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International Journal of Advanced Robotic Systems Customizable Rehabilitation Lower Limb Exoskeleton System Regular Paper

Riaan Stopforth1,*

1 Mechatronics and Robotics Research Group (MR2G) Bio-Engineering Unit, University of KwaZulu-Natal * Corresponding author E-mail: [email protected] Received 16 Jul 2012; Accepted 5 Sep 2012 DOI: 10.5772/53087 © 2012 Stopforth; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract Disabled people require assistance with the motion of their lower limbs to improve rehabilitation. Exoskeletons used for lower limb rehabilitation are highly priced and are not affordable to the lowerincome sector of the population. This paper describes an exoskeleton lower limb system that was designed keeping in mind that the cost must be as low as possible. The forward kinematic system that is used must be a simplified model to decrease computational time, yet allow the exoskeleton to be adjustable according to the patient’s leg dimensions. Keywords Lower limb exoskeleton, rehabilitation, customizable

1. Introduction

The 21st century has seen the realization of wearable robots. From their first introduction into the industrial workplace in the 1960s (Craig, 2005), robots have developed at an incredible rate and now encompass almost every aspect of modern society. Wearable robots are defined as “a mechatronic system that is designed

around the shape and function of the human body, with segments and joints corresponding to those of the person it is externally coupled with” (Mohammed and Amirat, 2008). A bio‐mechatronic system is needed for such wearable robots, which is the integration of biology, mechanical, electronic and computer engineering, as shown in Figure 1 (Naidu et al., 2012). Due to technological developments, robotic exoskeleton systems have evolved from rudimentary prototypes with limited application to highly sophisticated devices. These systems have the ability to enhance the performance of humans and enable disabled individuals to perform actions according to the Activities of Daily Living (ADL). There are approximately 250 000 cases of spinal cord injuries per annum in the United States of America alone (Koslowski, 2009). Severe trauma to the spinal cord may result in paraplegia or tetraplegia. Paraplegia is the loss of motor function in the lower extremities, usually with retained upper limb functions. Damage to the central nervous system or spinal cord injuries may result in such a loss of upper or lower limb motor functions (Stokes, 2010). An exoskeleton structure is required for

1Riaan Stopforth: Customizable Rehabilitation Lower Limb Exoskeleton Systemwww.intechopen.com

ARTICLE

www.intechopen.com Int J Adv Robotic Sy, 2012, Vol. 9, 152:2012

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2 Int J Adv Robotic Sy, 2012, Vol. 9, 152:2012 www.intechopen.com

Hip abduction/adduction and internal rotation do not play a significant role during the walking cycle (Hian Kai et al., 2009), and were omitted from the design. The design developed is seen in Figure 3, which permitted walking in a straight line. This straight line walking means that the hip, knee and ankle joints permit articulation of the limbs in the sagittal plane (Naidu et al., 2011b).

Figure 3. Lower limb design

The ranges of motion for the joints are constrained such that hyper‐extension and hyper‐flexion do not occur. These ranges are tabulated in Table 1 (Naidu et al., 2011b). Mechanical stops at the extremities act as a failsafe in the event of an electrical or software failure from the safety switches. Lower operational limits can be entered on a graphics user interface (GUI) should a patient need rehabilitation at lower angles.

Table 1. Joint range of motion

Both the hip and knee DOF were actuated, while the ankle joint was designed to be passive. A torsion spring mounted at the ankle was used to return the foot plate to a neutral position during the swing phase of the walking cycle. Data from clinical gait analysis (Riener et al., 2002) were evaluated to determine the joint torques for the actuated DOF. For a 100 kg system, the torque requirement for hip extension was 80 Nm. The torque required for knee extension during stair climbing was 140 Nm and 50 Nm during walking. Actuators were

selected such that the maximum torque was met, which allows for the operator to be raised or lowered from a seated position. Electric linear actuators from Phoenix Mecano’s LZ60 range were selected as they offered high speed/load capabilities and a less bulky design than direct mounted rotational actuators.

3. Customizable Kinematic Model

A kinematics analysis was undertaken for the lower limb exoskeletons. The Denavit‐Hartenberg (D‐H) convention was incorporated for assigning the reference frames. The transformation matrix shown in Equation (1), represents joint i relative to joint i‐1. The exoskeletons are rigid serial mechanisms, which allow for the end‐effecter to be represented relative to the fixed base frames (Craig, 2005).

�� 0 ��

�� 0 0 0 1

� (1)

Where: ��−1=distance from ��−1 to �� about ��−1 �� =angle from ��−1 to �� about ��−1 �� =angle from ��−1 to �� about �� =distance from ��−1 to �� along �� The lower limbs have identical kinematic chains, thus the fixed reference frame was defined at the hip, and the transformation matrices relating the ankle to the reference frame were found. These matrices can be seen in Equations (2) ‐ (4), which have been derived from Equation (1) (Naidu et al., 2011b).

�� 1 ��1 0 0�1 �1 0 00 0 1 00 0 0 1

� (2)

�� 1 0 0 �10 1 0 00 0 1 00 0 0 1

� (3)

�� 0 �� 0 00 0 1 00 0 0 1

� (4)

The forward kinematics of the exoskeleton leg were obtained using Equation (5) (Craig, 2005). This kinematics model relates the end‐effector to the origin of the base frame, which is represented by the GH joint.

� �� (5)


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ph of actuator l

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lower limb exo

system was twalking moti

within 3 seconnt angles wererage values wd Figure 13 re° were placed

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oskeleton being

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Conclusion

lower limb pwed for hip, ittal plane. Oskeleton framleg rehabilitatallow for adjson with a wriction allowe

D‐H notatiematics of thed to derive th workspace ofrange of mots used to solvwing determin

feed‐rate of m/s allowing thuired for the ggth of 150 mained for walkorithm and arknee joint o

hip flexionjoint

knee extensionj

ower of the acons were recoy.

prototype exosknee and ankOther DOF

me as these motion. The exoskjustability anweight less ted for a safety

on was usede mechanism. he relative joinf the design wtion. The Damve the inversening the angle

the linear acthe knee joint tgait cycle. Formm is requireking motion hchitecture havof 1° above

t angle within th

joint angle with

ctuators for thorded as bein

skeleton was kle joint articuwere eliminotions were ukeleton legs wnd the rehabithan 100 kg. factor to be in

d to derive The D‐H parnt transformawas simulatedmped Least Sqe kinematics oe of each joint

uators was foto rotate 90° inr a 120° revolued. The resulhave shown thve caused an the constrain

he gait cycle

hin the gait

he hip flexionng 110 W and

designed thatulation in thenated on thesed less oftenwere designedilitation of aThis weight

ncluded.

the forwardrameters weretion matrices.d to determinequare methodof the system,.

ound to be 28n 1 second, asution, a strokelts that werehat the controlovershoot fornt limit (60°)

n d

t e e n d a t

d e . e d ,

8 s e e l r )


indicated on the GUI. An undershoot was observed by the hip joint, which is possibly due to the load on the actuators that damped the chance of an overshoot. The operation of the biological leg and previous lower limb exoskeletons were researched. The mechanical properties of the biological leg were correlated to the design and development of the exoskeleton legs to allow rehabilitation in the sagittal plane. The objectives explained in the introduction were achieved. The integration of the electronic system to control and operate the mechanical system was explained in the paper. Safety implementation of the system was integrated mechanically, electronically and by means of software. The research has developed a prototype system that allows for the rehabilitation of a person’s lower limbs, which came to a total cost of under US$ 3,000.Satisfactory results were obtained to allow future work to be performed on the system.

6.1. Future Work

The actuators that were used in the prototype system had a low torque and speed which could be increased to allow rehabilitation of people with greater weight. Higher torque actuators that have a low weight ratio would be more beneficial, but would increase the cost. The prototype system could be improved and expanded for different types of applications. Adaptive control architecture could be implemented into the GUI model that will take into account the weight of the person. These variables could be determined by sensors placed on the exoskeleton lower limb system. The designed lower limb exoskeleton system will allow for rehabilitation in an up‐right position. Investigation of a lower limb rehabilitation system in a seated position could be considered, with the use of an impedance control system.

7. References

[1] Buss, S.R. (2004), ʺIntroduction to Inverse Kinematics with Jacobian Transpose, Pseudoinverse and Damped Least Squares methodsʺ, IEEE Journal of Robotics and Automation, 17 April 2004.

[2] Craig, J.J. (2005), Introduction to Robotics ‐Mechanics and Control 3rd ed. Upper Saddle River: Pearson Prentice Hall.

[3] Hian Kai, K., Missel, M., Craig, T., Pratt, J.E., Neuhaus, P.D. (2009), ʺDevelopment of the IHMC Mobility Assist Exoskeleton”, IEEE International Conference in Robotics and Automation (ICRA 2009), pp. 2556‐2562.

[4] Inc., C. (2011). “Hybrid Assistive Limb”, Available: http://www.cyberdyne.jp/english/index.html, 18 May 2011

[5] Kazerooni, H., Racine, J.‐L., Lihua, H., Steger, R. (2005), ʺOn the Control of the Berkeley Lower Extremity Exoskeleton (BLEEX),ʺ IEEE International Conference of Robotics and Automation (ICRA 2005), pp. 4353‐4360.

[6] Koslowski, H.M. (2009), ʺSpinal Cord Injury: Functional Outcomes in 2009 and Beyond,ʺ Northeast Florida Medicine, vol. 60, pp. 32‐35.

[7] Mohammed, S. and Amirat, Y. (2008), ʺTowards intelligent lower limb wearable robots: Challenges and perspectives ‐ State of the artʺ, IEEE International Conference on Robotics and Biomimetics, 2008, pp. 312‐317.

[8] Na, M., Yang, B., and Jia, P. (2008), ʺImproved damped least squares solution with joint limits, joint weights and comfortable criteria for controlling human‐like figures.,ʺ IEEE Conference on Robotics, Automation and Mechatronics, pp. 1090‐1095.

[9] Naidu, D., Stopforth, R., Bright G., Davrajh, S. (2011a), ʺA 7 DOF exoskeleton arm: Shoulder, elbow, wrist and hand mechanism for assistance to upper limb disabled individuals,ʺ AFRICON, 2011, Livingstone, Zambia; IEEE, pp. 1‐6, 13‐15 Sept. 2011

[10] Naidu, D., Cunniffe, C., Stopforth, R., Bright, G., Davrajh, S. (2011), “Upper and Lower exoskeleton limbs for Assistive and Rehabilitative Applications”, 4th Conference of Robotics and Mechatronics (RobMech), Pretoria, South Africa, November 2011

[11] Naidu, D., Stopforth R., Davrajh S., Bright G. (2012), “A Portable Passive Physiotherapeutic Exoskeleton”, International Journal of Advanced Robotic Systems, InTech, Vol 9

[12] Pons, J.L. (2008), “Wearable Robots: Biomechatronic Exoskeletons”, Chichester, West Sussex: John Wiley & Sons Ltd, 2008.

[13] Riener, R., Rabuffetti, M., Frigo, C. (2002), ʺStair ascent and descent at different inclinationsʺ, Gait & Posture, vol. 15, pp. 32‐44.

[14] Sankai, Y. (2006), ʺLeading Edge of Cybernics: Robot Suit HAL,ʺ International Joint Conference (SICE‐ICASE 2006), pp. 1‐2.

[15] Stopforth, R., Bright, G., Davrajh, S., Walker, A., (2011), ʺImproved communication between manufacturing robots, ʺSouth African Journal of Industrial Engineering, vol. 22, pp. 99 ‐ 107.

[16] Stroke, N. I. o. N. D. a. (2010), “NINDS Brachial Plexus Injuries Information Page”, Available: http://www.ninds.nih.gov/disorders/brachial_plexus/brachial_plexus.htm, 31 May 2010.


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