INTRODUCTION

Many welding operations rely on a skilled operator to monitor and control the process in order to produce quality welds. Human operators accomplish a variety of subtle tasks—weld placement, weld joint tracking, weld size control, and control of the weld pool, among many others—with relative ease. When human welders or operators are relieved of the welding process control function in favor of mechanized process control equipment, this subtle sense of the process is lost. However, the demands for increased productivity and improved quality, an increased emphasis on statistical process control, and the desire to remove the operator from the harsh welding environment continue to drive the need for real-time process monitoring and control.

The difficulties posed by measuring and controlling process variables in real time are major obstacles to the practical application of automatic controls. Welding processes are complex, and the variables that adversely affect the properties of the completed weldments are subject to change. The environment at the point of process application can be quite harsh. High temperatures, intense electromagnetic radiation, high electric and magnetic fields, molten metal spatter, and fumes are typically involved. Thus, sensors must be used to obtain physical information about the welding process.

Once an appropriate sensor has been selected, a monitoring device can be utilized to record and display the process variables. With respect to these variables, a process monitor can also be used to compare the output of the sensor to predefined parameter limits and sound an alarm when the measured value exceeds these limits. Process control goes a step beyond process monitoring in that adjustments are automatically made in the welding process to correct a variable that has deviated from the desired value.

This chapter provides information on the basic concepts and application of sensors, monitors, and controls in welding processes. The first section discusses the principles behind process sensing, monitoring, and control. The application of sensors, monitors, and process controls are then described in sections covering the arc, resistance, laser beam, electron beam, friction welding, and brazing processes.