Sensors for arc welding are devices which – as a part of a fully mechanised welding equipment – are capable to acquire information about position and, if possible, about the geometry of the intended weld at the workpiece and to provide respective data in a suitable form for the control of the weld torch position and, if possible, for the arc welding process parameters.
Introduction The quality of a weld depends, besides the weld parameters which are important for the welding process (e.g. voltage, current, wire feed and weld speed) also mainly from the type of input of process energy and of the used filler material. The positioning of the torch exerts a direct influence on the material flow. The heat input for the melting of the component edges and the steady heat flow are, furthermore, directly connected with the torch guidance and exert substantial influence on the weld quality and on the resulting residual stresses. In fully mechanised and automated shielded gas welding, the inaccuracies of torch guidance, workpiece handling, groove preparation and thermal distortion are adding to the variations of the edge position and edge geometry. In fully mechanised welding, the information which is required for the weld quality is detected via sensors. Sensors are applied for checking the position of the component (detection of weld start and end of weld), for joint tracking and for the adaptation of the process parameters to changes of the joints/grooves. It is possible to use the sensors online (together/at the same time with the welding process) or offline (in a separate working step before welding). Sensors are mainly used in online joint tracking.
Principles
All physical principles which are capable to provide information about the position of an object are suitable to serve as the starting basis for a sensor function. The ambient conditions prevailing during arc welding and also the requirements which are made by fully mechanised equipments have, however, many restrictions as a consequence. Figure 1 depicts the system overview. The monitoring strategy of the sensor (process or geometry) has been chosen as the superordinate criterion, the further subdivision is orientated on the measuring principle. A further distinctive feature of sensor systems is their design. Leading sensors are, thus, marked by the fact that measuring point and joining point are not located in the same position. Here, the measuring and joining process are mainly running in sequence. For making position-relevant statements about the welding process, those systems require calibration of the relative position. If process-oriented sensors are used, the measuring point and the joining point are identical. What the measuring principles all have in common is the fact that through the evaluation of the sensor signal, geometrical information about the joint and its relative position to the measuring head is provided. The individual active principles allow different processing speed for acquiring the information.
Geometry-oriented Geometry-oriented sensors acquire their signals from the geometry of the groove or from an edge or area the course of which is in accordance with the groove.
Tactile sensors Electric contact sensors for joint tracking and/or work piece measurement are representing one type of tactile sensors. The sensor makes electric contact with the workpiece, the electrically conductive workpiece is included into the measuring circuit of the sensor. The mechanical contact sensors belong to the second category of the tactile sensors. The mechanical deflection of a scanning element which makes contact with the workpiece is evaluated.
Electric contact sensors Following a determined searching strategy, the electric contact sensor systems are scanning the weld start or other track points via contacting the work piece with parts/components which have been subjected to voltage (direct voltage of several ten Volt up to 1 KV, depending on material and surface) of the welding equipment (shielding gas nozzle, welding electrode, stylus, or similar.) This means the offline-measuring of the weld start, the part position or part geometry before welding. Knowing the scheduled path, a transformation of the track points in accordance with the measured conditions is carried out. In this case, corrective action is not carried out during the welding process.
Thermal Here, the heat flow is measured with two thermo-couples which are arranged on the welding torch, the thermal flow is used for the side/lateral- and height control of the torch. The orientation of the torch towards the groove is detected via the comparison of the sensor temperature of the two thermo-couples. If the orientation of the torch is symmetrical, the difference of the radiated thermal flow equals to zero, so do the temperature differences of the thermo-couples. Dependent on the lateral misalignment of the torch the thermo-couples are subject to different heat flows, by the deformation of the arc and also by the changed position of the molten pool.
Mechanical contact
Mechanical contact systems transform the deflection of the scanning element directly into electric control signals. The following transformer principles are differentiated: micro-switch potentiometer optical transformer (light barriers or similar) inductive transformer Due to the required distance of the acting/break points in one level, transformers which are equipped with micro-switches have a control hysteresis in the working point which has the consequence of a restricted reproducible accuracy. Electric displacement of the working point is not possible. The other, above-mentioned transformer systems (the use of optical systems is probably limited due to design reasons) produce analogous signals in proportion to the scanning element deflection and allow thus the error-proportional weld head tracking and also the electric working point displacement through the superordinate control, e.g. in multiple layer welding. The output signals of the most commonly used inductive measuring transformer systems are between 0 and 10 V DC, depending on the scanning element deflection (Figure 2).
Boundary conditions Any impairment of the electric contact between sensor scanning element and workpiece is, in the case of electric-contact sensors, problematic, e.g. welding spatters at the shielding gas nozzle, scale and rolling skin on the workpiece surface or through a wire electrode end which has molten spheroidally and has adherence of slag. When mechanical-contact sensors are used, the scanning elements must be adapted to the respective groove shapes. Butt welds with a square butt joint preparation must have a groove gap of more than 3 mm; in overlap joints the top plate must have a thickness of more than 3 mm. The sensor must be mounted separately from the welding torch. Thus, the groove scanning is mainly carried out in a leading position in front of the torch. If the welds are mainly straight, this adjustment is no problem. It is also possible to use scanning element arrays (e.g. fork callipers) or separated scanning elements for height and lateral scanning which allow scanning in the torch level and thus weld scanning which is almost free from errors. Apart from the torch guidance along a weld groove, mechanical contact sensors can also be applied for detection of the weld start and end of the weld.
Optical Optical sensors belong to the group of non-contact measuring, geometry-oriented sensors (Figure 1). For information retrieval, the weld groove is scanned via a radiation detector which records the emitted optical radiation of the measured object. Semiconductor image sensors are applied for the detection of radiation. The optical measuring principles are differentiated into sensors with and without active structured lighting. If there is no active structured lighting, a camera is used for signal acquisition. The camera observes the workpiece and extracts the required information from the two-dimensional halftone picture. Active structured lighting means the application of a light source for the defined lighting of specified regions of the part. For the subsequent acquisition, single photo elements, lines or arrays can be used, depending on their design.
Operating mode
For optical measurement without active structured lighting, a camera is directed on the region of the weld groove and the scene of interest is observed directly. This is used, for example, for SA welding processes in order to provide the welder with a live photograph of the weld groove on the monitor. We know two semiconductor technologies for image sensing. The CCD camera (CCD: Charged Coupled Device) is the best known, most widely spread camera type, it is also used in standard video cameras. If a CMOS image sensor has been used, the high input dynamics allow, even with a burning arc, to record a usable image of the weld groove. The method of optical measuring technique with active structured lighting, mainly generated by a laser with a defined wavelength, is often used for the automation of welding processes. It is differentiated between 1, 2 and 3-D measuring systems. Since measuring in the arc directly is not possible, a defined distance (advance) which depends on the type and size of the arc itself must be maintained.
If one-dimensional measuring systems are used, the distance from sensor to workpiece surface is determined. This is carried out via measurement of the running time. A further, frequently used method is laser triangulation (Figure 4).
The distance of the workpiece is determined from the known dimensions of the sensor and the triangulation angle α. This type of one-dimensional optical distance measurement systems is widely used in the field of industrial automation technique and is therefore offered by many companies. In automated welding, they are often used for the detection of the part and/or groove position before the start of the welding process. There are different types of design of the two-dimensionally measuring sensor systems. From the 1D triangulation sensor, the two-dimensional laser scanner can be derived from the oscillation movement. Here, the groove geometry is detected via a scanning movement transverse to the groove (Figure 5). This is mainly carried out via a movable mirror unit which is integrated in the sensor head.
As an alternative, an oscillating movement of the entire sensor head can be carried out, this is, however, only considered a special application of a one-dimensionally measuring system. An advantage of the laser scanner is that, with according processing speed, the lighting conditions can be adapted for every single point-shaped distance measurement which results in illumination uniformity. Moreover, due to the point-shaped illumination, the laser point is through the concentrated laser power and also through appropriate optical filters, compared with the interfering arc radiation, easier to detect by the detection element. The light-section sensor avoids the disadvantage of moving parts in the sensor head (Figure 6). Here, the surface is not scanned pointwise, the entire geometry is, moreover, captured in one image. For this purpose, the point-shaped laser beam is expanded via an optics to a line which is projected onto the surface of the workpiece transverse to the groove in accordance with the scan line of the scanner. The laser line is, in accordance with the same geometrical principle of triangulation, again acquired with a detector element, this time, however, two-dimensionally. For the acquisition, CCD and CMOS cameras with the above-mentioned properties can be used.