Hao Ma

Recent developments and challenges of lower extremity exoskeletons

Summary The number of people with a mobility disorder caused by stroke, spinal cord injury, or other related diseases is increasing rapidly. To improve the quality of life of these people, devices that can assist them to regain the ability to walk are of great demand. Robotic devices that can release the burden of therapists and provide effective and repetitive gait training have been widely studied recently. By contrast, devices that can augment the physical abilities of able-bodied humans to enhance their performances in industrial and military work are needed as well. In the past decade, robotic assistive devices such as exoskeletons have undergone enormous progress, and some products have recently been commercialized. Exoskeletons are wearable robotic systems that integrate human intelligence and robot power. This paper first introduces the general concept of exoskeletons and reviews several typical lower extremity exoskeletons (LEEs) in three main applications (i.e. gait rehabilitation, human locomotion assistance, and human strength augmentation), and provides a systemic review on the acquisition of a wearers motion intention and control strategies for LEEs. The limitations of the currently developed LEEs and future research and development directions of LEEs for wider applications are discussed.

Human Gait Modeling and Analysis Using a Semi-Markov Process With Ground Reaction Forces

Modeling and evaluation of patients’ gait patterns is the basis for both gait assessment and gait rehabilitation. This paper presents a convenient and real-time gait modeling, analysis, and evaluation method based on ground reaction forces (GRFs) measured by a pair of smart insoles. Gait states are defined based on the foot–ground contact forms of both legs. From the obtained gait state sequence and the duration of each state, the human gait is modeled as a semi-Markov process (SMP). Four groups of gait features derived from the SMP gait model are used for characterizing individual gait patterns. With this model, both the normal gaits of healthy people and the abnormal gaits of patients with impaired mobility are analyzed. Abnormal evaluation indices (AEI) are further proposed for gait abnormality assessment. Gait analysis experiments are conducted on 23 subjects with different ages and health conditions. The results show that gait patterns are successfully obtained and evaluated for normal, age-related, and pathological gaits. The effectiveness of the proposed AEI for gait assessment is verified through comparison with a video-based gait abnormality rating scale.

Design of a lower extremity exoskeleton for motion assistance in paralyzed individuals

In this paper, we reported innovative design of a lower extremity exoskeleton developed by The Chinese University of Hong Kong (CUHK-EXO) that can help the paralyzed individuals to regain the ability to stand up/sit down (STS) and walk. CUHK-EXO is developed with the features of ergonomic design, user-friendly interface and high safety. The CUHK-EXO hardware design including mechanical structure, electrical system, and multiple sensors is introduced first. A pair of smart crutches and smart phone App are designed as part of the human-machine interface, which can improve the intelligence of the system and make the exoskeleton easier to be used by physical therapists and paralyzed patients. Then, the CUHK-EXO control is developed with STS motion assistance. Finally, pilot trial study is conducted for a healthy subject, and testing results show that the developed CUHK-EXO can help the subject perform desired STS motions.

HIP-KNEE control for gait assistance with Powered Knee Orthosis

A Powered Knee Orthosis (PKO) was developed for the elderly and patients with disordered gait to regain normal walking. In order to enhance the PKO performance and reduce system complexity especially for people with muscle weakness in their knee joints, an algorithm named HIP-KNEE control is proposed. This algorithm is based on the analysis of kinematic gait model, and the desired knee joint angle (KNEE) is estimated from the measurements of hip joint angle (HIP). The relationship between HIP and KNEE is modeled as a polynomial, which can be easily implemented to an embedded controller for real-time control. This control method is suitable to subjects with good function in hip joint, and it can provide help in walking without special training. An Inertia Measurement Units (IMU) is used for obtaining HIP input, and integrated with a footswitch for checking the heel condition; the gait assistance performance can be further improved.

Design of powered ankle-foot prosthesis driven by parallel elastic actuator

To improve the energy efficiency and downsize the powered ankle-foot prostheses (PAFP) but still provide sufficient instantaneous power output, elastic actuators (EAs) are widely used. Generally, there are two kinds of EAs: series elastic actuator (SEA), and parallel elastic actuator (PEA). Both of them can reduce the energy requirement and peak power of the motor in the PAFP by using elastic elements to store and release energy during walking cycles. Researchers working on prostheses fields have been focusing on the design of PAFP driven by SEAs. As a result, several PAFP driven by SEAs were proposed. Compared with the PAFP driven by SEAs, the PAFP driven by PEAs could have more benefits, such as lower copper loss and higher force bandwidth due to the smaller motor output torque requirement. In this paper, we aim at designing a PAFP driven by a PEA. Based on the energy requirement analysis, an energy-efficient PAFP is proposed. The designed PAFP can mimic human intact ankle with the minimum energy consumption that equals to the human ankle net positive work. Besides, with the motor output torque being decreased, the motor peak power of the PAFP is reduced by 64%.

Design optimization of a magnetorheological brake in powered knee orthosis

Magneto-rheological (MR) fluids have been utilized in devices like orthoses and prostheses to generate controllable braking torque. In this paper, a flat shape rotary MR brake is designed for powered knee orthosis to provide adjustable resistance. Multiple disk structure with interior inner coil is adopted in the MR brake configuration. In order to increase the maximal magnetic flux, a novel internal structure design with smooth transition surface is proposed. Based on this design, a parameterized model of the MR brake is built for geometrical optimization. Multiple factors are considered in the optimization objective: braking torque, weight, and, particularly, average power consumption. The optimization is then performed with Finite Element Analysis (FEA), and the optimal design is obtained among the Pareto-optimal set considering the trade-offs in design objectives.

A wearable exoskeleton suit for motion assistance to paralysed patients

Summary Background/Objective The number of patients paralysed due to stroke, spinal cord injury, or other related diseases is increasing. In order to improve the physical and mental health of these patients, robotic devices that can help them to regain the mobility to stand and walk are highly desirable. The aim of this study is to develop a wearable exoskeleton suit to help paralysed patients regain the ability to stand up/sit down (STS) and walk. Methods A lower extremity exoskeleton named CUHK-EXO was developed with considerations of ergonomics, user-friendly interface, safety, and comfort. The mechanical structure, human-machine interface, reference trajectories of the exoskeleton hip and knee joints, and control architecture of CUHK-EXO were designed. Clinical trials with a paralysed patient were performed to validate the effectiveness of the whole system design. Results With the assistance provided by CUHK-EXO, the paralysed patient was able to STS and walk. As designed, the actual joint angles of the exoskeleton well followed the designed reference trajectories, and assistive torques generated from the exoskeleton actuators were able to support the patient’s STS and walking motions. Conclusion The whole system design of CUHK-EXO is effective and can be optimised for clinical application. The exoskeleton can provide proper assistance in enabling paralysed patients to STS and walk.

Design and analysis of a regenerative magnetorheological actuator for gait assistance in knee joint

In this paper, we propose and analyze a Regenerative Magnetorheological Actuator (RMRA) for gait assistance in knee joint. The actuator has motor and magnetorheological (MR) brake parts working in parallel, and can harvest energy through regenerative braking. This novel design provides multiple functions with good energy efficiency. The design of the RMRA is first introduced, and its multiple functions are modeled. Different working modes of the RMRA are analyzed for mimicking knee joint functions within gait cycle. To achieve an effective and energy efficient braking function, cooperation between regenerative braking and MR braking is investigated. The energy harvesting (EH) circuit equivalent load is further optimized for maximal energy harvested within a gait cycle. Then impedance based braking control of the RMRA for gait assistance is proposed. Preliminary simulation shows that the developed RMRA with proposed control strategy can provide desired torque assistance during normal gait.