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# Toward A Soft Robotic Ankle-Foot Orthosis (SR-AFO) Exosuit for Human Locomotion: Preliminary Results in Late Stance Plantarflexion Assistance

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This paper presents the design of a soft robotic ankle-foot orthosis (SR-AFO) exosuit to aid in plantarflexion for gait rehabilitation in individuals who suffer from irregular gaits due to stroke or other injuries. The SR-AFO exosuit is a sock-like garment fabricated from compliant fabrics. The SR-AFO exosuit aids in late-stance of the walking gait in plantarflexion by contracting the actuator to pull the posterior end of the foot upward. This helps to reduce the muscle effort of the user during plantarflexion. The addition of a second actuator shows a 45.3% increase to 13.51 ± 0.31 kg payload capacity. The actuators are oriented at an optimal angle of 5 • to produce the highest pulling force. Three healthy participants are evaluated during walking trials with and without SR-AFO exosuit assistance while ankle angle and muscle activity are monitored. The gastrocnemius (GA) and soleus (SOL) muscle activity during late stance is reduced by 13.4% and 16.6% respectively. Tibialis anterior (TA) increases slightly during swing most likely due to the hysteresis in the system deflating during that window. The ankle range of motion remains within natural walking limitations and plantarflexion angle increases when the SR-AFO exosuit is active.
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Toward A Soft Robotic Ankle-Foot Orthosis (SR-AFO) Exosuit for
Human Locomotion: Preliminary Results in Late Stance Plantarﬂexion
Assistance
Carly M. Thalman, Student Member, IEEE, Tiffany Hertzell, and Hyunglae Lee,Member, IEEE
Abstract This paper presents the design of a soft robotic
ankle-foot orthosis (SR-AFO) exosuit to aid in plantarﬂexion
for gait rehabilitation in individuals who suffer from irregular
gaits due to stroke or other injuries. The SR-AFO exosuit is
a sock-like garment fabricated from compliant fabrics. The
SR-AFO exosuit aids in late stance of the walking gait in
plantarﬂexion by contracting the actuator to pull the posterior
end of the foot upward. This helps to reduce the muscle effort
of the user during plantarﬂexion. The addition of a second
actuator shows a 45.3% increase to 13.51 ±0.31 kg payload
capacity. The actuators are oriented at an optimal angle of 5
to produce the highest pulling force. Three healthy participants
are evaluated during walking trials with and without SR-AFO
exosuit assistance while ankle angle and muscle activity are
monitored. The gastrocnemius (GA) and soleus (SOL) muscle
activity during late stance is reduced by 13.4% and 16.6%
respectively. Tibialis anterior (TA) increases slightly during
swing most likely due to the hysteresis in the system deﬂating
during that window. The ankle range of motion remains within
natural walking limitations and plantarﬂexion angle increases
when the SR-AFO exosuit is active.
Keywords - Soft Robotics, Wearable Robots, Assistive Robots,
Rehabilitation.
I. INTRODUCTION
The ankle joint is responsible for 45% of the power behind
human locomotion, and plantarﬂexion is a critical motion
throughout the entire gait cycle [1]. Body propulsion during
a forward gait requires a propulsive force that pushes off of
the ground and creates forward motion. Ankle plantarﬂexion
produces propulsion by using ankle muscles to push off the
ground during late stance phase [2], [3]. The soleus (SOL)
and gastrocnemius (GA) muscles provide vertical support
during single-leg stance [4]. During mid single-leg stance,
the SOL and GA have an opposite energetic effect on the
leg and trunk of the body to ensure support and forward
progression of the leg and trunk [5]. Proper plantarﬂexion
ensures stable weight transfer and drives critical lever-force
behind the ﬁnal rocker action transitioning to pre-swing
[6]. Individuals suffering from hemiparesis after a stroke,
paralysis of pretibial muscles, or ﬁxed plantarﬂexion will
often experience a lack of shock absorption and loss of
deﬁnitive heel strike [6]. This can cause a loss of the ﬁnal
rocker action needed to propel the foot forward for toe-off
to transition to pre-swing [4], [7]. As a result, various forms
Corresponding Author
Carly M. Thalman, Tiffany Hertzell, and Hyunglae Lee are with the
Ira A. Fulton Schools of Engineering, Arizona State University, AZ, USA.
cmthalma@asu.edu, hyunglae.lee@asu.edu
Fig. 1. An illustration of the SR-AFO Exosuit for plantarﬂexion is shown
in (a), while (b) shows the posterior view of the actual device being worn
by a user.
of gait abnormalities arise that can cause further injury, pain,
or risk of trips and falls [7]. The aforementioned afﬂictions
to gait allude to reasons why a passive rigid orthotic device
may not be beneﬁcial in all cases. In typical rehabilitative
environments, treatments can be labor intensive for both
the therapists and patients performing the training. For gait
rehabilitation, therapies often require manually moving the
lower limbs and torso of the patient [8] or rigid AFOs
which create a ﬁxed ankle angle that can cause further
gait abnormalities [9]. To eliminate some limitations of
traditional AFOs, a range of wearable robotic solutions
has been proposed to provide more dynamic and tailored
methods of assistance to a wide range of users [10]–[15].
Robotics designed for lower-extremity assistance has been
shown to assist in restoring natural gait patterns through
assistive actuation and controlled perturbations [10], [16].
the inertia that can cause issues with balance, fatigue and
stride length [17]. Soft robotics is a rapidly growing ﬁeld
in addressing needs in human assistance, speciﬁcally related
to post-stroke therapy and rehabilitation strategies for the
lower body [11], [12]. Some beneﬁts of soft robotic solutions
for ankle rehabilitation are lightweight materials and reduced
issues with joint alignment due to increased compliance [13],
[18].
In previous work, a soft robotic ankle-foot orthosis (SR-
AFO) exosuit was developed to address the issue of foot
drop using soft pneumatic actuators to assist dorsiﬂexion,
inversion and eversion (IE) [19]. The sock-like design was
implemented to go over the user’s athletic shoe and is
2020 3rd IEEE International Conference on Soft Robotics (RoboSoft)
Yale University, USA
978-1-7281-6569-1/20/$31.00 ©2020 IEEE 801 Fig. 2. (a) A user with the donned SR-AFO Exosuit inactive and deﬂated, (b) the SR-AFO Exosuit when active and inﬂated. Fig. 3. Simpliﬁed diagram showing the lever force produced to assist in plantarﬂexion when going from inactive in (a) to active and inﬂated in (b), while (c) shows the plantarﬂexion actuator post inﬂation inspired by thin ﬁlm contracting actuator design. Further evaluation of the IE actuators was completed [20]. This paper presents new developments to the SR-AFO exosuit, including the addition of new actuators placed to assist plantarﬂexion, a more robust sensor placement, and a smaller, more reﬁned design of the control belt. Section II discusses the design of the new SR-AFO exosuit, the addition of the new plantarﬂexion actuators, and the characterization and experimental testing of the plantarﬂexion actuators is explored. Section III introduces the control strategy for the exosuit, user testing, investigating the range of motion (ROM) of the user with and without the SR-AFO exosuit, as well as an electromyography (EMG) study of the muscles used in plantarﬂexion at the ankle joint during walking, and the antagonistic muscles for dorsiﬂexion. Section IV includes a discussion of the overall performance of the SR- AFO exosuit. II. DESIGN AND CHARACTERIZATION OF THE SR-AFO EXOSUIT A. Design Overview The SR-AFO exosuit aims to assist ankle plantarﬂexion by utilizing the intrinsic compliance of the fabric-based design paired with pneumatic actuators anchored at speciﬁc points on the body. The SR-AFO exosuit has a low proﬁle and ﬁts tightly around the user’s foot and lower leg (Fig. 2). The low proﬁle makes the wearable robot less cumbersome and cre- ates an ease of application in different rehabilitative settings. The lightweight materials prevent unwanted alterations to the user’s kinematics during natural gait from becoming strewn by restricted motion or added weight [17]. The actuator runs along the back of the user’s calf and pulls the heel upward, producing a negative angle from a neutral position. TABLE I FUNCTIONAL REQUIREMENTS,CONSIDERATIONS,AND CONSTRAINTS Design Requirements Characteristics Design Considerations and Criteria [13][17] Soft, compliant material Neoprene, Spandex and Nylon Low proﬁle 5mm Easy don/doff 30 s Light weight 200 g Motion and Force Considerations [5], [22]–[25] Support Plantarﬂexion 1.6 Nm/kg PF Muscle Assistance GA, SOL muscles Assists Pre-Swing Active during late stance Minimum ROM 30Dorsiﬂexion-Plantarﬂexion (DP) Controls Criteria [1], [4] Perturbation Timing + 10% natural weight on toe Gait Cycle % Assisted 40% to 60% of gait Pressure Threshold 100 kPa Walk at a natural pace 1.0 m/s The controls and hardware for the SR-AFO exosuit are not carried or worn by the user. The maximum force output of the actuator is modeled similarly to previous work [19], [21] using the simple notion shown in Fig 3(a-b) to get torque: τ=FLf oot (1) where τis the assistive torque applied by the actuator, Fis the uniaxial tensile force during contraction and Lf oot is the distance from the center of the ankle joint to the posterior end of the foot. In previous work [19], the soft fabric actuators are modeled and tested in ideal conditions and ﬁxed boundary conditions through ﬁnite element analysis (FEA) and testing on a universal testing machine using rigid vice grips in perfectly ﬁxed positioning. The contraction percentage (ε) of the actuator was optimized via FEA using the following conditions: ε=LiLf Li ,(2) L(x1) = (n1)d+nx1+2d,(3) where the deﬂated chamber length Liis equal to x1, which is the initial length of each chamber, Lfis the ﬁnal length after inﬂation, dis the distance between each chamber created by the heat seal, and nis the total number of chambers. For this work, 8 chambers have been selected to achieve desired contraction. As contraction is a function of overall length, the selected length was sufﬁcient to pull the ankle to the desired angle. 1) Design Considerations and Criteria: A complete list of all considered design requirements and characteristics is outlined in Table I. The sock-like SR-AFO exosuit design allows the materials to lay close to the net-shape of the user. The SR-AFO exosuit has a proﬁle of 5 mm thickness, adding minimal bulk to the leg with layers comprised of neoprene and spandex. The materials and design of the SR- AFO exosuit make it easy for the user to don/doff. The SR- AFO exosuit is donned from the back of the leg, slipping the heel into an opening in the back of the SR-AFO exosuit. Straps with Velcro secure the SR-AFO exosuit on the top 802 of the foot, around the ankle, and below the knee (Fig. 2). The dofﬁng process is quick and easy, only requiring the user to undo the Velcro straps and removing the SR- AFO exosuit from the back. The actuators also have a low proﬁle when active and inactive. The fabrication of the soft actuators uses a thermoplastic polyurethane (TPU) coated nylon fabric (200 Denier Rockywoods Fabrics) which is thermally bonded with a 2 mm heat impulse sealer (AIE-500 2mm Impulse Sealer, American International Electric INC, CA) which applies uniform heat and pressure to the seam to create an air-tight seal [19]. 2) Motion and Force Considerations: The tensile force of actuation will match a percentage of assistance, which is typically measured at 1.6 Nm/kg for plantarﬂexion torque [25]. The SR-AFO exosuit will assist the primary plantar ﬂexor muscles, GA and SOL, to show a reduction of muscle effort during late stance. The range of motion (ROM) for plantarﬂexion and dorsiﬂexion is 30and 20, respectively. The actuator is pulled tight to lay ﬂat along the calf but does not restrict dorsiﬂexion. This ensures the actuator has ample tension to contract and produce plantarﬂexion when walking ensues. The tension of the actuator is critical to the amount of pulling force it provides to the ankle; this topic will be discussed in later sections. 3) Control Criteria: A tether connects the SR-AFO exo- suit to a control box containing all electro-pneumatic hard- ware. The SR-AFO exosuit allows the user to walk at their natural pace while the actuators engage just before the foot pushes off of the ground. This point varies across participants but the timing coincides with the natural pace of the user. The actuators inﬂate from 40% to 60% of the gait. This range was chosen because it is during this point in the gait cycle where the GA and SOL muscles are activated to produce plantarﬂexion. A pressure threshold of 100 kPa is chosen to reduce risk of mechanical failure due to the cyclical nature of operation. The natural walking pace of a healthy participant typically starts around 1 m/s, so the SR-AFO exosuit should operate within this range. B. Soft Actuator Design Principles While preliminary modeling in ideal conditions was re- quired for actuator design optimization in geometric pattern determinations, it is not enough to assume that the actuators will perform and yield the same force output when worn on a person and afﬁxed to inextensible fabric anchoring points on the SR-AFO exosuit. By allowing the actuator to interface with a fabric anchor, it is assumed that a more realistic force output can be measured to represent the behavior of the actuator when worn on the SR-AFO exosuit. The soft actuator is evaluated following the same testing protocol used in testing Mckibben actuators [26] [27], as the primary function is consistent between the two designs: uniaxial contraction. The experimental setup for the quasi- static isometric tests for varying angles consists of a vice clamp, load cell, and a custom housing structure. The vice clamp is ﬁxed to the bottom of the housing unit and the load cell is supported at the top at a ﬁxed height. Fig. 4. Single actuator testing overview and setup is shown in (a), and double actuators in parallel (b). (c) Double actuators at varying angle, α. Fig. 5. (a) Single actuator testing at varying contractions and pressure. (b) Double actuator testing at varying angles at constant 100 kPa at varying contractions. (c) Double actuator testing at varying contractions and pressure.. C. Quasi-Static Isometric Test Conditions A single actuator is evaluated in this setup to characterize the contracting and uniaxial tension forces produced with a fabric anchoring point (Fig. 4a). The actuator is held at a ﬁxed length for 0%, 10%, 20%, and 30% contraction. Pressure is increased from 0 kPa to 100 kPa in increments of 10 kPa and the resulting payload is recorded across a total of three trials for each pressure and contraction ratio (Fig. 5a). A linear trend in force output is observed past 50 kPa in each displacement evaluated and a maximum payload at 100 kPa. While previous work analyzed a single actuator, this paper investigates how parallel actuators contribute to force output and inﬂation time. To determine the resultant force difference, two actuators are placed in parallel (Fig. 4b). The comparison of a single actuator and parallel actuators is observed at a ﬁxed 0% contraction to track maximum force output for 0 kPa to 100 kPa in increments of 10 kPa. A single actuator results in a maximum payload capacity of 9.3 ±0.15 kg whereas two actuators in parallel in the same orientation and test condition result in a 45.3% increase to 13.51 ±0.31 kg payload capacity (Fig. 5b). The two actuators are tested in parallel to each other and used the same experimental setup as the single actuator. The pressure is varied from 0 kPa to 100 kPa at increments of 10 kPa. This test is repeated at different contraction percentages; 0%, 10%, 20%, and 30% contraction. The maximum payload observed is of 13.52 kg and occurs at 0% contraction. 1) Constant Pressure: To verify which angle the two actuators perform best at, the angle between the two actuators is varied while maintaining a constant pressure (100 kPa) and recording the tensile force produced as seen in Fig. 4c. The two actuators are ﬁxed to a load cell at the same contact 803 Fig. 6. Time delay in actuation to the target pressure for inﬂation (a) and deﬂation (b) a with a single actuator and two parallel actuators. Fig. 7. Time delay in resultant force from actuation to the target pressure of 100 kPa for a ﬁxed rise of (a) 0.1 sec, (b) 0.2 sec, (c) 0.3 sec, and (d) 0.4 sec for a period of 1 sec. The force output over four pressure levels is shown in (e) for a period of 1 sec and rise time of 0.3 sec. point, while the distance between the actuators is varied at the bottom to provide different angles. Initially, the actuator angle is set to 0, where one actuator is positioned vertically parallel to the adjacent actuator. The angle is adjusted in increasing increments of 5from a parallel vertical position until a total angle of 30between the two actuators is obtained. This process is repeated with the actuator being held at a ﬁxed contraction percentage of 0%, 10%, 20%, and 30% from its initial length. This allows the force output of the actuators to be analyzed quasi-statically as both a function of the placement angle and displacement. The results of the testing can be seen in (Fig. 5c), which show that varying the actuator angle is relatively insigniﬁcant. 2) Constant Angle: The experimental setup for the quasi- static isometric test for varying pressure uses the same housing unit as the quasi-static isometric tests for varying angles. A vice clamp is used to ﬁx the actuators in place at the bottom of the housing unit. The angle between the two actuators is ﬁxed at 5, which is the angle determined to have the greatest force by the previous quasi-static test for each level of contraction: 0%, 10%, 20%, and 30% (Fig. 5c). The two actuators are ﬁxed to the load cell at the top of the housing unit at the same point. The pressure is varied from 0 kPa to 100 kPa in 10 kPa increments, while the angle between actuators is held constant and the contraction of the actuator is again ﬁxed at 0%, 10%, 20%, and 30%. D. Actuator Performance Validation The SR-AFO exosuit needs sufﬁcient cycle time to inﬂate and deﬂate within a short window. To determine the inﬂation and deﬂation speeds of the soft actuators, a pressure sensor (ASDXAVX 100PGAA5, Honeywell Sensing and Produc- tivity Solutions, Charlotte, USA) is used to monitor the instantaneous pressure provided to the actuator. The pressure is held constant at the source and a solenoid valve (320 12VDC, Humphrey, USA) is opened to create inﬂation. The pressure is monitored at a baud rate of 9600 Hz and the output is evaluated for the rise time. Fig. 6 shows inﬂation and deﬂation time of both single and double actuators under no-load conditions. Observing the dynamic force response it can provide is critical in understanding how the actuator performs when used during walking. A universal tensile testing machine (UTM) (Instron 5565, Instron Corp., High Wycombe, United Kingdom) is used to measure the dynamic response of the actuators. Fig. 7a-d shows the actuator force output with various actuation times pre-deﬁned. For each instance, the actuator pressurizes for 0.1, 0.2, 0.3, and 0.4 sec for a ﬁxed period of 1 sec, averaged across 30 trials. A manual valve is used to ensure the actuators reach the desired pressure of 100 kPa set for each trial. Under 0% contraction and loaded conditions, the actuators are able to reliably reach the peak pressure and corresponding force output of 118.2 ±3.1 Nat an interval of 0.3 sec. Fig. 7d shows that the resultant force from the actuators becomes more accurate in both rise time and the deviation of the peak force at higher pressures when observed between 100 kPa and 40 kPa. The force returns to 0 Nat the end of each trial, suggesting that that inﬂation and deﬂation time of the actuators under load will be sufﬁcient for walking. III. CON TROL O F THE SR-AFO EXOSUIT AND PRELIMINARY HUMAN EXPERIMENTS The SR-AFO exosuit is controlled by a control box, which houses all of the electro-pneumatic components required to provide dynamic control to the SR-AFO exosuit, and allows the SR-AFO exosuit to function autonomously without con- tinuous monitoring or input from the user. The timing of the controlled perturbation for the soft actuators is critical for ensuring the user is receiving assistance in the right location at the proper interval. When this is achieved, muscle exertion of the GA and SOL muscles will be reduced in the forward swinging motion of the leg. This reduces the effort exerted during walking for the user, creating a useful solution to gait rehabilitation. The actuator will inﬂate during mid-stance, when the GA and SOL isometrically contract to activate the Achilles tendon to provide pre-tensioning for plantarﬂexion. A. Control of the SR-AFO Exosuit The control box houses the logic controller (Arduino Mega 2560 Rev3), ﬁtted with a ProtoShield to connect 804 Fig. 8. Logic control box for the SR-AFO exosuit, which controls the timing of actuation, monitors pressure levels, and collects kinematic behavioral data of the user through FSR sensors. and read analog signals from force-sensitive resistor (FSR) sensors (Interlink 406, Adafruit, New York, USA). The FSR sensors are embedded in the user’s shoe at the anterior and posterior ends of the shoe insoles to provide kinematic data information that is critical to time-events to detect heel strike and the weight shift preceding pre-swing. A 12V, 5000 mAh LiPo Battery is used to power the system. A custom voltage regulator board is used to step the voltage down to 5V to power both the logic board and the pressure sensors (ASDXAVX 100PGAA5, Honeywell Sensing and Productivity Solutions, Charlotte, USA), which monitor the resulting pressure throughout operation. A portable air com- pressor (Model 8010A, California Air Tools, USA) is used to provide a pneumatic actuation source and is controlled through the use of 3-way, 2-channeled solenoid values (320 12VDC, Humphrey, USA), which activate during speciﬁed times in the human gait cycle to provide instantaneous pressure to the actuators through the tether (Fig. 8). The valves are powered through MOSFETs (IRF520 MOSFET Driver Module) which connect the signal from the logic controller to the 12V power rail. The housing of the control box is made from a custom 3D printed PLA design to house all of the electro-pneumatic components. LEDs are used as indicators once the lid of the control black box is closed to provide information to the user regarding the current state or status of the system. A power switch is provided on the side of the box to shut down the system and safely disconnect from battery power when not in use. B. Human Experiments A total of three healthy participants are recruited for this preliminary study [ages: 21 - 26 years, weight: 60.5 - 65.8 kg, height: 1.67 - 1.72 m]. This study has been approved by the Institutional Review Board of Arizona State University (STUDY00004351). In preparation for the walking trials, surface EMG sensors (Bagnoli Desktop System, Delsys, Natick, MA) are used to monitor the tibialis anterior (TA), Fig. 9. Controller logic based on bang-bang control, based on input from the FSR, producing inﬂation in the actuator when FSR passes a threshold. Fig. 10. State of the system during a single gait cycle going from heel strike to heel strike showing the threshold being reached prior to the active range of the actuator which spans from mid-stance to pre-swing. GA, and SOL throughout the duration of the trials after the maximum voluntary contraction (MVC) and the muscle activity during relaxation is collected as per standard Interna- tional Society of Electrophysiology and Kinesiology (ISEK) protocols [28]. Ankle angle in the sagittal plane for ankle Dorsiﬂexion-Plantarﬂexion (DP) is tracked throughout the trials using a wireless dual-axis goniometer (SG110, Biomet- rics Ltd, UK). All data is streamed through a motion capture system (Bonita 10, VICON Inc., Los Angeles, CA), and the participant dons a custom shoe retroﬁtted with FSR sensors as described in section II.E. The participant is asked to walk on a split-belt treadmill (Bertec Treadmill, Columbus, OH, USA) for two minutes prior to data collection to allow for acclimation to the repetitive and controlled treadmill walking and speed. This time is used to let the participant select a comfortable walking speed which ranged between 1.0 m/s and 1.2 m/sfor the three participants. The participant is asked to stand naturally with even weight distribution on each foot. Bodyweight and applied force on the FSR sensor in the shoe is recorded, and the threshold is set at 10% above this calibrated value for the FSR in the toe to detect the weight shift and transition out of mid-stance during walking (Fig. 10). Three walking trials are performed for each participant with 1) no SR-AFO exosuit, 2) the donned passive SR-AFO exosuit, and 3) the SR-AFO exosuit active. During each walking trial, the participant is asked to walk on the treadmill at their selected walking speed 805 Fig. 11. Average EMG data for individual steps collected and averaged over 2 minute walking trials, shown here for a representative subject. The EMG signal for the GA muscle is shown in (a), for the SOL in (b) and the TA in (c). Fig. 12. Average ankle angle for DP across each two minute walking trial, with SR-AFO exosuit passive, active, and with no SR-AFO exosuit for all three healthy participants. for the aforementioned experimental setups with a 1 min resting period between trials. For each participant, the trials are averaged across the duration of each step, synchronizing heel strike as the start of the gait cycle for the right foot. Heel strike is detected via the force plates in the split-belt treadmill. The difference between muscle activity recorded with no SR-AFO exosuit and the passive SR-AFO exosuit was negligible. During the trials where the SR-AFO exosuit was active, muscle activity reduction is seen in the GA, and SOL while the TA increases slightly during swing most likely due to the hysteresis in the system deﬂating during that window. The reduction is calculated by taking the area under the curve between the average of the trials with and without SR-AFO exosuit assistance between 40% and 60% of the gait cycle. Average GA reduction observed for the three participants was TABLE II WALKI NG SP EE DS A ND M US CL E RE DU CT IO N FO R EAC H PART IC IPA NT 13.4%, and the average reduction for SOL was measured at 16.6% (Fig. 11). The reduction of each participant is shown in Table II. The reduction observed during this window of gait indicates that the SR-AFO exosuit is able to provide assistance to the user during late stance by taking effort away from the primary two muscles responsible for powering this critical stage of the gait cycle. The increase in the TA muscle is anticipated due to its antagonistic behavior in relation to the actuation for plantarﬂexion assistance provided. The angle of the ankle is monitored for DP and syn- chronized across the gait cycle starting with heel strike, and averaged across each 2 minute trial. With no SR-AFO exosuit, the average DP range of motion is observed at 31.27, while the passive SR-AFO exosuit trials showed an ankle DP ROM of 29.83. This falls within 0.57% difference from the 30required ROM for ankle DP angle during gait. When the SR-AFO exosuit is providing active assistance, the average ROM in ankle DP is observed at 30.54, which meets the minimum aforementioned requirement and falls within 2.3% difference from the original ROM with no SR-AFO exosuit. An increase in maximum dorsiﬂexion angle was recorded with the passive SR-AFO exosuit and active SR- AFO exosuit, while a decrease in maximum plantarﬂexion is observed. The increased dorsiﬂexion angle during active stance is likely a compensatory motion by the user due to the presence of the SR-AFO exosuit, which is supported by the increased TA muscle activity during this window (40% - 60%). Decreased plantarﬂexion during late stance can likely be attributed to an increased pretension effect caused by the actuators, where the user does not need to push off with their toe as far as they previously needed to without the SR-AFO exosuit. The negligible change in muscle effort for GA and SOL during this period can help support this hypothesis and suggests the SR-AFO exosuit was able to provide assistance to the healthy subjects for plantarﬂexion. IV. DISCUSSION This paper presents the design of the SR-AFO exosuit for plantarﬂexion assistance, characterization of actuator properties and behaviors, and preliminary results for walking trials of healthy participants. The design is lightweight and form-ﬁtting for the user and can be easily worn over a variety of athletic shoes of various sizes. The actuators are characterized for payload capacity for a single actuator, and two actuators in parallel. When an additional actuator is used, the payload capacity increases 45.3% to produce a 13.51 ± 0.31 kg payload when inﬂated to 100 kPa. Three healthy participants are recruited to participate in a walking study, 806 with no SR-AFO exosuit, the SR-AFO exosuit passive, and the SR-AFO exosuit active and providing assistance. EMG sensors and a wireless goniometer were used to monitor the muscle activity and kinematics of the ankle joint. The actuators are inﬂated during the window of late stance (40% - 60% of the gait cycle). During this window, a reduction of 13.4% is seen for the GA, and 16.6% for the SOL. There is a slight increase in the TA muscle during this stage as well as through the swing phase, however, this is anticipated as there may be some residual energy still stored in the system. The ankle angle is able to verify that the minimum ROM for ankle DP for normal walking (30) is still maintained throughout gait, with a slightly offset proﬁle that does not affect muscle reduction. These preliminary results suggest that the SR-AFO exosuit would be an effective assistive device for plantarﬂexion for impaired user testing in the future. Future work will consist of the inclusion of further sensory information about the system, such as embedding wireless Inertial Measurement Unit (IMU) sensors for continuous ankle angle monitoring, as well as developing algorithms to provide pressure sensor feedback control based on the ankle’s current position and stage of the gait cycle. Future studies will also investigate the integration of these algorithms with actuators proposed in future works to support additional degrees of freedom and stages of gait. Finally, this work will aim to begin testing with impaired users in entrainment studies to evaluate the usefulness of controlled perturbations from a completely soft, fabric-based actuation system for gait training and regulating stride length. ACK NO WL EDG EME NT S C. M. Thalman is funded by the National Science Foun- dation, Graduate Research Fellowship Program (NSF-GRFP) award #1841051. This work is funded by the Global Sport Institute of the adidas and Arizona State University (ASU) Global Sport Alliance. The authors would like to thank Kayleigh Gavin and Omik Save for contribution. REFERENCES [1] D J Farris and G S Sawicki. The mechanics and energetics of human walking and running: a joint level perspective. Journal of The Royal Society Interface, 9(66):110–118, 2011. [2] D A Winter. 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In 2019 International Conference on Robotics and Automation (ICRA), pages 8436–8442. IEEE, 2019. [20] CM Thalman and H Lee. Design and validation of a soft robotic ankle- foot orthosis (sr-afo) exosuit for inversion and eversion ankle support. In 2019 International Conference on Robotics and Automation (ICRA). IEEE, 2020. [Accepted]. [21] R Niiyama, D Rus, and S Kim. Pouch motors: Printable/inﬂatable soft actuators for robotics. In Robotics and Automation (ICRA), 2014 IEEE International Conference on, pages 6332–6337. IEEE, 2014. [22] A T Asbeck, R J Dyer, A F Larusson, and C J Walsh. Biologically- inspired soft exosuit. In Rehabilitation robotics (ICORR), 2013 IEEE international conference on, pages 1–8. IEEE, 2013. [23] J. J Eng and D A Winter. Kinetic analysis of the lower limbs during walking: what information can be gained from a three-dimensional model? Journal of Biomechanics, 28(6):753–758, 1995. [24] C. L Brockett and G. J Chapman. Biomechanics of the ankle. Orthopaedics and trauma, 30(3):232–238, 2016. [25] L M Mooney and H M Herr. Biomechanical walking mechanisms underlying the metabolic reduction caused by an autonomous ex- oskeleton. Journal of neuroengineering and rehabilitation, 13(1):4, 2016. [26] C. Chou and B. Hannaford. Measurement and modeling of mckibben pneumatic artiﬁcial muscles. IEEE Transactions on robotics and automation, 12(1):90–102, 1996. [27] M. Doumit, A. Fahim, and M. Munro. Analytical modeling and experimental validation of the braided pneumatic muscle. IEEE transactions on robotics, 25(6):1282–1291, 2009. [28] R. Merletti and P Di Torino. Standards for reporting emg data. J Electromyogr Kinesiol, 9(1):3–4, 1999. 807 ... These have seen use in exoskeletons to aid in abduction/adduction and flexion/ extension. They are also seen in papers such as [79] and [81] which use a bubble-like strip of PAMs attached to the knee and ankle respectively to act as walking aids, when filled with air they expand outwards and so can help to extend the knee or ankle, then when are is taken out they contract and allow the knee and ankle to flex. A different implementation is seen in [76] and [92] which use single, large inflatable pads place on the back of the upper and lower leg and the front knee, connected via a non-compliant cloth which also attaches to the leg, when inflated they help to contract the knee by changing the cloth angle. ... ... Example solenoids such as the Festo 5 V VOVG Solenoid used in [65], Sizto 2S012-020 in [69], and Humphrey Series 320 in [81] have been used in order to control the flow of pressurised gas within both pneumatic cylinders and muscles. Some of these designs, such as the VOVG Solenoid are modular, and as such their weight and size do not increase entirely proportionally with the number of valves, starting at 84 g for 2, they increase by ~ 21 g per additional valve [99, p. 17]. ... ... With some examples including the DeWALT D55146 225 PSI Compressor [96] used in [78], the Elektromotorenwerk ECS 80G 4-213 EMG Compressor [97] used in [79], and the California Air Tools 8010A [98] used in [81]. Each of these have output pressures varying from 120 to 225PSI, but require power in the kW's, have weights between 16.8 kg-37.7 kg, and noise outputs between 60-78 dB (60 dB ~ = sound of a conversation, 78 dB ~ = sound of a vacuum cleaner). ... Article Full-text available A common issue with many commercial rehabilitative exoskeletons and orthoses are that they can be prohibitively expensive for an average individual to afford without additional financial support. Due to this a user may have limited to the usage of such devices within set rehabilitation sessions as opposed to a continual usage. The purpose of this review is therefore to find which actuator implementations would be most suitable for a simplistic, low-cost powered orthoses capable of assisting those with pathologic gait disorders by collating literature from Web of Science, Scopus, and Grey Literature. In this systematic review paper 127 papers were selected from these databases via the PRISMA guidelines, with the financial costs of 25 actuators discovered with 11 distinct actuator groups identified. The review paper will consider a variety of actuator implementations used in existing lower-limb exoskeletons that are specifically designed for the purpose of rehabilitating or aiding those with conditions inhibiting natural movement abilities, such as electric motors, hydraulics, pneumatics, cable-driven actuators, and compliant actuators. Key attributes such as technical simplicity, financial cost, power efficiency, size limitations, accuracy, and reliability are compared for all actuator groups. Statistical findings show that rotary electric motors (which are the most common actuator type within collated literature) and compliant actuators (such as elastic and springs) would be the most suitable actuators for a low-cost implementation. From these results, a possible actuator design will be proposed making use of both rotary electric motors and compliant actuators. ... Pneumatic actuators are made of pneumatic cylinders or cylinder-like elements with enclosed pistons that can be powered and driven using external air compressors. Four types of pneumatic actuators were used in the reviewed PAEs: pneumatic artificial muscles [11,38,49,50,62,63,[88][89][90][91][92][93][95][96][97][98][99][131][132][133][134][135]146,[174][175][176]179,[190][191][192][193][194][195][196][197][198][199][200][201]203], pneumatic cylinders [43,[166][167][168][169][170][171], soft fabric actuators [156,232], and soft fiber braided bending actuators [211]. Pneumatic actuators are cheap and can provide high specific power. ... ... Electric actuators were the most popular actuators deployed in the reviewed PAEs. They were powered by on-board battery packs [37,41,56,57,61,[65][66][67][76][77][78][79][80][81][82][83][84]86,87,94,[100][101][102]112,113,115,[118][119][120]126,127,130,144,145,[156][157][158][159][160][172][173][174][175]177,184,187,189,234,235], DC off-board power supply units [10,39,109,116,121,122,128,136,[147][148][149], and AC off-board power supply units [72,181]. Eight different types of electric actuation elements were used in the reviewed articles: brushed DC motors [86,115,121,172], brushless DC motors [37,39,40,42,57,[64][65][66][76][77][78][79][80][81][82][83][84][100][101][102][103][104][105][106][107][108][109][110][111][112][113][116][117][118][122][123][124][125]127,129,143,[147][148][149][150][151][157][158][159][160][161][162][163][164][165]178,[184][185][186][187]210], servo DC motors [74,75,87,119,120,141,144,145,177], servo AC motors [53,58,[68][69][70][71][72][73][180][181][182][183]202,204], stepper motors [189], permanent magnetic synchronous motors [85], DC voice coil actuators [154,155], and hybrid drive systems [138][139][140]173]. ... ... Yes [131,146,[174][175][176] No [11,38,49,50,62,63,[88][89][90][91][92][93][95][96][97][98][99][132][133][134][135]142,179,[190][191][192][193][194][195][196][197][198][199][200][201]203] Pneumatic Cylinders Yes [43,[166][167][168][169][170][171] Exosuit Pneumatic Source (Soft Fabric Actuator) No [156,232] Soft Fiber Braided Bending Actuator No [211] Electric Brushed DC Motors Yes [115,172] No [86,121] Brushless DC Motors Yes [37,39,40,42,57,65,66,[76][77][78][79][80][81][82][83][84][100][101][102][111][112][113]117,118,122,123,127,[157][158][159][160]178,[184][185][186][187] No [64,[103][104][105][106][107][108][109][110]116,124,125,129,143,[147][148][149][150][151][161][162][163][164][165]210] Servo DC Motors Yes [74,75,87,119,120,144,145,177] No [141] Servo AC Motors No [53,58,[68][69][70][71][72][73][180][181][182][183]202,204] Stepper Motor No [189] Permanent Magnetic Synchronous Motors No [85] Electromechanical DC Voice Coil Actuator No [154,155] Electrohydraulic Hybrid Drive System Yes [173] No [138][139][140] Electric Motors (Type Not Specified) Yes [56,114] No [41,126,128] ... Article Full-text available Powered ankle exoskeletons (PAEs) are robotic devices developed for gait assistance, rehabilitation, and augmentation. To fulfil their purposes, PAEs vastly rely heavily on their sensor systems. Human–machine interface sensors collect the biomechanical signals from the human user to inform the higher level of the control hierarchy about the user’s locomotion intention and requirement, whereas machine–machine interface sensors monitor the output of the actuation unit to ensure precise tracking of the high-level control commands via the low-level control scheme. The current article aims to provide a comprehensive review of how wearable sensor technology has contributed to the actuation and control of the PAEs developed over the past two decades. The control schemes and actuation principles employed in the reviewed PAEs, as well as their interaction with the integrated sensor systems, are investigated in this review. Further, the role of wearable sensors in overcoming the main challenges in developing fully autonomous portable PAEs is discussed. Finally, a brief discussion on how the recent technology advancements in wearable sensors, including environment—machine interface sensors, could promote the future generation of fully autonomous portable PAEs is provided. ... The ankle robot, the Soft Robotic Ankle-Foot Orthosis (SR-AFO), is very light (0.203 kg) and thus has a substantially smaller impact on gait biomechanics due to added mass. Perturbations from the pneumatic actuator (Fig. 1b), controlled by a solenoid valve to supply instantaneous pressure, are gentle yet powerful enough to provide assistance to the user [21]. It is predicted that using the lightweight, pneumatically-driven, soft robot will make the entrainment process feel more natural, potentially extending its use in gait adaptation, in place of the heavy, motorized, rigid robots which apply an abrupt force over a smaller window. ... ... A pair of flat fabric pneumatic artificial muscles (ff-PAM) that contract and generate a pulling force at the heel run along the posterior side of the leg and connect to the anchor point just behind the knee. Preliminary studies and earlier developments of this work show the original design and optimization of the ff-PAM actuator [22], as well as further investigation into the dual ff-PAM and final SR-AFO exosuit design [21]. The torque output of the actuators is provided as, ... ... Applied perturbations consisted of 0.2-second pulses of inflation for the ff-PAM, at a consistent pressure level of 200 kP a supplied to the valve. During walking trials performed in previous studies where the valve was released for 0.2 seconds [21] and activated during late stance into push-off, the resulting pressure would reach only 152.3 kP a (Fig. 3b). According to Fig. 3a, this would yield an average force output of 177.8 ± 2.2 N on the participant's ankle to assist in plantarflexion if the subject was successfully entrained to the frequency of applied perturbations. ... Article Full-text available An entrainment study was conducted with a novel soft robotic ankle-foot orthosis (SR-AFO) consisting of a pair of flat fabric pneumatic artificial muscles (ff-PAM). Entrainment capabilities of a lighter soft robotic orthosis were compared with heavy rigid robotic counterparts reported previously. To measure the SR-AFO's capacity to manifest gait entrainment, periodic pneumatic plantarflexion perturbations equal to the calculated increase from the subject's preferred gait frequency were applied to the ankle. Two days of experiments were conducted. In the Day 1 experiment, perturbations were applied from the baseline to a 15% increase in the gait frequency with steps of 3% at a fixed treadmill speed of the subject's preferred walking speed. In the Day 2 experiment, in order to investigate the maximum entrainment capability with the SR-AFO, perturbations were applied from the baseline with steps of 5% with proportionally increasing walking speed until subjects failed to maintain phase locking for 50 or more consecutive steps. In the Day 1 experiment, all 10 subjects were entrained at the highest 15% condition. In the Day 2 experiments, the average basin of entrainment was 39.3$\pm\$ 9.2%. Importantly, phase locking was always observed in the push-off phase of the gait cycle in both days of experiments. The observed basin of entrainment with the SR-AFO was substantially higher than the previously reported value (+7%) with a heavy rigid ankle robot, confirming the potential of the SR-AFO to significantly extend the effectiveness of the entrainment paradigm in gait adaptation and rehabilitation.
... Needs an emergency stop button and a more powerful motor for safety the swing phase of gait as shown in Figure. 13(G) [87], [91]. The soft robotic device, which is placed at the posterior end of the foot such that its contraction pulls the foot, proved to increase the plantarflexion angle while maintaining the natural ankle range of motion. ...
... The robotic device delivers the assistive propulsive force through a bidirectional cable-driven actuation system. It stabilizes inversion-eversion using small and lightweight gear motors VOLUME 4, 2016 [81], (B) Pneumatic artificial muscle and pneumatic rotational actuator ankle assisting robotic device [82], (C) McKibben-type artificial muscle aligned in series with a tension spring AFO [83], (D) Portable powered AFO with compact custom compressor [84], (E) Optimized portable powered AFO [85], (F) Soft pneumatically actuated AFO exosuit [86], (G) Soft pneumatically actuated AFO exosuit [87], (H) Artificial muscle actuated compliant ankle robotic device [88], (I) Pneumatic artificial muscles mimicking muscle-tendon-ligament-skin system [89] which deliver a counter-electromotive force. T-Flex is another bio-inspired AFO actuation presented by Manchola et al., which mimics the behavior of the antagonist muscles by adjusting the stiffness of bio-inspired tendons according to the gait cycle, as seen in Figure. ...
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Drop foot is a pathological type of gait frequently exhibited by individuals suffering from stroke and other neurological conditions due to the weakness of the ankle dorsiflexor muscles. To avoid common negative compensations, such as foot-slap during the loading response and toe-drag during the swing phase of gait, various drop foot assistive robotic devices and technologies have emerged over the last couple of decades. This review summarizes the design, working principle, and application of robotic devices for drop foot assistance and rehabilitation in the last decade. The research findings describe the design aspects of 72 lower-limb robotic assistance devices for drop foot, including 21 studies that evaluated specific design aspects through experimental trials. All the designs reviewed here demonstrated the capability to successfully improve drop foot impairments in the sagittal plane. Some leveraged advanced functional features to achieve optimal performance without jeopardizing the user’s natural range of motion, comfort, balance, or safety. However, there remain certain limitations when combining all these functional features into one robotic device. Overcoming these limitations should add great value to the future of advanced robotic devices for drop foot assistance and rehabilitation.
... Exploiting the compressibility of air is another way to achieve stiffness modulation in exoskeleton robots. Such soft pressurizable actuators are advantageous due to their high strength-toweight ratio [20], [21]. Pneumatic actuators also open up the possibility of connecting to a network system and adjusting compliance and stiffness [22]. ...
... Several existing exoskeleton devices are based on lightweight pneumatic artificial muscles and reduce the energy expenditure of the user while walking [23], or enhance the strength of the upper body power while keeping the exoskeleton lightweight with small inertia [24]. Different groups have designed and fabricated their own pneumatic actuators using non-standard parts [21], Fig. 1. The quasi-passive mechanism can operate in several modes, as indicated in Fig. 2. In a), valve 1 is on, allowing free airflow between the chambers, resulting in free motion. ...
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State-of-the-art compliant actuators with variable stiffness, meet the requirements for exoskeletons only to a limited extent, usually due to their higher mechanical complexity and large mass. In this paper, we present a quasi-passive lightweight pneumatic mechanism that emulates stiffness modulation in the pneumatic cylinder using fast-switching valves and without the need of an air supply. Depending on the state of the valves, the mechanism can operate in different modes. For example, timely control of three fast-switching solenoid valves can modulate the internal equilibrium pressure and adjust the stiffness of the cylinder, or it can be turned off to allow free movement, which is sometimes critical in wearable robotic applications. The novel approach to stiffness modulation is mathematically described. Furthermore, we discuss how changing the initial equilibrium pressure of the mechanism affects the stiffness and energy storage capacity, which has been studied in several experiments. The experiments show successful modulation of the stiffness without the need for an external pressurized air supply. The obtained measured findings are satisfactorily consistent with the derived theoretical model.
... : Biomedical application of pneumatic soft robotics: (a) STIFF-FLOP [566], (b) novel STIFF-FLOP with fiber jamming [368], (c) soft robotic gastric simulator [567], (d) soft robotic heart sleeve [568], (e) soft robotic glove [569], (f) soft elbow exosuit [570], (g) soft robotic ankle-foot orthosis exosuit [571], (h) pneumatic force jacket [572], and (i) I-support soft arm for bathing assistance [564]. All figures are reproduced with permission. ...
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Soft robotics is a rapidly evolving field where robots are fabricated using highly deformable materials and usually follow a bioinspired design. Their high dexterity and safety make them ideal for applications such as gripping, locomotion, and biomedical devices, where the environment is highly dynamic and sensitive to physical interaction. Pneumatic actuation remains the dominant technology in soft robotics due to its low cost and mass, fast response time, and easy implementation. Given the significant number of publications in soft robotics over recent years, newcomers and even established researchers may have difficulty assessing the state of the art. To address this issue, this article summarizes the development of soft pneumatic actuators and robots up until the date of publication. The scope of this article includes the design, modeling, fabrication, actuation, characterization, sensing, control, and applications of soft robotic devices. In addition to a historical overview, there is a special emphasis on recent advances such as novel designs, differential simulators, analytical and numerical modeling methods, topology optimization, data-driven modeling and control methods, hardware control boards, and nonlinear estimation and control techniques. Finally, the capabilities and limitations of soft pneumatic actuators and robots are discussed and directions for future research are identified.
... Gears increase torque, however are not always suitable for exoskeletons because of their poor weight-topower ratio. Recent promising actuator solutions are soft pressurizable actuators [16], [17], [18], [19] as well as distantly located motors where force is transmitted to the limb by Bowden cables [20], [21], [22], [23], [24], [25]. Both solutions have been shown to be suitable in exoskeletons due to low limb inertia. ...
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Quasi-passive exoskeletons have emerged as a solution that avoids the high energy requirements that negatively affect the efficiency of exoskeletons. These exoskeletons do not deliver positive mechanical work to the joint, but accumulate and deliver energy in a viscoelastic element that is actively placed or removed parallel to the user's muscles. %One of the crucial aspects of the exoskeleton is the power-to-weight ratio of the actuator, as this inescapably affects the weight and thus the portability of the entire system. Previous research has investigated different strategies, mostly based on energy harvesting with compliant elements for mechanical energy storage and locking mechanisms to turn the elements on and off. This paper seeks to address the problem of bulky actuators in quasi-passive exoskeletons by experimentally evaluating the proposed quasi-passive mechanism consisting of a pneumatic cylinder, acting as an elastic element, and a solenoid valve replacing a mechanical clutch. The main advantage of the proposed mechanism is that the elastic element can be turned on and off at any time and position, with a high switching frequency, which improves the possibility to harvest the energy. Our aim was to investigate whether, with the proposed actuation, it is possible to achieve the force that has previously been shown to have a positive effect on ankle plantar flexion. Furthermore, we analyzed how the timely activation of the solenoid valve affects the force characteristics. We built the testbed equipped with sensors that allow measurements of torque, pressure, and angular deflection. The obtained measurements are in line with the theoretical model, where the force achieved is within the required range. In addition, it was shown that the time for the actuator to switch off from the peak force is about 100 ms, without major air leaks and energy bursts. The presented results highlight that the actuation of this type is a good candidate for designing a lightweight high-performance quasi-passive exoskeleton.
... Various methods have been used in the controller design problem of active AFOs, including trajectory tracking control, torque control, and variable impedance control. [21][22][23][24]. Control algorithms such as PID, impedance, and adaptive are used to solve this controller problem [25][26][27]. ...
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We present the design and control of a pneumatic ankle-foot orthosis (P-AFO) device powered via bi-directional pneumatic rotary actuator and a pneumatic artificial muscle for rehabilitation assistance and treatment of neuromuscular disorders. The rotary actuator and the pneumatic muscle assist with dorsiflexion and plantar flexion, respectively. The prototype is also equipped with simple sensor system for gait pattern analysis. The P-AFO has the capability of 20 degrees dorsiflexion from the plantar flexion and 12 degrees dorsiflexion from the neutral position of an ankle joint. The data-driven predictive control (DDPC) algorithm has been designed for P-AFO to follow desired gait cycle trajectories while rectifying the nonlinearity and uncertainties of the pneumatic actuators. The design of DDPC is realized from the subspace identification matrices acquired by the input-output values obtained as a result of an open-loop operation. The control structure is completely data-based without certain use of a model in the control implementation. In order to control the developed P-AFO prototype device, the suggested controller was implemented in a real-time operating system. Experimental studies are performed to compare the proposed controller with a three-term controller (PID) in trajectory tracking of the P-AFO.
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The field of robot-assisted physical rehabilitation and robotics technology for providing support to the elderly population is rapidly evolving. Lower limb robot aided rehabilitation and assistive technology have been a focus for the engineering community during the last three decades as several robotic lower limb exoskeletons have been proposed in the literature as well as some being commercially available. Numerous manufacturing techniques and materials have been developed for lower limb exoskeletons during the last two decades, resulting in the design of a variety of robot exoskeletons for gait assistance for elderly and disabled people. One of the most important aspects of developing exoskeletons is the selection of the most appropriate proper material. The material selection strongly influences the overall weight and performance of the exoskeleton robot. The most suitable fabrication method for material is also an important parameter for the development of lower limb robot exoskeletons. In addition to the materials and manufacturing methods, the actuation method plays a vital role in the development of these robot exoskeletons. Even though various materials, manufacturing methods and actuators are reported in the literature for these lower limb robot exoskeletons, there are still avenues of improvement in these three domains. In this review, we have examined various lower limb robotic exoskeletons, concentrating on the three main aspects of material, manufacturing, and actuation. We have focused on the advantages and drawbacks of various materials and manufacturing practices as well as actuation methods. A discussion on future directions of research is provided for the engineering community covering the material, manufacturing and actuation methods.
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Background Ankle exoskeletons can now reduce the metabolic cost of walking in humans without leg disability, but the biomechanical mechanisms that underlie this augmentation are not fully understood. In this study, we analyze the energetics and lower limb mechanics of human study participants walking with and without an active autonomous ankle exoskeleton previously shown to reduce the metabolic cost of walking. Methods We measured the metabolic, kinetic and kinematic effects of wearing a battery powered bilateral ankle exoskeleton. Six participants walked on a level treadmill at 1.4 m/s under three conditions: exoskeleton not worn, exoskeleton worn in a powered-on state, and exoskeleton worn in a powered-off state. Metabolic rates were measured with a portable pulmonary gas exchange unit, body marker positions with a motion capture system, and ground reaction forces with a force-plate instrumented treadmill. Inverse dynamics were then used to estimate ankle, knee and hip torques and mechanical powers. Results The active ankle exoskeleton provided a mean positive power of 0.105 ± 0.008 W/kg per leg during the push-off region of stance phase. The net metabolic cost of walking with the active exoskeleton (3.28 ± 0.10 W/kg) was an 11 ± 4 % (p = 0.019) reduction compared to the cost of walking without the exoskeleton (3.71 ± 0.14 W/kg). Wearing the ankle exoskeleton significantly reduced the mean positive power of the ankle joint by 0.033 ± 0.006 W/kg (p = 0.007), the knee joint by 0.042 ± 0.015 W/kg (p = 0.020), and the hip joint by 0.034 ± 0.009 W/kg (p = 0.006). Conclusions This study shows that the ankle exoskeleton does not exclusively reduce positive mechanical power at the ankle joint, but also mitigates positive power at the knee and hip. Furthermore, the active ankle exoskeleton did not simply replace biological ankle function in walking, but rather augmented the total (biological + exoskeletal) ankle moment and power. This study underscores the need for comprehensive models of human-exoskeleton interaction and global optimization methods for the discovery of new control strategies that optimize the physiological impact of leg exoskeletons.
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