Int. J. Modelling, Identification and Control, Vol. 34, No. 1, 2020 3 Copyright © 2020 Inderscience Enterprises Ltd. Improving the performance of medical robotic system using H ∞ loop shaping robust controller Shahad Sami Ali Department of Computer Engineering, Al-Nahrain University, Baghdad, 10072, Iraq Email: shahad.sami@eng.nahrainuniv.edu.iq Safanah M. Raafat Department of Control and System Engineering, University of Technology, Baghdad, 10066, Iraq Email: 60154@uotechnology.edu.iq Ayman Al-Khazraji* Department of Electrical and Electronics Engineering, University of Bahrain, 32038, Bahrain Email: aalkhazraji@uob.edu.bh *Corresponding author Abstract: To solve the relevant open problem of precise tracking, the current research synthesises a robust controller via H ∞ loop shaping for a human swing lower limb system. The human walking is sophisticated with particularly nonlinear dynamics. It is interpreted as a pendulum links representing human thigh and shank. Reliable and efficient controller for a human swing leg robot system faces serious problems due to weight variations, modelling uncertainties and different disturbing effects. This work considers these constraints through designing a robust controller based on loop-shaping framework. It provides an appropriate approach to compromise robustness and tracking precision despite variation of weight factors, and model uncertainty. To obtain muscle gestures, two necessary control actions were developed and applied at the joints. Robust stability and tracking were demonstrated rigorously. The obtained results from numerical examples confirm the effectiveness of the proposed controller as well as its remarkable simplicity. Keywords: robotic system; H ∞ loop shaping; SLS; swing leg system; pendulum links; biped robot; robust control. Reference to this paper should be made as follows: Ali, S.S., Raafat, S.M. and Al-Khazraji, A. (2020) ‘Improving the performance of medical robotic system using H ∞ loop shaping robust controller’, Int. J. Modelling, Identification and Control, Vol. 34, No. 1, pp.3–12. Biographical notes: Shahad Sami Ali received her BEng and MSc in Control and System Engineering from the University of Technology-Baghdad. Actually, she is carrying out teaching and research activities in the department of Computer Engineering at Al-Nahrain University. Her main research interests include automation, robotics, optimal control, optimisation, and robust control. Safanah M. Raafat (BEng, MSc Eng, PhD in Control and System Engineering), is an Assistant Professor at Control and System Engineering Department at the University of Technology- Baghdad. She is a Senior IEEE member and a member of IEEE Control Systems Society. Her main research interests include system identification, optimisation, robust control, AI and optimal control. Ayman Al-Khazraji (BEng 1998/MSc Eng 2000/PhD Eng 2008) has taught for years at Reims University (France) then Caledonian College of Engineering (Oman). He has served, in France, as a consultant with General Electric Energy, Alstom Power Systems and Valeo Automotive.