Automated Welding Improves Safety and Productivity in Nuclear Industry
Following the introduction of its new DLS range of 3D digital laser scanners, Oxfordshire-based Meta Vision Systems has put together a VistaWeld package designed specifically to automate welding jobs in the nuclear industry. These are characterised by multiple passes of the weld torch to fill deep, narrow grooves down to 8 mm across between component parts typically up to 350 mm thick, often of highly reflective stainless steel.
The company claims that this is the first time full automation has been made available for such applications, which until now have been carried out by an operator watching a camera image of the seam and using a joystick to position the weld torch manually. This is significantly slower and with up to 50 or more weld passes needed for the root, hot, fill and cap phases, there is risk of human error, with consequent safety and cost implications.
Factors that complicate the welding process are a need for the sensor to see from top to bottom of the groove and to illuminate the sides to ensure positional accuracy, even though the walls can be angled at one degree (i.e. are nearly vertical). Another typical requirement is to preheat the parts to around 300 degrees Centigrade, ensuring that there is sufficient heat to fuse the material but not enough to cause cracking and compromise structural integrity.
The VistaWeld - DLS seam searching and joint tracking system is typically used for submerged arc welding of butt, V, and U joints. It not only tracks the seam and positions the weld electrode accurately and automatically, but additionally can record position correction data for each pass to provide certification of the manufacturing process. This is an important feature for the nuclear industry, as traceability is mandatory.
In fully automatic, multi-pass control mode, during the first pass of each layer, the system measures the joint width at that depth and determines the number of passes required for the layer. During the last pass of each layer, the system measures the remaining depth and determines whether to continue with filling or go to capping. Advanced adaptive fill to compensate for joint area variations by controlling welding parameters directly to adjust the deposition rate at each point on the joint is available as an option.
Alternatively, in semi-automatic mode, the operator manually selects the type (root, hot, fill or cap) and position (left, right or centre) of the next pass.
Digital laser scanner is unaffected by reflections
The DLS scanning-spot laser sensor has proved to be ideal for this application. It uses a 2048 element, linear CCD and has much of the signal processing integrated inside the sensor head itself. Compressed air cooling is provided to keep the unit within its operating temperature range. Water cooling can also be used in the case of high temperature preheats.
Conventional laser vision systems based on triangulation use a laser stripe projected onto the weld joint. As constant laser power is used, it is not possible to vary the intensity along the stripe to increase the light reflected from dark areas and decrease the light received from lighter areas. The camera images the complete scene including either side of the stripe, which can lead to further problems with dynamic range and unwanted reflections.
In contrast, Meta's DLS sensor is based on a scanning spot rather than a stripe, solving the two main problems of stripe-based triangulation when dealing with shiny materials like stainless steel. First, it is easy to implement effective automatic gain control to compensate for variations in workpiece reflectivity along the length of the stripe. Second, imaging is via the CCD, which only looks at the region of interest and is not affected by reflections.
Additional benefits of the scanning technology include a programmable field of view and independence of the sensor's depth of field from its width of field, providing high quality data in both axes simultaneously. Moreover, at the touch of a button, the sensor can project the spot to measure and control standoff - the height of the end of the electrode from the weld - and to assist manual horizontal tracking. This is the normal procedure for capping on circumferential welds.
Overall, the use of VistaWeld / DLS provides numerous benefits including higher weld speeds, better quality irrespective of operator attention level, superior interpenetration of the weld zones, reduced scrap and rework, and lower manning levels. Meta has supplied over 2,700 laser seam tracking systems worldwide.
