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Please review my fault tester design.

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strantor

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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:

strantor said:
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.



So I've drawn up this little schematic which I believe should do the same thing (eagle schematic attached):
View attachment 63821

the comparator, the Optocouplers, the AND gate, and DC/DC converter were just picked at random from Eagle. 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..

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.


Things I'm not sure of:

*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.

Thanks
 
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hi,
A small point a LM393 output will go into inversion if an input exceeds ~ Vs-2V.

E
 
hi,
A small point a LM393 output will go into inversion if an input exceeds ~ Vs-2V.

E

Thanks, Good to know. I won't be using that one then. I anticipate that the input from the pot will be nearly Vs after adjustments are made. I guess I should be looking for a 'rail to rail' op amp?
 
you should use resistor from the gate output to base of transistor. you don't show supply for 74xx chip but you cannot use 74xx gates on 12V, that one only goes up to 6V. if you want to stick with 12V source, you are better off using CMOS gate, some of them work at levels up to 15 or 18V. (CD4093B for example, you can use remaining gates in parallel as an inverter to convert NAND to AND and get higher output current). if you use 74xx then you need voltage regulator to get 5V. when using gates, read their datasheets.

also comparators require pull up resistors on outputs (open collector outputs) - check datasheet.

alternatively you can connect outputs of both comparators together to a base of transistor and use just one pull-up resistor (2.3k or similar). if either comparator output is low, transistor will be off. only when both are high, resistor will bias transistor base and turn it on. but check datasheet

you don't need to invert signals by optocouplers, you can use positive logic, just connect pins 8,6 to positive, and use 7, 5 as outputs (with resistors to ground).

you have no resistor directly after DC/DC converter so faulty cable can send shocks to those that handle it or ruin the tester. 1kV at 1.5mA do not sound appealing to me. your schematic shows wrong part number for dc/dc converter (that one does not go to 1kV, check datasheet).

your converter may be wimpy, but it seam to be stressed a lot if it takes several seconds for it to reach specified output (part number on schematic does have overload and short circuit protection but this should not be abused, in general this is meant as protection from ocassional accident rathet than operation on every cycle, check datasheet to be sure). Chances are that it may not last and then (sooner or later) someone may replace it with one that is not so wimpy.

currently there is only start button and a dangerous one - someone leaning on the start button or wiring problem or start button that is stuck would mean continuous HV output, regardless of state of K1 or lamps or even estop buttons (i only see them on older design). that should NOT be allowed.

there is no indication that HV output is present or that DC/DC converter is running. i would want to know if this is the case - otherwise I'm pretty sure that I would not want to be the one to replace that spool of cable.

i would expect additional means to remove HV and ground all cable terminals during cable replacement (lockout disconnect with padlock provision is common for such things).

the image of last year project looks very bad. for example:
- terrible soldering (hope this is not production model),
- small pitch terminal blocks (probably rated only 300V, maybe even less),
- no HV isolation/lockout switch (should have at least one, preferably additional one at slipring) and no indication of HV presence or converter operation,
- HV output with no resistor
- there are two mushroom operator buttons, presumably estop buttons, going to unmarked terminal of arduino controller. if this is pair of estop buttons as i suspect, this is a prime example of very common mistake which would (should) be caught by any engineer or PHSR inspector. but i doubt that this ever went through any kind of inspection even though i am sure texas has some laws on this. in a nutshell: estop (or any other safety device) has to be hardwired, it must remove power to a 'dangerous device', not be a mere signal to a controller (such as PLC or in this case arduino). hardwired means it cannot rely on some software to turn off outputs. proper way is to have those estop contacts in series with relay contact A. if monitoring of estops by controller is needed, those buttons can be installed before relay contact A, then have voltage divider right after them to brings their status to arduino input. this way estop still works regardless if arduino code is ok or not, regardless if arduino froze or not, if it is being programmed at the moment or not, regardless if relay A is on or off, regardless if transistor has failed (short) and still keeps relay energised and regardless if relay contact A is welded.
btw. there are also safety PLCs (or in general safety controllers), they use various technologies to ensure safety that is comparable to hardwired safety circuits. for example they use redundancy, crosschecking, libraries of safety functions that are scrutinized and certified by proper authorities, and then they may get SIL ratings. arduino and any normal controller (PLC, PC) is not safety product and cannot be used as one.
- no mounting holes or eclosure and there are components at the very edge of the pcb on all four sides (and yet, this thing has AC and 1kV)
- no insulation on the bottom or at least stand-offs, solder side of the board sits on the the desk, while wires are hanging from it down the desk edge,
- diodes with very long leads, plus they are riding on top of relays and in close proximity to transistors (exposed tab), should be mounted close to pcb so that cannot be moved around and reach other components.
- reel of monitored cable is grounded (no way to check for ground leaks). this in itself is not too bad because in worst case they will just make a lot of scrap before they notice problem.
 
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Panic Mode,
Sorry I don't check ETO every day and didn't realize anybody had posted on this old thread.

you should use resistor from the gate output to base of transistor. you don't show supply for 74xx chip but you cannot use 74xx gates on 12V, that one only goes up to 6V. if you want to stick with 12V source, you are better off using CMOS gate, some of them work at levels up to 15 or 18V. (CD4093B for example, you can use remaining gates in parallel as an inverter to convert NAND to AND and get higher output current). if you use 74xx then you need voltage regulator to get 5V. when using gates, read their datasheets.

also comparators require pull up resistors on outputs (open collector outputs) - check datasheet.

alternatively you can connect outputs of both comparators together to a base of transistor and use just one pull-up resistor (2.3k or similar). if either comparator output is low, transistor will be off. only when both are high, resistor will bias transistor base and turn it on. but check datasheet
That's what I ended up doing, got rid of the gate and or'd the outputs straight to the base of the transistor and with a 2.2kohm resistor.
you don't need to invert signals by optocouplers, you can use positive logic, just connect pins 8,6 to positive, and use 7, 5 as outputs (with resistors to ground).

you have no resistor directly after DC/DC converter so faulty cable can send shocks to those that handle it or ruin the tester. 1kV at 1.5mA do not sound appealing to me. your schematic shows wrong part number for dc/dc converter (that one does not go to 1kV, check datasheet).
yes the part number is wrong. The actual part is Hitek GMA12-1KPE

your converter may be wimpy, but it seam to be stressed a lot if it takes several seconds for it to reach specified output (part number on schematic does have overload and short circuit protection but this should not be abused, in general this is meant as protection from ocassional accident rathet than operation on every cycle, check datasheet to be sure). Chances are that it may not last and then (sooner or later) someone may replace it with one that is not so wimpy.
good point, I will need to talk with the manufacturer about this.
currently there is only start button and a dangerous one - someone leaning on the start button or wiring problem or start button that is stuck would mean continuous HV output, regardless of state of K1 or lamps or even estop buttons (i only see them on older design). that should NOT be allowed.

there is no indication that HV output is present or that DC/DC converter is running. i would want to know if this is the case - otherwise I'm pretty sure that I would not want to be the one to replace that spool of cable.
There is a stack light wired to the the relay's other contacts, so the operator will know if voltage is applied. I should have drawn that.
i would expect additional means to remove HV and ground all cable terminals during cable replacement (lockout disconnect with padlock provision is common for such things).
That's worth bringing up. I'll ask and see if they want that.
the image of last year project looks very bad. for example:
- terrible soldering (hope this is not production model),
can't argue there, but it works.
- small pitch terminal blocks (probably rated only 300V, maybe even less),
.1" not sure the rating, but I left gaps between the HV stuff to be safe.
- no HV isolation/lockout switch (should have at least one, preferably additional one at slipring) and no indication of HV presence or converter operation,
- HV output with no resistor
If we think of the cable as a big capacitor, does it matter whether the circuit has a resistor (it does on the input, 20meg)? Because if someone were to contact the conductors, that big capacitor would be discharged through them to ground, totally uneffected by the circuit resistor.
- there are two mushroom operator buttons, presumably estop buttons, going to unmarked terminal of arduino controller. if this is pair of estop buttons as i suspect, this is a prime example of very common mistake which would (should) be caught by any engineer or PHSR inspector. but i doubt that this ever went through any kind of inspection even though i am sure texas has some laws on this. in a nutshell: estop (or any other safety device) has to be hardwired, it must remove power to a 'dangerous device', not be a mere signal to a controller (such as PLC or in this case arduino). hardwired means it cannot rely on some software to turn off outputs. proper way is to have those estop contacts in series with relay contact A. if monitoring of estops by controller is needed, those buttons can be installed before relay contact A, then have voltage divider right after them to brings their status to arduino input. this way estop still works regardless if arduino code is ok or not, regardless if arduino froze or not, if it is being programmed at the moment or not, regardless if relay A is on or off, regardless if transistor has failed (short) and still keeps relay energised and regardless if relay contact A is welded.
btw. there are also safety PLCs (or in general safety controllers), they use various technologies to ensure safety that is comparable to hardwired safety circuits. for example they use redundancy, crosschecking, libraries of safety functions that are scrutinized and certified by proper authorities, and then they may get SIL ratings. arduino and any normal controller (PLC, PC) is not safety product and cannot be used as one.
The mushroom buttons are not an e-stop, they are a start. I made them NC and inverted the logic in the arduino so that I could wire them in series so that I could use wires that were already ran in the conduit and not have to run one more wire. The entire circuit is energized/ deenergized through a seperate relay (not drawn).
- no mounting holes or eclosure and there are components at the very edge of the pcb on all four sides (and yet, this thing has AC and 1kV)
- no insulation on the bottom or at least stand-offs, solder side of the board sits on the the desk, while wires are hanging from it down the desk edge,
The board was mounted with standoffs in an enclosure after the fact. As far as the components near the edge, I had no idea that was a no-no and I promise I won't do it again. :) thanks.
- diodes with very long leads, plus they are riding on top of relays and in close proximity to transistors (exposed tab), should be mounted close to pcb so that cannot be moved around and reach other components.
That's a big DOH! Should be obvious, no? I never thought about the diodes leaning over and touching the transistor tab, but I am thinking about it now.
- reel of monitored cable is grounded (no way to check for ground leaks). this in itself is not too bad because in worst case they will just make a lot of scrap before they notice problem.
Really don't know what you're talking about here. You're right, the reel is grounded, but that's how I'm checking for ground leaks. After the initial charge-up, if any current is flowing, it's flowing to ground, and that's a fault. :confused:


Thanks for the great feedback sir!
 
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