monitoring.
generation or nugget growth, a number of adaptive control units have been developed that shut off the current at some predetermined voltage level (e.g., Nakata et al.,5). Welding current is another important variable to monitor. Some controllers sense only the welding current and signal a fault when the measured value does not fall within prescribed limits.
Similar to tip voltage, the welding current itself does not represent the input heat either.
In order to determine the heat input to a weld, both voltage and current must be measured. Consequently, controllers based on the so-called constant-power (heat) control algorithm have been developed and are now commercially available. However, due to the
variable energy loss of the resistance spot welding process, constant power still cannot guarantee consistent weld quality.
Infrared emission, acoustic emission, and ultrasonic signals have all been attempted for weld quality monitoring. However, the infrared emission method can only measure temperature far away from an actual weld. The variability of emissivity of materials is another reason that this kind of system failed. Acoustic emission can be used to detect expulsion, but it is not presently useful for monitoring weld nugget growth (Cleveland and O’Brien6). The ultrasonic method has been studied. However, it is intrusive in an assembly process and has not been proven to be reliable as an on-line monitoring means.
Both electrode displacement and electrode force during welding can reflect the nugget growth process. Electrode displacement is generally considered to be a better indication.
It is believed that the amount of thermal expansion, melting, and expulsion can be correlated to the slope and magnitude of a displacement curve. Several control strategies have been developed based on monitoring displacement curves (Stiebel et al.7,8; Hanfer et al.4; Tsai et al.2), even though many consider it applicable only to pedestal welders and not to portable gun welders. Electrode force during welding can also be correlated to the amount of thermal expansion, melting, and expulsion. However, the correlation may not be consistent due to the variability of welding machines’ characteristics. Some researchers have found that the dynamic force reflected the nugget growth process, while others have reported that the measurement provided little useful information (Gedeon et al.1).
Dynamic resistance is a measure of the electrical resistance change during welding. It can be calculated from the tip voltage and welding current. Dynamic resistance has been shown to have a good correlation to the nugget growth (Dickinson et al.9) and is currently receiving more attention.
Although electrode displacement is considered the most revealing signal for nugget growth, on-line resistance spot welding monitoring and diagnosis systems usually consist of tip voltage, welding current, and electrode force only, to describe nugget growth in a production environment, due to the intrusive nature of electrode displacement sensors.
A significant amount of research has been conducted on instrumentation of resistance spot welding processes. However, little has been done to understand the physical origins of the signal waveforms. Some efforts have been devoted to removing the induced noise from corrupted electrode force and tip voltage measurements. Yet it still remains an issue for signal processing of the monitoring and control systems.
Most of the work on monitoring and control has been focused on welding under nominal process conditions (e.g., perfect alignment, no edge-weld conditions); very limited work has been done to investigate the effects of different process conditions. In this chapter, the effects of abnormal conditions are also discussed, which serves as a liaison between developed monitoring and control systems and their application in the actual production environment.
5.3 Process Monitoring
Monitoring a welding process provides useful information on the physical processes involved in welding, and it is a necessary step toward successful control of the process. A
direct measurement of weld quality, such as weld size or weld strength, can sometimes be used as a means of monitoring welding process. However, such monitoring ignores the details in welding; it considers only the end results of welding instead, and its usefulness is very limited. A process monitoring of real meaning contains detailed observation of the process through the use of various sensors, and it is correlated with weld quality. In this section, common signals collected during RSW are discussed, and their use for welding process monitoring is presented.
5.3.1 Signals Commonly Monitored during Welding
Intuitively, welding voltage and current should be monitored, as they are directly related to joule heating, or the formation of a weld nugget. In addition, the thermal process during welding is reflected in the expansion or shrinkage of the sheet metal stack-up, or it can be monitored through the changes in electrode force and electrode displacement.
Figure 5.3 shows typical signals collected during an RSW. The voltage, current, electrode force, and displacement are each discussed in detail in this section.
The process signals are heavily influenced by the electric and magnetic fields surrounding the welder, and therefore, it is necessary to understand the electrical aspect of a welder. The electrical part of a welding machine involves mainly a transformer, which brings the high line voltage down to a low secondary voltage and provides a high current to the secondary loop.10–12 The system can be represented by a two-port transformer model, as shown in Figure 5.4. ro1 and Lo1 are primary resistance and inductance, respectively. Similarly, ro2 and Lo2 are the parameters for the secondary side