Re: Sensor?
All components involved in safety related functions have to be safety rated. These are special components with special expensive price.
Consider a motor spinning some blades chopping salad or whatever. The layman's approach is to enclose it in a cage, add a switch to a door. The switch is placed electrically so that it cuts power to the contactor that energizes the motor.
Motor 300 bucks, contactor 50 bucks, switch 10 bucks. 360.
However, since this is a dangerous machine, you need to use safety rated components. The switch Costs now 100 bucks. You need two contactos in series in case one of them fails. You need a 500 buck safety relay that evaluates that the switch is working properly, evaluates whether any of the contactor have failed, and also evaluates whether itself has failed. Failure should be safe and result in the safety relay denying the system from running.
Our system cost just went from 360 bucks to 1000 bucks.
For this reason it's common to enclose very large areas behind a single safety circuit. This results in larger areas than strictly necessary to lose power whenever an operator needs to access a small part. This unfortunately leads to crafty maintenance engineers devising ways to bypass safety systems and rely on just switching off the relevant motor from a control panel or PC or what have you..
It gets even worse if you want some actual "smart" safety functions. A safety rated PLC needs to be able to survive abuse and still able to shut down itself if its input or output circuitry gets damaged by surges or overloads, or the operator forcefully inserting a knife through its circuitry (happened at my workplace when the system refused to run with a vague safety-related error message).
It needs to have two redundant CPUs,from different manufacturers but running the same program.
So no, it's not a few dollars. A smart safety system costs many times more than the robot itself...