WheTLHLoc 4W: Small‐scale inspection ground mobile robot with two tracks, two rotating legs, and four wheels

Abstract WheTLHLoc 4W is the second version of a hybrid leg–wheel–track small‐scale ground mobile robot, developed for surveillance and inspection applications. It differs from its predecessor, WheTLHLoc 2W, in two main aspects: the number of wheels (four instead of two), and the new leg design. The overall size of the robot varies depending on the configuration: 500 × 420 × 140 mm (length × width × height) in tracked mode with lowered legs, 315 × 420 × 310 mm in tracked mode with raised legs, and 430 × 420 × 260 mm in wheeled mode. The robot is sufficiently small to explore narrow spaces, both indoors and outdoors, switching between wheeled locomotion on compact surfaces and tracked locomotion on irregular and yielding terrain. Due to its small length, the robot can stand on one tread of a stair. Nevertheless, it is capable of climbing stairs, and the step height limit has been augmented from 165 mm to 180 mm (+9.1%) with respect to the first version, keeping constant the robot overall size. This enhancement has been achieved through a mechanical redesign of the two rotating legs: each leg now features a pair of smaller wheels, actuated by the same actuator through a gear train, instead of a single larger wheel. The significant increase in the maximum step height expands the robot's capabilities for indoor environments, as stairs with a rise higher than 180 mm are extremely uncommon. The paper discusses the mechanical design of WheTLHLoc 4W, the trajectory planning for step/stair climbing, and provides an analytical comparison between the novel architecture and the first version in terms of stability against overturn and slippage. These analytical results are then experimentally verified. The findings demonstrate that the new leg design not only increases the maximum step height (+9.1%), but also reduces the minimum friction coefficient required for successful stair climbing (−19.1% for the maximum step).