Online measurement and closed loop control of penetration depth in laser welding processes

The key quality factor for reliable deep penetration laser welding processes is a correct penetration depth. Today, this parameter is only accessible by offline quality control methods. Since the reliability of a buttjoint strongly depends on the penetration depth, it is desirable to have a possibility to measure and control the penetration depth online during the welding process. A sensor for measuring the penetration depth has to fulfill several requirements: 1) Illumination and detection has to be coaxially to the high power laser, since the capillary can be very deep and slender. 2) The measurement has to work despite of a very high dynamic range of the detected light and despite of strong background radiation from the welding process. 3) The sensor should realize scan rates in the kilohertz range to enable real time closed loop control of the welding process with high control frequency. 4) The penetration depth should be measured with a resolution in the range of 1 00 m. Today no sensor exists that fulfills these requirements. Different approaches, like in [1] and [2], were discussed in the past, but none of them has emerged to a fast and reliable measuring product.