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 723 To achieve an optimal gripper design it is of interest to use similar coils and magnets, especially in terms of geometry and dimensions. Therefore, tests were conducted using a cylin- drical magnet and the coil described in Figure 5. As shown in Figure 8b, the tests were performed with a pitch-catch config- uration (separate but identical transmitter and receiver) to mimic communication between the eight EMATs in the LTI robot. In these tests, no additional normal force was applied to the coils and the magnets were in full contact with the surface to avoid liftoff between the EMAT transducer and substrate. Figure 9 shows the signals received using a cylindrical magnet in the nonconformed and conformed configurations. The signal was recorded when the two EMATs were located 100 mm away from each other. In all cases, the arrival time of the selected Lamb mode could be identified using the group velocity determined from the dispersion curves. For example, the plate signal (Figure 9a) could confidently be identified as the S0 Lamb wave mode due to its correct arrival time at 52 μs. This figure also reveals that the nonconformed coil resulted in slightly higher amplitudes compared to the conformed configuration in all pipe samples. Figure 10a compares the two configuration responses with respect to the curvature of each tested pipe. On an average, the amplitude of the received signal decreased by 9.2% when the coils conformed to the surface. Therefore, it may be stated that the conformed configuration did not improve signal amplitude when compared to the nonconformed configuration under similar test conditions. In the next part, this result is consid- ered in the gripper design for the LTI robot. The time trace in Figure 9 reveals that the waves have different propagation velocities in each specimen. The group velocity in each case can be determined using the arrival time of the received signal from the generated wave pulse (the time of arrival). Figure 10b shows the effects of curvature on the wave propagation velocity. It can be seen in the figure that group velocities in the conformed and nonconformed config- urations were almost consistent, which justifies that the coil conformation does not affect wave velocity. However, sample geometry significantly influences group velocity as shown in Figure 10b, there is an inverse relationship between specimen curvature and group velocity. Based on these tests and the complicated design requirements to embed and conform EMAT coils to the tube surface in the LTI robot, the second configuration was selected (nonconforming case) for tube inspection. 0 0 2 4 –2 –4 20 40 60 Time (μs) 80 100 120 0 0 2 4 –2 –4 20 40 60 Time (μs) 80 100 120 Nonconformed coil Conformed coil 0 0 2 4 –2 –4 20 40 60 Time (μs) 80 100 120 Nonconformed coil Conformed coil 0 0 2 4 –2 –4 20 40 60 Time (μs) 80 100 120 Nonconformed coil Conformed coil Figure 9. Lamb wave signals with spiral coils in nonconformed and conformed configurations: (a) single signal in plate (b) 73.025 mm OD pipe (c) 88.9 mm OD pipe and (d) 114.3 mm OD pipe. (a) (b) (c) (d) 16 1.5 2 2.5 3 3.5 4 4.5 18 20 22 κ (1/m) 24 26 28 15 3800 4000 4200 4400 4600 4800 5000 5200 5400 20 κ (1/m) 25 30 Nonconformed Conformed Nonconformed Conformed Figure 10. Effect of curvature on the wave propagation in materials with different diameters curvature versus: (a) maximum amplitude (b) group velocity. (a) (b) Amplitude (V) ×104 Amplitude (V) ×104 Amplitude (V) ×104 Amplitude (V) ×104 Maximum amplitude (V) ×104 Group velocity (m/s)

724 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 Pipe Inspection by Gripper with EMATs The dynamic movement of the gripper and its electrical components may affect the performance of EMATs. To analyze this phenomenon, in this study the modular gripper integrated with EMATs was used for testing pipes. Two sets of tests were carried out on both defect-free pipes and pipes with artificial defects in a semiautomatic manner. As discussed previously in this paper, the designed gripper was attached to a robotic arm, which can be programmed to move a tool and communicate with other machines. A full laboratory setup for pitch-catch experiments using the robotic arm is shown in Figure 11. The movements of the gripper were controlled using the robot manipulator touchscreen control panel. Once initial tests were conducted on the robot and after setting up the data acquisition system, a code was created to allow the robot to move in a more autonomous way between two points to better mimic the movements of the LTI robot in action. This could be accomplished using the pick-and- place option in the robot manipulator software. This coding simplified the process because the only manual action required was to move the robot to the desired place using the robotic arm touchscreen and lock it there. Therefore, the first position was above the pipe for clearance of the gripper subsequently, the manipulator moved down, allowing the gripper to close and wait for a desired amount of time to conduct tests before it moved to a second location. Figure 11 shows one gripper grabbing the intact pipe’s surface while the EMATs on adjacent fingers communicate with each other. In fact, the robot system moved in a longitudinal direction. At the same time, EMATs inspected the defect-free pipe by transmitting and receiving Lamb waves. The grabbing process was designed with an open-loop feedback control, in which the gripper motors were controlled by a simple user-interface which would send high-level commands to actuate. An instruction packet containing the operating speed required for closing the fingers was sent to the pipe. Based on signal characteristics (shape and ampli- tude), the operator sent another instruction packet to stop the motor and ensured that the gripper latched onto the pipe correctly. After recording and collecting data from the EMATs, the operator sent an instruction packet that allowed the gripper fingers to open up. It is worth mentioning that, based on the results described in the previous section, the sensing program was carried out using a nonconformed configuration. As shown in Figure 12, according to the inspection results recorded using the gripper, directly transmitted signals and received signals were successfully acquired in the time domain. The figure illustrates that as the sample diameter ME TECHNICAL PAPER w modular robotic gripper for tubular components Data acquisition system Software Touchscreen Cylindrical magnet Spiral coil Robotic arm Gripper Figure 11. Experimental setup for EMAT testing of the gripper. 0 0 50 100 150 –2 –4 2 Time (μs) 0 0 50 100 150 –2 –4 2 Time (μs) 0 0 50 100 150 –2 –4 2 Time (μs) Figure 12. Lamb wave time histories on pipes: (a) 73.025 mm OD (b) 88.9 mm OD and (c) 114.3 mm OD. (a) (b) (c) Amplitude (V) ×104 Amplitude (V) ×104 Amplitude (V) ×104