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. 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