# Development of a Capsule-Type Inspection Robot Customized for Ondol Pipelines
## 3 Conceptual Design
### Subsection Characteristic Analysis of Ondol Pipeline Installation
#### 3.1.2 Installation Specifications and Patterns  
Generally, onod pipes have a thickness and inner diameter of 2 and 12-25 mm, respectively; however, pipes with an inner diameter of 16 mm are primarily used in onod piping. Therefore, an inspection robot with a diameter of less than 16 mm must be developed. Additionally, the robot must pass through curved sections according to the installation patterns of the pipes. The representative patterns of the onod pipeline are shown in Figure 2. Pattern A in Figure 2 is the same as that in Figure 1 and is the most common pattern. Depending on the size of the room, they can also be installed in the form of patterns B and C.  
Figure 1: Onod pipeline system shown in the floor plan.  
The curved sections in the pattern are important because the original circular cross-section of pipes changes to an elliptical shape in the curved section. Therefore, the horizontal diameter inside the pipe is smaller than that in the curved section. Figure 3 illustrates the situation in which the horizontal diameter of the cross-section decreases, dividing the sections of the pattern into straight, 90-degree rotation, and 180-degree rotation.  
For example, in the case of a pipe with an inner diameter D, the inner diameter remains at D in the straight section. However, in a 90-degree curved section, the inner diameter of the pipe decreases to 95%, becoming 0.95D, and in the largest 180-degree section, it decreases to 75%, becoming 0.75D. Figure 4 shows an image of the cross-section of a 16 mm pipe in a 180-degree section that has been physically installed. The cross-section becomes elliptical, with a horizontal diameter of 12 mm. Although the vertical diameter is longer than 16 mm, the smallest diameter must be considered for the robot to pass through. Therefore, the robot must be designed to be smaller than 12 mm.  
Figure 4: Internal diameter changes in the bending section.  
Figure 3: Change in internal diameter according to the bending angle of the pipeline.  
Figure 2: Representative installation patterns of ondol pipeline.  
In the design of the robot, determining the size that can pass through the pipes is the most critical priority, which is a very important and fundamental requirement and has become the standard for determining the size of all components and the type of system applied to a robot. Incorrect determination of size can lead to serious trial and error. Therefore, a preliminary experiment was conducted to determine the size of the robot prior to its design. Figure 5a shows the dummy robot used for the test, while Figure 5b shows the piping used for the experiment. The piping with straight, 90-degree, and 180-degree sections was constructed in the same manner as in the actual installation, and dummy robots of various sizes were fabricated using 3D printing. A total of 65 dummies were designed with diameters ranging from 9 to 12 mm and lengths ranging from 26 to 34 mm, divided into approximately 0.1 mm increments. Starting with the smallest dummy, the dummies were repeatedly inserted into the pipe to determine the largest dummy size that could move smoothly. The test results determined that a robot with a diameter of 11.4 mm and a length of 32 mm or less was required.