# Mobile Robot for Security Applications in Remotely Operated Advanced Reactors
## 1 Introduction
Legged robots encounter significant challenges when navigating through unstructured terrains, particularly in terms of gait flexibility and trajectory planning under heavy payload conditions. To address these issues, the researchers present an innovative gait transition hierarchical control framework for their hexapod wheel-legged robot, BIT-6 NAZA (Beijing Institute of Technology, Beijing, China) [17; 18]. The framework incorporates a flexible gait planner (FGP) and a gait feedback regulator (GFR) with behavior rules. The FGP optimally selects footholds and adjusts gait types based on considerations such as the secure foothold, stability margin, and kinematic margin of legs. Meanwhile, the GFR modifies the foot-end trajectory of the chosen gait using terrain feedback information, enabling the robot to adapt seamlessly to unstructured terrains during its patrol. The BIT-6 NAZA robot is also featured in a study on a novel hybrid gait obstacle avoidance control strategy using a perception system for navigating uneven terrain [18]. This research considers the robot's motion state and gait unit, identifying the local terrain with a visual perception system. Wheel-legged hybrid gait types and parameters are selected based on terrain detection, with gait topology and planning matrices generated for each leg controller to enable obstacle avoidance. A feedback controller, incorporating posture and foot-end force sensors, is utilized for stability. Demonstrations with the BIT-6 NAZA robot confirm the effectiveness and feasibility of the hybrid gait obstacle avoidance control strategy. A novel hierarchical framework was developed [19] for the six wheel-legged robot (BIT-6 NAZA) to achieve stable high-level movement with payload delivery. Experimental results demonstrated the effectiveness of the framework, showcasing satisfactory stable performance in real-world field environments. The framework includes a speed consensus algorithm for wheeled motion, three designed legged motion sequences, and foot-end trajectories based on Bezier functions. A comprehensive whole-body control architecture ensures horizontal stability during obstacle avoidance through attitude, impedance, and center height controllers. Legged robots commonly employ sets of gait patterns to traverse diverse surfaces. Achieving stable transitions between these gaits is crucial for effective locomotion on changing terrains. In their work, [PERSON] and [PERSON] [20] demonstrate the benefits of implementing gait transitions on two distinct robotic platforms to enhance their behavioral capabilities. Using the RHex robotic hexapod, they showcase how gait transitions facilitate a seamless switch from a tripod walking gait to a metachronal wave gait, which is particularly advantageous for climbing stairs. Additionally, the researchers introduce the RiSE platform, a hexapod robot designed for vertical climbing, emphasizing the pivotal role of gait transitions in enabling efficient vertical mobility for this robot. [PERSON] et al. [21] present a path planning framework for legged robots in challenging terrains, emphasizing efficient and safe navigation. The approach computes terrain characteristics and utilizes a traversability map to assess the costs of robot footprints based on obstacle negotiation capabilities. Employing the RRT* algorithm, the planner optimizes path length and safety. Results from extensive simulations and real-world testing on the quadrupedal robot StarIETH (Swiss Federal Institute of Technology (ETH), Zurich, Switzerland) demonstrate successful integration of the proposed navigation framework. The hierarchical structure facilitates frequent path replanning, while the cascaded planning approach enables swift searches in simple environments and complex solutions for intricate terrains. [PERSON] et al. [22] explore the development, deployment, and efficacy of an autonomous navigation system designed for an agricultural robot.