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FEATURE


SENSORS & SENSING SYSTEMS


Fibre Optics: Reaching the PaRts OtheR sensORs can’t


With their long, slender fibres and tiny light spots, fibre optic sensors have a futuristic feel to them. The technology is helping scientists to conquer new frontiers all over the world – from exploring the craters


of volcanoes to the depths of the oceans. Closer to home, in industrial machines the ability of fibre optics to reach into the awkward areas


other sensors can’t gives them special ‘super-powers’. David Hannaby, SICK UK product manager for Presence Detection, comments


T


hey may seem more futuristic, but fibre optic sensors have actually developed in parallel to other industrial photoelectric sensor types


since the early days of both technologies. Often favoured for their high switching speeds and ability to detect tiny objects, fibre optic sensors can also fit into small or awkward spaces which overcomes mounting and installation headaches in machines and on conveyors. So, how do they work? It was the celebrated


Irish physicist John Tyndall who, in 1854, first observed the principle of ‘total internal reflection’. He noticed that light could be made to travel in an arcing stream of water. The discovery that light energy could be ‘piped’ to a destination in a highly refractive material paved the way for a global fibre optics communications revolution, and the ability to send signals through fibres no wider than a human hair. In industrial automation, fibre optic sensors


work on the same principle of total internal reflection. Light is transmitted along a central core, made of a single filament of either glass or polymer surrounded by a less refractive and protective sheath, and a robust outer housing. The light is emitted from the end of the core at an angle of around 60 degrees. Fibre optic amplifiers – such as SICK’s new


WLL80 – are an energetic system used for object detection. The amplifier transmits, receives and evaluates the light signal, as well as enabling the sensing parameters to be adjusted. The fibre is the optical component that transfers the sender light from the amplifier and back to a receiver in the amplifier.


light beam between a sender and receiver fibre. As soon as an object passes through the light beam, it is interrupted and the sensor switches. For example, the SICK WLL80 system with appropriately specified SICK LLX fibres can detect thin, fast-moving flat objects, such as the leading edge of a semi-conductor wafer, or a foil tea bag wrapper, extremely reliably, even where their exact height on the conveyor cannot be defined (Fig 2, below). A delay timer function in the WLL80 eliminates the possibility of false signals occurring between the expected detection points. Fibre optic sensors can also be configured


Fig 1 - Use Case: Precisely detecting the location of small components surface- mounted on an electronics board


SenSor typeS


There are two types of fibre optic sensor: proximity and through-beam. In a proximity system, the sender and receiver are combined in a single fibre head. The system is sensitive to the amount of light energy returned to the amplifier; and precise switching thresholds can be set up to provide an output to the machine control. There is a range of lens heads to direct light emitted from the end of the fibre, depending on how the sensor is mounted. These can be used to focus the light spot to detect very precise small objects, for example in electronics or small part assemblies. (Fig 1, above). Through-beam fibre optic sensors transmit a


16 DESIGN SOLUTIONS SEPTEMBER 2022


with heads that produce a detection grid, for example to count small components falling from a chute into a tray for packaging.


Fig 2 - Use case: High-speed


conveyor detection of tea bags on a leading edge to the rear edge. The tea bags are corrugated so that the front and rear edge position differ in height


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