1state key laboratory of robotics and system, harbin ... · 1state key laboratory of robotics and...

SWINGING LEG CONTROL OF A LOWER LIMB EXOSKELETON VIA A SHOE WITHIN-SOLE SENSING

Yanhe Zhu1, Chao Zhang1, Jizhuang Fan1, Hongying Yu2 and Jie Zhao11State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, China

2School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, ChinaE-mail: [email protected]; [email protected]

IMETI 2015 WB5007_SCINo. 16-CSME-21, E.I.C. Accession Number 3907

ABSTRACTA lower limb exoskeleton can help in weight-bearing and walking to assist laborers doing heavy work. Forexoskeleton-assisted walking, the wearing comfort and walking convenience are important so there mustbe minimal interference with leg movement. Hence, a peculiar design strategy based on an in-sole sensingshoe is presented to achieve real-time motion detection and follow-up control of the moving leg. Comparedto the elastic muscle extension, the sensor must exhibit minimal deflection under load. Therefore, an ultra-thin structure integrating 6 bar linkages and 3 cantilevers has been used in the design of the in-sole sensingshoe which can detect force in two directions and torque in one. A swing phase experiment and a randomleg motion test were carried out. Results show validity of human motion detection and follow-up controlstrategy based on this plantar surface sensor.

Keywords: swinging leg control; exoskeleton; in-sole sensing shoe; man-machine interface.

CONTRÔLE DU BALANCEMENT DE JAMBE D’UN EXOSQUELETTE DE MEMBREINFÉRIEUR À L’AIDE DE CAPTEURS DANS LA SEMELLE DE LA CHAUSSURE

RÉSUMÉUn exosquelette de membre inférieur peut être utilisé pour aider à la marche avec mise en charge ou pourassister les travailleurs qui manIJuvrent de lourdes charges. Pour la marche assistée par exosquelette, leconfort et la commodité de la marche sont importants, par conséquent il doit y avoir le minimum d’interfé-rence avec le mouvement de la jambe. Ainsi une stratégie inusitée de conception basée sur une semelle dechaussure munie de capteurs est présentée dans le but de réaliser la détection de mouvement en temps réelet le suivi de contrôle de la jambe en mouvement. En comparaison avec l’extension élastique du muscle, lecapteur doit présenter un minimum de déflection sous une charge. Par conséquent, une structure ultra minceayant une liaison à six barres et trois supports a été utilisée dans la conception d’une semelle de chaussuremunie de capteurs, lesquels peuvent détecter la force dans deux directions et la rotation dans l’une. Uneexpérience de la phase d’oscillation et un mouvement aléatoire de la jambe ont été performée. Les résultatsdémontrent la validité de la détection du mouvement humain et la stratégie du suivi de contrôle basé sur cecapteur à la surface plantaire.

Mots-clés : contrôle de mouvement de la jambe; exosquelette; semelle de chaussure munie de capteurs;interface homme-machine.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 4, 2016 657

1. INTRODUCTION

Lower limb power-assist exoskeleton, which could be worn by human beings, is a human-machine inte-gration system following the human motion and supplying power assistance. The main difference betweenwearable exoskeletons and other robots is the human-machine interaction, which requiring the considera-tion of the wearer’s leading function in the control circle [1]. The harmonious motion between man andexoskeleton is based on the optimized control of interaction force.

There has been great achievement in the exoskeleton researches, aiming at different applications. How-ever, wearing feasibility and walking convenience remain to be settled. The reasons lie in the large amountof moving joints and irregular walking environment. Especially for the swinging leg, it performs more ran-domness and requires more flexibility in some typical movement conditions such as uphill/downhill walking,obstacle climbing over, side stepping, knee bending, etc.

The Berkeley Lower Extremity Exoskeleton (BLEEX) is the first energetically autonomous exoskeletondesigned at U.C. Berkeley [2]. A special control method called sensitivity amplification control (SAC) wasdesigned based on the accurate dynamics models of three phases, including single stance phase, doublesupport phase and double support phase with one redundancy. The ultimate aim is to minimize the torquesimposed by the wearer to exoskeleton joints [3]. Its designer then developed another product named “Hu-man Universal Load Carrier” (HULC), which is closer to practical application. In Japan, a series of HybridAssistive legs (HAL) have been developed at Tsukuba University [4]. The EMG signals are detected for thehuman movement identification. And many other detecting measurements are also used for aided detection,such as ground reaction forces, exoskeleton joint angles and the body posture [5]. The MIT Media Labora-tory developed a quasi-passive exoskeleton with the combination of motor and magnetorheological damper[6–8]. The human movement intention is obtained by thigh interaction force and ground-exoskeleton inter-action force test. Information such as joint angles, angular velocity and the joint driving torque is also usedfor the division of controller states. The wearable robot by the Institute of Intelligent Machines in Chinahas two-dimensional force sensor on each thigh and calve, and one-dimensional force sensor under the foresole and heel of each foot [9]. A full-body exoskeleton, called the body extender (BE) developed at Italy,settles force sensors of 6-6 Stewart platform architecture at soles, wrists and back. All of the six-axis forcessensors are used for human movement identification and exoskeleton control [10–12]. This method is alsoused on the Power Loader developed by Active Link Co. Ltd. [13] The DSME wearable robot in Korea isdesigned for workers’ manual labor in complex environment. Furthermore, a particular foot sensor moduleis developed for the sensing system [14].

From the above exoskeleton detecting methods, we extract three human motion detecting ways for theswinging leg control, including leg binding detection, feet bottom pressure distribution detection and feetbottom human-machine interaction force detection. The first method based on leg binding requires strictparallel posture between humans and exoskeletons. All the variations of instant rotation center of the legjoints, strain or relax of leg muscles might lead to extra disturbances to the detection and lower limb control.The second method requires a relatively flat ground, and could not adjust to the random motion of swingingleg. The detecting method based on feet bottom human-machine interaction force could fit the randommotion of swinging legs. However, the multidimensional force sensors are expensive and difficult for thelectotype due to the limited installing space and suitable measuring range.

In this paper, a purpose built in-sole sensing shoe is designed for the swinging leg control of HIT lowerlimb exoskeleton, which could detect two-dimensional forces and one-dimensional torque. All the sensorstructures are installed at the bottom of feet with a flat ultra-thin design style and would provide sufficientinformation for the motion detection and follow-up control of the swinging leg.

658 Transactions of the Canadian Society for Mechanical Engineering, Vol. 40, No. 4, 2016

Fig. 1. DOFs distribution of the swinging leg.

2. KINEMATICS ANALYSIS AND IN-SOLE SENSING SHOE DESIGN

In the process of walking, the swinging and supporting phases are very different. The former requiresmore flexibility; and the latter needs greater joint driven torque. The wearing feasibility and walkingconvenience of exoskeleton mostly depend on the motion detection and control on the swing leg. In thissection, the kinematics of the swinging phase is analyzed for the plantar man-machine interaction forcedetection.

2.1. The Swing Phase Kinematics AnalysisThe motion pattern of lower limbs mainly exists in the flexion/extension rotation at the hip joint, knee jointand ankle joint on the sagittal plane. The HIT lower limb exoskeleton is an electrically actuated load-carrying robot-suit with four electric motors on the hip and knee joints. A passive spring is located at theankle joint, which only works during the supporting phase and would not be discussed here.

The structure of HIT lower limb exoskeleton and its wearing method is shown in Fig. 1, in which theperson is in the swinging phase with one leg dangling in the air. For the swing leg, there are 3 DOFs at hipjoint (Abduction/Adduction, Flexion/Extension and Internal/External rotation); 1 DOF at knee joint (Flex-ion/Extension) and 2 DOFs at ankle joint (Abduction/Adduction and Flexion/Extension). In considerationof the motion pattern of the swinging phase, the Flexion/Extension rotations at the hip, knee and ankle jointshave largest ranges, which are all on the sagittal plane.


Fig. 2. Establishment of the D-H coordinate of the swinging leg on the sagittal plane.

Moreover, the Abduction/Adduction DOFs at the hip and ankle joints are mainly used for sidestep, whilethe Internal/External rotation at the hip joint is mainly adapted to the direction changing on the support-ing leg. So it is a reasonable simplification for the swing leg research restricted to the sagittal plane, notconsidering the DOFs of Abduction/Adduction and Internal/External Rotation.

On the wearing method design, the exoskeleton is connected to the wearer by waistband, shoulder strapsand foot belts. A flexible bundling belt is also arranged at lower thigh for supplemental connection, whichhas no influence on the plantar man-machine interaction detection owing to its elasticity and floating mount-ing base on the exoskeleton.

The kinematics in the sagittal plane and the D-H coordinate establishment of the swing leg are shown inFig. 2. The exoskeleton is required to test the motion intention of the swing leg and follow human motionin real time. The sensitive motion intention detection and the anhysteretic servo control are the basis forthe wearing feasibility and walking convenience. The planar man-machine interaction on the sagittal planeincludes two-dimensional forces and one-dimensional torque, which is marked out at the point P in Fig. 2.

Because of the anthropomorphic designing structure and the fixed connection at the upper body, it couldbe considered that the rotation axis of the wearer and exoskeleton are overlapped at the hip joint. Duringthe D-H coordinate establishment, the torso of the exoskeleton could be seen as the base and the swingingfeet as the end effector. Then only the three Extension/Flexion DOFs at the hip, knee and ankle need tobe considered. As shown in Fig. 2, the whole swinging leg is similar with a 3-link planar manipulator.According to the D-H principle, the vertical standing condition is regarded as the initial condition. The


Table 1. The D-H parameters of the swinging leg on the sagittal plane.i αi−1 ai−1 di θi1 0 0 0 θ12 0 LThigh 0 -θ23 0 LShank 0 θ3

reference coordinate O0 is situated at the hip joint, which is fixed to the torso. The coordinates O1,O2,O3are connected to the exoskeleton thigh, shank and foot separately. The D-H parameters are shown in Table 1.All the z axes are perpendicular with the paper and directing inside.

According to the parameters in Table 1, the transformation matrix is derived as follows:

01T =

cθ1 −sθ1 0 0sθ1 cθ1 0 00 0 1 00 0 0 1

, 12T =

cθ2 sθ2 0 LThigh−sθ2 cθ2 0 00 0 1 00 0 0 1

, 23T =

cθ3 −sθ3 0 LShanksθ3 cθ3 0 00 0 1 00 0 0 1

03T =

c(θ1−θ2 +θ3) −s(θ1−θ2 +θ3) 0 LThighcθ1 +LShankc(θ1−θ2)s(θ1−θ2 +θ3) c(θ1−θ2 +θ3) 0 LThighsθ1 +LShanks(θ1−θ2)

0 0 1 00 0 0 1

(1)

The exoskeleton should follow the human motion correspondingly. Assume that the position of hu-man foot in coordinate O0 could be summarized by three parameters, xfoot,yfoot and φfoot. So in the ex-oskeleton follow-up control, the coordinate O3 should have similar motion parameters, (xO3 ,yO3 ,φO3 ) → (xfoot,yfoot,φfoot).

As an under-actuated system, only the hip and knee joints are supported by actuators, the angle is pas-sively adjusted to human motion. So the position of point at ankle joint is settled as the control target. Bycomparison of Eqs. (1) and (2), the position of O3 should satisfy the following equation:{

xO3 = LThighcθ1 +LShankc(θ1−θ2)

yO3 = LThighsθ1 +LShanks(θ1−θ2)(2)

It could be concluded from Eq. (2) that the control over the swinging leg could be regarded as a 2-linkmanipulator, which depends on the 2-dimensional force at the end effector. The Jacobian matrix for jointvelocity and the end velocity could be shown as

0J(θ) =

[−LThighsθ1−LShanks(θ1−θ2) LShanks(θ1−θ2)

LThighcθ1 +LShankc(θ1−θ2) −LShankc(θ1−θ2)

](3)

2.2. Detecting System Design of the In-Sole Sensing ShoeThe HIT lower limb exoskeleton adopts the force following method at the foot for swinging phase con-trol. The keys exist in the detecting method of man-machine interaction forces and the design of the sensordetecting devices. The design of the In-sole anthropomorphic sensor boot is shown in Fig. 3. The de-tecting devices include three film pressure force sensors situated at the rubber shock absorber sole, threestrain cantilever beams situated at the inside boot bottom, and two angle sensors for Extension/Flexion andAbduction/Adduction testing of the ankle joint.


Fig. 3. The detecting system design of the in-sole anthropomorphic sensing shoe.

Fig. 4. Principle of the 6-linked mechanism.

In the structure design, the in-sole sensing shoe includes 4-layer structure: the rubber shock absorber sole,the exoskeleton bearing bottom plate, the elastomer with strain cantilever beams in the middle and the footbinding plate at the top.

The exoskeleton bearing bottom plate is connected to the shank by two vertical support plates on bothsides and one U-shaped connecting link. The elastomer between the exoskeleton bearing bottom plate andthe foot binding plate has three cantilever beams with mechanical-limit protection structure. They are con-nected with each other by a 6-linked mechanism, which limits 3-DOFs movement of the foot binding plate,realizing the detection of the two-dimensional forces and one-dimensional torque. The structural schematicdiagram of the plantar man-machine interaction device is shown in Fig. 4. The measuring range of thecantilever beams is designed to be ±10 kg in the mechanical-limit range. The ends of the cantilever beamsadopt the line contact style and could make slightly relatively slip in a particular direction. The designing


method avoids coupling disturbances among dimensions. The principle of the 6-linked mechanism is shownin Fig. 4.

The foot binding plate is shown by the dotted line, which is connected to human foot; and the lowerfull line represents the exoskeleton bearing bottom plate; the lines on the both sides represent two shortlinks. The detecting values of three cantilever beams (A, B, C) could be represented by Fa,Fb. The two-dimensional force and one-dimensional torque could be computed as

Fx3 = Fa +FbFy3 = Fc

TO3 = (Fa−Fb) · l1(4)

where l1 demonstrates the distance between Fa and Fb.The results of Eq. (4) are based on the coordinate O3. In the swinging leg control, the detected two-

dimensional forces should be transferred to the coordinate O2. The ankle Extension/Flexion angle θ3 is andthe ankle Abduction/Adduction angle is θ4. During the swinging phase, the ankle Adduction/Abductionmotion could be neglected for its small measured value. So the transformation to the coordinate O2 couldbe summarized as {

Fx2−ext = Fx3cθ3−Fy3sθ3Fy2−ext = Fx3sθ3 +Fy3cθ3c

(5)

where Fx2−ext and Fy2−ext could be adopted as the swinging leg control input.

3. THE MOTION DETECTION AND CONTROL STRATEGY

During the swinging phase of walking, the wearing comfort and walking convenience are decided by themotion detecting sensibility and the fast follow-up control. The real-time judging of the start and end of theswinging phase is also the key factor for the control process design.

3.1. The State-Machine of the Human Swinging LegFor the human walking, the main performance of the foot includes the off-ground and landing ground stages.In the control of the exoskeleton swinging leg, it needs the precise detection of the start and end time of theswinging phase for the switch between the status of swinging and supporting phases. In this paper, twotransient statuses of entering the supporting phase and entering the swinging phase are defined with specificidentification method as shown in Table 2. It could been seen in the state-machine that state 1 and state3 are transient processes and could be regarded as the demarcation points for the swinging phase and thesupporting phase.

3.2. The Follow-up Control of the Exoskeleton Swinging LegIn this paper, a follow-up control strategy based on the plantar man-machine interaction device is adopted,which requires the real time two-dimensional force detection on the sagittal plane. Then the judgment ofhuman’s movement intention will be made, and the exoskeleton should quickly follow the human motion toeliminate the interaction force.

The control strategy based on the speed forecast in the Cartesian coordinate is designed to control thejoint angular velocities. By continuously revising the speed of the end effector and appropriate coordinatetransformation, the desired angular speeds in the joint velocity space could be obtained to realize the motionfollowing and eliminate the movement deviation between the exoskeleton and the human feet. The controlstrategy diagram is shown in Fig. 5.


Table 2. The description of states and triggers for the swinging leg control of HIT-exoskeleton.State Description of each state Triggers

0 Swingingphase

The pelma totally leaves the groundand accompanies human motion.

The measured values of three thin film pressure sen-sors are close to zero, and mean while the three can-tilever beams varies in the range of about ±5kg.

1 Interim intosupportingphase

Only the heel touches ground andthe forefoot gradually approachesground.

The measured value of the film pressure sensor atthe heel is bigger than the threshold value (such as1 kg), and the two values at the forefoot is zero orlower than the threshold value.

2 Supportingphase

The foot totally touches the ground. All the three film pressure sensors possess largerpressure value than the threshold value (1 kg) andthe detecting value of the vertical force on can-tilever beams is out of the negative threshold value(–5 kg).

3 Interim intoswingingphase

The heel leaves the ground. The detected value of the vertical force on can-tilever beams comes back to the negative thresholdrange; and then demonstrates the completely en-trance of swinging phase when rising to zero.

Fig. 5. The follow-up control of the swinging leg based on the Cartesian speed forecast.

4. EXPERIMENTS AND DISCUSSION

In order to testify the control strategy based on the plantar man-machine interaction force detection, twoexperiments of foot lifting and landing, and single-leg stepping forward are manipulated in the laboratoryenvironment. The dynamic processes are shown in Figs. 6 and 7.

In the second experiment, the two-dimensional forces are tested as the control inputs. Their variations arewithin the measuring range as shown in Fig. 8. It is also worth pointing out that the interaction forces mightbe further reduced by improving the control algorithm. What’s more, the wearer’s motion is flexible duringthe experiment, proving that the swinging leg control strategy based on the in-sole sensing shoe possessesgood wearing feasibility and walking convenience.

5. CONCLUSION

As important performance indicators of a wearable exoskeleton, the wearing feasibility and walking conve-nience are largely determined by the human motion detection and the follow-up control of the swing leg inreal time. An in-sole sensing shoe is presented to detect the plantar man-machine interaction forces using a


Fig. 6. Foot lifting and landing experiment.

Fig. 7. Single-leg stepping forward experiment.

Fig. 8. The plantar man-machine interaction forces in the second experiment.

6-linked mechanism. Accompanied with joint angle detection and plantar pressure detection, the accuratestart and end time of the swinging phase could be judged for the switch between different control states. Thespeed forecast control strategy based on Cartesian coordinate is used for the follow-up control. In the ex-periments of the foot lifting and landing, and the single-leg stepping forward, the exoskeleton could rapidlyidentify different working conditions and flexibly follow the swing leg movement. The results show thevalidity of the human motion detection and the follow-up control strategy based on the in-sole sensing shoeduring the swing phase.

ACKNOWLEDGMENTS

The work reported in this paper is funded by the National High Technology Research and DevelopmentProgram of China (863 Program) (Grant No. 2012AA041505) and the Self-Planned Task of State Key Lab-oratory of Robotics and System (HIT) (Grant No. SKLRS201201A02).


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