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Energy 171 (2019) 785e794

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Energy

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Pressure-type generator for harvesting mechanical energy fromhuman gait

Fang Deng*, Yeyun Cai, Xinyu Fan, Peng Gui, Jie ChenKey Laboratory of Intelligent Control and Decision of Complex Systems, School of Automation, Beijing Institute of Technology, Beijing, 100081, China

a r t i c l e i n f o

Article history:Received 28 May 2018Received in revised form19 December 2018Accepted 9 January 2019Available online 19 January 2019

Keywords:Human motionMechanical energyHuman gait analysisPressure-type generator

* Corresponding author.E-mail addresses: dengfang@bit.edu.cn (F. Deng), lp

fanzyzl@163.com (X. Fan), guipengbit@126.com ((J. Chen).

https://doi.org/10.1016/j.energy.2019.01.0390360-5442/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

This paper proposes a pressure-type generator that collects human mechanical energy by stepping, aprototype is finally designed and manufactured. The proposed pressure-type generator has a volume of82.8 cm3 when subjected to pressure. The average output power can reach to 97mW when under thewalking speed of 4 km/h. As a result we can get the power density of 1.17 mW=cm3, higher than mostelectromagnetically powered devices. Unlike some other generators, this pressure-type generator usesmagnets as the recovery device, making the durability of the device better. And also, the wearable designis completely made to let the device can be comfortable used. Finally, we did a series of tests to provethat the device can have great power output performance in the low frequency environment of thehuman foot movement.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

With the development of various portable electronic devices,human life becomes more and more inseparable from electricalenergy. The emergence of portable power effectively alleviates theproblem of outdoor electricity consumption. However, such powersupplies are usually bulky, not very easy to carry, and the batterycapacity is limited. Once the power is exhausted outdoors, it cannotbe used any longer. In order to solve the problem of outdoor elec-tricity consumption better, more and more scholars have focusedtheir research on new energy power generation. They combinedwearable technology as much as possible with human body, usingthe human body's own activities to generate electricity. In this way,the devices can meet the basic electricity demand and won'tbecome a burden on people's travel [1,2]. Analyzing the normalactivity of the human body can give different characteristics ofdifferent activities in different parts. The upper limbs are moreflexible and changeable, which is not conducive to the installationof equipment. Although the area of the back and the chest is large,the activity range is small and it is suitable for installing the deviceswhich require stable body movements and large installation area,

601810418@163.com (Y. Cai),P. Gui), chenjie@bit.edu.cn

such as wearable solar cloth [3]. Compared to limbs, the movementof leg and foot are more stable, and are more suitable for installingsmall energy harvesters. Researchers predicted that for a typicalrunner moving at 16.2 km/h, a step would consume 1.72e10.32 J ofenergy, and most of the energy would be generated during heelstrike [4]. If we can use this part of energy effectively and convert itinto electrical energy, it will be able to make an energy harvesterthat is driven by human walking motion and has a considerableamount of power generation. For example, F. Invernizzi et al.introduced different types of energy harvesting devices that canuse human motion to generate power, in particular, has beendescribed from the relationship between material properties andefficiency of the equipment in the paper [5]. The paper also intro-duced the application of new materials such as electromagneticpolymers (EAPs), triboelectricity (TENG) and reverse electrowetting(REWOD) in human energy harvesting devices. Based on thisbackground, this paper designs and manufactures a mechanicalenergy collection system harvesting energy from foot, which isnamed pressure-type generator.

From the principle of energy conversion, power generationshoes generally have three conversion methods: electromagnetic,piezoelectric, and electrostatic [6]. Electromagnetic harvester isgenerally composed of coils and magnets, and the induced elec-tromotive force is generated by utilizing a change in the magneticflux, converting mechanical energy into electrical energy. The en-ergy harvester is usually bulky, but the conversion efficiency isbetter than other types. The piezoelectric harvester uses the

F. Deng et al. / Energy 171 (2019) 785e794786

principle that the piezoelectric material can generate electricalenergy by force deformation, and finally converts mechanical en-ergy into electrical energy. Such harvesters usually install in theshoe heel, using stepping motion to generate electricity. This typeof energy harvesters is small in size and relatively easy to install inshoes, but it is in low efficiency, and is easily damaged due tolimited life causing by stress. Electrostatic harvesters will causechanges in capacitance when they receive vibration, then generateelectricity. Similar to the piezoelectric harvester, the small size iseasy to install in shoes, but the power generation efficiency is verylow.

The energy conversion method of pressure-type generatordesigned and manufactured in this paper belongs to the electro-magnetic type. This method is also the most popular way used inresearch. J. Lin and his team proposed a rotating electromagneticenergy collector, consisting of a cylindrical stator and a disk-shapedrotor [7]. Because of the magnetic interaction between the rotorand the stator, there is no surface contact between them. So therotor can freely rotate around the stator when the device is shaking.At a speed of 8 km/h, the maximum voltage and average powerdensity can reach to 1.92 V and 0.2 mW=cm3 respectively. A studyproposes two miniature energy harvesters using different humangait features [8]. The first one is a multi-coil swing energy harvesterbased on the foot's swing. When tested at 6 km/h, the averageoutput power is up to 0.84mW. The second one is based on ac-celeration of the heel stepping, the total volume of the device is 48cm3, and is integrated in the soles for use. When tested at 6 km/h,the average output power can reach to 4.13mW. The author alsoproposes that the output power of the existing acceleration-drivencollectors are generally limited due to the device size. And it isdifficult to reduce the size to combine it with shoes without losingthe power. Another study proposes a flat multi-coil energy gener-ator applied to a shoe sole, which converts the mechanical energyof the foot into electrical energy by swinging [9]. The authors usedthree sets of magnets and coils to construct the collector, eachgroup consists of repulsive magnets stacked on top of each otherand equally spaced coils. The total volume of the collector proto-type is 46.6875 cm3. When tested at 4 km/h, the average power of1mW can be generated. Y. J. Hayashida proposed a power gener-ation shoe using electromagnetic generator that requires a rockerto rotate the gear [10]. The designer placed the rocker on the soleand uses the pedal to drive the gear. However, from the figureprovided by the author, users should be able to feel the existence ofthe rocker clearly. H. O. C. Houng, S. Sarah et al. proposed a rotaryelectromagnetic energy harvester. The whole device is based on ahand crank generator removed from a flashlight [4]. When tested atthe speed of 0.477 km/h, it can harvest an average power of256.91mW. Another study also used a micro-manual torch to refitand produced power-generating shoes [11]. The author installed anelectromagnetic device with a handle on the heel and was able toharvest 250mW of energy. However, the author mentioned thatthere was a significant effect on the human gait when used, and therocker was broken after several measurements. X. Zhao et al. pro-posed an electromagnetic energy harvesting device based on limb'sswing [12]. A coil winding around a cylindrical housing is a stator,and a magnet that is freely movable in the column is a mover. Thetotal volume of the device is 6.77 cm3. When tied to the ankle at afrequency of 1 HZ (z6 km/h), 4.2mW can be obtained with a po-wer density of 0.62 mW=cm3.

In the previous work of the laboratory, we have designed andpublished an electromagnetic energy harvesting device driven byswinging [13]. The volume of that device is 25.1 cm3, and 20mWofenergy can be harvested at a speed of 5 km/h. In order to increasethe power density of the device, we continue to study the tech-nology of power generation, and proposed this new pressure-type

power generator. From the point of view of the current stepping-based power generators designed and manufactured, most of thedevices require springs to help restart. After stepping on manytimes, the spring is easily deformed and the restoring force isaffected, affecting the generation efficiency of the device. Moreover,although many devices have high power output, they do not have agood wearable design, and users can feel discomfort. However, thepressure-type generator proposed in this paper uses magnets tocomplete the reset after stepping. There are no problems of mag-netic weakening after multiple steps, and the durability of theequipment is improved. A good wearable design is performed andthe relationship between the output and the volume of theequipment is weighed. The pressure-type generator and a specialhalf-size insole are installed together, and can be easily put intoordinary shoes to achieve power generation.

In this paper, a pressure-type power generator is proposed.According to the gait analysis and wearability considerations whenthe human body is walking, the optimal installation site of thepower generation system is designed. The specific structure of theproposed pressure-type power generator is introduced in detail,and it is combined with the shoes tomake a power generation shoeprototype. Finally, a variety of performance tests are taken on theprototype in a laboratory environment. Test results and analysis arepresented. The final prototype of the pressure-type generatorprepsented in this paper can eventually output an average power of97mW with a matching resistance of 17U and a traveling speed of4 km/h.

2. Analysis of installation position

Engineering has a wide range of applications in various fields[14e16]. This paper also collects and analyzes actual humanmotiondata through the engineering method. During the research on thehuman foot wearable device, many scholars have conducted in-depth analysis of the movement rules of the human foot, to findthe installation position suitable for the power generators so thatthe foot mechanical energy can be collected more efficiently [17].The analysis of the human gait has tow assessment methods, (1)semi-subjective analysis techniques and (2) objective analysistechniques [18]. Compared to objective analysis techniques, thesemi-subjective analysis techniques are more commonly used inclinical practice and are mainly evaluated with reference to expertopinions. As for objective analysis techniques, there are three mainways: the thing based on non-wearable sensors (NWS), on wear-able sensors (WS) and the method of mixing the two systems. TheNWS system requires the testers to walk on a clearly markedwalkway during the experiment. The WS system can be tested indaily activities. The NWS system can be divided into two sub-systems: (1) those based on image processing (IP), and (2) thosebased on floor sensors (FS). The IP system captures gait featuresthrough visual sensors and analyzes them on digital images toobtain parameters. The FS system is tested by “force platforms”placed on the ground, typically by pressure sensors and groundreaction force sensors (GRF). According to the existing equipmentof the laboratory and previous testing experience, this paper selectsthe FS analysismethod in the NWS system, which is tested by “forceplatforms” placed on the ground.

2.1. Analysis of human gait model

In normal walking activities, the movement characteristics ofhuman walking have a periodic pattern. As shown in Fig. 1, the leftleg and the right leg alternate, and the action of “touching theground-supporting-lifting the leg-swinging-touching the ground”is repeated regularly with a certain cycle [19]. By observing such a

Fig. 1. The regularity of human normal walking [19].

F. Deng et al. / Energy 171 (2019) 785e794 787

gait model, we can know that when the human body is walkingnormally, the foot has both vertical and horizontal displacementsand accelerations. In general, we can feel the downward pressureobviously. Take a common example in life. When the insole is oftenstepped on for a long time, we'll find the noticeable deformationsoccur in the sole of the foot and heel, suggesting that downwardpressure on the foot must not be underestimated. This pressure iscaused by the lower limb's inertial changes when the sole and theheel touch the ground. The force is about 2e3 times the weight ofthe human body [6]. The triggering time of the whole action is veryshort, and a lot of repetitions appeared. As a result, some re-searchers have considered using the pressure generated by the footto design an electrical energy harvesting device. The design pro-posed in this paper is a good collector of the pressure generated bythe foot when stepping, converting it into electrical energy.

Fig. 2. One-step force in the Z direction.

Fig. 3. Two-step force in the Z direction.

2.2. Collect and analysis of human foot movement data

In this paper, three-dimensional force-measuring platform isused to collect foot-to-ground force data during walking. The force-measuring platform used in the experiment is the TAFS-P-060three-dimensional force measuring platform designed by theInstitute of Mechanical Research, Hefei Institute of Intelligent Sci-ence, Chinese Academy of Sciences. Using four separate three-dimensional force sensors, an effective test area of600mm� 600mmwas formed. The height of the force measuringplatform is 8 cm. In order to simulate the situation that the humanbody walks on the flat ground as much as possible, a toolbox that isbasically the same as the height of the force measuring platform isplaced at two opposite corners of the measuring platform. The174 cm tall, 630 N weighted tester walked diagonally in theexperimental area to collect the force generated by the single-stepstepping and two-step stepping on the Z-axis (see Figs. 2 and 3).

In order to analyze the changes in the force of the foot on theground easily, the testers walked very slowly in the experimentalarea. As can be seen from Fig. 3, in the time period when the centerof gravity of the person shifts from the hindfoot to the forefoot, thatis, the rear foot has not yet left the ground, and the heel of the frontfoot touches the ground, until the hindfoot toes off the ground, theforce of the foot on the ground reaches a maximumvalue. It's about1.2 times that of the gravity.

After knowing that the pressure caused by foot stepping is veryconsiderable, we need to know the specific pressure situation indifferent parts and design a reasonable installation position for the

power generation system. This article uses the RealTag sensor,Bluetooth adapter and the upper computer to form a human footmovement data acquisition system. The sensors are installed in thetoe, sole and heel. The collection and analysis of human foot

F. Deng et al. / Energy 171 (2019) 785e794788

movement data were conducted in an experimental environment.Since the design of this article is mainly based on foot steppingpressure, only the acceleration generated on the Z axis is focused.

As shown in Fig. 4, the trends of the curves on the three imagesare significantly different. The main reason is that during the samewalking cycle, the time and force of the three different parts aredifferent, resulting in different acceleration performance in the Z-axis direction. The first is the difference in the starting position ofthe acceleration. The general walking habits of the human bodyare in the order of the heel, the soles of the feet and the last toes.Therefore, at the same time scale, the acceleration of the heelposition appears first, followed by the sole of the foot, and finallythe toe. When the foot touches the ground, in order to slow downthe foot motion, the acceleration directions of the three are allalong the z-axis. After the touch of the ground, the center ofgravity gradually shifts forward, and the acceleration of the threedecreases. When the feet are all on the ground, they are subjectedto downward pressure from the human body, so a short reverseacceleration occurs. At this time, the foot is actively accelerated tothe ground, and the toes begin to exert force, so the acceleration ofthe toes has a small peak. Then the center of gravity of the body istransferred to the other leg, and the measured foot enters theswing state. At this time, in order to gradually slow down the footmotion, the acceleration direction is along the z-axis. However,since the toe is usually upward when the foot is lifted, the toeneeds to gradually change the direction of the acceleration tomake it fall during the swinging process, so the acceleration at thetoe is reversed along the z-axis when the foot is swung. Since themeasured foot is the supporting part when the first accelerationpeak appears, and the body center of gravity changes to anotherleg when the second acceleration peak appears, the peak of theprevious acceleration is greater than the peak of the second ac-celeration. In general, you can see that the foot can produceconsiderable acceleration in the Z-axis when walking. The placewhere the acceleration changes most is the toe, followed by theheel and finally the sole of the foot.

Fig. 4. RMS acceleration data of three positions

2.3. Wearable design analysis

This article needs to design a pressure-type power generator,which is a wearable energy device. So it needs a wearable designanalysis. Some scholars have also raised issues that need to beaddressed when designing wearable devices [20,21]. Combinedwith the specific conditions of this design, the following four mainfeatures need to be considered:

2.3.1. ComfortSince the pressure-type power generator is installed in shoes,

and the shoes are the basic clothes that the human needs to wearon a daily walk, if the installed power generator affects the comfortof normal human walking, the power generator based on humanwalking will lose its significance. Generally, the tip of the shoe isthin and the heel is thick. In comparison, installing the powergeneration system on the heel has the least impact on the wearingcomfort of the shoe.

2.3.2. StabilityAlthough compared with other limb movements, the pattern of

foot movement is relatively simple, there is still a lot of random-ness. So there is a great demand for the stability of the entire powergeneration system. The power generation system needs to dealwith the pressure from the human foot, the tide of the footwear,sand or dust and other conditions caused by environmental factors.

2.3.3. DetachabilityDetachability mainly considers that the body needs to remove

the power generation system for cleaning shoes and other activ-ities. The detachable design also brings great conveniencewhen theequipment fails and needs to be replaced.

2.3.4. DevelopabilityThe pressure-type power generator needs to be designed not

immutable, but can be continuously optimized according to the

of foot while walking at a speed of 3 km/h.

F. Deng et al. / Energy 171 (2019) 785e794 789

specific needs of users. Honestly, there are still many deficiencies inthe prototype of the first generation. It is necessary to bettercomplete the design of the power generation system by collecting alarge amount of experimental data and user experience.

Considering the analysis of the collected motion data and theanalysis of the wearable design, it was finally decided that thepressure-type power generator should be installed on the heel,which can not only generate large longitudinal acceleration whenthe foot moves, but also collect as much mechanical energy aspossible. It also meets the requirements of human wearable designand does not affect the normal movement of the human foot.

Fig. 6. Pressure-type generator parts.

Fig. 7. Pressure-type generator prototype.

2.4. Design and manufacture of pressure-type generator

This design focuses the energy collection on the amount ofmovement of the human foot in the vertical direction. The devicecan be driven by stepping. From the analysis results in the previoussection, the most suitable installation site for the power generatoris the heel of the shoes.

The pressure generator is mainly composed of a base, a pressuredevice, a transmission device and a small brushless DC motor. Thepressure device is composed of an auxiliary pressure magnet, apressure magnet and a pressure block. The auxiliary pressuremagnet and the pressure magnet are attracted by the attractingforce of the two magnets in the groove on the lower surface of thepressure block. A pressure-supporting magnet is embedded in thebase of the corresponding pressure-applying magnet. Thepressure-receiving magnet and the pressure-applying magnet arein the same polarity to each other, and the pressure-returningmagnet is reset by using the principle of like electric chargesrepel. The pressure block is in clearance fit with the pressure-receiving device housing, and is limited to the upper part of thecavity of the pressure-receiving device housing in the direction ofthe repulsive force. The pressure-receiving device housing islimited to the groove of the base so as to be fitted and fixed. Fig. 5isa parts drawing of various parts of the pressure-type generator (seeFigs. 6 and 7).

The magnetic block in the figure is a permanent magnet. Exceptfor themagnetic block and the generator, other parts are all made ofa 3D printer. The transmission is mainly composed of a trans-mission rod and a gearbox, and is connected with a pressure deviceand a small brushless DC generator. The transmission rod has a Z-shaped structure and includes two horizontal rods and one verticalrod. The upper horizontal rod passes through the hole in thepressure block, and the rotation shaft of the transmission box isnested in the horizontal rod below. The angle between the plane ofthe transmission rod and the horizontal plane is less than 90�,preventing the transmission rod from getting stuck. The horizontalbar below the transmission rod rotates as the rotation center, andthe up and down motions of the pressure block are converted intothe rotation of the transmission. Transmission gear shaft rotates at

Fig. 5. The model of pressure-type generator.

a low speed and changes speed, driving the output gear to rotate athigh speed. The gear meshes with the gear on the input shaft of thesmall generator, and the mover of the small generator produceselectrical energy at high speed and outputs it through thegenerator.

When walking around, the foot presses the ground and thepressure block. The pressure block moves downwards underpressure until the pressure is balanced. The transmission rodinterposed in the pressure block also follows to move downwards,thereby driving the rotation shaft to rotate. Through the accelera-tion of the gearbox, the rotating shaft of the small generator rotatesat high speed to generate electricity. When the foot lifts off theground, the pressure exerted on the pressure block disappears, andthe pressure block moves upward under the repulsive force causedby pressure magnet and bearing magnet, thus driving the trans-mission rod upward. The axis of rotation thus rotates in theopposite direction, the gearbox accelerates again, and the smallgenerator reverses the rotation to achieve power generation. Eachstepping cycle can achieve two power generation processes. So thatthe cycle is repeated and the device achieves power generationwhile walking. The alternating current generated by the motor is

F. Deng et al. / Energy 171 (2019) 785e794790

input to the energy processing module. After being rectified andstabilized, it is stored in the super capacitor of the energy pro-cessing module, and can also output electrical energy through theUSB interface.

Because this paper chooses to install the pressure-type gener-ator on the heel, the selected shoes need a thick heel and iswaterproof, strong and practical. In order to facilitate the experi-mental measurement, this article selects ordinary men's sportsshoes with a little high-heel. Using 3D printing technology, thepressure-type generator is manufactured, installed in a customizedhalf-size insole, and finally installed in the heel of the shoe.

The shaped pressure-type generator is a regular rectangle with abase of 4.5 cm � 8 cm. The height is 3.5 cm when it is notdepressed, while the height is 2.3 cm when it's stepped on. Such adesigned can convert the stepping pressure with short displace-ment and large amplitude into the high-speed rotational motion ofthe generator shaft, through the pressure-receiving device, thetransmission device and the gearbox. Thus, the mechanical energyis converted into electrical energy to achieve the purpose of col-lecting pedaling pressure from the human foot.

Fig. 9. Operating voltage changes over time.

2.5. Experimental results and analysis

In the laboratory environment, the testers put on the prototypeof the pressure-type generator, took a series of performance testson the treadmill. We finally obtained experimental results and did acorresponding analysis in the paper (see Fig. 8).

2.6. Stability test

After the prototype was created, we performed a series of sta-bility tests on the equipment, including work stability and struc-tural stability tests. The evaluation of the work stability is mainly toobserve the effect of the equipment under long-term workingconditions; the structural stability is evaluated mainly by observingwhether the increase in the use time and the number of uses willaffect the use of the equipment. First, for the stability of the work,we tested the equipment for half an hour of continuous operation,2 h of intermittent work, and 7 h of wearing work. By sorting outthe collected data shown in Fig. 9, the power generation of theequipment is basically the same when the walking speed is similarin three different situations, and the power generation is not

Fig. 8. Pressure-type generator installed in shoe.

reduced due to the change of working time.Secondly, for the structural stability, we have carried out more

than 1000 pressing experiments on the equipment, the comparisonexperiment between the old and new equipment and the experi-ment with the fit of the shoes. The wearable walking test is per-formed on the device before the start of the pressing test, and thenthe pressing test is performed. The pressing experiment wascompleted in 3 days, and the machine was pressed 400 times indifferent time periods every day, and thewearable walking test wasperformed on the device again 3 days later. Through results shownin Fig.10, it was found thatmultiple presses had no effect on the useof the device. It should be pointed out here that since the repulsiveforce between the magnets is used to achieve the reset, themagnetism of the magnet does not decrease with the increase ofthe use time and the number of uses. The stability of this structureis well guaranteed compared to a reset device that generally utilizesa spring structure. The second is a comparison experiment betweenthe old and new equipment. The prototype used in the laboratoryfor more than 9 months and the recently rebuilt prototype weretested at 6 km/h trotting. From Fig. 10, it is found that the powergeneration of new and old equipment is basically the same.

The third one is the experiment of fitting the fit with the shoes.This experiment is mainly reflected by the user's comfort test onthe power generation shoe prototype. Ten testers were put onpressure-type generator prototypes, and they were evaluated forthe prototypes in terms of appearance, wearing feeling, walkingfeeling and recognition level. As shown in Table I, we can see theevaluation results from 10 testers. Most of the testers were able toaccept wearing the pressure-type generator, and they thought theprototypes performed well in wearing comfort and walking feel-ings, and did not cause discomfort to the testers. However, we canalso see that there are still deficiencies in the shape and thickness ofthe heel, and need to continue to improve in the subsequentstudies.

This series of experiments shows that the pressure-type powershoes we designed can guarantee stable working conditions for along working time, and its working performance will not decreasewith time and usage. Most users are feeling good when using them.

Table 110 testers’ evaluation of pressure-type generator prototypes.

Tester number Appearance evaluation Wearing feeling Walking feeling Recognition level

1 Ordinary Normal Feel foreign body C2 Ordinary Normal Normal A3 Too thick Normal Normal A4 Ordinary High heel Feel foreign body B5 Too thick Normal Normal A6 Ordinary Uncomfortable Feel foreign body C7 Ordinary Normal Feel foreign body B8 Ordinary Normal Normal A9 Too thick High heel Normal B10 Ordinary Normal Normal A

Recognition level: A. Approved B. Highly accepted C. Need improvement D. Bad E. Terrible.

Fig. 10. Voltage output under different conditions.

F. Deng et al. / Energy 171 (2019) 785e794 791

2.7. Characteristics test

The tester wore pressure-type generator prototypes to walk onthe treadmill at a speed of 2 km/h. We changed the load resistanceand measured the output power of the system. As can be seen fromFig.11, when the load resistance is less than 18U, the power reachesa maximumvalue at a load resistance value of 17U. However, whenthe load resistance is greater than 18U, the output power does notdecrease. This is because the load current decreases as the loadresistance increases, and the input torque required by the generatoralso decreases, which is beneficial to the rapid pressure device.Recovery will increase the stroke of the pressure device, so theoutput power will increase.

Afterwards, the tester was put on a pressure-type powergenerator prototype and walked on a treadmill at different speeds.The unloaded RMS voltage and load (associated load) RMS voltageof the prototypewere tested at different speeds, shown in Fig.12. Ascan be seen from the figure, the RMS voltage and speed are posi-tively correlatedwhen the speed is less than 4 km/h. This is becausethe transmission rod can be completely depressed when the

Fig. 11. Load-power characteristic curve of the pressure-type generator.

Fig. 12. RMS voltage-speed characteristic curve of the pressure-type generator.

Fig. 13. Speed-power characteristic curve of the pressure-type generator.

Table 2Comparison results of some other electromagnetic type energy harvesters.

Reference Volume (cm3) Speed (km/h)

[4] e 0.48[4] e 0.48[7] e 8[8] 21 6[8] 48 6[9] 46.6875 4[11] e e

[12] 6.77 6[13] 25.1 5[22] 7.7 6[23] e 6[24,25] 2 4.8Our harvester 82.8 4

F. Deng et al. / Energy 171 (2019) 785e794792

walking speed is less than 4 km/h, so as the walking speed in-creases, that is, the moving frequency increases, the time requiredto change the same magnetic flux becomes smaller, that is, theentire magnetic flux change rate DF

Dt becomes larger, so the induced

electromotive force E ¼ nDFDt also becomes larger. Where E is the

induced electromotive force, n is the number of turns of the coil,and DF

Dt is the rate of change of the magnetic flux. When the speed ismore than 4 km/h and less than 5 km/h, the RMS voltage willdecrease. This is because in this speed range, the human body is in abrisk state, the distance from which the foot lifts off the ground isshort, and the frequency of stepping on the pressure-relief device istoo high. When the foot is lifted off the ground, the pressure-receiving device will have no time to return to its original posi-tion before be stepped on again, reducing the energy conversionefficiency. When the speed is greater than 5 km/h, the human bodybasically enters a trot. At this time, although the pedaling frequencybecomes higher, the height of the foot lifted from the ground is alsoincreased, and the pedaling pressure when the foot is set is largerthan that when walking. Therefore, the RMS voltage re-increase.The RMS voltage change rule is more obvious under no loadconditions.

Finally, measure the power output of the system at differentspeeds under matched resistance. The results are shown in Fig. 13.From the curve in the figure, we can clearly see that the outputpower of the system rises as the walking speed gradually increases.Therefore, the pressure-type generator has a good performance inthe low-frequency motion of the human walking (1e10 HZ).

Through multiple experiments conducted in the laboratory, theprototype of the pressure-type generator can eventually output anaverage power of 97mW with a matching resistance of 17U and atraveling speed of 4 km/h.

Some other electromagnetic type energy harvesters arecompared with ours, the results are shown in Table 2. Through thecomparison of the power density on Table 2, we can intuitively seethat the pressure-type power shoes designed in this paper havegreat advantages. Such amechanical energy harvestingmethod canalso be used in energy sensor systems in the future [26].

3. Conclusion

This paper proposes a pressure-type power generator that re-lies on stepping to collect the mechanical energy from the humanfoot. Using the self-designedmechanical structure, the 3D printingtechnology, a small brushless DC motor and a neodymium-iron-boron material magnet, the pressure-type power generator pro-totype is designed and produced. The pressure-receiving device inthis type of pressure-type generator uses the repulsive force

Power (mW) Power density (mW=cm3)

256.91 e

0.4867 e

e 0.20.84 0.044.13 0.0861 0.0214250 e

4.2 0.6220 0.822.46 0.3190.66 e

0.117 0.058597 1.17

F. Deng et al. / Energy 171 (2019) 785e794 793

caused by the same pole of the magnet. Unlike a device that relieson a spring to reset, the device can be reset without mechanicalwear. In the shoes, the magnetic properties of the magnet will notbe weakened, so the restoring force will not decrease with the useof time. In the laboratory environment, the testers wear thepressure-type generator prototype to perform a series of charac-teristic tests to provide effective experimental data for the futureimprovement. The proposed pressure-type power generator pro-totype has a volume of 82.8 cm3 when it is pressed. The averageoutput power can reach to 97mW when under the walking speedof 4 km/h and the matching resistance of 17U, which means it canperform well in the low-frequency environment of humanwalking. Subsequent research will further optimize the mechan-ical design and load capacity of the pressure-type generator,reduce its discomfort while installed in the shoes, and make itmore effective in practical use.

Acknowledgements

This work was supported by the Beijing NOVA Programxx2016B027 and Projects of Major International (Regional) JointResearch Program NSFC(Grant no.61720106011), NSFC(Grantno.61621063), Beijing Advanced Innovation Center for IntelligentRobots and Systems(Beijing Institute of Technology).

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Fang Deng received his B.E. degree and Ph.D. degree incontrol science and engineering from Beijing Institute ofTechnology, Beijing, China, in 2004 and 2009, respectively.He is a professor of the School of Automation, BeijingInstitute of Technology. His current research interestsinclude nonlinear estimation, fault diagnosis, control ofrenewable energy resources and wireless sensornetworks.

Yeyun Cai received her B.E. degree in control science andengineering from Minzu University of China, Beijing,China, in 2017. She is currently a Ph.D. student of theSchool of Automation, Beijing Institute of Technology. Hercurrent research interests includes energy and control ofrenewable energy resources.

Xinyu Fan received his B.E. degree and M.S. degree incontrol science and engineering from Beijing Institute ofTechnology, Beijing, China, in 2011 and 2013, respectively.He is currently a Ph.D. student of the School of Automa-tion, Beijing Institute of Technology. His current researchinterests include photovoltaic systems, wind energy sys-tems, and power electronics.

F. Deng et al. / Energy 171 (2019) 785e794794

Peng Gui received his B.E. degree and M.S. degree incontrol science and engineering from Beijing Institute ofTechnology, Beijing, China, in 2014 and 2017, respectively.His current research interests include sound sourcelocalization and pattern recognition.

Jie Chen received the B.E. degree, the M.S. and Ph.D. de-grees from Beijing Institute of Technology, Beijing, in 1986,1996, and 2001, respectively. He is a member of ChinaEngineering Academy and a Professor of the School ofAutomation, Beijing Institute of Technology, Beijing, China.His current research interests include complex systems,multi-agent systems, multi-objective optimization anddecision, constrained nonlinear control, and optimizationmethods.