SENSORS & SENSING SYSTEMS FEATURE
FIBRE PHOTOELECTRIC SENSORS I
The choice of fibre photoelectric sensors causes the most difficulties in the production workplace when considering the use of an optical sensor. In principle they operate in the same way as the ubiquitous standard optical sensor. As with standard optical sensors they operate with three basic principles, as Richard Thornton of Panasonic Electric Works UK explains
n diffuse reflective types the light emitter and light receiving element are
mounted in the same unit. An LED emits a light beam towards the object to be detected. Should the object be present a certain amount of light will be reflected back towards the sensor where the detection element is mounted. This type of sensor has the distinct advantage that it is self contained and all wiring can be contained in the single unit. Thru beam sensors have separate units
for LED emitters and the receiving element. An object to be sensed interrupts the light beam travelling from the emitter to the receiver. This allows a long sensing range since the LED output can be made very powerful if needed. This type of sensor is not affected by the shape, colour or material except in the case of a transparent material which may allow enough light to pass to escape detection. Of course because the sensor consists of two separate units correct placement and beam alignment is required and both units need external wiring that can affect mounting areas. The final type is retro-reflective. This is
a single unit sensor that contains both a LED emitting element and receiving
Above: cable presence detection
the sensor electronics and casing may be at some distance from the actual point of sensing since the light beams are transmitted through the fibre to the point of detection. This allows for variation in what can be achieved in terms of size of mounting space, resistance to hazardous or corrosive environments, extreme temperature ranges, and ease of replacement if the fibre head suffers damage.
element in the same housing. However, the beam is focused onto a separate reflector placed at the focal point of the beam. An object is detected by interrupting the beam along the light path. The advantage of this type of sensor is that only one side is wired with very easy beam alignment and the sensing is not affected by shape, colour or material. These three principles of operation are
mirrored in fibre photoelectric sensors. However, there is a difference in that
/AUTOMATION
Above: the FX500 Series Fibre Amplifier
FIBRE CONTROLLERS In many fibre product ranges a few sensor control units are used to operate with a large variety of fibres. As such many fibre control units are multi-functional in order to include both typical and specialised control features. Most fibre controllers are capable of all three detection methods and use a digital display to allow visual set- ups. The vast majority of fibre sensor controllers will include very basic functions such as dark on/light on selection, PNP/NPN output selection and timer options. Some may include a ‘teach’ mode in order to easily set-up the sensor. The teach mode uses a button to register sensing with an object and then sensing without an object and uses this information to determine detection parameters. More advanced teach modes can offer background suppression and light variation compensation. Use of an automatic attenuation function allows the reduction in the LED light level should the receiving detector receive saturated light as a result of the detected object being too close to the fibre emitter. Over longer periods dust and contamination can result in detection parameters altering and a threshold tracking control can help to avoid false triggering or no detection under these conditions. High specification fibre sensor controllers add to the basic functions by offering more specialised controls such as threshold windows that can judge the presence of an object only within the set distance window, ultra high speed (µs) response times and communication ports to allow transfer of information to external systems.
INFRARED One important option that the controller can offer is a built-in infrared interface.
This interface allows all settings to be copied from one amplifier to the next. This is very useful for applications with identical tasks, (for example to confirm all identical holes drilled in the sensing object). For production in lots it is often a requirement that the controller can store different ‘batch’ settings. The settings also need to be adjusted via an external controller. Using an RS422 infrared adapter, the intensity values of the light received can be analysed and all necessary settings made. Fibre selection is more specialised with
many hundreds of variants being available. Groupings include types in diffuse, thru- beam and retro-reflective configurations. Standard plastic fibre types meet the majority of applications and are usually the most cost effective types to consider. Typically the range offers fibre lengths from 0.5m to 5m and detection of objects’ down to 0.5mm wide. Detection distances range up to one metre for thru-beam types to 20-50mm for diffuse types. Wide and narrow beam types are
designed for use with particular sized objects and to cover specific areas. For wide beam types, the beams can cover a wide area of +30mm whilst minute object versions can detect very small items such as chip components or very small surface mount components. For diffuse reflective types, the focal point can be reduced to as little as 0.1mm with the help of a special accessory lens. A large number of types are available for
specific environmental situations. They include many specialised fibres suitable for liquids, vacuums, chemical exposure, fuel detection, tough types, glass detection or for very high flexibility as may be found in robotic applications. Heat resistant special fibre-optic sensors cover temperatures from -60°C up to 350°C. The sheer number of optical fibres
available can overwhelm the inexperienced user and so in many cases it is important to contact a reputable manufacturer in order to assess the requirement and get an educated suggestion on the best options available.
Panasonic
www.panasonic-electric-works.co.uk T: 01908 231 555
Enter 235 AUTOMATION | JULY/AUGUST 2014 33
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