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OPTIMIZATION OF PGMAW USING ONLINE OBSERVATION

AND STATISTICAL DATA

S. Nordbruch 12, A. Gräser
1,2

2

ABSTRACT

In this paper a monitoring system that simplifies the finding of optimal welding parameters, the analysis and the optimization of pulsed gas metal arc welding (PGMAW) is described.

The system allows the visual online observation of all states of the welding process, including the droplet transfer, without an additional lighting unit. Additionally, the synchronized measurement of the welding current and welding voltage signals during image recording and the extraction of characteristic parameters of the signals is possible. Furthermore, the system allows the visual analysis of the material transition images.

For an analysis and optimization of the process the systems computes statistical data of all collected and calculated visual and electrical data of a recording sequence.

KEYWORDS

PGMAW, droplet transfer, online observation, visual data, electrical welding data, statistical data

INTRODUCTION

The pulsed gas metal arc welding process is an important component in many industrial and manufacturing operations. It is highly suited to a wide range of applications. Due to the complex processes, the extreme brightness of the welding arc, the high number of different welding tasks, etc., the finding of optimal parameters, the test of new welding parameter combinations or the analyses by process errors is difficult.

For the solution of the problems the visual observation of the droplet transfer in combination with the measurement of electrical welding parameters is an approach. Typically, the droplets should be even and in uniform size and the material transition should be splashless.

The visual observation of the material transition has been used extensively. Normally digital Charge-Coupled-Device (CCD) high-speed cameras in combination with an optical laser are used. Due to the extreme brightness these approaches are using the shadowgraphing technique, described by Allemand et. al. (Ref. 1).

For the mentioned problems the systems are unsuitable due to a set of disadvantages. The most important are:

» The necessity of the lighting unit and the limited possibilities of observation caused by this (shadowgraphing technique).

The acquisition and maintenance costs are very high.

1

2

Friedrich-Wilhelm-Bessel-Institut Forschungsgesellschaft m.b.H, Postfach 106364, 28063 Bremen, Germany University Bremen, Institute of Automation (IAT), Kufsteiner Str. NW1, 28359 Bremen, Germany

SYSTEM

The monitoring system consists of a High-Dynamic-Range-CMOS camera (HDRC) with an external trigger input, an intelligent measuring board for the triggering of the image recording and the synchronized and simultaneous measurement of electrical welding parameters, a signal processing unit for the automatic calculation of characteristic welding process parameters, an image processing unit for the automatic visual analysis of the droplet transfer images and a statistical unit. Due to the properties of the HDRC camera the system requires no additional lighting unit. The basic set-up of the system is shown in figure 1.

HDRC cameras use an image sensor with a brightness dynamic of approximately 10°:1. This corresponds approximately to the intensity difference of the welding process. HDRC cameras are therefore able to observe all states of the material transition in all welding positions without an additional lighting unit.

The principle of HDRC cameras can be described as follows. The level of intensity, i.e. the color/gray values of an object, are essentially dependent on the exposure rate and the material properties of the object with its reflection properties. The information of an object is essentially dependent on the contrast. A CCD camera maps the absolute color/gray values caused by respective irradiation in the image. As opposed to the CCD camera the HDRC camera maps the contrast, which is caused by different reflections. I. e., brightness conditions (color/gray values of the object) are independently represented by the intensity of illumination as constant number differences. Before further processing, this high intensity dynamic is compressed logarithmically in every cell of the image sensor just like in the human eye. Due to this compression, the information content of the image that is included in the contrast is not reduced.

A further advantage of HDRC cameras is, that at every time each sensor cell can be accessed independently of all others. The constraint of CCD cameras that must read out complete images does not apply for HDRC cameras. This property of HDRC cameras allows the selection of sub areas of the image sensor instead of the complete image. Considerably higher image recording frequencies become possible.

Principle of Image Recording and Measurement of Electrical Welding Parameters

The system uses the periodicity of the droplet transfer of the PGMAW process. For the observation of the regularity of such processes it is sufficient to take one picture per period of the same

process phase. In order to observe the exactly same process phase in each case (also at period times varying), the image recording is synchronized on an electrical welding parameter and is not carried out in fixed time intervals. The image recording starts after a trigger criterion is detected and an additional variable delay time (see figure 2).

Figure 2: Procedure of image and electrical welding parameter recording

Due to the variable trigger criterion and the variable delay time, every state of the welding process (the melting of filler wire through the arc, the detachment of the drop and finally the immersion of the drop into the welding bath) can be observed in online mode, even if this state itself has no unambiguous trigger criterion.

With periodically recorded images, the virtual picture of a quasi-stationary process, i.e. of a stationary drop is generated. Irregularities within the welding process are immediately recognized as different images in a series. The influence of varying welding parameters on the process is immediately visible.

Furthermore the variable delay time between the trigger criterion and the trigger impulse for the camera in one recording series can be continuously enlarged. In this case a virtual picture of a continuous process, i.e. of a virtual droplet transfer through the sequence of the different consecutive drops, is recorded.

Simultaneous to the image recording the welding current and voltage are measured. The measurement starts at the trigger criterion and holds after a variable time within the period of the process (see figure 2).

Signal Processing

The signal processing unit allows the automatic calculation of the following typical characteristic PGMAW process parameters from the measured current and voltage signals (see figure 3).

Additionally, the unit calculates specific parameters of the used welding supply (Ref. 2). The

supply generates current and voltage signals that contain a so-called 'backpack' in the falling edge of the pulse (see figure 3). The calculated special parameters are:

>> Backpack current I, and voltage U,

>> Backpack time t