The principle of triangulation – the geometric measurement using three-point relationships – has been known for many centuries. In the 18th century, this land survey method established itself in Europe. However, the procedure can also be applied on a significantly smaller scale. Using laser line triangulation, geometries can be measured very precisely on different surfaces. Sensors from Micro-Epsilon are based on this principle and can be used in many applications.
In order to operate a triangulation sensor using laser light, a laser beam is focused onto a target. A sensor array with high spatial resolution, which is tilted at a fixed angle to the laser beam, detects the diffuse reflection on the surface. If the distance between the sensor and the target changes, the angle at which the reflected light is transmitted onto the sensor array changes too and therefore also the position of the reflected light on the sensor array. The distance between the sensor and the target surface can be calculated via the angular relationships in the triangle. This measuring technique enables measurement accuracies in the submicrometre range.
This measuring principle can also be extended to two dimensions by focusing the laser to a line rather than to a point. This procedure is then known as laser line triangulation. A two-dimensional sensor element instead of a sensor array acts as the sensor. When the laser line is projected onto a shaped surface, the straight laser line becomes a surface profile – this is why they are also called laser profile sensors. This surface profile can be calculated from the intensity values of the reflected light, which is transmitted onto a position-sensitive sensor element. Even if a third dimension is added, the target can be moved vertically to the laser line or the profile sensor can be moved vertically over the target. The sensor becomes a 3D laser profile scanner and so can produce a three-dimensional image of the surface.
Key factors to consider in laser triangulation
The performance of laser profile scanners depends on many factors. When all essential components including the laser source, the focussing lens for the laser line, the receiving optics and the sensor element, are housed in one body, the conditions are perfect. The thermal and mechanical stability of the laser profile scanner is therefore much easier to control. Also, the distance between the sensor and the target, as well as the lighting, influence the quality of the measurement results. In order to suppress light interference, a filter is installed in front of the receiving optics, which is only permeable to the wavelength of the laser light applied. In addition to these key factors, other technical characteristics of the laser profile scanner also play an important role. The quality of the lens and the laser diode used decide how sharp the line can be focused onto the target. The spatial resolution of the sensor matrix and the performance of the processor used are primarily responsible for how precise and fast a surface profile can be calculated from the reflected light. In addition, the wavelength of the laser light plays a part in determining the precision of measurement results. The laser line of a blue laser diode can be more sharply focussed onto the target than conventional red laser diodes. As blue laser light does not penetrate the surface too deeply, the measurement results are further improved. Only surfaces that are difficult to measure using conventional red laser profile scanners – e.g. organic materials such as wood and food, or semi-transparent materials – are significantly better detected using blue laser profile scanners.
Powerful scanners
Micro-Epsilon offers a wide range of laser profile scanners based on the laser line triangulation measuring principle. The scanCONTROL series is among the highest performing profile sensors in the world. The scanners operate using a CMOS sensor matrix that provides a resolution along the x-axis – the direction of the laser line on the surface – of up to 1,280 points. The scanCONTROL 29xx-10/BL version sets a new standard in terms of profile resolution, with a laser line length of 10mm. This results in a point distance of just 7.8µm, which enables the laser profile scanner to provide twice the resolution of previous laser scanners with a 25mm measuring range. The resolution in the z-axis can, depending on the sensor version, be less than 1µm. Their high accuracy enables the laser profile scanners to detect even the smallest of parts to the highest precision. The resolution along the y-axis essentially depends on how precise the target can be moved relatively to the scanner. With measuring rates up to 4,000Hz, surfaces can be measured very precisely and at very high speeds.
The scanCONTROL laser profile scanners are equipped with a powerful controller that is already integrated inside the housing. The controller calculates the two-dimensional surface profile from the intensity values on the CMOS sensor matrix. Also, an evaluation of the profile is possible by using the SMART sensor versions. These enable frequently recurring, easy and complex measurement tasks to be performed directly in the sensor and to output these as measurement values. Set up and configuration is carried out via the Configuration Tools PC software, which enables, for example, the measurement of steps, angles and grooves. Parameter sets are recorded in the sensor. In addition, output of an OK/NOK signal is possible. The user does not require any external control and evaluation unit. For example, the height of a step on a surface can be measured. The height can then be output as an analogue or digital signal via the interfaces. Alternatively, the scanner can output the “OK” signal if the value is in a predefined range; otherwise the signal is “NOK”. From a total of 26 measurement programs, an individual parameter set with up to eight programs can be used with the scanner (‘user modes’). Furthermore, it is possible to save up to 15 parameter sets in the scanner and to switch these via digital inputs.
If the user wishes to evaluate the profile data externally, the laser profile scanner can also output all raw profiles of the sensor matrix. An Ethernet interface with GigE Vision enables connection to a PC. In order to simplify the integration into a customer’s own software, Micro-Epsilon offers libraries for C, C++ and C#, as well as LabView drivers. Integration into Linux environments is not a problem either due to corresponding libraries.
Versatile application possibilities
The laser profile scanners from Micro-Epsilon are used in many different applications. Where high measurement accuracy and resolution are required, the scan CONTROL series demonstrates its strength. Typical applications can be found in precision mechanics, electronics and in the manufacture of precision parts. Also quality control in laser welding is possible. For these and other applications in harsh environments, Micro-Epsilon offers special accessories to protect the scanner. For example, a protective housing with exchangeable windows is available for welding applications. A special compressed-air purge system protects the optical components from dust. The scanCONTROL can be inserted into a cooling jacket for use in high ambient temperatures. The position of razor blades, the completeness of weld seams, the optimum dose of an adhesive and the correct gap size on a car chassis – all of these applications have been successfully solved in recent years using precision laser profile scanners from Micro-Epsilon.
For more information,
Website: www.micro-epsilon.com
