LIBRARY ı6ıul 0) - Cranfield University

Concerning the time when the sensor is used, three techniques can be
identified [ref. 51,116]: a) pre-weld sensing, b) real-time sensing, c) post-weld
sensing.
In the literature, three main applications of sensors in robotic welding can be
identified: a) joint location, b) joint tracking, c) joint recognition, d) weld recognition
2.6.1.2 Joint location
Joint location is defined as a form of adaptive control which recognizes, before
welding, the position of the joint to be welded and instructs the machine to take the
appropriate action [ref. 58].
Many approaches for joint location have been reported [ref. 114]; the most
common method is the wire touch sensing [refs. 117,118,119,120], in which the gas
metal arc welding wire is used to search for the start point. In order to accomplish
this, the robot is moved to its search start point and a high voltage, ranging from 300
to 600 Vac, low current (; 30mA), signal is applied to the welding wire. The robot
then moves according to a chosen search pattern, "probing" for each joint surface, in
turn on up to three planes (see Figure 2.18). The surface is detected by loss of voltage
as the welding wire earths on the component. Search times can be as short as three
seconds per direction.
Other joint location methods make use of ultrasonics (time of flight),
proximity sensors (capacitive, inductive) and optical sensors [ref. 114]. Special
attention has been given to optical sensors which are expected to give similar
performance as the human eyes in the future [ref. 121]. One such example was
presented by Cheung et al. [ref. 122] who have described a method of tracking an
object by analysing its real-time laser rangefinder data. The method consisted of
extracting three-dimensional lines and circles from edge points and then matching
these extracted curves to the model of each object, hence obtaining their poses. The
edge points were obtained from a pre-processing of each range image. Although the
method presented by Cheung et al. [ref. 122] has been devised for tracking moving
objects, it could be used off-line for locating welding joints and start points, before
starting welding in a robotic welding cell.
2.6.1.3 Joint tracking
Joint tracking is also defined as a form of adaptive control which monitors
changes in the location of the joint to be welded and instructs the welding machine to
take the appropriate corrective action. The process is based on the signals provided by
suitable sensors and can take place in a preliminary (off-line) scan or in real time [ref.
58]. Off-line joint tracking, however, does not take into account the effects of thermal
distortion during welding. Therefore, it is not suitable for solving problems related to
on-line joint movement.
Several joint tracking systems have been reported in the literature, the five
main types of sensors being [ref. 114,123]:
a) Electromechanical
sensors;
b) Inductive sensors;
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