# Collision Avoidance Using Finite Control Set Model Predictive Control for Unmanned Surface Vehicle
## 2 The Overview of the System Structure and Technology  
The Lanxin USV is a small unmanned mobile platform above water of Dalian Maritime University, which has many functions such as maritime rescue, marine monitoring, water sampling, and so on [20]. As a new type of intelligent marine equipment, it is equipped with many types of equipment, such as radar, camera, differential global positioning system (DGPS) navigation sensor, automatic identification system (AIS), etc. The radar, DGPS and AIS were provided by Simrad; the camera is by Hikvision; and the supplier of the host propeller was Mercury Marine. The information of these devices will be transmitted to STM32 (32-bit microcontroller integrated circuits by STMicroelectronics) and a computer, in which the CAS is implemented. The Lanxin USV is shown in Figure 1.  
In the paper, the Lanxin USV is used as the research object. Figure 2 describes the hierarchical structure of the USV system. The structure consists of four layers, which are the perception layer, task layer, decision layer and execution layer.  
The perception layer acquires environmental information including global and local information from the electronic chart and radar. The task layer arranges the mission of the USV based on the requirements. In general, a task can be defined as a set of path points in environmental information. According to the task and environmental information, the third layer, that is the decision layer, updates the appropriate control commands to complete the mission by FCS-MPC. The generated control commands will be passed to the execution layer, to calculate the specific trajectory for the USV. In order to minimize system error, the real-time state information (velocity, position) is fed back to the decision layer to modify the control commands by the state awareness system. The structure is a closed loop system to ensure the safety of the USV.  
Figure 1: The Lanxin USV. AIS, automatic identification system.  
Figure 2: The hierarchical structure of the Lanxin USV system. FCS, finite control set.  
From the whole structure of the USV to analyze, the decision layer is very important, and it serves as a link between the other three layers. In the decision layer, the most important task of CAS is to avoid collisions. In addition to the collision avoidance task, the CAS also needs to reach the required target point according to the mission. There are multiple objectives that need to be optimized, such as distance, speed, etc. The ultimate goal is to make the USV accomplish tasks as quickly and safely as possible.  
Although many research works have also been carried out on collision avoidance of some unmanned platforms (unmanned ground vehicle [21; 22], unmanned aerial vehicle [23], autonomous robot [24], etc.), in contrast, only a few articles have studied USV. This is possibly due to the relatively complex ocean environment and the robustness of the USV control system. Therefore, the challenge of CAS research is to propose a practical collision avoidance algorithm for USV in the real marine environment.