Laser Welding Tracking: Understanding the Working Principle

The head of the welding tracking sensor consists of a CCD camera and one or two semiconductor lasers. The laser stripe is projected onto the workpiece surface at a predetermined angle as a structural light source. The camera directly observes the stripe at the bottom of the sensor. The front of the camera has an optical filter that allows the laser to pass through while filtering out all other light, such as welding arcs. As a result, the sensor is positioned very close to the welding arc.

Fig. 1 The head of the welding tracking sensor.

Fig. 1 The head of the welding tracking sensor.

The sensor is typically mounted in front of the torch at a preset distance, known as the lead, in order to observe the weld. The installation height, or the distance between the sensor body and the workpiece, varies based on the type of sensor installed.

To ensure accurate observation, the welding gun should be positioned correctly above the weld so that the weld is near the center of the stripe, allowing the camera to observe both the laser stripe and the weld.

Fig. 2 The position of weld.

Fig. 2 The position of weld.

The laser stripe is projected at a specific angle. If the workpiece is too close to the sensor, the position of the laser stripe is relatively close. On the other hand, if the workpiece is far from the sensor, the position of the laser stripe on the workpiece surface is relatively shifted towards the rear.

The camera observes the position of the laser stripe and the sensor can measure the vertical distance from the workpiece. By analyzing the shape of the stripe, the sensor can also determine the contour of the surface and the position of the weld on the stripe, allowing it to measure the transverse position of the weld.

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Fig. 3 Workpiece with normal distance

Fig. 3 Workpiece with ordinary distance

Fig. 4 Workpiece with far distance

Fig. 4 Workpiece with long distance

Fig. 5 Workpiece with near distance

Fig. 5 Workpiece with close distance

The camera captures an image, which is processed by the controller and transformed into a digital laser stripe image. The software then segments the stripe into multiple lines to form the weld. Based on the position of these lines, the system can calculate the position of the weld and convert it into a distance in millimeters using the calibration data stored in the sensor head.

During the tracking process, the system uses the welding speed and forward-looking distance to determine the delay time, ensuring that the torch follows the weld and not the sensor. The control strategy is designed to provide a smooth forward-looking distance, resulting in a smooth weld. In case the sensor encounters a sudden change in the path, it will respond smoothly, as illustrated in the figure below.

Fig. 6 A smooth response.

Fig. 6 A smooth response.

The sensor consists of several key components, including a CCD camera and filter, a semiconductor laser and optical elements, and a microprocessor for temperature monitoring and storing calibration data. The temperature monitor helps to protect the laser from damage in case the cooling system fails. It is important to note that if the laser operates beyond its temperature limit, its lifespan will be greatly reduced.

The storage of calibration data makes it possible to interchange the sensor heads without incurring additional costs or modifications, ensuring minimum downtime in case of sensor damage or failure. The welding process is protected from soot and spatter by a black copper splash guard, which is equipped with a clear and replaceable plastic sheet that needs to be regularly replaced when dirt accumulates on its surface.

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The sensor must be cooled using welding protective gas or clean, dry, and oil-free air to maintain the temperature of electronic components below 50°C, prevent dust buildup, and protect the optical components. The typical gas flow rate used is 5 L/min.

If necessary, a water-cooled mounting plate can provide additional cooling for the sensor head. On the other hand, if the temperature of the semiconductor laser falls below +5°C, an optional heater should be installed on the sensor.

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