Christoph Franz

Real-time process control by machine vision

Quality assurance in laser processing is often provided by post process inspections with machine vision solutions. When process stability gain is needed, one-dimensional signals like diodes or pyrometers which measure the emitted radiation of the laser process may be applied to closed-loop-control of e.g. the laser power. Such signals mostly provide short latency and good signal to noise ratio, but represent only mean values of the whole processing area or parts of it.On the other Hand secondary process radiation can provide information about the process stability, but may not always show the cause for process imperfections. To overcome the lack of spatial resolution in closed loop control, this paper explains how real-time computer vision improves existing processes and may even enable new laser processes. Furthermore an example machine vision algorithm is presented which measures kinematic properties of the process zone in real time. Such signals provide reliable information of the process state by recovering the influences which cause the process to leave its stable state.The shown system can be adapted to most laser processes (e.g. welding, brazing, cutting, selective laser melting, etc.). For complexity reasons most examples in this paper will be based on remote laser welding only.Quality assurance in laser processing is often provided by post process inspections with machine vision solutions. When process stability gain is needed, one-dimensional signals like diodes or pyrometers which measure the emitted radiation of the laser process may be applied to closed-loop-control of e.g. the laser power. Such signals mostly provide short latency and good signal to noise ratio, but represent only mean values of the whole processing area or parts of it.On the other Hand secondary process radiation can provide information about the process stability, but may not always show the cause for process imperfections. To overcome the lack of spatial resolution in closed loop control, this paper explains how real-time computer vision improves existing processes and may even enable new laser processes. Furthermore an example machine vision algorithm is presented which measures kinematic properties of the process zone in real time. Such signals provide reliable information of the process state by reco...

Comparison of process monitoring strategies for laser transmission welding of plastics

Laser transmission welding of plastics in industrial applications is expected to be robust against material quality variations and cleanness variations1. The process has to be applicable tolerating all workpiece dimension variations and different assembling positioning within the given tolerances. The inevitable failures of the welding process have to be detected reliably in each part even in large lot sizes.Process monitoring offers an alternative to post-production quality testing which is often destructive, and provides a complete recording of the manufacturing process, as required by official quality standards such as DIN EN ISO 9001.Within the scope of this paper the specific informative capabilities of coaxially spatially-integrated and coaxially spatially-resolved monitoring as well as monitoring of secondary radiation and monitoring by using external illumination are being compared and evaluated. Therefore a welding head is being developed using the already existing modular optical CPC-system of the Fraunhofer ILT. The Coaxial Process Control (CPC) system monitors the machining process in coaxial alignment with the laser beam axis. The welding head allows simultaneous observation using both observation methods.The emitted radiation by the process itself is being detected by a spatially integrating pyrometer and can be spatially resolved by a thermal camera. The visible spectrum is being reflected to a CMOS camera. It allows spatially resolved and spatially integrated monitoring at the same time.The modular system allows also coaxial and off-axis illumination, so coaxial, brightfield and darkfield workpiece illumination is provided and offers different illumination strategies for different materials and welding methods.Laser transmission welding of plastics in industrial applications is expected to be robust against material quality variations and cleanness variations1. The process has to be applicable tolerating all workpiece dimension variations and different assembling positioning within the given tolerances. The inevitable failures of the welding process have to be detected reliably in each part even in large lot sizes.Process monitoring offers an alternative to post-production quality testing which is often destructive, and provides a complete recording of the manufacturing process, as required by official quality standards such as DIN EN ISO 9001.Within the scope of this paper the specific informative capabilities of coaxially spatially-integrated and coaxially spatially-resolved monitoring as well as monitoring of secondary radiation and monitoring by using external illumination are being compared and evaluated. Therefore a welding head is being developed using the already existing modular optical CPC-system of t...

Measuring process speed at tool center point

Advanced laser sources enable manufacturers to produce weld seams of high quality at high process velocities. These processes require high degrees of precision and furthermore, process parameters have to be kept in close limits. The welding speed is one of the most important parameters among laser power which has to be held constantly in order to achieve a stable quality of the manufactured products. Only a constant energy input per unit length may ensure steady qualities of welded or brazed seams.Even by highly sophisticated handling systems, the influence of mass inertia cannot be fully suppressed when moving with high accelerations. Small deflection angles at the processing head cause large displacements at the Tool Center Point (TCP).In this paper new approaches to measure the real velocity at the TCP seamlessly are being published. A new image processing technique provides the ability, to measure welding velocities at high precision and high sampling rates directly on the work piece. By combining the values of the measured laser power at the processing head with the actual velocity, this technology enables the direct measurement of the energy input per unit length at real time. This new measurement method offers a high degree of quality gain and may even empower the end user to keep welding depths in small boundaries. Especially laser welding at long focal distances and setups with handling systems of mere precision (i.e. remote welding) will benefit from this measuring method.Advanced laser sources enable manufacturers to produce weld seams of high quality at high process velocities. These processes require high degrees of precision and furthermore, process parameters have to be kept in close limits. The welding speed is one of the most important parameters among laser power which has to be held constantly in order to achieve a stable quality of the manufactured products. Only a constant energy input per unit length may ensure steady qualities of welded or brazed seams.Even by highly sophisticated handling systems, the influence of mass inertia cannot be fully suppressed when moving with high accelerations. Small deflection angles at the processing head cause large displacements at the Tool Center Point (TCP).In this paper new approaches to measure the real velocity at the TCP seamlessly are being published. A new image processing technique provides the ability, to measure welding velocities at high precision and high sampling rates directly on the work piece. By combining the...