A Light-weight Exoskeleton for Hip Flexion-extension Assistance

Francesco Giovacchini

, Matteo Fantozzi

, Mariele Peroni

, Matteo Moisé

, Marco Cempini

Mario Cortese

, Dirk Lefeber

, Maria Chiara Carrozza

and Nicola Vitiello

The BioRobotics Institute, Scuola Superiore Sant’Anna, viale Rinaldo Piaggio 34, 56025 Pontedera, Italy

Department of Mechanical Engineering, Faculty of Applied Sciences, Vrije Universiteit Brussel,

Pleinlaan 2, B-1050, Brussels, Belgium

Keywords: Lower-limb Assistive Exoskeleton, Wearable Robot, Powered Orthosis, Assistive Technology, Gait Aid,

Series Elastic Actuator.

Abstract: Wearable robots can represent a way to rehabilitate and/or assist people affected by gait disorders that are

common problems associated with ageing, following orthopedic surgery or diseases like brain injuries. In

order to improve their usability and effectiveness, exoskeletons aiming at assisting people affected by gait

impairments should be light-weight devices and have safe and low output impedance actuators. In this paper

we present a light-weight exoskeleton endowed with two series elastic actuators for hip flexion-extension

assistance.

1 INTRODUCTION

Gait disorders and lower-limb impairments are

common and often devastating companions of

ageing (Snijders, 2007). Old age is the most

important risk factor for gait disturbance (Stolze,

2005): several population-based study has shown a

35% prevalence of gait disorders among persons

over age 70 (Verghese, 2006), and 80% over 85

years of age.

Gait disorders lead to several major

consequences, including falling (leading to major

fractures or head trauma), the number of which is

expected to reach 500,000 by the year 2040, leading

to a total annual cost of 16 billion dollars (Burge,

2007). Another important consequence is reduced

mobility, which leads to loss of independence. Gait

disorders are also associated with reduced survival,

which can be attributed to a combination of fatal

falls, reduced cardiovascular fitness, and death from

an underlying disease (Snijders, 2007; Wilson, 2002;

Verghese, 2006).

Wearable robotic orthoses (namely

“exoskeletons”) capable to rehabilitate and/or assist

people affected by gait disorders have been proposed

as a solution by many research teams active in the

field of rehabilitation robotics (Pons, 2010; Ronsse

2011a; Ronsse 2011b). Both unilateral and bilateral

robotic exoskeletons are available in the current state

of the art and relevant for this study (Dollar, 2008).

Relevant unilateral orthoses – among many –

are: ALEX, a 2 DoF powered leg exoskeleton

(Banala, 2006); the ankle-foot and knee-ankle-foot

orthoses powered by McKibben type pneumatic

muscles developed by Sawicki (Sawicki, 2005),

SERKA, an active knee orthosis for gait training

(Sulzer, 2009); AKROD, a knee orthosis with an

electro-rheological fluid (ERF) variable damper

(Weinberg, 2007); the knee-ankle foot orthosis

developed at the Vrije Universiteit Brussel, driven

by pleated pneumatic artificial muscles (Beyl, 2010).

Among the many bilateral orthoses we can

mention: LOPES, a lower-limb exoskeleton for post-

stroke rehabilitation and driven by series-elastic

actuators (Veneman, 2007); ReWalk, a bilateral

robotic suit for the mobility of paraplegic patients

(Argo Medical Technologies, Israel); the Walking

Assistance devices from Honda; HAL, a powered

suit for motion assistance commercialized by

Cyberdyne (Tsukuba, Japan). Relevant for this study

are also the human performance augmenting

exoskeletons for the lower limbs such as: BLEEX

(Kazerooni, 2006), the Sarcos exoskeleton (Sarcos,

US), and the quasi-passive exoskeleton developed at

MIT by Walsh (Walsh, 2007).

In this paper, we introduce the design of a light-

weight exoskeleton for the assistance of hip flexion-

194

Giovacchini F., Fantozzi M., Peroni M., Moisè M., Cempini M., Cortese M., Lefeber D., Carrozza M. and Vitiello N..

A Light-weight Exoskeleton for Hip Flexion-extension Assistance.

DOI: 10.5220/0004663901940198

In Proceedings of the International Congress on Neurotechnology, Electronics and Informatics (RoboAssist-2013), pages 194-198

ISBN: 978-989-8565-80-8

 2013 SCITEPRESS (Science and Technology Publications, Lda.)

extension. The design of this device embeds two

main innovative solutions. First, it has a novel,

compact and light-weight series-elastic actuation

unit. Second, carbon-fibre linkages embedding

passive degrees of freedom (DoF) are used to ensure

good kinematics compatibility, thus enhancing the

comfort of the human-robot physical interaction.

Finally, in this paper, we also present the results of

the experimental characterization carried out to

assess the performance of the actuation and control

system.

2 METHODS

This exoskeleton is conceived for providing torque

to the user's flexion-extension hip joint. It is

constituted (Figure 1) by a frame carrying an

actuation unit and a upper-leg link for each side. The

frame is worn on the user's trunk, and is made up of

an extensible rear bar coupled with two carbon-fibre

lateral arms: it allows a fast don-doff procedure

thanks to a detachable pin and a fine tuning of the

size on medial-lateral direction thanks to a leadscrew

driven adjustment. Two thermoformed orthotic

shells (customized on the user’s body) interface the

frame with the user's trunk ensuring a comfortable

wearing and avoid slippage during the application of

the assistive torque. Two straps allow portion of the

structure’s weight to be supported by the shoulders,

avoiding excessive pressure on the wearer's trunk.

The two actuation units are mounted on the

frame lateral arms, and their position can be adjusted

in order to align them with the hip flexion-extension

human axes by means of two lockable sliders.

Figure 1: CAD model of the exoskeleton.

Coupled to the actuation axes there are two

carbon-fibre linkages, molded with a shape that

sweeps from the lateral to the back side of the thigh.

The shape of the links allows to swap the two,

connecting the actuation axes with the front side of

the thigh, without compromising the functionality.

This possibility turns useful when it is necessary to

maintain the rear of the thighs free from any

component, for instance to allow the user to sit

without hindrance.

The interface with the thighs is provided by

orthotic shells tightened around the limbs by means

of elastic belts. In order to fit different length of

lower-limbs the vertical position of the plastic shells

is adjustable thanks to lockable sliders. Thigh links

are also endowed with a passive rotational DoF for

abduction-adduction: this joint is located in a distal

position with respect to the flexion-extension joint in

order to let it completely passive and not loaded by

the weight of the actuation unit; this passive axis is

not collocated with the anatomical one, but still

contributes to provide a comfortable interaction.

The entire system has a total weight of 4.2 kg

(this weight excludes the control unit which is still

remotely located in this prototype).

2.1 Actuation Units

The actuation units are two Series Elastic Actuators

(SEA) (Pratt and Williamson, 1995). SEAs have

been successfully applied in the field of wearable

powered robots mostly to solve safety issues and

reduce the inherent output impedance (Vitiello,

2013); (Veneman, 2006); (Zinn, 2004). In this case,

the actuation is not rigid and allows relatively low

joint impedance across the entire frequency

spectrum. Furthermore, variations in the output

impedance can still be achieved by means of closed-

loop interaction control strategies (Pratt, 1995).

The design for the actuation units took into

account hip angle and torque profiles given by

Winter dataset, considering the natural cadence of

105 steps/min and a user weight of 80 kg (Winter,

2009). The target amount of assistance was set at

50% of the human torque during level walking and

the maximum value of assistive torque was set at 35

N·m.

A customized torsional spring was developed to

achieve a stiffness of 100 N·m·rad

-1

, which is a

value comparable with the average stiffness of the

human hip during level walking (Walsh, 2007): this

value prevents the subject from an uncomfortable (or

even painful) interaction with an excessively stiff

device in case of high frequency movements

ALight-weightExoskeletonforHipFlexion-extensionAssistance

195

(a) (b)

Figure 2: Lateral view (a) and exploded view (b) of the actuation unit.

(e.g. spasms, interaction with the ground). The same

spring was used for the actuation of the elbow

exoskeleton (NEUROExos) developed at The

Biorobotics Institute (Cempini, 2013: see the

reference for details on the torsional spring).

Each actuation unit (Figure 2) is configured

around two parallel axes. On one axis there is a DC

motor (Maxon Motor® EC60, 100W) equipped with

an incremental encoder and coupled with a

Harmonic Drive (Harmonic Drive® CPL-17A-080-

2A). On the other axis (which is the actual hip joint

axis) there is the torsional spring in series with a 32-

bit absolute encoder (Renishaw® RESOLUTE™,

ring: RESA30USA052B, readhead:

RA32BAA052B30F), which actually measures the

hip joint angle. The transmission between two

parallel axes is obtained by means of a 4-bar

mechanism. This configuration was chosen in order

to have a small lateral encumbrance, however the

swinging of the arms could be not as natural as

desired. In order to overcome this limitation in the

next version of the exoskeleton the actuation units

will be placed on the rear part of the support frame.

2.2 Control

The control system runs on a real-time controller, a

cRIO9082 (National Instruments, Austin, Texas,

US), endowed with a 1.33 GHz dual-core processor

running a NI real-time operating system and a Field

programmable gate array (FPGA) processor Spartan-

6 LX150. Motor velocity is controlled by means of a

commercial servo (Maxon EPOS2 70/10). On top of

the velocity control, a closed-loop 2-pole-2-zero

control is used to control the joint torque. Joint

torque is estimated by measuring the deformation of

the torsional spring by means of the two encoders.

3 RESULTS

Initial experiments were carried out to assess the

performance of the torque control, and the usability

of the device under a zero-torque control mode, i.e.

with the device controlled to be as transparent as

possible. Experiments showed that the torque control

– when a healthy subject displaces the exoskeleton

joint over a frequency range of 0.3-1.5 Hz – has a

parasitic stiffness ranging from 1 to 10 N·m·rad

-1

Furthermore, experiments with a subject walking

with the exoskeleton on the treadmill showed that

the passive DoFs allow a comfortable interaction:

the parasitic interaction torque at a cadence of about

0.7 cycle/s was in the range ±1 N·m (Figure 3).

Figure 3: Joint angle profile in zero-torque control mode.

Top panel: right hip joint angle vs. time; down panel:

measured vs. desired torque.

NEUROTECHNIX2013-InternationalCongressonNeurotechnology,ElectronicsandInformatics

196

Figure 4: A subject wearing the exoskeleton.

4 DISCUSSION

Recorded data and feedback from subjects (Figure 4)

that tested the device showed promising

performance for the developed exoskeleton and

encourage to progress with a more extensive

experimental characterization. The target users for

the presented exoskeleton will be elderly people that

need assistance to recover a more stable gait pattern;

in addiction the device is addressed to reduce the

metabolic consumption in lower-limb transfemoral

amputees providing them with assistance during

walking and other tasks. In general several kinds of

patients affected by gait disorders could take

advantages from the presented device.

ACKNOWLEDGEMENTS

This work was supported in part by the EU within

the CYBERLEGs project (FP7-ICT-2011-2.1 Grant

Agreement #287894) and by Fondazione Pisa within

the IUVO project (prog. 154/11).

REFERENCES

Banala, S. K., Agrawal, S. K., Fattah, A. et al., 2006.

Gravity-Balancing Leg Orthosis and Its Performance

Evaluation, IEEE Transactions on Robotics,

22(6):1228-1239.

Beyl, P., Van Damme, M., Van Ham, R., et al., 2010.

Design and control of a lower limb exoskeleton for

robot-assisted gait training. Applied Bionics and

Biomechanics, 6(2):229 – 243.

Burge, R., Dawson-Hughes, B., Solomon, D. H., et al.,

2007. Incidence and Economic Burden of

Osteoporosis-Related Fractures in the United States,

2005–2025. Journal of Bone and Mineral Research,

22: 465–475.

Cempini, M., Giovacchini, F., Vitiello, N., et al., 2013.

NEUROExos: A Powered Elbow Orthosis for Post-

Stroke Early Neurorehabilitation. In Proceedings of

35th Annual International Conference of the IEEE

EMBS, 342-345.

Dollar, A., Herr, H., 2008. Lower extremity exoskeletons

and active orthoses: challenges and state-of-the-art.

IEEE Transactions on Robotics, 24(1):144 158.

Kazerooni, H., Steger, R., 2006. The Berkeley Lower

Extremity Exoskeleton. Journal of Dynamic Systems,

Measurement and Control, 128(1):14-25.

Pons, J. L, 2010. Rehabilitation Exoskeletal Robotics.

IEEE Engineering in Medicine and Biology Magazine,

29(3): 57-63.

Pratt, G., Williamson, M. M., 1995. Series elastic

actuators. In Proc. IEEE Int. Conf. Intell. Robots Syst.,

Pittsburgh, PA, 339–406.

Ronsse, R., Vitiello, N., Lenzi, T., et al., 2011. Human-

robot synchrony: flexible assistance using adaptive

oscillators, IEEE Transactions on Biomedical

Engineering, 58(4):1001-1012.

Ronsse, R., Lenzi, T., Vitiello, N., et al., 2011. Oscillator-

based assistance of cyclical movements: model-based

and model-free approaches, Medical and Biological

Engineering and Computing, 49(10):1173-1185.

Sawicki, G., Gordon, K., Ferris, D., 2005. Powered lower

limb orthoses: applications in motor adaptation and

rehabilitation. In 9th International Conference on

Rehabilitation Robotics, 206-211.

Snijders, A. H., van den Warrenburg, B.P., Giladi, N., et

al., 2007. Neurological gait disorders in elderly

people: clinical approach and classification. Lancet

Neurol., 6(1):63-74.

Stolze, H., Klebe, S., Baecker, C., 2005. Prevalence of gait

disorders in hospitalized neurological patients. Mov

Disord, 20(1): 89–94.

Sulzer, J., Roiz, R., Peshkin, M., et al., 2009. A Highly

Backdrivable, Lightweight Knee Actuator for

Investigating Gait in Stroke. IEEE Transactions on

Robotics, 25(3):539-548.

Verghese, J., Levalley, A., Hall, C. B., et al., 2006.

Epidemiology of gait disorders in community-residing

older adults. J Am Geriatr Soc, 54: 255–261.

Walsh, C. J., Endo, K., Herr, H., 2007. A quasi-passive

Legacy Exoskeleton for load-carrying augmentation.

International Journal of Humanoid Robotics,

4(3):487-506.

Weinberg, B., Nikitczuk, J., Patel, S., et al., 2007. Design,

Control and Human Testing of an Active Knee

Rehabilitation Orthotic Device. In Proceedings of

IEEE International Conference on Robotics and

Automation, 4126-4133.

Wilson, R. S., Schneider, J. A., Beckett, L. A., et al., 2002.

Progression of gait disorder and rigidity and risk of

death in older persons. Neurology, 58: 1815–1819.

Winter D. A., 2009. Biomechanics and Motor Control of

ALight-weightExoskeletonforHipFlexion-extensionAssistance

197

Human Movement, 4th ed.: Wiley.

Veneman, J. F., Ekkelenkamp, R. Kruidhof, R., et al.,

2006. A series elastic- and Bowden-cable-based

actuation of use torque actuator in exoskeleton-type

robots, Int. J. Robot. Res., vol. 25, no. 3, pp. 261–281.

Vitiello, N., Lenzi, T., Roccella, S., et al., 2013.

NEUROExos: A Powered Elbow Exoskeleton for

Physical Rehabilitation, IEEE Transactions on

Robotics, vol. 29(1): 220-235,

Veneman, J. F., Kruidhof, R., Hekman, E. E. G., et al.,

2007. Design and Evaluation of the LOPES

Exoskeleton Robot for Interactive Gait Rehabilitation.

IEEE Transactions on Neural Systems and

Rehabilitation Engineering, 15(3):379-386.

Zinn, M., Roth, B., Khatib, O., et al., 2004. A new

actuation approach for human-friendly robot design.

Int. J. Robot. Res., vol. 23,no. 4–5, pp. 379–398.

NEUROTECHNIX2013-InternationalCongressonNeurotechnology,ElectronicsandInformatics

198