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Posted on 01 November 2019

New Applications for Optical Sensors

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Triangulation sensor instead of camera

Infrared sensors that work using the triangulation principle also measure accurately on reflective surfaces and are not affected by either light scatter or high temperatures. When used with a pyroelectric resistor, the wide-angle versions can even distinguish between people and objects. They can therefore be used in applications for which only image processing has previously been possible.

By Andreas Biß, Product Marketing Optoelectronics, Sharp Microelectronics Europe

 

Up to now optical sensors have mainly been used in printing and copying systems. Depending on the model and design, however, detectors of this type can also be used in other applications in which it is necessary to recognise objects. They could be used in industrial plants, vending machines and cash dispensers, in sanitary applications or in vehicle interiors. Infrared (IR) sensors can trigger predefined switching processes and can be used with wide-angle sensors to control robots. Previously, only cameras were able to record objects and environments in three dimensions. What is more, the efficiency of camera technology is undisputed and image resolution is high even over a distance of several hundred meters. The optical imaging method has disadvantages, however. For example, maintenance costs are high as the appliances are sensitive to contamination. In addition, processing image data takes up computer capacity and cameras are more expensive than IR sensors. They are therefore not suitable for price-sensitive applications. Many applications also do not need high-resolutions sensors, for example if they only need to detect objects and estimate their distance.

Angle measurement increases accuracy

Many object detection applications can make do with a simple IR distance meter. Sharp offers a wide range of such meters with analogue and digital distance-measuring sensors (DMS). Depending on the model, they have a range of between 4 mm and 500 cm. Unlike traditional IR sensors, which use the intensity of the reflected IR beam to determine the distances, Sharp sensors use trigonometry. This means that they determine the distance to the object using the angle at which the reflected light reaches the detector. This new generation of sensors is based on the "Position Sensitive Device" (PSD) developed by Sharp, which records the reflected beam at different points on the PSD surface. The electrical currents I1 and I2 (Figure 1) that are generated are directly proportional to distances d1 and d2 between the point of impact of the beam and the outside edge of the PSD. This means that the distance can be clearly assigned to an output voltage. In analogue sensors, an internal circuit converts the PSD signal into an equivalent voltage between 0.4 and 2.4 V. In digital sensors, the logic state of the initial output pin changes as soon as a predefined distance threshold is reached (Figure 2). Compared with traditional distance sensors this mode of operation offers very accurate measurements and long lifetime. Furthermore, measurements are not affected by light scatter or temperature fluctuations or distorted by colours or reflective surfaces. Even rapidly-moving objects cause little or no distortion, depending on the application and type of sensor. Such distinctions are easy for the human eye, but this is an outstanding performance for an optical sensor, especially since it only operates with the monochrome infrared light of an LED.

Position Sensitive Device

Digital sensors changes as soon as a predefined distance threshold is reached

Short reaction time, wide temperature range

A distance-measuring sensor tolerates different surfaces and movements. This has advantages for many measurements. DMS are used to check the filling level of containers in filling systems, for example. As many different products are processed in modern plants, each product has to be monitored reliably. It is important to avoid overfilling with apple juice, for example, and bottles of cherry juice should not leave the factory half-full. The filling system needs to be reset for each change of product if gravimetric methods are used. However, this is not the case with IR sensors, as the system does not need to be reset even if the products to be checked have different densities. As long as the bottle shape remains the same, there is no change to the filling level for a predefined volume. On the other hand, the originally weighed in quantity may be different for each product. Another important factor is that the filling level is the same for the 100,000th bottle or the first bottle.

Measuring, counting and switching

In addition to an operating temperature of – 40 to + 130°C, the short reaction time of only 8 m/s is also an advantage for distance sensors. This enables them to achieve a switching cycle of at least of 32 ms and record up to 30 parts per second. This is sufficient for many industrial production processes. This means that as well as measuring filling levels in filling systems and drink vending machines, the distance meters are also suitable as counters on conveyor belts or as sensors for controlling industrial robots. As a result of the wide operating temperature range Sharp DMS can be used even in situations where there a great amount of process heat is generated. In addition, DMS can be used as proximity switches to activate a vending machine or information terminal as soon as it is approached by a user. When the user moves away, the machine switches back to electricity-saving standby mode. DMS are therefore a simple and cost-effective way of saving electricity, particularly for machines that are only used sporadically. In vehicles, distance sensors could be used as dimmers for interior lighting or for selectively controlling ventilation and air-conditioning. The system could automatically detect which seats in a car are actually occupied and direct the cooling flow of air only towards the occupied seats. Optimising the use of the airconditioning system in this way will reduce petrol consumption, especially if the driver is the only occupant of the vehicle. Sharp now also offers a battery-operated DMS for sanitary applications. They are suitable for use where there is no power supply available or where electricity cannot be supplied for safety reasons - in rooms where water is present, for example. They are already used in automatic toilet flush systems, taps, hand-driers and paper dispensers.

Infrared eyes instead of a camera

Wide-angle sensors (WAS) are a further technical development of distance meters. Wide-angle sensors contain five DMS units, which are arranged horizontally with an aperture of 25°. This means that a WAS is not only able to detect the fact that an object is approaching, but also the direction from which it is approaching. If a number of DMS units are used, the aperture can be widened to up to 90°. Distances of 4 to 30 cm, 20 to 150 cm or 40 to 300 cm can be determined, depending on the type of sensor. The sensor that is suitable for long distances measures 53 x 20 x 21 mm3. Shorter-range models measure 41 x 20 x 21 mm3. If a WAS is also equipped with a pyroelectric sensor, it can distinguish between people and objects based on the differing levels of thermal radiation. The heat radiation is detected using pyroelectric crystals, which respond to changes in temperature with a change of status. Intelligent wide-angle sensors are therefore suitable as a cost-effective alternative to camera systems, for controlling self-propelled robots, for example. Sharp is currently supporting a development project, in which autonomous robots are used to measure buildings. This shows how useful it is for a WAS to be able to distinguish between people and objects and not to be sensitive to different surfaces and movements. If a robot encounters obstacles such as walls or items in a room during measurement, it will avoid them. If a person approaches, the robot will detect that it is a person on the basis of thermal radiation and wait until the person has moved out of the reception range before continuing its work. This means we are that much closer to the concept of robots not just measuring but also cleaning buildings as if by magic.

Wide-angle sensors (WAS)

Distance-measuring sensors (DMS)

Summary: you don’t always have to go for the expensive solution

The high-resolution recognition of objects and people is not needed in many areas of automation technology. For this reason, distancemeasuring sensors and wide-angle sensors are often a perfectly adequate and, above all, cost-effective alternative to camera systems. By combining them with pyroelectric sensors to distinguish between people and objects, the DMS or WAS can also function as sensors for self-propelled robots. 

 

 

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