# Path Planning for Laser Cladding Robot on Artificial Joint Surface Based on Topology Reconstruction
## 1 Introduction  
Artificial joints are widely used in medical treatment, which can effectively help patients with joint injury to eliminate pain and rebuild joint functions [1; 2]. With the aggravation of aging problems and the increase of joint injuries caused by accidents such as traffic accidents, the demand for artificial joints is increasing day by day [3]. Titanium alloys are commonly used to make artificial joint prosthesis, due to its superior mechanical properties, excellent corrosion resistance and good biocompatibility [4]. However, artificial joint prosthesis, as a body implant in direct contact with bone or tissue fluid, needs to have excellent biological activity [5], and the contact surface of the joint friction pair needs to have extremely high wear resistance [6]. In order to meet the special requirements of artificial joint application, titanium alloys are usually selected as the substrate, and one or more layers of high performance coating is fabricated on the surface of titanium alloys. In recent years, laser cladding as a novel and advanced surface modification technology has been gradually used to prepare ceramic coatings on the surface of titanium alloys to improve the wear resistance or impart bioactive properties [7; 8]. However, the geometric shape of artificial joints is complex [9], so it is difficult to plan the laser cladding path. How to design a suitable laser cladding path on such a complex surface and improve the efficiency and quality of coatings preparation has become an urgent problem to besolved. At present, most research of coatings prepared on artificial joint surfaces using laser cladding are roughly divided into two categories: one is to study the effects of process parameters on laser cladding coatings [10], and the other is to study the selection and ratio of the materials for coatings preparation [11]. Although the laser cladding path is an important factor for coatings preparation in laser cladding, it is seldom discussed. [PERSON] et al. [12] used a five-axis CNC (Computerized Numerical Control) machine tool to generate the processing trajectory through the relative motion between the powder nozzle and the substrate. A dense, bioactive glass coating on the surface of a ceramic acetabular cup was prepared by superimposing multiple laser cladding trajectories. [PERSON] et al. [13] created a metal coating with the porous structure on the surface of the artificial joint acetabular cup by using laser-aided direct metal tooling (DMT) technology. The pose of the acetabular cup was determined by programmatically combining the mobile platform with the positioner. However, either in the case of using multi-axis CNC machine tool or in the case of using both mobile platform and the positioner, there are problems such as complicated and expensive processing equipment, large processing space required, and difficult programming and operation. The multi-joint industrial robot is gradually introduced in laser cladding because of the characteristics of high degree of freedom, flexibility and programmable [14; 15]. [PERSON] et al. [16] analyzed the geometric information of the part CAD (Computer Aided Design) model, and the cladding paths were automatically generated layer by layer according to the CAD model. The effectiveness of this method was verified using a cladding experiment on a blade surface. [PERSON] et al. [17] prepared good quality coatings on continuous curved surfaces according to the trajectory obtained by using point cloud slicing. The motion mode that a robot equipped with a laser head moves along a predetermined trajectory is adopted in most cases of robot laser cladding, which still brings the problem of requiring a large processing space. Currently, the hip joint femoral head prosthesis with a diameter of 28-40 mm and the acetabular cup prosthesis with an external diameter of 44-56 mm are the most widely used in clinical applications [18; 19].