16. Sensors 217. eye hand control. br-er16-01e.cdr

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Peregrine Greene
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1 16. Sensors

2 16. Sensors 217 The welding process is exposed to disturbances like misalignment of workpiece, inaccurate preparation, machine and device tolerances, and proess disturbances, Figure sensor strategy brain eye hand control welding process The manual welder notices them by eyesight and corrects them manually according to strategies learned and gained by experience. To record process br-er16-01e.cdr Figure 16.1 process disturbances Adaptive Process Control Manually - Fully Mechanised irregularities and path deviations, a fully mechanised welding plant requires sensors providing control signals which are then used in accordance with implemented rules. Using corresponding control elements, the control loop is closed for the welding process. Scopes of duty of the sensors is finding the weld start point and seam tracking. In addition, with the help of information about joint geometry, process parameters can be adapted online and offline. The ideal sensor for a robot application should measure the welding point (avoidance of tracking misalignment), detect in advance (finding the start point of the seam, br-er16-02e.cdr Figure 16.2 Sensors for Arc Welding Systems Survey recognising corners, avoiding collisions) and should be as small as possible (no restriction in accessibility). The ideal sensor which combines all three requirements, does not yet exist, therefore one must select a sensor which is suitable for the individual welding job. Figure 16.2 shows different sensor

3 16. Sensors 218 principles used in welding engineering. The most frequently used systems in practice are tactile, optical, and arc based sensor systems with mechanical arc adjustment. V, X Y seam with ball-probe overlapp seam with ball-probe With tactile scanning systems, the simplest type of I seam with blanc-probe fillet seam with ball-probe edge seam with edge-probe scanning is a mechanical sensor. Pins, rollers, balls, or similar devices may be used as sensors. br-er16-03e.cdr multilayer seam with ball-probe Scanning Principles With Tactile Sensors Such scanning systems show a long distance between Figure 16.3 sensor and torch, the application range is limited. Only grooves with large dimensions and relatively straight seam path can be scanned with these systems. Figure 16.3 shows some examples of different groove geometries. Tactile sensors can recognise 3-dimensional offsets of the workpiece. Through scanning of three levels the 3-dimensional point of intersection can be calculated and the robot program for correcting the deviation can be shifted accordingly thus finding the start point A A' of the weld. In this case, the gas nozzle of the torch serves as a sensor, Figure 16.4, which is charged with electrical tension. As soon as the torch touches the workpiece, a current C' B' B flows, which is then taken by the robot control as a signal for obtaining the level to be scanned. br-er16-04e.cdr Figure 16.4 Inductive sensors are graded as non-contact measurement systems. Due to their function

4 16. Sensors 219 principle, they can be applied for metallic and electrically conductive materials. The simplest type is a ring coil. If alternating current flows though the coil,,a magnetic field is generated Figure 16.5 close to the workpiece. When the coil approaches the workpiece surface, the magnetic field weakens. Figure 16.5 shows the distance-dependent electrical signal. Such simple sensors are used to recognise the workpiece position. Using several distance sensors, also a welding groove can be scanned. With multi-coil arrangements in one sensor, the position of the welding groove, the angle between sensor and workpiece surface and the distance can be recorded. Figure 16.6 shows a principle arrangement. A transmitter coil generates an magnetically alternating field which induces alternating currents in the two receiver coils. In the undisturbed case, these currents are phase-shifted by 180 and neutralise each other. If the sensor is moved crosswise to the groove, magnetical asymmetries will occur in the scanning area, which will show in the presented signal shape. The output signal will be zero, if the coils are positioned exactly above the centre of the groove. The radar sensor in Figure 16.6 uses Doppler's effect to generate a signal. Here the phase difference between transmitter signal and receiving signal is evaluated. coil arrangement for groove position transmitter coil reception coil br-er16-05e.cdr coil arrangement for distance measurement A mathematical process groove position Principle of an Inductive Sensor (Single Coil and Multicoil Arrangement) A sensor signals functional principle of continuous wave doppler s radar radar sensor br-er16-06e.cdr Figure 16.6 distance radar sensor oscillation workpiece B transmitting wave receiving wave phase difference signal path Functional Principle of a Radar Sensor work piece

5 16. Sensors 220 transforms such signals into distance values. To record the position and the depth of the groove, the sensor must be continuously moved along the seam. Radar sensors form a so called radar baton, which is focussed onto a measurement spot of about 0,7 mm diameter for this application. Figure 16.6 shows the sensor signal, which represents the relative movement along the workpiece. At the moment, the characteristic values of the weld groove can be determined with a resolution in the range of 1/10 mm. U br-er16-07e.cdr Figure 16.7 I O l 2 l 0 I 1 I I l l 2 l 1 side correction l = 0 Arc Sensor l 1,2 height correction ΣI = 2 x I Soll Arc sensors evaluate the continuous change of the welding current with a change of the contact tip-to-work distance, Figure A signal for side control of the torch is determined by measurement and subtraction of the currents on the flanks of a groove. A comparison between actual welding current and programmed rated current provides a signal for distance control of the welding torch. To let this sensor method work, a divergence of the arc or the use of a second arc is required. To realise this principle, mechanical oscillation twin wire welding there are numerous possibili-ties. Figure 16.8 shows some variants of signal recording. The most br-er16-08e.cdr magnetical oscillation rotating wire frequently used method is a mechanical oscillation of Figure 16.8 Arc Sensor - Signal Detection - the welding torch, which is carried out by a rotor

The advantage of this method is a high divergence frequency of about 15 Hz. A disadvantage is the size of the electromagnets and the limited accessibility to the workpiece.

6 16. Sensors 221 movement with an oscillation frequency up to 5 Hz. The second method is mainly used with submerged arc welding. Both wires are aligned crossways to welding direction and the difference of the two currents is evaluated. Magnetic fields can diverge only the arc itself. The advantage of this method is a high divergence frequency of about 15 Hz. A disadvantage is the size of the electromagnets and the limited accessibility to the workpiece. The last variant of an arc sensor incorporates a mechanical rotation of the welding wire. In this case, the divergence frequency of the arc can reach up to 30 Hz. The signal recording is continuous during the movement. In this way, information about orientation of the torch and groove width is also provided. The arc sensor principle is limited to groove shapes with clear flanks. Together with the tactile torch gas nozzle sensor, it provides a frequently used combination for seam finding and seam tracking during robot welding. Optical sensors can be used for a great number of jobs. The easiest method is the recognition of the radiation intensity, which is reflected during welding. E.g. with laser beam welding, this is carried out through recording the reflected laser radiation with simple sensors for control of penetration depth, Figure The procedure is based on the line-up between the degree of reflection and shaft relation (penetration depth/focus position) of the capillary. The amount of back-reflection of the laser br-er16-09e.cdr Figure 16.9 NdYAG-Laser Back Reflection Procedure for Laser Beam Welding CO2-Laser beam power is measured, which due to multireflection is not absorbed by the workpiece. Changes of penetration depth due to modified laser power or a shifted focus position can be identified by the signal of reflected laser power and can be used for control of the penetration depth.

7 16. Sensors 222 However, optical sensors can also be used for measuring geometrical values. Such information may be used for finding the start point of a seam, for seam tracking, and for identification of groove profile. The two last mentioned functions provide the possibility to use the information for filling rate control and/or quality control. Geometry-measuring optical sensors are normally external systems, which are positioned in front of the torch as a leading element. It is practical to equip the sensor with additional axes, because both, torch and sensor, must be moved along the groove. Without additional axes, a robot would be limited in its accessibility to the workpiece and in its working range. Another problem is the tremendous effort to introduce the control-technical integration into the robot control. Among other things, information must be exchanged in real time. Most of geometry-measuring sensors use the triangulation principle or a variant of this measurement procedure. The triangulation measurement procedure provides information about the distance to the workpiece surface. A light spot is projected onto the workpiece surface and displayed to a line-type receiver element under a certain angle. With distance changes laser CCD camera lens laser beam emerge corresponding positions on the receiver element, Figure Sensors which use this triangulation principle are applied for recognition of workpiece position and for offline seam finding. depth measurement range br-er16-10e.cdr Figure Triangulation Principle Both, the laser scanner and the light-section procedure are based on the triangulation measurement principle. With the laser scanner, Figure 16.11, this principle is complemen-ted by an oscillating axis in parallel to the groove axis. The measurement of a sequence of distances along a line becomes possible and provides a 2-dimensional record and evaluation of the groove contours.

8 16. Sensors 223 Sensors as part of the light-section procedure, also provide information about the 2- dimensional position of the groove. As a function of this system, one or more light lines are projected onto the workpiece surface and displayed to a CCD matrix under a certain angle, Figure In contrast to scanning, information about the groove profile is provided by taking a picture scene. Using sensors, it is pssible to obtain additional 3-dimensional information through evaluation of more, in succession taken, while the camera moves over the grooves. Systems, which generate their information through a projection of several light lines, provide additional information about the path of the seam and the orientation of the sensor related to the workpiece surface. Both, scanning systems and sensors based on the light section procedure, can be used for recognition of the welded seam to make an automised quality control of the outer weld characteristics possible. motor angle transmitter beam deflection mirror oscillation laser focusing lens lens mirror workpiece CCD camera Another optical measurement principle uses, similar to human sight, the stereo procedure to record geometry information across the weld groove. Two independent optics photograph the interesting groove area and displays them onto two image converter elements (CCD-lines or CCD-matrix). Based on the corresponding image points in both picture scenes, the 3- dimensional position of object points is evaluated. Figure br-er16-12e.cdr Figure br-er15-20e.cdr laser ligthing workpiece Laser Scanner Principle 2-D detector Detektor Diodendiode Laser laser line projection CCD camera Principle of Light-Section Procedure

16. Sensors 224 Figure 16.13 shows the measurement principle, which uses CCD lines as image converter elements, and idealised signals for generating information.

13 reflection intensity between workpiece surface and gap.

The width of the groove is taken from the width of the signal drop.

9 16. Sensors 224 Figure shows the measurement principle, which uses CCD lines as image converter elements, and idealised signals for generating information. The grey scale drop in the signal is ideally used as corresponding image area, which occurs with butt welds due to different CCD lines workpiece Figure reflection intensity between workpiece surface and gap. Both, the lateral position of the groove and the distance to the sensor can be determined by evaluating the centre positions of both signal drops. The width of the groove is taken from the width of the signal drop. Optical sensors may also be used for geometrical recognition of the weld pool, to adapt process parame-ters in the case of possible deviations. Figure depicts such a system for use with laser beam welding. The welding process is monitored by a CCD camera through a filter system. An optical filter allows to observe the weld pool surface without disturbing effects of the plasma in the near infrared spectrum. Picture data are transferred to an image processing computer which measures the geometry of Figure stereo measurement principle br-er16-13e.cdr br-er16-14e.cdr laser display levels light line projection side position : distance : groove width : Weld Pool Recognition stereo measurement principle left signal right signal Principle of Stereo Measurement Procedure signal drop the weld pool. Geometry data contain information which is used online for control of the welding process. Among others, penetration depth and focus position can be controlled. The system also provides the recognition of protrusion-welded joints and welding defects like e.g. molten pool ejections.

16. Sensors 225 During electron beam welding, the beam is in combination with a detector used for both, to

For this, the beam can be diverged as well as bent, Figure 16.15.

The line or area surface scanning by the spotted functional principle (evaluation of back-scattered

15 seam tracking monitoring Sensor Principle of Electron Beam Welding electron beam provides a progressive

10 16. Sensors 225 During electron beam welding, the beam is in combination with a detector used for both, to carry out a seam tracking and a monitoring of the welded seam. For this, the beam can be diverged as well as bent, Figure Backscattered electrons are recognised by a special detector and converted into grey values. The line or area surface scanning by the spotted functional principle (evaluation of back-scattered electrons) br-er16-15e.cdr Figure seam tracking monitoring Sensor Principle of Electron Beam Welding electron beam provides a progressive series of greys across the scanned line or area. During electron beam welding, these signals can be used for seam tracking by scanning an edge which is parallel to the groove. The area-type scanning provides the possibility for observing the welded seam or the focus position.

QIROX Sensor systems. Top quality with each weld seam

QIROX Sensor systems. Top quality with each weld seam QIROX Sensor systems Top quality with each weld seam Precision work! Looking into space requires maximum optical precision. Just as your production processes with automated welding technology. Sensors