Page 89 - Welding Robots Technology, System Issues, and Applications

cases several possibilities to control the welding process and fulfill requirements
defined in the WPS. Sensors for Welding Robots 75

It should be noted here that the concept of specifying the weld to be produced
within a WPS document is a method to define a procedure to, in a consistent and
reliable way, work out the welding process. It should include information about the
actual weld to produce, like joint geometry and material, but also joint preparation,
consumables like shielding gas composition and flow, and welding wire, and
nominal operating parameters and the productivity and quality to achieve. In most
cases, however, we can only measure some of the parameters needed and from the
available observations make judgments on how to control the process to reach the
specifications defined in the WPS.

In this way, the WPS is both a specification of the functional specifications of the
weld (quality, productivity) and operating data (nominal operating parameters) to
reach the specifications. To use sensors, however, means to measure the real
process, extract features from these measurements and, through a control action,
override these pre-set parameters in order to achieve the functional specifications,
where quality and productivity issues have to balance each other on a case by case
basis. For a typical case, nominal operating data is defined based on specified weld
quality and productivity. This is used to pre-set data of the welding power source
and to instruct the robot to generate the trajectory in accordance with defined
velocity, welding torch orientation with respect to the weld joint and the distance to
the weld joint. If a sensor is used which through a feed-back loop will alter one or
several of these settings, the WPS should include allowed tolerances for all
nominal data. This is also the case if sensors are used for the purpose of monitoring
the robotic welding operation.

3.1 Sensors for Technological Parameters

Technological parameters include voltage, current and wire feed speed. In this
section sensors to measure those parameters are reviewed.

3.1.1 Arc Voltage

The measurement of the arc voltage should, in principle, be made as close to the
welding arc as possible. The current is delivered to the wire at the contact tube and
this could be assumed to be a good measuring point for arc voltage. However, there
is a voltage drop between the contact tube and the wire tip where the arc starts of
about 0.3 V, depending on the process characteristics [1]. In practice, it is difficult,
if not impossible in a production environment, to measure the true arc voltage. This
is also the case for measuring the voltage at the contact tube in the weld torch, and
a better and more reliable way is to measure the voltage on the wire inside the wire
feeding system. As the wire does not carry a current between the feeder and the