SENSOR APPLICATIONS IN ROBOTS FOR WELDLINE DETECTION

SENSOR APPLICATIONS IN WALL-CLIMBING ROBOTS FOR WELDLINE DETECTIONPresented By Priyanka Balhara2K13/CSE/18

INTRODUCTIONTo guarantee the security of the employees and to prevent aggressions to the environment, industries frequently carry out the inspection of fillet welds of the storage tanks.Quality control and above all, human and environmental safety are great reasons that stimulate the investments in technologies like automatic inspection.

The automatic inspection of weld lines in storage tanks is of special interest, due to the fact that such tanks are currently used to store harmful products.To carry out a confident inspection it is necessary to exactly identify the fillet weld, thus getting the necessary data for the adequate positioning of the sensor. Sensors based on different physical principles are used . In order to perform testing of weld lines, we often employ wall-climbing robots with vision sensors to support detection and measurement tasks.Some approaches have been proposed to detect the weld line, which includes Distance measurement [2],[5] Vision based approaches and [3]Structured light based approaches[1],[4]1. DETECTION OF THE WELD LINE USING DISTANCE MEASUREMENTFillet Weld Identification for Automatic Inspection of Spherical Tanks - Elyson Carvalhol, Lucas Molina, Eduardo Freire, Raimundo Freirel, Benedito Luciano

Distance Measurement ApproachThe fillet weld has a bigger thickness than the steel plates that composes the spherical tanks structure. This weld quantity that exceeds the steel plate level is known as "reinforcement" and is about 5 mm higher than the steel plate.

Ways to detect the fillet weld consists in using distance sensors- In Fig. 2, two possible sensory arrangements are presented.

Frequently the fillet weld do not present the profile shown in Fig. 1 and Fig. 2.In these cases the sensory arrangement illustrated in Fig. 2.b is not useful.

PROPOSED APPROACHIn this paper , optical (infrared) distance sensors were used.The chosen optical (infrared) distance sensors selected was the TCND5000 from Vishay.This sensor gives output as current values to indicate intensity of reflected light received.Distance is measured through the measurement of this current.The current values were transformed into voltage values.To characterize the infrared distance sensor a range from 10 to 30 mm was chosen, because this is the range of interest to detect the fillet weld.The procedure was carried out moving a white colour bulkhead from 10 to 30 mm with steps of 1 mm. At each step the voltage value was measured. With the data measured using this procedure, the graphic in Fig. 6 was drawn.

Spherical tanks are usually of white colour, but as they are almost always exposed to the environment conditions, other colours can be found in their structure, as the brown colour of rust and accumulated dirt on their surface. The sensor responses when using white and brown bulkheads are very similar.

2. VISION BASED APPROACHRobotic Vision System Using a Modified Hough Transform to Perform Weld Line Detection on Storage Tanks- Lucas Molina, Elyson A. N. Carvalho, Eduardo O. Freire, Jugurta R. Montalvo-Filho, and Flvio de A. ChagasThe Proposed ApproachThis paper presented a sensing system based on computer vision to allow the correct detection of the weld line even in the absence of reinforcement structures.It takes into account two intrinsic characteristics of images from weld lines, its thickness and differentiated texture of the weld line with respect to the image background.

As can be seen in the figure the textured part of image has locally repeated patterns of intensities.We can use statistical moments of the gray level histogram of an image.

The first order moment gives mean and second order gives variance.The variance describes relative smoothness. The textured points can be differentiated from non-textured by variance.The robot with VGA resolution camera was used to take pictures.Then the image of weld line was segmented based on texture to a binary image.Then Hough transform was used to detect the position and orientation of weld line on storage tank.

The images show the results of segmentation and Hough transform respectively.

3. STRUCTURED LIGHT BASED APPROACHESA Cross Structured Light Sensor for Weld Line Detection on Wall-climbing Robot-Liguo Zhang, Wei Ke, Zhenjun Han and Jianbin Jiao Robust Weld Line Detection with Cross Structured Light and Hidden Markov Model-Liguo Zhang, Jianbin Jiao, Qixiang Ye, Zhenjun Han, Wei Yang

ApproachIn these papers, structured light is used which captures the convex shapes of weld reinforcement.In the system, a CSL platform is designed to project a red cross laser stripe on the weldment surface, which is then captured into an image with a CCD camera.

When the stripe meets the weld line, it appears like a convex arc because of the height of weld line.

Model of CSL SensorThe weld line detection and tracking system platform is composed of a climbing robot, a weldment, an ultrasonic device and a CSL sensor(as shown in fig.).The laser projector is fixed on the robot and perpendicular to the weldment. Two stripes s1 and s2 are formed by intersection lines between weldment and two orthogonal laser planes L1 and L2.

The laser projector projects a cross laser beam on the weld line, forming convexity light stripe around weld line.When the robot moves along the straight weld line, only a convex arc exists on stripe s2, as shown in Fig. 1(b). When the robot closes to a T-intersection of vertical and horizontal weld lines, two convex arcs will appear on the strips, as showed in Fig. 1(c).As we have seen, by deformation of stripes it can detect two typical scenarios of weld lines: straight weld line and T-intersection weld lines.

CONCLUSIONWith the distance sensor arrays, the depth points are discrete even though distance sensor arrays have complex mechanical structuresAlthough vision based approaches are simpler and more intuitive than those based on distance sensor arrays, it is difficult to obtain the 3D information of weldment surface by a single camera. Moreover weld lines on painted surface often have similar color to their background, which makes it difficult to distinguish the weld lines from their background.Structured light based approach has advantages compared to the other two methods: it can obtain 3D information and is visible.ReferencesA Cross Structured Light Sensor for Weld Line Detection on Wall-climbing Robot-Liguo Zhang, Wei Ke, Zhenjun Han and Jianbin JiaoFillet Weld Identification for Automatic Inspection of Spherical Tanks - Elyson Carvalhol, Lucas Molina, Eduardo Freire, Raimundo Freirel, Benedito LucianoRobotic Vision System Using a Modified Hough Transform to Perform Weld Line Detection on Storage Tanks- Lucas Molina, Elyson A. N. Carvalho, Eduardo O. Freire, Jugurta R. Montalvo-Filho, and Flvio de A. Chagas Robust Weld Line Detection with Cross Structured Light and Hidden Markov Model-Liguo Zhang, Jianbin Jiao, Qixiang Ye, Zhenjun Han, Wei YangFault-tolerant weld line detection for automatic inspection of storage tanks based on distance and visual information fusion - E. Carvalho, B. Luciano, R. Freire, L. Molina, E. Freire