or smaller?

Re: Team Update #11
 

Now we are going to get stuck on it and not be able to reach the tower with our 1 inch off the ground bumpers.......j/k :ahh:

Re: Team Update #11
 

This has been part of the rules since 09.....

Re: Team Update #11
 

My understanding is that the ridge will be the height of the 3/16" HPDE Plus the Carpet that covers it. I'd imagine that it would be no more than 3/8" but I cannot be sure. The description is a bit confusing, a drawing would be nice.

Re: Team Update #11
 

Basically they removed the 1/4" because there is no real way to say exactly how big the bump will be.

A hard number like that will only cause headaches with teams complaining about it being to high when it's not exactly 1/4"

So they are saying "this is how it's constructed plan accordingly YMMV."

Re: Team Update #11
 

:) :)

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:

Re: Team Update #11
 

What good will that do a week before ship?

Re: Team Update #11
 

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.

Re: Team Update #11
 

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 802.11n@2.4GHz.

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.

Re: Team Update #11
 

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.

Re: Team Update #11
 

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?

Re: Team Update #11
 

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.:) :)

Re: Team Update #11
 

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.

Re: Team Update #11
 

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?

Re: Team Update #11
 

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.

Re: Team Update #11
 

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.

Re: Team Update #11
 

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.

Re: Team Update #11
 

This may be a closer match to the stock inductor:

http://www.mouser.com/ProductDetail/...ZBsTfTG7 Q%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/Product...42-5900-390-RC

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

Re: Team Update #11
 

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.

Re: Team Update #11
 

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

Re: Team Update #11
 

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.

Re: Team Update #11
 

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.

Re: Team Update #11
 

So does anyone have a distributor for the stock inductor?

Re: Team Update #11
 

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!

Re: Team Update #11
 

Sounds good! Thanks!

Re: Team Update #11
 

Sorry so late:

http://www.mouser.com/Search/Product...lkey434-22-3R9

Re: Team Update #11
 

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

Re: Team Update #11
 

I previously repaired a broken terminal on one of these motors. It was broken off down in the plastic. After getting the motor apart, I found an old on/off switch which had little flat terminals the same size and shape of the motor terminal. I broke the switch open and stole one of its terminals, used it to replace the broken-off motor terminal. It did not have a locking tang, so after installing and soldering it inside, I gave the new terminal a quarter-turn twist on the outside to lock it in place. When I was trying to figure out how to get the motor apart, I decided to use a hobby motor gear puller, and then slide out the armature with the end bell still on it. That worked OK, next time I will try the method above with the holding pins.

Now it looks like I will be doing more motor work. We are purchasing new motors to replace two that burned out this week, and will get the motor-protecting cabling. The motor that had been previously repaired is one that burned, it is starting to look a little rough due to all the handling. I might experiment on this one, just to see if I can make a fault-protected practice motor. The companies that make polyswitch fuses make some for low voltage motors, the leaded ones are about the size of the motor's original inductor.

For some background, here is one mfg's site

http://www.circuitprotection.com/app...brushdcmotor=1

I have not picked a fuse yet, probably go for one with about a 30V rating, a hold current in the 3 to 5 amp range, trip current in the 5 to 10 amp range, trip time in the 1 to 2 second range. But these are expensive components, I may have to spend as much as 50 cents. If I can make it un-burnable, I will update this.

Re: Team Update #11
 

WARNING: We just got those inductors from Mouser, and they are NOT identical. They have much more internal resistance than the ones on the motors, and burn up under relatively nominal load.

Sorry, everyone, for giving you bad information... We were certain we'd hunted down an inductor with the right specifications.

Re: Team Update #11
 

Tom,
This is not a legal repair. However, if you assemble the brush/endplate assy to the armature prior to installing the armature in the motor housing, your step is not needed. For those who are attempting this repair, the magnets will try to draw the armature into the housing, so a device will be needed to control the attraction. I found that a wooden swab works for this purpose.

Pat,
The inductor you link to is only 400 ma, there are others in that package that are 900-950 ma. They are non-stock on the Mouser website so you will need to find another source.

Re: Team Update #11
 

Al,

In your opinion, is the suggestion to replace the inductor on the outside of the motor (so it is easy to replace if it burns up again) a legal repair? (I understand that your opinion is not everywhere "legally binding", but it is afforded a great deal of respect!)

Thanks!

Re: Team Update #11
 

Patrick,
The TU makes repairs replacing defective parts with "identical" parts legal. Adding a replacement in a different location seems to disagree with this ruling.
Thanks for the kind words. I had originally suggested that teams just be allowed to remove the inductor from Tetrix motors, if they were making repairs in order to eliminate the cost of replacing the motor. The GDC felt very generous and allowed teams to make repairs on all components. I have to thank them for that.

Re: Team Update #11
 

Fair enough -- we'll do that for our already-burned-out ones for continued testing, and keep the others as-is.

Re: Team Update #11
 

Al:

Can you please clarify why you feel an identical replacement part that is physically in a slightly different location would not be legal? This seems like a technicality because it is clearly electrically equivalent.

Also, you suggest assembling the brush/endplate to the armature prior to installation... I don't have a motor in front of me, but doesn't that require removal / re-installation of the pinion gear? I would think it easier to drill a couple of retaining holes than to R&R the pinion...

-Tom

Re: Team Update #11
 

Tom,
The pinion pushes through the motor casing, after you remove the transmission of course. The illegal part is drilling holes in the motor in addition to the external mounting of the inductor. While electrically equivalent, the RF generated by the motor is shielded by the motor casing. Moving the choke to the outside reduces it's effectiveness as a filter, hence not "an identical replacement" repair.

Re: Team Update #11
 

FTC teams have been burning these motors out since Tetrix replaced VEX. Before abandoning FTC after last season, we had a bag of about 20 dead Tetrix motors -- a combination of motors that had burned out, had broken gear heads, and or had snapped-off power tabs. If FIRST didn't know about it, it's because they never asked FTC teams. I'm not quite so sure it is the GDC's fault.

All you FRC teams should make sure to have plenty of spares. The gear heads are just about as likely to fail as the motors are to burn out during a stall.

Re: Team Update #11
 

Due to the nature of the circuit there is a continuous switching in it, which has a DC transient characteristic repeating every time the brushes leave one rotor coil and get to the next one. The 3.9 µH adds to the rotor inductance and limits the transient current running through the motor. It also limits the current through the motor considering its resistance gets in series with the rotor's resistance. The presence of the capacitor in parallel with the rotor limits the noise due to the switching.

Replacing the inductor with a simple piece of wire , or an inductor with less than 3.9 µH, will allow a higher current through the rotor, which will increase the torque providing an advantage over competitors running with motors which have not been modified at all.

The motor expected life will be reduced with smaller inductors, or without inductor at all.

Hugo J. Salom, P.E.
Engineering Lead Mentor - Team 3337

Re: Team Update #11
 

I ordered 10 of the inductors listed on the Mouser site. They are the same value as the one in the motor, BUT, they are not an identical part. They appear to be a bit larger. I will have these with me in St. Louis if anyone needs one (or two). I believe they will be legal to use, but I will get approval first.
Here is a picture of what was delivered.