# Design of a Lower Limb Exoskeleton: Robust Control, Simulation and Experimental Results
## 1 Introduction  
Robots can be classified by the functions they perform in cooperation with humans; however, in this article, only exoskeletons for rehabilitation are discussed. Exoskeletons are very useful in the rehabilitation of patients with motor deficiencies. Some examples of these include upper and lower limbs that are designed to guide movement, support weight, align body structures, protect joints, or correct deformities. Rehabilitation can be classified into the following:  
* Passive rehabilitation: does not involve movement by the patient themself. * Active rehabilitation: involves the participation of the patient, who is the one performing the movement. The proposed prototype is intended to treat Anterior Cruciate Ligament (ACL) strains and tears because they are the most common type of knee injury [1]. ACL injuries are more common in athletes who play high-demand sports like basketball, football, and soccer. * Strains of Grade 1. They cause little ligament injury. Despite the fact that it has been considerably stretched, it could still aid in preserving the stability of the knee joint. * Strains of Grade 2. The ligament is stretched to the point of becoming loose. This is frequently referred to as a partial ligament tear. * Strains of Grade 3. This is a total ligament tear. The knee joint is unstable because the ligament has been split in half. In [2], the authors presented some rehabilitation exercises for ACL injuries, which include movements of flexion/extension of the knee and ankle joints. To develop rehabilitation exoskeletons, it is important to consider the analysis of movement of the biomechanics of the human being; for example, in [3], the main characteristics of the gait phases were determined to identify each gait phase. A long short-term memory-deep neural network (LSTM-DNN) algorithm was proposed for gate detection. There are a lot of prototypes used for rehabilitation, as in [4], where the exoskeleton was used for gait rehabilitation, or for patients with cerebral palsy as in [5]. In [6], the suggested robotic architecture considered the patients' kinematic performance in addition to their psychophysiological condition, for example, heart and respiratory activity and galvanic skin response. Ref. [7] presented the development of assist-as-needed robotic rehabilitation. The main objective was to minimize robotic device intervention when following an established trajectory in order to promote patient engagement and improve training sessions. It is very important that the prototype has the quality of being able to adjust to different patients, as in [8], where the authors discussed the lack of exoskeletons for minors, in particular the compatibility with children 6-11 years old. Ref. [9] presented a prototype that includes six degrees of freedom (DOF), divided into four for the lower limbs (two for knee joints and the other two for hip joints) and two for the system that permits standing and sitting activities. The exoskeleton was mechanically separated into two major components: the lifting system and the lower limb system, Figure 1 shows the exoskeleton prototype where we can observe that the joints corresponding to the ankles are missing. The actuation system is one of the most important variables influencing the design of an exoskeleton since it impacts the general performance, efficiency, and portability [10]. Electrical motors, pneumatic actuators, hydraulic actuators, and elastic actuators are the  
Figure 1: Exoskeleton for gait rehabilitation to assist in the task of standing–sitting the patient and the hip and knee movements [9]. most common types of actuators utilized in exoskeletons. Electric motors are used in most exoskeletons because they can be precisely regulated, and the energy source is easily available. However, some people prefer alternative choices because they have a greater power/weight ratio [11].