Testing rear window defroster grid

[Lans mnet]

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... :)
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.

Ate

 

[anorlunda]

A thermal image camera would be the simplest method by far. There are even smart phone apps to do that. There are also inexpensive USB thermal imagers.

 

[jim hardy]

Here's an approach, not a solution

Are you testing these in a manufacturing environment before application to the window?

I'd think a hall sensor could detect the field from an amp or maybe less
especially if you apply AC to the heater.

5mm away from an amp gives 0.4 gauss per this page
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c3

and this gizmo would give a couple millivolts of signal at that field strength
http://www.allegromicro.com/en/Products/Magnetic-Linear-And-Angular-Position-Sensor-ICs/Linear-Position-Sensor-ICs/A1324-5-6.aspx

you'd take the sensor output , filter it for the excitation frequency, amplify , and indicate on a meter or go-nogo light.

i'm thinking a test jig with one sensor per wire
but it could as easily be a single sensor in a handheld battery powered wand that you sweep over the heater array.

That sort of opamp project is a LOT of fun, and good experience for a young engineer.

I've used this filter up in th 25khz range with excellent results
from http://www.ti.com/lit/ds/symlink/lm359.pdf

more here http://www.ti.com/lit/an/snoa666b/snoa666b.pdf

The humble LM386 audio amp has pins that let you tailor its frequency response, and it's designed for battery operation as well as AC signals..who knows, you might even be able to make it work at line frequency
that'd sure simplify your power driver .

Anyhow - my point is, let the heater show you its magnetic shadow. A wire that's open will have no magnetic shadow.

old jim