●Unlike adaptive human behavior, robots cannot independently make autonomous  corrective decisions and have to be supplemented by the use of sensors and a robust control system for decision- making。

●Robotic welding cannot easily be performed in some areas like pressure vessels, interior tanks, and ship bodies due to workspace constraints (Robotics Bible 2011)。

●The majority of sensor-based intelligent systems available in the market are not tightly   integrated

with the robot controller, which limits the perform- ance of the robotic system as most industrial robots only offer around a 20-Hz feedback loop   through

the programming interface。 Consequently,  the

robot cannot respond to the sensor information quickly, resulting in sluggish and sometimes un- stable performance。

Sensors in robotic welding

Need for sensors in robotic  welding

At present, welding robots are predominantly found in automatic manufacturing processes, most of which use teach and playback robots that require a great deal of time for training and path planning, etc。 Furthermore, teaching and programming needs to be repeated if the dimensions of the weld workpieces are changed, as they cannot self-rectify during the welding process。  The seam position in particular is often  disturbed  in  prac- tice due to various problems。 The use of sensors is a way to address these problems in automated robotic welding processes (Xu et al。 2012)。 The main use of sensors in robotic welding is to detect and measure process features and parameters, such as joint  geom- etry, weld pool geometry and location, and online con- trol  of  the  welding  process。  Sensors  are     additionally

used for weld inspection of defects and quality evalu- ation (Pires et al。 2006)。 The ideal sensor for robot ap- plication should measure  the  welding  point  (avoidance of tracking misalignment), should detect in advance (finding the start point of the seam, recognizing  cor- ners, avoiding collisions), and should be as small as possible (no restriction in accessibility)。 The ideal sen- sors, which combine all three requirements, do not exist; therefore, one must select a sensor which is suit- able for the inpidual welding job (Bolmsjö and Olsson 2005)。 Sensors that measure geometrical parameters are mainly used to provide the robot with seam-tracking capability and/or search capability, allowing the path of the robot to be adapted according to geometrical devia- tions from the nominal path。 Technological sensors measure parameters within the welding process for its stability and are mostly used for monitoring and/or controlling purposes (Pires et al。 2006)。 Table 1  pre- sents different sensor applications, and summarized ad- vantages, and drawbacks for a specific time during welding operation。

Contact-type sensors, like nozzle or finger, are less expensive and easier to use than a non-contact。 How- ever, this type of sensors cannot be used for butt joints and thin lap joints。 Non-contact sensors referred as through-the-arc sensors may be used for tee joints, U and V grooves, or lap joints over a certain thickness。 These types of sensors are appropriate for welding of bigger pieces with weaving when penetration control is not necessary。 However, it is not applicable to materials with high reflectivity such as aluminum。 Great attention has been paid to joint sensing by welding personnel since the 1980s。 The principal types of industrial arc- welding sensors that have been employed are optical and arc sensors (Nomura et al。 1986)。 Some of the most important uses of sensors in robotic welding are discussed below:

Seam finding Seam finding (or joint finding)  is  a process in which the seam is located using one or more searches to make sure that the weld bead is precisely de- posited in the joint。 Seam finding  is done  by  adjusting the robotic manipulator and weld torch to the right pos- ition and orientation in relation to the welding groove or by adjusting the machine program, prior to welding (Servo Robot Inc 2013a)。 Many robotic applications, especially in the auto industry, involve producing a series of short and repeated welds for which real-time  tracking is not required; however, it is necessary to  begin each weld in the correct place, which necessitates the use of seam-finding sensors (Meta Vision Systems Ltd 2006)。