Here’s a real-world application where the reliability of the sensors is directly related to the reliability of the process in producing quality results.
Pictured below is a pneumatic actuator for a vacuum valve. Inside the actuator, a magnetic ring is installed around the moving piston by the manufacturer of the actuator (this is an option that must be requested when placing the order for the actuator). The magnet acts as a target to activate the sensors as it moves under them during operation. It’s important to note that the wall material of the actuator must be non-magnetic in order for this concept to work properly; typically aluminum or non-magnetic stainless steel is used.