Glenn, thanks for that troubleshooting story. I love hearing stories about someone who uses logic to solve problems. Troubleshooting skills, which included logical thinking, was one of the things I learn in community college but not much was taught to students when I attended engineering school. As an engineer, I find more times than not we are expected to troubleshoot design and other issues where troubleshooting skills are definitely needed. Thanks to my community college school training, I have solved numerous longstanding problems with just the simple skills I learned in community college.
Sounds like you have some great instructors, Gsmith120. This is a consistent theme in the Sherlock Ohms blogs. Logic used with discipline and thoroughness seems to be the specical sauce for solving engineering problems.
Some of the power wires went to operator panels, some to sensors, some to solenoid valves. Some proxi sensors have an LED indicating the sensor has power, and some others have an LED to indicate it is detecting the target. Any section of the machine that had LED's on had to be okay. When I noticed sensors without LED's on, I just had to look more closely at that area. It just happened that the sensor had obvious damage from weld expulsions, and was in a location where it was readily visible. If the sensor had been hidden, or there had been another sensor that was also shorted, the story would have been longer.
putting a higher wattage lightbulb is a trick that I was taught way back in the 1950's, when we had fuses at home and it was a great saver. I used that exact technique just this past Monday for servicing a house-wiring compaint that the circuit breaker would not stay reset.
The faster way to find the short would have been using the "divide and conquer" approach, which would be to split the hundred terminals into two groups and check each for shorts, then split the shorted group in half and check again. Just two checks and disconnecting three places cuts the number of suspects down to 25 from 100.
Of course, in some machines the switches circuit feed wire is daisy-chained through the remote wiring, which can make it a bit more work because of needing to open remote boxes.
The one other technique that I have seen is to use a hundred watt lamp with a flasher and a clampon ammeter to determine which wire is drawing the current. That method allows for checking groups of wires without any unhooking, which sometimes reconnecting wires can be a pain.
Of course, good controls design would always separate output device power feeds from sensor switch feeds. That way the E-Stop could switch off all the motion drive functions.
I agree. It's nice to see an example of a well run and methodical investigation into a failure. So often, someone goes charging off towards a fix without proper information: an approach I've often heard described as "ready"-"shoot"-"aim". Well done and well described.
and listen for the sound. That would identify a short to ground (or shielding if shielded cable, which would then be terminated to ground at the source point.)
I like to use the diode check, as do not have to look at the screen. Using this and the "bundling" technique, and you can soon locate the short. Have done this in many control cabinets.
Can use Ohm mode to see level of resistance as you described. The light bulb check does help to identify a short as opposed to a flow to ground through another component, but I have seen where using the bulb actually created enough working load that it burned the few wire strands that were contacting the ground, and then went out. Now you measure to ground, and the wires that had shown a short do not, and you have still identified the faulty wire.
Of course, back in the old days, many of us did not have multimeters and it was standard practice to use bulbs for testing, including to trace wires, etc. on our automobiles.
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