applied sciences  
Active Ankle-Foot Orthosis Design and Computer Simulation with Multi-Objective Parameter Optimization [PERSON]\(\copyright\), [PERSON]\(\copyright\) and [PERSON]  
also found downsides of FES in the literature, including their complexity of setup, skin irritation, and pain. Regarding AFO solutions, we can mention other orthosis devices for foot-drop, like the cable-driven wearable robots studied by [PERSON] and [PERSON] [7]. These devices are used more often as exoskeletons, and the authors concluded that cable-driven devices solve problems of excess weight and misalignment, having successfully addressed the issue of foot-drop. Moreover, end-effector-based robots have been used exclusively for rehabilitation purposes, and a significant disadvantage of this type of power transmission is that the amount of applicable shear force is limited by the comfort level of the wearer. Among the solutions presented with this technology, [PERSON] et al. [8] proposed a bidirectional tendon driven actuator to assist dorsiflexion and plantarflexion, where they presented successful results, but with limited validation. Technologies using electrical motors as actuators can be found in the literature; for instance, [PERSON] et al. [9] presented a lightweight but strong and wearable AFO with dorsiflexion assistance, weighing just 0.5 kg, reporting good results. This work does not mention a compliance element in the orthosis design, resulting in rigid actuation, which could be dangerous since external disturbances might damage the servomotor. An important matter in the design of orthosis devices is their simulation by computers. The simulation of the gait cycle has been addressed and explored by different authors using various tools. For example, [PERSON] et al. [10] simulated a robot foot in Simulink, where they proposed a platform to study human gait behavior, ensuring that it is possible to provide the main dynamic features of various cases of foot drop. Another standard software used for simulations and optimizations of orthosis devices is OpenSim, as in the case of [PERSON] et al. [11], who used dynamic optimization of anchor point positions in a cable-driven exosuit. Three main complications must be solved in AFO design for foot-drop conditions: two of them are the slapping of the foot after the heel-strike (foot slap) and the dragging of the toe during the swing (toe drag) [12], and the third complication is ensuring that the natural push-off movement is not interrupted [13]. To address these challenges, we investigate the possibility of designing an ankle-foot device that will assist with walking only during the dorsiflexion stages of the gait, avoiding interference with other stages, and we propose that this design can be improved by the use of multi-objective optimization to obtain a static set of parameters that are applicable to varying initial conditions. Hence, we present the design of a light, low-cost, and functional active AFO for foot-drop conditions. In addition, we simulate our device acting on a lower leg model with a foot-drop condition during the gait cycle, in a controlled environment, using the Multi-Joint Dynamics with Contact (MuJoCo) software, version 2.3.5. This simulation is used to address improvements and execute optimizations of the active AFO design parameters. The proposed simulation makes the design configurable through an optimization process. Three objectives were proposed to warrant ankle angle accuracy, minimize oscillations, and reduce energy consumption. At the beginning of the study, we assumed that a user would achieve a gait pattern similar to a normal one after some time wearing the device. Like [PERSON] et al. [14], a database was used to obtain a reference signal to track the ankle angle. We apply two multi-objective optimization (MOO) algorithms to tune the parameters of two PD controllers and the mechanical properties of the suspensions. The sets of solutions obtained with each algorithm are compared to analyze their convergence and diversity.