High-speed profile measurement for industrial applications

Laser triangulation is similar in principle to the method that is used for surveying land using a theodolite. A laser projects a spot of light, which is monitored by a camera, onto the surface to be measured. If the distance between the sensor and the surface changes, the angle also changes at which the camera records the light spot. This change of distance can be determined very accurately using simple trigonometric calculations. The resolution possible using this method is down into the sub-micrometer range.

Extension to two or three dimensions

If, instead of a single spot of light, a line of an infinite number of points is used, the method can be extended to cover two dimensions. From the image of the laser line on the surface, which is recorded by the camera, a height profile along the laser line can be calculated. If the sensor is traversed over the object, an exact three-dimensional height profile of the surface is obtained. The scanCONTROL 2800/2810 Laser Profile Sensors from Micro-Epsilon Messtechnik operate precisely using this measurement technique. The system consists of an ultra- compact sensor, in which the laser and the image sensor are housed and an intelligent controller (Figure 2). The system is ideal for tasks in industrial environments. The integrated FireWire interface enables full control from a PC, also giving high data rates while simultaneously preserving the bandwidth.

To record the image, a CMOS matrix is used with a global, electronic shutter. This so-called "high-speed shutter" facilitates the simultaneous reading of all pixels in the complete image. The measurement therefore occurs synchronously along the laser line, whereby any distortion of the measurement image due to moving targets is eliminated. In addition, the shutter can be controlled in real time via an external trigger input located on the controller. In this way, a number of measurement systems can be synchronised with each other. The shutter enables the measurement system to work virtually independent of the speed and acceleration of the target. This is crucial in ensuring fast throughput times, particularly when the process speedvaries.

The specially developed CMOS matrix has an extremely high performance and supports the exposure of the next image while the current image is being read out. The advantage here is that a higher measuring frequency is possible, even if the surface of the target changes between shiny and strongly-absorbing. Depending on the application, either the data processing rate or the pixel resolution (x or z-axis) can be increased through the variable measurement field. Each second up to 256,000 pixels is processed. With a resolution of 1,024 pixels per profile, 250 profiles per second can therefore be recorded. If, however, the resolution is reduced to 64 pixels, the measuring rate increases to a maximum of 4,000 profiles per second.

Measuring profiles at high speed

The scanCONTROL 2810 system was specially developed for the measurement of beads (e.g. seams) or grooves (e.g. gaps or channels). By internally evaluating the measured profiles, the system determines various parameters directly in the controller. Using the digital and analogue outputs, control signals can be produced, which for example, are proportional to the gap width or other typical measurement parameters. Since the signal processing and evaluation take place directly in the controller, it is not essential to use a PC. The system can be easily configured via the set up and configuration software ‘ICONNECT’ which is included in the supplied items (Figure 3). With the software, the user can set various test criteria and boundary conditions, which are then saved in the controller.

If the measurement tasks are more demanding, the system can also be operated using a PC. In this respect, Micro-Epsilon offers a special industrial PC for DIN rail mounting, which apart from parameterisation, can also be used for visual displays and storage. In this way, complete measurement sequences can be logged and archived. Different parameter settings can also be saved so that the system can be switched quickly and easily between different measurement tasks.

Application for fitting windscreens

In car production today, windscreens are normally glued. The production process of applying glue to the glass is critical and is often difficult to control. The precise application of the special adhesive, which is carried out by robots, is of critical importance (Figure 4). However, it is not sufficient just to check whether the adhesive has been applied. The height and width of the adhesive bead and its position relative to the border strip must also be checked to ensure they lie within tight tolerance bands. The robot that applies the adhesive to the strip also carries out the inspection in a second movement. The system, scanCONTROL 2810, is used here. The system measures the profile of the adhesive beading and calculates the important parameters online directly in the controller. The system assesses the position of the adhesive beading relative to the strip, determines the width and height and provides deviations of the tolerance values as control signals via digital outputs. The tolerance limits for height, width and position of the adhesive beading are directly configured in the controller using the parameterisation software. The complete system therefore runs independently of an external PC.

In order to correctly detect beads of adhesive along curves in the beading track, or bulges and indentations in the material, an anchor point in the profile can be defined. This anchor point serves as a reference feature to provide orientation on the target, e.g. the tracking of the areas of the adhesive beading to be monitored. For this reason, the scanning line does not need to exactly follow these dents or raised sections. This simplifies inspection, particularly with adhesive beading with changing radii or a very uneven base material.

Apart from the high speed, the main reasons for selecting scanCONTROL for this application, were primarily its simple and rugged design. Other contributing factors are its compactness the analysis and evaluation of measured data takes place directly in the controller and a direct digital output is possible.

Flexible system

Potential applications for this measurement method are numerous. These universally-applicable sensors come into their own anywhere where surfaces or profiles need to be measured precisely and at high speed. Typical applications range from quality assurance in car tyre production, through to the inspection of welded seams and the measurement of wear on railway lines.