Photoelectric Sensors operate on the general principle of light being reflected from (or blocked by) a target, which then triggers a response from a photo-sensitive receiver – turning on or off an output based on the presence or absence of an object. They come in many flavors, including diffuse-reflective, through-beam, retro-reflective, and background suppression. One large subset of photoelectric sensors is fiber optic sensors, which allows for use in tight spaces or harsh environments.
Proximity (“Prox”) Sensors operate on the principle of a target interfering with a generated electro-magnetic field when close to the sensor – most commonly a metal target and an inductive proximity sensor, but options also exist for capacitive proximity sensors, which can detect the presence of other non-magnetic materials such as water (and thus body parts). These typically have a short range (up to about 20mm), and can be very cost-effective in quantity.
Another broad category, these are sensors that measure something – for example, a laser sensor that measures distance to an object very precisely and relays that information via analog outputs or communications to a PLC. There are 2D measurement sensors for profiling a workpiece, contact sensors for detecting object size, and many, many other options. These can range from low-hundreds to many thousands of dollars.
Ultrasonic Sensors operate by emitting a low-frequency vibration, and measuring timed response echo as that vibration bounces off a target. These work well in applications like tank level monitoring, where you have a large, flat surface (liquid in the tank) to reflect the sound waves from. Measurement ranges commonly go from a few inches to about 50 feet.
Vision Sensors and Vision Systems operate by taking a picture of the target and applying software to inspect for various features such as position, orientation, defects or missing parts in an assembly, bar-code markings or writing, and various other things. They are commonly used for quality control – making sure caps are on bottles, for example – and in coordination with robotics, detecting the location and rotation of a part to be picked. Vision Systems are more complicated than other sensor technologies and require careful attention to lighting and lensing, as well as specialized software and programming. Basic Vision Sensors (“smart cameras”) start at around $1500. High-speed, multi-camera Vision Systems can be in the tens of thousands of dollars.