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Design and Analysis of a Mechanical Device to Harvest Energy from Human Footstep Motion

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CHINESE JOURNAL 0F MECHANICAL ENGINEERING ・738・ Vo1.24,No.5,2011 DOI:10.3901/CJME.2011.05.738,available online atwww.cjmenet.com;www.cjmenet.com.cn Design and Analysis of a Mechanical Device to Harvest Energy from Human Footstep Motion XIE Longhan ’ 、 and DU Ruxu 1 School ofMechanical&Automotive Engineering,South China University D厂Technology,Guangzhou 510640,China 2InstituteofPrecisionEngineering,theChinese University ofHongKong,HongKong,China ReceivedDecember 23,2010;revisedMay10,2011;acceptedMay13,2011;published electronicallyMay18,2011 Abstract:Portable electronics is usually powered by battery.which is not sustainable not only to the long1[ime outdoor use but also to our living environment.There is rich kinetic energy in footstep motion during walking,so it is ideal to harvest the kinetic energy from human footstep motion as power source for portable electronic devices.In this paper,a novel mechanism based on dual—oscillating mode is designed to harvest the kinetic energy from footstep motion.The harvester contains two oscillating sub—mechanisms:one is spring.mass oscillator to absorb the vibration from external excitation.i.e. the footstep motion。and the other is cantilever beam with tip mass for amplifying the vibration.Theoretic analysis shows that the dual—oscillating mechanism can be more effectively harness the foot step motion.The energy conversion sub—mechanism is based on the electromagnetic induction.where the wire coils fixed at the tip end of the cantilever beam serves as the slider and permanent magnets and yoke forln the changing magnetic field.Simulation shows that the harvester.with total mass 70 g.can produce about 1 00 mW of electricity at the walking speed of 2 steps per second. Key words:energy harvesting,dual—oscillating mechanism,human motion approximately equivalent to 384 MJ Thus,if even a very 1 lntroduction In recent years,humans have become increasingly smal1 fraction of this stored energy could be extracted.a portable device would have a large and renewable resource to draw on.Some researchers have explored to extract dependent on elec仃onic devices.such as mobile phones PDAs.and etc.Especially for soldiers in remote area,they need to use electronic devices for communication or other energy from body heat ,breathing[5]j typing 们,arm motion[7】-and walking .Walking is a main energy consumption activity which also has meehanieal power to be exploited.It has been calculated that up to 67 W 0f necessary functions.such as the US Army Land Warrior system providing radio,navigation,computer for soldiers… Nowadays,most of these mobile electronic devices are powered by batteries.Although substantial progresses have power are available from heel strike during normal walking for a 68 kg person with the walking frequency at 2 steps per second There are mainly two methods to harvest the heel strike energy during human walking.One is to use piezoelectric effect to convert the pressure generated when the foot strikes the ground to electricity,but the energy been made in reducing the power requirements of the electronic devices and increasing the power densities of batteries,the limited energY storage of battery and its considerable weight hinder the extensive use of electronic devices.For example.the above.mentioned Land肋rrior system needs 5 of battery to reach the design goal, density of piezoelectric harvester is very low(on the scale of milliwatts) J.Another is to us electromagnetic induction to convert body motion into electricity.CHEN【“ and LAKIC【 reported electromechanical generators to which is inconvenient for soldier application.Furthermore。 discarded baRery generates billions of wastes every vear- resulting in negative environment impacts.Therefore,it is necessary to find alternative methods to solve the energy problem for portable electronic devices. The human body is a tremendous resource of chemical energV.some of which is converted into kinetic energv to support human activities.The store chemical energy of an convert the footstep motion.However these designs are very complex with many parts,and can only harvest the press.down motion,which make these device is fragile, expensive and lower eficifency. This PaDer is focused on how to harvest the kinetic energy from human footstep motion based on dua1.oscillating mechanism and electromagnetic induction. average person of 68 kg with 15%bodv fat is In order to avoid the fragile contact between the transmission components,the design presented in this PaOer adopts a dual-oscillating mechanism to harvest footstep motion.Because the harvester only absorbs the Corresponding author.E-mail:xielonghan@gmail.corn This paper is supported by Fundamental Research Funds for the Central Universities of China(Grant No 2011ZM0061),and National NaRn'al Science Foundation ofChina(GrantNo 51105146) CHINESE JOURNAL OF MECHANICAL ENGINEERING acceleration from footstep motion and needn’t transmit motion directly from hee1 strike.it is stronger and more reliable than other designs with similar functions.And what’s more.the design is based on the mass—spring ・739・ oscillating weight,and the weight at its tip end,make up of the second tier of oscillating mechanism to amplify the vibration from the oscillating weight.The wire coils are wound at the tip end of the cantilever beam as the proof mass for the second tier of the oscillating mechanism.With the magnetic field produced by permanent magnets,the oscillating mechanism,so it can harness not only the heel strike motion,but also the acceleration during foot swing, which can contribute to higher energY eficiency.The restf vibration from the two tiers of oscillating mechanism of this paper is organized as follows.Section 2 illustrates the physical model of the harvester.Section 3 conducts the Yoke mad ofelectric steeI makes the wire coils generate induction voltage. system analysis.including electromagnetic and kinematical analysis.Section 4 shows the system performance during xed norma1 walking.Section 5 contains the conclusions. 2 Physical Model of the Harvester The bottom rear part of a shoe.i…e the hee1 is designed to suppo ̄the heel of the foot,which is usually higher than the shoe sole.Therefore it is ideal to employ the room of the shoe bee1 to set up a harvester to harness the footstep motion.Fig.1 shows the appearance of the harvester, which is a box container where the harvester is fixed.The container is made of non—magnetic mental to endure human body weight and the impulsive force from heel strike. There are two helical springs on the harvester:one is a ̄ached to the bottom base of the container,and the other is attached to the upper cover of the container.And the container cover is fined via screws to the container body. Fig.1.Overview of the dual—oscillating harvester The harvester is symmetric and there are independent magnetic fields in the harvester.As shown in Fig.2.there are two sets of components to produce magnetic field arranged at the two ends of container body.Each set is composed of two pairs of NdFeB permanent magnets arranged in opposite magnetization direction and yoke made of electric steel,Permanent magnets are a ̄ached to yoke,which forms a magnetic circuit with the counterpart. The oscillating mechanism is used to produce vibration from external excitation.as shown in Fig.3.There are a proof mass,called oscillating weight,and two helical springs attached to the oscillating weight which forills the ifrst tier of oscillating mechanism to absorb the external excitation.Two identical cantilever beams a ̄ached to the C Fig.2. Structure for producing magnetic field Fig.3. Structure of the oscillating mechanism Abstracted from the physical model,Fig.4 shows the schematic diagram of the harvester,which can be divided into three functional pams:vibration absorber,vibration amplifier and harvesting mechanism. B I II II1 vibration Vibration Hanesting absorber ampliifer mechanism Fig.4. Schematic diagram of the dual—oscillating harvester ’740‘XIE Longhan,et al:Design and Analysis of a Mechanical Device to Harvest Energy from Human Footstep Motion The vibration absorbe r_consisting of an oscillating types of material areas:permanent magnets magnetized weight and two helical springs attached to the oscillating from left to right(in red),permanent magnets magnetized weight and the container base,transmits vibration from from right to left(in deep blue),the electric steels(in pink) external excitation to the oscillating weight.The vibration and the air area(in light blue). amplifier consists of a cantilever beam and a tip mass composed of the wire coils and their iron core.which is ; 1 st magnetic pair similar to spring.mass system,to amplify the vibration ;; 2nd magnetic pair rfom the oscillating weight.The harvesting mechanism mainly consists of permanent magnet pairs and coils.The 嚣Yoke 幢I A magnets are arranged to produce changing magnetic field, from which the moving coils can produce induced voltage due to the electromagnetic induction effect. 3 System Analysis The energy conversion,from kinetic energy to electricity, is based on electromagnetic induction.The schematic diagram of magnetic structure and magnetic flux path are Fig.6.FEA model and its mesh in Ansys shown in Fig.5.There are two pairs of permanent magnets separated by plastic spacers,arranged with diferent The density of 2D flux lines,shown in Fig.7,also magnetization direction to produce diiferent flux directions. indicates that the flux density is concentrated around the A11 permanent magnets are attached to yoke made of magnetic circuit.It also shows the flux distribution of the electric stee1.Yoke and permanent magnets constiutte the magnetic field,from which the flux density at the center of magnetic flux path,shown as dash lines in Fig.5.In this pole pitch is higher than that at the fringes. harvester,the magnets,yokes and spacers are all symmetric, so the coordinate is created at the symmetric center where Z direction is perpendicular to the ground and X direction is the magnetization direction of the permanents.When the harvester is excited by extema1 motion.the coils will vibrate on the YOZ plane where the magnetic flux is at its maximum. Z Hm N S N S | Hs D |j Fig.7.2D Flux distribution ofthe magnetic field s N S For a single coil lying on the YOZ plane at the fposition x=x0,the densiyt of magnetic flux through the  lWy Ij  Wm f  icoil is changing along Z direction,so the total flux through 口Pemanent magnet 一M g “。 g“ the coil should be calculated by integral by the following 口 Plastic spacer Hs——5paccrheight equation,where B(xo,z)is the flux density at point "goke wv—Yokewidth (Xo,:)and there is the assumption of equal distribution of Wm—Magnet width 腿一Gapwidth lfux density along Y direction: Fig.5.Schematic diagram ofmagnetic structure andthe fluxpaths (1) FEA models were built in Ansys to investigate the magnetic field.The structures related to magnetic field are Where D is the thickness of the permanent magnet,and simpliifed as Fig.6,where the areas with different material H is the height of the coil.Based on the Faraday’s Law, properties are displayed in different color.There are four the induction voltage of a single coil is CHINESE J0URNAL OF MECHANICAL ENGINEERING ・741・ = =警:警詈|.df  dZ出 ddZ /l Jr.  ze x,t7c I . z^)I.’ 。.’出 uz :。’。 attached to the oscillating weight m l,which can be combined together and has a combined spring constant k ̄, ・Z z一 calculated by kl=[kl1+kl2],where 1 and毛2 are the (2) spring constant of the upper and lower spring respectively. For simplification,the height of winding coils can be equivalent to the height of the middle layer Hc ,and The vibration absorber is also subject to mechanical damping force cm1. Fig.8 shows the coupled schematic model of the assumed that there are N coils,so the total induction voltage at one end of cantilever beam is :Ⅳ竺:Nd ̄dz.: Ⅳ B(xo,"2) ̄D ̄d2卜 ㈣ The electrical damping force converts the kinetic energy to electricity,so the power done by the electrical damping force should be equal to the one done by electirciyt,that is ・ = R +Re+RL Where Rc,Re,and RL are the resistance of the coils,the resistance in the control circuit,and the load resistance respectively.Inserting Eq.(3)into Eq.(4),then the electrical damping force is Ⅳ・。・ d f r z+ ̄  )・ F= 争,0 z 手 手 Define the electrical damping coefifcient as |v.D. dZ(『rg z H m ( , ) . 2= 十 十RL There are two coupled sub.oscillating mechanism in this harvesting system:the vibration absorber and the vibration ampliifer.The vibration amplifier is a rectangular cantilever beam with a tip mass m,which made up of a non magnetic material core and the wire coils wound around the core.For analysis convenient,the cantilever beam can be equivalent to a spring with the spring constant =Ebh /(4L3),where b,h and L are the width, thickness and the length of the beam,respectively;and E is the Young’s module of the beam.Since the mass of the beam is much less than the tip mass,its mass is neglected for analysis convenience.The vibration ampliifer is subject to both the mechanical damping force ,and the electircal damping force which represents the energy converted into electricity. For the vibration absorber,there are two helical springs oscillating mechanism,where base excitation is q1(f),and the displacements of the oscillating weight of the vibration absorber and tip mass of the vibration amplifier are q2(f)and q3(f),respectively,then the kinetic energy, potential energy and Rayleigh potential can be obtained as following.With the Lagrangian method and let z=q3一ql, Y=q2一g1 and x:q1,the governing equation of the harvester can be expressed as O 十2 2Cm, + + 0 2m 一2Cm, 2(Cm,+ ) kl+2k2—2k2 1X—m1g ——2k2 2k2 一2m2 一2m2g Fig.8.Schematic model of the oscillating mechanism 4 System Performance This dual—oscillating harvester is used to harvest the vertical acceleration of human footstep motion not only during the foot swinging process but also when the foot hee1 stirke the ground.Therefore there are two major components of the footstep acceleration:one is from the f0ot stride along with the body trunk.which is equal to the vertical acceleration of the body center;the other is from heel strike against the ground during walking,which can be expressed as a step function.Since the body center can be modeled as an inverted pendulum,therefore the vertical acceleration of body center can be expressed as a sine wave function as =an sin(cot),where ao can be obtained from experiment and is related to body weight,foot stride and rfequency.Then the foot acceleration on the vertical direction can be expressed as by combining a sine function with a step function.Fig.9 shows the sine acceleration from foot swing,step acceleration from foot strike and their ’742。XIE Longhan,et al:Design and Analysis of a Mechanical Device to Harvest Energy from Human Footstep Motion combination speed up the vibration from oscillating weight,which Call contribute to higher power output. 墨 墨 O Timet/s 墨 墨 Timet/s (b)Step acceleration from foot strike 墨 2O 鼋 l5 1O i 5 0 呈 5 0 O Timet/s (c)Combination of sine and step acceleration Fig.9.Acceleration (【 .g一performance dur\ 五 0一 ing foot swinging First of all,the system kinematical performance is studied at the condition of open circuit,that is,there is no power output.The system parameters for obtaining the kinematical performance and simulation are shown in Table. Table.System parameters for simulation Parameter Value Rigidity modulus ofthe helical spring G/GPa Wire diameter ofthe helical spring 1 d/mm Mean diameter ofthe spring D/mm 8 Number of active coils of the spring n Spring constant k.一Gd4/8nD3 Young’s module ofcantilever beam 71 E/GPa Width of the cantilever beam b/mm 1.5 Height ofthe cantilever beam h/mm 0.5 Length ofthe cantilever beam L/mm 15 Equivalent constant ofthe cantilever :Ebh3/4L3 beam如 Mass of oscillating weight m,/g 1O Mass oftip mass at cantilever beam 20 mz/g Damping coefifcient of the oscillating weight c . Cm =2 Damping coefifcient of the cantilever ( =beam Cm, ,2 √hm2 Fig.1 0 shows the relative velocity of oscillating weight and coils in terms of time(upper figure)with the external input of sine function(1ower figure)from the footstep acceleration during normal walking(2 steps per second). From the figure,the vibration amplifier can obviously Timet/s Fig.1 0.Velocity response from footstep acceleration With the relative velocity of the coils and the electrical damping coefifcient ,the rtansient power output can be calculated by P=c_・三 .Fig.11 shows the仃ansient power output of the harvester from the footstep acceleration.Since the acceleration from hee1 strike is bigger than that from the foot swing,the power output performance also shows that there is higher power output at the time of heel strike. ≥ 200 拿 2 l 100 O O Timet/s Fig.1 I.Transient power output Since the power output is fluctuant and the charge for electronics usually requires steady DC power supplier,a direct AC/DC converter developed bv DⅥ,ARI【”j is used to rectify and boost the voltage.Neglecting the energy loss in the power rectiifer,the average power output can be calculated by the integral of the electrical damping force and the relative velocity.The average power outr)ut from Fig.11 is about 1 00 mW at the walking speed of 2 steps per second. 5 Conclusions (1)A novel mechanism for energy harvesting from human footstep motion is presented.It adopts the dual—oscillating mode to harness acceleration from footstep, CHINESE J0URNAL OF MECHANICAL ENGINEERING including the mass・・spring oscillating sub・・mechanism absorb externa1 excitation and the cantilever beam with tip ・743・ H.MENET C G,CHING H,et a1.The automatic winding 【7] XIE L device of a mechanical watch movement and its application in mass to amplify the vibration,and the electromagnetic induction to convert kinetic energy to electricity. energy harvesting[J].Transaction ofAsME.Journal ofMechanical Design,2009,131(7):071005.  a1.Generating electricity [8] RoME L.FLYNN L GOLDMAN E M etf21 Electromagnetic and kinematical analyses are conducted to study the harvester’s performance.Analysis shows that the dua1.oscillating mechanism can more effectively amplify the vibration which con ̄ibutes to higher power output. while walking with loads[J].Science,2005,309:l 725-1 728.  M.Biomechanical energy harvesting:generating [91 DONELAN Jelectricity during walking with minimal user effort[J].Science,2008 319:807-810. N,PARADISO J.Energy scavenging with shoe—mounted 【10】 SHENCK piezoelectrics[J].IEEEMicro,200l,2l(3):3o_42. (31 The dua1.oscillating mechanism does not transmit motion and not directly contact with the input component, so it is stronger and more reliable than other designs of CHEN S H. Compiler." US, ,495,682[P/OL]. 1996—05—03 r2Ol1。05—081.http://www,freepatentsonline.com/5495682.htm1.  Compiler: US, 4,845.338[P/OL]. 1989—07.04 LAKIC N.similar ifmction. f4、The harvester is designed to insert into shoe heel as power supplier for portable electronics,especially for the use in wild area.This energy harvesting principle can also be applied to extract kinetic energY from other kind of movement. References 『l1 ANONYMOUs:Land Warrior.United States:Wikipedia,2011 『201 1.5.8].http://en.wikipedia.org/wild/Land Warrior. [2】STARNER L PARADISO J.Low-power electronics design[M]. New York:CRC Press,2004. 『31 FLEURIAL J P’OLSoN,N B0RSHCHEVSKY A,et a1. Compiler. "US, 6,288,321[P/OL]. 2001-09—11 [2011-05—08]. hUp://www,patents.com/us一628832 1.htm1. [4]KANESAKA T Development of a thermal energy watch[J]. Micromechatronies,l999,43(3):29-36. 『51 HAUSLER E, STEIN L, HARBAUER G. Implantable physiological power supply with PVDF film[J].Ferroelectrics, 1984.60:277—282. [6]CRISAN A. Compiler: US, 5,911,529[P/OL]. 1999—06—15 『2Ol1—05—08].http://www.patents.corn/us.59ll529.htm1. [12] r2Ol1.05一o8].http://www.freepatentsonline.com/4845338.htm1. [13】 DWARI S.An efficient AC—DC step—uD converter for low—voltage energy harvesting[J].IEEE Transaction on Power Electronics,2010, 25f81:2 188—2 199. Biographical notes XIE Longhan is currently an assistant professor at School of Mechanical&Automotive Engineering,South China University of Technology,China.He received his PhD degree from the Chinese University of Hong Kong,China.His main research interests include precision engineering,mechanical design and energy harvesting. E—mail:xielonghan@gmail.tom DU Ruxu is currently a professor at Department of Mechanical and Automation Engineering and the director of Institute of Precision Engineering,the Chinese Univers Hong Kong, China.He received his PhD degree from the University of Michigan,USA.His main research interests include precision engineering,metal stamping and cutting,and contro1. E—mail:rdu@mae.cuhk.edu,hk 

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