Page 91 - Welding Robots Technology, System Issues, and Applications

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Sensors for Welding Robots

schemes, but the general principle is to keep the voltage at a preset constant value.
As a result, the current will vary during the process as a result of changes in the
environment of the arc welding process, like changes in the distance from the weld
torch to the work-piece.

These changes can occur as a result of the movement of the robot or geometrical
variations in the weld joint. However, changes of speed can also originate from the
wire feeder system and in the end produce poor results with respect to quality.

The wire is fed through a conduit and the wire feed speed is generated by a feeding
unit. The unit usually has driving wheels that feed the wire through the conduit at a
specified speed. In normal robot systems, the wire feed unit is mounted on the
robot arm rather close (the order of 1 m) to the weld torch, giving a reliable push-
feed of the wire during the welding process. However, in some cases, longer
conduits must be used and since the wire diameter must be slightly less than the
inner diameter of the conduit, a varying wire speed will result during welding when
the conduit is bent and twisted. In practice, a push-pull wire feed system should be
used to counteract this problem.

Measuring the wire feed speed is a big problem and for laboratory purposes
custom-made solutions can be build that measure the speed at the contact tube. A
more realistic approach in a production system is to measure the controlled speed
of the drive wheel of the feeder unit. However, this must be complemented with
securing the functionality and reliability of the feeder system as its robustness is
important for the resulting quality of the welds.

3.2 Sensors for Geometrical Parameters

Sensors for geometrical parameters must be able to obtain information about the
weld that relates to the geometry of the weld joint. This information is of great
importance in order to both perform seam tracking and use this information for
quality control of the weld. This can be done in many ways but in most cases, a
seam tracker is able to extract some information about the weld joint besides the
target positions for the weld joint during welding. Examples of such data include
deviations from a nominal path, orientation changes and gap size.

To apply sensors in robotics welding means, in general, to use the sensors during
welding. In some cases sensors can be used to measure the position and orientation
of the weld joint or work-piece before welding. In such cases, different location
techniques can be applied in a similar way as for locating any work-piece such as
image recognition or binary sensors to detect the position of some plates of the
work-piece.

The challenge, however, is to use sensors during welding. Due to the harsh
environment with high temperature, intensive light and high currents, purpose built