Working Principle of Laser Welding Tracking

The head of the welding tracking sensor consists of a CCD camera and one or two semiconductor lasers.

As a structural light source, the laser stripe is projected onto the workpiece surface at a predetermined angle.

The camera observes directly at the stripe at the bottom of the sensor.

The front of the camera is an optical filter that allows the laser to pass through but filters out all other light, such as welding arcs.

Therefore, the sensor is 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 usually mounted in front of the torch at a preset distance (lead), so it can observe the weld.

The distance between the sensor body and the workpiece, that is, the installation height, depends on the type of sensor installed.

When the welding gun is correctly positioned above the weld, the weld should be close to the center of the stripe, so that the camera can observe the laser stripe and weld.

Fig. 2 The position of weld.

Fig. 2 The position of weld.

The laser stripe is projected at a certain angle.

If the workpiece is too close to the sensor, the position of the laser stripe is relatively close.

In contrast, if the workpiece is far away from the sensor, the position of the laser stripe on the workpiece surface is relatively backward.

The camera observes the position of the laser stripe, and the sensor can measure the vertical distance from the workpiece.

From the shape of the stripe, the sensor can also measure the contour of the surface and the position of the weld on the stripe, which allows the sensor to measure the transverse position of the weld.

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 image observed from the camera is processed by the controller, and the image is first captured and formed into a digital laser stripe image.

The software then uses specific settings to divide the stripe into many lines that form the weld.

From the position of these lines, the system can measure the position of the weld and convert it into a distance in mm.

The conversion is accomplished by using the calibration data stored in the sensor head.

When the system is tracking, the welding speed and forward-looking distance are used to calculate the delay time.

This ensures that the torch, not the sensor, moves along the weld.

It should be pointed out that the control strategy provides a smooth forward-looking distance to ensure the formation of a smooth weld.

Therefore, if the sensor encounters a step change in the path, it will provide a smooth response, as shown in the figure below.

Fig. 6 A smooth response.

Fig. 6 A smooth response.

The key components of the sensor include: CCD camera and filter, semiconductor laser and optical elements, microprocessor for temperature monitoring and stored calibration data.

The temperature monitor provides protection for the laser in case of failure of the cooling system.

If the laser is used at temperatures beyond the limit, its lifetime will be significantly reduced.

The storage of calibration data makes the sensor heads completely interchangeable without additional cost and modification.

Thus, the minimum down time is ensured in case of sensor damage or failure.

The soot and spatter in the welding process are protected by a black copper splash guard.

The splash baffle is equipped with a clear and replaceable plastic sheet, which must be replaced regularly when there is dirt on its surface.

The sensor must be cooled by welding protective gas or air (clean, dry and oil-free) to maintain the temperature of electronic components below 50 ℃, prevent dust and protect optical components.

The gas flow rate used is typically 5 L/min.

If necessary, a water-cooled mounting plate can be used to provide additional cooling for the sensor head.

Conversely, if the temperature of the semiconductor laser is below +5 ℃, the optional heater should be installed on the sensor.

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