I have an application that uses a armat plc and a 2500 ppr encoder to cut a product to length on the fly. The problem that I have to over come is that the actual cut is done via pneumatic cyclinders. The system work great when the speed stays constant, but when there are slow downs or speed ups on the line the product length gets longer or shorter. Although the PLC is cutting on the correct amount of pulses. then when the speed levels off the length comes back. Can anyone give me an idea on if there is a way to over come this problem.

 

Start with a count a fixed number less that tne length and add a delay adjusted to the line speed. And hope that your pneumatics stay consistant.

Regards

cww

 

I have a similar setup on a cast line, using a proportional soleniod valve (4-20ma input) we adjusted the exhuast of the cylinder to contol the speed of the cutter as it extends. The
4-20ma comes from an analogue output card from the PLC and is proportional to the line speed. The max line speed range I deal with is 25 to 65 ft per min.

Good Luck
ramadilljr

 

Try monitoring the product speed using the counter and using this value to calculate a correction for cutting position.

 

Hi List:

The cut takes some finite time to complete its cycle once the pneumatic cylinder is triggered. It sounds like this cut cycle time is relatively
constant at least within allowable tolerances. The cut cycle time relates directly to a period of additional product travel down the line after the cut is triggered. The amount of travel is directly related to line speed. As long as the change in line speed is not significant within a specific cut distance, the way to compensate for changes in line speed is to adjust the trigger point by adding pulses to the trigger setpoint value (so the cut mechanism triggers later) if the line speed is below the line speed where length comes out correctly and subtracting pulses from the trigger setpoint value if the line speed is faster than normal, (so the cut mechanism triggers sooner). Take a speed sample within the cut distance that insures you'll have enough time to calculate the required adjustment and get the adjusted trigger count setpoint in place before that count is exceeded. You'll need to develop an algorithm based on the results you're seeing to determine the correct trigger count setpoint adjustment to bring your cuts within allowable tolerance. If line speed is varying significantly over the cut distance you may need to average the line speed over at least part of the distance to determine the speed used to calculate the adjustment.

Best Regards,
Robert D. Wagner, P.E.

 

If you can speed up the cylinder to reduce the cutting time, you would be able to minimize the problem. This would minimize the amount of possible variation.

Bill Sturm