Testing rear window defroster grid

IF money is not too much of an issue then a thermal camera would instantly show you the broken wire

this may set you back 1000$ but would return that pretty quickly in time saved

Provided you can disconnect it, I'd be tempted to just make an oscillator.

With a 1nF cap in series, the resonant freq may be around 1MHz, and a single wire break may change the frequency (i.e. increase it) by around 5%

(back of envelope calculations). You'd need to keep any additional wiring short to maximise this change in frequency.

I've no idea if it is feasible, but could be worth a try.

EDIT: A more modern way could be to try the LDC1000 - there is a pre-built eval board for the LDC1000eval board for the LDC1000 worth a try I suspect.

Your window defroster is likely just in the right ballpark for the LDC1000. Snap off the coil part of the LDC1000 eval board, and attach the remainder

to your defroster, with approx 100pF cap in series.

Note: I've not used the LDC1000, so I don't know if it is appropriate. But it looks ideal to me.

Hi Lansm

I have a heat sensor that I use in the lab that has a very low mass thermistor. I suspect that it could be moved across a wire to detect if any heat was being produced. None of our solutions so far will tell you where in the wire there is a break, just that there is a break somewhere. I suspect that some sort of a capacitive sensor dragged along a broken wire might be able to detect the break where there is a potential gradient across the break but I have no empirical experience with this.

John

Actually my post to use a thermal camera / imager would show you

Hi Peter,

I actually like your idea the best and if it is a partial break in the heating element your thermal scan would show the high resistance area as a bright point. However if there is a total break in the wire there will be no current and no heat and the actual point of the break will not show in the IR camera.

John

Very true, you will not be able to see where the break is for a particular element on the window but it will easily show it is broken so from a QA perspective it will be very efficient to weed out the broken ones

if further diagnosis is required on a broken one then that is alo more complex and the camera wil not help

measuring the current, temperature etc will also not help (All of the above examples), now a visual inspection would also be quicker because you atleast know the broken element

down side with the inaging camera is it is not automated (Could be but would be expensive), if as mentioned in the post an automated process is required then one solution may be to use a hall effect sensor and measure the magnetic field created on each element when current flows, this is contactless and can easily flow to a PLC for taking readings, small coils may also be an option, of course this will require a little more circuitry but totally possible

A strip of thermal paper across the screen might work.

We use thermal printers for Flight Strips and they are reasonbly sensitive.

Mark

I think Ate is saying that the operator forgets to visually check the strip (or there is a belief it could damage through contact - but I would have thought that would

be a valid test), but it could all be automated, either with a visual camera examining the strip output, or FLIR has industrial thermal cams (e.g. just as an example, FLIR Axx series

spotted at a trade show, but other models may be more suitable) and they have an API where you can select the region of interest for example.

However, it costs (very high resolution might be needed to identify hot spots from a thinned out track, much like a CCTV camera has for it's field of view limited pixels) -

the cost may be negligible for an industrial use-case though.

There are cheaper ways for broken wire  - Panasonic has GRIDeye - twelve of the sensors, one aligned to each wire, would be sufficient to monitor the twelve wires are a

cost of under $500, and low-cost microcontroller(s) would be sufficient to process the data (I2C).

It would identify which of the 12 wires is not heating. There is also Melexis but I've had difficulty obtaining their products easily - although generally they work well from what I could tell.

Also Honeywell and Omron Omron and GEGE (no longer available) have single-pixel or low count linear arrays of thermal imaging type sensors (thermopile).

For a go/no-go indication for the entire window (e.g. if it is not for repair in a production line, but for complete replacement) where you may not care which of the 12 elements

are faulty, I think any resonant circuit method could be worth a shot simply because it doesn't need a jig for thermal alignment, (and so is a quick and cheap method to

eliminate quickly if it doesn't work) and possibly the easiest (with the LDC1000LDC1000) although this is a pure guess.

Thank you all for your kind support and creative ideas.

I would like to try to use analoge hall effect sensors because this seems the fastest method and could be easily automated and evaluated with plc. Could you recommend any appropriate sensor, small and sensitive enough? You can see the tool used for window checking and cleaning. I have 4cm gap between the window and the tool if I place window into that. Would it be better to use higher voltage?

Hi Iansm,

The sensors will be most effective if they are as close as possible to the wires carrying current. Perhaps they can be put on flexible fingers. The intensity of the magnetic field you are trying to sense will decrease with the square of the distance from the wire. In other words if you double the distance between the sensor and the wire you would have to increase the current 4 fold to compensate. From your specifications it looks like each wire is designed to carry 1 amp. I do not have the experience necessary to recommend a specific Hall Effect sensor but perhaps you can get a couple and experiment to see how close you have to get to obtain reliable readings.

John