Laser eyes for robots

11 February 2019

Laser profile sensors act as the ‘eyes’ of a robot and are now being used in a variety of applications, says Chris Jones.

As well as inspecting one-dimensional parameters, there is now an increasing need for multi-dimensional quality control in industrial production. Laser profile sensors (laser line scanners) are designed to carry out complex 2D/3D measurement tasks in robotics and other automated, high-speed production environments, where the sensors are increasingly used to measure profiles and contours.

The operating principle of a laser line scanners is based on the laser triangulation technique for two-dimensional profile detection. They detect, measure and evaluate the profiles on different object surfaces. By using special lenses, a laser beam is enlarged to form a static laser line instead of a point and is projected onto the target surface. An optical system projects the diffusely reflected light of this laser line onto a highly sensitive sensor matrix. In addition to distance information (z-axis), the controller also uses this camera image to calculate the position along the laser line (x-axis). These measured values are then output in a two-dimensional coordinate system that is fixed with respect to the sensor. In the case of moving objects or a traversing sensor, it is possible to obtain 3D measurement values. Laser line scanners are equipped with an integrated, highly sensitive receiving matrix that enables measurements on almost all industrial materials, largely independent of the surface reflection.

A typical application for laser profile scanners can be found in the inspection of adhesive beads in smartphones. The challenge here is the very fine contours inside the smartphone and extremely thin, semi-transparent adhesive beading. Here, absolute reliability and 100% control of the completeness of the beading, the height and width of the applied adhesive are required. This also applies to logos on tablets and laptops: grooves are milled into the aluminium housing, in which the logo elements are glued afterwards. The latter must be flush with the housing. Haptics (touch sensation) is a critical factor, as the customer would immediately feel any protruding logo or depression. Using laser line scanners, these depressions can be measured to determine the planarity as well as the depth. The parts being glued are also measured in order to ensure a perfect fit.

Laser line scanners are also carrying out important inspection tasks in the automotive industry. For example, airbag covers on steering wheels should be functional and not impede the airbag, while opening during an accident. They also form part of the interior decor of the car. As interior parts, the covers must therefore harmonise with the overall appearance. The high aesthetic requirements placed on the cockpit, seats, interior trim and steering wheel are particularly important for this functional component.

 While assembly gaps cannot be measured prior to installation, like many other elements of a car, the airbag stitching can be inspected during production. Guided by robot arms Micro-Epsilon scanCONTROL laser profile sensors can evaluate several features synchronously. The laser scanner continuously inspects the distance between the stitching and the separating point between the single stitches and outputs the evaluation directly as ‘O’ (NOK) or ‘1’ (OK) via its Ethernet interface. Beyond that, the height difference between two single stitches is directly inspected in order to recognise any faulty assemblies immediately.

Gap monitoring in car interiors
Car interiors present a number of gaps, for example, between the single cockpit elements such as the centre console or door trim. From a technical point of view, it is often irrelevant if the width and height of a gap are constant. However, the interior is an area of the car that the customer sees first. Even minor faults can therefore catch the buyer's eye and negatively influence their purchasing decision. The gapCONTROL profile scanner can inspect the gaps of all parts in the car interior during final assembly.

Depending on the inspection situation, a single scanner applied on a robot arm can measure different gaps in a static or dynamic mode; alternatively, a special frame on the robot arm can be used to enable the scanner to detect a number of different gaps in the interior in static mode within fractions of a second. The sensor then evaluates the measurement values and sends a signal to the control system if the values measured lie within the tolerances defined by the customer.

Robotic weld inspection systems
Laser profile sensors have also been installed on an automatic weld inspection system that is designed to inspect thin section steels and thin gauge welds on ships and other marine vessels. The system inspects the weld and classifies it by means of laser, ultrasound and electromagnetic methods. The system uses a crawling robot that deploys remote volumetric surface and visual inspection to verify the integrity of welds on the hull plates of ships during manufacture and in the servicing of shipping vessels.

Other inspection techniques have proven to be disruptive to the manufacturing process and are far from cost effective. New generations of ships are being built from thinner section steels in order to lower the cost of build and ship operations and so typical assessment methods are not as effective as they were for thicker sections. There is, therefore, is a genuine need for a more reliable, faster, cost effective and safer inspection technique.

Chris Jones is managing director of Micro-Epsilon UK.

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