Sensors

[Ken]

hi guys. have always wondered how the plcs can be programmed to know when to reject a fully enclosed package running on a conveyor belt. What are the sensors they used? heard it's something like pressure pad or something. Care to elaborate further?
Another question is how do you guys know which type of sensor to use? Do you use a optical sensor or the normal kind to detect a reflective surface at an angle? tried using one by fuji electric but it did not give a stable result. Any suggestions?
Thanks

[Mel]

I had a similar application with large objects(100# plus). In this case, the solution was easy, because we used load cells and eyes on the 'dump' section of the conveyor track. The dump section is separate from the rest of the track, and tilts to dump the product in to another bulk container.

The eyes are mounted at track level, and they detect the product when it arrives fully on the dump.

The load cells are mounted on the footing of this dump section. When the PLC (CLogix) gets the arrival signal, it then compares the load cell input(with some calibration and conversion math) with a preset number in memory. (The whole section is counted as tare wt. and subtracted from the sensor input.)

The operator is alerted by an alarm if the weight is out of tolerance compared to the preset.

There are quite a few choices of cells out there. Since this application weighed heavy objects, the accuracy of the sensor less important than durability. It works quite well in this application. If you have lightweight objects, this may not work as well for you.

[awalinski]

For many years we used Banner sensors, always with good results.
They have polarized and convergent beam photoeyes which can be used in reflective applications. Also their sales reps are pretty good at advicing the best suited sensors.

[TimeFluxCap]

Hi Ken

have a look at this site

http://www.ifm-electronic.com/ifmgb/web/techlex.htm

and have a look at the stuff in the technical glossary.

IFM are a big player in sensors so its a chance you might be able to find a solution.

[KMG]

There is no one answer to either of your questions. Everything changes with the details of your particular application. For instance…
Why are you rejecting the package? Too light, too heavy, box not closed, box out of position, missing product code, etc. different sensors are used to detect different conditions.

Where in the process is the defective product? You answered that on already. It’s on a moving conveyor belt.

How do you get the defective product out of the flow of good product? In our factory we reject many types of products from a moving conveyor belt. How we do it depends on the product and the speed of the conveyor. We will use an air blast to push product off the line into a reject container. We will use an air actuated arm to push the product into a reject container. We will open a section of the line and the defective product will fall off. We will use a diverted to guide the defective product from the main conveyor onto a reject lane.

How do you sense that the defective product is at the reject point? Again that depends on many things. Most of which you would have answered in the paragraphs above. If you were rejecting do to the wrong weight on a slower moving line, the scale could be part of the conveyor at the reject point. Sometimes the defect is sensed upstream and the product rejected later. If the line speed it constant you can use a simple timer to delay the reject until the defective product is at the reject point. In an assembly line type of application we know that there are 10 packages (example) between the point that we detect a defect and the reject point. We simply count 10 steps of the process and reject the package.

As to the question of which sensor to use…The reason that there are hundreds of them on the market is because no one sensor or brand of sensor will work in all applications. You will need to find the right fit for your application.

If you are having problems with a particular application, post the details and you will have no shortage of helpful advise.