July 2021 - Volume 79, Issue 7 Page 89 (89 of 116)

J U L Y 2 0 2 1 • M A T E R I A L S E V A L U A T I O N 737 Conclusion and Future Work A robotic crawler has been developed that can navigate through 5 cm diameter tubes similar to those found in fossil energy power plants. The base modules for navigation include two grippers and two extenders. The maximum pull force of the system is limited by the strength of the extenders, which is 40 N. Once the drag force of the tether reaches this value, an additional crawler would need to be inserted to assist in the load distribution. Additional modules have been developed that include an electronics module, an ultrasonic transducer sensor module, a surface preparation module, and an instru- mentation module. Testing of the crawler system demon- strates its ability to navigate through multiple straight sections and 180° bends. Initial testing of the modules demonstrates the system’s ability to inspect the integrity and conditions within 5 cm diameter tubes. A number of issues will continue to be investigated in efforts to improve the performance of the system. Since the distance the crawler can navigate is limited by its pull force capability, efforts will be made to improve the extenders and grippers and reduce the drag on the system. Design modifica- tions will include incorporation of the stabilization mecha- nism in the gripper and extender modules to reduce drag and improve the performance in elbows. Efforts will also continue to focus on improving the wire management by incorporating slip rings in the rotating modules. The individual microcontrollers in each module currently utilize serial protocols for communication between the modules. Additional protocols will be evaluated, including RS-485, CAN Bus, and SPI, which may provide more efficient and streamlined communication. This will also establish the controls for supplying power, streaming sensor data, and analog video feedback. Data from the camera and sensors will be collected and managed using a control box that will house a user interface for live video feedback. The software that will manage both the communication protocol and the interface is currently being developed. Future engineering-scale testing will be conducted with a fully developed communication system and integration of the sensor modules. Testing of the sensors in the instrumentation module was conducted with simple sensors that were commercially avail- able off the shelf. Future efforts will also include improving the accuracy and resolution of the sensors by investigating alternate sensors with improved performance. A high-fidelity model will also continue to be developed. Accurate models of each module will be integrated together and provide a platform for developing a virtual environment for evaluation of the system. This will allow for the assessment of module design changes, system additions to augment the crawler’s functionality, and control strategies to maximize the performance of the system. In addition, multiple crawlers will be integrated to maximize the distance the system can navigate. ACKNOWLEDGMENTS The authors would like to thank DOE-NETL for its financial support. This research effort was funded under Award No. DE-FE0031651. REFERENCES Abraham, G.J., V. Kain, and N. Kumar, 2018, “Cracking of Superheater Tube in a Captive Power Plant,” Corrosion Engineering, Science and Tech- nology, Vol. 53, pp. 98–104, https://doi.org/10.1080 /1478422X.2017.1386018 ASTM, 2014, ASTM D1894: Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, ASTM International, West Conshohocken, PA, https://doi.org/10.1520/D1894-14 Balaguer, C., A. Giménez, J.M. Pastor, V.M. Padrón, and M. Abderrahim, 2000, “Climbing Autonomous Robot for Inspection Applications in 3D Complex Environments,” Robotica, Vol. 18, No. 3, pp. 287–297, https://doi.org/10.1017/S0263574799002258 Boxerbaum, A.S., K.M. Shaw, H.J. Chiel, and R.D. Quinn, 2012, “Contin- uous Wave Peristaltic Motion in a Robot,” The International Journal of Robotics Research, Vol. 31, No. 3, pp. 302–318, https://doi.org/10.1177/0278364911432486 Debenest, P., M. Guarnieri, and S. Hirose, 2014, “PipeTron Series – Robots for Pipe Inspection,” Proceedings of the 2014 3rd International Conference on Applied Robotics for the Power Industry, pp. 1–6, https://doi.org/10.1109/CARPI. 2014.7030052 Dehnavi, F., A. Eslami, and F. Ashrafizadeh, 2017, “A Case Study on Failure of Superheater Tubes in an Industrial Power Plant,” Engineering Failure Analysis, Vol. 80, pp. 368–377, https://doi.org/10.1016 /j.engfailanal.2017.07.007 Gargade, A.A., and S.S. Ohol, 2016, “Development of In-pipe Inspection Robot,” IOSR Journal of Mechanical and Civil Engineering, Vol. 13, No. 4, pp. 64–72, https://doi.org/10.9790/1684-1304076472 Hadi, A., A. Hassani, K. Alipour, R. Askari Moghadam, and P. Pourakbarian Niaz, 2020, “Developing an Adaptable Pipe Inspection Robot Using Shape Memory Alloy Actuators,” Journal of Intelligent Material Systems and Structures, Vol. 31, No. 4, pp. 632–647, https://doi.org/10.1177 /1045389X19898255 Ito, F., T. Kawaguchi, Y. Yamada, and T. Nakamura, 2019, “Development of a Peristaltic-Movement Duct-Cleaning Robot for Application to Actual Environment – Examination of Brush Type and Installation Method to Improve Cleaning Efficiency,” Journal of Robotics and Mechatronics, Vol. 31, No. 6, pp. 781–793, https://doi.org/10.20965/jrm.2019.p0781 Kapayeva, S.D., M.J. Bergander, A. Vakhguelt, and S.I. Khairaliyev, 2017, “Remaining Life Assessment for Boiler Tubes Affected by Combined Effect of Wall Thinning and Overheating,” Journal of Vibroengineering, Vol. 19, No. 8, pp. 5892–5907, https://doi.org/10.21595/jve.2017.18219 Kishi, T., M. Ikeuchi, and T. Nakamura, 2015, “Development of a Peri- staltic Crawling Inspection Robot for Half-Inch Pipes Using Pneumatic Artificial Muscles,” SICE Journal of Control, Measurement, and System Inte- gration, Vol. 8, No. 4, pp. 256–264, https://doi.org/10.9746 /jcmsi.8.256 La Rosa, G., M. Messina, G. Muscato, and R. Sinatra, 2002, “A Low-Cost Lightweight Climbing Robot for the Inspection of Vertical Surfaces,” Mechatronics, Vol. 12, No. 1, pp. 71–96, https://doi.org/10.1016 /S0957-4158(00)00046-5 Longo, D., and G. Muscato, 2004, “A Modular Approach for the Design of the Alicia3 Climbing Robot for Industrial Inspection,” Industrial Robot, Vol. 31, No. 2, pp. 148–158, https://doi.org/10.1108 /01439910410522838 Nagase, J.-Y., F. Fukunaga, and Y. Shigemoto, 2016, “Cylindrical Elastic Crawler Mechanism for Pipe Inspection,” Advances in Cooperative Robotics, pp. 304–311, https://doi.org/10.1142/9789813149137_0037 Nagase, J., K. Suzumori, and N. Saga, 2013, “Development of Worm-Rack Driven Cylindrical Crawler Unit,” Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol. 7, No. 3, pp. 422–431, https://doi.org/10.1299/jamdsm.7.422

738 M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 1 Nayak, A., and S.K. Pradhan, 2014, “Design of a New In-Pipe Inspection Robot,” Procedia Engineering, Vol. 97, pp. 2081–2091, https://doi.org/10.1016/j.proeng.2014.12.451 Omori, H., T. Nakamura, and T. Yada, 2009, “An Underground Explorer Robot based on Peristaltic Crawling of Earthworms,” Industrial Robot, Vol. 36, No. 4, pp. 358–364, https://doi.org/10.1108 /01439910910957129 Park, S., H.D. Jeong, and Z.S. Lim, 2002, “Development of Mobile Robot Systems for Automatic Diagnosis of Boiler Tubes in Fossil Power Plants and Large Size Pipelines,” IEEE/RSJ International Conference on Intelligent Robots and Systems, Vol. 2, pp. 1880–1885, https://doi.org/10.1109 /IRDS.2002.1044030 Qiao, J., J. Shang, and A. Goldenberg, 2013, “Development of Inchworm In-Pipe Robot based on Self-Locking Mechanism,” IEEE/ASME Transac- tions on Mechatronics, Vol. 18, No. 2, pp. 799–806, https://doi.org/10.1109 /TMECH.2012.2184294 Shang, J., B. Bridge, T. Sattar, S. Mondal, and A. Brenner, 2008, “Development of a Climbing Robot for Inspection of Long Weld Lines,” Industrial Robot, Vol. 35, No. 3, pp. 217–223, https://doi.org/10.1108/01439910810868534 Tavakoli, M., C. Viegas, L. Marques, J.N. Pires, and A.T. de Almeida, 2013, “OmniClimbers: Omni-Directional Magnetic Wheeled Climbing Robots for Inspection of Ferromagnetic Structures,” Robotics and Autonomous Systems, Vol. 61, No. 9, pp. 997–1007, https://doi.org/10.1016 /j.robot.2013.05.005 Tesen, S., N. Saga, T. Satoh, and J. Nagase, 2013, “Peristaltic Crawling Robot for Use on the Ground and in Plumbing Pipes,” eds. V. Padois, P. Bidaud, O. Khatib, Romansy 19 – Robot Design, Dynamics and Control, CISM International Centre for Mechanical Sciences, Vol. 544, pp. 267–274, https://doi.org/10.1007/978-3-7091-1379-0_33 Yeo, H.-J., 2012, “Development of a Robot System for Repairing an Under- ground Pipe,” Journal of the Korea Academia-Industrial Cooperation Society, Vol. 13, No. 3, pp. 1270–1274 [in Korean], https://doi.org/10.5762/KAIS .2012.13.3.1270 ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes

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736 M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 1 the measurements were consistent between 2.5 and 2.6 mm. As noted previously, the flat sensor head does not mate perfectly with the internal tube surface due its curvature. Thus, an offset must be subtracted from the measurement to obtain a more accurate reading. Instrumentation module: To evaluate the LiDAR sensor in the instrumentation module, a template ring was used to simulate a 5 cm diameter surface with a variety of irregularities. During the testing, the module is positioned at the center of the template frame and rotates to scan the surrounding irregularities on the ring. Figure 11 shows the template ring and the instrumentation module positioned at the center. Preliminary results demonstrate the potential for the detection of anomalies in tubes and pipes using a LiDAR sensor. Data from the environmental sensor is also shown and includes pressure (p), altitude (a), and temperature (t). It should be noted that the camera was not installed during this testing. Engineering-Scale Testing To evaluate the crawler’s performance in a larger-scale testbed, a mock-up similar to the superheater tubes found in fossil energy power plants was constructed. The testbed was manufactured with acrylic plastic tubes so the crawler could be observed while navigating through the straight sections and 180° elbows. As shown in Figure 12, the crawler was able to navigate through multiple straight pipe sections and bends and was limited only by the length of the tether. An additional testbed is being constructed with metal tubes and contains sharper bends, as well as 90° elbows. This setup will not allow for visualization of the crawler but will provide a more realistic testbed to evaluate its pull force capability. ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes 270 240 210 300 330 30 60 90 120 150 180 0 Radius (mm) Figure 11. Instrumentation module testing: (a) module placed in template ring (b) template ring with anomalies (c) distance measurements obtained in template ring. (a) (b) (c) Figure 12. Crawler navigating the superheater tube mock-up with magnified images.

J U L Y 2 0 2 1 • M A T E R I A L S E V A L U A T I O N 737 Conclusion and Future Work A robotic crawler has been developed that can navigate through 5 cm diameter tubes similar to those found in fossil energy power plants. The base modules for navigation include two grippers and two extenders. The maximum pull force of the system is limited by the strength of the extenders, which is 40 N. Once the drag force of the tether reaches this value, an additional crawler would need to be inserted to assist in the load distribution. Additional modules have been developed that include an electronics module, an ultrasonic transducer sensor module, a surface preparation module, and an instru- mentation module. Testing of the crawler system demon- strates its ability to navigate through multiple straight sections and 180° bends. Initial testing of the modules demonstrates the system’s ability to inspect the integrity and conditions within 5 cm diameter tubes. A number of issues will continue to be investigated in efforts to improve the performance of the system. Since the distance the crawler can navigate is limited by its pull force capability, efforts will be made to improve the extenders and grippers and reduce the drag on the system. Design modifica- tions will include incorporation of the stabilization mecha- nism in the gripper and extender modules to reduce drag and improve the performance in elbows. Efforts will also continue to focus on improving the wire management by incorporating slip rings in the rotating modules. The individual microcontrollers in each module currently utilize serial protocols for communication between the modules. Additional protocols will be evaluated, including RS-485, CAN Bus, and SPI, which may provide more efficient and streamlined communication. This will also establish the controls for supplying power, streaming sensor data, and analog video feedback. Data from the camera and sensors will be collected and managed using a control box that will house a user interface for live video feedback. The software that will manage both the communication protocol and the interface is currently being developed. Future engineering-scale testing will be conducted with a fully developed communication system and integration of the sensor modules. Testing of the sensors in the instrumentation module was conducted with simple sensors that were commercially avail- able off the shelf. Future efforts will also include improving the accuracy and resolution of the sensors by investigating alternate sensors with improved performance. A high-fidelity model will also continue to be developed. Accurate models of each module will be integrated together and provide a platform for developing a virtual environment for evaluation of the system. This will allow for the assessment of module design changes, system additions to augment the crawler’s functionality, and control strategies to maximize the performance of the system. In addition, multiple crawlers will be integrated to maximize the distance the system can navigate. ACKNOWLEDGMENTS The authors would like to thank DOE-NETL for its financial support. This research effort was funded under Award No. DE-FE0031651. REFERENCES Abraham, G.J., V. Kain, and N. Kumar, 2018, “Cracking of Superheater Tube in a Captive Power Plant,” Corrosion Engineering, Science and Tech- nology, Vol. 53, pp. 98–104, https://doi.org/10.1080 /1478422X.2017.1386018 ASTM, 2014, ASTM D1894: Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, ASTM International, West Conshohocken, PA, https://doi.org/10.1520/D1894-14 Balaguer, C., A. Giménez, J.M. Pastor, V.M. Padrón, and M. Abderrahim, 2000, “Climbing Autonomous Robot for Inspection Applications in 3D Complex Environments,” Robotica, Vol. 18, No. 3, pp. 287–297, https://doi.org/10.1017/S0263574799002258 Boxerbaum, A.S., K.M. Shaw, H.J. Chiel, and R.D. Quinn, 2012, “Contin- uous Wave Peristaltic Motion in a Robot,” The International Journal of Robotics Research, Vol. 31, No. 3, pp. 302–318, https://doi.org/10.1177/0278364911432486 Debenest, P., M. Guarnieri, and S. Hirose, 2014, “PipeTron Series – Robots for Pipe Inspection,” Proceedings of the 2014 3rd International Conference on Applied Robotics for the Power Industry, pp. 1–6, https://doi.org/10.1109/CARPI. 2014.7030052 Dehnavi, F., A. Eslami, and F. Ashrafizadeh, 2017, “A Case Study on Failure of Superheater Tubes in an Industrial Power Plant,” Engineering Failure Analysis, Vol. 80, pp. 368–377, https://doi.org/10.1016 /j.engfailanal.2017.07.007 Gargade, A.A., and S.S. Ohol, 2016, “Development of In-pipe Inspection Robot,” IOSR Journal of Mechanical and Civil Engineering, Vol. 13, No. 4, pp. 64–72, https://doi.org/10.9790/1684-1304076472 Hadi, A., A. Hassani, K. Alipour, R. Askari Moghadam, and P. Pourakbarian Niaz, 2020, “Developing an Adaptable Pipe Inspection Robot Using Shape Memory Alloy Actuators,” Journal of Intelligent Material Systems and Structures, Vol. 31, No. 4, pp. 632–647, https://doi.org/10.1177 /1045389X19898255 Ito, F., T. Kawaguchi, Y. Yamada, and T. Nakamura, 2019, “Development of a Peristaltic-Movement Duct-Cleaning Robot for Application to Actual Environment – Examination of Brush Type and Installation Method to Improve Cleaning Efficiency,” Journal of Robotics and Mechatronics, Vol. 31, No. 6, pp. 781–793, https://doi.org/10.20965/jrm.2019.p0781 Kapayeva, S.D., M.J. Bergander, A. Vakhguelt, and S.I. Khairaliyev, 2017, “Remaining Life Assessment for Boiler Tubes Affected by Combined Effect of Wall Thinning and Overheating,” Journal of Vibroengineering, Vol. 19, No. 8, pp. 5892–5907, https://doi.org/10.21595/jve.2017.18219 Kishi, T., M. Ikeuchi, and T. Nakamura, 2015, “Development of a Peri- staltic Crawling Inspection Robot for Half-Inch Pipes Using Pneumatic Artificial Muscles,” SICE Journal of Control, Measurement, and System Inte- gration, Vol. 8, No. 4, pp. 256–264, https://doi.org/10.9746 /jcmsi.8.256 La Rosa, G., M. Messina, G. Muscato, and R. Sinatra, 2002, “A Low-Cost Lightweight Climbing Robot for the Inspection of Vertical Surfaces,” Mechatronics, Vol. 12, No. 1, pp. 71–96, https://doi.org/10.1016 /S0957-4158(00)00046-5 Longo, D., and G. Muscato, 2004, “A Modular Approach for the Design of the Alicia3 Climbing Robot for Industrial Inspection,” Industrial Robot, Vol. 31, No. 2, pp. 148–158, https://doi.org/10.1108 /01439910410522838 Nagase, J.-Y., F. Fukunaga, and Y. Shigemoto, 2016, “Cylindrical Elastic Crawler Mechanism for Pipe Inspection,” Advances in Cooperative Robotics, pp. 304–311, https://doi.org/10.1142/9789813149137_0037 Nagase, J., K. Suzumori, and N. Saga, 2013, “Development of Worm-Rack Driven Cylindrical Crawler Unit,” Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol. 7, No. 3, pp. 422–431, https://doi.org/10.1299/jamdsm.7.422

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J U L Y 2 0 2 1 • M A T E R I A L S E V A L U A T I O N 739 ME TECHNICAL PAPER w A B S T R A C T The aim of this paper is to develop the concept and a prototype of an intelligent mobile robotic platform that is integrated with nondestructive evaluation (NDE) capabilities for autonomous live inspection and repair. In many industrial environ- ments, such as the application of power plant boiler inspection, human inspectors often have to perform hazardous and challenging tasks. There is a significant chance of injury, considering the confined spaces and limited visibility of the inspec- tion environment and hazards such as pressuriza- tion and improper water levels. In order to provide a solution to eliminate these dangers, the concept of a new robotic system was developed and proto- typed that is capable of autonomously sweeping the region to be inspected. The robot design contains systematic integration of components from robotics, NDE, and artificial intelligence (AI). A magnetic track system is used to navigate over the vertical steel structures required for examination. While moving across the inspection area, the robot uses an NDE sensor to acquire data for inspection and repair. This paper presents a design of a portable NDE scanning system based on eddy current array probes, which can be customized and installed on various mobile robot platforms. Machine learning methods are applied for semantic segmentation that will simultaneously localize and recognize defects without the need of human intervention. Experiments were conducted that show the NDE and repair capabilities of the system. Improvements in human safety and struc- tural damage prevention, as well as lowering the overall costs of maintenance, are possible through the implementation of this robotic NDE system. KEYWORDS: AI-enabled robotics, structural damage assessment, eddy current Introduction Modern advances in robotics, especially artificial intelligence (AI) based on neural networks, and nondestructive evaluation (NDE) allow for automated and reliable maintenance of components, structures, and systems subject to surface and subsurface anomalies (Gibb et al. 2018 Ullmann et al. 2020 Ali and Cha 2019 Zhu et al. 2019 Caccamo et al. 2017 Bogue 2010 Meyendorf et al. 2017). The integration of these technologies is favorable for designing robotic systems that may conduct difficult evaluation tasks that may otherwise be hazardous for human inspectors. Such is the case for the examination of power plant boilers and heat exchangers. Power plants are used as a general means of providing energy to consumers on a worldwide scale (Byers et al. 2018). However, boilers and heat exchangers are susceptible to several types of damages as a result of thermal cycling, such as corrosion, fracturing, and aging (Natesan 2002). If these damages are left unchecked, catastrophic failures may occur, which can cause serious problems such as property destruc- tion, hefty repair costs, and loss of life (Musyafa and Adiyagsa 2012). Current available examination methods to prevent these catastrophes are done through human inspection. This method of power plant health analysis is a suboptimal AI-Enabled Robotic NDE for Structural Damage Assessment and Repair by Xiaodong Shi*, Anthony Olvera†, Ciaron Hamilton*, Erzhuo Gao†, Jiaoyang Li*, Lucas Utke†, Andrew Petruska†, Zhenzhen Yu†, Lalita Udpa*, Yiming Deng‡, and Hao Zhang§ Materials Evaluation 79 (7): 739–751 https://doi.org/10.32548/2021.me-04214 ©2021 American Society for Nondestructive Testing * Nondestructive Evaluation Laboratory, Michigan State University, East Lansing, MI 48824, USA † Colorado School of Mines, Golden, CO 80401, USA ‡ Nondestructive Evaluation Laboratory, Michigan State University, East Lansing, MI 48824, USA dengyimi@egr.msu.edu § Colorado School of Mines, Golden, CO 80401, USA hzhang@mines.edu

740 M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 1 solution, as it is a risky, expensive, and time-consuming endeavor due to several hazardous environmental conditions that may cause injury or death to inspectors (Lian et al. 2018 Ali and Habibullah 2019). Lack of training and experience for inspectors cause improper inspection results, which may cause equipment damage and injuries due to operation errors. This is amplified by environmental conditions such as opera- tion within confined workspaces, limited visibility, and inade- quate construction or maintenance of structures under test. Improper inspection results may cause damage to structures and equipment as well. Other fatal hazards from boiler inspec- tion include over pressurization, improper water treatment, flameouts, temperature conditions, and furnace explosions. State-of-the-art NDE methods and systems used for detecting discontinuities inside of power plant boilers and heat exchangers have been successfully implemented to minimize human-related risks for required inspections, while both automating the inspection process and decreasing diag- nosis errors and data uncertainties. This in turn reduces the risk of failure of power plant system components as well as lowers the cost of maintenance. Several NDE methods and techniques have been developed for this inspection environ- ment that include but are not limited to ultrasonic testing (Mandeliya and Vishwakarma 2018 Zhang et al. 2020), radi- ographic testing (Baba et al. 2009 Zhang et al. 2020), microwave testing (Korhonen and Ahola 2018 Zhang et al. 2020), and eddy current testing (Zhu et al. 2019 Liu et al. 2020 Yu et al. 2021). The eddy current testing technique was selected for AI-enabled robot integration due to providing a low-cost solution that does not require physically large or hefty measurement equipment to be onboard the robotic device. It is also possible to place eddy current coils in an array configuration to scale the number of data points, increasing resolution within the timeframe of a scan. For the robot system design, configuration depends entirely on appli- cation. Robotic development using NDE techniques including eddy current have been also proposed for similar operations for both mobile platforms and robotic arms. A bespoke configuration with NDE equipment was used for mapping the thickness of water pipelines made of metal, allowing for near-constant liftoffs against pipe walls (Miro et al. 2018). A wall-climbing robot platform was developed for thickness measurements of circulating fluidized bed boilers (Meng et al. 2018). In laboratory or inspection workspace settings, a static robot arm that may reach around a sample under test provides consistent measurement settings in terms of NDE probe placement. However, robot arms placed in static positions are limited without workspace boundaries hence, mobile platforms would be more applicable for larger samples. The size of the robot matters, considering if the robot is placed within a sample such as a pipe. If the robot attaches to the surface (Mattar and Kalai 2018 Kharkovsky and Zoughi 2007) or requires flight (Mattar and Kalai 2018), then the system should be lightweight or have strong adhesion to make up for heavier equipment. For robot arms, end-effector weight is a consideration as well. Finally, the system should be long-lasting. If the system is required to handle severe conditions on a regular basis, then it should be built to handle these conditions. There is a need for a mobile robot platform that may climb on power plant boilers and heat exchangers for inspec- tion and repair. To provide for this need, a novel robotic NDE system is proposed in this paper that allows for autonomous evaluation and repair within hazardous spaces inside of power plants. The autonomous system will use cyber-physical system (CPS) implementation to sweep across a steel plate while analyzing and repairing potential damages. A magnetic track system is implemented to allow the robot to adhere to the surface of the boiler and move using a tracked wheel platform. Such systems have been used for steel-based surfaces in the past (Kharkhovsky and Zoughi 2007 Kumar et al. 2018). To help with navigation, multisensory inertial odometry methods are implemented to localize the robot within the workspace environment. The robot carries both the NDE sensor and the tooling for live inspection and repair. The NDE sensor will be used in conjunction with machine learning capabilities to autonomously classify and analyze damage to the boiler. Finally, the robot will carry, as its payload, the required tooling necessary for live repair of the damage. This will consist of a wire cleaning brush as well as a friction stir welder to repair cracks as they are located. Mobile Robotic System for Damage Analysis and Repair Figure 1 shows a flowchart depicting the functionality of the robotic software subsystem for the proposed CPS, which is called the AI-enabled autonomy stack. The entire subsystem runs in a loop as soon as the robot is powered on, starting the AI system. The robot will then begin the exploration of the boiler until it has swept the entire workspace, collected and stored all necessary data, and made all needed repairs. When finished, the program will terminate and notify the operator that the task is complete and the post-repair evaluation can be conducted. A robotic system for power plant boiler and heat exchanger inspection must be designed to match the chal- lenges within the examination environment. Figure 2 depicts the components of the system designed by the authors. The upper left-hand corner of Figure 2 shows an earlier prototype of the design with a preliminary NDE sensor and no repair capability. The design consists of a magnetic track system to adhere the robot chassis to the walls of the boiler. This gives enough magnetic force to keep the robot and its payload attached to the boiler wall this capability is illustrated in Figure 2. The NDE sensor is mounted to the underside of the robot’s scanning gantry where it is pressed against and swept along the steel surface by a spring-loaded mechanism. By keeping the sensor close to the surface during operation, vari- ations in liftoff are minimized, which in turn increases NDE ME TECHNICAL PAPER w ai-enabled robotic nde for structural damage

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J U L Y 2 0 2 1 • M A T E R I A L S E V A L U A T I O N 735 Testing The development of the crawler system started with creating general concepts and initial prototyping and proceeded with bench-scale testing and engineering-scale testing of the system. This section presents the bench-scale testing used to validate the concepts and testing of the system in a tube with multiple bends and straight sections. Bench-Scale Testing Robotic crawler: To evaluate the pull force capability of the crawler, pull-force tests were conducted on the gripper and extender modules. The grippers were found to be capable of pulling approximately 84.5 N of force and the extenders were found to generate 40 N of force. These values demonstrate that the forces obtained previously in this paper are reason- able approximations of the actual pull forces. The pull-force tests were conducted using a digital weight scale attached to the ends of the modules. The value for the gripper was found by finding the maximum pull force before the gripper pads began to slip along a steel 5 cm diameter tube. The pull force for the extender was found by clamping the module to a flat surface and allowing the linear actuator to pull the scale. After the initial system was assembled, bench-scale tests were conducted to evaluate the crawler’s ability to navigate through both straight and 180° elbow sections. The elbow testing was conducted in a custom-built 5 cm diameter tube with a 7 cm bend radius. Although the system was designed to navigate through bends with a 5 cm radius of curvature, the smallest bend radius found in a transparent acrylic pipe was 7 cm. With this experimental setup, the crawler was easily able to navigate through the bend. The average speed of the crawler in straight sections was found to be 50 cm/min. The speed was slightly slower when navigating through the bends. Ultrasonic transducer module: To demonstrate the functionality of the ultrasonic transducer module, tests were performed in a clear acrylic 5 cm diameter straight tube with a wall thickness of 1.6 mm. The ultrasonic transducer gauge was calibrated to measure the thickness of PVC by adjusting the velocity of sound to 2390 m/s. Since the module was not inte- grated with the crawler for the bench-scale testing, the stabi- lization mechanism was adapted to be used at both ends of the module. Wall thickness was measured at three different locations around the inner circumference of the tube. The circumferential rotation using the spur gear set allowed the module to obtain measurements at different loca- tions along the inner wall of the tube. As shown in Figure 10, TABLE 1 Sensor specifications Sensor Measurement Range Resolution Unit Environmental • Temperature –40 to 85 ±1.0 °C • Pressure 26 to 126 ±0.02 kPa Inertial measurement • Acceleration ±2 to ±16 ±0.004 g • Angular velocity ±125 to ±2000 ±10.0 °/s Camera • Surface imaging 640 × 480 VGA pixel LiDAR • Circumferential mapping 10 to 60 ±1.0 mm Figure 10. Measurements performed on the tube’s inner surface showing the rotation of the crawler.

736 M A T E R I A L S E V A L U A T I O N • J U L Y 2 0 2 1 the measurements were consistent between 2.5 and 2.6 mm. As noted previously, the flat sensor head does not mate perfectly with the internal tube surface due its curvature. Thus, an offset must be subtracted from the measurement to obtain a more accurate reading. Instrumentation module: To evaluate the LiDAR sensor in the instrumentation module, a template ring was used to simulate a 5 cm diameter surface with a variety of irregularities. During the testing, the module is positioned at the center of the template frame and rotates to scan the surrounding irregularities on the ring. Figure 11 shows the template ring and the instrumentation module positioned at the center. Preliminary results demonstrate the potential for the detection of anomalies in tubes and pipes using a LiDAR sensor. Data from the environmental sensor is also shown and includes pressure (p), altitude (a), and temperature (t). It should be noted that the camera was not installed during this testing. Engineering-Scale Testing To evaluate the crawler’s performance in a larger-scale testbed, a mock-up similar to the superheater tubes found in fossil energy power plants was constructed. The testbed was manufactured with acrylic plastic tubes so the crawler could be observed while navigating through the straight sections and 180° elbows. As shown in Figure 12, the crawler was able to navigate through multiple straight pipe sections and bends and was limited only by the length of the tether. An additional testbed is being constructed with metal tubes and contains sharper bends, as well as 90° elbows. This setup will not allow for visualization of the crawler but will provide a more realistic testbed to evaluate its pull force capability. ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes 270 240 210 300 330 30 60 90 120 150 180 0 Radius (mm) Figure 11. Instrumentation module testing: (a) module placed in template ring (b) template ring with anomalies (c) distance measurements obtained in template ring. (a) (b) (c) Figure 12. Crawler navigating the superheater tube mock-up with magnified images.

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