Tricky application...suggestions welcome

Hi guys. I would like to sense coated web running on a chained conveyor. The application looks something like this. Material fed through a coater which coats the web with one of several chemicals. As web exits the coater, it lies on a steel oven belt, which carries the web through the oven. I need to sense the the web coming out of the coater, as it sits on this steel belt. The belt has a fairly rought surface, and it tends to get dirty with whatever chemical they are coating with. Webs can get close to 1/4" thick. What I need is some kind of a sensor that will be able to ignore the rough surface of the oven belt, but still pickup the web riding on top of it. This sensor should be mounted outside the coater enclosure to keep it GP. If we try to mount inside the enclosure, we need to go explosion proof. This was suggested, but we cannot find an explosion proof laser sensor, and we are unsure of how a laser sensor mounted inside an EP box will react to looking though a glass window. Also, a see though style sensor is not an option as the reflector underneath the belt will buildup quickly with whatever it is at the time that they are coating with. Anyone have any ideas?

I can only vaguely picture your scenario, but how about a diffuse sensor?

I would suggest a mechanical converter

You could let a steel harm hang down in such a way that it gets lifted by the web. Then let an ordinary prox sense the arm.
Make the arm height adjustable.

Try using a camera system and detect for color contrast. If it is too expensive, try wet coating reflective sensor, whose principle is based on absorbtion of light wave length. In our case the camera system worked well

I have done this. I have used vision for this type application but it is expensive. What I have used the most is photoelectric sensors that allow me to sense material within a specified range.

http://www.sick.com/gus/products/product_catalogs/industrial_sensors/reflexswitches1/en.html

baumer electric makes "learning laser photo sensors that I think would do the trick

Thanks guys.

I apologize for the vague visual. I can cut a hole in the top of the enclosure and look down at the belt carrying the web and keep this GP, but, this puts me about 4 feet from the belt.

The problem with a steel hanger, or dancer, or roller, is contamination from product to product. While possible, it would require cleaning.

I am not sure what a diffuse sensor is?
The vision system sounds like a possibility. Cost is always a problem, but we may be able to justify it in this case. Thanks for all the suggestions thus far. I will do some reasearch on the links posted.

Another option.
http://www.bannerengineering.com/products/photoelectrics/qs30/qs30ld/qs30ld_specs.php

Get your coating supply person to mix a little UV in with the coating and get a UV sensor such as a lut3 (sick) and away you go.

The Lut3 is outstanding. I use two of them and never here a word about them. They just work.

How about a PEC with a short sensing distance which can be extended with steel wrapped fibre optic. The sensor is thenm external but the sensing head internal and close.

What material is the web, steel?

I have a couple of vision sensors from Banner they work very well

If there is any difference in the material compared to the belt surface you can detect it.

http://www.bannerengineering.com/products/vision/

So I also vote Banner

Diffuse Sensing, see here.

http://www.ab.com/sensors/sensorstoday/nov98/nov98_techtalk.html

Read specifically fixed focus diffuse and background suppression diffuse. I don't know about accuracy from a distance of 4 feet, but it should work based on the difference in reflectivity between the web and the belt.

If you have an analog input you could use an ultra sonic sensor then in the program use a more than / less than logic to change a bit to give you a signal to stop the process or lamp or horn.

Just thinking,