Sensor Technology
Piping light to the heart of machines
David Hannaby, SICK UK product manager for presence detection, outlines the ability of fibre optic sensors to reach the parts of machines that others can’t
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n 1854, the celebrated Irish physicist John Tyndall made a discovery that presaged a global technological and communications revolution. He observed how light could be made to travel in an arcing stream of water. The discovery established the principle of “total internal reflection,” that light energy can be ‘piped’ to a destination through a highly refractive material. Tyndall’s discovery paved the way for the field of fibre optics and the ability to send signals through fibres no wider than a human hair. One field of the technology to develop has been fibre optic sensors, whose unique capabilities are helping scientists to conquer new frontiers all over the world, from exploring the craters of volcanoes to reaching the depths of the oceans. Closer to home, in industrial machines, fibre optic sensors reach into the awkward places/areas other sensors cannot, which gives them special “super-powers”.
With their long, slender fibres and tiny light spots, fibre optic sensors look futuristic, but they have developed in parallel to other industrial photoelectric sensor types since the early days of both technologies. Fibre optic sensors are often favoured in electronics applications for their high switching speeds and ability to detect tiny objects. Because they fit into small or awkward spaces, they can overcome mounting and installation headaches.
38 November 2022 Components in Electronics How do they work?
Fibre optic sensors work on the same principle of total internal reflection observed by John Tyndall. 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 (Fig 1) 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. Vendors offer an extensive range of fibres and heads with versatile options for light dispersal and orientation, as well as for easy mounting. The SICK LLX range (Fig 2), for example, offers 500 different fibre types and a wide range of threaded and smooth-sleeve heads, with both axial and radial head alignment available. A useful innovation is that now any length of LLX fibres up to 20m can be specified. There are two types of fibre optic sensor: proximity and through-beam. In a
Fig 1: SICK WLL80 fibre optic amplifier
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. So, 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 3). Through-beam fibre optic sensors transmit a 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. So, for example, a fibre optic amplifier with appropriately specified fibre head can detect thin, fast-moving flat objects, such as the leading edge of a semi- conductor wafer, extremely reliably, even where their exact height on the conveyor cannot be defined. A delay timer function in the sensor eliminates the possibility of false signals occurring between the expected detection points.
Fibre optic sensors can also be configured with heads that produce a detection grid (Fig 4), e.g. to count small components falling from a chute into a tray (Fig 5).
Why use a fibre optic sensor? The ability of fibre optic sensors to transmit light along a flexible fibre means they can be threaded into narrow locations. They
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