In recent decades, many researchers have focused on the design and development of exoskeletons. Several strategies have been proposed to develop increasingly more efficient and biomimetic mechanisms. However, existing exoskeletons tend to be expensive and only available for a few people. This paper introduces a new gravity-balanced upper-limb exoskeleton suited for rehabilitation applications and designed with the main objective of reducing the cost of the components and materials. Regarding mechanics, the proposed design significantly reduces the motor torque requirements, because a high cost is usually associated with high-torque actuation. Regarding the electronics, we aim to exploit the microprocessor peripherals to obtain parallel and real-time execution of communication and control tasks without relying on expensive RTOSs. Regarding sensing, we avoid the use of expensive force sensors. Advanced control and rehabilitation features are implemented, and an intuitive user interface is developed. To experimentally validate the functionality of the proposed exoskeleton, a rehabilitation exercise in the form of a pick-and-place task is considered. Experimentally, peak torques are reduced by 89% for the shoulder and by 84% for the elbow.

Robotics have the potential to become a revolutionary technology in the next future and if we want to spread it among the population we must start accounting for affordability in robot research and design. Plastic robots are an emerging example of affordable design where expensive metal structures are replaced by low-cost plastic materials. Plastic materials are not only cost effective but, thanks to their inherent compliance and lightweight, lead to significant advantages in terms of safety and force controllability. Inspired by the idea of Series Elastic Actuator (SEA) this paper introduces the new concept of Series Elastic Link (SEL) which exploits the inherent flexibility of plastic links to implement series compliance. This paper elaborates on the concept of SEL and highlights a parallelism with SEAs. Then, focusing on a single-DOF setup it shows that, beyond their economic advantage, SELs leads to enhanced safety and more accurate force control with respect to traditional SEA implementations. The proposed argumentations are theoretically supported and experimentally validated.

The benchmarking of force control algorithms has been significantly investigated in recent years. High-fidelity experimental benchmarking outcomes may require high-end electronics and mechanical systems not to compromise the algorithm’s evaluation. However, affordability may be highly desired to spread benchmarking tools within the research community. Mechanical inaccuracies due to affordability can lead to undesired friction effects which in this paper are tackled by exploiting a novel friction compensation technique based on an environment-aware friction observer (EA-FOB). Friction compensation capabilities of the proposed EA-FOB are assessed through simulation and experimental comparisons with a widely used static friction model: Coulomb friction combined with viscous friction. Moreover, a comprehensive stability comparison with state-of-the-art disturbance observers (DOBs) is conducted. Results show higher stability margins for the EA-FOB with respect to traditional DOBs. The research is carried on within the Forecast project, which aims to provide tools and metrics to benchmark force control algorithms relying on low-cost electronics and affordable hardware.

Friction phenomena may significantly deteriorate the performance and reliability of force-controlled systems. Despite existing solutions, achieving accurate and robust friction compensation is challenging due to model inaccuracies and parameter identification issues. This letter proposes a force control architecture with accurate friction compensation and friction shaping capabilities, based on a novel model reference approach. Since passivity is a fundamental requirement for interaction control, this work investigates under which conditions the proposed approach preserves passivity at the environment, control, and friction ports. This implies that the proposed architecture can passively compensate for any type of friction and can be combined with any passive force controller while guaranteeing a stable interaction with any passive environment. The approach is validated using simulation and experimental trials.

The increasing adoption of upper-limb exoskeletons in industrial, domestic, and clinical settings calls for assistive strategies capable of adapting to human-in-the-loop requirements, including dynamic payload variations. This paper presents a myography-based adaptive gravity compensation (AGC) method that relies solely on biceps and triceps activity, without requiring large training datasets, subjectspecific anthropometry, or lengthy calibration. A theoretical analysis shows that the combined effects of acceleration and anthropometric uncertainties introduce inherent disturbances in payload estimation. To address this issue, two implementations of the proposed adaptive law are introduced: a continuous update strategy that compensates inertial effects, and a thresholdbased strategy that temporarily suspends adaptation when acceleration exceeds a predefined limit. Both approaches are validated through simulations and experiments on a 1-DoF elbow exoskeleton with fifteen healthy participants and compared against the only existing method that neglects acceleration effects. Results show that the threshold-based strategy yields the most accurate mass estimates, significantly reduces muscular effort, and provides the most favorable user experience.