Chris Jones explains why laser line scanners are the eyes of your robot
Robots are becoming increasingly independent and are able to tackle more complex tasks. Laser profile sensors play a crucial role in ensuring that robots move safely, as well as precisely guiding and locating their positions. The sensors act as the ‘eyes’ of the robot and are used in a variety of applications, from gap measurement in car interiors and automatic weld inspection systems, to industrial pick-and-place systems.
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 therefore 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.
The Scancontrol series of laser line scanners from Micro-Epsilon offers a number of technical advantages in robotics applications. Their compact, lightweight design with integral controller means the sensors can be mounted on robot arms and in locations on machines where space is limited. The sensor and integrated evaluation electronics are space-saving, which simplifies cabling. Measured data is generally available in real time and so can be used to automatically correct and control the production process.
All laser profile sensors in the range are equipped with a Gigabit ethernet interface for transferring profile data, as well as a multi-purpose connector for RS422, encoder input triggering, Digital In (HTL/TTL), power supply and synchronisation. The sensors support power-over-ethernet (PoE), which means they can be operated with only one cable, thus simplifying installation even further. A simple and intuitive configuration interface is provided with all sensors to allow sensor configuration, as well as full SDK for more detailed machine integration. The sensors are configured with a unique IP address (user changeable), which enables future remote configuration and diagnostics. A Modbus protocol (supported via Ethernet and the RS422 interface) also enables direct connection between the sensor and a PLC.
The Smart version of the Scancontrol series offers a ‘plug & play’ solution with integrated controller that is suitable for simple-to-complex measurement tasks, eliminating the need for any external controller or PC. Set-up and configuration of the sensor is carried out via the Configuration Tools PC software, which enables the measurement of steps, angles, seams and grooves. The parameter sets are stored in the sensor, which is why the sensor autonomously performs measurements without requiring any external control devices or PCs.
Measurement of adhesive beads on smartphones
A typical application for laser profile scanners is 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 (i.e. touch sensation) is a critical factor, as the customer would immediately feel any protruding logo or depression. Using laser line scanners, these depressions are measured to determine the planarity as well as the depth. The parts being glued are also measured to ensure a perfect fit.
Stitching position of car airbags
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.
Although assembly gaps cannot be measured prior to installation, like many other elements of a car, the airbag stitching can be inspected during production. Scancontrol scanners are being used to detect the contour of the stitching while being guided by robot arms, evaluating 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 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 inspects 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 is used that enables 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 are also installed on a groundbreaking automatic weld inspection system that is specifically 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.
The Scancontrol LLT 2700-100 laser profile sensor is an integral part of the measurement system, providing the laser profile data required to verify and analyse the quality of thin steel and welds.
Most of the inspection techniques used today 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 to lower the cost of build and ship operations and so typical assessment methods are not as effective as they were for thicker sections. Therefore, there is a genuine need for a more reliable, faster, cost effective and safer inspection technique.
By combining ultrasonic phased array, electromagnetic alternating current field measurement (ACFM) and laser optical methods, the robotic inspection system enables the detection and sizing of surface breaking and sub-surface flaws or defects. By developing laser-based tracking and a self-controlled robot, the system enables automatic inspection following the weld run.
Chris Jones is MD of Micro-Epsilon UK
