FEATURE Sensors
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Fibre optic sensors: piping light to the heart of machines
By David Hannaby, Market Product Manager for presence detection at SICK UK I
n 1854, Irish physicist John Tyndall discovered that light could be made to travel in an arcing stream of water. The breakthrough established the principle of “total internal refl ection”, that light energy can be ‘piped’ to a destination through a highly refractive material. Tyndall’s discovery paved the way for the fi eld of fi bre optics and the ability to send signals through fi bres no wider than a human hair.
Why use a fibre optic sensor? The ability of fi bre optic sensors to transmit light along a fl exible fi bre means they can be threaded into narrow locations. Because they fi t into small or awkward spaces, they can overcome mounting and installation problems, and can detect tiny objects.
Requiring little power, fi bre optic sensing systems can reach very high switching speeds. With no electrical circuitry or mechanical parts, the fi bres can safely transport light to a target through dusty, dirty or explosive environments, such as wood processing. Fibre optic sensors are highly reliable even under severe environmental conditions, such as extreme temperatures, vibration, shock, moisture, corrosive chemicals or electromagnetic ‘noise’. The detection principle is independent of object colour, shape or surface, so they can detect virtually any material – even transparent foils. Some fi bres can withstand repeat bending without signal degradation, making them useful, e.g., in robotic gripper applications.
SICK WLL80 Fibre Optic amplifier
High-speed conveyor detection of tea bags
Fibre optic sensor operation Light is transmitted along a central core, made of a single fi lament 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. A fi bre-optic amplifi er is used to transmit, receive and evaluate the light signal, as well as enabling the sensing parameters to be adjusted. The fi bre is the optical component that transfers the sender light from the amplifi er and back to a receiver in the amplifi er. Vendors off er an extensive range of fi bres and heads with versatile options for light dispersal and orientation, as well as for easy mounting. There are two types of fi bre-optic sensor: proximity and through-beam. In a proximity system, the sender and receiver are combined in a single fi bre head. The system is sensitive to the amount of light energy returned to the amplifi er. Precise switching thresholds can be set up to provide an output to the machine control. Through-beam fi bre optic sensors transmit a light beam between a sender and receiver fi bre. As soon as an object passes through the light beam, it is interrupted and the sensor switches. Through-beam fi bre optic sensors can
24 February 2023 | Automation
also be confi gured with heads that produce a detection grid, e.g., to count small components falling from a chute into a tray.
Fibre optic sensor considerations First, look for a versatile amplifi er that minimises inventory and check that sensors can be networked together without the risk of interference. SICK WLL80 sensors can be organised in banks in a master-slave confi guration, to enable automatic cloning of parameter settings for multiple devices. The SICK WLL80 also off ers the advantage of continuous threshold adaption, which adapts the signal strength, say, in a dusty environment, or because of contamination build-up on the fi bre heads, to maximise periods between maintenance. There are some limitations associated with using fi bre optic sensors: it can take a little more eff ort to install and set the sensor parameters, for example. However, fi bre optic sensors are versatile for applications in almost any industry, so it’s always worth considering whether they are the best solution.
CONTACT:
SICK (UK)
www.sick.co.uk
automationmagazine.co.uk
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