Main breaker tripping, dead CIM
 

After having nearly no problems with our drive for all of Greater DC, our robot started tripping our main breaker in matches today (problem arose as the day went on, in our last two matches we tripped the breaker about halfway through). Upon testing, we discovered a few things:

1) From a cold start, it takes ~20 seconds of pushing against a wall to trip our main breaker.
2) The wires going to the breaker from the battery heated up noticeably during this time.
3) One of our CIMs was badly damaged; the front ring was able to turn independently from the body of the CIM.

We replaced the CIM (and for good measure replaced the breaker), but are wondering why the problem cropped up so suddenly and if the dead CIM could have been exacerbating it. Any thoughts? We have not been able to test since replacing the parts, unfortunately.

We're running a 6-CIM modified KOP drive with HiGrip wheels geared for 14 fps.

Re: Main breaker tripping, dead CIM
 

At CIM free speed?

Or 80% of CIM free speed?

Or something else?

Re: Main breaker tripping, dead CIM
 

6.11:1 gearing with 4'' (probably now more like 3.75'' due to wear) wheels.

Re: Main breaker tripping, dead CIM
 

A dead CIM certainly could be the source of the problem.

The faulty cim along with the 6 cim drive train, I've heard of 6 cim drives popping breakers.

Re: Main breaker tripping, dead CIM
 

Damage to the CIM could certainly increase its power draw either through increased drag or due to an internal short.

The breaker is rated to trip in 20 sec with an aprox 200% over current situation. The breaker is thermal so short bursts of much higher current will cause the temp to rise and it to trip eventually. If enough heat is building up in the breaker to cause it to trip the wire will certainly be heating up as well. Extended use at near trip conditions will eventually cause fatigue of the internal materials and eventually lead to an early trip situation.

At peak power, ~2600 rpm a CIM draws 68 amps and as it speed decreases that current increases to 133 amps at stall assuming a full constant 12v is applied.

Re: Main breaker tripping, dead CIM
 

I imagine this likely has something to do with it suddenly getting a lot worse. We'll be sure to keep a few spare.

Re: Main breaker tripping, dead CIM
 

We also had 4" higrip kit setup. We were 7.08:1 in high gear with 3 cim ball shifters (6 full cims), and we never needed low gear even when we played heavy D as we never had any main trips through our two regionals.

Re: Main breaker tripping, dead CIM
 

Another thing that could compound the issues stated above is that when CIMs break, they sometimes short to their chassis, which in turn can short to your robot's chassis depending on how you have the CIM mounted. This in turn can cause some strange issues with other components. I've seen this happen several times before, where an electrically hot chassis can be traced back to a broken CIM.

Re: Main breaker tripping, dead CIM
 

My team also had a problem with our breaker tripping with our 6 CIM drive train. It only became a problem in the later matches when we were playing heavy defense or were getting heavily defended ourselves. What we ended up doing was cooling down our main breaker right before our matches. We used a canned computer air duster held upside down and then sprayed it into the opening of the breaker. This got it cold enough that it wouldn't trip in the match for the most part. Its not the best solution but it ended up working decently well for us.

Re: Main breaker tripping, dead CIM
 

Well, swapping out the dead CIM unfortunately did not fix the issue; we were plagued by main breaker trips for all of our matches today.

After our last match, we went out to the practice field and tested a few things; during this time, I noticed that with a perfectly topped-up battery and no compressor running, we'd slip the wheels while pushing (as intended) and only draw 180 amps, which isn't a problem; this caused no heating of the wires and no breaker trip.

If the battery was slightly low and/or the compressor was running, we'd be unable to slip the wheels and our CIMs would stall, drawing 250 amps and causing massive heating and an eventual main breaker trip.

I'd imagine the problems we're having now, as opposed to at Greater DC, are due to an amalgam of factors; due to our robot's mechanisms being in a state of semi-operability, we weren't running our compressor during matches at DC. Moreover, our robot has gained ~6 lbs since then, and our gearboxes (they're WCP 3CIM SS gearboxes, so they're open) have been steadily accruing a layer of carpet fluff mixed in with the grease (we didn't have time to fully clean them out between matches).

It seems with our gearing/wheel COF/weight we're right at the boundary of what's operable. Does anyone have any advice (other than ditching a bunch of weight and cleaning out the gearboxes) for ensuring we're not tripping our breaker at worlds?

Re: Main breaker tripping, dead CIM
 

I was always under the impression that once a main breaker had tripped you should replace it since it took less energy for subsequent trips. Is that actually true?

Re: Main breaker tripping, dead CIM
 

You could try implementing a voltage ramp in software. We did this in-between Central Illinois & St. Louis and never had another issue.

We did some testing on our practice drive at our school and found we were able to force the robot's radio or CRIO to reboot using 6 full sized CIMs geared similarly to what you described. Once we implemented the voltage ramp the problem was solved and we couldn't re-create the symptoms.

Re: Main breaker tripping, dead CIM
 

How did you implement the voltage ramp?

If we were to use the simple slew rate limiter described by Ether, do you have any idea of what a sensible value for the limit should be? I'm not sure how fast the main loop is executing, and it seems it depends critically on that. We're coding in java.

Re: Main breaker tripping, dead CIM
 

Another thing to check is whether the terminals on the main breaker are tight. I saw two teams this year with circuit breaker trips with a root cause of loose wires at the breaker.

I agree with the suggestion that you replace the breaker if it has been tripping regularly.

Re: Main breaker tripping, dead CIM
 

I think you could be geared a little fast. If its not too hard to swap, a 50:24 gearkit and some 14T or 13T pinions would fix it, though you lose some top speed. Checkout WCP website for ratio options.

Re: Main breaker tripping, dead CIM
 

The problem is that doing this on a KOP chassis is about 2-3 hours of work. We might go this route if we don't think any other option will suffice, but it's not a particularly favorable option.

Does anyone have any advice (other than ditching a bunch of weight and cleaning out the gearboxes) for ensuring we're not tripping our breaker at worlds?[/quote]


Pull one cim off each side and replace them with mini cims. Making a 2cim/1mini cim gear box on each side.

It seems to be true in our case. Switching out the breaker solved almost all our problems. We also copied the poofs and used a upside down can of compressed air to freeze the main breaker before a match.

Re: Main breaker tripping, dead CIM
 

Programming options:

1: easy: set a maximum current flow lower than 100%
2: more challenging: limit current to CIMs at a gearbox when it is not turning and you've been at high power for a bit. This requires distinguishing "starting at rest" from "been pushing for a while".
3: more challenging: like 2, but turn off other motors/compressor on robot when in pushing situations
4: more challenging: like 2, but implement this as a "current budget" where you estimate current flows and heat dissipation, with the latter decaying over time.
5: easy: like 1, exception implement a driver controlled "pushing mode" on your robot, that lowers maximum current levels
6: more challenging: like 2, but only when driver selects "pushing mode"

Next year's control system should open up some interesting options in this scenario.

Re: Main breaker tripping, dead CIM
 

We've observed this on multiple occasions, and my understanding is that the tripping action is caused by the bending of a bimetal strip inside the breaker. I imagine that the strip could be subject to plastic deformation near the trip point and wouldn't have the same characteristics afterwards.

Whenever we trip a main breaker in competition (twice so far this year) it gets thrown into the nearest trash can and a new one goes on the robot. As Marcus said, between matches with tight turnaround we cool it down with compressed air.

Re: Main breaker tripping, dead CIM
 

+1 for voltage ramp. Doing this will lower the probability of all of your motors drawing peak current. We too do this on all of our 6-cim robots.

What language do you use? I am sure many teams on here can provide an example for the implementation in your programming language.

I know im side tracking here, but I am curious, do you know what was causing you to trip the main breaker?

Re: Main breaker tripping, dead CIM
 

Usually it's stalling a six-CIM drivetrain while trying to get out of a pin or T-bone, sometimes exacerbated by having had another match right before for the breaker to heat up.

Perhaps it's time for FIRST to consider distributing a higher-amperage breaker in the Kit of Parts? With all the motor power available to teams now, this seems like a pitfall that a lot of teams are falling into.

Re: Main breaker tripping, dead CIM
 

Or perhaps it is time for teams to back down on the number of motors they stall at the same time. Our experience with 6 CIM drive in 2013 was that we lost more matches due to blowing the main breaker than we won by having that little extra in our drive train. We went back to 4 CIMs in the 2013 off season and this year and have been very happy with the results (IE no blown breakers, just fine pushing and speed). The extra 2 CIMS in the drive isn't really adding 50% more performance. I would be interested to see some testing/analysis (which is on our todo list) of how much of a difference 6 CIMs vs 4 CIMs makes on performance given the limitations of the battery.

Re: Main breaker tripping, dead CIM
 

But to allow a higher amperage breaker would mean requiring larger gauge wire, higher capacity Anderson Connectors and a battery that could sustain a higher continuous current. I'm sure many teams would be upset as some have a rather significant investment in the current batteries and associated pieces.

Re: Main breaker tripping, dead CIM
 

The crucial bit we noticed is that at our gearing, our breaker is fine unless we stall the CIMs, i.e. while slipping the wheels we have no problem, and the CIMs do not stall on a fully topped-up battery if the compressor is not running.

So, it seems our short-term options (barring changing our gear ratio - if anyone thinks this is really our best solution even with the 2-3 hour investment to get it done, please let me know) are to not turn on our compressor if we are going to be playing defense and to make sure our batteries are in absolute top shape before going out on the field.

Does anyone know if there's a reliable way to test if batteries are actually charged? We have all new batteries this year, but the charge indicator on the charger can be misleading.

(As a side note, I'm wary about swapping two of our CIMs for mini-CIMs, as this could have the opposite of the desired effect by making it absolutely impossible for us to slip our wheels in a pushing match.)

Re: Main breaker tripping, dead CIM
 

Here's a possible software solution:

Measure the voltage drop across the main breaker with an analog input. Sample it and square each sample. Run the squared samples through a low-pass filter (a simple IIR would probably suffice). If the filtered value exceeds some experimentally-determined threshold, reduce the commands to the drive motors until the filtered value drops below the threshold.

Re: Main breaker tripping, dead CIM
 

This certainly sounds like it's worth trying. Unfortunately, I'm a mechanical mentor, and am not sure how to implement this; what would be the best way to actually wire that, and what sort of low-pass filter code would you recommend (I know how to do simple exponential smoothing, but that's about it)?

Re: Main breaker tripping, dead CIM
 

Perhaps something like this.

Re: Main breaker tripping, dead CIM
 

Amusingly, that's essentially exactly what my naive approach would have been, software-wise. Thanks.

I'm still partially leaning towards swapping out our gearkit to 7:1 or 8:1; we've got two spare gearboxes which should expedite the process greatly (much easier to switch out the whole gearbox than to change the gearkits of the ones on the bot). We can probably absorb the hit in top speed without too much trouble, as our defensive strategy is very much based on positioning and pushing.

Re: Main breaker tripping, dead CIM
 

Proper gearing is the correct solution.

Re: Main breaker tripping, dead CIM
 

If you work with a fully charged battery, you may be able to get more headroom by optimizing your electrical system to minimize voltage drop. Make sure that all wires are the minimum length necessary, and as large as possible. Make sure all electrical connections are tight.

Re: Main breaker tripping, dead CIM
 

A tripped breaker cost us the Waterford district. As was said above once it's tripped trash it.

http://www.cooperindustries.com/cont...UITBREAKER.pdf

Heat is a huge factor as can be seen in the spec sheet, if it starts the match hot you're in trouble. We run fans on ours between matches. We've now implemented some type of monitoring with our drive wheel encoder counts to limit our current draw, not exactly sure what the details were, I'll see if I can get someone who knows more about it to post. I think it basically says, if you're stalling either downshift or reduce power to motor.

Re: Main breaker tripping, dead CIM
 

Good for testing. Not legal to have any custom circuits not going through the main breaker in competition. The millivolts across the breaker is very close to the current passing through it in amperes.

I found that putting a good sized digital voltmeter display on an open PDB port and visible during test driving goes a long way toward understanding the electrical dynamics of your machine. Panel mount meters are 5 or 6 dollars. Seeing the meter drop into single digit range is eye opening indeed. I know the guys were working on coding that withheld the shot motor if the compressor was running after they saw the system voltage drop when it started.

Re: Main breaker tripping, dead CIM
 

OK, once again for effect, six motor drives are likely to trip the main breaker. Every report I have received of repeated trips of the main breaker were on six motor drives. Yes they are cool and yes they may achieve your design criteria but they come at a cost. Do you want to go fast or play an entire match? When the #6 wire is hot, that is your robot telling you something is wrong. Listen to it. To address some things from previous posts. Yes the breaker is a thermal device so when the wire heats up, some of that heat is conducted to the breaker and that adversely affects the trip point. Once cool, it returns to normal operation. A trip uses the same procedure as the red button on top of the breaker, so it is not a requirement to change out a breaker once it trips. Otherwise you would have to change it out every time you use it. Use ramping software will help quite a bit but will do nothing in a pushing match. Your results will vary with the type and number of wheels you have on the ground and how much you turn during a match. Add to this the fact that FRC robots are already drawing excessive currents that shorten battery life. If you are drawing currents that trip the breaker, consider that your battery life is cut by more than half. Do not be surprised when a battery suddenly appears to be discharged when you are on the field.

Re: Main breaker tripping, dead CIM
 

We are using 6cims in our Vexpro single speed, single reduction gearboxes. First time using 6 drive CIMs, and we will never go back. We havent had any problems yet, and we push all the time (and 95% win). I asked our electrical lead and she did notify me that at a few points the wires around the main breaker warmed up. We have never tripped the main breaker though even after 4 quarterfinal matches in Groton with very little break time. We have no software assistance either. Reading this thread worries me though.

Should we mount a large cooling fan above it just like we do for our compressor?

Re: Main breaker tripping, dead CIM
 

Would upping the gauge of the wires running from the PD board help at all? Say running 10 gauge from the PD board to the motor controllers, and keeping the 12 gauge lines coming out of the CIMs as short as possible? Is there a software solution that might also help, say by turning off a CIM or reducing power when a stall condition is detected? Do any of the motor controllers have a way of reporting current draw or load back to the main system?

Not with the breaker tripping. You'll lose less voltage, but 120 amps is 120 amps.

Your battery will drain less quickly, but as I said 120 amps is 120 amps.

That's an idea, but it would be hard to implement without impacting performance.

Jaguars with CAN do, and you can add external current monitors to any motor controller.

Re: Main breaker tripping, dead CIM
 

Thanks for all the advice. We've decided the safest/best option is to switch our gearing to 7.44:1 (or possibly 8:1 - any thoughts on which of these two would be better?) - we'll lose some top speed over where we were at DC, but gain a bit of pushing power and not compromise our ability to play offense. The only other solution that doesn't compromise drive performance would be to take off our roller arm and stop using the compressor, to put us back in the situation we were in at Greater DC, and we don't think that is acceptable. After all, we were only in that situation at Greater DC due to massive failure of multiple parts on the arm; it's ironic that the hardware failures, in hindsight, ultimately helped us, but we do not wish to intentionally remove most of the robot that our students spent so much effort building.

In the future, we will likely pursue 2-speed 6CIM drives; I am sold on the 6CIM drive after our performance at DC this year (we were a major contributor on the winning alliance due to our drive power and little else), but I had not realized how much strain it puts on the FRC batteries.

Re: Main breaker tripping, dead CIM
 

I agree. Changing the gear ratio is the right way to go. Don't think of it as losing top speed, think of how you are gaining acceleration. For short moves and evading defense, acceleration is what you want. We gear more conservatively than most, and haven't tripped the main breaker once in the last two seasons with a 6 CIM drive.

Re: Main breaker tripping, dead CIM
 

Could you tell me your robot weight, wheel COF, and gearing?

Re: Main breaker tripping, dead CIM
 

Here are some specs for our robot this year
weight 115 lbs
wheels 5" Colson
coefficient of friction approx 0.8-0.9 (not sure)
motors 6 CIM
gear ratio 10.4 : 1
top speed 10-11 ft/s
time to traverse field 5.1s

Re: Main breaker tripping, dead CIM
 

Is that a calculated number or did you measure it?

Full throttle from a dead stop with a freshly charged battery and only the drive motors powered?

Re: Main breaker tripping, dead CIM
 

The compressor draws a hellacious amount of current, especially when it starts. Would it be possible for you to replace the compressor with additional stored air? Another option, and this would be a fair amount of programming, would be to selectively run the compressor when the total draw isn't all that high.

Other than that I agree that gearing things down would help significantly. At least for this year speed is less important than control and power (IMO).

Re: Main breaker tripping, dead CIM
 

Changing from #12 to #10 and making everything shorter will increase your current draw, not decrease it. As you increase current demand, there is a voltage drop increase across everything in the path, wires, PD, breakers, speed controllers, and connectors. Jaguars when using the CAN bus can report current at the input to the controller. Be advised that the Jags have an over current and an under volt fault. Just like putting fans on a closed motor, trying to cool the breakers externally does little to change the trip point.
The compressors draw significant current when starting but in general are one of the least demand motors we use. The old Thomas KOP compressor would draw 25-27 amps at start and will run at 12 amps. The new Viair starts at something like 20 amps and runs at a little over 10.

Re: Main breaker tripping, dead CIM
 

It's calculated. I threw that number in there to emphasize that top speed isn't the most important parameter.

Re: Main breaker tripping, dead CIM
 

If I may ask: How did you calculated it?

Re: Main breaker tripping, dead CIM
 

How will reducing resistance increase current draw?

Re: Main breaker tripping, dead CIM
 

Ohm's Law.

Re: Main breaker tripping, dead CIM
 

Here's some quick data points, since this is turning into a "talk about your 6 CIM setup and your issues with the main breaker" thread - 4" wheels, 6 CIM singles, 6.1:1 gear ratio. Never tripped the breaker, playing heavy defense and whatnot. We did not use an on board compressor and we worked to minimize all other motor load to keep overall current draw low. Since this is the same ratio / setup as the OP, I would assert that they are either running a lower efficiency drive than us, a CIM is defective, or the rest of their robot draws too much current.

Something interesting we noted with 6 CIMs this year. We noticed a significant advantage in acceleration, but more notably we noticed that our actual top speed was higher than we expected. Everyone is aware of the 81% speed loss constant for a 4 CIM drive, but our 6 CIM drive's actual speed loss coefficient was around 90-91%. We measured an actual robot speed of 13.7 FPS with 4" wheels and a 6.1:1 gear ratio - 81% speed loss puts us in the upper 12 FPS range. It's possible that more motors actually results in less top speed loss.

Re: Main breaker tripping, dead CIM
 

We are running 6 CIM's on our drive this year. It was one of the best decisions we have made this year. We have played two districts this year, and have never popped the main breaker once. We are running 4" wheels with a 7.4:1 ratio.

One thing I've noticed is that many teams were gearing high in the past with just 4 CIM's. With 4 CIM's its harder to pop the main breaker. As soon as you bump that up to 6 CIM's, if you run the same gearing that wouldn't pop the breaker with 4 will pop for sure with 6. 6 CIM's really does need to be geared lower then 4 in order for this not to happen. And in essense, because of the MUCH faster acceleration, the time it takes to move across the field doesnt increase much as you lower the ratio and add 2 more CIM's, but the time it takes to move 1/4 of the field speeds up alot.

Re: Main breaker tripping, dead CIM
 

We were running a compressor for all of Chesapeake, which certainly would increase our current draw by a lot. We do not really wish to go back to not having a compressor, though - we were that way at DC because our ball-acquisition was non-functional.

Our drive efficiency is, as far as I can tell, quite good; there's not too much friction in the KOP belt setup. Our wires are all way longer than they should be, though, and it's not particularly feasible to fix the wiring at this point in time.

Re: Main breaker tripping, dead CIM
 

There is no reason to expect the "speed loss constant" to be the same for all drivetrains and floor surfaces, as it is substantially affected by the floor surface and the type of drivetrain design (chain and sprocket, belt and pulley, direct drive, type of gearbox, type of wheels, etc) and workmanship (chain or belt tension, proper assembly and lubrication of gearbox, wheel alignment including toe-in and camber¹, wheel axial offset², etc).

¹Toe-in and/or camber of a wheel causes the wheel sprocket (or pulley) to be non-coplanar with the driving sprocket (or pulley), and thus contributes to friction between the chain (or belt) and the sprocket (or pulley). Toe-in also causes scrubbing friction with the floor surface.

²"wheel axial offset" in this context means that the wheel sprocket (or pulley) is axially offset from the plane of the driving sprocket (or pulley), causing the chain (or belt) to be non-planar, thus creating additional friction between the chain (or belt) and the sprockets (or pulleys).

Re: Main breaker tripping, dead CIM
 

Its amazing to me how little the compressor demands. Last year we would drop a few volts after a match with our shooter only running for a short period of time, but this year the compressor is almost constantly running and we only drop a few tenths of a volt through the match.

Re: Main breaker tripping, dead CIM
 

While you're right that tripping the breaker once won't make it unusable (IE, you can still turn the robot on with a breaker that has tripped), it has been observed by my team, team 254, 67, 971, 1678 and many many others that breakers that have tripped before trip at much lower currents the second time around. The reason people are telling saying to throw away after they have tripped once is because they are much easier to trip again, not because they are literally unusable. If you trip a breaker once, it will trip at a lower amperage from then on. We've also observed that different breakers trip at different current levels, but our evidence for this is only really anecdotal.

Re: Main breaker tripping, dead CIM
 

We ran Lone Star with 6 CIMs and 2 MiniCIMs on our drive plus the old Thomas Compressor running on our robot. We were on 4 omni wheels most of the time so we have very little scrub. We have a single gear 12:72 reduction when in that configuration. When we shift to traction we are on 4" traction wheels with 2" wide rough top tread and a 18:42 belt reduction off the omni-wheel shaft. We got into several pushing matches and never popped the main breaker.

Re: Main breaker tripping, dead CIM
 

Roger,
That is anecdotal. There is nothing that happens to a breaker that trips for over current that doesn't happen during normal operation. It is possible to produce some pitting of the contacts during a high current trip but that is not likely significant unless repeated trips occur. A warm breaker will trip at lower currents whether it is new or not. That is why I encourage teams to take a full cool down period between finals matches. Warm wires, warm robot frame, motors near the breakers, etc. all will add to this phenomena.

Kevin, do you understand the Ohm's Law reference above? If not, just ask in a PM and I will explain it further if you would like.

Re: Main breaker tripping, dead CIM
 

For the past three years we have been using a spreadsheet that was made by the lead mentor of 294. It's probably posted on CD somewhere, but a quick search didn't find it for me. Anyway, the spreadsheet takes inputs such as weight, wheel diameter, motor parameters, gear ratio, wheel coF, etc, and produces distance vs time graphs among other things. We have it to be an effective tool for optimizing your gear ratio based on the distance you expect to be driving most frequently during matches.

Re: Main breaker tripping, dead CIM
 

Off topic, but if you do happen to find that again, I would love to take a look at it.

Re: Main breaker tripping, dead CIM
 

Me too.

There are many FRC drivetrain spreadsheets and models out there, and they are indeed useful - as long as their limitations are understood and taken into account.

Most of them contain at least one fudge factor to close the gap between theory and practice. Unfortunately, one size does not fit all: the correct factor(s) for one drivetrain design may be quite wrong for another. And it's not just the type of drivetrain that can substantially affect performance; craftsmanship also plays a major role.

Re: Main breaker tripping, dead CIM
 

I've been trying to read up on this - I don't quite understand this concept. How you can use an op amp to measure the voltage drop (without a resistance)? How does this work?

Re: Main breaker tripping, dead CIM
 

There is resistance associated with the main breaker. So therefore there is a voltage drop across the main breaker associated with the current flowing through it.

The resistance of the main breaker is quite small, and thus the voltage drop across the breaker is quite small.

The op amp doesn't measure the voltage drop. It just amplifies it to a level suitable to be digitized by the A/D.

Instead of measuring the voltage drop across the main breaker, you could measure the voltage drop across a length of wire.

Re: Main breaker tripping, dead CIM
 

What rule are you looking at?

Re: Main breaker tripping, dead CIM
 

Thanks. Eagle-eye Ether does it again. That is great to be able to do. I think we were called out for an extra connecter on the battery-side of the main breaker once and I just carried the incident forward. I'll be checking specifically next year for the same verbiage. It is very good to have your program know how much current is being drawn real-time. You might even consider alteration of drive power on the supposition that 120+ amps means you're stalled. YMMV of course.

Re: Main breaker tripping, dead CIM
 

FWIW, with 6 CIMS geared 6.1:1 we were drawing 180 amps while slipping the wheels.

Re: Main breaker tripping, dead CIM
 

Bill,
1 foot of #10 or 2 feet of #6 will drop 0.1 volt for every 100 amps. These two are equivalent to 1 mohm resistance.

Re: Main breaker tripping, dead CIM
 

Although your method does make mathematical sense, it will run into an error if you use this on the 120 amp breaker. You forgot to account for the fact that even the best op amps cannot read more than 95% of the reference voltage. This means that even if you used a differential resistor setup, which I assumed from your drawing, you could not use the 5 volt supply from the analog breakout. You would need somewhere around 16 volts, which means this would have to have an independent power supply. You could get around this by using a differential op amp setup on a length of wire attached to your main ground wire, but that adds unnecessary resistance.

A completly diffrent solution would be to use a linear hall sensor such as this part (http://www.jameco.com/webapp/wcs/sto...01_1915940_-1).

Place a steel washer or ring(any ferrous metal works) around one of your main wires. Then cut a slot in this steel washer and place the hall sensor in the slot. We epoxied ours in to keep it intact. The hall sensor outputs voltage which is directly related to the total robot current, and it can be plugged directly into the analog breakout. This is the system we have implemented into our practice robot and the results so far look good.

If you have any questions feel free to PM me.

Re: Main breaker tripping, dead CIM
 

You don't need to add unnecessary resistance. Just measure the voltage drop across the existing ground wire:

Or use an isolation amplifier. I'm sure a EE could figure it out.

I like your Hall Effect solution too.


[EDIT] By the way, your link is broken: it has a trailing ")"

Re: Main breaker tripping, dead CIM
 

Except that you are running several hundred amps through it and it is quite hot, while there is minimal current and temperature increase when you manually trip the breaker via the red button. Pat's suggestion that a phase change in the material takes place (some sort of plastic deformation) seems likely, considering also the difference in temperature and current through the conductive element of the breaker. Perhaps the increase in temperature and current brings the lattice above a threshold energy that allows it to plasticly deform more easily than when the red button is pressed.

Re: Main breaker tripping, dead CIM
 

Aren,
The breaker is a simple bimetallic device. As the current flows through it (and heat is generated), the contact latch cam deforms due to the expansion of two different metals. Theoretically, the breaker could exist forever at just under the trip temperature without damage. Pushing the red button merely forces the latch cam back to a position that prevents contact.

Re: Main breaker tripping, dead CIM
 

I'm aware of the operational principle. Pressing the red button to manually release the latch cam does not increase its temperature, and perhaps does not even force any deformation (elastic or plastic). We all know that this mechanical action, typically performed with the breaker closer to room temperature, does no harm to the breaker internals, since we have successfully used the breaker for years.

But things change under a trip condition when all the components are much hotter, and perhaps plastic deformation thresholds are different. It's also not clear to me whether there is any material deformation at all when the button is pressed, while there is of course significant deformation due to heating in an over current condition. This difference in conditions could conceivably lead to component damage in a trip condition while there is no damage when the button is pressed.

Re: Main breaker tripping, dead CIM
 

Remember that the deformation takes place in the latch cam. The latch is actually ties directly to the flag. It turns as the flag is moved in or out. What change may take place is on the contact area of the two parts. When a high current through an inductive load is interrupted, arcing usually occurs. While the contact is designed to survive multiple interruptions without damage, some pitting will occur with each arc. This effect is worse when the space between the contacts is very small. So the design of this device makes the opening of the contact large and quick when either tripped or opened manually.