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 715 ME TECHNICAL PAPER w A B S T R A C T Tubular structures are critical components in infra- structure such as power plants. Throughout their life, they are subjected to extreme conditions or suffer from defects such as corrosion and cracks. Although regular inspection of these components is necessary, such inspection is limited by safety- related risks and limited access for human inspec- tion. Robots can provide a solution for automatic inspection. The main challenge, however, lies in integrating sensors for nondestructive evaluation with robotic platforms. As part of developing a versatile lizard-inspired tube inspector robot, in this study the authors propose to integrate electro- magnetic acoustic transducers into a modular robotic gripper for use in automated ultrasonic inspection. In particular, spiral coils with cylin- drical magnets are integrated into a novel friction- based gripper to excite Lamb waves in thin cylindrical structures. To evaluate the performance of the integrated sensors, the gripper was attached to a robotic arm manipulator and tested on pipes of different outer diameters. Two sets of tests were carried out on both defect-free pipes and pipes with simulated defects, including surface partial cracking and corrosion. The inspection results indicated that transmitted and received signals could be acquired with an acceptable signal-to- noise ratio in the time domain. Moreover, the simulated defects could be successfully detected using the integrated robotic sensing system. KEYWORDS: electromagnetic acoustic transducers, Lamb waves, robotic sensing, nondestructive evaluation, tubular structures Introduction Power plants, which provide energy to surrounding cities, are important to everyday life and work, and tubular structures are critical components of these plants. Throughout their life, these tubular structures experience extreme conditions or suffer from defects such as corrosion, cracks, and stress corro- sion cracks. Therefore, it is necessary to regularly inspect them to ensure structural integrity. This is especially impor- tant because even small defects in the tubes can result in cata- strophic failure and power outages for weeks. However, frequent inspection of these structures is problematic due to various reasons. For example, most power plants that need to be inspected cannot be shut down as they are critical to the functioning of surrounding communities. In addition, safety risks, high temperatures, and limited access to these structures (hard-to-reach places) inhibit human inspection. In this context, robotic inspection is a potential route to mitigating some of the safety restrictions faced by humans furthermore, robotic inspection would facilitate structural inspection in a timely and cost-effective manner. Tubular structures can be difficult to inspect because they often involve time-consuming testing methods for verifying their integrity. Two major types of robots being used for this purpose are in-pipe and out-pipe robots (Roh et al. 2009 Roh and Choi 2005 Lee 2013 Qiao et al. 2013). The first type, in-pipe robots, focuses on inspecting the insides of pipe the main limitation faced by these robots is that the power plant must be shut down during inspection, which affects plant Integrating Electromagnetic Acoustic Transducers in a Modular Robotic Gripper for Inspecting Tubular Components by Hamidreza Nemati*, Fernando Alvidrez*, Ankit Das†, Nihar Masurkar†, Manoj Rudraboina†, Hamid Marvi†, and Ehsan Dehghan-Niri‡ Materials Evaluation 79 (7): 715–727 https://doi.org/10.32548/2021.me-04223 ©2021 American Society for Nondestructive Testing * Intelligent Structures and Nondestructive Evaluation (ISNDE), Civil Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA † School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287, USA ‡ Intelligent Structures and Nondestructive Evaluation (ISNDE), Civil Engineering Department, New Mexico State University, Las Cruces, NM 88003, USA niri@nmsu.edu

ME TECHNICAL PAPER w modular robotic gripper for tubular components 716 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 productivity. Meanwhile, an out-pipe robot inspects tubular structures from the outside, thus allowing the plant to continue its operation. However, the main challenge facing the application of out-pipe robots is their limited mobility. For example, out-pipe robots must attach themselves to the structure using either friction or magnetic force (Lee et al. 2014), or use other methods like vacuum- or thrust-based systems and, at the same time, maneuver around other obstacles. Because an out-pipe robot does not require a plant to be shut down, assuming that the robot can traverse obstacles on the structure, this solution becomes the most cost-effective method of inspection. Multiple wheel-based out-pipe inspec- tion robots have already been developed (Xu and Wang 2008 Kawasaki et al. 2008). However, the disadvantage is that they can only be placed on a single pipe at a time. Once these robots are placed on a tube, workers will have to disassemble it before attaching it to another pipe. This exercise can be time-consuming and ineffective if a large tubular structure needs to be inspected. Therefore, the authors adopted a bioin- spired approach and proposed a robot with modular grippers with integrated noncontact ultrasound sensing elements to climb on vertical and horizontal surfaces using friction-based mobility. Thus, the objective of this study is to design and test a modular robotic gripper with embedded electromagnetic acoustic transducers (EMATs) as the main component of a versatile lizard-inspired tube inspector (LTI) robot (Marvi et al. 2020) (see Figure 1). Ultrasonic testing (UT) is the most effective nondestruc- tive testing (NDT) method that can be integrated with robotic platforms for inspecting tubular components (Liu and Kleiner 2013). Robotic ultrasonic inspection is sensitive to both surface and subsurface discontinuities and different UT techniques may be used for different types of tests. For example, UT can be used to evaluate surface rust, thickness, and deficiencies in the structure being tested. Furthermore, the size of the equipment required for UT is relatively small, which results in light payload for the robot. Contact UT based on bulk or guided waves is time-consuming and requires prepared surfaces with adequate couplants for point-by-point scanning, whereas EMATs are noncontact ultrasound trans- ducers that can effectively solve this problem. Because EMATs do not need a couplant, they can excite ultrasound waves through extreme temperatures and in remote locations (Salzburger et al. 2012). An EMAT works by using a coil that carries a current when a magnet is placed over the coil, a Lorentz force, which transmits the ultrasound wave through the material, is created. These transducers can generate different types of ultrasound waves, such as bulk, Lamb, and shear horizontal waves (SH waves), depending on the magnet-coil configuration (Lee et al. 2014). The generated waves can be used to detect and measure defects, cracks, and other discontinuities by transmission and reflection analyses. In addition, EMATs are less sensitive to surface conditions if the liftoff (the gap between the transducer and surface) is maintained within an acceptable range (Green 2004). These advantages make EMATs the perfect candidates for integra- tion into robotic platforms for inspection purposes. Park et al. (2002) described two mobile robotic inspection systems using different types of EMATs. They pointed out that some pipelines transport hazardous chemicals therefore, automation of discontinuity detection using the proposed robotic system minimizes the danger of human exposure to such a harmful environment. Kwon and Yi (2012) designed a caterpillar-based pipeline robot capable of inspecting 80 to 100 mm indoor pipelines. Another highly mobile robotic ultrasonic inspection system was proposed by Lee et al. (2014). With the help of a two-module mobile robotic platform, SH waves were applied with additional signal- processing methods to inspect pipe structures. Ren et al. (2017) developed a machine that uses an EMAT as the primary source for conducting ultrasound waves to detect discontinuities in materials. They also developed a special EMAT that could use high-sensitivity Lamb waves and SH waves simultaneously. Lissenden et al. (2017) developed a novel robotic system for crack detection in dry storage casks for spent nuclear fuel. For the sensing part, they used EMATs and laser induced breakdown spectroscopy (LIBS). Pei et al. (2018) integrated a robotic arm with an EMAT for C-scan UT to detect delamination a small coil and an enhanced magnet configuration were developed as a modified shear- wave EMAT to achieve good transduction efficiency in a small sensing area. Choi et al. (2017) investigated crack detec- tion in a stainless steel plate using shear horizontal guided waves generated from EMATs and dry-coupled magnetostric- tive transducers (MSTs). After comparing the advantages and drawbacks of each transducer, they selected EMATs for the inspection of stainless steel canisters. In another study, EMATs and robotics were integrated for high-temperature inspection of stress-corrosion cracks in the heat-affected Front camera Linear actuators Backbone active joint Tail Figure 1. Schematic of the lizard-inspired tube inspector (LTI) robot (adapted from Marvi et al. [2020]).