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AReviewofLowerExtremityAssistiveRoboticExoskeletonsinRehabilitationTherapy

ArticleinCriticalReviewsinBiomedicalEngineering·January2013

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A Review on Lower Extremity Assistive Robotic

Exoskeleton in Rehabilitation Therapy

Gong Chen, Chow Khuen Chan, Zhao Guo, & Haoyong Yu*

Department of Biomedical Engineering, National University of Singapore, Singapore

*Address all correspondence to Haoyong Yu, Department of Biomedical Engineering, National

University of Singapore, 9 Engineering Drive 1, Singapore 117575 (Tel: +(65) 6601-1590; Fax: +(65)

6872-3069; E-mail: bieyhy@nus.edu.sg).

ABSTRACT: The rapid advancement of robotics technology in recent years has pushed

the development of a distinctive field of robotic applications, namely robotic exoskeletons.

Due to the aging population, more people are suffering from neurological disorders such

as stroke, central nervous system disorder and spinal cord injury. As manual therapy

seems to be physically demanding for both treated patient and therapist, robotic

exoskeletons have been developed to increase the efficiency of the rehabilitation therapy.

Robotic exoskeletons are capable of providing more intensive patient training, better

quantitative feedback and improved functional outcomes for patients compared to manual

therapy. In this review, emphasize is placed on treadmill-based and over ground

exoskeletons for rehabilitation. Analyses of their mechanical designs, actuation systems

and the integrated control strategies are given priority as the interactions between these

components are crucial for the optimal performance of the rehabilitation robot. The review

also discusses the limitations of current exoskeletons and technical challenges faced in

exoskeleton development. A general perspective of the future development of more

effective robot exoskeletons, specifically real-time biological synergy-based exoskeletons,

could help promote brain plasticity among neurologically impaired patients, and allow

them to regain normal walking ability.

Keywords: Neuro-rehabilitation, robotics, exoskeleton, control strategies, rehabilitation

therapy, robotic rehabilitation, mechanical design, actuation system

ABBREVIATIONS: BWS, body weight support; CoP, center of pressure; DOF,

degree of freedom; EEG, electro-encephalograph; EMG, electromyography; FES,

functional electrical simulation; IMU, inertia measurement unit; PAM, pelvic assist

manipulator; POGO, pneumatically operated gait orthosis; SCI, spinal cord injuries; SEA,

series elastic actuator

1. Introduction

Computer technology has led to advancements in the development of lower extremities

robotic exoskeletons for rehabilitation therapy. There have also been an increasing number of new

exoskeletons which are designed to perform rehabilitation dedicated to certain patients. Hence, there

is no question that in future there will be an improvement in the medical rehabilitation therapy as it

seems no exact or proper medical cure for physiologically challenged subjects especially the paralytic,

neurologically damage [1] and spinal cord injured patients.

The designation of the lower limb robotic exoskeleton has been a complicated task as the

human walking is relatively complex, and rhythmic or likewise. The human walking involves the

coordination of brain, nerves and muscles whereby the lower and upper limb are systematically

controlled to generate the necessary force for locomotion. It is common knowledge that no two

individuals have identical walking styles. Diseases such as altered walking posture of the elderly,

reduction in walking ability caused by orthopedic disease (e.g. fracture or amputated limb), central

nervous system (CNS), and spinal cord disease or injury [1] make the rehabilitation process more

complicated and difficult to be treated either manually or with the assistance of devices. Therefore,

considerable number of factors needs to be taken care of in order to obtain a desirable design that

covers a wide range of DOFs in improving the mobility besides improving the neuroplasticity of the

patients [2-4]: severity of the impaired lower limb, mechanical design, actuation system, and control

strategy of the robotic exoskeleton.

One of the drawbacks of manual rehabilitation training, especially for the treadmill training, is

that it is not a very intensive training as the training time is limited to the personal trainer. With this

limitation of time, the gait pattern of the patient is not reproducible and thus the entire therapy is not

optimum. As for some severe SCI patients, the patient’s leg movements need to be assisted by at least

two therapists and in certain cases, a third therapist might even be needed to assist with the

stabilization of the pelvis movement. Moreover, the therapists will experience physical constraint and

ergonomically bad positions, in some circumstances [5]. With these physically demanding conditions,

the therapist will be exhausted and this further explains that manual rehabilitation training requires

great physical efforts [6-11].

Due to the difficulties in the manual rehabilitation therapy in correcting the ambulation of the

patients, robotic exoskeletons are suitable in taking over strenuous and repetitive task. The therapists

are then able to focus on more meaningful task such as interacting with patients, assessing the

outcome of the therapy, and intervene the training session if necessary. Optimal robotic training

programs can be designed according to the patient’s condition as this will improve the training quality

and motor recovery level [2-4]. Considerable studies have been reported to reveal the feasibility and

efficiency of the robotic exoskeletons [6-11], particularly on mechanical design point of view together

with its associated actuation system and control strategy.

The paper is organized as follows: Section 2 gives a description of the gait rehabilitation

devices that are currently available in the content of robotic rehabilitation exoskeleton. Section 3

presents the various types of gait rehabilitation exoskeletons including the treadmill-based gait

rehabilitation exoskeleton (Section 3.1) and exoskeleton for over ground gait training (Section 3.2). A

remarkable focus is given in the design consideration of the robotic exoskeleton in Section 4. Section

5 provides a description of the future challenge that can be a new paradigm shift in improving the

robotic exoskeleton technology.

2. Gait Rehabilitation Device

Robots can be designed to rehabilitate certain parts of the lower limb with different functions

and purposes. On the whole, the device can be classified into two groups: non-mobile robots and over

ground rehabilitation robots (Figure 1).

In the first group, immobile robots are devices that enable the users to receive gait training in

a fixed and confined area which include treadmill-based BWS system, treadmill-based exoskeleton,

BWS foot plate trainer and stationary device. Robotic gait rehabilitation trainer (RGR), an example of

treadmill-based based BWS system, offers the patient with the assistance of a BWS system but

unactuated joints [12]. Conversely, exoskeleton employs the treadmill-based mechanism with the

assistance of BWS system while the subject is walking on the treadmill. In this design, the BWS

system is required to maintain the balance while the exoskeleton is used to provide assistance to the

movements of the legs. The rehabilitation device, called as the treadmill-based exoskeleton, has been

widely used as this device has been implemented in Lokomat [13], ReoAmbulator [14], ALEX [15],

LOPES [16], University of Auckland system [17], POGO [18] and PAM [18]. The third gait

rehabilitation device, which is known as the BWS foot plate trainer, has an objective to provide tactile

feedback to patient during the gait rehabilitation therapy. In this foot plate system, the foot plate is

considered as the end effector of the robot instead of a wearable exoskeleton. Nevertheless, this BWS

foot plate mechanism, which is implemented in Gait Trainer GTI [19], GaitMaster 5 [20] and

LokoHelp [21], has the same objective in providing the controlled movements to the patient’s feet

besides giving assistance via BWS. Also, considerable attention is given in controlling the gait

trajectory of the feet in the BWS foot plate system. Nevertheless, there is no assistance provided to the

joint of the lower limb. The fourth device, which is called the stationary device, provides guided

movements in obtaining the effective muscle strengthening and endurance, instead of providing focus

to the locomotion function. As per date, Motion Maker [22], Lambda [23] and AIST Tsukuba [24] are

the devices that utilized this stationary device.

Figure 1.(a) treadmill-based BWS system, (b) treadmill-based exoskeleton, (c) joint level device,

(d) portable exoskeleton, (e) mobile robotic trainer, (f) exoskeleton with mobile platform.

In the second group of gait rehabilitation device, over ground rehabilitation robots are

designed to allow the subjects to walk over ground and increase the independence of gait training. In

this over ground rehabilitation robot, the mobility of the patient is improved tremendously as the

patient is not confined in the walking area or restricted to the sagittal plane movement. Examples

types of robot are portable exoskeleton, mobile robotic trainer, joint level device and exoskeleton with

mobile platform, are displayed in Figure 1. Joint level device is designed for single joint rehabilitation,

for example MIT ankle robot [25, 26], Ankle-Foot Orthosis [27] and SUkorpion AR [28]. Portable

exoskeleton is an over ground orthosis for rehabilitation training and assistance in daily life, which is

widely applied in robots such as ReWalk [29], Ekso [30], Indego [31], KAFO [32], NUS robot [33],

BLEEX [34-36], HAL[37-38] and PGO [39]. It provides assistive force to individual’s lower limb

joints or both legs and the user may require using crutches in maintaining the balance. Mobile robotic

trainer, on the other hand, provides both BWS and guidance in the patient’s motion especially on level

ground. The existing device that applies this concept is KineAssist [40]. Exoskeleton with mobile

platform is an exoskeleton system combining robotic mobile platform, a body weight support system

and a lower limb exoskeleton for over ground training. WalkTrainer [41], SUBAR [42], SJTU mobile

system [43] and NaTUre-gaits [44] are several examples of exoskeletons with such mechanisms.

3. Gait Rehabilitation Exoskeletons

Systems that are associated with the robotic gait rehabilitation exoskeleton are capable of

providing assistance and guidance to the joints of the human lower limbs. To ease the understanding

of the exoskeleton system, an in-depth explanation is provided for two types of gait rehabilitation

exoskeleton systems: treadmill-based exoskeleton and over ground exoskeleton. On the contrary,

exoskeletons which are designed for human performance augmentation or joint level robotic orthoses

are excluded.

3.1 Treadmill-based gait rehabilitation exoskeleton

Treadmill-based exoskeleton composed of a pair of powered leg orthoses, a BWS system and

a treadmill. This BWS treadmill training has proven to be effective for gait recovery by reducing the

gravitational force on the human legs. ReoAmbulator is a treadmill-based gait training system

commercialized by Motorika Ltd. (marketed in USA as the “AutoAmbulator”) [45]. ReoAmbulator

has a BWS system, actuated hip and knee exoskeleton joints (Figure 2). Additionally, its BWS system

provides suspension to the patient over a treadmill in which it is essential to retain the trunk in the

upright position.

Similar design is found on the Lokomat [5], a commercially available device and also the

most widely adopted rehabilitation robot around the world. Lokomat is a treadmill-based gait

rehabilitation device with exoskeletons and a virtual reality environment of audio and visual

biofeedback (Figure 3). This is developed from the prototype called driven gait orthosis (DGO) [13].

The exoskeleton has hip and knee joints for both legs, which are driven by DC motors and ball screw.

The BWS system can deliver programmable uplifting force while a rotatable parallelogram on the

pelvic provides a DOF in vertical direction.

Figure 2.ReoAmbulator [14]

Figure 3.Lokomat [Picture: Hocoma, Switzerland]

Another noteworthy treadmill-based exoskeleton is ALEX (active leg exoskeleton), which is

designed in University of Delaware (Figure 4) [46]. It is developed in the basis of a passive Gravity

Balancing Orthosis (GBO) [47]. ALEX possesses four DOFs in pelvic motion and two active DOFs

on the exoskeleton for hip and knee joints, which are actuated by linear actuators. Force-torque

sensors are integrated to measure the interaction force between the exoskeleton and user limbs. In

2011, a modified version of ALEX II was proposed [48] in which the rotary motors are applied to

actuate the hip and knee joints directly instead of linear actuators. With the rotary motors, the joints

achieve larger ranges of motion which will further enhance the treatment. ALEX, which has been

tested on both healthy and stroke subjects, has joints on one leg only [49-51].

Figure 4.ALEX [15]

Force torque sensors were used in Lokomat and ALEX to estimate the interaction force

between human limb and exoskeleton. However, the increased structure complexity and the absence

of compliance in exoskeleton contribute to potentially hazardous condition. To address this concern,

University of Twente has developed a treadmill-based robotic system with compliant actuator for gait

rehabilitation and motor function assessment, which is known as LOwer-extremity Powered

ExoSkeleton (LOPES) as displayed in Figure 5. LOPES has a BWS system with three DOFs for hip

(forward, sideways and vertical) and a mechanical design of Bowden-cable-based SEA is

implemented to actuate both legs (knee flexion, hip abduction and flexion) [16, 52]. The two pre-

tensioned springs in the SEA play the role as force sensor and bring intrinsic compliance to the

structure. The Bowden cable design allows the motor to be placed on the frame instead of the

exoskeleton, thus reducing the moving mass. LOPES has been tested with healthy subjects to prove

the design and control performance of the system.

Figure 5.LOPES [16]

A research group from University of Auckland, New Zealand, proposed a robotic orthosis

powered by pneumatic artificial muscle actuators (Figure 6) [17]. This system is composed of an

exoskeleton for one leg only. PAM differs from SEA in working principles, but has the compliance

characteristics due to the fact that air is compressible. This orthosis has a BWS frame with two DOFs

on the waist namely lateral and vertical translations; while hip, knee rotations are actuated in the

sagittal plane. A single PAM can provide peak joint torque of 50Nm, which is sufficient for

rehabilitation. Impedance control was implemented with pneumatic muscle actuator. However, due to

the high nonlinearity effect of the pneumatic system, modeling and control become foremost concerns

in influencing the performance of the exoskeleton [53].

Figure 6.University of Auckland system [17]

In the latest technology through the EU project, a treadmill-based exoskeleton called the

MINDWALKER [54], is proposed. This thought controller exoskeleton, which is displayed in Figure

7, is a device developed for SCI patients in allowing them to recover the walking abilities besides

improving the normal social life in today’s society. In this system, the brain provides the control of

the supporting exoskeleton by excitation signals from the brain which will able to provide kinematics

for controlling the exoskeleton. In this exoskeleton, steady state visually evoked potential modality of

control based on visual simulinks. In this intention based control exoskeleton, the blinking state of

LEDs at four different frequencies are used to indicate the intention of the patient in initiating,

terminating, halting and walking faster or slower pace [54]. The lower body mobility control lies on

the extraction of signal from the muscles. By extracting the signals from the shoulder when the

subjects are walking on the treadmill, the movement’ signals are processed to control the kinematics

lower limb of the exoskeleton.

Figure 7.MINDWALKER [54]

3.2 Exoskeletons for over ground gait training/assistance

At present, there are generally two types of over ground exoskeleton for the lower extremities:

one is the portable lower limb orthosis while the other is the mobile rehabilitation robot. The most

remarkable difference compared to the treadmill-restricted exoskeleton is that mobile rehabilitation

robot allows patients to regain natural gait training. With this relatively simple and small structure

exoskeleton, the patient will experience less visual impact. However, several important factors should

be taken into consideration during the designation of the exoskeletons. For instance, weight of the

device, balancing and power supply should be given priority so that the subject will not experience

additional burden and hazard during therapy.

There are some commercially available over ground exoskeleton systems: ReWalk [29], Ekso

[30] and Indego [31]. ReWalk is a device designed to treat individuals with SCI problems, especially

the patients with total thoracic or low level motor function in walking (Figure 8, Argo Medical

Technologies Ltd.) [29]. The ReWalk has motorized hip and knee joints for both legs. Also, battery

and controller set are integrated in a backpack besides the necessity of using the crutches in retaining

the balance while walking. In addition, there is a wireless pad controller on the wrist that is able to

command and instruct the exoskeleton to perform transition movements such as from standing to

sitting or likewise, stairs climbing or normal walking postures.

Figure 8.ReWalk [29]

Another remarkable exoskeleton in treating the neurologically impaired patients is Ekso

(Exoskeleton Lower Extremity Gait System) which was originally named as eLEGS [30]. This design

has been developed by Ekso Bionics (Figure 9) [30]. Ekso is powered by hydraulic actuators, with a

hydro-cylinder and battery integrated on a backpack. Ekso is suitable for user less than 220 pounds,

and it is adjustable to fit user whose height is ranging from 5ft 2in to 6ft 4in. Ekso provides transition

motions such as sit-to-stand, stand-to-sit and walking postures. In the later version of Ekso, control of

the movements via buttons has been introduced in which three modes of the transition movements can

be selected. The first mode is the initial step in allowing the therapist to initiate the patient’s step. The

user progresses from the sitting position to standing position. The patient subsequently continues to

walking position with the assistance of crutches in which this is often treated as their first session. The

second mode is the Active Step whereby the patient takes the control to actuate their steps via buttons

on the crutches or walker. As for the third step, which is known as the Pro Step, the patient achieves

the successive step by moving their hips forward and shifting them laterally (from side-to-side) [30].

Figure 9.Ekso [30]

Indego, a well-known over ground exoskeleton, has been developed by Vanderbilt University

(Figure 10). The said exoskeleton, which has been commercialized by Parker Hannifin Corp, is used

to treat paraplegic individuals. Due to its compact and modular design, the 27 lbs-Indego allows

patient to wear this device while sitting in a wheelchair. The design of this exoskeleton consists of

actuated hip and knee joints for both legs. Moreover, additional forearm crutches are needed to

maintain the balance of the patient [55]. The system is different from the active orthosis as the

actuation system for this robot exoskeleton obeys the principle of power dissipation braking system at

hip and knee joints. The gait is induced by functional electrical stimulation (FES), controlled by the

locking and releasing of the brakes [56].

Figure 10.Indego [31]

The compact design of the aforementioned three devices enables the patient to sit in a

wheelchair while wearing the exoskeleton. Without the restriction in the sagittal movement from

treadmill, over ground exoskeleton provides more freedom in motion, such as sit-to-stand, stand-to-sit,

walking, turning and stairs. Targeted therapy can be designed for patients with specific function

impairment. In addition, they are treated as rehabilitation tools and assistive robots in patient’s daily

activities.

Interestingly, the three portable exoskeletons are actuated with three different actuation

principles. ReWalk is powered by electric motors while Ekso system adopts hydraulic system as the

power source. On the other hand, Indego implements hybrid FES system to stimulate muscle activity,

and brakes on the joints to control the user’s motion. Compared to the unitary treadmill-based

exoskeleton designs, portable exoskeleton has proven its versatility and flexibility.

It should be noted that the ankle joint in the aforementioned three designs is left unactuated in

order to acquire an uncomplicated structure. Indeed, the ankle joint plays an important role in

biomechanics during walking [57-63].

There is also an introduction of exoskeleton for over ground gait training with mobile

platform in supporting the exoskeleton. The system often comes with a BWS so that the system can

provide balance support, body weight unloading and joint level assistance concurrently, making them

suitable for rehabilitation for patients ranging from acute to chronic stages. Examples of robot

exoskeleton that utilize such systems are WalkTrainer [41], EXPOS, SJTU system [43] and NaTUre-

gaits.

WalkTrainer, which is a commercially available over ground mobile device, was developed

by the Laboratoire des Systemes Robotiques (LSRO) at the EPFL (Ecole Polytechnique Federale de

Lausanne, Figure 11) [41]. It comprises a mobile frame that can follow the user during exercise. On

the frame, there is a BWS system that exerts lifting force to prevent the user from falling. This device

has a motion guide on pelvic with six actuated DOFs. A force sensor is implemented to monitor the

interaction force between robot and user. The exoskeleton, on the other hand, is powered by linear

actuators through crank mechanisms. In addition, a twenty-channel real time muscle stimulator is

implemented to stimulate muscles for cycling, rowing and walking activities.

Figure 11.WalkTrainer [41]

Sogang University developed a tendon-driven exoskeleton system EXPOS (exoskeleton for

patients and the old by the Sogang University), which is depicted in Figure 12 [42]. This device

integrated an automatic platform as caster walker to retain the balance during the rehabilitation

process. The handle bar on the platform is pneumatically actuated to synchronize with the up-down

motion of the user during walking. The exoskeleton, which has powered hip and knee joints for both

lower limbs, has force transmission through tendons and pulleys. The transmission is placed on the

walker to reduce the moving mass on exoskeleton. Another prominent property of EXPOS design is

the usage of air bladders, whereby the pressure sensors are implemented on thigh and in shoes. The

purpose of the pressure sensor is to detect human intention and muscle movements. To distinguish the

design of EXPOS with the rest of exoskeleton designs, the air bladder is tightly wrapped around the

key muscles on the legs so that the air pressure varies during muscles contraction. Recently, a newer

version of EXPOS was proposed, whereby the name was altered to SUBAR (Sogang University

biomedical assistive robot) [64]. SUBAR follows the same concept as EXPOS, but with an increment

in output torque (from 7 Nm to 44.0 Nm) and reduction of weight (from 3.2 kg to 11.0 kg). Thus, an

impedance compensation algorithm was implemented to reduce the resistive force.

Figure 12.SUBAR [42]

Researchers from Shanghai Jiao Tong University have proposed an exoskeleton device named

Mobile Lower Extremity Exoskeleton Robot (Figure 13) [43]. The device comprises of a pair of

orthosis with powered hip and knee joints, a moveable platform and a BWS system. The distinguished

feature of this device is the adoption of air pressure sensor which is wrapped on shank to measure the

interaction force based on pressure change. Moreover, the EMG is employed in order to detect human

motion intention during the lower limb muscle contraction [65].

Figure 13.SJTU mobile robot [43]

Nanyang Technological University (NTU) has introduced an over ground walking

rehabilitation device known as NaTUre-gaits (Natural and TUnable rehabilitation gait system). The

device consists of an exoskeleton with actuated hip-knee-ankle joints, BWS system and a mobile

platform [66]. One of the remarkable features of this device is that the system provides five actuated

DOFs on pelvic to assist in the pelvic movements. The advantage of this additional feature is to

promote motor recovery via repetition of therapy. In spite of the said features in which the mobility of

the patient is improved, these actuators however make the system relatively complicated and

cumbersome [67].

4. Discussion

The robotic exoskeletons can be viewed from three aspects: mechanical design, actuation

system and control strategy. Also, summary of the existing robotic rehabilitation exoskeletons and

the associated mechanical designs, actuation systems and control strategies are reviewed in the next

sections and tabulated in Table 1.

4.1 Mechanical design

Mechanical design plays an important role in determining the performance of gait

rehabilitation system as it affects the implementation of rehabilitation strategies. There are basically

two types of mechanical design which have been employed in many exoskeletons.

Exoskeleton of treadmill-based is considered as the conventional method of rehabilitation

exoskeleton mechanical design. By wearing the robotic exoskeleton, the patient is required to walk on

a treadmill during the rehabilitation treatment. In this design, the neurological impaired patient will be

able to perform a particular walking pattern without the direct assistance by the therapists. In another

words, there is no direct patient-therapist interaction compared to the physically demanding manual

therapy. The requirement of exorbitant force exerted on the patient will not be a major issue as the

robot arms are of lightweight and small. There is a claim that the treadmill bears the weight exerted by

the patient during the locomotion rehabilitation [68]. Notwithstanding the benefits mentioned, this

design is restricted to the flat plane ground walking. By employing this design, patients are only

allowed to move on the treadmill, in which the leg motion will be substantially limited to the sagittal

plane.

BWS system is widely adopted in treadmill-based exoskeletons, which provides uplifting

force to keep balance and provide suspension to the patient while walking on the treadmill. A

noteworthy exoskeleton that employs the aforementioned system is Lokomat. The suspension offers

an adjustable uplifting force to improve the gait pattern during training, and the support on pelvic

provides a freedom in vertical motion. Nonetheless, essential gait movements in pelvic, like rotating,

inclining and moving horizontally are inhibited [16]. In order to overcome the restrictions on pelvic,

LOPES group has enhanced its BWS system by providing three DOFs in pelvic motion: forward,

sideways and vertical. The vertical motion is unpowered but a spring mechanism is applied to

compensate gravity. The other DOFs are actuated by motors. The rotations, nevertheless, are still

constrained although the pelvic motion is improved with this configuration. The effort of reducing the

constraints tremendously seems to be not a perfect solution. To prevail over this problem, the

NaTUre-gaits device is invented whereby five powered assistance in pelvic movements are offered.

This device is superior in terms of comprising all significant during locomotion [69], but inferior in

terms of costing and conceptual design. It is discovered that the NaTU re-gaits device is a complex

and expensive; while its bulky design limits its optimal gait recovery. Thence, mobile walker seems to

be a solution to overcome this dilemma. The platform is used in providing some DOFs such as the

horizontal motion. The structure of BWS system is then simplified and adequate freedoms are

provided to promote natural gait motion.

In order to increase the mobility of the treated patients, the over ground exoskeleton design is

introduced. This portable exoskeleton provides more motion freedom compared to treadmill-based

exoskeleton in which patient can be treated and rehabilitated on stairs climbing and walking on flat

plane ground with different speeds and trajectory paths. This will be able to improve the brain

plasticity of the patient as the rehabilitation therapy for specific function is made possible. It is noted

that the over ground training is superior to the treadmill-based treatment [11, 68], and it is essential to

increase training independency among the patients.

In most of the exoskeletons designed till date, ankle joint is left unpowered. This is clearly

revealed in Lokomat, ReoAmbulator, LOPES, ALEX treadmill-based systems, ReWalk, Indego,

EXPOS over ground exoskeletons. The absence of actuated ankle joint brings the advantages of

simple structure and smaller moving mass on exoskeleton. In spite of that, ankle joint plays an

important role in biomechanics point of view [57-63]. The whole body weight is loaded on ankle joint

in a particular phase during walking and stairs. Absence of actuated ankle joint contributes to an

unnatural gait pattern. There are several exoskeletons with actuated ankle joint, such as WalkTrainer

and NaTUre-gaits that consist of complex mechanical structure with actuators attached to the mobile

platform. However, there seems to be a challenge in determining the method to achieve compactness

and effectiveness in ankle joint actuation. The key solution, however, lies in the actuator design of the

exoskeleton, which will be further explained in the next session.

4.2 Actuation system

The design of the actuation system is crucial for exoskeleton design as limitations in the

existing systems are due to the inefficient actuation system. Three types of actuators have been

utilized in exoskeleton systems: hydraulic, pneumatic actuator and electric motor. Hydraulic system,

which was adopted by Ekso, has the advantage of providing large output, but requires an additional

energy supply system. On the contrary, pneumatic artificial exoskeleton is known for its lightweight

and compliance. It appears that the nonlinear characteristic makes the pneumatically powered

exoskeleton difficult to be controlled. It is a fact that the hydraulic and pneumatic exoskeletons are not

portable as the actuators are usually bulky and difficult to be controlled. In order to overcome the

burdensome, electric motor is opted and used extensively in many existing designs. The usage of

electric motor produces the desired and controllable output with relatively fast and better response.

It must be borne in mind that the rehabilitation robots possess direct interaction with humans

physically. Thence, safety is a critical concern for human robot interaction. Conventional rigid

actuator is not inherently safe due to its high output impedance. Conversely, compliant actuator seems

to provide a better solution in which the famous ones are the pneumatic muscle actuator (PAM) and

series elastic actuator (SEA). PAM actuator, nonetheless, suffers a high nonlinearity effect which

leads to difficulty and inaccuracy in control system.

SEA actuator, on the other hand, places an elastic component between the power source and

output shaft. By measuring the deflection of the elastic component, the output force is then measured

based on Hooke’s law [70, 71]. LOPES uses a Bowden-cable-based SEA whereby two pre-tensioned

springs are adopted to drive the rotation of the joint. With this actuator, a desired torque is attained

besides the lightweight joint and incomplex structure. With the elastic component, SEA is back-driven

and intrinsically compliant. It works as a buffer between human limbs and exoskeleton which reduces

the impact of external shock to the subject. Furthermore, it is not necessary to apply force sensor and

this, of course, further simplifies the structure. However, SEA faces a dilemma of compliance and

force range, for which, a tradeoff has to be made. Softer component increases the force resolution but

reduces the compliance whereby the maximum output force is decreased. On the other hand, stiff

component yields larger range of output force, but extremely high stiffness will reduce the force

resolution. Recently, the NeuroRehabilitation Robotics research group from National University of

Singapore has recommended an approach by designing a novel linear SEA to overcome this limitation

[33, 72]. In this novel design, two springs with different stiffness ratio are placed in series; the softer

one enables the actuator to remain a low intrinsic compliance of the actuator, while the stiffer one

extends the range of output force. Experimental results have proven its excellent performance and

credibility in force tracking.

4.3 Control strategy

The main purpose of control strategy in the rehabilitation therapy is to engage the movement

sequences to assist in patient’s gait recovery. Different control strategies have been widely used in

the exoskeletons. Nonetheless, the discussion on control strategies in this paper is limited to pre-

defined trajectory based and intention based control strategies. The details of the mentioned control

strategies are presented in the following sections.

4.3.1 Trajectory based control strategy

In the early stage, exoskeletons move the patient’s limbs passively to follow a predefined gait

trajectory in rehabilitation therapies. However, the efficacy of this rehabilitation training is still

unknown due to the lack of human initiative [73]. Impedance control is then implemented in order to

engage human voluntary work in the rehabilitation treatment. This control provides assistive force

when human limb deviates from gait trajectory, in which restoring force that is proportional to the

deviation is provided [5, 16]. Clinical trials also affirmed that robot-applied resistance can improve

the performance of body-weight-supported treadmill training [74]. A number of control strategies

have been developed based on impedance control. This is clearly shown in LOPES exoskeleton as the

group proposed Virtual Mode Control whereby the selection of different gait functions is allowed [75].

On the other hand, ALEX used a force-field controller with visual guidance for gait rehabilitation [76].

The exoskeleton exerts both tangential and normal forces at the ankle joint with respect to the speed

of the reference trajectories. It should be borne in mind that assistive force varies with different

patients whereby this encourages them to have more engagement in the training process. Alexander et

al. developed a gait rehabilitation device, Lokomat, by constructing a path in space instead of gait

trajectory [77]. In this control strategy, the motion of the patient’s leg is constrained in both compliant

virtual tunnel and specific joint space. Thus, the guidance and assistance from the exoskeleton is

diminished besides interleaving effort and initiative from the patient.

Defining the gait trajectory reference for both passive and impedance control is a significant

issue to be taken into consideration. It is inaccurate to rely on a predefined gait pattern as reference

because every human has a unique gait pattern. Moreover, using fixed gait trajectories will prone to

the challenge in synchronization of desired gait cycle and subject’s gait phase. To prevail over this

problem, several approaches have been suggested: LOPES group proposed a method called CLME to

generate trajectories for the impaired leg with the motion of the healthy leg as reference [78]. ALEX

group also prepared a series of templates which are sorted according to the deviation from the

subject’s gait pattern to desired one [76]. By shifting the templates, the patient’s gait pattern is

improved when certain criterion is satisfied. To synchronize the reference and subject’s gait, Evryon

group has proposed an oscillator-based strategy [79]. Riener’s group has provided another approach in

allowing the subjects to adjust their pace initiatively by providing visual guide [77]. Adaptive

technique can also be applied to tune the trajectories in reducing or regulating the coordination

between human motion and exoskeleton assistance [80].

4.3.2 Intention based control strategy

A number of research groups have developed the control strategies of integrating the motion

intention of the subject (i.e. sit-to-stand, stand-to-sit, stairs climbing, walking in varying paces) in the

rehabilitation therapy.

Firstly, the physiological EMG and EEG signals are required to detect the intention of the

subjects. EMG is the mostly used technique in measuring muscle activity. Currently, there are several

exoskeletons that employ the EMG signal based assistance: HAL [37], KAFO [32] and NEUROExos

[81, 82]. However, it is plausible that noises in EMG signals increase the intricacy of muscle activity

evaluation [83-85]. Furthermore, EMG signals are very sensitive to the electrode placement,

disturbance from the neighboring muscles and skin properties (e.g. sweat on the skin) will further

make the detection unstable. Therefore, it is suggested that the EMG-based assistance should be tested

and calibrated for different patient in order to determine a better assistance.

In recent years, there has been much interest in introducing the EEG-based intention control

in treating the impaired lower limb [83]. The researchers of KAFO have revealed the potential of

high-density EEG to be implemented into the locomotors control [84]. The usage of EEG in providing

intention will be a challenge as the algorithm of the controller and the mapping limitation during

locomotion sources are no located in the brain cortex [86] and the relatively slow speed for real time

application [87]. Hence, the robustness and accuracy can be improved by fusing both the EEG and

EMG signals in providing assistance to the patient. With this combination technique, the EEG and

EMG signals are suitable in increasing the accuracy of features extraction [88, 89] and classification

which will improve the efficiency of the robot exoskeleton [90]. The mentioned fusion technique has

been employed by the MINDWALKER [54] in which further improvement can be suggested by

excluding the utilization of Brain Neural Computer Interface (BNCI). Hence, fusion of both EEG and

EMG is belief to be a new challenge in implementing human intention based control strategy.

Besides the physiological signals, human intention can be estimated from kinematics

information. Referring to Indego [55], changes of CoP on the ground are tracked to predict human

movements, such as sit-to-stand or stand-to-sit. Achievement via CoP is obtained but in-depth gait

patterns or biomechanics are required for better control. In order to achieve the gait patterns, inertia

measurement unit (IMU) can be implemented as a promising sensor. This can be noticed in [91],

whereby Peruzzi has estimated the stride length by double integrating the coordinate acceleration

components. With the increased number of IMUs, related gait pattern can be extracted [92] and the

motor abilities of human limbs are assessed [93, 94].

5. Conclusion

In this paper, an insight review has been conducted on the mechanical designs, actuation

systems and control strategies that are currently utilized in robot exoskeletons for rehabilitation

therapy. It should be borne in mind that rehabilitation related robotic exoskeletons are not only

designed to rehabilitate the lower limbs but also for the upper extremities. Nevertheless, significant

emphasis is given on the lower extremity rehabilitation in this review. As per date, there are several

types of exoskeletons which have their own advantages during rehabilitation therapy. Notably, our

discussion is constraint to the treadmill-based and over ground exoskeletons. On the contrary, a

detailed description of the compliance of both the mechanical design and actuation system to the

patients is offered: pneumatic-based, hydraulic-based and SEA actuator systems. Moreover, the

trajectory and intention based control strategies are given priority in this review.

Although research has been done extensively, the technology of robotic exoskeleton is

evolving rapidly. The question of the optimal effectiveness of the treadmill-based and over ground

exoskeleton is still remained open. It is predicted that the over ground exoskeleton has relatively high

potential in expanding its usage as it helps to increase the mobility of the patients, such as to perform

sit-to-stand, stand-to-sit and stairs climbing postures.

Through this study, it broadly reflects that the intention based control strategy is desired in the

next technological shift of robotic rehabilitation. With this new technology push, there is chance to

detect the human intention like gait cycles, stairs motion (both ascent and descent) and movements to

avoid obstacles based on the IMU system. Development of a control strategy by employing the fusion

of non-invasive human physiological signals, such as EMG and EEG, is worth being fully considered

in enhancing the rehabilitation treatment. With this relatively new intention based control strategy, the

rehabilitation outcome is desirable as this strategy engages more initiative from the patient.

Establishing the new concept of real-time control strategies and providing appropriate assistance to

the patients in a timely manner appear to be a new promising technology paradigm in yielding an

optimal rehabilitation therapy.

Table 1. Summary of existing robotic rehabilitation exoskeleton and its associated

mechanical design, actuation system and control strategy

Robot

Device’s

Name

Type of

Mechanical

Design

Type of

Actuation

System

Type of Control

Strategies

Description of the

Control Strategies

Evaluated

and

Targeted

Part of the

Lower

Limb

References

Loko-

mat

Treadmill-

based

exoskeleton,

BWS

Electro

Actuator

Position and

Impedance

control

Position, velocity

and force data are

obtained from

subject and the

correct force is

exerted to the

patient’s leg

movement.

Knee and

hip (pelvic)

[13]

LOPES Treadmill-

based

exoskeleton,

BWS

SEA

compliant

actuator

Impedance

control

Provide assistive

impedance control

while walking

Knee and

hip (pelvic)

[16]

ALEX Treadmill-

based

exoskeleton,

BWS

Linear

electro

actuator

Force, impedance

control

Based on a given

trajectory, the error

between the

reference trajectory

before and after

experiment is

evaluated in order

to apply desirable

force to the hip and

knee joints.

Knee, hip

and foot

[15,

46,47,49, 50,

51]

EXPOS

(SU-

BAR)

Over ground

exoskeleton

Cable

driven

Impedance

control

(a)Exhibits low

impedance and

generates torque

accordingly when

interacting with

patient.

(b)Desired assistive

torque is exerted to

the patient (real-

time) based on the

measurements of

joint angle and

ground contract

forces.

Hip and

knee joint

[42, 64]

Ekso Over ground

exoskeleton

Hydraulic

actuator

Force, impedance

control

Assistive force is

provided in order to

relieve the

deviation of the hip

and knee joint.

Hip and

knee

[30]

Table 1. Summary of existing robotic rehabilitation exoskeleton and its associated

mechanical design, actuation system and control strategy (cont)

Robot

Device’s

Name

Type of

Mechanical

Design

Type of

Actuation

System

Type of Control

Strategies

Description of the

Control Strategies

Evaluated

and

Targeted

Part of the

Lower

Limb

References

Mind-

walker

Treadmill-

based

exoskeleton,

BWS

SEA

compliant

actuator

EEG based

control

Steady state

visually evoked

potential modality

of control based on

visual simulations.

For LEDs which

are blinking at four

different

frequencies to

indicate whether the

patient is going to

start-stop and to

walk faster or

slower.

Knee and

hip

(pelvic)

[54]

RGR

Trainer

Treadmill-

based

exoskeleton,

BWS

Back-

drivable

linear

actuator

Impedance

control

The position of the

subject is compared

with the reference

trajectory.

Corrective force is

exerted to the

patient if there is

any deviation from

reference trajectory.

Hip

(pelvic)

[95]

PAM

and

POGO

Treadmill-

based

exoskeleton,

BWS

Pneumatic

compliant

actuator

Impedance

control

Corrective force is

applied to the end

point of the

exoskeleton by

referring and

comparing the

deviation from the

reference trajectory.

Hip

(pelvic)

[18, 96]

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