## 1 Introduction  
Composite materials are artificially manufactured from two or more materials that complement their most attractive properties while maintaining their macroscopic characteristics, which is in contrast to the metal alloys that have been widely used in the past as raw materials for various structures. Representative composite materials include carbon fiber reinforced plastic (CFRP), glass fiber reinforced plastic (GFRP), and epoxy resin materials [1, 2]. Composite materials are stronger, lighter, and more resistant to corrosion and fatigue than conventional metal materials [3]; therefore, they have attracted significant attention as materials for structures that require weight reduction, such as automobiles, aircraft, and structures for defense and aerospace industries. However, the initial production costs of these composite materials are higher than those of metal materials. This cost encompasses all the expenditures incurred from the sourcing of raw materials to the manufacturing process. From a technical perspective, the reliability of structures composed of these materials may be lower because the mechanical and physical properties of such structures deteriorate due to the occurrence of delamination,voids, inclusions, fiber misalignment, fiber fracture, debonding, etc., which are volumetric defects present in composite material structures [2]. In particular, delamination is regarded as a defect because it can significantly reduce the mechanical properties of the structure [4]. A void is the most common defect in the manufacturing process and affects mechanical properties, such as lowering the shear strength [5]. To address this problem, various nondestructive inspection methods have been applied [6; 7; 8; 9]. Among the nondestructive inspection methods, ultrasonic inspection methods have been most widely applied due to the nature of composite materials. Inspection in water for large structures has been excluded due to their sheer size and methods that efficiently use space with water injection have been widely used instead [10]. Composite materials exhibit higher levels of anisotropy and acoustic attenuation than other materials, which makes it difficult for ultrasound to pass through them. To address this problem, the through-transmission ultrasonic (TTU) method has been widely used. This method generates ultrasound on one side of a structure and receives it on the other side using two sensors; consequently, the ultrasound needs to pass through the specimen only once. In addition, as composite materials have already been used in large structures such as aerospace structures, automobiles, and submarines, numerous studies have been conducted using robot systems to inspect them with nondestructive inspection methods [10; 11]. Previous research has focused on developing multi-joint robotic ultrasonic inspection systems for the inspection of large composite material structures and verifying their validity [12]. However, multi-joint robotic ultrasonic inspection systems that use the TTU method may misinterpret an intact part as a defect if the two sensors are not accurately aligned because of the change in ultrasonic transmission intensity. To address this problem, two robot systems are precisely adjusted during the production of a multi-joint robotic ultrasonic inspection system with the objective of accurately aligning the two sensors. For the existing multi-joint robotic ultrasonic inspection systems, however, deformation occurs in many areas over time; for instance, at the floor level and in the machine assembly. Changes in the level values affect the ultrasonic inspection results. Regular adjustments to system levels and assembly are required to solve this problem; however, these require considerable time and are not cost-efficient. Therefore, this study focused on the development of a software program for automatic control of a multi-joint robot and correction of the level values by measuring the coordinates of the deformed mechanical part and applying the results to pre-developed software.