best prox sensor for potato counting

lesmar96

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Join Date
May 2017
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PA
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Hey All,

I am looking for advice on the best prox sensor or device to use in this application.

Customer wants to be able to count potatoes coming down a conveyor. Conveyor will be very narrow so the potatoes are in single file. Potatoes will be travelling past the sensor at approx. 2-3 second. They are not looking for a high tech system, but something that will work. I was thinking of using a prox sensor as an input into a Dwyer counter. I think this would work well as I have done this before, but I am struggling to find the best sort of sensor to use to detect a potato. Was looking into ultrasonic? is that the best? With where the sensors will be mounted and to allow for various sizes of potatoes, we are looking for a sensing distance of 150mm-200mm.

Let me know if you have any advice. Maybe there is a better way to do the whole setup?!
 
Please include explanation for why you think what would be best. I am trying to sort out all the different styles and how they work.
 
I do not know of a proximity sensor that will work well with a potato. A readily available through beam (opposed emitter and receiver units) or retro-reflective photocell will detect anything that isn't transparent and do it quickly. Either type of photocell will require a gap between the products. The duration of the gap will need to exceed the switch off time of the photocell. The time that each product blocks the photocell beam needs to exceed the switch on time of the photo cell.

The drawback to any photocell is keeping its lens clean and undamaged. Both of the types I recommend are mostly immune to false triggering off of reflective surfaces. You would be surprised how reflective common surfaces like cardboard can be. The polarized sensor requires a polarized reflector and will only react to light that it emits that reflects off of that reflector.

The opposed mode (through-beam) uses a matched emitter light source and the receiver won't react to other types of light. Those in my links are wash-down rated.

Here is an article I googled up with some general explanation about photocell types:
https://www.automation.com/library/...ologies/fundamentals-of-photoelectric-sensors
 
Maybe a stupid question. I thought products like this were all bought and sold on weight. Would an inline weighing conveyor be more appropriate?

That said, I think counting something with an organic varied shape such as potatoes is probably harder than first glance especially if there is any bouncing of the product. I suppose a rough count would be not too hard but an accurate count might take more sensors and/or computing to account for all the variability in the product.
 
Another problem you might have is in maintaining separation between potatoes. If you don't have some sort of gap between them, i.e. two potatoes are toughing each other as they pass by, a thru-beam will not be accurate. I might consider a simple camera based sensor, "teach" it what a potato is supposed to look like.
 
Thanks for your replies.

The reason they want to count these is because this is a test plot machine where they are counting and weighing the potatoes that come out of certain test plots compared to others.

I do see a potential for dust and dirt, so that makes the equation more difficult.
 
Another factor no one has mentioned is size. If the potatoes are a variety of sizes and shapes, this is a much more difficult application to do with a sensor. If the potatoes are more of a uniform size (within a few cm) and shape, than it wouldn't be as difficult to figure a height on the conveyor which will almost always have a gap between potatoes.

Is there any quality control before this count? Or is it just every potato pulled from the field.
 

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