applied sciences  
The Design of a Lightweight Cable Aerial Manipulator with a CoG Compensation Mechanism for Construction  
Inspection Purposes  
[PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] The Design of a Lightweight Cable Aerial Manipulator with a CoG Compensation Mechanism for Construction Inspection Purposes. Received: 10 December 2021  
Accepted: 20 January 2022  
Submitted: 23 January 2022  
Typically, inspections are done manually by a human operator. This method exposes the operator to hazardous work areas, such as high elevations, to reach the inspected surface of the structure. Whilst traditional safety equipment greatly minimize the risk involved in such operations, failing to abide by safety requirements poses potential life-threatening risks. Thus, an interest has risen in automated inspection methods, which eliminate the aforementioned risks while decreasing running costs and conveniently mapping inspection points [2,3]. Unmanned aerial vehicles (UAVs) have been extensively used by both individuals and industries, mostly for recreational and surveillance purposes. However, due to several limitations, there have been few practical implementations of manipulators attached to UAVs. Existing industrial implementations of aerial manipulators include high voltage [4] and surface contact [5] inspections, as well as specific tasks, such as bolt tightening [6] and valve turning [7]. These limitations exist due to aerial manipulators being more susceptible to disturbances since they are not fixed on stationary rigid frames. The two main disturbances that challenge aerial manipulators are the center of gravity (CoG) shifts and the inertial forces caused by the movement of the links [8]. Several attempts have been made to overcome these challenges, each having their own advantages and limitations. Some methods used a slider with a counterweight in addition to propeller thrust to compensate for the CoG shift [9,10]. Others have proposed model-based control schemes that rely on propeller thrust, such as a decoupling transformation [11] and feedback linearization [12]. A solution proposed in [8] utilized a slider that moved the entire manipulator in a direction opposite to the arm motion. This solution, while being lightweight and compact, is not suitable for construction inspection where the manipulator's end-effector must remain stationary. Another solution was proposed in [13] which showed a counter-balance mechanism that utilized a pantograph-like structure to stabilize the manipulator. However, such a mechanism is mainly used in pick-and-place operations. A suspended aerial manipulator (SAM) was also proposed in [14], which operates by splitting the system into two sections: a main aerial carrier and a winch-suspended manipulator. Thus, the aerial manipulator can easily reach cramped areas, a task which traditional manipulators struggle with due to the large propellers on the attached UAVs [14]. However, this solution requires the development of a separate entity with its own propulsion units which adds to the cost and complexity of the system. Finally, a decentralized approach was proposed in [15] based on the compensation system which involves the utilization of an object slider, similar to the manipulator slider mechanism. However, this system slides only one part of the system, i.e., the battery, to compensate for CoG shifts instead of sliding the entire manipulator. This paper proposes a lightweight design of a cable aerial manipulator with a CoG shift compensation mechanism that can be used for construction inspection purposes, such as in Figure 1. The purpose of this design is to overcome some of the aforementioned challenges faced by aerial manipulators in construction. The lightweight design and cabling system reduces the CoG shift caused by the manipulator motions compared to other conventional manipulators connected to the UAV and the CoG shift compensation slider mechanism ensures the drone's stabilization while decreasing the rotor power required for stabilization in conventional methods.