grant-proposal thhsieh 1

8/11/2019 Grant-proposal THHsieh 1

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Muscle Inspired Actuator Using Solenoids: Design of A Powered

Ankle-foot Exoskeleton

Tsung-Han Hsieh

Department of Bio-Industrial Mechatronics Engineering,

National Taiwan University

Taipei, Taiwan, ROC

e-mail: [email protected]

I. INTRODUCTION

One of the biggest challenges in developing powered exoskeletons or active orthoses for either

able-bodied users or people suffering from lower extremity pathology is the way that the device works in

concert with the operators movements. The additional forces that the device provided, if not optimized

enough, might result in resisting operators motion rather than assisting [1]. Among the state-of-the-art

lower limb exoskeletons or active orthoses, the most commonly used actuators including: hydraulic

cylinders [2], electromagnetic dc motors with harmonic drives [3], and pneumatic actuators [4], were

shown in Fig. 1. These actuators are all stiff actuators which will lock themselves while idled, in other

words, the operators motion is limited by this locking mechanism. Therefore, for those powered

exoskeleton and active orthoses, detail sensory information of human motion is essential for

human/exoskeleton or human/orthotics interaction such as electromyography (EMG) to assess neural

activation of muscles, accelerometers for joint motions, and force sensors for detecting contact forces on

the ground. Furthermore, these actuators all have some drawbacks. Pneumatics and hydraulics actuators,

although were able to imitate the performance of natural muscle action and have a shape and feel similar

to natural muscles, they are noisy, difficult to control, and require additional pump to provide fluid energy,

which could be heavy and cannot be portable. Electromagnetic dc motors, on the other hand, do not offer

adequately high peaks or average power outputs, resulting in devices that are heavier than desired [5], [6].

In this proposal, a new type of actuator system, which was inspired by muscle physiology, was

proposed. Similar to the fundamental units in skeletal muscles called sarcomeres, the proposed actuator

system consists of many cellular units made of solenoids. This novel actuator system has several

advantages as follows: 1) Each solenoid has its own ONor OFFstate just like muscle fibers which

are able to produce tension during ON state and simply relaxed (no tension generated) during OFF state

and is therefore easier to control for some simple tasks. 2) Solenoids are able to produce high peak forces

with fast speed and 3) they will not be interfering with operators movement during the OFF state since

the plungers of the solenoids can move freely during OFF state. In order to reduce the complexity of

analytical effort, a uni-articulate mechanism was designed, which functioned as an additional soleus

muscle and can therefore provide additional moment of plantar flexion for ankle joint during push-off.

Although having the potential to implement it in different tasks such as stair ambulation or jumping, the

application for this powered ankle-foot exoskeleton in this proposal is narrowed down to assist push-offduring normal ambulation. In the future, this actuator has the possibility to be implemented in robotics,

exoskeletons, and other orthotic devices.


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Fig. 2. System architecture. The sbRIO serves as system I/O, it can control whether the voltage booster should charge or not,

analyze the signals from FSRs, and control the firing time of solenoids. As soon as the push-off event is detected, the sbRIO will

make driver circuits release the stored energy, making the solenoids to generate pulling force.

B. Electromechanical Characteristics and Mechanical Design of Solenoids

The low-profile solenoid, SMT-3018SL (Tai-Shing Electronics Corp., Taipei, Taiwan), was selected

to work as the contractile element in the cellular units. This low-profile solenoid, as shown in Fig. 4 only

weighs 97 grams and is 3.4 centimeters long in diameter, is able to generate more than 7 kilograms peak

pulling force under 10% duty cycle [8]. The duty cycle is the ratio between the ON and OFF period and

can be calculated as (1):

" "(%) 100%

" " " "

ON timeDuty cycle

ON time OFF time

(1)

One drawback of using solenoid as actuator is magnetic hysteresis. However, it can be compensated

with appropriate selection of spring or providing an inverse current. In this proposal, however, the effect

of magnetic hysteresis was neglected.

Fig. 3. The single-board RIO (sb-9642), manufactured by National Instruments, integrated I/O ports, a real-time processor, and a

re-programmable FPGA on a printed circuit board (PCB) [7].


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Fig. 4. Electromechanical characteristics and mechanical design of solenoids. While weighted only 97 grams and 3.4 centimeters in

diameter, the solenoid can generate more than 7 kilograms under 10% duty cycle [8].

C.

Hardware Design

The concept of hardware design is shown in Fig. 5. The mechanism was designed via SolidWorks

(Dassault Systmes SolidWorks Corp., Waltham, MA, USA) based on the dimensions provided by the

datasheet of solenoids. SolidWorks is 3D computer-aided design (CAD) software that is widely used by

engineers to create 3D models for machine parts.

As shown in Fig. 5, a pair of solenoids was arranged in a bipennate form, with a pennation angle of

o30 . By having this pennation angle, more solenoids can be placed in the limited space. In this project,

theo

30 pennation angle was chosen under the consideration of mechanism design, the relation between

pennation angle and overall force output should be optimized in the future. As a result, a pair of solenoid

can create a 5.2 millimeters stroke, since its original stroke was 6 millimeters. The total stroke that the

cellular units can provide can be modified base on different operators. In Fig. 5, three pairs of cellular

units were illustrated, which are able to provide a total of 15.6 millimeters of stroke. The cellular units

were expected to be attached onto a custom made ankle-foot orthosis (AFO). The AFO will be made from

carbon fiber tubing to minimize its weight.

Fig. 5. Hardware design of the system. The cellular units were modeled in SolidWorks based on dimensions provided by the data

sheet. Attached by linkages, a pair of solenoids was arranged in a o30 pennation angle, in a bipennate form. The packages were

expected to attach on an ankle-foot orthosis (AFO) to provide additional moment on ankle joint. Image credit of AFO: Carbon

Express LLC [9].


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III. EXPECTED OUTCOMES

A. Physical Characteristics of System

In order to find out what the cellular units can contribute to during the push-off phase, the mass and

the moment of inertia of the system should be provided. Once the construction of the system is completed,the total mass can be easily measured, and the moment of inertia can be measured from experiments. For

the following assessment for system outcome, only symbols and hypothetical data will be used rather than

actual values.

B. Assessment for System Outcome

Considering the additional moment that the system can contribute solely, the free body diagram can

be drawn as in Fig. 6(a). Since pennation angle iso30 , the maximum total force, F, which the device can

generate during push-off, can be calculated as equation (2), assuming the solenoids is operating in 10%

duty cycle:

70 cos30 6 363.73 ( )oF N (2)

The net moment can be calculated in equation (3):

1 2-M F d mg d I (3)

To evaluate the hypothetical outcome of the system, we assigned values for the following parameters:

d1= 0.1 m, d2 = 0.03m, =12 rad/s2, I =0.05 kgm2, and mg = 24.5 N, then M = 35.04 N-m. For a

person weighed 75 kg, the device can provide 0.47 Nm/kg during push-off, as shown in Fig. 6(b). The

calculation above was based on hypothetical data; however, it can be inferred that the additional torque

the device provides should have the ability to overcome its own kinetic characteristics.To truly evaluate the performance of the system; the metabolic cost of transport (COT) should be

measured. By measuring the oxygen consumption and carbon dioxide production of human breathing

during an assigned task, a measure of how physically taxing the activity for the subject can then be

acquired [10].

Fig. 6. (a) The free-body diagram of the device and (b) the hypothetical outcome assessment. Only the contribution of the device

was evaluated. The outcome was estimated based on hypothetical assumptions. Image credit of the ankle moment graph: Dr. Jim

Richards [11].


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REFERENCES

[1] A. M. Dollar, and H. Herr, Lower extremity exoskeletons and active orthoses: challenges and

state-of-the-art,IEEE Trans. Robot.vol. 24, pp.144-158, Feb. 2008.

[2] H. Kazerooni and R. Steger, The Berkeley lower extremity exoskeleton, Trans. ASME, J. Dyn.

Syst., Meas., Control, vol. 128, pp. 14-25, Mar. 2006.

[3] Y. Sankai, Leading edge of Cybernics: robot suit HAL,SICE-ICASE Int. Joint Conf., Oct. 2006.

[4] G. S. Sawicki, K. E. Gordon, and D. P. Ferris, Powered lower limb orthoses: applications in motor

adaptation and rehabilitation,IEEE Int. Conf. Rehabil. Robots ., pp. 206-211, July, 2005.

[5] H. Herr and R. Kornbluh, New horizons for orthotic and prosthetic technology: artificial muscle

for ambulation, Smart Struct. Mater. Electroactive Polym. Actuator Devices, Mar. 2004.

[6] G. K. Klute, and B. Hannaford, Accounting for elastic energy storage in McKibben artificial

muscle actuators,ASME J. Dyn. Syst. Meas. Control, vol. 122, pp. 386-388, June, 2000.

[7]

NI Single-Board RIO,National Instruments, 2013. [Online]. Available:http://www.ni.com

[8] Low-profile solenoid, Tai-Shing Electronics Corp., 2013 [Online]. Available: http://www.

tai-shing.com.tw

[9] Knee Ankle Foot Orthosis, Carbon Express LLC, 2013 [Online], Available: http://www.

mycarbonexpress.com

[10] A. M. Dollar, and H. Herr, Design of a Quasi-Passive Knee Exoskeleton to Assist Running,

IEEE/RSJ Inter. Conf. Intell. Robots Sys., Sept. 2008.

[11] J. Richards, Biomechanics in clinic and research. 1st ed. Boulevard,PA: Elsevier, 2007, ch. 5.

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