best sensing methodology for an automation project

I am looking for someone who can suggest the best sensing methodology for an automation project that is to do with automating the movement of steel rebars. The challenge is to automate the job of a human who used to be using his eyes in order to stop the movement of steel rebars, having had stopped the movement, then the human operator will push a button to do another function. I have written a PLC code and tried proximity sensors but the sensing range a proximity sensors is not more than 5cm. Please imagine the steel rebars being moved to a table (you can imagine it as a converyer of length 70m and width of 1m. so i have designed a gate that is free and attached it to a small rod out of the converyor. Then when the steel rebars falls above this verticle gate, the gate will flip clock wise to be positioned horizantly. and the rod will then be in front of the proximity sensor therefore the sensor will sense the rod. as a result the sensor will know that there are steel rebars now. However, the problem with this idea is that the proximity sensor got damaged because the distance is 2cm only and due to the viberation caused by the movement of the load of rebars the sensor got damaged. Therefore, i want another idea of sensing the rebars without using expinsive technology. challenges: 1- the steel rebars are hot and more than 70c. 2- the application needs a large number of sensors therefore, cost is important. expectations: I want someone who can provide me an idea to automate this process either by modifying my method or you are welcome to come up with a different idea for sensing.