Sensors for Arc Welding

The collective term sensors for arc welding refers to devices that collect information about the location and possible geometry of the welded seam on the workpiece as part of a fully mechanized welding system and provide appropriate data in a suitable form for controlling the position of the welding torch and if possible the welding process variables.

• 3.5.1 operation
• 3.5.2 Boundary conditions
• 3.5.3 Problems with the use of optical sensors

• 4.1 arc sensors TIG welding
• 4.2 arc sensors in GMA welding
• 4.3 Boundary conditions

Introduction

The quality of a weld depends not only on the most representative for the welding process welding parameters (eg voltage, current, wire feed speed and welding speed ) to a large extent also from how the process energy and the filler material are introduced. The positioning of the burner has a direct effect on the flow of material. The heat input to melt the component edges and uniform heat flux are directly linked to the burner management and have significant impact on the quality of the stitching and resulting residual stresses. When fully mechanized and automated arc welding, inaccuracies of the burner control, the workpiece handling, joint preparation and thermal distortion to deviations of the joint position and joint geometry add up. When fully mechanized welding necessary for the weld quality information required in each case can be determined by sensors. Sensors are therefore used to determine the position of the component (seam beginning and Nahtendfindung ), for joint tracking and adaptation of process parameters on current joint changes. The sensors can also be used both online ( at the same time with the welding process ) and offline ( in a separate step prior to welding ). The majority of sensor applications is the online joint tracking.

Principles of Operation

Each physical principle, is capable of providing information about the position of an object, is used as a base for a sensor function, in question. The prevailing environmental conditions in arc welding and the requirements that the mechanized system, but lead to many restrictions. Figure 1 shows the system overview. The overarching criterion, the observation strategy of the sensor ( process or geometry) is selected, the further subdivision is based on the measuring principle. Another distinguishing feature of sensor systems lies in the consideration of the design. Thus leading sensors are characterized in that measurement and joint are not the same location. Measuring and joining process running in this case mostly from sequentially. In order to make statements position relevant for the welding process, with these systems, a calibration of the relative position is required. In process-oriented sensors measuring and joint are identical. Common to all of the measuring principles is the fact that a geometric information about the joint and its position relative to the measuring head is obtained by the evaluation of the sensor signals. Here, the individual active principles allow different processing speed with which this information can be obtained.

Geometry -based sensors

Geometry -based sensors win their signals from the geometry of the joint or a defined to extending edge or surface.

Tactile sensors

A kind of tactile sensors are electrically contacting sensors for seam finding and workpiece measurement. The sensor provides an electrical contact with the workpiece ago; the electrically conductive workpiece are included in the measuring circuit of the sensor. The mechanical contact sensors constitute the second category of tactile sensors. For them, the mechanical deflection of a work-contacting probe element is evaluated.

Electrical contacting sensors

Electrical contacting sensor systems grope, a specific search strategy following the start of the seam or more path points by using untensioned ( DC voltage of a few tens of volts to 1 KV depending on the material and surface ) components of the welding ( shielding gas, welding electrode, stylus, etc.) contact the workpiece. This means an offline measurement of the initial seam, the component location or geometry before welding. From the knowledge of the planned trajectory of the path points, a transformation is performed according to the measured conditions. During the welding process takes place no corrective action in this case.

Thermal sensors

These sensors is measured by means of two welding torches placed at the thermocouples of the transmitted heat flow and is used for side sheets and height control of the burner. By comparing the temperatures of the two thermocouples sensor alignment of the burner relative to the joint can be detected. With symmetrical alignment of the burner, the difference of the incident heat flow equal to zero, and also the differences in temperature of the thermocouples. Depending on the lateral misalignment of the burner, the thermocouple, on the one hand exposed to the deformation of the arc and on the other hand by the changed position of the molten bath, different heat flows.

Mechanically -contacting sensors

Mechanical systems put in contact with the deflection of a probe element directly into electrical control signals. We distinguish the following transducer principles:

• Microswitch
• Potentiometer
• Optical transducer (photocells, etc.)
• Inductive transducer

Transducer with micro switches have due to the required distance between the switching points in a plane, a control hysteresis at the operating point and thus a limited tracking precision result. An electrical shift of the working point is not possible. The other said transducer systems (optical systems may be restricted depending on the model ) generate analog signals proportional to Tastelementauslenkung and thus enable error - proportional Schweißkopfnachführung and electrical work offset by the higher-level control such as when multi-pass welding. The output signals of the most common inductive transducer systems from 0 to 10 V DC depending on Tastelementauslenkung ( Fig. 2).

Boundary conditions

For electrically contacting systems, each affecting the electrical contact between Sensortastelement and workpiece is problematic, for example by welding spatter at the inert gas through the scale layer and mill scale on the workpiece surface or by a spherically be fused and is encumbered by a slag wire electrode end. In mechanically contacting sensors, the sensing elements are adapted to the respective groove shapes. Butt joints with butt joint preparation should have a joint gap of more than 3 mm, with lap joints, the thickness of the top sheet should be greater than 3 mm.

The sensor must be mounted separate from the welding torch. Thus, the joint sampling is usually Running forward in front of the burner. Despite mostly straight seams, this arrangement not a problem are also Tastelementanordnungen feasible (eg fork button or separate sensing elements for vertical and lateral scanning) which can be done a scan in burner level and thus almost of overrun errors free seam sampling. In addition to the burner control along a weld mechanically contacting sensors can also be used for finding the seam beginning and seam end detection.

Optical sensors

Optical sensors are a type of non-contact measuring, geometry -based sensors ( Fig. 1). In optical sensors is the weld for obtaining information by means of a radiation detector that detects the emitted optical radiation from a measurement object is scanned. The radiation detectors Halbleiterbildaufnehmer be used. The optical measuring principles can be distinguished in sensors with and without active structured lighting. Without active structured lighting means that for signal acquisition, a camera is used which considers the workpiece and extracted from the two-dimensional gray-scale image, the information sought. An active structured light using a light source means for selective illumination of particular regions of the component. For subsequent collection of these can be used according to design individual photo elements, rows or arrays.

Operation

For optical measurements without active structured lighting a camera on the area of the weld is directed and looks at the scene of interest directly. This is for example used in submerged arc welding processes to represent the worker on a monitor a live image of the weld. As imager two semiconductor technologies are known. The CCD camera (CCD: Charge Coupled Device) is the older, the most common type of camera, it is also used in standard video cameras. When using a CMOS image sensor, it is possible even with burning arc by its high input dynamic range to include a usable image of the weld. The method of optical metrology with active structured light, usually generated by a laser with a defined wavelength, is often used for the automation of welding processes. Here are 1 -, 2 - and 3- to distinguish dimensonal measuring systems. Since a measurement is not possible directly in the arc itself, must be a certain distance ( forward), which depends on the type and size of the arc itself, are complied with.

In the one-dimensional measuring systems, the distance from the sensor is determined from the workpiece surface. This can be done by time of flight measurement. Another widely used method is the laser triangulation (see Figure 4).

From the known dimensions of the sensor and the triangulation angle α, the distance of the workpiece can be determined. Such one-dimensional, optical distance measurement systems have a large spread in the field of industrial automation technology and therefore are offered by a variety of manufacturers. In automated welding, they are often used for detecting the component and / or joint location prior to welding. In the two-dimensional sensor measuring systems, in turn, different versions are known. Immediately from the 1D triangulation can be derived by a pendulum motion of the two-dimensional laser scanner. This is performed by a transversely to the joint scan movement, the joint geometry detected ( Fig. 5). This is usually implemented by an integrated in the sensor head movable mirror device.

Alternatively, also a swinging movement of the entire sensor head can be made ​​, but this can be considered as a special use of a one-dimensional measuring system. An advantage of the laser scanner is that with appropriate processing speed for each individual point-like distance measurement, the exposures can not be re-adjusted, thus resulting in an evenly illuminated image. In addition, due to the point-like illumination of the laser spot focused by the laser power as well as by suitable optical filtering compared to the disturbing arc radiation is better for the detection element visible. The disadvantage of moving parts in the sensor head avoids the light section sensor ( Fig. 6). Here, the surface is not scanned point by point, but the entire geometry detected in a picture. For this purpose the point-like laser beam is expanded by optics to a line which is projected transversely of the joint according to the scan line of the scanner on the surface of the workpiece. The laser line is in the same geometric principle of triangulation back to a detector element, but now two-dimensional, collected. For detecting CCD and CMOS cameras can be used with the above properties.

As an output signal after pre-processing of the sensor signals obtained with laser scanners and light section sensor known as the altitude profile of the measured joint geometry. This provides the surface of the workpiece along the section of the projected laser line dar. Dimensional measuring systems with active illumination often utilize the light-section procedure in conjunction with the projection of a plurality of parallel laser lines. This gives a height profile per line. By arranging several lines along the weld obtained a further dimension, which shows the change in the height profiles of the joint geometry. Due to the number of lines, the resolution increases in joint longitudinal direction, but also so that in turn the data processing effort. A plurality of parallel lines similar to the projection is measured by a projected circle or other geometrical figures on the surface of the workpiece possible.

Boundary conditions

All optical measurement methods have in common that the determined joint points have to be transformed from the sensor coordinates of the cameras in the machine or workpiece coordinate. For this, these are previously calibrated to test workpieces and to deposit Kalibriermatrizen. Furthermore, must be deposited for the use of image processing algorithms prior information about the joint profile. This can be done by teaching templates, entering geometric parameters or learning by means of test workpieces. A more comprehensive image processing for 2 - and 3-D sensor systems usually needed to evaluate a PC system, which is why the standard PC interfaces are used for data exchange, but there is still no single sensor interface.

Problems with the use of optical sensors

Problems occur in optical sensor systems inherently on by scattered light of the open arc. Therefore, a measurement is directly at the operating point with most optical sensors is not possible, it must be adhered to a certain flow. Other disorders stir ago by weld spatter, can take the negative impact on the detection result. Here provide shielding between the sensor and burner to some extent remedy. One exception is the direct observation of the arc with special cameras for process monitoring. The advance of the arc due to a limited accessibility of vertices in components. In order to reduce this problem is to pay attention to a highly compact structure and a short lead distance. Furthermore, given the orientation of the sensor means a limitation of the working space for the robot. For the smooth operation of the optical components also increased pollution should be avoided ( by dust and sweat that builds up smoke particles ) as possible. Remedy replaceable protective glasses and protective shields by compressed air curtains. A significant influence on the measurement result, the quality of the surface to be measured. Is this highly reflective, so it can lead to unwanted reflections and faulty measurements, matt surfaces give less trouble. Problems also arise with changing surface qualities. Since optical systems are equipped with semiconductor detectors and extensive electronics, must always be taken to ensure good electromagnetic shielding. This applies to the sensor, the image processing unit and the connection lines. Sensor systems with active laser illumination are particularly sensitive to fluctuations in temperature, as the laser diodes used in the emitted wavelength of light depends on the temperature of the laser. Change the ambient temperature and therefore the wavelength of the active lighting, so can not penetrate through the narrow-band optical filter to the photodetector this light. An appropriate shielding from the welding process or a cooling of the sensor head is therefore provided. Depending on the laser power used, special care should be exercised in dealing with sensors with active illumination. Often, the wavelengths of the systems used in the visible range. This means a classification in the hazard classes 3A and 3B. The relevant accident prevention regulations must be observed. The following points should be considered when using optical sensors:

• Take the restriction of accessibility and of the working space
• Shield stray light through the open arc and weld spatter
• Note the reflection properties of the measurement surface
• Avoid contamination of the optical components
• Provide electromagnetic shielding of electronic components
• Compensating temperature fluctuations of the sensor
• Caution in using laser radiation

Inductive Sensors

Inductive sensors evaluate the induced eddy currents in the workpiece attenuation of high-frequency electromagnetic field. In single-coil models, one side or height correction is possible. Mehrspulige sensors allow a correction in two coordinate directions and in addition to influence the orientation of the welding torch.

Capacitive sensors

Capacitive sensors measure the capacitance between the workpiece and an electrically conductive plate of small dimensions. They offer the possibility of a distance measurement in media constant dielectric constant.

Process-oriented sensors

Process-oriented sensors get their signals from the primary or secondary process variables.

Arc sensors use the primary process parameters (welding current and / or voltage) of a moving or ungependelter two arcs to generate elevation and windage correction signals. Of course, these sensors require a scannable geometry of the joint, but fall relative to geometry -based sensors measuring and Fügeort together.

Arc sensors

Stable operating points during arc welding face as the intersection between the process characteristics and the current source characteristic curve (Fig. 7). The process characteristic it describes the relationship between a stable arc voltage and the corresponding current intensity of the process at constant boundary conditions. By variation in the arc length / distance of the burner to obtain a characteristic field.

Arc sensors TIG welding

TIG welding is one of the welding process with non-consumable electrode. Therefore, the process characteristic is often referred to as the arc characteristic. A direct change of the working distance is compensated by the length of the arc. As a result changed the arc resistance. Short arcs have a lower electrical resistance as long arcs. In TIG welding power sources typically be used with a steeply falling characteristic. Therefore, a change in the arc length leads directly to a change in the process voltage. A comparative measurement allows the determination of the distance to the workpiece.

Arc sensors in GMA welding

When MIG welding is a process characteristic in the voltage -current diagram results from the interaction of the electrical properties of the free end of the wire and the arc. Basically stable operating points are obtained by the use of suitable power source characteristics or by superimposed control strategies.

In point 1 of the image 8, there is a stable equilibrium, in which the energy supplied to the process is sufficient to melt the continuously fed wire electrode. During a rapid change in the distance of the arc to compensate for the change in length, point 2 The lower resistance of the short arc causes an increase in current, which leads to faster melting of the free end of the wire until a stable operating point is taken, point 3 This compensation process takes between 100 and 200 ms. The arc sensor rated the permanent change of current between point 1 and point 3 to win a distance proportional to size. Basically, this valuation concept is also applicable for the pulse arc welding. The above presented concept of distance measurement is extended with most arc sensors with a transverse scan of the joint geometry. The deflection of the process on the joint edges allows comparative measurement of the standoff. By forming the difference of the distance values ​​, the lateral positioning of the burner can be measured. The average of the two distance values ​​are at the height of the torch on the joint. To come deflection various concepts used ( Fig. 9). The mechanical oscillation is the most common and is used especially often in robots. Basically offer fast deflected systems eg with magnetic or rotational displacement to improve the signaling rate and signal quality, but in these systems, a higher expenditure on equipment must be taken into account. In the double- wire technique both joint edges can be scanned simultaneously, each with a wire.

Boundary conditions

Arc sensors evaluate the stable operating points in arc welding. Disturbances of the process must be compensated by a suitable filtering is not susceptible evaluation strategies. With a simultaneous vertical and lateral guidance is to be noted that only such joint geometries are suitable for arc sensors that allow their geometry from a lateral position determination by comparative measurement of the joint edges. V- welds and fillet welds are absolutely suitable. I seams without gaps are not suitable for cornering. Industrially available arc sensors have not been used on aluminum materials.

Secondary process variables evaluating systems

For sensor types, watching the molten material of the range of application is limited by the fact that molten pool and arc radiation of geometry factors such as material density or composition ( alloying elements ) are dependent.

Optical analysis of the Schmelzbadbereichs: A visual observation of the Schmelzbadbereichs notes changes in the Schmelzbadkontur. The deviation from a defined ideal contour is interpreted as a wrong position or a change in process behavior and compensated.

In the spectral analysis of the process signals emission spectra of the arc or melt pool with assumed ideal values ​​are compared. Deviations indicate a change in chemical composition or an energetic change in the process zone.

Applicable technical rules

• DVS 0922-1 industrial robot system for inert gas welding - Definition and definition of parameters
• DVS 0922-2 industrial robot systems for arc welding - control and programming functions
• DVS 0922-3 industrial robot systems for arc welding - welding equipment for MIG / MAG welding
• DVS 0922-4 industrial robot systems for arc welding - Planning and Setup
• DVS 0922-5 industrial robot systems for arc welding - Positioning of workpieces and industrial robots
• DVS 0927-2 sensors for fully mechanized arc welding; Influence of part geometry and the manufacturing conditions on the possibilities
• DVS 0939 tolerances in fully mechanized MIG welding and welding with industrial robots