A novel computation method of the 3D weld pool surface from specular reflection of laser beams is presented in this paper. The mathematical model for the three-dimensional surface measurement technique has been developed. The structured pattern of a laser is projected on the weld pool in a molten state and the reflection observed using a CCD camera. These reflected patterns can be tracked using image processing techniques including optical flow and moving point tracking. A simulation model has been developed implementing these techniques. The movement of the reflected laser during the welding process is indicative of the change in state of the weld pool. The surface slope field is calculated from the law of reflection and is used to compute the 3D surface of the weld pool. The measurement technique is tested on objects with a priori knowledge of geometry having a specular surface to test the effectiveness of the measurement technique.
Automated monitoring and control of the weld pool surface has been one of the goals of the welding industry. This paper presents a technique which uses a calibrated charge-coupled device (CCD) sensor and structured light to extract the surface information as depth of pool from captured images. It projects a laser line from a predetermined position onto the specular weld pool surface. A reflected laser beam from the specular surface is captured by a calibrated CCD sensor to form the image. The image is then processed based on the ray-tracing technique to calculate the depth of the weld pool surface using the position of the laser and its fan angle along with the intrinsic parameters and extrinsic parameters of the CCD sensor.
Observation and measurement of a weld pool surface is a key towards the development of next generation intelligent welding machines which can mimic a skilled human welder to a certain extent. However, the bright arc radiation and the specular surface complicate the observation and measurement task. This paper proposes a novel method to turn the difficulty of the specular surface into an advantage by exploiting the difference between propagation of an illumination laser and the arc plasma. The governing law is simply the reflection law which can provide the base for the computation of the weld pool surface. Experimental results verified the effectiveness of the proposed method in acquiring clear images in the presence of the bright arc.
Weld pool surface can change dynamically during welding and is indicative of information critical to controlling the process. Research has picked up in the field of observing the weld pool surface to understand the dynamics of the welding process. This paper will help visualize and understand the physics involved in observing the weld pool surface. A study of laser properties, weld pool and camera optics was incorporated in developing a model to describe the mechanism of observing the weld pool surface from specular reflection. This observation method projects a laser beam on the pool surface through an optical grid with a frosted glass attached. The corresponding specular reflection is calculated, which is derived based on the reflection law. The reflected laser beams are then captured by the camera to form the image. The model can be used to predict the outcome of experiments with grids placed in front of the laser and to determine the position where the camera should be placed to acquire the best image. Preliminary results showed that the camera should be placed with the weld pool along the optical axis, and the aperture should be as large as possible to allow as many rays into the camera as possible. The model can be used to find the optimal location of the laser and camera for materials of different thickness, by moving the electrode higher in the simulation, and adjusting the laser and camera location accordingly. The paper will give some insight into problems that might be encountered in observing the weld pool, and suggest the setup of the laser and camera for obtaining the best image.
A substantially successful technology that has emerged is the use of electro-optics, which has been expanded into the realm of 'machine vision' with the rapid advance in optical sensor technology and computer processing power. The basic principle of the optical sensor has remained unchanged over the years. Charge-Coupled Devices (CCDs), laser diodes and microprocessors form the basis of this type of sensing technique. CCDs have found their use in the observation of weld pool. It is known that the surface of the weld pool contains information that can be used to control the welding process. Researchers have used cameras to observe the weld pool and study the shape of the weld pool. Optical sensors have also been used to measure the arc light to determine the state of the weld pool. This paper summarises limited research done in the field of sensing the weld pool visually or optically and is intended to give the reader a brief overview of what optical technologies and techniques have been used to sense the weld pool state.
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