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Research Papers

An Ankle Exoskeleton Using a Lightweight Motor to Create High Power Assistance for Push-Off

[+] Author and Article Information
Jiazhen Liu

Department of Mechanical and Energy Engineering,
Southern University of Science and Technology,
Shenzhen 518055, China;
Naval Logistics Academy,
Tianjin 300450, China;
Department of Mechanical Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: ljz15@tsinghua.org.cn

Caihua Xiong

State Key Laboratory of Digital Manufacturing Equipment and Technology,
Huazhong University of Science and Technology,
Wuhan 430074, China
e-mail: chxiong@hust.edu.cn

Chenglong Fu

Department of Mechanical and Energy Engineering,
Southern University of Science and Technology,
Shenzhen 518055, China
e-mail: fucl@sustc.edu.cn

1Corresponding author.

Contributed by the Mechanisms and Robotics Committee of ASME for publication in the Journal of Mechanisms and Robotics. Manuscript received October 12, 2017; final manuscript received March 29, 2019; published online May 17, 2019. Assoc. Editor: Xilun Ding.

J. Mechanisms Robotics 11(4), 041001 (May 17, 2019) (10 pages) Paper No: JMR-17-1349; doi: 10.1115/1.4043456 History: Received October 12, 2017; Accepted April 03, 2019

Active exoskeletons have capacity to provide biologically equivalent levels of joint mechanical power, but high mass of actuation units may lead to uncoordinated walking and extra metabolic consumption. Active exoskeletons normally supply assistance directly during push-off and have a power burst during push-off. Thus, the requirements on power of motors are high, which is the main reason for the high mass. However, in a muscle-tendon system, the strategy of injecting energy slowly and releasing quickly is utilized to obtain a higher peak power than that of muscle alone. Application of this strategy of peak power amplification in exoskeleton actuation might lead to reductions of input power and device mass. This paper presents an ankle exoskeleton which can accumulate the energy injected by a motor during the swing phase and mostly the stance phase and then release it quickly during push-off. An energy storage and release system was developed using a four-bar linkage clutch. In addition, evaluation experiments on the exoskeleton were carried out. Results show that the exoskeleton could provide a high power assistance with a low power motor and reduced the requirement on motor power by 4.73 times. Besides, when walking with the exoskeleton, the ankle peak power was reduced by 25.8% compared to the normal condition. The strategy which imitates the working pattern of the muscle-tendon system leads to a lightweight and effective exoskeleton actuation, and it also supplies ideas for the designs of lightweight actuators that work discontinuously in other conditions.

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Figures

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Fig. 1

The main idea of our exoskeleton. To explain the idea clearly, we define a gait cycle starting with the swing phase and ending with the stance phase. The solid line represents the ankle power. Push-off lasts 85–100% of the gait cycle, and the ankle power has a burst obviously. If energy can be stored during 0–85% rather than supplied directly during push-off, working time will be extended as the bar shows and the peak power will be reduced significantly compared to the direct actuation as the shadow shows.

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Fig. 2

3D model of the exoskeleton. (a) The exoskeleton comprises rigid struts, a four-bar linkage clutch mechanism, and a tension spring act in parallel with the Achilles tendon. A trigger system acts with the ankle rotation and controls the state of the clutch. (b) A pair of gears connects the motor and the pulley. When the clutch is locked, the gears engage, and when the clutch is unlocked, the gears disengage.

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Fig. 3

Working principle of the exoskeleton. Drawings of the ankle with the exoskeleton illustrate the working state at the phase which is marked by dashed lines. The arrows represent motion directions of motor, PO spring, and clutch bars, respectively.

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Fig. 4

Force analysis of the four-bar linkage. (a) The external force on the four-bar linkage. F1 means the external force applied by the ES rope, and F2 means the force applied by the trigger bar to destroy the equilibrium. λ means the angle between the bar and the vertical direction and it ensures the stable equilibrium. (b) The force condition of the bar in dashed box. F3 and F4 refer to external forces applied by other bars. Since λ is small, we can consider the angle between the direction of F3 and the vertical direction as 2λ approximately.

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Fig. 5

Analysis of the trigger. Angle of the ankle joint is defined as θ, and the angle between the trigger and the clutch is defined as η. η varies with the variation of θ. In essence, the ankle frame and the trigger system form a quadrangle which is shown with a dashed box, and α, β, δ, and γ, respectively, represent the interior angle of the quadrangle. There is a constant difference between θ and γ, and also between η and α. We define the ankle moment as τ which makes the trigger bar create a force called F2.

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Fig. 6

Angles and moment in the gait cycle. Dashed line means the phase when maximum push-off occurs, and at this phase, ankle angle, moment, and trigger angle reach the extreme value. The data of the ankle angle are obtained by human walking experiments. Based on the angle of ankle, the angle of trigger can be obtained through Eq. (6). Ankle moment is obtained by inverse dynamic analysis.

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Fig. 7

Adjustment of the trigger angle. (a) Curve of the ankle angle during the gait cycle. (b) The length of the intermediate bar L2 can be adjusted to allow the individual difference of the maximum ankle angle. Dense dashed line depicts the curve of the trigger angle when length of L2 increases. Sparse dashed line depicts the opposing situation.

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Fig. 8

The effect of design parameters on the output peak power. The contour refers to the output peak power, and the red line refers to the result of parameter selection.

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Fig. 9

The effect of design parameters on the device mass and the peak power amplification factor. The contour in (a) refers to the device mass and the dashed line refers to the isoline of target output power.

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Fig. 10

Evaluation of PPAF. (a) The manufactured exoskeleton used in evaluation. Two load cells shown in dashed boxes were placed in ER and ES ropes, respectively, to measure input and output forces. (b) Subject wearing the exoskeleton walked on the pathway, and walking frequency is 1 Hz.

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Fig. 11

Input and output power in a complete gait cycle. The input power is the power generated by the DC motor when the PO spring was stretched. The output power is the power generated by the PO spring when it is released.

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Fig. 12

Experimental environment of ankle power evaluation. The motion capture system (including six Raptor 4 cameras and 15 makers) was used to collect joint kinematics information, and three force plates in the pathway were used to measure the ground reaction force. Load cell was used to measure force of the PO spring and obtain the power of the exoskeleton. Subject wore the exoskeleton only on the right shank, and markers were placed on lower limbs as a Helen Hayes marker set.

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Fig. 13

Effect of the exoskeleton on the ankle power: (a) curves of the human ankle power in three conditions and (b) ankle peak powers under three conditions

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