Standing and Sitting Motion Assistance using a Compliant KneeExoskeleton

Nikos C. Karavas, Nikos G. Tsagarakis and Darwin G. CaldwellIstituto Italiano di Tecnologia (IIT)

Sit-to-stand movement is the one of most important tasksin daily life for individuals with impaired legs and elderlypeople who do not have the required physical strength[1], [2]. This work presents an compliant knee exoskeleton(see Fig. 2) which can provide motion assistance to theuser during standing-up and sitting-down based on the userintent. The knee exoskeleton is energized by a series elasticactuator with offline reconfigurable stiffness (CompAct-RS)[3], [4]. Introducing passive compliance to the actuation unitdecouples the inertia of the motor drive from the outputlink and thus the exoskeleton presents low impedance to thewearer. Additionally, compliant robots can intrinsically ab-sorb impacts and associated with dedicated control strategiescan achieve safety and adaptability of physical human-robotinteraction (pHRI).

CompAct-RS as mentioned above is a series elastic actua-tor with the ability to regulate off-line the level of stiffness inas wide a range as needed. This feature permits the experi-mentation with different compliance levels and the adaptationof the joint to fit specific task requirements. The eliminationof active tuning of the spring stiffness through a secondmotor was performed to reduce the weight and dimensionsof the unit. The working principle of the CompAct-RS isbased on the CompAct-VSA (Variable Stiffness Actuator)[5], which uses a lever arm mechanism with a variable pivotaxis.

Fig. 1. View of CompAct-RS.

Fundamental starting point of the design was to derivethe mechanical requirements of the actuator from simulation

The authors are within the Department of AdvancedRobotics, Istituto Italiano di Tecnologia, via Morego, 30, 16163Genova {nikolaos.karavas, nikos.tsagarakis,darwin.caldwell}@iit.it

and experimental data of the human standing-up and sitting-down. Acquiring this data we selected the design variablesof the actuator and analysed its performance and stiffnesscharacteristic.

An assistive control strategy is proposed which allows theactuator to generate assistive torques towards to the directionof motion based on estimated user torques. Particularly,electromyographic signals are used in order to derive theestimated user torque while the equilibrium position of a vir-tual spring-damper network is being updated in accordanceto this estimated user torque.

Fig. 2. Knee exoskeleton first prototype.

REFERENCES

[1] A. Tsukahara, Y. Hasegawa, and Y. Sankai, “Standing-up motionsupport for paraplegic patient with robot suit hal,” in RehabilitationRobotics, 2009. ICORR 2009. IEEE International Conference on, june2009, pp. 211 –217.

[2] A. Tsukahara, R. Kawanishi, Y. Hasegawa, and Y. Sankai, “Sit-to-standand stand-to-sit transfer support for complete paraplegic patients withrobot suit hal,” Advanced robotics, vol. 24, no. 11, pp. 1615–1638,2010.

[3] N. C. Karavas, N. G. Tsagarakis, J. Saglia, and D. G. Galdwell, “A novelactuator with reconfigurable stiffness for a knee exoskeleton: Designand modeling,” in Advances in Reconfigurable Mechanisms and RobotsI, J. S. Dai, M. Zoppi, and X. Kong, Eds. Springer London, 2012, pp.411–421.

[4] N. C. Karavas, N. G. Tsagarakis, and D. G. Galdwell, “Design,modeling and control of a series elastic actuator for an assistive kneeexoskeleton,” in Biomedical Robotics and Biomechatronics. The FourthIEEE/RAS-EMBS International Conference on, 2012.

[5] N. Tsagarakis, I. Sardellitti, and D. Caldwell, “A new variable stiffnessactuator (compact-vsa): Design and modelling,” in Intelligent Robotsand Systems (IROS), 2011 IEEE/RSJ International Conference on.IEEE, 2011, pp. 378–383.