# A Digital Twin System for Adaptive Aligning of Large Cylindrical Components
## 2 DT Aligning System for Cylindrical Components
### Aligning Process of Large Cylindrical Components  
As shown in Figure 3, the aligning process of the large cylindrical component involves seven sub-processes, i.e., initialization work, installation and clamping of large components, global coordinate system calibration, off-line simulation, rough aligning, finish aligning, and aligning quality evaluation. Each subsystem of the aligning system performs its function in each step of the aligning process and collaboratively realizes the adaptive aligning of large components. Figure 3 illustrates the operational logic of each subsystem in the process and their interrelationships with each other.  
Figure 2: Overall framework of the DT aligning system.  
* **Initialization work:** the aligning system is turned on and initialized, and the large components are ready to be initially installed and positioned.
* **Installation and clamping of large components:** the large components (i.e., the target component and the components to be docked) are respectively placed on the static and dynamic aligning platform and clamped. Meanwhile, the DT process system is opened, and thus, the initial work is completed.
* **Global coordinate system calibration:** the vision measurement system is used to measure the 3D coordinates of the key measurement points on the large component and the aligning tool system to construct the local coordinate system of each component (such as the local coordinate system of two large components) and then to construct the global coordinate system of the aligning system, providing a unified coordinate reference for the aligning of large components.
* **Off-line simulation:** in the DT process system, the aligning process is simulated based on the measured data, and thus, the error of the aligning trajectory is calculated, which is used to correct the aligning trajectory of the large component to be docked.
* **Rough aligning:** based on the off-line simulated and corrected aligning trajectory, the aligning motion control program is generated to perform the rough aligning process. During the rough aligning process execution, the process sensing system collects process information in real time and transmits it to the DT process system for sensing and monitoring the aligning status and quality of large components.
* **Finish aligning:** the difference between finish aligning process and rough aligning process lies in the axial feed rate of large components. In the finish aligning stage, the axial feed rate of large components is slower, which effectively protects them from collision and damage. Repeat the above process steps to complete the aligning task of large components.
* **Aligning quality evaluation:** after the aligning of large components is completed, the key process data are collected to evaluate the aligning quality, such as whether the axis of the large components is collinear and whether aligning stress is generated. When the aligning quality is satisfied, the aligning process is finished.  
Figure 3: Aligning process of large cylindrical components.