1726 Electric Problem

[Matt H.]

As anyone who watched the Arizona webcast (or was at the regional) knows team 1726 put on a great show, but would intermittently stop working. We believe this is an electrical problem (something we have never encountered before). When the robot stops working the victor lights turn off and the victor fans stop, but the pneumatics system and compressor continue to run and operate. Jostling the robot will result in the victors receiving power for an intermittent amount of time before the robot dies again. This originally led me to believe that the problem was a loose connection in the maxi fuse panel wiring (from power distribution to fuse panel to victors back to power distribution), but after re tightening all the connections the problem persists. There is no obvious evidence of a short such as melted wires and when examined the connecting wires appear to be in excellent condition.

Even more confusing when operating with the robot on blocks we are unable to recreate the problem.

So now I have several questions

Could this be a problem with our forty amp breakers, on Thursday we used nitrile tread and due to the high torque tripped our breakers continuously for several matches before recognizing and fixing the problem. Is it possible that some of the breakers are now damaged and trip very easily? It seems strange that all four would fail at once (as we observed however tripping one breaker could result in others tripping as we use two motors/gearbox) and that bumping the robot would reset them, does anyone think this is the problem?

If the above could not be the problem how would you recommend we trouble shoot. A multimeter simply isn't enough because it is highly possible that we have conductivity, but not enough current carrying capacity. Is there a proper way to trouble shoot this situation?

[Vikesrock]

I'll throw my two cents in here, I'm sure other more knowledgeable people will pop in with some more suggestions later.

Regarding the breakers, it is possible that they are now easier to trip after being tripped repeatedly. The thing that makes me wonder if this is really the issue is the jostling the robot thing. Either that is a coincidence or your issue lies elsewhere.

It makes sense that you cannot recreate the issue on blocks as your motors will raw far less current when on blocks than it will on the carpet when pushing or turning.

If it is just the victors that are losing power, one thing I would make sure to check is the Rockwell Distribution block. If the wire to the maxi panel comes out of a different block than the wire coming in from the main breaker, check and recheck the jumper block. That yellow block can cause all sorts of issues if not seated well and tightened down properly (my team had this issue this year).

Hopefully some others can add to these suggestions.

[Matt H.]

Wow I can't believe we didn't check that... I'll add that as one of the first things we need to check once the robot comes out of the crate in LA. If the yellow jumper block is the problem how would be best fix that and ensure that it does not become a problem again?

[Vikesrock]

Originally Posted by Matt H.

Wow I can't believe we didn't check that... I'll add that as one of the first things we need to check once the robot comes out of the crate in LA. If the yellow jumper block is the problem how would be best fix that and ensure that it does not become a problem again?

On our robot, the problem was simply not tightening down all the screws on the jumper block, tightening them down solved it. If the jumper block looks "fried" in any manner (I would pull it out and test it for resistance if the screws are tight) then I would replace it (I believe there are replacements available at Pit Admin if you or other teams don't have one).

[ay2b]

I don't know if any of this will help, but:

1717 had what I think is a similar problem - they would be driving and then all four of their drive motors would blink out simultaneously. I believe they eventually fixed the problem, but I don't remember the solution. They were also noticing a difference between on tether with the OI running off AC power, on tether with the OI running of the robot's battery power, and on radio with the OI running off the competition port power.

980 was also having electrical problems - we'd be driving along and then the RC would just reboot (sometimes several times in a second) or randomly turn off relays. (This reeked enough havoc to our auton mode, that the ref gave us a warning and we decided not to run autonomous until we'd fixed the problem.) We discovered that the red wire to the Anderson connector on the robot was very warm, so we think we were getting an intermittent connection there. We also discovered that the ground wire to the backup battery was loose within the plastic housing (although the "bad backup battery" light never came on). We replaced the Anderson connector on the robot, fixed the loose backup battery ground wire, re-downloaded the master code to the RC, added some printf() statements to the user code, and the problem seemed to go away.

[dtengineering]

We had some odd electrical problems in Oregon this year. Fortunately, most of them occurred on Thursday.

The first one was due to the RC main power spade connector ground being loose. This was due to an incident during build where the spade connector was pulled out of the RC. We thought we had it soldered back in well, but apparently we were wrong. We re-soldered it (just from the top this time) and it has held so far. We re-check it regularly now.

The second was due to a poor connection between the battery and the 120A circuit breaker. This resulted in our main breaker getting hot and tripping. The key that it was a poor connection (that we eventually clued in to) was that the stud on one side of the 120A breaker was hotter than the stud on the other side. The challenge in diagnosing this may have been compounded by loose connections in the Rockwell block, as mentioned in a post above, as we found a few of them needed an extra turn of the screwdriver or two.

We are fortunate to have a clamp on ammeter, as that allows us to easily monitor current flow in the motors/circuits. (If anyone is thinking of buying one... make sure you get one that does DC current as well as AC.) Ours has two settings 4A and 80A, so at maximum load we can only monitor the current to one CIM. Using the ammeter we were able to recreate medium to high current draw situations in the pit by having a couple of team members kneel in front of the robot and draw the bumper up tight to their legs. The driver would then gradually add "forward" to the robot until the motors got up to about 40A (in stall) and hold it there for about ten or 20 seconds. Another option would be to drive the robot up against a wall and push for a while. Again, it is nice if you can monitor current draw for at least one motor so you have an idea of how hard you are pushing the motors/electrical system.

We discovered (technically these probably aren't discoveries to the rest of the world, but they were enlightening to us) that with two CIMs hooked up to a toughbox that when the wheels run freely, one CIM would be drawing 5A and another would draw pretty close to zero. Differences in the motor? The Victor? We haven't figured that out yet, but the discrepancy seems disappear as the load is increased.

We also discovered that the eventual solution to our wiring problem, although obvious in retrospect, was the very last thing we checked... even going so far as to pull a gearbox apart to check if anything was wrong in there. (Okay, we just pulled a motor and peeked in... but we were most definitely looking in the wrong place.)

We also had a weird situation where the robot died during a match... but came back to life after hitting "robot reset". This is one of the advantages of experimenting with buggy code... the drive team gets to know the location of the "reset" button very well! If you conk out in the middle of a match, give it a push... you've got nothing to lose and it might just get things going again.

Good luck getting the wiring sorted out, intermittent wiring problems are a real pain and it is a shame they held you back. Mind you, it still looks like you shared a pretty good playoff run with a couple of rookie teams. Well done, and good luck,

Jason

[RyanN]

I would definitely check the wire connections from the Distribution Block to the Circuit Panel, the wires tend to come loose after a lot of matches. A way to diagnose this would be to run the robot for a while (or at least try) and then feel the wires... or you could just try to tighten the wires up.

As for the breakers becoming weak; YES! it can happen. In fact, this happened to us last year during Nationals, and the simple fix was to replace all of them, but if all the Victors turn off with their fans, this points to a wiring problem. I believe all you need to do is tighten the screws that hold the wires in place. I would also recommend that you move all the heavier loads to the same block that the main power is fed, then all the smaller stuff run off of the jumped block, because I see a lot of power being fed through such a small piece of metal (80 AMPs with 2 CIMs at stall, 160 AMPs with 4 CIMs at stall; also those numbers are likely to be higher as the breakers aren't instantaneous)

[Qbranch]

Also, check your crimps on your battery connector. This is probably the most mechanically stressed connection in most people's machines as it is the only one which gets moved every time a battery is changed. Also, it is the highest electrical load connection in your robot. I suggest checking both your ground and positive terminals going into your distribuition and breaker blocks respectively.

Our positive terminal came off of our robot at the end of our first semifinal match in Chicago... thankfully we noticed it when it fell out of the robot while changing the battery!

Then again, we also had the eh.. small problem... of one of our ultrasonic sensors catching fire... but that's another story entirely.

Hope you find your problem! If all else fails, tug on wires and see what pulls out/moves easily.

-q

[David Brinza]

Connectors are generally the weak link in electrical systems.

In the space business, people are trained and certified to mate and demate connectors. Before connectors are mated, the contact mating force is verified for each pin/socket, the connectors are visually inspected for bent pins/sockets, both sides of the connectors are cleaned to remove any debris (a little shard of metal can be disastrous) and the proper alignment of plug and socket is verified.

I'd check all connections (including the power distribution block) carefully before blaming the problem on other components in the electrical system.

[Tapoore]

Originally Posted by David Brinza

Connectors are generally the weak link in electrical systems.

Funny story: this year, our hurdle bot weighed only 87 lbs when we got to the competition. We decided to add some weight to get it closer to 120lbs using a couple of dumbbells. We figured that the easiest way to "secure" them would be to use zipties. We put two dumbbells in the front and one in the back near to our electronics board. During our final qualification match, we slammed into the alliance wall pretty fast and the dumbbell came loose and unplugged all of our motor connections! Ooops...

[Qbranch]

Originally Posted by Tapoore

Funny story: this year, our hurdle bot weighed only 87 lbs when we got to the competition. We decided to add some weight to get it closer to 120lbs using a couple of dumbbells. We figured that the easiest way to "secure" them would be to use zipties. We put two dumbbells in the front and one in the back near to our electronics board. During our final qualification match, we slammed into the alliance wall pretty fast and the dumbbell came loose and unplugged all of our motor connections! Ooops...

As a quick aside:

Have you tried running your robot without the extra weight? Since there isn't as much defense this year, you may gain critical extra performance from your robot's drive system.

-q

[EricH]

Troubleshooting: Find source of problem.

This means checking all the wires going into the Victor. Trace them back to wherever they originate. Test the breakers, the connections, everything. All wires includes the PWM cables, by the way, even though I'm fairly certain they aren't the problem.

If you don't find the problem before the breaker panel, figure out where everything on that panel goes. Do any of those have problems? If so, it's probably in the panel or "upstream" somewhere. If not, check the breakers.

[DonRotolo]

Troubleshooting:

When you have a circuit that loses power, there are several methods of identifying the problem. One method, while not simple, is very fast and effective:

Determine how many different things there are in the circuit. For example:
Battery, anderson connector, main breaker, wiring block, yellow thingus, circuit breaker panel, victor, motot connectr(?) and motor. That's 9.

Get several 12 volt LEDs (use a 680 Ohm dropping resistor). Put all of the LEDs in a row and make them visible to an operator or observer.

Connect an LED using clip leads at each point in the circuit where there is a change in the path - at the input to the Anderson connector, the input of the main breaker, the input of the power block, etc.

Duplicate the condition - perhaps on the practice field, I can't say these LEDs are competition-legal.

Watch which LEDs are not lit when the condition occurs, the will lead you right to the problem. Works every time, this is a common technique in automotive repair when time is of the essence. Videotape the LEDs if necessary, to catch very fast transients.

Don

[Al Skierkiewicz]

Wow, this thread went pretty far in just a few hours.
If the drive motors are all cutting out at once, this points to something in the 40 amp wiring, if you are using that. Remember that the black wiring is just as important here as the red wiring. That being said, the problem could be in either the 40 amp block or the Rockwell block. In the order of failures these problems are:
1. Improperly stripped and terminated wiring in the Rockwell Block. Strip 5/8" and do not tin the wires before insertion.
2: Improper termination of the input side of the 40 Amp Maxi Block. Better termination can be made by stripping the input wire about 1-1/4" and folding over the stripped wire before insertion in the Maxi-Block. If you use a #6 wire straight into the block, the strands tend to ride the threads of the clamp screw up the side of the hole and prevent them from becoming part of the connection. Folding over the wire, fills the hole and allows the best clamp when the screw is tightened.
3. Loose jumper strip mounting hardware.

If power is being lost to other robot systems or if your Anderson connector is getting hot, the likely cause is scratched surfaces on the Anderson connector. Often teams use the alligator clips on the charger to connect to the open Anderson connector on the batteries. This raises scratches that reduce the amount of current carrying metal to a fraction of the design surface. The resulting high resistance connection causes heat and a voltage drop across the connector. Spend the money to buy an extra Anderson connector kit. Remove the alligator clips and replace them with the Anderson connectors. You will have to strip the wires from the charger at least two inches and fold them twice to be able to solder into the connector if you con't have the proper crimper.
Breakers do go, bad but rarely. Hot breakers trip more easily than those at room temperature. These are temperature contolled devices after all. (that includes the main breaker.) Hot wiring conducts it's heat to the breaker it is connected to.
If the problem exists on the floor and not on blocks you should suspect loose wiring that is pulled or pushed by the robot frame when on the blocks. Or on rare occasions, wiring is contacting the frame and is shorting out that branch circuit. Suspect any wiring that passes through frame tubing. An errant drill may have put a hole in the tube and damaged the wiring running through it.
As to unbalanced current in a two motor transmission, suspect that the speed contollers are not calibrated. Although rare, a motor does go open and therefore no current flows. If the current draw does approach the same current at a higher speed or load, check the calibration. It is simple to move your joysticks while watching the LEDs on the Victors. Watch to see that both light the LED fully at the same point on the joystick.

[MrForbes]

Thanks for the suggestions, Al. I didn't really get a chance to get into trying to figure out what was going on during competition Saturday afternoon, but we do have some things to look over when we get to LA. We have Andersen connectors on our chargers, we have not used the clips at all.