TWI and Coherent collaborate on real-time laser weld monitoring using fibre-optics

Welding research and technology specialist TWI has been working alongside laser giant Coherent and automotive manufacturer Sodecia to produce a real-time fibre-optic cable-based monitoring system for laser welding.

A working principle of the active sensors inside a fibre-optic cable. (Image: TWI)

The ‘ActFast’ project being undertaken by the collaboration aims to address the continuing need for quick, reliable, and cost-effective detection of weld defects, and intends to do so using both existing and newly developed sensors offering fast algorithm process monitoring.

The sensors developed for the project reflect a range of visible, UV and infrared wavelengths back to the process head from the workpiece in order to detect changes indicative of misalignments, weld defects, and the contamination or damage of optics.

Four types of sensor will be used in the fibre-optic system: Photodiodes, to detect intensities and wavelengths of radiation inside the connector using ultraviolet, visible light and infrared wavelengths; ΔT sensors capable of measuring temperature changes as small as 0.01°C, to measure the difference in temperature between the inlet and outlet cooling water to the connector; absolute temperature sensors, to gain status information about the general condition of the connector; and humidity sensors, which give information about the environmental climate inside the connector.

The ActFast project not only seeks to improve the detection speed, accuracy, and real-time data processing of weld monitoring, but also looks to develop a database for system calibration and teaching based on information generated from test samples with zero defects and other samples with known, deliberately introduced defects. The project will also compare the results of new the fibre-optic system with the technological and economic factors of other existing laser weld monitoring and control solutions.

The benefits of the new fibre-optic system include the sensors being integrated to the interlock circuitry of the laser, allowing the system to be shut down in an emergency without risking damage to the expensive optical components. In addition, the ActFast system will offer process monitoring through a closed-loop interface between the sensors and the laser controller to improve weld quality and minimise scrappage and rework.

The ActFast project will also enable the quick and accurate alignment of optical components without the need for external equipment. Raw data for each workpiece will also be recorded for future improvements and offline analysis.

While the project will initially examine sensory capabilities in relation to automotive components specified by Sodecia, Coherent expects that ActFast could be useful in other industries, including aerospace and power generation.

The ActFast project is now entering its closing stages, with the final results expected to be shared by the end of 2018.

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