# Design and Motion Planning of a Biped Climbing Robot with Redundant Manipulator
## 2 Design of the Robotic Systems
### Mechanical Configuration  
The CMBOT consists of a 5-Dof biped climbing mechanism with two magnetic feet and a 7-Dof series redundant manipulator, as shown in Figure 2. Since the surfaces of the steel bars are too narrow to use the wheeled or tracked climbing mechanisms, the CMBOT uses the legged type of climbing mechanism. Among the existing legged climbing mechanisms that can cross irregular gaps, as shown in Figure 1, the 5-Dof biped climbing mechanism is most suitable since it has the fewest degrees of freedom and can improve the stability of motion. Five rotational joints, which are labeled J1 to J5, form the 5-Dof biped climbing mechanism. The rotational axes of the rotational joints are marked with purple arrows in Figure 2.  
Figure 1: Steel guardrails and brackets in a railway bridge. Figure 2: Overall structure of the climbing manipulator robot (CMBOT). To achieve stable and accurate walking, the magnetic feet of the CMBOT consist of electromagnet modules, limiting mechanisms, and laser range finders, as shown in Figure 3. Two electromagnet modules are mounted on both ends of the climbing mechanism to ensure the CMBOT climbs on the highest horizontal angle steel bar of the guardrail. There are two laser range finders on the front and back of the electromagnet module to measure the distance between the bottom surfaces of electromagnet modules and the top surfaces of the steel guardrails. According to the distance information, J2 and J4 can adjust their angles to ensure a parallel relationship between the contact surfaces of the electromagnet modules and steel guardrails, and these actions can avoid uneven contact between the contact surfaces. In addition, limiting mechanisms are mounted on the side of both electromagnet modules. Consisting of a driven motor and limiting plate, the limiting mechanism can seize the guardrail when the magnetic feet maintain contact with the upper surface of the guardrail and release the guardrail before the electromagnet modules lift. The main objective of the limiting plate is to reduce the surface clearance between the electromagnet modules and guardrail since the surface clearance has a significant influence on the magnetic force. The control cabinet and battery packs of the CMBOT are fixed on the other side of the electromagnet and can balance the weight of the manipulators. A rotating two-dimensional (2D) Light detection and ranging (LIDAR) sensor is mounted on the front of the climbing mechanism to acquire the three-dimensional (3D) characteristics used to plan the motion of the climbing mechanism. The rotating 2D LIDAR sensor, which consists of a servo motor and a 2D LIDAR, can produce a 3D point cloud of the work scene by rotating the 2D LIDAR and fusing the angle information of the servo and 2D point information of the 2D LIDAR [20]. An example of a 3D point cloud acquired by the rotating 2D LIDAR sensor is shown in Figure 4.  
Although the CMBOT can move on the guardrail flexibly with the climbing mechanism, the operating mechanism is still needed since the workplace is far beyond the reachable space of the climbing mechanism. Considering the characteristics of steel guardrails and brackets, a 7-Dof series redundant manipulator is used as the operating mechanism of the CMBOT. The redundant degrees of freedom can help the manipulator avoid collision with the guardrails and brackets while operating. Seven rotational joints, numbered from J6 to J12, are used for the manipulator and carbon  
Figure 4: Three-dimensional (3D) point cloud of a part of the bracket acquired by the rotating two-dimensional (2D) Light detection and ranging (LIDAR) sensor. Figure 3: Schematic diagram of the magnetic feet. fiber tubes are used as the manipulator links to reduce weight. The rotational axes of the manipulator joints are marked with yellow arrows in Figure 2. The manipulator is connected to the climbing mechanism through J6, whose rotational axis is collinear with the rotational axis of J3.