3。2。1Optical Sensors

Optical sensors use the following basic principle for detecting the weld joint during arc welding; (i) a laser beam that is projected in a scanning motion across the seam and (ii) a CCD-array that is used to measure features of the weld joint in combination with a laser stripe。 Variations of this method are in use and, as an example, the laser stripe may not be a linear line on the weld joint but circular instead。 In such a case, the sensor is more flexible to detect weld joints in corners from one location of the torch, or point of view of the sensor。 To measure the distance, the method of triangulation is used which is of great importance in welding, see Figure 3。1。

Figure 3。1。 The working method of the triangulation method [2]

A laser beam is focused on an object, and then the reflection from the object as seen from a lens in the laser sensor is determined by the distance between the sensor and the object。 If the object is close to the sensor then the angle between the outgoing beam and the reflection through the focusing lens of the detector is large, while it is small if the object is farther away。 The detection of the distance between the sensor and object is made by focusing the incoming beam on a detector, in most cases a CCD array。 Depending on which of the pixels of the array are illuminated, it is possible to calculate the distance to the object。

Depending on the weld joint preparation and geometrical shape, the laser beam can produce reflections like mirrors。 Consider for example a V-groove  weld joint where the laser light will produce several reflecting positions but with different intensities depending on the surfaces of the weld joint。 Therefore, these sensors must have real time image processing capabilities to filter out reflections that do not belong to the point of interest。 It should be noted in this context that highly reflective materials may cause problems during welding and a real test may be needed to verify the functionality。

Figure 3。2。 Scanning principle of a seam tracking combined with the triangulation method [2]

The basic functionality of a triangulation sensor is to measure the distance to the spot of the object the beam is pointing at。 In some cases this can be useful, e。g。 to control the height during an operation of a robotized process like welding or cutting。 But the general use of triangulation in welding is for seam tracking and this requires measuring the weld joint geometry。 This is achieved through a scanning technique of the beam across the weld joint, see Figure 3。2。 During the

scanning, the sensor acquires a two-dimensional picture of the joint profile as an array of 2D coordinates。 When the robot is moving, a weld joint geometrical model can be made that contains a full 3D description of the joint which is created during the welding operation when the sensor is moved along the joint。

If a laser stripe is projected onto the object and sensed by a 2D CCD array, the image information can be used directly without moving the robot。 This is a technique that is useful if the stripe is circular and aimed at a corner。 Then, the corner and its walls can be located from one position of the robot only, compared with the more time consuming traditional technique of measuring the location of one wall at a time。

In most cases, optical seam trackers based on triangulation are used to keep the robot “on track” with the weld joint during welding in real time。 However, these sensors have a capability for more than that, and information that can in most cases be achieved that include joint volume, gap size, misalignment, tack welds, etc。 This information is useful for adaptive feed-back control of both the welding power source and the robot to perform the task in accordance with the predefined specifications of the welds to be produced。 As an example, the travel speed of the welding gun defined by the robot can be controlled with respect to the gap of the weld and the welding power related parameters in combination。