# Collision Risk Assessment and Operation Assistant Strategy for Teleoperation System
## 1 Introduction  
With the development of in-orbit manufacturing technology [1, 2], in-orbit printing-assembly has become a trend for large spacecraft. In-orbit assembly can be divided into automatic and teleoperated assembly. Fully autonomous robots have certain drawbacks in complicated and unstructured environments, such as in-orbit assembly, due to mechanical, sensor, and control technology limitations [3]. When compared with automated robots, teleoperation robots can combine the initiative of operators with the high precision of the robot, which remain superior to the fully autonomous approach for rapid decision making and the handling of emergency cases [4]. Safety is critical in the field of teleoperation assembly. Due to limited vision, deficient control technology, and the physiological limitations of the operator, the teleoperation assembly robot can easily collide with the surrounding assembly parts, and even slight negligence may cause unnecessary loss [5], especially for high-risk, high-value aerospace exploration tasks. Therefore, collision risk assessment and obstacle avoidance strategiesbecome primary issues in teleoperation applications. This not only boosts the safety of teleoperation systems, but also enhances the efficiency of safely accomplishing the assembly task. The most straightforward and brief method to avoid collisions with surrounding assembly parts is the virtual fixture, which assists a teleoperation robot to accomplish assembly task by restricting the robot's movement to certain areas [6] or guiding the robot along a preset path [7]. The virtual fixture can be custom generated [8] or automatically generated [9]. The virtual fixture ensures the safety of the robot system while maintaining a certain degree of operational freedom, but it also constrains the robot from moving beyond the allowed range, thereby limiting the flexibility of teleoperation. In addition, artificial potential field-based control [10] may be a better option to address issues in path planning and real-time control [11; 12]. Each surrounding assembly part is considered as the origin of the repulsive potential field; the repulsive force is inversely proportional to the distance from the obstacle, and the risk can be directly assessed by the repulsive force. A drawback of this approach is that the classical potential field cannot capture the relative motion between the robot and the obstacle and is mostly applicable in two-dimensional space. The above-mentioned methods are more operationally oriented and, for that reason, cannot directly capture the collision risk of the operation. Therefore, teleoperation-assisted control methods based on safety assessment are attracting increasing attention. [PERSON] et al. [13] used the minimum separation distance criterion (MSDC) to evaluate the safety of the robot and to improve operational safety by constraining the robot speed. [PERSON] et al. [14] estimated the collision probability by obtaining the distance between the robot and the obstacle and translating it into a conservative speed limit. A so-called distance-controlled velocity was proposed [15], the key idea of which is to adjust its speed on the specified path according to the robot's distance relative to environmental obstacles. This concept was further clarified in [16]. These studies evaluated the collision risk of the robot by calculating the closest distance between components. The evaluation results were not sufficiently accurate, because only the influence of a single factor was considered. To comprehensively assess the collision risk, [PERSON] et al. [17] considered velocity in the collision assessment model and proposed the concept of a dynamic envelope, which is a region around the surface of an obstacle whose size depends on the risk assessment results. Nevertheless, this method is only applicable to known simple-shaped obstacles, and a similar concept was proposed in [18]. [PERSON] and [PERSON] et al.