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Testing rear window defroster grid

Former Member

Hello,

I would like to develop a testing method to detect window defroster grid failures. Rear windows has 12 heating wires painted to the glass and cca. 2% are faulty pieces.

I tried to measure the resistance, current and voltage drop of good parts and compare results with those where was at least 1 broken wire, but the difference was smaller than the tolerance specified by the supplier. (Other problem is that the resistance is greatly depending on the temperature)

Now we are testing visually with a thermo foil but it takes more time and sometimes the operator mistakes or can forget to correctly check thermo foil. It is also very important to not hurt parts during the testing process, so best option would be contactless testing if possible... image

I'am thinking about to make a tool with 12 sensors, 1 sensor to every each heating wire to sense a flowing current in a wire. But I'am not sure that there is any sensor avaible in the market for such a purpose. Finally PLC have to check the outputs of every sensors, if all the 12 wires are good (current is flowing through them) gives visual signal or in case of failure sound alarm or red light flashing.

 

Here is the specification of rear window defroster:

Power: 167,37W ±10%
Current: 12,87A ±1A
Resistance: 1,0 Ohm -10%/+20%
Wire thickness: 0,48mm
In the attachment you can find technical drawing of the window.

Any ideas and suggestions are welcome.

Best regards,

Ate

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  • 0

    Hi Iansm,

    I borrowed a Fluke 80T-IR Infrared Temperature probe from my sons shop to see how accurately I could read the temperatures of the individual wires in my rear window. The 80T-IR will plug into any handheld DMM. It outputs the temperature to the multimeter at 1 mV per degree. The scale is selectable C or F by a switch in the battery compartment. I was able to easily read the temperature of each of the wires in the window as they heated up. This temperature probe is 22 years old and they are still available on EBay for $60.00 and up. The more modern probes are probably more sensitive and accurate. A person could easily check each of the 12 wires in the window in the time it would take to position a test jig that would check all 12 simultaneously. Something to think about.

     

    image

     

    John

  • 0 in reply to jw0752

    Just to add to your thoughts....

     

    If the voltage/current was regulated, any broken wires would mean it falls below spec.

    Would this change of 1/12th of the resistance show up.

     

    The other thought was using something like a tuned circuit over each wire.

    If the wire is intact, it will detune it like a shorted turn.

     

    Mark

  • 0 in reply to mcb1

    Hello,

    Thank you so much for your time and efforts.

    I uploaded a video to show you how we are testing heating wires currently. Visually it is totally OK, fast and accurate, thermal paper almost instantly changes its color. I have to extend our PLC controlled Poka-yoke system with Go/no-go testing to automatically block broken windows from further processing. With a regulated power supply the difference between 12 and 11 wires is very small. I measured current consumption using a non regulated power supply and the difference was clearer, 5-8% less Amps but this is still much smaller than the tolerance defined by the manufacturer. So I am afraid that 2 weeks later for example from the next delivery all parts could be evaluated as a broken ones. That's why I'm trying to detect wires separately. Than summarize outputs into one digital go/no-go signal.

    Thermal camera I think would be very problematic, can catch fingerprints, or glass can be accidentally preheated, in this case the contrast will be insufficient. We are using infrared thermometers, pyrometers and heat switches in other departments in the factory but I thought that is difficult to measure glass surface, and these kind of sensors detects bigger area not just 0,5mm thickness.


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  • 0 in reply to mcb1

    Hello,

    Thank you so much for your time and efforts.

    I uploaded a video to show you how we are testing heating wires currently. Visually it is totally OK, fast and accurate, thermal paper almost instantly changes its color. I have to extend our PLC controlled Poka-yoke system with Go/no-go testing to automatically block broken windows from further processing. With a regulated power supply the difference between 12 and 11 wires is very small. I measured current consumption using a non regulated power supply and the difference was clearer, 5-8% less Amps but this is still much smaller than the tolerance defined by the manufacturer. So I am afraid that 2 weeks later for example from the next delivery all parts could be evaluated as a broken ones. That's why I'm trying to detect wires separately. Than summarize outputs into one digital go/no-go signal.

    Thermal camera I think would be very problematic, can catch fingerprints, or glass can be accidentally preheated, in this case the contrast will be insufficient. We are using infrared thermometers, pyrometers and heat switches in other departments in the factory but I thought that is difficult to measure glass surface, and these kind of sensors detects bigger area not just 0,5mm thickness.


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