The paper discusses the state-of-the-art of locomotion systems for ground mobile robots comprising tracks. Tracked locomotion, due to the large contact surface with the ground, is particularly suitable for tackling soft, yielding, and irregular terrains, but is characterized by lower speed and energy efficiency than wheeled locomotion, and lower obstacle-climbing capability than legged locomotion. Therefore, in recent years academic and industrial researchers have designed a wide variety of hybrid solutions, combining tracks with legs and wheels. The paper proposes three possible parallel taxonomies, based on body architecture, track profile, and track type, to help designers select the most suitable architecture on the basis of the operative necessities. Moreover, modeling, simulation, and design methodologies for tracked ground mobile robots are recalled.

This paper presents the conceptual and functional design of a novel hybrid leg-wheel-track ground mobile robot for surveillance and inspection, named WheTLHLoc (Wheel-Track-Leg Hybrid Locomotion). The aim of the work is the development of a general-purpose platform capable of combining tracked locomotion on irregular and yielding terrains, wheeled locomotion with high energy efficiency on flat and compact grounds, and stair climbing/descent ability. The architecture of the hybrid locomotion system is firstly outlined, then the validation of its stair climbing maneuver capabilities by means of multibody simulation is presented. The embodiment design and the internal mechanical layout are then discussed.

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).

Fractional Calculus is usually applied to control systems by means of the well-known PI^λD^μ scheme, which adopts integral and derivative components of non-integer orders λ and µ. An alternative approach is to add equally distributed fractional-order terms to the PID scheme instead of replacing the integer-order terms (Distributed Order PID, DOPID). This work analyzes the properties of the DOPID scheme with five terms, that is the PII^1/2DD^1/2 (the half-integral and the half-derivative components are added to the classical PID). The frequency domain responses of the PID, PI^λD^μ and PII^1/2DD^1/2 controllers are compared, then stability features of the PII^1/2DD^1/2 controller are discussed. A Bode plot-based tuning method for the PII^1/2DD^1/2 controller is proposed and then applied to the position control of a mechatronic axis. The closed-loop behaviours of PID and PII^1/2DD^1/2 are compared by simulation and by experimental tests. The results show that the PII^1/2DD^1/2 scheme with the proposed tuning criterium allows remarkable reduction in the position error with respect to the PID, with a similar control effort and maximum torque. For the considered mechatronic axis and trapezoidal speed law, the reduction in maximum tracking error is −71% and the reduction in mean tracking error is −77%, in correspondence to a limited increase in maximum torque (+5%) and in control effort (+4%).

The application of Fractional Calculus to control mechatronic devices is a promising research area. The most common approach to Fractional-Order (FO) control design is the PI^λD^µ scheme, which adopts integrals and derivatives of non-integer order λ and µ. A different possible approach is to add FO terms to the PID control, instead of replacing integer order terms; for example, in the PDD^1/2 scheme, the half-derivative term is added to the classical PD. In the present paper, by mainly focusing on the transitory behaviour, a comparison among PD, PD^µ, and PDD^1/2 control schemes is carried out, with reference to a real-world mechatronic implementation: a position-controlled rotor actuated by a DC brushless motor. While using a general non-dimensional approach, the three control schemes are first compared by continuous-time simulations, and tuning criteria are outlined. Afterwards, the effects of the discrete-time digital implementation of the controllers are investigated by both simulation and experimental tests. The results show how PDD^1/2 is an effective and almost cost-free solution for improving the trajectory-tracking performance in position control of mechatronic devices, with limited computational burden and, consequently, easily implementable on most commercial motion control drives.