314c-2dp positioning with sensors (without encoder) !!!

I must do a program with 4 robots in which must be controled only with sensors. Robots are going left-right...Each robot have one sensor on it.
When it is going the sensor give the plc signals, when it pass one of five points..
My first idea is with hardware interrupt but the problem is that on that PLC I have got only one hardware interrupt (OB40) and my program must control 4 robots...

Why? This sounds like someone trying to save a few bucks on sensors and wasting thousand of dollars getting the system to work poorly if at all.

what is your objective? 'control four robots that go left-right and pass one of five points'? you need to be more specific. why do you consider interrupt instead of standard task?

I will try to explain on English. Sory it isn't good....

Why not encoder ??
In place where is that line (with 4 robots), there many elotronic waves and program with encoders never run as we wish..That mean that that solution we try....What I mean with that...Program work fine sometimes one hour, sometimes two, but after that positioning starts wrong...Depends of activity other electronic in room....We try with different type of cables but nothing hapeness...

I'll try here to find out is it possible to that without encoder. How I say I try with hardware interrupt. But the problem here is if one hardwere interrupt is trigered the other will not be trigered till the first will finish. In PLC that I use have got a choice of only one hardware interrupt, but I have four robots...

Or my question can be how people do that in the past when they have not got encoders ??