Seam tracking Seam tracking enables the welding torch to follow automatically the weld seam groove and    adjust

Table 1 Applications and quality of sensors

Operation time

Type of sensors

Advantages Drawbacks

Sensing independent

Preview sensing

On-the-spot sensing

Touch sensing

Contact sensing

Inductive sensing

Capacitive sensing

Acoustical sensing

Optical sensing

Weld pool observation

Through- the-arc sensing

Can recognize 3-dimensional offset of the workpiece。 The wire tip or the gas nozzle can serve as a sensor。 Can be used for accurate learning of the path before welding。

Relatively low cost。 The mechanically probes leads the welding spots。

Largely used in industry, configurations with one pick-up coil can provide a cross-seam or vertical path correction signal。

Offer the opportunity to measure the distance between the workpiece and an electrically conduction plate of small dimension。

Apart from seam-tracking application, an acoustical sensing system can be used to explore the workpiece for obstacle and maybe to inspect a produced weld。

Can be used for seam tracking as well as for geometrical recognition of the weld pool, to adapt process parameters in the case of possible deviations。

Dedicated to welding pool geometry and properties。 The obtained image is processed and pattern recognition algorithms are used to extract the dimensions and form of the weld pool。 Different sensors can be applied: optic sensing, thermal sensing, real-time radiography, weld pool oscillation sensing,

No additional voluminous sensor needs to be fixed to the weld torch。 Its simple operation and implementation have made arc sensing a commonly accepted off-the-shelf technique。

Can defect elastically, using tactile probes it is difficult, if not impossible, to provide information on the joint fit up。 Poor weld joint repeatability。

Not adaptable to suit a variety of joint geometries。

Different sensor is needed for each type of joint, should stay very close to the joint

It is hard to extract a correction signal in two direction from the capacity variations

Line of sight must not deviate from the surface normal; another limitation is the temperature dependence of the speed of the sound。

To prevent accessibility limitation, it may require additional axes for seam tracking, tremendous effort to introduce technical integration, regularly check the lens protection。

There should be a clear interpretation of the image by the system, in order to give torch corrective changes accordingly

The torch has to be weaved during welding。 The dimension of the joint must exceed some critical dimension, e。g。, it is not applicable for sheet metal。 In addition, a signal can be obtained only after the arc has been established。 Therefore, it cannot be used for finding starting point of the weld。

the robotic manipulator accordingly; to counter  the effects of variation in the seam caused by distortion, un- even heat transfer, variability of gap size, staggered edges, etc。 (Xu et al。 2012)。

Reliable seam-tracking sensors provide the following advantages (Björkelund 1987):

●Automatic vertical and horizontal correction of the path (even path changes necessitated by thermal distortion)

●Less stringent accuracy demands on objects and fixtures

●Welding parameter adaptation

●Reduced programming time

●Lower rejection rates

●Higher welding quality

●Viability of short  series

Adaptive control In adaptive control welding, i。e。, a closed loop system using feedback-sensing devices and adaptive control, there is a process control system that detects changes in welding conditions automatically with the aid of sensors and directs the equipment to take appropriate action。 Sensors are needed in adaptive con- trol  welding  to  find  the  joint,  assess  root   penetration,