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 719 Because the gripper mount is modular, it can be easily detached from the gripper and its design can be changed depending on the robotic arm. We used a robot manipulator to evaluate the performance of the gripper in inspecting tubular components using EMATs. The gripper was attached to the mount using six 8-32 flathead screws, as illustrated in Figure 3. The mount was then screwed onto the manipulator end-effector using four M6 screws. This allows designers to position the gripper onto any part of the tube. A representa- tion of the gripper operation along with the robotic arm is shown in Figure 4. Power Train and Load Characteristics The power train was designed to meet the torque and load requirements of the LTI robot. The weight of the LTI robot was estimated to be 7 kg and a single gripper is expected to hold on to the pipe with a force (applied to the fingers) greater than the robot weight. Therefore, assuming the load applied at the tip of a 70 mm long finger to be 7 kg (#68.67 N), the torque required on the finger shaft is #4.8 N·m. As there are four fingers, a total torque of 9.6 N·m is required for each shaft and a torque of 19.2 N·m is required for the two shafts combined. However, to achieve a minimum factor of safety (FoS) of 4, a torque of #76 N·m is required. To achieve this high torque level, a smart servo motor (8.4 N·m) was used as the source with a worm-gear conversion ratio of 1:10, which in turn resulted in a torque of 84 N·m (#4.375 FoS). Based on load calculations, static finite element analysis (FEA) was carried out using a modeling software for the finger to determine material strength and deformation. The FEA results suggested an FoS of 7, which makes the design safe for the applied load and selected material. Control of the Gripper Gripper fingers were connected to a smart servo motor using a worm and spur gear combination. These smart servo motors are all-in-one actuator systems with a servo mechanism that can precisely control the angular or linear position, velocity, and acceleration of the output shaft. The electric motor in the servo motor was controlled by an internal microcontroller unit with a simple proportional-integral-derivative (PID) control loop taking the velocity, position, torque, load, temperature, and voltage as feedback to improve control over the motor. EMAT Systems Using EMATs is a promising UT technique for the nonde- structive evaluation of electrically conductive materials. Although the signal-to-noise ratio (SNR) in EMAT systems is inherently weak, high amplification, filtering, and narrow- band excitation can be used to enhance the SNR (Mirkhani et al. 2004). Despite some of their limitations, including a low efficiency, liftoff sensitivity, and limited frequency range, EMATs have the advantage of not requiring any couplants for Mount Robotic arm M6 screw Figure 3. Manipulator mount for the robotic gripper. Robotic arm Tube Robotic gripper Figure 4. Demonstration of the grasping of a tube by the robotic gripper.

ME TECHNICAL PAPER w modular robotic gripper for tubular components UT. This permits the easier deployment of probes, which is a remarkable advantage in automated inspection. This tech- nique also facilitates the rapid scanning of components with complex geometries and rough surfaces, as described earlier. Traditional EMATs consist of a coil and magnet and are used to generate ultrasonic waves in test metallic samples. The coil is excited by an alternating electric current in the presence of a uniform magnetic field near the surface of an electrically conductive or ferromagnetic material. This alternating Lorentz force plays a major role in transmitting waves (as a source of oscillating stress waves) (ASTM 2017). Depending on the design and orientation of the coils and magnets, different wave modes can be generated in EMAT applications (Kundu 2004). In other words, the orientation, size, and shape of the coil and magnet determine the type of ultrasonic waves and modes required in different inspections. In the following subsection, the authors shall describe in detail the characteristics of the EMATs used in the LTI robot. EMAT Design and Setup In the LTI robot, Lamb waves are generated and received by integrated EMATs to eliminate the need for a point-by-point cross-sectional inspection of tubular components. The main idea behind using Lamb waves in the LTI robot is that as the robot climbs, the network of eight EMATs in the two grippers can communicate and image the entire cross section of the part of the tubular component between the grippers. As a type of guided ultrasound wave, Lamb waves can propagate in plate-like structures. In tubular structures with a small wall thickness-to-diameter ratio, circumferential waves in the tube wall can be considered to be similar to Lamb waves by replacing the cylindrical structure with an equivalent unwrapped plate (Li and Rose 2006 Pierce and Kil 1990). Lamb waves have unique properties that enable sophisticated inspection schemes in general, these waves can propagate over long distances and are more sensitive to discontinuities than traditional bulk ultrasound waves. Thus, the capacity of Lamb waves to inspect the entire cross section of a plate from a long distance makes them a practical choice for the real-time monitoring and inspection of plate-like structures (Rose 2014). Lamb waves are inherently multimodal and dispersive. Therefore, before using Lamb waves to detect discontinuities in pipes, it is necessary to represent their theoretical disper- sion curves to select an effective mode. These curves can be obtained from the governing differential equations and are functions of material properties and sample thickness (Rose 2014). The stainless steel (316L) material used to determine curvature dispersion had an elastic modulus of 200 GPa, Poisson’s ratio of 0.25, a density of 8000 kg/m3, and a thickness of 3.048 mm. A spiral coil (pancake coil) was used to generate omnidirectional Lamb waves to propagate in helical paths and cover the entire cross section of the tube between the two grippers. A permanent magnet generates a static magnetic field, whereas a spiral coil into which 720 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 Permanent magnet Spiral coil 12.7 mm N N S S B 0 Figure 5. EMAT components: (a) spiral coil (b) cylindrical magnet and (c) schematic diagram of an EMAT probe. (b) (c) (a) 28.5 mm