# Modular Design and Decentralized Control of the Recupera Exoskeleton for Stroke Rehabilitation
## 2. Mechatronic System Design
### 2.1. Mechanical Design
#### 2.1.1. Sub-Mechanism Modules  
The design of a submechanism depends on its position in the overall system and the requirements regarding range of motion (ROM) and forces. Figure 1. Recupera Exoskeleton system configurations. The aim of the design is to obtain a fully actuated 3 DOF mechanism with a workspace similar to the human shoulder [39]. The three joint axes should intersect orthogonally at a central point (CP) in the human shoulder such that no two joint axes become coincident in the allowed motion range to ensure a singularity free operation. This central point is coincident with glenohumeral joint in the human shoulder. In contrast to serial shoulder designs [41; 42], where a collision of the motors with the patient is possible, the novel shoulder design hides the first two motors (M1 and M2, refer Figure 3a) behind the shoulder by employing a variant of Watt six bar mechanism with the geometry of a double parallelogram. This linkage creates a virtual center of rotation and provides a 1:1 transmission between actuator (M2) and virtual axis of rotation (see Figure 2 for ROM). The shoulder design provides large clearance between the shoulder motors and patient's head to avoid any collisions. To reduce the weight, the forearm mechanism is designed with reduced DOF as shown in Figure 3b. The joint for elbow flexion/extension and the forearm pronation/supination are active. Both joints are implemented as serial elastic actuators (SEA), with a measured spring between gear and joint output. The elbow actuator is a combination of a BLDC Maxon EC-flat 45 drive and a harmonic drive with a reduction of 100:1 and delivers 9 Nm nominal torque. The pronation and supination is provided by a Dynamixel MX28 servo drive with 2.5 Nm. The wrist is designed with a passive DOF that are fixable to prevent pathological movements between forearm and hand. All joints are placed in a serial kinematic configuration using the open joint concept [42]. Finally, a force controlled gripper allows to perform simple grasping tasks. Figure 2: Recupera exoskeleton configurations (left and right: full-body, middle: wheelchair) with its ROM and force/torque capabilities. Modules highlighted in green are used in both systems, mechanisms highlighted in blue only in the fullbody system. Full-body system (weight = 42 kg, height = 1.70–1.90 m) is driven by 28 brush-less direct current (BLDC) and 4 servo motors. Wheelchair system (weight = 8.6 kg, 29.2 kg including commercial wheelchair) is driven by 8 BLDC and 4 servo motors. Hip and ankle joints are designed as 3 DOF orientational joints in the exoskeleton. Here, a novel almost spherical parallel mechanism (ASPM), also known as Active Ankle, has been invented [37; 38; 43; 44]. In mechanism theory, it is typical to identify parallel mechanisms using their type (for details, see [45]). In this convention, different letters denote the joint type (R for revolute, P for prismatic, S for spherical etc.) and the underlined letter shows the active joint. The mechanical type of ASPM is 3-[R-[2-SS]] as depicted in Figure 2(c) featuring three rotative actuators fixed to the base. Each of the three motors drives a spatial quadrilateral consisting of a symmetric crank, two rods, and a line segment on the mobile platform. The mechanism's homogeneous and simple design leads to a low link diversity, permits a low-cost construction, and provides robustness against production inaccuracies. A crucial feature of the mechanism's design is the stress distribution among the structure. The six rods that transmit the forces from the cranks to the platform are only loaded with forces along their axes due to the spherical joints attached to them. Moreover, any force applied along the direction of the platform's torsional axis can be supported without an active torque in the motors. Figure 3: Various mechanisms in Recupera full-body exoskeleton. Knee Joint  
Instead of a bendable knee joint, a novel mechanism has been developed that allows the user to stand, sit (see Figure 3d) and move with the exoskeleton.