A wide range of single-purpose point sensors such as proximity sensors, lasers and photocells have long played a key role in virtually all factory automation systems by detecting the presence of parts or assemblies, performing measurements and identifying colors, among other tasks. Machine vision systems, on the other hand, perform more complicated operations such as complete inspection of a part. The work performed by machine vision may include multiple dimensions and presence or absence of various components or determining the position and orientation of a component for a subsequent operation. Simpler, less expensive machine vision sensors have recently emerged to target operations currently performed by point sensors, as well as many other inspection tasks, at a similar price point. Here we review the capabilities and potential applications of vision sensors.
For many years, developers of automated systems have used traditional point sensors to detect the presence or absence of parts or assemblies for controlling machines and processes. For example, photoelectric sensors generate a beam of light that is typically aimed at a detector. At some point in the process, the part is expected to make an appearance at just the right position to interrupt the light circuit. The sensor provides a binary output depending on the presence or absence of light at the detector. Point sensors are still relevant in applications where only verification of the presence or absence of a part is required. But the capabilities of photocells are limited; by restricting the inspection to a single point, photocells have no way of knowing if the part is the one that was expected, nor can they assess the quality of the part or whether it has been correctly assembled. Point sensors also require tight alignment between the source and the detector. If unforeseen variation is added to the process, there is no way to adapt.