Exoskeletons are mechanical structures that are mostly employed in industrial and rehabilitation fields. In the first case, mechanical structures are used to help the operator to execute heavy tasks. In the second case, rehabilitation requires structures that help the patient to restore or recover lost motion abilities. Depending on the application, exoskeletons can be designed for power amplification [
1], neuromuscular impairment compensation [
2,
3] and rehabilitation [
4,
5] and to support disabled people in activities of daily living (ADL) [
6].
In the context of rehabilitation, to fully or partially recover physiological motion capabilities, patients receive rehabilitation treatment based on active and repetitive exercises [
7,
8,
9,
10]. In this context, upper-limb exoskeletons have attracted great attention over the years, and several surveys on upper-limb rehabilitation robotic devices can be found [
11,
12,
13,
14,
15]. In fact, exoskeletons can be employed to assist the medical operator, increasing rehabilitation performance [
16,
17,
18,
19] and diminishing healthcare expenses. Therapy sessions can be performed in parallel supervised by a single therapist, patients can benefit from a prolonged therapy time (increasing therapy effectiveness), and a greater repeatability can be reached during the motion task execution. Even though several classifications are proposed in the literature [
20] (e.g., field of application, active or passive, degrees of freedom (DOFs) of the system, type of actuation and portable or fixed), two main categories of devices can be found when comparing the mechanical structure: end-effector-based systems and exoskeleton-based systems [
21]. End-effector-based systems are in contact with the human limb at its most distal part only (e.g., the hand). For this reason, they have a simpler mechanical structure. However, it is not possible to impose specific movements to a particular human joint. Typical end-effector-based systems include serial manipulators (e.g., MIT Manus [
22] and ACRE [
23]) and parallel (e.g., CRAMER [
24] and InMotion ARM [
25]) and cable-driven robots (e.g., NeReBot [
26] and MACARM [
27]). Unlike end-effector manipulators, exoskeletons have serial link chains, allowing one to exactly reproduce the physiological movement of each joint of the limb. Moreover, they can fully or partially compensate upper-limb weight. To reach these aims, they require adaptation of the length segments to the patient limb, which may take a significant amount of time. Furthermore, to avoid damage to the patient, proper designs are required to match the position of the center of the rotation of the human limb with that of the mechanical structure [
28]. Even if a high-performance and a comfortable exoskeleton can be found [
29], very little consideration has been given to the minimization of system costs. To the best of our efforts, we were not able to find specific literature on upper-limb complete systems (i.e., comprising the mechanical structure, the software and the electronics) developed with the intention of being low cost for rehabilitation purposes: a few low-cost exoskeleton mechanical structures were found but in an embryonic state [
30,
31]. As a result, the vast majority of existing devices are cost prohibitive for most people, preventing any personal or domestic use. Starting from these considerations, this paper introduces a low-cost exoskeleton concept developed for the rehabilitation of the upper limb. The main objective of the presented concept is to show a significant cost reduction with respect to existing designs. This contribution describes our mechanical, electronic and sensing designs. Although the main focus of this work is on the mechanics, we need to consider that the choices of electronic architectures and sensing can also lead to a significant cost reduction. From the point of view of electronics, high costs are usually associated with real-time Oss (RTOSs), which are cost demanding in terms of the required computational resources and/or in terms of licenses. From the point of view of sensing, we need to acknowledge that commercial force sensors are usually very expensive components.
As the first step in the direction of designing affordable exoskeletons, we consider a simplified target application: rehabilitation exercises characterized by only two DOFs in the sagittal plane (namely, shoulder and elbow flexion/extension). After a brief literature review on the available exoskeletons (
Section 1), the present paper focuses on the proposed exoskeleton design—including the mechanics and electronics—and briefly describes advanced rehabilitation features (
Section 2). Then, preliminary tests conducted on one healthy subject are reported (
Section 3), and conclusions are drawn (
Section 5).
State of the Art
In this section, the latest developments in upper-limb exoskeleton technology for rehabilitation are discussed. Some of them have been commercialized, while others are research prototypes. The Harmony [
5] exoskeleton consists of an active five-DOF shoulder mechanism, one DOF elbow mechanism and one DOF wrist mechanism powered by series elastic actuators (SEAs). It exhibits good kinematic compatibility with the human body with a wide range of motion, and it performs task-space force and impedance control. The Alex 2 exoskeleton [
32] was developed based on a similar concept, but, unlike the Harmony exoskeleton, it uses series elastic tendon transmission, which guarantees intrinsic mechanical compliance. Hsieh et al. [
33] has proposed a mechanism specifically designed for shoulder rehabilitation. It consists of two spherical mechanisms, two slider crank mechanisms and a gravity-balancing mechanism. The side-by-side disposition of the actuators ensures not only lower inertia properties but also compactness and less weight. A mechanism with a passive joint is introduced to compensate the misalignment of the exoskeleton with the human limb in the case of variable physiological parameters of the user. Linear SEAs have been used. Hunt et al. [
34] proposed a spherical joint based on a 3-UPU (U and P are universal and prismatic joints, respectively) Gough–Stewart platform. The three prismatic joints are actuated. Two additional passive joints (corresponding to a translation and a rotation) connect the platform to the human limb. NEUROExos [
35] is an elbow-powered exoskeleton designed for post-stroke rehabilitation of the arm, ensuring comfort for the patient. Misalignment problems have been addressed by mounting the active rotational joint on a moveable translational passive mechanism, which decouples the robot joint rotations from axis translations. Undesired forces resulting from a rigid connection between the limb and the system are avoided, while assistive forces can be performed. Oguntosin et al. [
36] proposed the EasoftM, a 3D printed exoskeleton with passive joints to compensate for gravity and with active joints to rotate the shoulder and elbow joints. It resulted in a lightweight system that assists planar reaching motion. Wu et al. [
37] proposed a gravity-balanced exoskeleton with a flexible Bowden cable transmission system for active rehabilitation training of the upper limb. Gravity balancing of the human arm and mechanism is achieved using auxiliary links and zero free length. Those described here are only a small selection of all the exoskeletons commercially available or presented in the literature: high-performance and comfortable exoskeletons can be found [
29]. A more detailed description of the state of the art can be found in [
38,
39,
40,
41]. As mentioned before, to date, very little consideration has been given to affordability arguments, and the vast majority of existing devices are cost prohibitive. High costs are associated with compact high-torque actuation [
42,
43]. (which often includes expensive harmonic drive reduction stages), cable-driven systems [
32] (which require high-precision manufacturing), commercial force sensors [
44] and high-performance RTOS solutions (which usually require high computational resources to execute hard real-time processes at suitable control frequencies) [
45], [
46]. For reference, the average cost for a complete upper-limb exoskeleton with five–six actuated joints is in the order of USD 130,000 [
47]. This price hinders the availability of exoskeletons in small therapeutic centers or in domestic settings, which, otherwise, could lead to longer lasting and more effective therapies.