Ok guys, you might remember almost a year ago I built this device for monitoring cables in production. I used an arduino. I am going to be building some more of them, and as I started to get back into the project I realized that it's a pretty simple operation and not much need for a μC.
To give some background, I'll quote the important parts from last year's post:
Explanation:
EDIT: Failed to mention, the comparator, the Optocouplers, the AND gate, and DC/DC converter were just picked at random from Eagle. I don't know what to use there. The DC/DC coverter will be a HITEK GMA, I already have on order, but it was not in the eagle library, and the optocoupler will be a Avago HCPL-4731-300E but was not in eagle..
To give some background, I'll quote the important parts from last year's post:
So I've drawn up this little schematic which I believe should do the same thing (eagle schematic attached):I have designed this little board which will monitor a 7 conductor cable during a specific stage of production. During this stage, the cable (between 3,000ft & 40,000ft) is having heavy duty steel armor wires wrapped around the outside of it and there is a potential for one or more of these steel armor wires (if the tension is not set properly) to cut into the cables, resulting either in a direct short from conductor to armor wire, a high resistance short from conductor to armor wire (think armor wire compressed almost all the way through conductor insulation) or an open caused by armor wire severing one or more of the conductors. The purpose of this board is to detect any of the above scenarios and shut down the machine so that a repair can be made before it's too late, or so the cable can be scrapped before all the supplies are used up. The armor wire contacts the machine, which is bolted to the ground, so armor wire = chasis ground = earth ground. It is controlled by an arduino microcontroller and has 2 relay outputs; one for the 120V machine run signal, and one for a 120V red/green stack light.
The basic theory of operation is that 1,000v from the DC/DC converter will be sent out on the center conductor, weave back and forth through the rest of the conductors, and return on 2 conductors. the DC/DC converter can only supply 1.5mA, so it should be able to push all 1000V across 666KΩ; any lower resistance and voltage starts to drop off. In the circuit, I have 10MΩ of resistance, (plus 500Ω-5KΩ for the conductor), so that means that theoretically I should be able to start to detect a DC/DC voltage output drop (leakage from conductor to armor wire) around 715KΩ. The voltage recieved back on the 2 conductors is conveyed to the microcontroller via a darlington optocoupler. I am aware that the output of the opto is not linear, but at the specific current resultant of the 20MΩ resistors, it seems to be the "butter zone" and testing has shown that I get a useable analog output from it - doesn't need to be exact. The μcontroller recieves the 2 analog signals (inverted signals) from the opto and compares them to a value from a potentiometer; this value would be the user-set tripwire - if the value of either opto channel is > than the pot value, there is a problem: either there is a short to ground or an open.
Explanation:
- Operator holds down the START/CHARGE button until he sees the green light. It takes some time for the wimpy output of the DC/DC converter to charge the big capacitance of the cable.
- As the voltage on the cable increases (charging the capacitance), the voltage at the + input of the comparators drops. When it drops below the threshold set by the pot both of the comparators should go high.
- Because of the AND gate, when both comparators go high, the 4PDT relay should switch.
- When the relay switches, is when the operator sees the green light, and then the relay bypasses the push button and the operator can let it go.
- It should stay in this mode until there is a fault, at which one or both of the comparators will go low, and the relay will drop out.
- Whether I got the comparators hooked up right.
- What kind of caps I need to use & where.
- If the HCPL-4731-300E optocoupler will function in the same "butter zone" with 12V as it did with 5V.
- what comparator to use
- what AND gate to use
- If the idea is sound.
EDIT: Failed to mention, the comparator, the Optocouplers, the AND gate, and DC/DC converter were just picked at random from Eagle. I don't know what to use there. The DC/DC coverter will be a HITEK GMA, I already have on order, but it was not in the eagle library, and the optocoupler will be a Avago HCPL-4731-300E but was not in eagle..
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