Researchers have given attention to lower limb exoskeletons in recent years. Lower limb exoskeletons have been designed, prototype tested through experiments, and even produced. In general, lower limb exoskeletons have two different objectives: (1) rehabilitation and (2) assisting human work activities. Referring to these objectives, researchers have iteratively improved lower limb exoskeleton designs, especially in the location of actuators. Some of these devices use actuators, particularly on hips, ankles or knees of the users. Additionally, other devices employ a combination of actuators on multiple joints. In order to provide information about which actuator location is more suitable; a review study on the design of actuator locations is presented in this paper. The location of actuators is an important factor because it is related to the analysis of the design and the control system. This factor affects the entire lower limb exoskeleton’s performance and functionality. In addition, the disadvantages of several types of lower limb exoskeletons in terms of actuator locations and the challenges of the lower limb exoskeleton in the future are also presented in this paper.
1. Introduction
Nowadays, people work even more strenuously and require stronger and longer lasting muscle movements. However, human muscles have a fatigue limit when doing regular and repetitive activities. To help overcome this fatigue limit, some researchers have suggested that humans use an external wearable device, i.e., an exoskeleton. An exoskeleton, also known as a wearable robotic, is a system that can be worn to help human beings to support and protect parts of their bodies [1]. Such a device has been used for many applications, including enhancing workers when doing their jobs or as medical tools for rehabilitation. In many industries, exoskeletons have been used to increase worker strength for walking on long journeys [2] or lift heavy items [3]. In the medical field, exoskeletons have been used to assist patients who have lost their ability to walk due to spinal cord injuries, stroke, and other trauma [4]. Coenen et al. [5] reported that rehabilitation exoskeletons can improve the quality of exercises during rehabilitation and can accelerate recovery process.
The application of the exoskeleton to the human body can be divided into three locations: (1) throughout the human body [6], (2) at the upper part of human body, such as the torso and arms [7], and (3) at the lower part of the human body, i.e., from the waist down [8]. Various parts of the human body simultaneously play certain functions in supporting movement during walking. However, the lower limbs of the human body have more important roles than the other parts. This is because the lower limbs generate more torque than other parts while walking. This paper reviews a number of existing published papers related to lower limb exoskeletons. However, this paper limits its discussion to the classification of joint motions and types of actuators of the exoskeleton.
The joints in the lower limb of the human body are the hips, knees, and ankles. Each joint has different abilities to move or degrees of freedom (DoF), as shown in more detail in Table 1. The types of lower limb exoskeletons based on joint motions are differentiated into several types based on how the actuators drive the exoskeleton. The actuators can drive just the hips, the knees, or the ankles. In a small number of studies, exoskeletons have multiple actuators to drive a combination of joints. These combinations of actuators are hips and knees, knees and ankles, and all three joints (hips, knees, and ankles).
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Figure 4. (a) exoskeleton developed by Giovacchini et al. [16]; (b) HiBSO (hip ball screw orthosis) [17]; (c) PH-EXOS [18]; (d) Exosuit [19].
TBI Rehabilitation 