Photocell choice?

[rmarotta]

I'm using Allen Bradley 42MRU-5000 photocells that intermittently fail to operate reliably in a cold, high-humidity environment. The photocells detect pallets of cold vegetables on a conveyor coming out of a hydro-cooler tunnel. They stop the conveyor to await forklifts used to haul pallets away. Clouds of moisture periodically cause annoying, premature stops. Is there some other method of photo-detection that will penetrate a cloud at a distance of about 20 feet?
The photocells replaced limit switches that needed constant maintainence due to the wet environment.
Thanks for any suggestions.
rmarotta

 

[MikeHalloran]

If the photocells are being fooled by condensation on their surfaces, local heating, e.g. by an incandescent bulb underneath, might suffice to prevent it.

Next choice is removing moisture from the optical path. ... perhaps by putting a tube around the optical path to the extent possible, and flushing the tube with dry air.



Mike Halloran
Pembroke Pines, FL, USA

 

[rmarotta]

Mike,

Thanks for your input.
Lens condensation is a sometimes issue, presently cured with a quick wipe of the lens and/or reflector at the opposite side of the tunnel. What really causes the problem seems to be the occasional clouds of "steam" breaking the beam... even when the lenses/reflectors are clear.
I'm having a similar problem with cold room sliding doors. The same type photocells are used to guard against collision with forklifts passing thru. (Doors occasionally hang open for long periods due to a cloud of steam.)
Regards,
Ralph Marotta

 

[btrueblood]

"Is there some other method of photo-detection that will penetrate a cloud at a distance of about 20 feet? "

Not with any reliability, as you've found. Some types of radar (microwave radiation) can penetrate water clouds, but it depends on the droplet size. I'd think a mechanical sensor (limit switch) or a proximity sensor would work better (if a suitable target exists for the prox., like a hard/metal spot on the pallet?) An acoustical system (like the old Polaroid auto-focus rangefinder) might also work.

 

[VEBill]

Something with higher power, and perhaps IR, should be able to penetrate any cloud of vapour. Obviously the beam would still be blocked by a solid pallet. The OEM (now Rockwell Automation?) should have a Sales Engineer that would be able to direct you to the most appropriate product. It should be solvable.

 

[ScottyUK]

Speak to the apps guys at Erwin Sick - excellent products and backed up by knowledgeable people. I'm pretty sure they will have a solution available.

http://www.sick.com

 

[IRstuff]

If there's liquid or aerosolized water, then IR isn't going to be sufficiently immune to be fully reliable. Generally, in that case, you should opt for some sort of RF or EM field approach. Doppler radars exist for even home motion detection systems, so something should be available for this type of application. Or, even something buried in the ground. Some shopping carts are configured for locking their wheels if they pass over a buried wire. The converse could be an option. Or, even just the standard traffic induction loop detectors used at intersections. Or, weight sensors at the appropriate location. Or, RFID tags on the pallets could be detected, ala retail loss prevention systems.

TTFN
faq731-376

 

[VEBill]

A beam-breaking type of detection system (an IR transmitter on one side and the matching receiver on the other) should be capable of being perfectly reliable in the circumstances described.

 

[OperaHouse]

"periodically cause annoying, premature stops" Indicates to me that it is reflecting off the clouds. Likely because the sensitivity has to be set ultra high because of the distance and poor target reflective properties. It seems odd that the sensor has to be placed 20 feet away. Somehow getting it closer could solve those problems. While likely put at those distances to eliminate moisture problems, those could be easier to solve than detection.

 

[rmarotta]

Thanks for all the help. I'll be doing some investigation into all the suggestions presented.
Getting sensors/reflectors closer together isn't an option, though. (There are actually four pallets abreast of each other, transported by the conveyor.) Upon exiting the hydro-cooler tunnel, pallets are delivered to a set of rollers at the end of the conveyor, where they are detected by the photocell. An adjustable time delay built into the photocell allows advancing pallets to push each other to the end of the rollers before stopping conveyor movement. Pallets then await the next available fork truck to be transported to a distant cold storage room. As stated earlier, this system has replaced mechanical limit switches that were causing constant problems.
Regards to all,
Ralph Marotta

 

[MiketheEngineer]

How about a couple of fans???

 

[controlsdude]

Suggestions: (dealt with this on -20 freezers)
More Positive pressure on Refrigated space would push more air thru the tunnel and might push that cloud away from the conveyor. Also, only open the tunnel on the refrigator end to charge the tunnel and not mix the air from the shipping area, put a humidity and temp sensor to monitor to get an idea of the time delay needed before releasing the pallets into this area. Once the cloud is gone, release pallets into area. Now close the refrigator door and open the shipping side door of the tunnel, wait for a time delay on when to move the pallets based on no clouds hanging in area. Now move pallets out fo the tunnel into the shipping.

Go to a thru beam 120vac photo eyes (120vac will act as a heater and keep the photo cell clear, thru beams more reliable to push thru dust and other materials to sense what you need) If you already have retro reflective, try to get a reflector that is heated to keep the water vapor off the reflector. The heated reflector is a specialty item sold by the phot sensor manufacturers.

 

[itsmoked]

The receiver and transmitter type sensor 'pairs', as others have mentioned, are considerably better at handling these sensing insults. That's the very first thing you should try.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MiketheEngineer]

I like the fan idea. Also - there are some very cheap radars that may do the trick...

 

[KllrWolf]

Maybe a simple sonar set would work. I have used them for simple distance checking. As long as the water vapor is not dense enough to return a solid signal or the object absorb it instead of reflect, I dont see an issue.