AWS WHC-10.1.11:2018 pdf download.MONITORING AND CONTROL OF WEL DING ANDJOINING PROCESSES
In its most basic form, a sensor is a transducer that converts a property from one physical form to another,most often to an electrical signal. Welding process sen-sors obtain information about the welding process by converting physical phenomena from the input and pro-cess response variables into signals that can be utilized by monitoring or control equipment. Because several welding processes generate high frequency and other electrical noise, signal conditioning, amplification, and isolation are often required before sensor output can be safely and reliably fed into a monitor or controller.
Sensors can be simple or complex in design. Exam-ples of simple sensors include current shunts, which convert the current flowing in the welding circuit to a proportional voltage, and thermocouples, which con-vert temperature into a voltage signal. Complex sensors are composed of several individual sensors that operate together. An example is the machine vision sensor which is now reliably used for many welding applica-tions. This complex sensor is an entire subsystem,including a video camera (a transducer that converts light intensity into a video signal), a video signal digi-tizer, and a microprocessor, which extracts information from the video image. Machine vision sensors, which are described in more detail later in this chapter, pro-vide information on the location and geometry of weld joints and the size and shape of weld pools.
The physical and operational characteristics of some sensors make them advantageous for use with specific welding processes as compared to other sensors. For instance, sensors that provide a direct measurement of the process variables are more desirable than those that provide indirect measurements. Sensing devices that require contact with the process or weldment are less desirable than those requiring no direct contact. In addi-tion, sensors that can be applied from the face of the weld are more desirable than those requiring access to the interior or back of the weld. Overall, sensors that can be applied to multiple processes are highly desirable and have become commonplace in the welding industry.
Optical Pyrometer
Optical pyrometers are noncontact devices that mea-sure the thermal radiation emitted by a source. The radiation may be in the infrared or visible light range,depending on the temperature. Infrared pyrometers employ the infrared portion of the spectrum by using a thermal detector to measure the temperature of the sur-face of the body emitting the infrared waves. An exam-ple of an infrared sensor mounted to a weld head to monitor interpass temperatures is shown in Figure 11.2.
This figure shows a complex weld head used for thick section pipe welding applications in the oil and gas industry. The GTA torch in the middle shows an extended electrode to reach the bottom of thick joints.There is a hot wire torch mounted in front of the GTA
torch to provide higher wire feed fill rates (right side).Directly behind the GTA torch is a magnetic oscillator which allows the arc to be magnetically oscillated to contact and melt each side wall. The optical pyrometer (silver) is mounted behind the oscillator and below the weld camera (black). The sensor will provide tempera-ture data of the part as the part rotates clockwise under the arc. This pyrometer data is integrated within the process control to automatically stop and start the pro-cess ensuring that maximum part and interpass temper-ature requirements are met.AWS WHC-10.1.11 pdf download.