Special Issue on Exoskeleton Robotic Systems

Over the past 20 years, the therapies available in rehabilitation clinics have improved substantially, aided by robotic rehabilitation systems, which have seen rapid progress and increased performance. This progress has contributed to the diversification of the applications of exoskeleton robotic systems, namely, to provide assistance: in climbing up and down stairs, to workers in various activities (carrying weights, working with raised arms or in a bent position), and even in executing sit-to-stand movements. Exoskeleton robotic systems are used in a wide range of fields, both medical (rehabilitation, paraplegics, and amputees) or non-medical (rescue operations, construction work).

The progress of exoskeleton robotic systems is notable, and is visible in both lower-limb and upper-limb exoskeletons. However, there remain challenges, such as optimizing control systems and human–machine interaction.

This Special Issue presents a collection of articles highlighting recent achievements, applications, studies, and review articles within the field of exoskeleton robotic systems. These contributions cover a wide range of the applications of these systems, from the rehabilitation of the upper and lower limbs to assisting human gait when ascending and descending stairs, actuator selection criteria, and the implementation of actuation moment control algorithms.

The topics addressed within this Special Issue—“Exoskeleton Robotic Systems”—are varied. One such topic is the investigation of daily variations in the physical effects of using exoskeletons with the upper limbs [1]. The study aimed to evaluate the long-term biomechanics of wearing an exoskeleton at work to assist the upper limbs. Four activities are proposed for assistance, namely, lifting and lowering a box, moving while holding a box, moving without the box, and static bending during mechanical work. The data collected from the wearer while performing these activities are kinematic (movements using a full-body inertial motion capture system) and electromyographic (with sEMG sensors placed on the subject).

Review articles such as [2] aim to study the impact of COVID-19 on the evolution of patent proposals for lower limb exoskeleton robotic systems. The authors conducted a comparative analysis of the situation during the pandemic compared to the previous decade. The main purpose of the study was to provide a comprehensive overview of the efforts to innovate exoskeleton robotic systems during the pandemic. The results of the study confirm the trend of a decreasing number of patent proposals during the pandemic.

Another review study [3] provides a summary and overview of the use of the Hybrid Assistive Limb (HAL) robotic system in patients with spinal cord injuries. The conclusion of this study is that the use of this robotic system demonstrates a wide variety of applications, in both acute to chronic patients. Its use is generally safe, with no serious user injuries reported. Additionally, the vast majority of users experienced functional improvements, showing increased walking distances, gait speeds, and pain reduction.

Recent developments in exoskeleton robotic systems are presented in [4]. Namely, an exoskeleton robotic system has been developed which provides support for all three joints of the human lower limb. In order to design the motion laws of the drive motors, an experimental analysis of human gait with the Vicon equipment was used as a starting point, and a database of joint angle variation laws for a group of 30 people was created. A dynamic study of the exoskeleton was carried out using the Newton Euler method supplemented with Lagrange multipliers, in conjunction with a dynamic study of the virtual model using the ADAMS software for dynamic analysis of multibody systems. Experimental tests of the physical model were also carried out, demonstrating the feasibility of the prototype.

In this collection of Special Issue articles, there is also one [5] which proposes a new selection criterion for linear pneumatic motors used to drive wearable assistive devices, namely, the energy-to-mass ratio. The requirements for a wearable assistive device are compactness, lightweightness, and energy efficiency. The authors propose a criterion for choosing an actuator that meets the above-mentioned requirements, namely, the energy-to-mass ratio.

In the field of locomotor rehabilitation, prostheses are commonly required by people with trans tibial amputations. The authors of [6] propose the constructive solution of an ankle prosthesis based on an intelligent fluid joint (magneto rheological). Additionally, the sole of the prosthesis is equipped with eight pressure sensors which register the pressure between the sole and the ground, which is necessary to determine the kinematic and dynamic parameters of the person’s gait.

Robotic rehabilitation systems also involve the upper limbs. Upper limb exoskeletons are used for two types of activity: as an assistive system that provides physical support and reduces the wearer’s strain at work, and as a robotic system for the rehabilitation of stroke patients.

Thus, in [7], a dynamic analysis of a mechanism in the structure of an upper limb rehabilitation robot, called ParReEx, is presented. The dynamic analysis was performed in the first phase, considering the rigid kinematic elements, and later, the elasticity of the elements, as well as the friction in the kinematic couplings. A method of structural optimization of the constructive shape of the mechanism’s elements is also presented, with the aim of reducing mechanical stress concentrators as well as minimizing the mass of the rehabilitation device.

A similar system designed for upper limb rehabilitation is the ASPIRE robot, a dynamic study of which is presented in [8]. The purpose of this system is the rehabilitation of brachial monoparesis. The dynamic analysis was performed using MSC.ADAMS software, a powerful tool for the study of mobile multibody systems. The robot is composed of three modules: the first providing the flexion/extension movement for the shoulder, the second providing the adduction/abduction movement, and the third providing forearm pronation/supination. The results obtained from the ADAMS simulation computed torque for the flexion/extension module and the adduction/abduction module are compared with those obtained experimentally. The similarity of these results validates the dynamic model, confirming that the connection forces determined from the dynamic simulation are valid. These data are used to study the mechanical strength of the flexion/extension modulus using the finite element method.

The outstanding performance of exoskeleton robotic systems is evidenced by their use as assistive systems for stair ascent/descent, as presented in [9]. Stair climbing involves slow movements within the joints but with greater amplitudes than in normal walking. The objective of this study was to evaluate the biomechanics of a new robotic exoskeleton system intended for stair climbing. The results of the study show that an individual application of assistive force is required for adequate assistance when climbing stairs.

Exoskeletons have also been developed that provide rehabilitation of a specific joint of the human lower limb. Thus, in [10], the energy consumption and torque control of such an exoskeleton are studied. The study focuses on sit-to-stand and walking activities. The constructive design of the exoskeleton hip robot was performed using Autodesk Inventor software, and the kinematic analysis was based on Denavit–Hartenberg parameters inputted into Matlab computational software. The Linear Quadratic Regulator (LQR) method was considered in order to obtain the optimum controller for energy consumption and walking activity.