Elsevier

Robotics and Autonomous Systems

Volume 95, September 2017, Pages 181-195
Robotics and Autonomous Systems

Biomechanical design of an agile, electricity-powered lower-limb exoskeleton for weight-bearing assistance

https://doi.org/10.1016/j.robot.2017.06.010Get rights and content

Abstract

This paper proposes the design of an electricity-powered lower-limb exoskeleton called “Human Universal Mobility Assistance (HUMA)”. HUMA was developed as a research platform with the objective of providing its wearer with weight-bearing assistance for human strength/endurance augmentation. It has 12 degrees of active/spring passive/free passive freedom to assist human locomotion. The artificial leg has two electricity-powered degrees of freedom (DoFs) for hip/knee flexions/extensions, passive spring-installed two DoFs for ankle inversion/eversion and plantarflexion/dorsiflexion, and two free, passive DoFs for hip roll/yaw movements. HUMA has mechanical structures for active artificial hip and knee joints; the hip actuator is not directly connected to the robot’s leg system, but a universal joint is installed between the actuator and the leg system to allow a free coaxial hip yaw/roll DoF for the wearer. Therefore, the hip-actuating torque is transferred solely for hip flexion/extension. Its active artificial knee is structured by a four bar-based polycentric linkage, and is power=driven by an actuator in the middle of the robot’s thigh segment through the other four bar-based power transmission linkage. This powered knee structure yields several advantages related to (1) human–robot knee alignment during leg motion, (2) the expansion of the zone of voluntary knee stability, (3) the angle-dependent variable knee torque/velocity amplification ratio, and (4) a reduction in the total moment of artificial leg inertia. The exoskeleton was tested for dynamic gait by using assistive torques determined by a control algorithm. Experiments were conducted on the robot while it walked at 5 km/h (1.39 m/s) with/without a 20 kg load, as well as for a 10-km/h (2.78 m/s) run.

Introduction

A number of technological challenges have been encountered and surmounted in research on electricity-powered robotic suits in recent years. Despite recent progress at an unprecedented rate in electricity-powered exoskeleton technology [1], [2], such metrics of mobility performance as natural speed of locomotion and metabolic cost remain too poor to be viable for application [3]. It is challenging to develop electric actuators of exoskeletons that are not sluggish, but light and compact. Moreover, the energy supplies are too bulky and heavy to deliver satisfactory operational time. Hence, there is a need for interdisciplinary efforts in such fields as biomechanics and physiology to solve the above problems [4].

In particular, the mechanical design of lower-limb exoskeletons introduces critical attributes that determine their functions based on wearable robotics and human–robot physical interfacing. The human musculoskeletal system followed by the exoskeletal parts in parallel has complex structures that permit various degrees of freedom (DoFs), but the space/weight for/of robotic suits is limited to allow or actuate all the DoFs of the human body. Therefore, many powered lower-limb exoskeletons have exhibited semi-anthropomorphic mechanical design that are simplified or approximated according to human kinematics.

A representative design approximation for powered lower-extremity exoskeletons is evident in the design of single axis-type artificial knee joints [5], [6], [7]. However, the human knee joint exhibits polycentric rotational behavior [8] that cannot be reproduced by these single-axis joint structures. Furthermore, this kinematic misalignment between artificial legs and human legs can be a more critical issue for fast and dynamic gaits due to the loss of power by force transmission in unintended directions to the human limbs, making the wearer uncomfortable [9].

Therefore, artificial polycentric knee joints utilizing various mechanical mechanisms are often applied to prosthetics for transfemoral amputees and orthoses for patients of knee injuries. Polycentric knee joint mechanisms are especially well known to expand the region of voluntary knee stability, which allows patients to have a more natural gait in the stance gait phase with knee flexion [10]. Further, better motion alignment, higher ground clearance, and cosmetic superiority are typical advantages of polycentric knee orthoses, despite their weak structural durability. In order to expand these advantages to powered, full lower-limb exoskeleton technology, various polycentric mechanical mechanisms have been applied to modular-type knee exoskeletons in the literature [11], [12], [13]. However, it remains rare, to the best of our knowledge, to find a powered, full lower-limb exoskeleton that uses a polycentric knee mechanism.

This paper introduces a light-weight, electricity-powered lower-limb exoskeleton called Human Universal Mobility Assistance (HUMA), and is shown in Fig. 1. HUMA was developed to provide its wearer with weight-bearing assistance that allows individuals, including the elderly, to augment their endurance/strength, such that they can support their own weights as well as an additional payload. For a similar purpose, previously developed and notably agile lower-limb exoskeletons include BLEEX, which attained an average speed of 4.68 km/h with a 34-kg load [14], HERCULE that attained a maximum walking speed of 5 km/h [15], HULC that recorded a maximum speed of 11 km/h for long durations [16], and the quasi-passive MIT exoskeleton tested with a pilot for 3.3-km/h walking [17]. Note that the gait speed of healthy people was recorded as 6.63 km/h on average as fast, self-selected gait [18]. Except for hydraulic-actuated HULC, it is quite challenging to find exoskeletons covering the human agility observed in walking and running. In other words, it is difficult to provide a pilot with the required agility assistance, especially using electric motor-driven exoskeletons. We believe that the biomechanical design for HUMA powered by electric motors is essential for agility to rival that of HULC.

The artificial leg system of HUMA has unique mechanical structures for the hip, the knee, and the ankle to follow and assist degrees of freedom (DoFs) allowable in the lower limbs of humans: (1) a powered artificial hip-transferring hip flexion/extension torque through a universal joint, (2) a powered, artificial polycentric knee reinforced by an array of roller bearings, and (3) a two-DoF passive spring-loaded ankle.

Based on the above design, two identical hip and knee actuators are built and controlled by a control algorithm with sensory information concerning HUMA. The ultimate goal of the control is to determine effective hip and knee torques for weight-bearing assistance that can help support the wearer’s body weight as well as an additional load while walking or running at various speeds. In experiments, HUMA was tested on a pilot for agile and consistent gait-assistive performance. To the best of our knowledge, this is the first implementation of a powered, polycentric-structured knee on a full lower-limb exoskeleton.

The remainder of this paper is structured as follows: Section 2 explains the mechanical design of HUMA with its kinematic characteristics from top to bottom. Section 3 briefly introduces the control architecture and addresses the leg control algorithm implemented on HUMA. Subsequently, experimental data obtained from several implementations are provided, and the gait characteristics of a pilot using HUMA are discussed in Section 4. We summarize the paper and propose directions for future work in Section 5.

Section snippets

Overall configuration

The main components of HUMA are shown in Fig. 2. Its weight, including the battery pack, is approximately 10 kg. Its leg system consists of (1) a three-DoF hip joint with active hip flexion/extension, with passive hip roll and yaw, (2) a one-DoF knee joint for its flexion/extension, and (3) a two-DoF ankle joint with spring-loaded ankle eversion/inversion and dorsiflexion/plantarflexion.

Therefore, the total number of DoFs provided by HUMA is 12. The length of the shank link is discretely

Control architecture—partitioned left/right leg control

To determine assistive torques and apply them to the artificial hip and knee joints, a controller package containing a 48 V LiPo battery was developed and mounted on the back panel of the robot, as shown in Fig. 2. The controller package consisted of two low-cost, identical controllers. Each had an ARM cortex 667 MHz dual-core processor with a field-programmable gate array (FPGA). The controllers were operated on a Linux real-time operating system and interfaced with the sensors and motor

Experimental results and discussion

To verify the gait performance of HUMA, several experiments were conducted in a laboratory. Prior to the experiments, the adjustable shank length of HUMA was prepared for a 175-cm tall male weighing 75 kg and aged 36. The model parameters for the control algorithm were derived from the CAD file and applied to both leg controllers. Overall, the experiments consisted of (1) 5 km/h walking with/without the universal joint in the hip structure, (2) 10 km/h running, (3) 5 km/h walking with a 20 kg

Conclusion

This paper introduced a biomechanical design for an agile electricity-powered lower-limb exoskeleton called HUMA that uses (1) a powered artificial hip transferring hip flexion/extension torque through the universal joint, (2) a powered artificial knee structured by a double four-bar linkage, and (3) a two-DoF spring-loaded ankle joint. Using such design advantages as minimal impedance on non-actuated hip motions, voluntary knee stability, reduction of the moment of inertia of the leg, and

Dong Jin Hyun received the B.S. degree from the School of Mechanical and Aerospace Engineering, Seoul National University, Korea, in 2006, the M.S. degree in mechanical engineering from the University of Michigan, Ann Arbor, MI, USA, in 2007, and the Ph.D. degree in mechanical engineering from the University of California, Berkeley, CA, USA in 2012. He was the Postdoctoral Associate in mechanical engineering at the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA in 2013. Since

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    Dong Jin Hyun received the B.S. degree from the School of Mechanical and Aerospace Engineering, Seoul National University, Korea, in 2006, the M.S. degree in mechanical engineering from the University of Michigan, Ann Arbor, MI, USA, in 2007, and the Ph.D. degree in mechanical engineering from the University of California, Berkeley, CA, USA in 2012. He was the Postdoctoral Associate in mechanical engineering at the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA in 2013. Since 2013, he has been a Senior Research Engineer with Hyundai Motor Company, Gyeonggi-do, Korea. His research interests include dynamics & control of legged locomotion, human–robot interaction for wearable robots, and biomechanics.

    Hyunseok Park received the B.S. and M.S. degrees in electrical engineering from Pohang University of Science and Technology (POSTECH), Pohang, South Korea, in 2008 and 2010, respectively. He is currently a Research Engineer of Hyundai Rotem Company, Uiwang, South Korea. His research interests include signal processing, system identification, and control on wearable robots.

    Taejun Ha received the B.S. degree in mechanical engineering from Kyunghee University, Suwon, South Korea and the M.S. degree in mechanical engineering from Hanyang University, Seoul, South Korea in 2009. He is currently a Senior Research Engineer of Hyundai Rotem, Uiwang, South Korea. His research interests include mechanical design on wearable robots, power-electronics, and actuation systems.

    Sangin Park received the B.S. degree from the Department of Mechanical Engineering, Hanyang University, Seoul, Korea, in 2006. He was with the Korean branch of National Instruments as an Application Engineer, PAC specialist and Field Sales Engineer from 2005 to 2013. In 2013, he joined the MIT Biomimetic Robotics Laboratory as a Research Engineer. Since 2014, he has been a Senior Research Engineer with Hyundai Motor Company, Gyeonggi-do, Korea. His research interests include control architecture design, and power electronics.

    Kyungmo Jung received the B.S. and M.S. degrees from the Department of Mechanical Engineering, Korea University, Seoul, Korea in 2006 and 2008, respectively. And he received the Ph.D. degree in robotics from Korea University, Seoul, Korea, in 2013. Since 2013, he has been a Senior Research Engineer with Hyundai Motor Company, Gyeonggi-do, Korea. His research interests include mechanical design on wearable robots, and actuation systems.

    This project was conducted & sponsored under the “Human Gait Assistance Project” in the Central Advanced Research and Engineering Institute of Hyundai Motor Group.

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