Proximity sensor technologies vary with operating standards, strengths, and determining detection, proximity, or distance. There are four major options for compact proximity sensors useful in fixed embedded systems. It is necessary to understand the basic principles of operation of these four types for determining which to select.
Most proximity sensors offer an accurate means of detecting the presence of an object and its distance, without requiring physical contact. Typically, the sensor sends out an electromagnetic field, a beam of light, or ultrasonic sound waves that pass through or reflect off an object, before returning to the sensor. Compared with conventional limit switches, proximity sensors have the significant benefit of being more durable and, hence, last longer than their mechanical counterparts.
Reviewing the performance of a proximity sensor technology for a specific application requires considering the cost, size, range, latency, refresh rate, and material effect.
Ultrasonic proximity sensors emit a chirp or pulse of sound with a frequency beyond the usual hearing range of the human ear. The length of time the chirp takes to bounce off an object and return determines not only the presence of the object but also its distance from the sensor. The proximity sensor holds a transmitter and a receiver in a single package, with the device using the principles of echolocation to function.
Photoelectric sensors are a practical option for detecting the presence or absence of an object. Typically, infrared-based, their applications include garage door sensing, counting occupancy in stores, and a wide range of industrial requirements.
Implementing photoelectric sensors can be through-beam or retro-reflective methods. The through-beam method places the emitter on one side of the object, with the detector on the opposite side. As long as the beam remains unbroken, there is no object present. An interruption of the beam indicates the presence of the object.
The retro-reflective method requires the emitter and the detector to be on the same side of the object. It also requires the presence of a reflector on the other side of the object. As long as the beam of light returns unimpeded, there is no object detected. The breaking of the beam indicates the presence of an object. Unfortunately, it is not possible to measure distances.
Although expensive, these are highly accurate, and work on the same principle as that of ultrasonic sensors, but using a laser beam rather than a sound wave.
Lasers require lots of power to operate, making laser rangefinders non-suitable for portable applications or battery operations. Being high-power devices, they can be unsafe for ocular health. Although their field of view can be fairly narrow, lasers do not work well with glass or water.
Inductive proximity sensors work only with metallic objects, as they use a magnetic field to detect them. They perform better with ferrous materials, typically steel and iron. A cost-effective solution over a huge range, the limited use of inductive proximity sensors to detect objects reduces their usefulness. Moreover, inductive proximity sensors can be susceptible to a wide range of external interference sources.