Introduction The demand for better control and sensing in welding has increased with automation and welding processes involving new and advanced materials。 This requires precise control of the welding process to produce the desired weld with respect to productivity and quality。 Consequently, there is a need for different technologies to control precisely the process with respect to the different welding operating parameters。 In doing so, sensors play a crucial role as the major source of input to the control system that manage and control the behavior and output of the welding system。 Here, the term “sensor” is used for devices that measure observable parameters related to the welding process which is used to control the process in accordance with defined specifications。79895

In general, most robotized welding processes that produce a continuous weld are based on the MIG/MAG process, or GMAW (Gas Metal Arc Welding)。 Within this application field, the use of sensors has been modest。 However, the development and introduction of new welding processes like high speed welding, laser welding, etc。, emphasize the importance of accurate control of the process。 The development of new products also makes use of new materials with possibilities to decrease thicknesses。 A result of this is a need to be able to work with tighter tolerances。 Thus, the need is increasing for sensors that can meet the requirements from new processes and product specifications and is in many cases a necessity。

The main task of the sensors is to provide the control system with information to generate proper actions to produce a result that corresponds with defined specifications。 In welding this is not as easy as we might think。 From a welding process perspective, the process is performed mainly by two subsystems; the welding equipment and the robot。 The welding equipment includes the welding power source and the devices that deliver the energy from the welding power source, like the wire feed system, conduit, welding torch and so on。 The robot

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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 pided 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