Page 88 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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produces the relative positioning of the energy and the work-piece that is to be
welded through a weld torch attached to the end effector mounting plate.

From a control point of view, both the welding equipment and the robot are
important to produce the weld with the specified quality and productivity. They are
normally controlled by two different and loosely coupled control systems,
controlling the welding power source and the robot arm. In the following, both the
sensors and the purpose of utilizing sensors to control the weld process will be
discussed. Concerning sensors, they are in most cases used for one of the control
systems, the welding equipment or the robot, but for the purpose of using sensors,
the information can preferably be used for both controlling the welding power
source and the robot arm as will be discussed later in this chapter. However, the
purpose of the sensor and how it will be used will affect the specification of the
sensor which therefore can be divided into two groups, technological and
geometrical sensors, measuring technological parameters with respect to the
welding process and geometrical parameters with respect to the weld joint
geometry.

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 deviations 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.

As will be discussed later, information from both technological and geometrical
sensors provides a basis for a qualitative control of the welding process to make it
possible to conform to specifications defined within a WPS (Welding Procedure
Specification) concerning quality and productivity measures. Another issue of
importance is the mapping problem between observable parameters and
controllable parameters with respect to the sensor. In the simple case with a sensor
based seam tracking of almost straight welds, the feedback control is
straightforward, but applying the sensor data for integrated control related to the
WPS will require a more sophisticated model-based control approach. This is
because a controllable parameter is not always a parameter that a sensor can
observe during the welding process and thus, a model-based mapping must be
made to be able to control the weld. In this context, a model-based approach may
transform a parameter or set of parameters not directly observable by the sensor
into the known set of parameters which can be calculated from a model of the
welding process using the information of the sensor. However, the welding process
includes many interrelated parameters with included tolerances, which means that
such models will predict a number of data not observed directly and with a degree
of uncertainty. In practice, such models work better close to defined nominal data
where process conditions are similar to those anticipated in the WPS. In the same
way, actions to control the process are usually defined by a combination of a set of
parameters which together counteract deviations from specifications defined in the
WPS. In general, such a set of parameters are not unique and there are in most