Introduction

This section provides a discussion about the various types of exoskeletons. Exoskeletons are classified by the location of the power sources of the lower exoskeleton. The actuators of the lower exoskeleton are usually placed on the joints of the human. The purpose of this part of the exoskeleton is to move the joint of the user. The actuator can be placed only one joint on each leg, such as the hip, knee or ankle; a combination between two joints (hip and knee, or knee and ankle); or a combination between three joints (hip, knee and ankle).

Hip Exoskeleton

Hips connect the upper limbs and the lower limbs. Human hips enable their owner to perform the motions of flexion/extension, abduction/adduction, and medial/lateral rotation (three DoF motions). These motions are required for a human to walk or run. Most researchers have placed actuators on the hips of the users for their exoskeletons. Moreover, Lenzi et al. [14] concluded that the hip exoskeleton enables a reduction of hip and ankle muscle activities.

Honda developed an exoskeleton called the Honda Walking Assist [15]. This device has one Direct Current (DC) motor on each hip. The force from the motor is passed to the thigh of the user through the straps, resulting in a light and neat exoskeleton. In another design similar to Honda Walking Assist [15], Giovacchini et al. [16] developed a hip orthotic whose actuator is located near the user’s hips. This exoskeleton helps the user to move their hip in the flexion and extension directions (Figure 4a). Moreover, this wearable robot is equipped with a passive actuator that enables the user to move in the abduction–adduction direction, resulting in user comfort.

The HiBSO (hip ball screw orthosis) exploits a ball screw in each leg for transmitting the force from a DC motor [17]. At the end of the ball screw is a strap that passes the actuation movement to the thigh (see Figure 4b). This structure has other movements beside the flexion/extension action. The HiBSO enables the user to move in the abduction/adduction direction in the hips, while allowing for the rotation of the thigh. Another wearable robot, Powered Hip Exoskeleton or PH-EXOS also added abduction/adduction motions and internal and external rotation [18]. These motions enrich its primary movements, namely flexion/extension, with the abduction and adduction motions being passive actions. The motors are placed on the waist of the user and are connected to the pulley through a Bowden cable, as seen in Figure 4c.

Asbeck et al. [19] constructed an exoskeleton called the Exosuit. They constructed the webbing straps with a geared motor that is carried on the user’s back. These straps are linked to the thigh of the user, as seen in Figure 4d. These straps perform by contracting and expanding on the leg during the heel strike until the terminal stance.

Figure 4. (a) exoskeleton developed by Giovacchini et al. [16]; (b) HiBSO (hip ball screw orthosis) [17]; (c) PH-EXOS [18]; (d) Exosuit [19].

Knee Exoskeleton

The human knee is an important object of study by researchers, because this part of the human body generates significant torque for walking [12], running [20], and movement from squatting to standing and vice versa [21,22]. Moreover, knees also restrain impact during those activities. Additionally, the position of the knee is between the hip and the ankle. Compared to the hip and ankle, the knee has a more straightforward movement—flexion/extension as well as rotation movements. However, for the sake of simplification, most studies of exoskeletons have modeled only one DoF for the knee exoskeleton, dedicated entirely to moving the knee in the flexion/extension actions.

A soft inflatable cushion is used as the actuator in an exoskeleton [23]. The inflatable part is placed behind the knee of the user. This component allows for the reduction of the weight of the exoskeleton. To inflate and deflate the component, a pneumatic system is used. This exoskeleton is inflated during the swing phase of the walking gait and deflated during the other phases of the walking gait cycle. Figure 5a shows this exoskeleton. Two DC motors are used to actuate two Bowden cables [24]. One of the cables is connected to a strap behind the lower thigh, while another cable is connected to a strap in front of the top of the thigh, as presented in Figure 5b.

Another activity that is often used while working is squatting. Human knees have an essential role in the squatting motion. The squatting action requires a high torque from the knees [21]. Moreover, several activities are required in the squatting movement. However, most wearable robots are not designed for this action. A passive one-DoF knee exoskeleton was developed by Ranaweera et al. [10] to help humans lift loads from the squatted position. This device employed two helical elastic springs on each knee. This component was connected to the pulley disk, which was placed behind the knees of the user. Figure 5c shows the prototype of this exoskeleton. Huang et al. [25] designed an exoskeleton to prevent injury because they observed that the squatting motion makes one susceptible to personal injury. This device can help the user to squat and walk without carrying any load. They utilized a motor and transmitted the torque to the gear and pulley by a flexion cable, as presented in Figure 5d. Meanwhile, an exoskeleton was designed for walking and kneeling. Wang et al. [26] developed a knee exoskeleton actuated with a motor and transmitted to a double pulley on the user’s knee. This configuration helps the user to walk and to assume a kneeling posture. The design of this exoskeleton is presented in Figure 5e.

Figure 5. Knee exoskeletons using (a) an inflatable actuator [23], (b) a Bowden cable put in the front and back of the leg [24], (c) springs [10], (d) a DC motor and pulley [25], (e) double pulleys [26].

Ankle Exoskeleton

According to Figure 2, the ankle has the most significant torque during the walking gait compared to other joints. This has caused most studies to place actuators in the ankle. This joint has four bones in three planar motions (three DoF). However, plantar or dorsiflexion movement is the primary movement during the gait cycle. Some researchers have simplified the motions on their exoskeletons to only one DoF for this main ankle movement.

Mooney and Herr [27] developed a one-DoF exoskeleton, as seen in Figure 6a. This equipment was intended to help the user to walk while carrying a load. This exoskeleton uses a brushless DC motor (BLDC) placed in the shank of the user, while the motor controller and batteries are attached to a vest. The device activates a fiberglass strut to pull the ankle of the user; the struts are connected to the boots. Another design called a soft exosuit was proposed by Asbeck et al. [28]. This equipment’s purpose is to assist the user in walking while carrying a load. Their exoskeleton is actuated by an electric motor; the motor, batteries and controller are placed in a backpack. This motor is connected to a Bowden cable. The purpose of the cable is to pull the heel of the user (see Figure 6b). The cables behave similarly to the human calf muscle. Another exoskeleton was designed by Bai et al. [29]. This device is used for the therapy of subjects suffering from ankle injuries. Figure 6c shows this exoskeleton. An electric motor is mounted in front of the shinbone, while the motor control and batteries are carried on the subject’s waist. The belt is used to transmit the movement of the motor to the gear. This system functions to activate the subject’s ankle. An ankle exoskeleton powered pneumatically was devised by Shorter et al. [9]. The movement of the subject’s ankles is actuated by a rotary pneumatic actuator. This device is attached at the ankle of the user. This exoskeleton utilizes two valves, with the air source placed on the subject’s waist.

Some studies have attempted to add additional DoF of the ankle exoskeleton, such those of Carberry et al. [30], Agrawal et al. [31], and Park et al. [32]. A two-DoF ankle exoskeleton was proposed by Carberry et al. [30]. They enhanced their exoskeleton with eversion/inversion movements. This device is intended for post-stroke rehabilitation. The mechanism of the exoskeleton is pneumatically actuated, with the air source placed separately from the exoskeleton, as shown in Figure 6d. Agrawal et al. [31] developed another two-DoF exoskeleton which has a motion similar to Carberry’s. They combined an active joint with a passive joint. The flexion/extension ankle motions are controlled by a DC servomotor, while the inversion/eversion motions are controlled use a spring and damper mechanism. Three pneumatic synthetic muscles are used to simulate the human leg muscles [32], as seen in Figure 6f. This system enables the user to move their ankle with two DoF using these devices with movements similar to the natural ankle.

Figure 6. One-DoF ankle exoskeleton using: (a) DC motor and struts [27]; (b) motor and Bowden cable [28]; (c) DC motor and belt [29]; (d) two-DoF with active actuators [30]; and (e) two-DoF active pneumatic actuators [32].

Multiple Joints Exoskeleton

To actuate a joint, more than one muscle that passes through multiple joints may be required. Some studies have utilized more than one actuator to actuate joints in their exoskeletons. The actuators actuate a combinations of joints, namely hip and knee; knee and ankle; and hip, knee and ankle. These multiple joint exoskeletons need to have a more advanced control system than single joint exoskeletons, because the movement of the joints has to be controlled in such a way that results in a harmonious gait. These exoskeletons are discussed in this section.

Some researchers have proposed a knee–ankle, two-DoF exoskeleton for rehabilitation. The National University of Singapore (NUS) developed a device to rehabilitate stroke patients. They used a series elastic actuator (SEA) to actuate the knee and ankle joints [33]. The SEA is a connection between a motor with a serial spring. The actuators are placed between the joint and the thigh or shank of the user, with each actuator using a crank and a connecting rod to move the leg of the user. Figure 7a shows this exoskeleton. The WAKE-up (wearable ankle knee exoskeleton) also utilized an SEA [34]. The actuators are chosen to prevent direct contact between the user and the actuator. To transmit power, a timing belt is used. This device is a modular exoskeleton, which can be worn for single joints or for multiple joints at once.

In 2006, a prototype hip–knee exoskeleton was developed by University of Twente. This exoskeleton is called the LOPES (lower extremity-powered exoskeleton) and is intended as a rehabilitation device [35]. This device has actuators on the hip and knee. This design enables the user to move the hip on the flexion/extension and abduction/adduction directions, as well as the flexion/extension direction on the knee. The actuator is actuated by a motor and transmitted to the SEA using a Bowden cable. The LOPES is shown in Figure 7b. For rehabilitation purposes, the clearance of the foot must be sufficient so that this exoskeleton does not actuate the ankle during the gait.

Another type of combined exoskeleton is the hip, knee and ankle exoskeleton, in which all joints of the user’s lower limb are actuated. In 2004, the University of California developed an exoskeleton called the Berkeley Lower Extremity Exoskeleton (BLEEX) [3]. This exoskeleton is actuated using linear hydraulics and has the ability to provide additional power for carrying heavy loads. The two DoF of the hip are actuated using active actuators, while the rotation of the hip is passively actuated by using springs and elastomers. One motion each is actively actuated in the knee joint and the ankle joint. The BLEEX is shown in Figure 7c. The exoskeleton constructed by the University of Salford is an example of an exoskeleton that uses pneumatic muscle actuators (PMA) [36]. This equipment is intended for paraplegic patient rehabilitation. They equipped their exoskeleton with PMA to actuate the hip with three DoF—only one each for the knee and ankle.

Figure 7. Multiple joints exoskeletons: (a) the National University of Singapore (NUS) exoskeleton [33]; (b) the lower extremity-powered exoskeleton (LOPES) (all lower limb joints) [35]; and (c) the Berkeley Lower Extremity Exoskeleton BLEEX [3].