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 731 key components and dimensions. The gripper module consists of a total of 31 bodies and uses 14 joints to incorpo- rate the constraints needed to generate the required motion. In addition, 16 constraints are used to implement limitations on the motion. The bodies include two end caps that can be used to mount the flexible connecting cable between the modules, a lead screw, a lead nut, and a helical spring. Addi- tionally, there are nine links connecting the spring and the bottom end caps that support the gripper pad with a revolute joint. This joint provides the constraints so that the pads can contact the internal pipe surface, regardless of the orientation of the linkage. A micro gearmotor is used to rotate the lead screw and expand the linkages that mate the pads with the pipe surface. Similarly, the extender consists of 22 bodies and uses eight joints to generate the motion. However, only one motion constraint is required, which limits the module’s extension distance. Two micro gearmotors are used in the extender to provide the necessary torque on the lead screw that extends to the top of the module. The stroke length for each module is approximately 2.58 cm, resulting in 5.16 cm of displacement for each cycle of the crawler. Motion Analysis To generate peristaltic motion of the crawler, the gripper in the rear extends and fixes its position within the tube while the other modules are collapsed. With the rear gripper position fixed, the two extenders simultaneously extend, moving the front gripper forward 5.16 cm. The front gripper then extends, fixing its position within the tube, and the rear gripper collapses. The two extenders then retract, pulling the rear gripper forward 5.16 cm. The cycle generates 5.16 cm of motion for the crawler and is repeated continuously until the crawler reaches its destination. In addition, a simplified force analysis of a single gripper and extender in a static state was conducted. The gripper must be capable of generating a friction force greater than the drag force generated from the tether. If the friction force is lower than the drag force, the gripper will slip backward. Similarly, when the extender is retracting, it must be capable of overcoming the tether load or there would be no motion generated. A simulation of the gripper and extender was created using a motion simulation package. The analysis included extending one gripper that was resting on the bottom of a 5 cm tube. A motor drives the lead screw, which extends the gripper arms until the three pads reach the tube’s inner wall, as shown in Figure 4. The normal force produced from the simulation is also shown in Figure 4. A peak force of approximately 60 N was obtained for one gripper. The result also shows smaller peaks in the normal force as the linkage arms extend. These peaks were due to the weight of the gripper on the lower arm. The friction force generated by the gripper is dependent on the normal force and the coefficient of friction between the pad and tube wall. For this analysis, a coefficient of friction of 0.6 was used for the interface between the rubber pad and steel tube surface. This was experimentally determined in accor- dance with ASTM D1894 (2014) and is in agreement with standard values provided in the literature. This provides a maximum static friction force of 36 N per pad. With the three pads, this results in a simulated force of 108 N. Additional data from the simulation with the gripper shows the axial motion of the lead screw, the radial displace- ment of the arm linkage, and the radial displacement of the gripper pad (Figure 5). The simulation was conducted from the closed position to the extended position. As the nut is moved forward, the angle of the linkages changes and moves the gripper pad toward the inner pipe. A simulation was also conducted of the extender module. Similar to the gripper, a motor drives a lead screw, causing the lead nut to extend toward the module’s end cap. The maximum extension of the module is 2.58 cm. Both the front and back gripper modules will be connected to the extender modules via short flexible wires. 0 0 0.2 0.4 0.6 0.8 1 Time (s) 10 20 30 40 50 60 X Y Z X Y Z Figure 4. Motion analysis of the gripper: (a) gripper closed (b) gripper extended (c) plot of gripper pad contact force. (a) (b) (c) Gripper pad reaction force (N)

732 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 Inspection Modules The purpose of the crawler is to provide information regarding the structural integrity of key pipeline components in fossil energy power plants. Although cameras will be incor- porated into the front and rear modules, additional modules have been developed to house inspection sensors and are described in the following subsections. Ultrasonic Transducer Module One of the sensors that will be used to assess the integrity of the tubes is an ultrasonic transducer sensor. A module has been developed that will house and deploy the ultrasonic transducer sensor for tube thickness measurements. The module will be used to measure the thickness at any location within the tube. To move the sensor radially to the inner surface, a deployment mechanism was developed. The mech- anism utilizes two micromotors connected to a gearbox and a lead screw. The lead nut is attached to a housing for the ultra- sonic transducer sensor and translates along the lead screw, converting the rotary motion from the motors to a linear motion for the sensor. To aid in obtaining thickness measurements at different circumferential locations, a spur gear system was added. A stationary spur gear is mounted on the front end of the module and acts as the output shaft. The input shaft gear, connected to a micromotor, spins with the module. A set of bearings permits the rotation and reduces the friction between the moving parts. This mechanism provides a full 360° rotation of the modules and provides a means for the ultrasonic transducer sensor to measure the tube thickness at any radial location. A schematic of the module highlighting the major components is shown in Figure 6. To keep the module centered and maintain stability during the rotations, a stabilization system is included that ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes 0 0 2 4 6 0.2 0.4 0.6 Time (s) 0.8 1 1.2 0 0 2 4 6 0.2 0.4 0.6 Time (s) 0.8 1 1.2 0 100 120 140 160 0.2 0.4 0.6 Time (s) 0.8 1 1.2 l θ l Figure 5. Gripper component motion: (a) lead screw displacement (b) linkage rotation (c) gripper pad displacement. (a) (b) (c) Ultrasonic sensor Spur gear housing Linear actuator Stabilization system Figure 6. Ultrasonic transducer module schematic: (a) isometric view (b) side view. (a) (b) Displacement (mm) Displacement (mm) Rotation (degrees)