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 717 zones of welds in stainless steel dry storage canisters (Choi et al. 2018). The researchers were able to successfully use this robotic system for inspecting storage casks at temperatures as high as 121 °C. Although EMATs have been used in robotic inspection applications, they still face some limitations. For instance, most of these robots use wheels for mobility, which reduces their maneuverability. Moreover, the current robotic systems cannot be used on components with complex geometries or on rough surfaces. In addition, in some UT inspections (such as thickness measurement), point-by-point inspection is required, which implies the need for circumferential access. In contrast, the LTI, a robot capable of friction-based mobility, can move on tubes with complex geometries, obstacles, and rough surfaces. The other main advantage of designing such a robot is that it eliminates the need for point-by-point scanning of tube surfaces for crack and corrosion detection and evaluation by integrating advanced Lamb wave–based imaging. In the following section, the inspector gripper system will be discussed in detail. It is anticipated that the current robotic inspection protocols can benefit substantially by implementing the proposed in-process method. If successful, the proposed system will be the first bioinspired robot that uses Lamb waves generated by EMATs to inspect pipes and other tubular components. The integrated mobility and sensing of the LTI robot are dependent on three main processes, which are: l fabrication and control of modular friction-based grippers with integrated EMATs, l fabrication and control of the robot consisting of two grippers, a body, and a tail, and l advanced Lamb wave–based imaging for crack detection and characterization. Among these listed processes, the current study focuses on the first aspect. The remainder of this paper is organized as follows: first, the design process of the robot, power train, and gripper control is described. The next section discusses the EMAT system and describes the design and setup associated with EMATs, followed by the description of the experimental setup. Next, the experimental results related to pipe inspec- tion using conventional EMAT testing and the gripper are detailed. Finally, the conclusions are presented. Robotic System To evaluate the structural integrity of pipes, an LTI robot is proposed in this study. One of the main advantages of this robot is that it can travel along boiler tubes without having to shut down the plant. The robot comprises two grippers to grasp the tube and linear actuators, which connect the grippers and act as the spine of the robot, to allow linear motion. The joints (three rotational joints and one prismatic joint) and actuators allow four degrees of freedom in each half of the robot. There is another rotational joint connecting the two halves, taking the total number of degrees of freedom to nine for the entire robot. These joints and actuators allow the robot to maneuver through bends and turns in the boiler tubes. In this study, the modular design of the gripper used for testing is discussed. External grasping of a tube can be achieved through various robotic mechanisms, such as wheels, clamps, fingers, or limbs (Chattopadhyay et al. 2018). The biggest issue that most robotic grippers face is modularity when it comes to grasping tubes of varying diameters. Another factor to consider is the placement of EMAT sensors on the gripper for NDT. The addition of EMAT sensors means that wheeled grippers are not suitable for this application and therefore, clamps and fingers should be used. Several clamp-and-finger designs can be found in the avail- able literature. For example, the 3DCLIMBER developed at the University of Coimbra includes two unique multi-fingered V-shaped grippers (Tavakoli et al. 2008). The V-bend is the primary contact surface and a layer of rubber is added to this bend to provide friction. The length of the V-shaped structure ensures that the gripper can overcome the torque generated by the robot’s motion. A gripper mechanism consisting of two clamps actuated linearly by motors (Li 2017) would have trouble grasping tubes of different sizes. PiROB (Kim et al. 2018) is a vision-based pipe-climbing robot with a gripper actuated by a motor and gear system. The inner surface of the gripping finger is lined with a rubber layer for friction. However, this system also faces issues with tubes of different diameters. The gripper in the authors’ robotic system was originally inspired by the Yale Open Hand Project Model T, which is an underactuated four-finger hand with compliant flexure joints driven by a single actuator (Ma et al. 2013). Aramid synthetic fiber strings were used to set up a pulley mechanism for actu- ating the gripper fingers. When testing the system, it was observed that the tension in the strings was not strong enough to overcome the torque required for our application, and assembling the strings led to manual errors. Therefore, the authors switched to a worm-gear pair mechanism to overcome the torque and load requirements of the robot. Gripper Design The current design of the robotic gripper includes four fingers equipped with extrusions for integrating the magnet of the EMAT sensors. For the NDT experiments, pads made of polydimethyl-siloxane were used. However, the material and design of the friction pads are currently being optimized as part of a separate study. The difference between this gripper design and the grippers discussed earlier is that the curvature required to grasp the tube comes from the friction pads rather than fingers or clamps. This gives the proposed gripper a sense of modularity when it comes to adjusting to tubes of different sizes. The curved surface of the friction pad allows the gripper to maximize the contact surface between itself and the tube. Changing the gripper fingers each time a new tube

718 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 of a different size is to be evaluated is a cumbersome process. Instead, it is much simpler and efficient to replace friction pads when they wear off. The thickness of the friction pad between the tube contact surface and EMAT sensor was mini- mized (less than 1 mm) for effective NDT. The gripper consisted of three major components (Figure 2): l Base plate: The base plate is a circular disc-shaped component designed to accommodate the arch and motors. It also acts as a connecting point between the gripper and the rest of the body of the LTI robot or any robotic arm’s end- effector through the robotic arm mount. The base plate consists of mounting provisions for two smart servo motors and the arch. However, only one motor is required for gripper finger actuation and the other is used for the LTI robot. l Arch: The arch is split into two halves. The bottom half is connected to the base plate using six 8-32 flathead screws. The bottom half and base plate combination act as a sturdy base for the gripper construction. The top and bottom halves of the arch are connected to each other using six M6 bolts and nuts. These are designed to hold minor components, such as bearings, shafts for finger actuation, and worm wheels. l Fingers: The fingers are the primary contact points for sensing. The gripper consists of two pairs of fingers, and each pair is connected to two separate shafts actuated by the same motor. Each finger consists of an EMAT sensor, friction pads, and other sensor provisions. The contact surface of the finger is designed to be flat, similar to the gripping curvature on friction pads. These parts were fabricated by 3D printing using a fiberglass-reinforced composite base material. Although our initial intention was to design this gripper for LTI robots, its modular characteristics enable its use in different robotic platforms. In this study, we focused on the evaluation of the gripper using the robotic arm platform. ME TECHNICAL PAPER w modular robotic gripper for tubular components A A ∅130 mm ∅130 mm 68 mm 80 mm 43 mm 60.79 mm 18 mm 16 mm 70 mm 20.53 mm 28.32 mm 5.08 mm 20 mm 60 mm Gripper finger Friction pads Gripper base plate Worm gear Extrusion for EMAT sensor Arch Worm wheels Section A-A Smart servo motor Figure 2. Two-dimensional view of the gripper design: (a) front view (b) side view (c) cross-sectional view. (a) (c) (b) 5 mm