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 729 availability of space. They also can require the system to be shut down prior to the inspection. Several designs and concepts for in-pipe inspection systems have been discussed in the literature (Hadi et al. 2020 Kishi et al. 2015 Tesen et al. 2013 Omori et al. 2009 Debenest et al. 2014 Nagase et al. 2013 Ito et al. 2019 Gargade and Ohol 2016 Boxerbaum et al. 2012 Qiao et al. 2013 Yeo 2012 Nagase et al. 2016). These systems perform a variety of tasks including internal cleaning of ducts (Ito et al. 2019), measurement of surface thickness, mapping of tubes, and visual inspection (Gargade and Ohol 2016). One of the significant challenges for internal crawler systems is the ability to generate traction within the limited space of the tubes. Earthworm-type robots (Boxerbaum et al. 2012 Qiao et al. 2013) offer larger traction output, but require a number of degrees of freedom. Wheeled (Yeo 2012) and treaded (Nagase et al. 2016) systems offer simpler designs but generate less traction. The compromise between maneuverability and design simplicity is a major challenge in developing a robotic inspection system. However, there is little research that has been conducted on the development of internal crawlers for superheater tubes and small-diameter pipes in general. This is likely due to the limited space avail- able and the coiled nature of the tubes. In this paper, a novel robotic inspection tool is presented that can navigate through small-diameter pipes and provide information on the structural integrity of tubes typically found in power plant superheaters. The system consists of a tethered pipe crawler that can navigate through coiled tubes with 180° bends and diameters as small as 5 cm. The primary crawler will contain modules that house inspection sensors including a light detecting and ranging (LiDAR) sensor, environmental sensors, cameras, and an ultrasonic sensor for measuring tube thickness. Multiple auxiliary crawlers will also be utilized for load distribution of the tether as the system navigates through the multiple bends and straight sections. A schematic of the concept is shown in Figure 1. Robotic Crawler Movement of the crawler is generated using a set of gripper and extender modules that propel the crawler forward using peristaltic motion, similar to earthworms that travel by Crawler Auxiliary crawler Auxiliary crawler Camera Camera Instrumentation module Extenders Tether Tether Superheater header access point Superheater coil Coil connection Flexible links Grippers Crawler Auxiliary crawlers Entry point Deploying tool Figure 1. Conceptual design of the crawler inspection system.

730 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 contracting their body segments sequentially. Each module holds a linear actuator consisting of a rotating lead screw and nut. The basic design is composed of five modules: two grippers, one at the front and one near the rear of the system two extenders between the grippers and one electronics module. The modules are connected via a flexible cable that has the strength to handle the push/pull loads and is also flexible enough to allow for significant rotation between the modules. Each gripper contains linkage arms that push small pads radially outward and engage the inner pipe wall. The radial symmetry of the design allows for three sets of linkage arms and gripper pads. These linkages are driven by a mechanism attached to the nut of the rotating lead screw. Similarly, the extenders utilize a nut at the center of the module that expands and contracts. Two electric motors on the sides of the module utilize a gearbox to power the lead screw. The selection of actuators was critical to the design of the crawler. The choice of electric linear actuators versus pneu- matic actuators involved the consideration of several factors. Reliance on compressed air throughout the crawler posed a challenge in the tether design management. Additionally, electric motors are much smaller and require thinner wiring than pneumatic motors. It should be noted that there was a clear trade-off between reducing the module’s overall dimen- sions versus simplifying the controls of the system. Pneumatic actuators provide a much simpler method of producing the linear motion. This challenge was addressed in developing the electronics and communication system for the crawler. A module has been incorporated into the system that houses the major elec- tronic components. This includes the embedded microcon- troller, voltage regulator, current sensors, and motor controllers, which are mounted onto printed circuit boards. The electronics module was added to the rear end of the system and controls the movement of each gripper and extender. Figure 2 shows a prototype of the base crawler system with the five modules. Simulation Model The development of the crawler significantly depended on the coil geometry. In an initial geometric analysis, efforts were made to design the system to be capable of navigating through 5 cm radius bends. This led to the to the modules having a maximum diameter of 3.5 cm and a maximum length of 7 cm. To improve the design of the peristaltic crawler and set the framework to evaluate the controls of the system, a high- fidelity model is being developed. Figure 3 shows a detailed schematic of the gripper and extender modules and includes ME TECHNICAL PAPER w robotic inspection of small-diameter superheater pipes Figure 2. Prototype of the base robotic crawler. 7 cm Lead screw Lead nut Spring Stroke Stroke End cap 7 cm 2.58 cm 2.58 cm 2 cm 1 cm 1.5 cm 3.5 cm 3.5 cm 1.5 cm 5.16 cm Lead screw Micro gearmotor Figure 3. Schematic of the primary modules: (a) gripper (b) extender. (a) (b)