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Sean O Matic

Sensing on a conveyor line

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Hi, I'm a newbie to PLC's. I have built several machines using antiquated methods such as cam wheels and timing switches. I'm comfortable building relatively complex systems with those methods. However, I recently took the plunge and used a Click PLC to build an automated boring machine - the flexibility and ease of "tweaking" the various timings and cycles was amazing. Now I'm hooked! I should tell you that I am not an engineer by any stretch of the imagination and have no business doing this type of work - except that I love it and enjoy the challenge and learning! My next project involves sensing parts on a conveyor line which sprays glue onto the part as it moves under an application station. The parts will vary in width and placement on the conveyor which will be about 24" wide. So there are 2 hurdles that I am contemplating. The first is I need to sense the width of the parts. One might be 6 inches wide, the next 12 inches. My plan is to use a proximity sensor that travels in unison with the spray head back and forth across the width of the conveyor so I can sense where the part (or parts) start and stop as the sensor moves across the belt. (The belt will not move constantly, it will start and stop for each cycle.) So the first thing I need to do is learn how to record that start and stop information. Secondly, I need to sense a few steps ahead of where the glue is being applied. So, I think this will involve some sort of shift register type function whereas the sensing will be done at position 1 and the glue application will be in position 4 with 2 intermediate or "travel" positions between sensing and the actual application of the glue and use of the sensed part information. So, any suggestions or ideas would be helpful and appreciated. Please remember that I'm new to this so I'm unfamiliar with a lot of the jargon. I'm not afraid to learn though, just knowing what concepts or parts to read up on would be a help. Thanks for your time. Sean

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position tracking can be done using sensor at conveyor. resolution depends on application, if you do not need very precise information, you can use low resolution sensor such as proxy counting gear teeth for example. for every increment in conveyor position, shift data. you don't say anything about product shape, type, material etc. proxy sensors are nice and simple to use but if you use such sensor on a sweeping arm, you still need to keep track of the arm position (another sensor). i would try to avoid moving parts if possible (cameras, laser distance sensors, etc.). for example pair of laser distance sensors (one on each side of conveyor) should give you all information you need. also interfacing and programming will be very simple.

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Not so simple for a novice, probably. Also, installing any kind of optical sensor (and laser is such) is questionable in glue spraying area.

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