Today is...
Saturday, March 24, 2018
Welcome to, the global online
community of automation professionals.
Featured Video...
Featured Video
Watch an animation of a conveyor stacking operation demonstrating the use of a move on a gear command.
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
Need Help - Change sensor to accurately detect box jams
Looking to use a different sensor to detect jams. On a conveyor belt, when boxes are pushed together and there is no gap between them it thinks there's a jam even when the boxes are moving past.

Hi guys....first post here. It's been about 6 years since I worked heavily with PLC's and automation, but I'm getting back into it. I was hoping you might be able to help me with an issue I've noticed at work.


The sensor we use just reads if there's something there, or not. A make or break if you will. When boxes are pushed together and there is no gap between them it thinks there's a jam (after 6 seconds) even when the boxes are moving past (as they should). A jam stops the conveyor belt requiring someone to physically move the boxes to clear the sensor and restart the equipment.

I'm trying to find a sensor that will see the distance changes from box to box (mounted on the side of the conveyor). Only when there is a true jam and the distance isn't changing will it send a jam signal.

I've tried researching sensors and have been overwhelmed by the endless possibilities and combinations (not all of which I understand).

We are currently using an Allen-Bradley 42EF-P2RJB-A2H

It would be ideal to have a similar style plug, but it isn't a deal breaker.

Here's an idea of what I'm dealing with. The actual conveyor the jams are occurring on is a decline belt, not a roller accumulator. There are varying box sixes, but they are all packed together. Creating a gap between boxes or using metering belt is not an option here.

Thank you for any help/guidance you can offer!


With the challenges you face it might be better to think mechanically as well. Could you design an arm to ride the top of boxes with an accumulator roller to read motion so it moves up and down
following the various size of boxes?


Look for laser distance sensors which give you an analogue (e.g 4-20mA). Build in some logic in your control checking the analogue signal:

"If it remains for x amount of time unchanged and not looking at open end (no box) then jam occured."

Like that?

It's not clear whether you are trying to sense lack of a gap or lack of motion. Lack of a gap could be an simple optical switch across the conveyor closing as gaps go past and timed. A jamb would be no changes.

If its a solid stream of boxes all touching, how about a light (as in low force) wheel producing a pulse train from simple reed switches or again an optical wheel?

Monitor both Open and Closed states each with a timer, a time-out would be triggered by staying in either state too long.