ThruBeam

This is a job for good old fashioned trial and error.

Since the PLC cannot know or control the ball speed, it can't calculate an expectation. I would expect that a typical ball passing through the beam is going to be much less than 500ms (where I would start the trial).

A really good cheater might be able to foul and hop out of the way almost as fast, so it is unlikely you wil ever get it perfect.

Set up your logic and pitch the ball at the slowest potential legitimate speed. Trap the timer accumulator in your logic to measure that duration and set your preset a little below that.

Repeat a few times and adjust until it is as good as you can get it. Too low, and you will false trigger on slow balls, Too high and you will fail to detect really nimble cheaters...
 
I think a single time delay with somewhere around 50 - 100 mS delay would be sufficient. Even if the delay needs to be a little longer it would be pretty tough for a person to break the beam with a foot and reverse direction within that time period. Well nigh impossible when you factor in reaction time. On top of that, a person concentrating on the foul line sensor to see whether he needs to jump out of the way is focused on the wrong thing at the time and isn't likely to be much competition the other team.

If the timer turns out to be an unworkable solution, how about mounting two photocells a short distance apart. Both have to be simultaneously tripped to generate a foul. Keep the beam as close to the floor as possible and the rolling ball wont be able to block both at the same time, but a foot will.
 
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