Sensing light is immensely useful in many applications. Want to check everything on your production line is stood up and oriented correctly? Need to count small items your manufacturing system is making? And want to do it all reliably, cost-effectively, and quickly? Then light sensing is going to be your technology of choice.
Photoelectric sensors take light, typically visible or infrared, and convert it into an electrical signal. Originally developed around the middle of the last century, the technology has developed substantially since the original phototransistors and photodiodes.
Today, sensors have become far smaller and more sensitive, while often integrating analogue-to-digital conversion and a digital interface. Developments are being partly driven by new applications, such as ambient light detection in smartphones, but manufacturing and packaging remain the biggest market.
Photoelectric sensors can detect objects many metres, or even tens of metres, away. They can pick up targets less than a millimetre across, and can even be used to analyse the colour of an object.
Three basic types
For most applications, the choice of photoelectric sensor comes down to one of the three commonest types: through-beam, retroreflective and diffuse.
Through-beam
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Through-beam sensors use an emitter that sends out a beam of light towards the sensor, which detects when an object obstructs the beam’s path. Through-beam sensors are reliable, but require the emitter
and sensor to be accurately installed in two different places, which can be awkward and costly.
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Retroreflective sensors emit a beam of light that is sent back by a reflector, and an object is detected when it breaks this light beam. Like through-beam sensors, retroreflective sensors still require two items to be installed (the emitter/receiver, and the reflector). They typically have lower precision and sensitivity, and can struggle to spot shiny objects like metal cans. They are, on the other hand, often a little lower in cost.
Diffuse
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The third type is the diffuse sensor, which also has its light emitter and receiver in the same place, and often in the same casing, thus making it easier and cheaper to install. An object is picked up when it reflects some of the light back to the receiver – so the performance depends on the colour and reflectivity of the target object.
With diffuse sensors, there is also the risk of light that has been reflected by a background being erroneously detected. Various techniques have been applied to overcome this, including using multiple receivers and focussing one where the object is expected and one on the background, and then comparing the signals.
In terms of operating distance, through-beam sensors have the advantage of the longest range, often working at 30m or more. Retro-reflective sensors can typically operate up to about 10m distance, while diffuse sensors usually work over much shorter ranges. Of course, in practice you may only need a very short range, and sensors working over just a few millimetres are widely used.
Slimline yet robust
Whichever type you choose, for manufacturing automation and industrial automation one of the key criteria is something we’ve not mentioned yet – ruggedness, and ability to function reliably in harsh conditions. Bulgin has designed a range of slim line photoelectric sensors specifically to meet the tough demands of these kinds of applications,
The Bulgin sensors provide high levels of mechanical and electrical stability, with a watertight and dustproof seal to any standard M5 interface, and a Stainless Steel 316 shell, sealed to IP67. The sensors are only 4.5mm thick, and optionally supplied with a 2m cable. They offer a sensing range of up to 40mm.
For more information, visit the Bulgin website, for help and advice, please visit the Connectivity Community