Actuators and transmission mechanisms in rehabilitation lower limb exoskeletons: a review

Introduction

Over the past decade, rehabilitation lower limb exoskeletons (RLLEs) have become indispensable due to the rapid increase in the number of patients suffering from neurological diseases, such as spinal cord injuries (SCIs) and muscle weakness [1]. A Malaysian epidemiology study conducted by Ibrahim et al. [2] indicated that vehicle accidents are the main contributors to SCIs. Similar trends worldwide were reported in the National Spinal Cord Injury Statistical Center [3] and in Kang et al.’s [4] work, where vehicle accidents dominate SCI cases, which are gradually increasing in line with the increase in human activity.

Patients suffering from spinal cord injuries normally lose motor skills and self-balancing ability while walking. Appropriate therapy or motion training through active muscle activation during a chronic phase can increase the opportunity for healing. Thus, utilizing an RLLE device is one approach that can be implemented to improve recovery or regain lower limb function. RLLE devices can be used to address the weaknesses of traditional rehabilitation interventions, such as repeatability, to provide consistent and comprehensive observations and monitoring of a patient’s progress and training duration, and provide data analysis and recoding, thus reducing the burden on clinicians in hospitals and clinics [5].

RLLEs are human-oriented mechatronics systems that directly interact with patients to optimize the retention of physiological movement patterns via activity-based interventions. The RLLE device consists of a mechanical structure, actuators, sensors, power supplies and controllers [6]. The actuator system employs power transmission mechanisms, which must comply with human biomechanics. For this reason, the actuator and power transmission mechanism are the components that are crucial for ensuring that the RLLE operates in an effective, safe, comfortable and smooth motion. Therefore, regarding the use of actuators and power transmission mechanisms, characteristics such as linearity, compliance, torque, shock load effects, backlash, compactness, reliability, controllability, and noise need to be considered [7, 8].

To investigate the appropriate actuator criteria, a review of the technical characteristics and the status of advanced multiple joints of RLLE is presented. We analyze the actuator characteristics, and the power transmission mechanism technology from 1996 to 2022 is discussed. Finally, development issues and possible research directions related to actuators and power transmission mechanisms are presented. To obtain a collection of publications within the scope of our review, we performed a search using Scopus databases. The search was conducted using the following set of keywords: lower limb exoskeleton, exoskeleton, rehabilitation robotic, wearable robot and rehabilitation exoskeleton.

The number of initial articles obtained was 20,349. After reviewing them, the number of articles related to the field of review reached 500. Papers with insufficient information on actuator and power transmission designs and those pertaining to studies published in languages other than English were eliminated. As a result, 210 were selected, incorporating active actuators and power transmission mechanisms within the realm of RLLE. Subsequently, 45 research papers that met the criteria for this review underwent in-depth investigation. The focus criteria encompassed the exoskeleton’s target joints (hip, knee, and ankle), rigid structure, actuation types (electric motor, pneumatic, hydraulic, and hybrid actuators), and mechanical transmission of power (gears, belts, linkages, or other mechanical systems).

Types of RLLE joints

An RLLE is a mechanical structure that mimics the lower limb extremities specifically and is designed to help patients undergo physical rehabilitation. The RLLE joint is used for enabling movement and providing stability. By replicating the RLLE joint based on human joints, the mechanical structures of an RLLE can be designed to closely match the human limb’s movement. In RLLEs, the joint can be divided into two categories: multiple joints and single or partial joints. Multiple joints mean that more than one joint or all joints (hips, knees, and ankles) are actuated, while a single joint is actuated by only one joint, at the hip or knee, or ankle joint.

Types of RLLE structures

In RLLE design, the structure type can be categorized into two groups: overground devices and immobile devices [9]. Each category has different operation principles and technical requirements. In the overground types, RLLEs have been designed for patients who are able to stand and walk but require assistance with stability and mobility. These RLLEs typically provide power assistance to the patients’ legs, allowing them to walk on the ground. Therefore, the walking area is not restricted. Two operation modes are employed in overground RLLEs: body weight-supported (BWS) and unsupported body weight (UBW). Among the overground types, RLLEs have been developed with the aim of increasing the independence of gait training.

However, immobile RLLEs are used for patients who are unable to stand or walk on their own due to conditions such as spinal cord injuries or severe neurological conditions. Gait training is conducted in a fixed area and a confined space. Training applications include supporting the body weight or trunk of a patient with a device structure, walking on a treadmill, and using a stationary device, such as a therapeutic exercise device. Two modes of rehabilitation exist in an immobile RLLE. The first mode is the fully assisted mode, in which a patient does not actively participate or no effort is needed in a rehabilitation session. The second mode is the assist-as-needed mode, in which an RLLE provides appropriate motion on the lower limb of the human joint by supplementing the effort already being applied by the human.

Rehabilitation approaches

RLLEs are widely used by therapists in clinics or hospitals for treatment or rehabilitation. Some devices previously at the research stage, such as ReWalk, REX Rehab, Indego Ekso, Therapy and HAL, have successfully obtained approval from the U.S. Food and Drug Administration (FDA) for clinical use and have been commercialized [10]. From a clinical perspective, for the rehabilitation of patients with ambulation impairments, such as SCI, foot drop, stroke and Parkinson’s disease, two rehabilitation approaches are generally utilized: restoration of the neuromuscular system and behavioral compensation [11].

Restoration of the neuromuscular system is aimed at regaining function using preinjury motor behaviors [12]. RLLEs for rehabilitation, regaining locomotion and treating muscular weaknesses are categorized as restorative devices. The motion of the devices can be realized by trajectory-based control or non-trajectory-based control [13]. Many controller strategies have been proposed for these training approaches, such as sensitivity implication, predefined gait trajectory, and model-based control [14]. LOKOMAT [15] and ALEX [16] are examples of devices that belong in the restoration category.

Behavioral compensation refers to the use of typical motor patterns, behaviors, body segments, and assistive devices to compensate for postinjury neurologic deficits to accomplish the same or similar functions [17]. The device that assists with movement for people with chronic neuromuscular motor impairments is also a compensatory device, which is used by nondisabled subjects, such as elderly individuals, for movement augmentation. ReWalk [18] and REX [19] are examples of devices that belong in the compensation category. However, these devices have a rigid structure, are designed to be parallel to the legs of users, and are bulky. Moreover, their weight can make them difficult to use and transport. They also have limited capabilities to support rehabilitation exercises at home; thus far, these devices have been used mostly in clinical environments [20]. The other aspects that need to be considered are safety and dependability, ease of implementation and removal, and comfortability [21, 22]. Moreover, compensatory use in activities of daily living environments leads to additional requirements regarding robustness against dirt, humidity, and misuse.

Actuator selection in RLLEs

The choice of the actuation type plays an important role in RLLEs since it generally determines the performance, such as the speed, torque and portability. There are two considerations of actuator selection in RLLE development: the structure types and actuator characteristics.

Power transmission mechanism

The power transmission mechanism is designed to deliver torque from the actuator to the RLLE joint. Some RLLEs have different power transmission designs between joint locations, i.e., hip, knee, and ankle. Each design also has advantages and disadvantages. The following discussion addresses the power transmission mechanism of the active actuator, i.e., electric motors and pneumatic, hydraulic and hybrid actuators in RLLEs from the perspective of design.

The design of the power transmission mechanism of an RLLE is categorized based on the types of actuators used. First, exoskeleton joints are actuated by a single-type actuator, such as an electric motor, pneumatic or hydraulic. This category consists of (i) a direct-driven mechanism; (ii) a fixed-hinge slider mechanism; (iii) a tendon-driven mechanism; (iv) a push–pull cable (PPC) mechanism; (v) a chain-and-gear mechanism; (vi) an antagonistic mechanism; and (vii) a double-ended-rod mechanism. Second, exoskeleton joints are actuated by a hybrid-type actuator, that is, (i) a passive-electric motor direct mechanism; (ii) an antagonistic and direct mechanism; (iii) an actuator-spiral spring mechanism; and (iv) an elastic-tendon mechanism. All these types of mechanisms are illustrated in Figure 1(A–Q).

Figure 1:

Power transmission mechanism (A) directly mounted via a speed reducer or a torque amplifier, (B) directly mounted via a gear train, (C) directly mounted via a rotation-to-linear converter, (D) fixed-hinge slider configuration, (E) tendon-driven configuration, (F) PPC configuration, (G) chain-and-gear configuration, (H) antagonistic pair of pneumatic actuators connected by linkages, (I) antagonistic pair of actuators connected by a pulley and belt system, (J) antagonistic pair of actuators connected by the pulley and belt system, (K) antagonistic pair of actuators connected by a lever arm mechanism, (L) antagonist pneumatic actuators mounted on brackets, (M) double-ended-rod configuration, (N) passive-actuator direct configuration, (O) antagonistic and direct power transmission, (P) actuator-spiral spring mechanism, and (Q) elastic-tendon mechanism.

Discussion and future directions

Studying previous studies is useful for realizing a more advantageous RLLE. The actuator type and the power transmission mechanism play key roles in RLLE development and thus greatly impact motion and human‒machine interaction. When actuators are compared for RLLE applications, electric motors still offer a considerably better choice that is easy to control and compact in joint configuration than fluidic-based actuators. Electric motors are a promising technology for achieving portable devices. Batteries can be easily used to power such devices, facilitating portability and silence operation that result in comfortable patient-device usage. Pneumatic and hydraulic actuators are costly due to the frequent service needed to maintain satisfactory actuator efficiency. In addition, the auxiliary systems are complex, and the power transfer mechanism is bulky, thus limiting their portability. Table 2 summarizes the types of power transmission mechanisms with compatible actuators and their advantages and disadvantages for RLLEs.

Researchers have focused on compact pneumatic actuators. Sun et al. [96] introduced an electric motor, micro-pump, tank, and other components, which were integrated into a module. However, it was still bulky compared to electric motors. Actuators with characteristics such as a small volume, light weight, high power-to-weight ratio, compactness, silent operation, low bulk, low maintenance, low cost, and high-power storage capacity are in demand. Moreover, the smaller auxiliary systems that need low energy that can be supplied by batteries provide enough of the necessary power output, which will be demanded in the future.

Next, we show the comparison of actuator characteristics for RLLEs by 10 criteria, as shown in Table 3. The results show that DC motors are more suited to 10 criteria for suitable and relevant use in RLLE applications for immobile and overground application domains. To improve RLLEs, such as by improving the compliance, a hybrid actuator that contains a DC motor and passive element is also the best choice for rehabilitation domains. In contrast, hydraulic actuation is an unsuggested choice for RLLEs for both immobile and overground application domains.

In RLLE safety, a restricted range of joint rotation can be achieved through three approaches: mechanical methods, electrical methods and software-based methods. Mechanical approaches use a mechanical stopper to limit the joint’s range of motion and prevent the exoskeleton from damaging itself in the case of sensor or software failure. The electrical approach is based on the control system, which limits the range of motion. The joint is equipped with a limit switch or sensor to send a signal to the control system, instructing it to halt the rotation when it is over the limit. The software-based approach involves the control algorithm, monitoring the forces and torques applied to the joint, and limiting the motion when the forces and torques exceed a certain threshold. When the forces and torques exceed a specified limit, the controller instructs the motor to shut down and sends a brake command to hold or free the joint. Other safety features need to be considered, i.e., a failsafe feature due to power failure. For example, in commercialized RLLEs, i.e., Indego and Ekso, the hips become free, and the knees lock. In ReWalk, the device slowly collapses [23]. These measures are very important for protecting a wearer from serious injury. These conditions require that an actuator be equipped with internal mechanical breaking or free mechanisms. Therefore, it is crucial to design a safety mechanism for actuators that is integrated with a power transmission mechanism to minimize risky situations.

The actuators and transmission mechanisms of RLLEs are not fully adjusted for ergonomic and biological features. Four points are suggested to be considered. First, the current RLLE actuators and power transmission mechanisms still face challenges regarding the misalignment between RLLEs and the joints of patients. The rigid structures of RLLEs do not replicate the human anatomy perfectly, thus causing misalignment [97, 98]. For example, the knee joint is a complex joint that is not monocentric. It provides a combination of rolling and sliding motions between the femur and the tibia, and the size of the femur and tibia bone differ among people [99]. Knee anatomy requires the development of a mechanism and configuration that matches and aligns with the human knee joint. Even if a small misalignment between the RLLE and the patient’s joint occurs, it can result in unnatural and undesired translation forces, thus causing undesired parasitic forces on the patient’s skeletal system during therapy sessions.

There are three sources of misalignment: migration of the instantaneous center of rotation or slippage during operation, inaccurate alignment while wearing the RLLE, and an inaccurate RLLE replicating the human anatomy [100]. This effect induces undesired interaction forces that can potentially constrain the movements of the user and cause discomfort, pain, or even injury. There are several joint mechanism designs, including self-alignment approaches: Schmidt coupling on the knee joint [101], parallel self-alignment on the ankle joint [102], adjustment of the structure length via slots [103] and three DOFs via sliders and hinges [104]. A study by Bartenbach [105] has shown that misalignment compensation can be achieved by allowing the interfaces to move along the rigid exoskeleton structure. Linear ball bearings are integrated into each cuff attachment, enabling the cuffs to slide up and down the tubes of the leg device to avoid constraining forces resulting from misalignment between the patient’s joint and the exoskeleton’s joint. Research is also being conducted on the use of soft robotics in RLLE design. Soft robotics are made of materials that are compliant and can adapt to the shape and movements of the body, which may reduce misalignment issues [106].

Second, RLLE actuators and power transmission mechanisms should be designed in the position corresponding to the representative muscle in the human limb and the trunk to mimic the functions of muscles during motion. Thus, in RLLEs, actuators and their mechanisms should be designed with the objective of achieving natural characteristics and not interfering with physiological motion. The tendon sheath artificial muscle is an example configured with consideration of this characteristic [107]. Therefore, the flexion and extension motions of RLLEs can be employed by double-acting linear actuators and a fixed-hinge slider mechanism. The double-acting linear actuator should be arranged to mimic the function of the biceps femoris on the knee and the rectus femoris on the hip. However, if a single-acting actuator is utilized, an antagonistic mechanism should be considered to mimic the rectus femoris, the gluteus maximus on the hip and the vastus maximus and biceps femoris on the knee.

Third, we suggest that future studies on RLLE actuators and power transmission mechanism designs should focus on compliance or spring-like characteristics. The requirements of the compliance features are important for reducing mechanical impedance due to physical interactions among the exoskeleton, the patient and the ground. Usually, RLLEs that are primarily driven by single-type active actuators and configured as shown in Figure 1(A)–(P) are confronted with rigid actuation without compliance, leaving the impression of large vibration and shock. Compliance actuators, such as SEAs and variable stiffness actuators, have emerged to overcome weaknesses with regards to the compliance issue [108]. Indeed, SEAs offer the advantages of energy reduction, back-drivability and smooth motions [40, 109]. Additionally, compliance behaviors can be achieved with pneumatic actuators [33, 110]. However, designing compliance actuators remains challenging due to the complexity of adjusting the stiffness of the elastic element and spring stiffness. Furthermore, actuators with compliance characteristics should meet torque control stability and low intrinsic output impedance requirements. The light weight and compactness of overground exoskeletons are essential. Therefore, solutions to these issues are still in high demand [7, 111, 112].

Fourth, an immobile RLLE-type device using a treadmill as a walking platform does not require precise positioning, but mechanical safety is inherently important. Many studies have shown that pneumatic actuators have properties similar to human muscles. Due to these advantages, we suggest that applications of the pneumatic actuator on immobile RLLEs should be explored and subsequently marketed. To reduce the weight exerted on the patient’s limb by the immobile RLLE-type, actuation systems can be placed proximally. Thus, demanding remote actuation can be realized through the pneumatic type of actuation or using Bowden cables if actuated by the electric motor. Myosuits have become prominent; thus, further research is necessary to assess their potential for replacing the rigid and heavy structure of RLLEs [106].

Conclusions

In this paper, the selection criteria of the actuator and the power transfer mechanism design were reviewed. Moreover, 210 studies on RLLE devices from 2009 to 2022 were studied. The information regarding actuators and power transmission mechanisms provided in this study will be useful guidance for researchers and developers to investigate technological implementations of RLLEs. From the state-of-the-art perspective, the actuator type and the power transmission mechanism play key roles in RLLE development and thus greatly impact its motion and safer human‒machine interaction. After reviewing the wide variety of RLLE devices, electric motors and pneumatic and hybrid actuators have been used in RLLE devices in research and commercial devices. Electric motors offer a considerably good choice for both RLLE structure types, i.e., overground and immobile devices. It is in line with the actuator characteristic, in which electric motors offer a good choice due to being easier to control, compact in design, and configurable at a joint rather than fluidic-based actuators. Moreover, muscle-like actuators have the advantages of flexible structures and light weight. However, there is a demand to improve the low stiffness to ensure that the weights of bodies are supported and provide safety in human‒machine interactions. Furthermore, the designed power transmission mechanism, which is classified into two categories, was investigated in this paper. First, RLLE joints are actuated by a single type actuator, that is, an electric motor, pneumatic or hydraulic. Second, exoskeleton joints are actuated by a hybrid type. There are many types of power transmission mechanisms that are being used. The characteristics of backlash, less friction, simple mechanism configuration, bulkiness, and reduced energy usage should be considered when designing the power transfer mechanism. Therefore, the trade-off between the advantages and disadvantages of each type of power transmission mechanism must be considered. Future research and development of actuators and power transmission mechanisms are expected to considerably increase simultaneously with the evolution of RLLE applications. There is a trend in developing RLLEs to assist patients with rehabilitation activities outside the clinic environment, such as at home. Therefore, more sophisticated adoption of the technology is needed. With this shift in focus, a new design or improvement of an actuator of the RLLE with characteristics such as compactness, lightness, less energy usage, and compliance is also needed.