Page 98 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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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.

3.2.2 Through-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 = E , E E , E l 3.1

where the constants E - E 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.

In practice, the current is measured using a Hall Effect sensor or a current shunt. A
low-pass filter is used to depress noise from the signal. From a control point of
view, sensor data can be analyzed continuously using template matching or only at
the turning points using differential control. Due to the relatively low accuracy of
the sensor data regarding precision, etc., a differential control is in most cases
sufficient. This should be seen in the context that the sensing principle needs a few
mm weaving amplitude to be able to measure a reliable change and difference in
arc current. It also indicates that achievable tolerance are restricted for those cases
which can accept a weaving motion, which produce a wider weld rather than a