Hybrid offline programming method for robotic welding systems

Highlights

• Current automatic programming approaches for robotic manufacturing systems have been reviewed.

• A hybrid offline programming method that combines CAD-based and vision-based offline approaches is proposed.

• The vision-based and CAD-based activities of the proposed programming method with the support techniques are presented.

• Vision & CAD interactive activities are proposed to identify the workplace, detect the deviation, and compensated the path.

Abstract

Offline programming is an intuitive and automatic programming generation technique that does not use real robotic systems, thus greatly decreasing the downtime required for system programming, and resulting in enormous savings in terms of labor costs. Currently, offline programming can be generally categorized into computer-aided-design-based (CAD-based) and vision-based approaches; these two types of offline programming approaches have been widely applied in robotic welding systems. However, owing to the highly complex and diverse workpieces needed in the shipbuilding industry, neither of the aforementioned offline programming approaches can fully support the automatic generation of welding programs.

In this paper, a hybrid offline programming method systematically combining CAD-based, vision-based, and vision & CAD interactive activities is proposed to overcome the limitations of current automatic program generation methods for robotic welding systems. In the vision-based activities, the positions of the workpieces are obtained by using geometrical features gathered from the workpieces’ images, whereas in the CAD-based activities, the welding tasks are assigned to different mobile components of the welding torch; then, their welding paths are planned according to the workpieces’ CAD models. The vision & CAD interactive activities enable the mapping between the point cloud of a workpiece and its CAD model, so that the deviations caused by assembly errors can be detected and path compensation data can be determined. The effectiveness of the proposed hybrid offline programming method is demonstrated by integrating it into a subassembly welding robotic system. The experimental results indicate that this method can significantly improve the efficiency, accuracy, and flexibility of the robotic welding system.

Introduction

Robotic welding systems have played an increasingly important role in shipbuilding enterprises, owing to their higher production efficiency, better welding quality, and longer working hours. However, programming for robotic welding systems has long been considered as a challenging task, as it requires specific engineering expertise [1]. To overcome this challenge, two different programming modes (online and offline) have been developed to achieve automatic programming, and are implemented in most current robotic welding systems [2]. Online programming is typically realized using a teaching programming method [3]. Such a methods is always time-consuming, as the robotic system must be manually controlled by an operator to achieve a series of required motions. Moreover, a completely new program must be written if a new workpiece is being processed. As a result, the online programming mode becomes no longer suitable for the current shipbuilding industry, as the ships’ diverse workpieces, often with small batch sizes, can lead to critical losses in programming efficiency.

Offline programming is an automatic programming generation technique that does not require operations in real robotic systems [4], thereby greatly decreasing the downtime required for system programming, and resulting in enormous savings in labor costs [5]. Currently, the offline programming mode can be generally categorized into two types: computer-aided-design-based (CAD-based) offline programming approaches and vision-based offline programming approaches [6]. However, owing to the diverse workpieces with high complexity levels needed in the shipbuilding industry, neither type of offline programming approach can fully support automatic welding program generation. Although a CAD-based offline programming approach can provide the complete geometrical features of workpieces to automatically generate a program, the deviations between the workpieces and CAD model are not considered; therefore, the welding accuracy cannot be guaranteed. With regard to the vision-based programming approach, the welding seams are ideally selected from numerous lines in the image captured by the vision sensor; this is a time-consuming task that largely limits the welding efficiency, especially for workpieces with complex structures.

In this study, a hybrid offline programming method systematically combining CAD-based and vision-based offline approaches is proposed, aiming to overcome the limitations of current automatic program generation methods for robotic welding systems. The remainder of this paper is organized as follows. Section 2 presents a literature review of current programming approaches for robotic manufacturing systems. Section 3 introduces the proposed hybrid offline programming method. The effectiveness of the proposed programming method is demonstrated by integrating it into a subassembly welding robotic system, as described in Section 4. A detailed discussion of the proposed hybrid offline programming method is provided in Section 5, and conclusions are presented in Section 6.