Resurect a kr150L150 with KRC1 [OVERCURRENT ERROR AXIS 4]

axis over current indicates problem in one or more of:

1. with motor,

2. internal cable harness in the robot arm

3. robot power cable (X20-X30)

4. cabinet motor interface (drive to X20)

5. drive (PM-600)

troubleshooting is supposed to narrow it down....

for example if you unplug motor connector for that axis directly at the drive (PM-600), do you still get over current fault? if so problem (at least one) is internal to PM-600.

This is the fourth PM6-600 I know of that throws overcurrent errors specifically on A4, regardless of any motor and cable swaps, and if I remember correctly, most forum posts asking about this problem refer to A4 as well. Makes you think, ain't it?

When testing, remember that:

- KRC1-era robots control motor brakes in two groups of A1-3 and A4-6, can't open them individually and need to energize all motors in a group to be able to move just one, as the motors have to hold the axes in place when brakes disengage

- encoders have to be swapped when doing motor cable swaps, this is best done at the RDC (robot base, where the thinner cable from KRC goes in), otherwise the robot will go bonkers for a split second, then e-stop due to excessive position error

- it is safest to swap A4 and A6, both have no mechanical limits to exceed by accident

- A1-A3 are influenced by gravity and/or the counterbalance as soon as the brakes are released, but A4-A6 aren't, so you can leave a motor disconnected during jog testing of the other two (technically speaking, they are influenced, but the force is so low it can't backdrive the harmonic drives...)

In any case, it looks like you already ruled out any cabling, motor or drivetrain problems by swapping stuff around and/or leaving the A4 output completely disconnected and still getting the error. (BTW, no, you won't damage anything by leaving outputs unconnected)

On mine, which did the exact same thing and resisted the exact same attempts at fixing it, I swapped the power PCB along with the IGBT packs - I got two untested PM6-600 units very cheap on an auction site and cannibalized one of them. This helped, ruling out any problems in the logic board (on top of the power board) or the IGBT driver cards (in the middle). Before I found the donor units, I started probing stuff, both off- and online. What I managed to figure out is that the current sensing transformers on A4 looked fine, so it had to be one of the auxiliary components in the sensing circuit. This might be different for you. Then again, with this being yet another A4 overcurrent, I suspect a design fault in the power PCB causing a consistent, repeatable problem.

Now, remember, poking around in a powered, half-disassembled PM6-600 while it's connected to the robot isn't the safest thing to do even with personal protection equipment. Don't even think about trying it without *proper* PPE. You can and most likely will die if you do this. There's no galvanic isolation from the mains input in a PM6-600. That huge capacictor bank in the middle is charged up to ~550V DC (thankfully, it does discharge very quickly when power is cut). It will kill you a hundred times over without even noticing any extra load and keep running in front of your slightly smoldering, spasmodically twitching corpse. At minimum, buy certified insulated gloves. They're surprisingly cheap for how much safety they offer. SECURA ELSEC gloves should be readily available in France, for example. Use multimeter and scope probes with insulation running up to the very tip, to avoid shorting stuff. Get a grinding face shield (the ones designed to stop shrapnel from an exploding angle grinder disk, not the flimsy COVID things) and a welding apron, for when you *do* short stuff and get a splatter of the molten probe in your face. Figure out how to power and ground your oscilloscope to safely measure mains circuits, so you don't fry it. You can bench-test devices like this if you know what you're doing (guys at KUKA had to do it somehow), but it's no joke. Be safe.

And before you go any deeper, just look on eBay or whatever local French auction sites there are for cheap PM6-600s as replacements or part donors. They do show up.

this is why i suggested as probably quickest and safest tests is to just disconnect A4 motor directly at PM600 and see if the problem is still there.

of course disconnected axis will not be able to move but thisshould help distinguish if the problem is in the PM600 or not.

and if the same overcurrent fault is still shown (even with no cable or motor for that axis connected to PM600), then problem is in the PM600 itself... so spolution is to replace or repair the PM600

but if the fault is NOT due to PM600, everything else should be able to function (enabling drives, jogging other axes) and ... there will be no overcurrent fault displayed any more (other faults may still be there). in this case check the wiring, is the internal wiring between PM600 and X20 ok? check for corroded, deformed, pushed out contacts, on each section. check if the motor connections to PM600 are in correct order for that robot type. brake circuits are jouned check brake circuit resistance with and without affected motor. it must be lower when all three motors are connected, disconnecting any of the three motors in the group means fewer brakes are connected in parallel so measured resistance will increase. if there is no change, brake circuit is not ok so replace that motor.

Torque and current are directly related on synchronous servo motors, so the current measurement is in fact the same as torque measurement. What you found looks like it could be the cause.

I'll try to dig up the PCB from my old broken PM6-600 and check the A4 transformers. Which one of the two is broken on yours? They should be labeled I11 and I12, I think. I'm suspecting there might be something causing an intermittent open circuit between the transformer's secondary and opamp inputs on the logic board. Some current transformers are susceptible to damage from operating with no load on the secondary. I'm not sure if that applies to the type used in PM6-600, though. EDIT: this might be a bit more complicated, as those are active transducers (LEM LE 55, https://www.lem.com/sites/defa…_datasheets/la_55-p_e.pdf ) but the internal schematic suggests they could still be vulnerable to high voltages being induced in the secondary with the output disconnected.

PM6-600 schematics are available online if you know where to look. I have them, but I'm not sure what the forum policy is for uploading things that certainly weren't meant to be published in the open and probably got leaked at some point.

good job.

what kind of spring movement? is the robot bolted down? is there anything attached to the robot flange?

Sudden stops when jogging manually are jerky and springy because the robot has no way of anticipating the need to decelerate before a stop. Doubly so when you let go of the dead man's switch on the pendant while the robot is in motion - if I understand KRC1's logic, this is a category 0 stop as the drives contactor opens and motor brakes slam shut with no advance warning whatsoever. The arm will get jerked - and that's an actual term in physics - by inertia and there's nothing you can do about it, several hundred kilos of steel and aluminum swinging on a long stick can't be stopped instantly and can't be perfectly rigid.

You can start being concerned if you see the same problem with programmed movement.

tmuthesius I just had the same thing happen to my KRC1. Did you find out what caused it? And were you able to repair it? I don't have a schematic for that board, but I am suspicious that a pair of dead batteries (computer backup power) might have caused too high of a current flow. Looks like that board generates the 27V rails that charge the batteries

.

hmmm.... how did you get clue like that? reason dead batteries are called 'dead' is because not enough juice left in them and they cannot deliver high current. when batteries get old, internal resistance increases and acts as a current limit. this causes large voltage drop too.

but this high internal resistance is not just there when drawing current from battery, it has the same effect when trying to charge battery.

Yes, typically batteries fail high impedance, but not always (loose lead plates and internal corrosion can cause low impedance failures)- but seeing how there are two in series, you're most likely correct that the impedance is high and unlikely to cause a failure in that direction. Alternatively (again, just stipulating - don't actually have a schematic) a high impedance battery on a current controlled charging circuit could cause a rail to go beyond expected voltage. (Yes, it's a total BS guess- but i'm looking for reasons why [what i think is was a MOV] the component failed). Time and heat can also dry out electrolytic capacitors, but they look fine externally. Power supplies often fail because of inappropriate or damaged loads attached to them, so that was my first suspicion. Open to alternative ideas as well.

you are correct, batteries can fail to low impedance too but this is far less likely as even cells within one battery are in series. don't think i have ever seen one but... i have seen plenty of batteries that fail to high impedance.

yes that looks like is is/was an MOV. can you post more pictures? top and bottom of the PCB, and some nearby components including their rating/values? collected dust could lead to problems too. Alu capacitors are least stable type of capacitors, they lose electrolyte and capacitance values becomes lower, can short as well. even dry capacitors like paper etc. decay over time as the dielectric decays, but that is on a significantly longer time scale.

very good....