A Novel Reconfigurable Ankle Rehabilitation Robot for Various Exercises

8/11/2019 A Novel Reconfigurable Ankle Rehabilitation Robot for Various Exercises

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AbstractThis paper presents a novel reconfigurable ankle rehabilitation robot to cover

various rehabilitation exercise modes.The designed robot can allow desired ankle and foot

motions including toe and heel raising as well as traditional ankle rotations since the

mechanism can generate relative rotation between the fore and rear platforms as well as pitch

and roll motions. In addition, the robotic device can be reconfigured from a range of motion

(ROM)/strengthening exercise device to a balance/proprioception exercise device by simply

incorporating additional plate. Further, the action of the device is two folded in the sense that

while a patients foot is fastened firmly to the ROM/strengthening device for task specific

training, s/he can also stand on the balance/proproception device. To perform each mode of

ROM, strengthening, and proproception exercises, a unified position-based impedance control

is systematically developedtaking into account of desired position and velocity.

Index TermsAnkle rehabilitation, exercise modes, metatarsophalangeal joint, parallel

mechanism, reconfigurable robot, and impedance control.

A Novel Reconfigurable Ankle Rehabilitation

Robot for Various Exercises

Jungwon Yoon1

, Jeha Ryu*2

, and Kil-Byung Lim3

1School of Mechanical and Aerospace Engineering and ReCAPT

Gyeongsang National University, Jinju, Gyeongnam 660-701, Korea.

email:[email protected]

2Department of Mechatronics, Gwangju Institute of Science and Technology

Bukgu, Gwangju 500-712, Korea

e-mail: [email protected]

3 Department of Rehabilitation Medicine, Ilsan Paik Hospital,

Inje University, Ilsangu, Goyang, Korea


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1. INTRODUCTION

Rehabilitation aims to bring back the patient's physical, sensory, and mental capabilities that

were lost due to injury, illness, and disease and to support the patient to compensate for deficits

that cannot be treated medically.1Primary types of rehabilitations are physical, occupational,

and speech therapy. Physical therapy aids the patient to restore the muscles, bones, and nervous

system through the use of thermal treatments and exercises. Among physical therapy, exercises

are the most widely used method.2 Exercises are used to restore the original range of motion

(ROM) and mobility and to strengthen the affected limbs or parts.

Among lower limb injuries, the ankle sprain that occurs with ligaments stretched or torn is

the most common in sports and daily life in general. Full functional activities such as ROM,

progressive muscle strengthening, and proprioceptive exercise after acute cure plays a pivotal

role in speeding up return to preinjury level.3Achilles tendon stretching should be performed

within 48 to 72 hours of injury, regardless of weight-bearing capacity and ROM must be

regained. Once ROM is achieved, strengthening of weakened muscles is essential to rapid

recovery and is a preventive measure against reinjury. Manual therapy such as toe raises, heel

walks, and toe walks, toe curls, and marble pick-ups as well as resistance exercises to ankle

joints are attempted to regain strength and coordination.As the patient achieves full weight

bearing capability without pain, proprioceptive exercise is initiated for the recovery of balance

and postural control. Finally, advanced exercises should be performed to regain functions

specific to normal activities.

3

Modal exercise procedures are summarized in Fig. 1.

In order to cover some of the exercises satisfying the above procedures, several devices for

ankle rehabilitation have been developed. Traditionally primitive passive devices 4 such as

elastic bands and the active reflex treatment unit for strength exercises, wobbles board and

foam rollers for balance exercises have been in use.These devices, however, can allow very

simple rehabilitation exercise by only patients efforts and cannot store the past information of

the exercises. Therefore, task specific training by programmable robotic systems will be very

helpful not only to provide patient with more diverse and useful exercises in a suitable manner


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but also to reduce the physical efforts due to repetitive nature of exercise and furthermore to

store the patients exercise histories. To meet the need, the design and development of

advanced exercise tools are in progress. Representatives of some commercial developments

are biodexs multi joint system3 and balance system which can facilitate to achieve

ROM/strengthening, and balance exercises, respectively. Girone et. al suggested the Rutgers

Ankle5 that can provide active 6-dof motions at ankle with a Stewart platform structure to

cover various exercise modes. Using this, the patients foot is fixed firmly to the platform of the

Rutgers Ankle and is synchronized to virtual reality scenarios.6,7 Recently, the Rutgers

Ankle is extended to Dual Stewart platform 8with 12-dof motions for gait rehabilitation.

Even though presently most of the exercise modes are possible by the use of two Stewart

platforms8, balance/proprioception exercises can also be achieved with a single device. Since

ankle movement in most exercises needs less than 4-dof motions, Dai and Zhao proposed a

sprained ankle device9 using a 3-or 4- dof parallel mechanisms with a central strut and

analyzed the orientation and stiffness of the mechanism with considerations of the central strut.

Commercial developments and the existing robotic devices 5-9 as mentioned above with a

single rigid upper platform, however, cannot generate the relative rotation i.e.,

metatarsophalangeal (MTP) joint motion (shown in Fig. 2) between fore and rear foot for

natural foot motions. The relative rotation around MTP joint entails more natural exercises

such as toe raise, heel raise, etc. Therefore, more diverse exercise modes should be available

with the robotic devices in order to be able to facilitate more natural MTP joint motions.

Further, in order to accommodate the desired exercise modes, a reconfigurable platform is

inevitable. Eventually a versatile single platform-based reconfigurable robot mechanism with

satisfactory MTP joint motions and less than 6-dof motions should be developed for allowing

more natural foot and ankle motions to cover various exercise modes as shown in Fig. 1.

Girone et. al 6 commented that adequate programs should be developed to perform desired

rehabilitative motions satisfying exercise parameters and constraints, and suitable control

methods should be implemented for certain exercise modes. As regards to transduction


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scheme and methods, impedance-based combined position and force control is preferred for

such interactive type applications. A short review on impedance control especially applied to

rehabilitation devices follows.For upper limbs, Noritsugu and Tanake10 and Richardson et. al11

applied a position-based impedance control using pneumatic actuators. Krebs et. al12

implemented a force-based impedance control on a physiotherapy robot using electric motors.

For lower limbs, Yoon et. al 13presented position-based impedance control for the Rutgers

Ankle rehabilitation device that is driven by six double-acting pneumatic actuators. Jezernik et.

al 14suggested a novel gait-pattern adaptation algorithm based on the impedance control for

adapting the reference walking trajectories according to the physical interaction of the patient

with the robotic orthosis. Even though these controllers satisfied their specific purposes, design

and development of unified stable controller that can realize various exercise modes including

flexibility, strength, and proprioception, however, draws attention for effective control of

versatile reconfigurable rehabilitation robots.

In this paper, we present design and control of a novel ankle rehabilitation robot to cover

various exercises modes with natural foot and ankle motions using a parallel mechanism that

can generate pitch and roll motions as well as relative rotation between fore and rear platforms.

In addition, we propose a simple reconfiguration in the sense that it can be used as a

ROM/strengthing exercise device as well as a balance/proprioception device simply by adding

an extra large plate. For systematic implementation of the exercise, a unified position-based

impedance controller is suggested that can cover desired exercise modes. The following

section describes requirements specification based on ankle anatomy. It follows with the

mechanism design. Section 3 presents a position-based impedance control. Section 4 presents

impedance parameter design for each exercise mode. Section 5 presents experimental results

for each exercise mode. Finally, conclusions and future research scenarios are summarized in

Section 6.


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2. MECHANISM DESIGN FOR ANKLE REHABILITATION EXERCISE

2.1. Ankle Anatomy and Design Requirements

Based on ankle anatomy, this section identifies the design requirements for ankle

rehabilitation device. Normally the ROM (flexibility) exercise of ankle joint is performed

before other functional rehabilitation exercises are initiated. ROM exercise is followed by

strengthening exercise to increase muscle strength and endurance. The device should generate

pitch motions: dorsiflexion (pull foot upward toward the head) and plantar flexion (push foot

downward away from the head), as well as roll motions: inversion (push foot inward toward the

midline of the body) and eversion (push foot outward away from the midline of the body) about

the ankle joint. In addition, since a patients foot will be fastened to the device platform during

flexibility and strength exercises, the platform should generate MTP joint motion between fore

and rear foot to facilitate natural foot motions (see Fig. 2).

Even though the ankle joint can generate three rotations in all three pitch, roll, and yaw axes,

two rotations about ankle axes are generally utilized for the sprained ankle rehabilitation.3

Research in biomechanics areas showed the importance of inter-segments foot motions based

on the extent of forefoot motion during stance phase15, and foot could be simply modeled as

two segments with one revolute joint to represent more closely natural foot motion during

gait.16For ankle rehabilitation, this motion will allow more diverse ankle exercises such as toe

raise, heel raise, etc, which cannot be achieved with the single rigid platform.

Further, lack of sufficient ankle proprioception is responsible for subsequent sprains in the

ankle.17Therefore, a proprioceptive exercise is desirable. This reduces the risk of re-injury and

increases the quality of the training. A hemispheric base is a suitable choice to increase

proprioception. Moreover, the device should have enough orientation workspace for wobble

bode motions17and should support full human weight. The designed angle orientation should

be more than 20and the device should generate at least continuous 1000N to allow patient to


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proposed 4-dof mechanism is much smaller than that of general parallel mechanisms.

Furthermore, the platform of the mechanism is equipped with only two serial limbs instead of

four as seen in other parallel mechanisms, providing parallel mechanisms increased orientation

workspace as well as high stiffness to support human weight. The mechanism also inherits

characteristics that are intermediate between those of parallel and serial types, and provides

adequate solution for MTP motions.

Moreover, reconfiguration can be utilized to implement various exercise modes with single

device. Modular design has already been pursued for both serial and parallel mechanism robots.

22-25 Yoon and Ryu26proposed a kind of top-down approach, in which a base multiple

degree-of-freedom mechanism is invented first and then reconfigurability is sought by more

specifically changing extra part and/or by changing the direction of a revolute joint with adding

an extra part. Similar to Yoon and Ryu26, it is suggested that if a large plate is attached to both

fore and rear platforms with one actuator located at front platform having detachable feature,

the robot as shown in Fig. 4(a) with two platforms for ankle and foot motion control can act as

3-dof robot as shown in Fig. 4(b) with single platform, on which a patient can stand for both

proprioception and balance exercises. This facilitates reconfigurablity.

2.3. Kinematic Performance

Based on the design requirements computer simulations have been performed and the design

values are:La= 5 cm,Lb = 15 cm,Lbase= 10 cm, and actuator stroke = 20 cm, whereLaandLb

are the distances from the middle limb to the point of the fore and rear prismatic actuators, and

Lbase is the distance between the two active prismatic joints of the 2-dof mechanism (Fig. 2(a)).

For ROM exercises, necessary orientation workspaces are obtained with the designed

parameters and Fig. 5 shows the resultant workspace envelope. The workspace analysis uses

the inverse kinematics of the mechanism and actuator length constraints. Fig. 5(a) shows the

orientation workspace about pitch angle fand roll angle versus the front platform heave

motionZf, while Fig. 5(b) shows the orientation workspace of pitch angle rand roll angle


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versus the rear platform heave motion Zr. The maximum heave motion is 20 cm, and the

maximum pitch angles of the front and rear plates exceed 50 and 70, respectively. The

maximum roll angle of the platforms exceeds 55. Since the relative rotation angle m

(rfm

= ) can be obtained by relative rotation of the front and rear platform, the maximum

allowable MTP joint angle can be generated up to 70 when r=0 and f = maximum or

minimum. Fig. 6 shows the geometric representation at roll angle of 55 and MTP joint angle

of 70.

After design of desirable workspaces, it is necessary to satisfy adequate force-bearing

capability requirement throughout range of motionsfor strengthening exercises. In addition,

when a patient stands on the device platform during balance exercises, the device need to

support the human weight. In the content of analysis and design for load-bearing capability,

given maximum force of actuators, the maximum force/torque at the device platforms can be

computed by using force/torque relationship between actuators and platform as

iT

i FJ ( rorfi= ) (1)

where T ],,,[ 21 mmrf is the vector of the linear forces of the actuators,

iFT

zr TFTT if],,,[ = is the vector of the generalized forces of each platform, and J is the

Jacobian matrix21 of the designed mechanism, subscript f and r represent front and rear

platform, respectively. For bounded actuators force, maximal force and torques applied on the

upper platform of the mechanism can be determined by using numerical discretization. 27The

maximum force and torques at each axis of the platform are measured by checking whether or

not the maximum force of each single actuator is over with respect to platform rotations. For a

pneumatic cylinder (max force: 1000N at 100psi) with a 50mm diameter and actuator force

bounded by 1000N, force-bearing capability requirements for both strengthening and

balancing exercises can be satisfied. For strengthening exercises, the maximum allowable

torques that can be applied on the upper platform are shown in Fig. 7. As two rations vary

within prescribed workspace, the maximum torque T about the roll axis in Fig. 7(a) exceeds


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104Nm, while the maximum torquer

T about the pitch axis in Fig. 7(b) is above 248Nm. It

should be noted that the maximum torquer

T about the pitch axis are independent on the roll

motions since the roll motions generated by the 2-dof driving mechanism are independent on

the pitch motions. Fig. 8 shows that the maximum force rzF at z-axis exceeds at least 1100N at

the center of the rear platform with respect to the two rotations for balancing exercises. It

should be noted that the maximum forcerz

F at z-axis is 3000N at zero angles of two rotations,

which is the sum of the bounded force of the three actuators. Finally, the torque at MTP joint

can be obtained by relative torque of the front and rear platform until 300Nm. Based on the

specification requirements, the design (Table I) can satisfy motion range and force-bearing

capabilities for ROM/strengthening device and balance/proprioception device. Fig. 9 shows

each device that is designed based on these requirements.

3. IMPEDANCE CONTROL FOR VARIOUS EXERCISE MODES

For physical therapy, impedance control of pneumatically-driven robotic devices can be

integrated depending on specific rehabilitation situations. Pneumatic actuators have been in

use because of their high power-to-weight ratio, good compliance, ease of maintenance,

cleanliness, and the ability to maintain high forces without overheating.28 Since the

force-based scheme generally provides low impedance compared to position-based scheme29

and lower limbs generate higher forces/ torques than upper limbs, the rehabilitation device

entails high impedance suggesting position-based adoption. Additionally, position-based

impedance may be more appropriate for pneumatic systems because the valve dynamics to

generate desirable torque as actuators have large lag compared with electrical motors and

unlike torque based control, the controller requires no dynamic model of the robot or the

environments, simplifying the control.30 For patients who want to exercise from small to large

torque to increase their muscular strength, a wide range of achievable impedance must be


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provided without causing any stability problem. The suggested impedance controller is

therefore very convenient and systematic way of implementing various exercise modes

requiring pure position, pure force, and hybrid position and force control of the system at given

specific rehabilitations without causing any instability during rehabilitation trial by setting the

impedance parameters within a stable range.

3.1. Position-based Impedance Control

3.1.1. Outer loop (position compensation loop with a force feedback)

The position-based impedance controller (Fig. 10) is composed of a position controller (inner

loop) with a force feedback loop (outer loop) that modifies position control command in order

to satisfy target impedance. The desired forceFdand desired foot positionXdin the Cartesian

space are related as29;

)()( mdmdd XXBXXKF && += (2)

whereK=diag {iz

K ,i

K , K } is the desired stiffness matrix,B=diag { izB , iB , B } is the

desired damping matrix, and Xm is the measured platform position. The desired stiffness,

damping matrices, and foot position Xd should be selected (explained later). The position

adjustment vectorXimp can be computed by following relations

impimpmXBKXF &+= (3)

whereFmis the measured interaction forcethat is estimated by pressure sensors. The desired

set point position commandXcand the position adjustment vectorXimp are related by

impdc XXX (4)

Thus, the position commandXcis modified by the position adjustment vectorXimpcalculated

from the target impedance relationship in equation (3) with the measured force Fm. If the

position control of the robot is perfectly operating (Xc=Xm), the desirable forceFd in equation


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(2) become ideally equal to the measured force Fm in equation (3), which means that the

desired impedance have been achieved ideally.Note that if there are no interaction forces, the

desired foot positionXdbecomes the desired position commandXc and the impedance control

is operating as only a position control. Also, the larger impedance implies the harder movement

and the smaller impedance implies the easier movement of the platform through the foot.

3.1.2. Inner loop (position control loop)

The position control loop should be designed to operate as ideally as possible to achieve

desirable impedance more closely. The position control is implemented with feedback control

of PD type with feedforward control input.10 If the desired command input Xc is given, the

resultant measured platform coordinates Xm can be obtained from the measured actuator

lengths by forward kinematics (FK). The position control input will be transformed into the

desired actuator inputLcafter being mapped through inverse kinematics (IK).

Then, the independent joint control will be applied to actuator, which is controlled by the

four-way proportional valve (Enfield Technology). The control input of the double acting

pneumatic cylinder with special low-friction type (Bimba Corp.) is proportional to differential

pressure. The generated differential pressure is expressed as

)())(/( ulmmcidp PPKLLsKsKKP ++= (5)

whereLc andLm are the commanded and measured cylinder displacements,Kp,Kd, andKiare

proportional, derivative, integral gains,Pl andPuare lower and upper pressures of the cylinder

chamber, respectively, and Km is the pressure feedback gain. The pressure feedback is

proportional to the external force applied to the piston. The system stability can be improved

by augmenting differential pressure feedback to a position controller.31 We define instability

as the situation in which vibration occurs while maintaining a fixed pose (i.e. at constantXd)

during foot interaction with the robot. The pressure sensor can directly detect the forces such

as friction or external disturbances, and can efficiently reject force disturbances that are


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generated by patients legs during trials. The PID scheme with pressure feedback gives

comparable results with other robust controllers. 32

3.2. Achievable Impedance

For the suggested impedance controller, the maximum stiffness33-34, the ability to create

force in response to a disturbance, can be obtained by measuring the changed positions from

home position and the resultant forces of the device with the largest possible position gain

while not inducing control instabilities. Similarly, the minimum DC stiffness can be achieved

by measuring the changed positions and the resultant forces. In the case of position control

with pressure differential feedback, the achievable stiffness range was increased about 20%,

which shows the importance of the position controller performance for wider impedance range.

When the minimum achievable impedance experiments are performed at the position

controller with and without pressured feedback, Fig. 11 shows that the platform driven by

position controller without differential pressure feedback becomes unstable at a stiffness gain

of rzK =10kN/m, whereas the platform driven by position controller with differential pressure

feedback is stable at the same stiffness gain. The minimum achievable stiffness with pressured

feedback was stable until 7kN/m without causing instability. Since damping coefficient is not

utilized for implementation of each exercise of ankle rehabilitation stages in the later section,

this coefficient is set to zero. When the position controller is set to its maximum and minimum

stable gains, the achievable maximum and minimum stiffness were measured (Table II).

4. IMPEDANCE DESIGN FOR EACH EXERCISE MODE

Parameters of the position-based impedance control such as the desirable foot position Xd,

the desirable foot velocityXd, and the desired impedance are designed for each exercise mode.


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4.1. Range of Motion Exercise

There are three types of ROM exercises: passive, active-assist, and active. Passive ROM

(PROM) is the movement that is applied to a joint only by efforts such as another persons or a

machine while the patient does not resist against the motion. To realize PROM, a fine position

control should be guaranteed to keep the range of motion within acceptable range without

inducing any pain even though some external force is exerted. This demands implication of

infinite impedance Zinf . By setting the desired impedance infinite, the position can be

controlled through the impedance controller. This reduces Ximp to become zero. Since the

resultant impedance generated by setting of the desirable impedance of the robot cannot exceed

the maximum achievable impedance, the desirable impedance was set to the maximum

achievable impedance. Active assist range of motion (AAROM) represents a joint movement

that needs partial assistance from an external force. Active range of motion (AROM) is the

movement of a joint that is provided totally by the patient without external assistance. Thus, the

amount of selected impedance determines the effect of assistance suggesting minimum and

maximum achievable range of the specific device. In PROM and AAROM mode, a desired

staircase position command Xd can be designated by a physical therapist who can directly

diagnose the patients conditions during testing as well as operations, keeping in view of static

movements. The staircase command input may be more convenient to generate necessary

change of the patients current ROM with respect to the patients direct response.

4.2. Strengthening Exercise

Strengthening exercises enhance muscle strength by generating more strain on a muscle than

it is in normal condition. Strengthening exercises can be classified into the form of isometric,

isotonic and isokinetic strengthening.

4.2.1. Isometric exercise

Strengthening can be applied to the proper joint angle with exact isolation about the region of


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weak muscles. Isometric training is efficient to develop overall strength of particular muscles.

Even though muscles contract, there is no movement in the affected joints during isometric

exercises. To satisfy this mode, the infinite impedanceZinfshould be applied keeping position

command.

4.2.2. Isotonic exercise

Isotonic exercise is a dynamic form of exercise that is carried out against a constant or

variable load through the available ROM. Isotonic exercise is different from isometric exercise

in that there is movement of a joint during the exercise. Weight training with dumbbells and

barbells, or heel raise are standard examples of an isotonic exercise. The muscle can be

shortened or lengthened with a constant weight throughout the range of motion. The isotonic

controller is implemented when the human moves the foot, while the desired force/torque is

maintained constant. By using (5) withB=0, the desired foot positionXdcan be represented by

mdd XKFX / (6)

Then, the desired position control commandc

X can be represented by

mmdmmdimpdc XKFFKFXKFXXX /)(// (7)

Eq. (7) implies thatXcwill be changed to keep the measured forceFmto desired forceFd.

4.2.3. Isokinetic exercise

Isokinetic exercise allows constant preset speed with variable resistance as the muscle moves

through full ROM. This exercise can provide maximal loading throughout. Despite how hard

and fast the patient works, the isokinetic properties of the robot should permit patients to move

only as fast as preset speed. To satisfy isokinetic motions, the desired velocity command

dX& should be preset. In addition, the infinite impedance Zinf should be applied to generate

velocity control, which should prevent the robot from deviating from the desired speed by

external disturbances like patients muscle force. Since the impedance controller with the

infinite impedance Zinfbehaves a position controller, the desirable foot position Xd can be


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represented by the desirable foot velocity dX& and time t

Xd=X0+ dX& t (8)

Where X0 is the initial foot position. The desirable foot velocity dX& should be constant in

piecewise periods to provide isokinetic mode.

4.3. Proprioception Exercise

The loss of proprioception will cause a patient not to recognize his joint motions, while people

with a strong proprioceptive sense can have good balance and coordination. A common

progression when performing balance exercise is to move from a position of non-weight

bearing to weight bearing, bilateral stance to unilateral stance, eyes open to eyes closed, firm

surface to soft, uneven or moving surface.3For static balancing exercise, motions similar to

wobble boardare required to be generated by impedance control. The wobble board allows

only rotational motions of pitch, roll, and yaw about a hemispherical pivot at the center of

lower side. In order to mimic the wobble board motions, the impedance of translational

motions should be maximized to fix the platform at a constant position, while the impedance of

the orientation motions should be minimized to give free orientation motions at platform center.

Then, the dynamic balancing exercises are performed to improve effectiveness. Dynamic

balancing can be implemented by varying the desired positions and changing the desired

impedance of the platform. Table III summarizes the design parameters of impedance Z,

desired position and velocity command, and the desirable exercise examples with respect to

each exercise mode.

5. EXPERIMENTS OF EXERCISE MODES

For experiments, the total ankle rehabilitation system is suggested for patient to perform each

exercise mode with visual feedback, which composed of the robotic device, the controller with

pneumatic actuators, the host PC, and a patient. The host PC displays virtual environments and


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connects the controller through RS232C serial communication. The rotation and torque

information of the robotic platform will be transferred to the virtual environments and will be

stored for the patients histories at DB. The suggested system is shown in Fig. 12. In this

section, the experiment results of each exercise mode that is designed based on impedance

parameters will be shown.

5.1. ROM Exercises

Based on the patients condition, initial commands utilizing stair type exercise scenario are set.

Fig. 13 shows rotation and torque of the inversion (negative roll motion of the platform) in the

AAROM and PROM modes. For AAROM mode, desired stair input was displayed at

computer monitor for visual feedback of a patient. Fig. 13(a) shows the desired and measured

stair response for each ROM mode. PROM mode with infinite impedance setting follows the

desired trajectory well, while AAROM mode with K =150Nm/rad and B =0 has bigger

trajectory errors due to the lower impedance setting. Fig. 13(b) shows the measured torque for

each case. The relatively small measured torque value at AAROM means that the subject

applied his own torque in order to follow the desired trajectory. Conversely, the PROM mode

needs larger torque because of inactive efforts from the subject.

5.2. Strengthening Exercise

5.2.1. Isometric exercise

For isometric exercise, virtual environments have been developed in the previous research

for ankle and hand rehabilitation.35-36 Fig. 14 shows the image viewers that are used to give

motivation for isometric exercise. There are two images for which the front image will be

widened proportional to the torque exerted by a patient. If the device driven by human foot

exceeds beyond threshold (torque) selected by a therapist, the hidden image will appear at the

front. Then, another mage will be positioned on the back. The torques of the inversion and


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eversion directions will divide the front image into horizontal direction, while the torques of

the dorsiflexion and plantar flexion directions will divide the hidden image into horizontal

direction. Fig. 15 shows that even though the position variations are relatively small (within 2

degrees), which is required to be kept constant for isometric exercises, the exerted maximum

torques about inversion and eversion increases or decreases according to a patients efforts at

the ankle joint. If the exerted torque is higher than the present threshold (e.g. 50Nm), the next

scene will appear in the front. The torque threshold is determined by physical therapists

considering the patients conditions.

5.2.2. Heel raise for isotonic excise

Heel raise motion is a standard isotonic exercise since there is constant torque to ankle axis

due to a human weight. This exercise can be generated by setting the desired position control

commandXcat equation (7) with desired force and stiffness. In order to generate heel raise

motion, the only pitch motion r of the rear platform should be moved, while the pitch motion

f of the front platform and roll motion should be zero since toe attached to the front platform is

steady. Fig. 16 shows that even though the pitch motion r (platar flexion or dorsiflexion) is

changing, the device can keep the measured torque constant; 25Nm with small oscillations,

wherer

K =500Nm/rad, rB =0, andXd(r)=0.5 rad (28.65).

5.2.3. Gait trajectories training for isokinetic exercise

The ankle and foot motions during walking at the ground level is generated based on normal

gait.37 The task specific and repetitive gait training of a patient at foot and ankle can increase

the strength of the lower limbs with an isokinetic exercise. Since the mechanisms for the

unrolling foot during stance phase could generate the development of ankle sprains, Willems et.

al38 suggested that effective prevention and rehabilitation of inversion sprains should include

attention to gait patterns and adjustments of foot biomechanics in subjects at risk of a sprain.

Therefore, the test of ankle and foot motions about the repetitive normal gait trajectories was


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executed while the subject was sitting on the chair.

When the heel-striking angle, heel-off angle, and toe-off angle are 30, respectively, the foot

trajectory configurations at planar surface with respect to gait cycle are illustrated in Fig. 17.

Fig.18 (a) shows the desired and measured rotation value of pitch angles r of the real platform

in order to generate desirable trajectories with respect to gait cycle. Fig. 18(b) shows the

desired and measured angular velocity r& of the rear platform. At piecewise periods, the

angular velocities of the rear platform generated constant values, a desirable characteristic for

isokinetic exercises. Through experiments, the ioskinetic motions were well generated

regardless of subjects intentions. These kinds of advanced functional training including MTP

joint motions will not be possible by using Cybex and Biodex, which may be helpful to

increase the coordination of the lower limbs during real walking.

5.3. Proprioception/Balance Exercise

A ball stabilization game may be used for these exercises (See Fig. 19). By standing on the

large plate and shifting his/her weight from left to right or from fore to rear, the patient can

make the board tilt about the two ankle axes. The balance by maintaining center of gravity

within bodys base of support and the coordination by moving patients two limbs can be

improved simultaneously. Fig. 20 shows experimental results for the condition that a subject

was standing on the platform with double legs and shifted his center of gravity from fore to rear

while keeping his bodys positions stable. The impedance values are set tor

zK =80kN/m for

hard surface,r

K =0.2kNm/rad, and rK =0.1kNm/rad for free rotations.The desirable position

commands are set to theXd (z)=150mm,Xd (r)=0 andXd ()=0 for keeping the plate stationary.

Fig. 20 shows that the rear platform z-axis reaction force of about 800N is kept almost constant

(Fig. 20(a)) with small displacements changes (Fig. 20(b)). On the other hand, there are large

pitch and roll motions with relatively small torques as shown in Fig. 20 (c) and Fig. 20(d).

Decreasing rotational motions in Fig. 20(d) showed that the subject standing on the center of


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the platform was changing his weight to keep the balance of his body. Patients will try to

maintain constant height and zero rotations to improve proprioception/balance. The recorded

motion histories should be useful for progress reports of patients. Comparing to conventional

wobble board, the suggested proprioception/balance type robot can adjust the level of motions

stability by changing the impedance of the platforms. Also, the more difficult dynamic balance

training can be achievable by moving the platform with the desired foot position Xd for

generations of moving surfaces.

6. CONCLUSIONS AND DISCUSSIONS

This paper presents a novel reconfigurable ankle rehabilitation robot for allowing various

exercise modes. The proposed 4-dof robot is composed of two upper platforms and three limbs

driven by four pneumatic actuators. This new design can generate MTP motion between the

fore and rear foot and two rotations about the ankle joint including the heave motion. These

motions are adequate for natural foot and ankle motions. The mechanism has very simple

kinematics and allows wide workspace enough to cover the required ROM. Additional

exercises such as heel and toe raise that were not possible with the previous rehabilitation robot

systems can be implemented. Furthermore, simple reconfiguration of the upper platforms

allows covering more exercises such as proprioception and balance training.Position control

method utilizing impedance parameter of each exercise mode is adopted.

The developed ankle rehabilitation robot can substitute not only the traditional therapy in

various exercise modes but also supply the advanced functional exercises such as desirable gait

training during stance phase by generating natural foot trajectories with the MTP and ankle

joints motions and dynamic balancing by controlling levels of unstable surfaces with the

variable impedance and varied movement of the platform. Some standard exercise examples

are experimented and presented.

As regards to further work, comprehensive clinical trials with real patients should be


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performed to tune the impedance parameters and to evaluate the usefulness of the proposed

system. Moreover, more entertaining virtual environments for each exercise mode should be

developed for motivating patients in order to shorten recovery periods. Clinical test for the

advanced functional exercises with the developed ankle rehabilitation robot and extension to

gait rehabilitation should be considered too.

ACKNOWLEDGMENT

Research reported here was supported by grant (No. R01-2002-000-00254-0) from the Basic

Research Program of the Korea Science & engineering Foundation. The authors would like to

thank T.S. Song and J. W. Choi for their discussions, especially on experiments.

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Figure 1. Functional Ankle Rehabilitation

Figure 2. Ankle and foot motions

(a) CAD model (b) fabricated robot

Figure 3. New 4-dof mechanism with two platforms


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(a) ROM/strengthening type (b) balance/proprioception type

Figure 4. Novel reconfigurable ankle robot

(a) Front plate (b) Rear plate

Figure 5. Workspaces at ankle joint for ROM exercises

(a) roll ( ) (b) MTP joint

Figure 6. Geometric representation of maximum allowable rotations


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(a) T (roll axis) (b) rT (pitch axis)

Figure 7. Maximum torques of the two axes for strengthening exercises

Figure 8. The maximum z-axis forces at center of the rear platform for balancing exercises

(a) ROM/ strengthening type (b) Balance/proprioception type

Figure 9. Ankle rehabilitation device


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Figure 10. Impedance control block diagram for rehabilitation exercises

(a) position (b) force

Figure 11. Minimum achievable stiffness atrz

K =10kN/m

Figure 12. Total Ankle Rehabilitation System


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(a)Xm(axis) (b)Fm(axis)

Figure 13. AAROM and PROM for inversion

Figure 14. Image viewer window35

a)Xm(axis) b)Fm (axis)

Figure 15. Isometric exercise for inversion and eversion


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(a)Xm(raxis) (b)Fm(raxis)

Figure 16. Isotonic exercise at heel raise

(a) Heel-strike (0%) (b) Foot-flat (10%) (c) Heel-off (50%)

(d) Toe-off (60%) (e) Initial swing (70%) (f) Terminal swing (90%)

Figure 17. The mechanism configuration according to gait cycle at ground level


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(a) measured angles r (b) measured angular velocity ( r& )

Fig. 18. The walking motions for isokinetic exercise

Figure 19. Ball Stabilization Virtual Environment

(a)Fm (Zraxis) (b)Xm(Zraxis)


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(c)Fm(r and axes) (d) Xm(r and axes)

Figure 20. Balance /coordination exercise results

TABLEI

REQUIREMENTS OF ANKLE JOINT AND DESIGNED RESULTS OF THE DEVICE.

Motion

DF

(ROM)

PF

(ROM)

Inv

(ROM)

Eve

(ROM)

MTP joint

(ROM)

DF

(Torq)

PF

(Torq)

Inv

(Torq)

Eve

(Torq)

Spec 20 40 35 25

Flex (45)

Ext (70)

40.7-

97.6Nm

20.3-36.6

Nm

Max

48Nm

Max 34Nm

Designed 50 55 70 104Nm 248Nm

TABLE II

MINIMUM AND MAXIMUM ACHIEVABLE STIFFNESS

fzK

fK K rzK rK

Minimum 4.5kN/m 0.25kNm/rad 0.05kNm/rad 4.5kN/m 0.1kN/rad

Maximum 200kN/m 6.5kNm/rad 2kNm/rad 200kN/m 2.5kNm/rad


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TABLEIII

IMPEDANCE PARAMETERS FOR EACH EXERCISE MODE

ROM (stretching) Strengthening ProprioceptionImpedanceparameter

PROM AAROM AROM Isometric Isotonic Isokinetic AROM + Isometric

Z Zinf Zzero-Zinf Zzero

Zinf Zconst Zinf Zzeroat orientations

Zinf at position

Xd Vari Vari Const Vari Vari Const (static)

or Vari (dynamic)

dX& Zero Zero

N/A

Zero Vari Const Zero (static)

or Const (dynamic)

Control

type Position

Variable

Impedance

N/A

Position

Position

based force

Velocity Wobble board motions

Exercise

Example Stair input

Stair input

with visualfeedback

N/A

Image

viewer

Heel

raise

Gait

generation Ball stabilization

A Novel Reconfigurable Ankle Rehabilitation Robot for Various Exercises

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