Optical Coherence Tomography for laser welding

The innovative OCT system is finally breaking ground, providing tailor-made solutions for real-time seam tracking, µm-accurate process monitoring and quality assurance for different processing applications, primarily for laser welding.

  • Real-time process seam tracking, process monitoring and quality assurance for laser welding and brazing
  • Direct high resolution height measurements and multidimensional visualization
  • Fast and precise online detection of seam joint location, focal position, weld penetration depth as well as profile of the cooled seam
  • Online detection and auto-correction of weld faults before the weld fails, solving weld irregularities faster, thus reducing scrap rate, test costs and rework
  • Compact and lightweight design, small interfering contour

 

                             OCT IPC

                  OCT sensor                                   OCT scanner                                    OCT IPC

     

  • Easy adaption of the OCT scanner to conventional laser processing heads through the camera port
  • Measurement, evaluation, and control facilities are at the operator site and coupled to the OCT scanner
  • Robust industrial design (dustproof construction (IP-64)) with robot-drag adapted cables and interfaces
  • Communication via standard fieldbuses (Profinet, Profibus, Ethercat, Devicenet, Ethernet, Ethernet/IP, 0–10 V analog signal)
OCT system setup; example for system integration to Lessmüller LSO welding optics

OCT system setup; example for system integration to Lessmüller LSO welding optics.

 

OCT enables high accuracy no-contact seam tracking and opens up new possibilities for modern online process monitoring and quality assurance.

In the OCT sensor the light beam from an integrated broadband source is split into a reference and a sample arm. The split light beams are reflected at the reference reflector and at the surface being worked, recombined, and evaluated by spectrometer.

By comparing the optical distance from reference arm and the measured surface a depth measurement is acquired. Three-dimensional profiles are obtained by lateral scanning of the workpiece with OCT scanner and simultaneous acquisition of depth measurements with OCT sensor.

During the active welding process, the OCT scans the workpiece surface with the rate of 98 400 measurement points per second per scan and with µm-accuracy. One scan covers the surface of the joint directly before welding, the melt pool and the cooled seam.

Schematic representation of the OCT measurements (doted blue lines) performed on the fillet weld in 0° incidence configuration. Left and right are OCT images with respective cross-section photographs of the joint (pre-process) and the weld bead (post-process)

Schematic representation of the OCT measurements (doted blue lines) performed on the fillet weld in 0° incidence configuration. Left and right are OCT images with respective cross-section photographs of the joint (pre-process) and the weld bead (post-process).

 

Evaluation algorithms, which are integrated in the OCT, detect the seam joint position proximate to processing zone in real-time with high degree of accuracy.

The OCT output signals are fed back into the processing head, the robot or the guiding system for a full closed loop operation and the exact positioning of the laser beam. Simultaneously, the weld penetration depth is determined by depth profiling of the melt pool along the process direction, thus acquiring the maximal depth.

This value can be used for online adjusting the power of the processing laser (if applicable). In addition, the surface profile of the cooled seam is measured and gives insight into the seam quality.

 

Adaptation to laser remote welding

Using OCT, a processing head is capable to follow joints or traces on the workpiece self-guided, which means independent of the actual movement of the processing head and without any prior calibration or information of the trajectory.

Example for system integration to scanner-based remote welding optics (here: SCANLAB intelliWELD 30 FC V)

Example for system integration to scanner-based remote welding optics (here: SCANLAB intelliWELD 30 FC V).

 

Since the probe beam is coaxial to the processing beam, it guarantees measurements in machine coordinates irrespective of the welding direction.

OCT offers the flexibility in choosing the scan region and in positioning of the beam within the scan region.

OCT has a long working distance and is therefore not influenced by the focus changes caused by tilting of the scanner mirrors or changes in the focusing distance.

The system utilizes a length modulator that allows determining the distance to every single point on the workpiece surface at variable focusing of scanner-integrated remote welding system in the working range of 390 mm.

OCT affords a relatively wide tilt tracking angle without requiring any recalibration. Thus it can provide “on the fly” tracking and height based image analysis in particularly for the geometries that create problems for traditional structured light measurements.

OCT can be applied at any orientation and any user defined distribution of weld seams.

 

Advantages:

  • Precise closed loop controlled detection of the seam joint position
  • μm-accurate process monitoring and quality assurance for premium quality welds
  • OCT accuracy is not limited by the process glow
  • Obstacles like fastening fixtures in process surrounding do not have any influence on the tracking results and the cycle times
  • High axial measuring accuracy regardless of focusing
  • In-process seam depth measurement (and adjustment if applicable)
  • Acquisition, evaluation, video recording and documentation (including quality assurance) as well as review of the OCT data of past processes
  • Processing speed is compatible with high welding speed permitting to realize time efficient “on the fly” processing and enabling flexible production
  • Simple QA-solution for various laser welding and brazing processes
  • Operating reliability for high accuracy applications