Hybrid Actuator Design for a Gait Augmentation Wearable
Fang Wan, Zheng Wang, Brooke Franchuk, Xinyao Hu, Zhenglong Sun, Chaoyang Song
HHybrid Actuator Design for a Gait Augmentation Wearable
Fang Wan , Zheng Wang , Brooke Franchuk , Xinyao Hu , Zhenglong Sun and Chaoyang Song Abstract — We describe a fluidic actuator design that replacesthe sealed chamber of a hydraulic cylinder using a soft actuatorto provide compliant linear compression with a large force( ≥
100 N) at a low operation pressure ( ≤
50 kPa) for lower-limb wearable. The external shells constrain the deformationof the soft actuator under fluidic pressurization. This enables usto use latex party balloons as a quick and cheap alternative forinitial design investigation. We found that the forces exertedby the soft material deformation are well-captured by therigid shells, removing the necessity of explicitly describing themechanics of the soft material deformation and its interactionwith the rigid structure. One can use the classical Force, Pres-sure and Area formula factored with an efficiency parameterto characterize the actuator performance. Furthermore, weproposed an engineering design of the hybrid actuator usinga customized soft actuator placed inside a single shell cavitywith an open end for compression force. Our results show thatthe proposed design can generate a very high force within ashort stroke distance. At a low input pressure of 50 kPa, theexerted block force is approaching only about 3% less thanthe classical equation predicted. The actuator is fitted to a newgait augmentation design for correcting knee alignment, whichis usually challenging for actuators made from the purely softmaterial.
I. I
NTRODUCTION
Soft robotics is an emerging area in robot design by utiliz-ing material elasticity for compliant, light-weight, and cus-tomizable actuation during human-robot interactions, makingit a superior choice of design for wearable devices [1]. Tostimulate soft material deformation, fluidic pressurization isusually adopted for actuation, leading to the majority of softrobot designs that involves a chamber wrapped by specif-ically designed soft material for programmable motions.This poses a major challenge in engineering design, whichwere mainly built on the assumption of rigid mechanics.Soft robot design need to account the soft matter for bothmotion actuation and power transmission with a changingform factor. This present a major drawback in many currentsoft robots which can only product a relatively small forceor torque, which can only be used for upper limb or handwearables [2], [3], [4], [5], [6]. Fang Wan is an independent researcher, Wuhan, Hubei, China. Zheng Wang is with the Department of Mechanical and AerospaceEngineering, Monash University, 3800 VIC, Australia. Brooke Franchuk is with the Department of Mechanical and AerospaceEngineering, University of British Columbia, Canada. Xinyao Hu is with the Institute of Human Factors and Ergonomics,Shenzhen University, Shenzhen, Guangdong, China. Zhenglong Sun is with the Institute of Robotics and IntelligentManufacturing, Chinese University of Hong Kong, Shenzhen, China. , ∗ Chaoyang Song is with the Department of Mechanical and AerospaceEngineering, Monash University, 3800 VIC, Australia. Corresponding Au-thor. [email protected]
The majority of current soft actuators are presented with arelatively long shape with a regular cross-section, generatingforce under 10 N between a relatively high operating pressurebetween 200 kPa and 600 kPa ([2], [3], [4]). Such constrainedoutput, on the other hand, limited the application of suchactuators in mainly the hand wearable as assistive gloves.There are a few soft actuator designs aim at overcomingsuch limitations. For example, the soft actuator designed in[7] presented with a high force output of 14 N at 40 kPa inputpressure, where a layer of thin, patterned, and unstretchablematerial is used to wrap around the soft cylinder chamberfor mechanically programmable motion generation. In arecent work in [8], another two-stage soft actuator designis presented which is capable of grasping 40 N payload in athree-finger configuration.The fluidic soft actuators share many similarities withtraditional fluidic actuators, such as the hydraulic cylinders.However, a significant amount of pressure is dissipated bythe deformed material, especially those made from purelysoft material. It has been established that reinforcement byinelastic material can significantly improve the engineeringperformance of the soft actuator [4], [8], [7]. There is a re-search gap in an optimized design methodology between theutilization of fluidic pressurization and the use of soft, elasticmaterial for a light-weight, safe, yet powerful fluidic actuator,which can be bridged by the involvement of material withdifferent elasticity properties.In this paper, we propose a fluidic actuator design thatreplaces the sealed chamber of a hydraulic cylinder usinga soft actuator to generate compliant, powerful and light-weight actuation for lower-limb assistance, as shown in Fig.1, the soft actuator inside the rigid shells seals the pressurizedfluids even under a high pressure. Once pressurized, thesoft actuator can interact with human body through directinteraction. The rigid shell constrains and guide the softactuator to deform in a desirable way, which significantly im-proves to use of fluidic pressure. Effectively, one can describethe overall performance of the actuator by examining theoutput from the rigid components without specific modelingof the soft matter. Our experiment results shows that theresultant actuator remains efficient in delivering the hydraulicpressuring into actuation force in a compact form factor anda relatively lower input pressure. We further implement ourproposed hybrid actuator design into a wearable device forthe lower limb knee augmentation through selective actuationof multiple actuators on two sides of the leg in a light-weightdesign. a r X i v : . [ c s . R O ] M a r ig. 1. Conceptual design of a wearable on the knees for gait augmentationusing multiple hybrid actuators made from soft bladders enclosed inside3D printed rigid cylinder shells for selective compression: (a) misalignmentof the knee; (b) conceptual design of the gait augmentation wearable; (c)proposed hybrid actuator design compress under fluidic actuation; and (d)3D printed wearable prototype. II. E
XPERIMENT D ESIGN AND A NALYSIS
We used a common latex party balloon instead of acustomized, molded actuator to investigate the workingmechanism of the proposed actuator design. As shown in Fig.2, a small latex party balloon is placed inside the chamber ofthe sliding cylinders. The lower one is mounted on the test rigand the upper moving one is attached to a loading cell, whichis also fixed on the test rig. Different cross-section shapes.The balloon is loosely placed inside. After pressurization,the balloon is inflated to fill the space inside the cylinderwith a tendency to push the sliding cylinder on the top tomove outwards. Under a limited stroke of 5 mm, the balloonis quickly inflated to fill all the cavity inside the rigid shellsat a low pressure and starts to push the sliding part againstthe loading cell, generating force.The exerted force from the interaction between the inflatedballoon and the sliding shell can be directly measured by theloading cell, which is the system output as shown in Fig. 3.One can compare the measured force as an ideal pressure-force-area model factored with a force loss parameter η .As shown in Eq. (1), P in is the input pressure, A is the Fig. 2. Conceptual hybrid actuator design validation using a balloonplaced inside 3D printed shells with different geometries but the same areas,including circular, triangle, square and rectangle shapes. interaction area, and F out is the measured force exerted bythe moving cylinder shell, which can be measured using aloading cell. The simplicity of the design enables one tomodel the actuator without dive into the non-linear mechan-ics of the soft actuator while effective characterizing theperformance of the overall actuator in a light-weight design. Fig. 3. Modeling the hybrid actuator during rigid-soft interaction, includingthe input pressure, output force, and energy loss during the nonlinearmaterial deformation interaction with the rigid shells. Note that the softactuator inside tends to fill all available volume inside the rigid shells underfluidic pressure. η = 1 − F out P in · A × (1)Four experiments are carried out to explore the characteris-tics of η with different geometries of the interaction area us-ing the same small, thin, and latex balloon, including circle,triangle, square and rectangle. All shapes of the interactiongeometry shares the same area as a radius 25 mm circle. Suchballoons can be cheaply sourced but their elasticity will bepermanently changed once inflated. Therefore, the balloon isnflated and deflated continuously for 10 times before puttinginto the rigid shells for experiment. During each experiment,the pressure is supplied at an increment of 5 kPa until 60 kPa.We observed that given our design, further pressurizationis at the risk of fraction the experiment platform with 3Dprinted plastic parts. At each pressure increment, the outputforce is measured three times. The averaged measurement isplotted in Fig. 4, where the efficiency of the force exerted, η , increases as the pressure increases. Fig. 4. The increasing efficiency of the hybrid actuators with differentcross-section geometries while pressure increases.
As shown in Fig. 4, the η started with a relatively low valuebetween 30% and 50% for different interaction geometriesat the beginning. After 30 kPa, the η starts to emerge witha linear relationship to the input pressure, reaching about77% at 60 kPa for all shapes. One possible explanation forthe behavior before 30 kPa is the pre-pressurization of theballoon to fill all available cavity of the rigid shells withdifferent interaction area. After this pre-pressurization, theoverall actuator increase with a tendency towards a hydrauliccylinder with decreasing loss of force exerted.Among the four different interaction geometries tested, itis observed that the square and rectangle ones exhibited arelatively lower force loss at a similar level. The square onewas found to be the relatively low performing one, with thecircular one performs in the middle. This provides designguidelines for the soft actuator design when replacing theballoon with custom made soft actuators, where a square orrectangle with rounded corners might be a preferred choicefor improved performance, which will be further addressedin the next section.The average force exerted by the actuator between 30kPa and 60 kPa ranges between 36 N and 90 N, which issignificantly larger than soft actuators made from purely softmaterial or the reinforced ones. The average force loss ofthe actuator between 30 and 60 kPa can be described usingEq. (2) by fitting the measured data, reaching a high r of97.8%. As shown in Fig. 5, the exerted force within a rangeof relatively lower pressure can reach up to around 90 N at 60 kPa, which is much higher than most actuators made frompurely soft material. This enables the possibility of adoptingsuch light-weight, low-cost and high performance hybridactuator design to be implemented for lower limb wearabledevices, which will be introduced in the next section. η = − . · x + 0 . (2) Fig. 5. The comparison between the ideal and average measured forceexerted by the hybrid actuator.
III. L
OWER - LIMB W EARABLE FOR G AIT A UGMENTATION
An engineering design of the proposed actuator is devel-oped by using soft actuator instead of balloons for robust,reliable and improved performance, which can be integratedfor lower-limb augmentation. Commonly gait abnormalitiesare caused by the misalignment of the femur, patella andthe tibia, others may be caused by nerve system. Thesepathological gaits ultimately need surgical operations torectify. However, some mild gait issues such as patientssuffering from patellofemoral arthritis can alleviate the painthrough applying medical wearable devices such as kneebraces and supports.Commonly any misalignment of the lower limp can berectified by exerting an external corrective moment throughimposing 3-4 forces system along the side of the leg. Toexert this force, many different styles of knee braces havebeen developed. Most braces offer similar aspects suchas mechanical support, lateral stability, proprioception andprotection for damaged ligaments. They also typically workto change the system of external forces and moments abouta joint, as well as restore the balance between the externaland internal moments [9]. Common lower-limb assistance isusually powered by electric motors [10]. Recent work in [11]shows that it is possible to pack multiple soft actuators togenerators high force output.here are a few design challenges for lower-limb wearable,including safe, compliant and powerful actuation, light-weight and flat design, ergonomics as a wearable devicewith active control. We adapt the principals of the proposedhybrid actuator design to address these design challenges byredesigning the actuator with a light-weight brace structure asshown in Fig. 6. The interaction area takes the geometry of arectangle with rounded corners, which intends maximize theefficiency of the soft-rigid interaction under fluid pressuriza-tion. The soft actuator is replaced by a customized one fabri-cated through two stage molding with an increased thicknessof 3 mm to safely hold the pressure inside. Furthermore, weremoved the rigid, sliding shell by introducing a pop-up areaon the top of the soft actuator so that the compression forceis generated directly by the soft component that is contactwith the leg. This enables us to reduce the overall thicknessof the actuator to be the same as the mounting shell at thebottom, which is to be fixed to the bracelet structure. As aresult, the modified actuator presents a compact form factorthat works under a relatively lower input pressure and a smallstroke distance to provide a compression force to the humanleg for active augmentation. The overall bracelet structuretakes an “X” shape that wraps around the leg, which canbe individually customized during the design stage. Eachbracelet comprises of two “X” structures tightened to thethigh and leg side of the knee. One actuator is placed oneach side of the thigh, knee and leg. According to differentaugmentation need, different pairs of the actuators could beselectively pressurized to augment the alignment of the kneeduring a walking cycle or at rest.We also conducted experiments to characterize the perfor-mance of the modified hybrid actuator, as shown in Fig. 7.The force output between the two different cross-sectionalgeometries are almost identical, where the exerted forcesexhibit a linear relationship to the input pressure. The lossof force decrease exponentially from 70% to about 3% at50 kPa. In fact, the force exerted from the actuator is morethan enough for the wearable to be effective, which suggesta further design iteration to reduce the size and output withina more appropriate range as a lower-limb wearable.Each brace is equipped with six actuators, with three ofthem placed on each side of the leg. One pair is placed on thetwo sides of the knee, one pair on the thigh and one pair onthe leg. To correct the knee alignment, different actuators areengaged at each phase of a gait cycle, aiming at correctingthe alignment angle during active walking exercises. Theinteracting surface is through direct contact with the softactuator, which is safe and compliant as a wearable device.Further development is currently on-going for a controlsystem of this wearable device during active walking.IV. C
ONCLUSION
In this paper, we proposed the conceptual design of afluidic soft actuator with rigid shells that is capable ofgenerator large force at a lower operating pressure. Weexperimented with a quick, easy and cheap validation methodby using latex party balloons as replacement of soft actuators
Fig. 6. Engineering design of the hybrid actuator for lower limb wearable asa gait augmentation device. (a) and (b) are two different interaction geometrywith the same effective area. (c) is an illustration of the revised interactionmechanism of the hybrid actuator to produce compression against the leg.(d) is an illustration of overall wearable design with two ’X’ structure tomount a total of six actuators on two sides of each leg.Fig. 7. Measured force output of the engineering designed hybrid actuatorwith different interaction geometry but similar output, where a high force ofmore than 100N can be generated at a low input pressure of 50 kPa with avery small loss of force (3%) comparing to the ideal fluidic cylinder model. or initial design exploration. We developed an engineeringdesign of the actuator for a lower-limb wearable to activelycorrect the knee alignment angle in a light-weight structure.The resultant actuator performs very well with more than100 N force exerted at only 50 kPa with only 3% loss offorce given the input pressure and actuator design.There are still many limitations with our proposed design.For example, the current engineering design generated toomuch force, giving us the design space to further reduce thesize of the actuator as a wearable device. We are currently inthe process of developing a control system for the wearable,and a user interface for more intuitive usage and operation.A portable version of the pneumatic power source is also tobe developed to make this device wearable for daily usage.ACKNOWLEDGMENTThis work was supported in part by the Science,Technology, and Innovation Committee of Shenzhen City[JCYJ20160422145322758].Ror initial design exploration. We developed an engineeringdesign of the actuator for a lower-limb wearable to activelycorrect the knee alignment angle in a light-weight structure.The resultant actuator performs very well with more than100 N force exerted at only 50 kPa with only 3% loss offorce given the input pressure and actuator design.There are still many limitations with our proposed design.For example, the current engineering design generated toomuch force, giving us the design space to further reduce thesize of the actuator as a wearable device. We are currently inthe process of developing a control system for the wearable,and a user interface for more intuitive usage and operation.A portable version of the pneumatic power source is also tobe developed to make this device wearable for daily usage.ACKNOWLEDGMENTThis work was supported in part by the Science,Technology, and Innovation Committee of Shenzhen City[JCYJ20160422145322758].R