If, instead, the robot is only instructed where to start and where to end, it must be able to follow the path measured by the sensor on the fly。 This put some additional requirements on the robot system as it must be able to calculate the trajectory including both target positions and orientations of the weld torch。 In doing so, the robot might easily get into control problems for the specific task that is related to close to singular areas, joint limit and collisions with the work-piece。 However, the benefits high and more flexible control of seam tracking can be obtained using available sensors。

In order to use the sensor data and let the robot follow the measured and generated trajectory, the target data from the buffer of target positions must be filtered。 A suggested method is to use about five target positions and generate a polynomial curve for x, y and z and a vector tangent at the current position。 This is useful for several reasons: (i) a generalized description of the weld joint makes it easy to reuse the generated trajectory for calibration purposes, (ii) the generated trajectory can be used as a nominal path for subsequent weld passes, (iii) the vector tangent can be used for subsequent target positions even if target drop-outs occur such as at tack welds and (iv) the vector can be compared with the current trajectory vector and from that an optimal orientation of the weld torch can be calculated with respect to the weld joint as measured by the laser scanner。

Typical operating data of a laser scanner is a scan sweep frequency of 10-50 Hz。 If we assume a welding speed of 20 mm/s, this means about one sweep  per mm during welding。 This is in most cases more than sufficient。 However, new welding processes such as laser welding will increase the welding speed considerably and for high requirements careful analysis and trials must be made。 The accuracy of the laser scanner is high and is better than 0。1 mm。 However, it should be noted that single scan sweeps may generate outliers as the weld joint and the environment during welding generate severe disturbances。

From a practical point of view, laser scanners are accurate and robust sensors that meet most requirements within the welding process。 However, they must be mounted on the weld torch and take up some space。 They also put additional requirements on programming and positioning of the robot and the weld torch。 Laser scanning sensors are also still relatively expensive and if alternative methods can be used like through-arc sensing as described in the next section, they are usually preferred。

3。2。2Through-arc Sensing

Seam tracking using a weaving motion and the arc itself as the sensor, sometimes referred to as through-arc sensing, was introduced in the 1980s。 The principle behind the method is to make use of the change in current when the distance between the contact tube and the work-piece varies。 The underlying principle is relatively easy and cost-effective to use and is a common sensor for tracking methods in robotic welding based on gas metal arc welding and related processes, like flux-cored arc welding, submerged arc welding, etc。 According to [4], the approximate relationship between arc voltage (U), arc current (I) and the contact tube to work-piece distance (l) is expressed by

U = El , +  E2  + E3 / , + E4 l 3。1

where the constants El - E4 are dependent on factors like wire, gas and the characteristics of the welding power source。 In most cases, the welding power source is set up to maintain a constant voltage and thereby a more stable welding process。 Thus, when the value l varies, the arc current I will also change, mainly as a proportional change with opposite sign。 This can be used in mechanized welding and specifically in robotized welding to perform a weaving motion during welding。 When doing so for a weld joint as shown in Figure 3。6, the distance between the weld torch and the weld joint will vary during the weaving motion and so will the current。 Hence, a longer contact tube to work-piece distance will result in a lower arc current than a shorter distance, given that all other parameters are the same。 This can be utilized during an overlayed weaving motion, normally a sinus or triangular type of motion, but more complex motions also exist。