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 733 utilizes three lever arms for support. The three arms have linkages with a pair of springs attached to each link. The springs provide a constant opposing force to a set of wheels mounted on the end of the arm. This spring force offsets gravity during the rotation of the module and establishes continuous surface contact for each of the wheels. The stabi- lization system allows for accurate positioning of the sensor while obtaining wall thickness measurements of the tube. During the evaluation of different ultrasonic transducer sensors, it was clear that even the smallest sensor head of 7 mm used in this study did not make significant contact with the curved internal tube surface. The small gap between the two surfaces, as shown in Figure 7a, led to an offset in the thickness measurements of the 5 cm diameter tube. To evaluate the consistency of the thickness offset, a stepped steel tube section was machined to create gradually varying thick- nesses along the length of the tube. The wall thickness decreased 0.198 mm at each step. Figure 7b shows the machined tube and the thickness steps created. Thickness measurements were taken from both the inside and outside surfaces of the machined tube. Since the probe had more surface contact on the outside surface, these measurements were found to represent the actual thickness. For each thick- ness step, 20 measurements were obtained and averaged. The results, shown in Figure 7c, were plotted with the blue line representing the measurements from the inside surface and the red line representing the measurements from the outside surface. Several factors can affect the accuracy of measurements using an ultrasonic transducer sensor: the angle of incidence, the couplant, diameter of the sensor head, and the curvature of the pipe. Results from this analysis show that although the measurements taken from the inside surfaces were off, the offset from the true thickness was fairly consistent and could be used to obtain reasonably accurate measurements. Surface Preparation Module The ultrasonic sensor used in the ultrasonic transducer module requires a liquid couplant to obtain measurements. The gap between the sensor tip and the surface should be filled with a fluid to permit the ultrasonic signals to travel to and from the wall surface. Water can be used as the couplant however, denser liquids allow for a higher refraction of the signals. Thus, a gel couplant offers a better alternative and was utilized in this system. Couplant-free ultrasonic sensors were considered for this study however, they can require large application forces to compensate for potential gaps. Due to space limitations within the crawler, these requirements are difficult to achieve with the small electric motors currently available. In addition to the surface gap, moisture and elevated temperatures in superheater tubes increases levels of corro- sion and creep in the tubes. To address these challenges, a separate module was developed for application of a gel couplant and surface-cleaning brush as shown in Figure 8. The design of the module includes four motors. The motors rotate and extend a surface brush, drive a pump to apply a couplant, and rotate the module. To apply the gel couplant needed for the ultrasonic transducer sensor, a 0 2 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 4 6 Pipe step 8 10 12 Inside measurements Outside measurements Figure 7. Thickness measurements obtained from the inside and outside surfaces of a tube with varying thickness: (a) measurement from inside the pipe (b) cut pipe used for measurements (c) thickness measurements from inside and outside of the pipe. (a) (b) (c) Wall thickness (mm)

734 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 peristaltic pump is used to transfer the gel from a reservoir to the tube’s inner surface. A chamber at the front of the module houses a reservoir containing the gel and the pump. Due to the available space in the module, the pump selected was based on its size, flow rate, pressure, and compatibility with the liquid. The surface-cleaning brush is attached to a motor mounted perpendicular to the module. Radial extension of the brush and its motor is accomplished using a rack and pinion gear. This allows the components that make up the brush to fit within the diameter of the module. The brush can then be positioned on the tube surface for cleaning when needed. Since the ultrasonic transducer sensor will need to obtain measurements around the circumference of the tube, this module also contains a spur gear system for rotation. This will allow the module to rotate and prepare the internal surface for measurements using the brush and couplant. Testing of the module was conducted in a clear 5 cm diameter tube to evaluate surface-brush contact and application of the gel couplant. Instrumentation Module To improve the functionality of the crawler, an instrumenta- tion module was developed that contains a variety of sensors to evaluate the conditions within the tube. Similar to the previous modules, this module uses a carousel-type motion with a spur gear system (Figure 9). Six modular panels spin 360° concentrically about the center of the tube. Each panel design can be modified to support a different sensor. The module currently includes three sensors for assessing the tube conditions, although there are three more panels available for additional sensors in the future. The three current sensors include an analog video camera, an environmental sensor for temperature and pressure measurements, and a LiDAR sensor. The LiDAR sensor can provide information on potential surface anomalies and defects. An inertial meas- urement unit is also included in the module and provides the angular position and acceleration of the crawler. An embedded microcontroller manages the communication between the sensors and the electronics module discussed previously. Table 1 shows the specifications of the sensors currently installed in the module. ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes Spur gear housing Surface brush and linear actuator Miniature peristaltic pump Couplant reservoir area Stabilization system Figure 8. Surface preparation module: (a) isometric view (b) side view. (a) (b) Sensors Spur gear housing Modular panels Camera Stabilization system Figure 9. Schematic of the instrumentation module: (a) isometric view (b) side view. (a) (b)