This work addresses the design and numerical characterization of a new exoskeleton solution for human leg motion assistance and rehabilitation. The exoskeleton solution is anthropomorphic, simple, low cost and easy to adapt on the human subject. The design aspect concerns the exoskeleton mechatronic structure, achieved in SolidWorks virtual environment. Numerical simulation is performed in MSC.ADAMS simulation environment. Obtained results for the exoskeleton computed motion are compared with those obtained from experimental walking of healthy subject. The prototype feasibility is studied both for design and operation aspect.
Nowadays, the importance of measuring and analyzing gait variability has increased and it is more and more recognized and used in biomechanics, in robotics, in rehabilitation and in clinical research field. Clinical gait analysis usually consists of measurement of gait parameters, kinematic analysis, kinetic measurement and electromyography. Spatial and temporal parameters of gait provide useful diagnostic and therapeutic information, if they are accurately measured (Begg et al., 1989). The most analyzed parameters of normal and pathological human gait are the following: travelled distances, velocity, gait phases, step length, joints angles, swing time, support time, ground reaction forces, forces and momentum in joints (Tarnita, 2016). In the medical field, the knowledge of gait characteristics, the monitoring and evaluating changes in human gait reveal important information about quantitative objective measurement of the different gait parameters, about the evolution and early diagnosis of different diseases and about physical therapy, involving rehabilitation, which helps to improve the walking function (Sutherland et al., 2002; Tao et al., 2012; Muro-de-la-Herran et al., 2014). The main goal of the rehabilitation therapy is to minimize functional deficits of disabled patients, this procedure requiring a repetitive motion performed by the patient. For that purpose, the modern rehabilitation techniques use mechanical systems (as robots or exoskeletons) to assist the lower limbs in their movements' rehabilitation. The main advantages of robotic rehabilitation are: reducing dependence on clinical staff, providing adequate rehabilitation movements and adjusting the level of treatment according to patient requirements, allowing control of joint movement, helping ensure controlled repetitive preparation at a reasonable cost (Banala et al., 2007).
An exoskeleton is defined as an active mechanical device that is essentially
anthropomorphic, in nature is “worn” by an operator and it augments the
performance of an able-bodied wearer (Dollar et al., 2008). This purpose is
realized by providing a supplementary force to the legs (Viteckova et al., 2013).
Due to the increased interest upon the development of rehabilitation
procedures based on mechatronic and robotic technologies, in recent years,
many review studies which evaluate the progress and future directions in
rehabilitation, were published (Onose et al., 2013; Young et al., 2017; Chen et al., 2013; Lajeunesse et al., 2016 and
Louie et al., 2016). Ones of the most popular exoskeletons are:
BLEEX (Berkely Lower Extremity Exoskeleton), pointed as the first load
carrying exoskeleton (Anama et al., 2012). BLEEX exoskeleton actuates the hip
abduction/adduction, hip flexion/extension and knee flexion; HAL (Hybrid Assistive Limb), described as a wearable robot intended for
multiple applications, from rehabilitation purposes to heavy works
additional support, (Díaz et al., 2011; Askani et al., 2016); LOPES exoskeleton combine an actuated pelvis segment with a leg exoskeleton,
with three actuated joints: two for hip and one for knee joint (Veneman et al., 2007). LOKOMAT system consists into a treadmill and a powered exoskeleton. In order
to use this rehabilitation system, the patients that are affected by spinal
cord injuries are sustained by a body weight support (Hesse et al., 2003). REX exoskeleton by REX Bionics is a wearable exoskeleton capable to support
the weight of the patient and is able to self-balance (Barbareschi et al., 2015).
Multi joint exoskeletons are described by Tingfang et al. (2015), and most
of them are based on the principle of predefined gait trajectory control.
This principle according to which the exoskeleton replicates the desired
joint trajectory corresponding to an experimental data set acquired from a
healthy person is implemented to exoskeletons like HAL, Rewalk and
Mindwalker (Wang et al., 2015a).
Exoskeletons for industrial application are presented in de Looze et al. (2016), powered exoskeletons in post – stroke rehabilitation are studied in Louie et al. (2016) and for arm rehabilitation in Schorsch et al. (2014); Jarrassé et al. (2014). The aspect of control systems of active exoskeletons is studied in Anama et al. (2012), while aspects of exoskeleton design, path planning, modeling and simulation, experimental tests are studied by Carbone et al. (2007); Ashkani et al. (2016); Wang et al. (2015b); Ceccarelli et al. (2009).
Recently, numerous review articles presenting the state of the art concerning passive devices used for rehabilitation of lower limb are published. These types of device are represented by passive functional upper-limb orthoses (Rahman et al., 2006).
According to the rehabilitation principle (Díaz et al., 2011), the active rehabilitation devices can be grouped in five groups: (a) treadmill gait trainers; (b) foot-plate-based gait trainers; (c) mobile robotics based solutions; (d) stationary gait trainers; (e) ankle rehabilitation systems, as active foot orthoses.
Some studies (Onose et al., 2016) evaluate the progress and future directions in the field of robotic rehabilitation technologies, from the perspective of professionals, engineers and users, while the main improvements needed for the development and functional optimization of practical exoskeletons are highlighted in Lajeunesse et al. (2016).
Another research theme from this subject area is the development and evaluation of a new leg robotic exoskeleton, called H2, intended for gait rehabilitation of the stroke survivors (Bortole et al., 2015). In order to guide the development of lower limb exoskeletons, some studies (Kao et al., 2010) had, as purpose, the understanding of how humans adapt to powered assistance.
From studied literature it results that there are many devices and robotic systems for human gait rehabilitation. These systems are intended to patients affected by spinal cord injuries, muscular dystrophy, spinal muscular atrophy, cerebral palsy and stroke. These disorders produce muscular weakness and this is the reason for the development of locomotion assistance robotic devices and necessity of gait rehabilitation (Rahman et al., 2006). A category of exoskeletons is intended to a single joint motion assistance (Ceccarelli et al., 2016), and other category assists the motion of two or more leg joints. Exoskeletons in this category use actuators for joints, command and control architectures, and are costly solutions that are difficult to deploy on a large scale (Geonea et al., 2013). As a conclusion these existing solutions are not accessible to disabled persons, although they assure proper requirements for rehabilitation.
The low-cost solution does not perform rehabilitation movements for all of the leg joints because they generally provide active rehabilitation movement only for the hip and knee joints. Other devices are used as passive solutions to provide rehabilitation movements that correspond to a specific rehabilitation therapy designed to recover walking for a particular joint such as the hip, knee or ankle. Several devices solutions are designed to reproduce human bipedal locomotion, but generally few of them provide approximately anthropomorphic movements (Geonea et al., 2015).
This research proposes a new leg exoskeleton design that provides support for the movement of the hip and knee joints. An experimental human walking test is used to obtain reference movement laws for the exoskeleton's joints during walking. Then, a dynamic study of the walking exoskeleton attached to a human virtual mannequin is performed. The dynamic virtual model is designed using SolidWorks, and numerical simulation is done using the MSC.ADAMS software. A prototype of the exoskeleton is manufactured and subjected to experimental tests. Finally, a comparison of motion laws performed by a healthy human subject and by the mannequin-exoskeleton assembly is made.
Human gait analysis represents a large interest subject in the literature (Varela et al., 2015). Also experimental characterization of human falling down, represent a subject presented in recent studies (Meng et al., 2017). According to some studies (Winter et al., 1979; Tarnita et al., 2013), human gait represents a cyclic motion between heel strike on ground and next ground contact of same heel. This cycle consists of two important phases, stance phase and swing phase. Intermediate phases are also studied, a complete gait cycle phases being described by Perry et al. (2010).
Goniometers and Datalog devices mounted on the human subject, during the test.
For human gait experimental analysis, a Biometrics data acquisition system
based on electrogoniometers is used (
The block schema of the acquisition system is shown in Fig. 2.
Block schema of the acquisition system.
Experimental data acquired from a 35 years old healthy subject for normal walking during 30 s are reported as angle variation in time, for the ankle, knee and hip joints, in sagittal plane, for the right leg (Fig. 3) and for the left leg (Fig. 4). Experimental tests performed by the male subject consist in 25 consecutive gait cycles repeated for five times in the same walking conditions.
The anthropometric data of the human subject are: body weight
1 – right knee flexion/extension; 2 – right hip flexion/extension; 3 – right ankle flexion/extension; Healthy human measured hip, knee and ankle joint angle, in sagittal plane, for right leg.
1 – left knee flexion/extension; 2 – left hip flexion/extension; 3 – left ankle flexion/extension; Healthy human measured hip, knee and ankle joint angle, in sagittal plane, for left leg.
The knee joint angular maximum amplitude is by 65
The second set of experimental results is obtained, as data files, for the
test of self-speed walking during 20 s, from a disabled 50 years old
male patient, suffering from osteoarthritis (OA) at left knee. The
anthropometric data of the patient are: body weight
Human gait variability from one cycle to another for the same subject, from one subject to another and from one healthy subject to a diseased one, imposes the normalization of gait cycles. Normalization is done using SimiMotion software where the data files are transferred (Tarnita et al., 2017). For the accuracy of the final results, there were eliminated 2 cycles from the beginning and from end of data files. To compare results, it is necessary to determine the average cycle for each acquired data file.
1 – knee flexion/extension; 2 – hip flexion/extension; 3 – ankle flexion/extension; The experimental angles variations for disabled human, in sagittal plane, for right leg.
1 – knee flexion/extension; 2 – hip flexion/extension; 3 – ankle flexion/extension; The experimental angles variations for disabled human, in sagittal plane, for left leg (disabled leg).
Taking into account existing solutions of leg mechanisms used for human leg motion assistance and rehabilitation, presented in reviews and research studies, (Dollar et al., 2008; Rajesh et al., 2013; Bruzzone et al., 2012; Dumitru et al., 2015; Copilusi et al., 2015), for this research a single DOF, simple and light solution of a mechanism which assists the human gait is developed. There are developed similar one DOF leg solutions at LARM Laboratory Casino, Italy (Li et al., 2013; Wang et al., 2015b).
The starting point in developing of a new leg mechanism is that the ankle joint trajectory must assure an ovoid path, to assure human leg motion during swing and also to assure proper angular variations of the joints. The proposed structural solution is optimized from kinematic perspective, using virtual modeling and simulation methods and principles (Ilhem et al., 2013; Carbone et al., 2007) and path planning (Carbone et al., 2008; Tedeschi et al., 2015). The optimal design solution is materialized into an experimental prototype. The mechanical solution is completed with the command and control architecture. For actuation a DC gear motor is used. The motor speed is controlled with a hardware architecture based on pulse wave modulation method (PWM).
Leg exoskeleton:
A virtual model of the exoskeleton in SolidWorks:
The structure of the leg exoskeleton and the virtual model designed in SolidWorks software package are presented in Fig. 7. This low cost solution uses only a rotary actuator mounted at the joint A of the link 1 which is fixed to the upper frame. The solution consists in a planar mechanism with 9 links and 13 revolute joints. The actuator rotates the link 1 of Cebyshev linkage 1–2–3. The femur link, 5, consists of two orthogonal segments: FG and IG. Joint G, representing the hip, is connected to the upper frame, considered as a fixed element.
The link 4 transmits the motion, being connected by joint E to Chebyshev linkage, and by joint F to femur segment. Joint B is a multiple one, here being connected links 1 and 2 and link 6 with 1. Through link 6 and the quadrilateral linkage ILKH the motion is transmitted from link 1 to the link 9 (materialized by exoskeleton tibia). Based on this innovative structure of the rehabilitation exoskeleton, a 3-D model is designed as it is presented in Fig. 8.
Mechanism for exoskeleton leg:
Computed joints angle variation:
Computed plot of ankle joint M position:
Computed plot of ankle joint M velocity:
Computed plot of ankle joint M acceleration:
In order to synthesize the lengths of the mechanism links we started from
the patient dimensions (femur and tibia). The trajectory of ankle joint is
tracked by performing 2-D simulations in MSC.ADAMS (Wojtyra et al., 2003). Using
ADAMS “
A virtual model of the exoskeleton assembly, consisting of two legs attached
to an upper frame and of one chain transmission for both legs actuation, is
designed in SolidWorks in order to perform a dynamic simulation. The links 1
of left and right legs are opposite positioned to 180
The proposed exoskeleton mechanism is characterized by some novel characteristics:
Comparing with other rehabilitation exoskeletons, the present proposed design has only one actuator. For that reason, it's a low-cost design solution and simple operation, because don't needs a complex command and control hardware.
The design solution can generate a suitable gait by its construction and gait planning is not necessary. By using a parallelogram in the knee joint, the foot moves parallel to the ground during propelling phase (as see in Fig. 9b, where is presented the computed ADAMS trajectory). Being a low-cost solution, the ankle joint is not actuated, but a torsion spring is mounted in order to be used for simulation.
It has an anthropomorphic structure, because the achieved joint motion of the exoskeleton is comparable with those of human subjects. A detailed comparison is presented in the paper.
The mechanism is characterized by simple operation. More, the exoskeleton is able to walk on inclined planes, because its structure assures a proper stride height.
In order to evaluate the prototype leg characteristics, a kinematic analysis considering the mechanism operation on a supporting stand is performed. The reference coordinate system XY has its origin placed in joint A. The design parameters of the leg mechanism are indicated in Fig. 9. The lengths of the mechanism elements and the coordinates of fixed joints A, D, G are shown in Table 1.
For a proper dimensional synthesis of the mechanism, the virtual simulation tools available in Solid Works software are used. The prototype of the exoskeleton is intended to be used by a young disabled person, with 1.58 m height. For this purpose, in order that the exoskeleton to be suitable to wear, are imposed the length of the segment GI to 350 mm, that corresponds to the subject femur and the length of the segment IM to 315 mm corresponding to the tibia length. Another condition for the dimensional synthesis is to impose a minimum 90 mm horizontal distance between point G (hip joint) and joint A (where the engine shaft is located), allowing the exoskeleton to be suitable to wear by the patient (the patient must have sufficient space to fit with the pelvic basin). The link AB is designed with a minimum dimension of 12 mm in the first phase for minimum power requirements and in final design with length of 13.5 mm, in order to increase the knee and hip joints angular amplitude motion. Link BJ is designed with 355 mm, because, in initial position, the segments GI and IM have to be a straight line, as the human leg in vertical position. Linkage IHKL is designed as a quadrilateral mechanism. Finally, the lengths of links 2, 3 and 4 are adjusted in SolidWorks parametric design, following to obtain an ovoid trajectory of the point M resulted by virtual simulation tracking.
Exoskeleton virtual model worn by a human mannequin:
Dynamic model of leg exoskeleton for simulation:
Snapshots of simulation outputs of the exoskeleton and mannequin walking.
Computed exoskeleton joints angle (dotted line-right
leg, continuous line-left leg):
Computed displacement of the left leg exoskeleton ankle
joint:
Computed vertical reaction force on the exoskeleton hip
joint:
Computed vertical reaction force on the exoskeleton knee
joint:
Computed exoskeleton ground reaction forces (GRF):
Computed actuator parameters:
A view of exoskeleton manufactured prototype:
Marker tracking for exoskeleton kinematics:
Compared variation of knee joint angle for human subject and for exoskeleton model.
Compared variation of hip joint angle for human subject and for exoskeleton model.
Design parameters of leg mechanism.
The kinematics of the mechanism is described by the loop closure Eq. (1).
The mathematical model characterizes the kinematics of the leg exoskeleton
when it operates on a supporting stand. This kinematical model is solved in
Maple software, and it is useful in order to validate the engineering
feasibility of the proposed leg mechanism. Numerical results of the
mathematical model are reported in plots in Figs. 10–13. In Fig. 10a, the
computed variation of the knee angle is shown, and in Fig. 10b the computed
variation of the hip angle is presented. By comparing the obtained motion
laws with those obtained in experimental gait analysis, for the healthy
subject, presented in Figs. 3–4, one can remark that the variation in time
is almost similar. The numerical angular amplitude for the knee joint varies
in the interval [
The leg exoskeleton realizes a displacement of 220 mm, from
The walking velocities on
A virtual model of the exoskeleton, consisting of two legs attached to an
upper frame, and a chain transmission for actuation of the legs is designed
in SolidWorks for dynamic simulation. The links 1 of left and right legs are
opposite positioned to 180
In the literature are available several studies on the importance of the
foot-ground contact definition. Valiant (1990) published studies on
the dynamic characteristics of the plantar surface of the foot. Then, this
work has been continued and developed by Meglan et al. (1992), who studied
the load-deformation parameters of the foot-ground and developed a general
equation to compute vertical ground reaction force, using optimization
techniques in order to fit Valiant's data results to a simple polynomial
equation. The obtained equation was (Patton et al., 1993):
The torsion spring damper, placed at ankle joint uses the
coefficients: (
The simulation settings for the mannequin – exoskeleton assembly were considered as real. The model of the mannequin was positioned in the bipedal position when the left foot touches the ground and the right leg is in the balancing phase. This is the same starting position of the man subjected to experimental walking analysis. Simulation of the exoskeleton in MSC.ADAMS is proposed for the case of assistance to a patient with a leg disability. A sequence of exoskeleton with mannequin walking positions is shown in Fig. 16.
For the simulation, input angular velocity is set up to 4 rad s
Numerical results of the mannequin – exoskeleton assembly gait simulation
are presented in the plots from Figs. 17–22, related to the snapshots of
virtual simulation, shown in Fig. 16. When the exoskeleton is worn by a
human mannequin and performs gait, the obtained numerical results for the
hip and knee joints angular variation are shown in Fig. 17. Presented plots
show that the computed angular amplitude of the right hip joint vary in the
range [
Figure 18 shows computed displacements of the left leg exoskeleton ankle joint, upon horizontal axis Fig. 18a and vertical axis Fig. 18b. In the first phase the leg is on the ground during 1.5 s, and then the leg is performing the swing phase, with a step length by 440 mm. The step length performed when the exoskeleton is performing gait is bigger than in situation when the mechanism operates at stationary with 220 mm (is double). This fact is confirmed also by the experimental analysis of the exoskeleton gait.
Figures 19 and 20 show the reaction forces calculated at the hip and knee joints of the exoskeleton. The vertical reaction force reaches a maximum of 1000 N for the hip joint and 1100 N for the knee joint. The weight of the mannequin – exoskeleton assembly is 950 N, so that the horizontal force component of the exoskeleton knee is approximately 1.15 times the weight of the model. The vertical reaction forces computed reaches the maximum amplitude when the exoskeleton leg it detaches from the ground. Figure 21 shows the exoskeleton ground reaction forces for both legs. The maximum value is 1130 N, also a value correlated with the weight of the assisted virtual mannequin (patient). Figure 22 shows computed torque of driving motor on rotation axis and computed power of the actuator. As is observed the time for a step is about 1.5 s, and the time period of one walking operation cycle (two steps) is 3 s. The peaks values are 14 Nm, corresponding to the ground contact phase of the legs. For the swing phase of the exoskeleton the values for the forces and computed torque are smaller. Computed diagrams for the legs show repetitive variations. Simulation results allow concluding that the leg exoskeleton design is suitable for human gait rehabilitation.
The proposed exoskeleton design is subjected to experimental tests, to validate the dynamic simulation. For that purpose, based on the optimal design, the experimental prototype of the exoskeleton is manufactured (Fig. 23). For the exoskeleton actuation, a DC electric motor is used. The electric motor design uses a gearbox in order to deliver a high torque at a low rotational output. In addition, the exoskeleton design uses a chain transmission, which can be customizable to adopt different supplementary transmission ratios. The maximum torque delivered by the motor is 15 Nm at 38 rpm rated speed. The motor is powered by a 12 volts accumulator. To adjust the electric motor speed, a pulse wave modulation (PWM) controller is implemented based on an Arduino board. In first stage of experimental tests, the exoskeleton walks without a human subject. During the walking, the exoskeleton stability is guaranteed by two links, which run on the ground with two self-directional wheels. In this way there are assured two additional ground contact points in order to have dynamic stability conditions. The walking of the exoskeleton is performed on laboratory floor, and the motion is analyzed based on video cameras motion analysis equipment CONTEMPLAS. Reflective markers on different interest points, like hip, knee and ankle joint are attached and their captured motion trajectories are shown in Fig. 24a and b.
Joints angle variation is computed based on marker tracking (Fig. 24c, d). Numerical results, obtained for the exoskeleton joint variations are compared with those achieved by the human subject. For the human subject hip and knee joint angular variation a medium cycle is calculated.
A comparison of the achieved left knee joint angular variation for the
exoskeleton and human healthy subject is presented in Fig. 25. Also because
of the human gait variability for a relevant comparison are presented on the
same diagram the variations for the exoskeleton knee joint and the human
subject medium cycle, for a gait cycle. It can be observed that the maximum
angular amplitude for the human and exoskeleton knee joint is 65
By comparing the five maximum values of the mean cycles of knee
flexion-extension with the maximum value of the final mean cycle
corresponding to the test, we can conclude that, these are very close with
non-significant differences. There were not big differences in the shape of
the flexion angle. The minor differences obtained by this comparison show a
good repeatability of the imposed test for all the five trials. The maximum
values of the knee angle determined during the performed trials were
compared and tested with a Student
The angular variations diagrams in case of human hip joint and exoskeleton
hip joint are presented in Fig. 26. For the hip joint, the angular variation
achieved by the human and exoskeleton is between [
A new exoskeleton mechanism, including the design model, the simulation results and the experimental prototype, is presented. The proposed exoskeleton is designed based on a low cost and easy-to-use design. The functionality of the exoskeleton is studied for the case when it is worn by a virtual human mannequin. For that purpose a dynamic simulation model in ADAMS is developed. The results obtained by numerical simulation are discussed and compared with experimental results obtained on healthy human subjects. Based on the CAD design an experimental prototype of the exoskeleton is manufactured in order to perform experimental studies and to validate the results obtained by numerical simulation. The exoskeleton real model motion analysis is performed using Contemplas ultra speed video cameras equipment and analysis software. The motion of the experimental prototype is compared with results of virtual simulation motion and with results of the human gait experimental analysis. Obtained results are used to characterize the operation of the leg proposed exoskeleton. Finally is presented a comparison of exoskeleton motion compared results. The exoskeleton hip and knee joints angular motion it is compared with human healthy subject motion and the obtained variation graphics are similar. In conclusion, the mechanism operation is suitable for human motion assistance and rehabilitation purposes.
All the data used in this manuscript can be obtained by requesting from the corresponding author.
Equations (1) are projected on the coordinate system axis, and the Eqs. (A1)–(A4) are obtained, where the unknowns are the angles
Solutions of Eqs. (A1)–(A4) are found by solving nonlinear Eq. (A5), with variable coefficients. These loop closure equations are solved
using a package program developed on Maple environment.
The components of the point M velocity on both axes are given by the Eq. (A8):
The components of the point M acceleration on both axes are given by the Eq. (3):
The authors declare that they have no conflict of interest.