Photoelectric Sensors
(8 230)Photoelectric sensors available in the TME catalogue are designed to be used primarily in industrial automation systems. However, optoelectronic components can also be found in our offer. Such components are used in the manufacture of other, more complex devices, such as alarm systems sensors, automatic lighting controllers, etc. Describing every single component and solution provided by our suppliers would be a well-nigh impossible task. To make things simple, we will focus on the basic division of photoelectric sensors and the characteristic features of individual product groups.
Photoelectric sensor types
Firstly, it is important to note that the type of a sensor can refer to several parameters – the production technology of an emitter or a sensor, the way it works, its structure, purpose, mounting method, etc. Several examples are listed below, however they are not mutually exclusive categories. For instance: reflective sensors can be PCB-mounted and use infra-red light at the same time.
What’s the difference between reflective sensors and the through-beam transmitter-receiver set-up?
The main division in the photoelectric sensors group when it comes to how they work is related to the electromagnetic radiation propagation. Most sensors emit light (you can read more about its range soon), which is then picked up by the sensor. The light can be reflected from a detected object and return to the source module (reflective sensors) or transmitted to a receiver located away from the transmitter (through-beam transmitter-receiver set-up). In the latter case, when light with specific parameters (i.e. the same light that was emitted) does not reach the sensor, the output of the sensor will return the status informing the user that there is an object disrupting the light’s travel line between the light source and a receiver. This kind of devices boast extremely high precision and a large maximum work distance (especially if it employs the laser technology).
Sometimes, however, placing separate transmitter and receiver modules is not an optimal or doable solution. In such cases, it might be better to opt for a reflective sensor with a reflector – it combines both the emitter and the sensor in a single housing, but instead of detecting light reflected from the object’s surface, it reacts to the interruption of the beam reflected by the mirror. Reflectors can be found in a separate branch of the TME catalogue.
Infra-red, UV, laser and microwaves
Electromagnetic radiation of varying lengths is used in photoelectric sensors. Each wavelength and type of radiation have their own benefits to them, fit for appropriate, desired applications. For instance, microwaves boast excellent propagation in polluted or smoky air. They can also penetrate some materials – this feature makes it possible to detect large objects (solid matter), while ignoring materials such as foil or liquids at the same time. Moving on, infra-red radiation (IR) can be used in environments which are intensely illuminated by different types of light (background disturbance), even under fairly strong insolation. It is a crucial attribute in places such as production lines and warehouses, where comprehensive lighting systems are installed for the safety and comfort of workers. Another type of electromagnetic radiation used in sensors is ultraviolet. It is important to note here that all the aforementioned sensors use radiation that is invisible to the human eye, which is another reason why they are a good solution for applications such as alarm systems.
Regardless of the light frequency used, the sensor category that deserves to be highlighted are laser sensors, which employ a narrow beam of concentrated light. Such solutions boast high precision and response time, facilitating the control of mass production in fields that require utmost precision, such as manufacturing of electronic components. Another important asset of lasers is their extremely long effective range.
Motion sensors
Motion sensors often don’t require a transmitter, since their operation is based on detecting more and less subtle changes in the natural light illuminance (or the illuminance of light emitted from different sources). The most basic example of such devices are automatic light switches, commonly used in residential buildings, corridors, parking lots, etc.
Photoelectric liquid level sensors
There are several technologies commonly applied in liquid level detection. The oldest ones are switches or reed relays used in pair with a float. The use of a photoelectric sensor for this purpose has a certain advantage, namely excellent safety and no need to tamper with the tank/reservoir’s structure. What is more, this method can be used to monitor the status of virtually every substance, since no component comes into direct contact with the liquid and electromagnetic radiation remains neutral to most chemical compounds.
Colour and contrast sensors
A unique type of sensors are devices that can detect colours and contrast. They are used for automatic segregation, discolouration detection and label reading. Colour detection in its most common form is made possible through the alternating emission of light in different colours (i.e. RGB, that is red, green and blue) and measuring how effectively the surface can reflect the radiation of different wavelengths. Based on the information obtained in this way, the digital system can then define the colour of the object with high precision. It is important to note, however, that due to the method of operation, such sensors have a rather low maximum sensing range and a slightly slower reaction time. They also require to be provided with optimal optical conditions during the measurement (to eliminate the risk of interference).
Barcode readers
Barcodes have become a key element of supply chains, logistics systems and warehouse/depot management. A special type of photoelectric sensors is applied to make reading those barcodes possible, using a laser beam reflected from an oscillating mirror. A barcode reader is therefore a module combining several elements, including electromechanical and electromagnetic ones. Depending on the model, the scanner can read the codes using a built-in microprocessor system and transfer data through a digital bus, or transmit the data related to the currently read code fragment to other transducer that is responsible for data processing (a transistor output is most commonly used in that case to ensure the required transmission speed).
Light curtains
Light curtains (laser light curtains) are another type of integrated photoelectric sensors. It is a solution that is most commonly found in elevators and other environments with an automatic open/close system. They are used for detecting objects in a singular plane of a relatively large surface. Light curtains are composed of transmitter and receiver modules (sometimes it can be a pair of components that combine both functions). They use an array of laser transmitters emitting a dense beam stream, which allows a curtain to detect even small objects. Such devices are mainly used as security systems, since they can be employed to, among other things, detect unauthorised access to the area, which can pose a threat of health or life loss (productions lines, chemical sprays, etc.) or to confirm that there are no obstructions or people at the door/shutter closing point.
PCB photoelectric sensors
Both modules for industrial applications and components for manufacturing more complex equipment can be found in the category of photoelectric sensors. Their design allows sensors to be PCB-mounted (usually using the THT technology). The motion, reflective, colour sensors mentioned above, and more, are available in this format. A specific element that is present almost exclusively in the compact version is a (through-beam) transmitter-receiver set-up with a slot. It is used in the production of encoders, which can detect a position of engine shafts, and in circuits measuring the rotational speed of said shafts. The way such module works is very simple – in the orifice between the emitter and a sensor, a disc with extruded radial slots is placed. Then, by counting interruptions of a light beam in a specific period of time it is possible to tell how fast the disk is rotating and, by extension, which axis it was mounted on. The slot element can also be used to control the flow of perforated tapes (such technique is used e.g. in automatic feeders of SMD components).
Applications of photoelectric sensors
Detecting objects and their location using light waves has a range of benefits. It’s non-invasive, safe, soundless, resistant to numerous environmental hazards and can be performed at large distances. What’s more, photoelectric sensors boast excellent reliability and long service life. Thanks to all those features, they enjoy unwavering popularity in fields such as industrial installations (production lines), warehouses/depots and logistics.
Important parameters of photoelectric sensors
Depending on the type of the sensor, it will be characterised by its specific parameters. However, four of them remain unchanged. The first one is the sensing range, which determines the minimum and maximum allowed distance between the face of the sensor and the detected object (for the through-beam transmitter-receiver set-up, it will be a distance between the modules). The values can range between several millimetres up to dozens of metres. Picking the right solution will always depend on the required precision, available space and the size of the monitored objects.
Operating frequency determines the intervals in which the sensor’s output state is refreshed. It is related, but not tantamount to reaction time. The delay always depends on several aspects, for instance: type of the sensor, output interface (a transistor output will always be faster than a digital bus), etc.
When choosing photoelectric sensors, one needs to take into account the output configuration (type). It will be obvious when constructing a new installation, but slightly more complicated when searching for replacement parts or expanding the original system. A module can have a transistor output (in NPN or PNP configuration), a relay output (with a normally closed or normally open circuit), as well as an analogue output (current, voltage output). The output type is related to another specific parameter of photoelectric sensors, i.e. the operating mode. It can be described as “DARK-ON” or “LIGHT-ON”, which tells us quite literally whether the output will activate when it becomes dark or when light is detected.
Other characteristics
A range of parameters which are used to describe the capabilities and operation ranges of photoelectric sensors does not require an additional comment. In product’s documentation, manufacturers specify parameters such as maximum allowed supply voltage, thermal tolerance of the module, its power consumption and the maximum allowed load capacity of the outputs. Due to varied requirements of industrial processes, a body material will also be specified (e.g. materials with unusual properties, such as high resistance to oils or corrosive substances). Naturally, there is also information about the terminal type (sensors are often manufactured with pre-installed wiring) and the protection rating of the sensor’s body. The latter will rarely go below IP65, which stands for full dust ingress protection and high splash resistance (often, the sensor body is fully waterproof).
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