conveyor jam detection

[plcdp]

Hello all, I am currently working on a machine that cuts the dome off the tops of large mouth plastic bottles, commonly called a spin trimmer. What I would like to do is implement some jam up detection on this unit. I am using an FX3U. My first thought was using an infeed sensor and a out feed sensor and time the duration the bottle is in the trimmer. The issue I am thinking I am going to have is the fact that multiple bottles can enter the machine before the first bottle exits. This will interfere with the starting/stopping of the timer. I was thinking of a bit shift perhaps, but I am not sure how to integrate the timer. Another thought I had was multiple timers to handle as many bottles as could possibly be in the trimmer at once, but I feel like I am going to run into the same issues with resetting the timers as the bottles exit the machine. I was hoping someone here might have an idea or have been here before? Thanks in advance, Dave

[JRoss]

My brainstorm is to use some kind of free-running timer and a FIFO. Each time a bottle enters the machine, pop the timer value into the stack. Each time a bottle exits the machine, pop it back out. Compare the oldest value in the stack with the current timer value, and if the difference gets too big, you've got a jam. You'll have to deal with timer rollover, but this should work.

[plcdp]

Jeremy, Thank you for the reply. I like this idea and I will give it a try. One question....what do you mean by timer rollover? Thanks again, Dave

[JRoss]

Eventually the timer will have to reset to zero, it can't keep incrementing up forever. That's called rollover. When you subtract the current time by the saved time, if you get a negative result add the preset of the timer.

[Ron_S]

You could also increment a counter by the same method - a 32 bit counter would count for a very long time before it needed resetting.

[plcdp]

I see. Thanks for the ideas. Regards, Dave