Hip-mounted electromagnetic generator to harvest energy from human motion

Abstract

A type of electromagnetic hip-mounted generator (EHG) is proposed to harvest energy from human hip motion to generate electricity. Based on the law of electromagnetic induction, the EHG generator can convert the kinetic power of the thigh swing into electrical energy during walking or running. To demonstrate the feasibility of the present method, a prototype of the EHG has been designed and fabricated. A theoretical analysis has been conducted to interpret the working behavior of this prototype. In addition, the performance of the system has been experimentally tested through a rotary motor and human body motion. When driven by the rotary motor at a rotation speed of 100 r/min, the open-circuit voltage of this prototype is 2.5 V. Further, a maximum open-circuit voltage of approximately 1 V and a maximum output power of 284 μW could be produced respectively when walking with an EHG at a speed of 1.47 m/s. This handy renewable energy technology is promising as a pervasive electricity generation system for a group of wearable or implanted sensors, actuators and mobile electronics.

Keywords

human power harvesting / pervasive energy / electricity generation / wearable energy / renewable and sustainable energy / mobile electronics

Introduction

Providing power for implantable biomedical devices has always been a challenge from the time when they were investigated. Presently, most of such devices are driven by batteries, either rechargeable or not. Although these batteries can effectively solve quite a few practical problems, there are still many situations in which replacing or recharging a battery is extremely troublesome. To extend the life span of an implantable device, it is necessary to equip it with an autonomous and sustainable energy source. In this regard, the renewable and environment friendly human kinetic energy has proven to be rather attractive for many mobile electronic devices. In fact, such power is strong enough to drive the wearable sensors in the implantable system [1]. Generally, the main approaches to convert human motion into electricity include piezoelectric effect or electromagnetic induction. The former works through piezoelectric materials with internal electrical charge generated from an applied mechanical force. A typical piezoelectric generator contains a cantilever which consists of one or several piezoelectric inserts [2-6]. During the past decades, many harvesters have been investigated to power the implants in this way [7-10]. In addition, based on the high efficiency, electromagnetic generators have also been frequently employed to power implantable medical devices. Determined by Faraday’s law of electromagnetic induction, a typical magnetic generator is comprised of one or more magnets and coils. Saha et al. [11] reported an electromagnetic generator which could harvest human energy for driving the body worn sensor or other electronic devices. Through spring and magnets, such generator can produce an electricity of 0.3-2.46 mW during human walking or slow running. In addition, Sardini et al. [12] proposed an efficient electromagnetic power generator for converting low-frequency mechanical energy into electricity. The test results showed that at a frequency of approximately 100 Hz, the generator could produce a power of approximately 290 μW with a harvesting efficiency of 0.5%. For the frequency of around 40 Hz, a power of approximately 153 μW was generated with an efficiency of 3.3%. Based on the analysis of the power output of knee while walking, Donelan and his colleague [13-16] built and tested a knee-mounted harvester (1.6 kg) which can produce an electricity of approximately 5 W by two sets of such devices while walking. Furthermore, an autonomous sensor was presented for force measurement in human knee implants in Ref [17], which was powered by a coil inside the implants induced by an externally applied magnetic field. Through such power supply system, the sensor can transmit data directly from the inside of the implant to an external readout unit wirelessly. Morais et al. [18] described an electromagnetic power generator for smart hip prosthesis via harvesting the energy from the human gait. The result showed that this permanent magnet vibration power generator could harvest a usable energy of 1912.5 μJ during normal walking. Bian et al. [19] proposed a magnetoelectric transducer for wireless power receiver which was promising for magnetoelectric energy conversion. The output power density produced by this transducer was 0.956 mW/cm³ under 0.3 Oe root-mean-square AC magnetic field. Besides, the technology of transcutaneous energy transmission system for all implantable devices was also studied. In 1992, based on the Class-E topology, a multi-frequency transmitter coil driver with a closed-loop controller was proposed to power and transmit data to implanted electronic devices [20]. The test result showed that several amperes could be easily and efficiently gained at the radio frequency. Two years later, Joung et al. [21] designed and built an energy transmission system to power an artificial heart using a transcutaneous transformer. After that, inherent advantages of such converter like high-voltage gain, minimum circulating current and high efficiency were reported. Hmida et al. [22] described an inductive system which could provide power to the implant device wirelessly through an external transmission coil and an implanted receiving coil. The test results indicated that a power of approximately 136 mW was gained with Class-E power amplifier. At the same time, a bit rate of 1 Mb/s was also gained for sending command data to the implanted electronic devices. Wang et al. [23] proposed a power transfer system with adaptive control technique which could reduce the fluctuations caused by coil displacement or load change in transmitted power. Based on the Class-E power amplifier, a maximum power of 250 mW could be transmitted to the implanted electronic devices. Riistama et al. [24] presented an active implantable device for measuring electrocardiogram (ECG) with wireless data and power transfer. In that work, the ECG instrumentation was tested successfully in vivo via implanting it in cows for 24 h. Watada et al. [25] developed a core type transcutaneous energy transmission system (TETS) with high efficiency for powering the implantable devices in a non-contact transcutaneous way.

In this paper, a kind of electromagnetic generator has been proposed to provide electricity power for the hip prosthesis based on harvesting human kinetic energy during walking. Besides, the prototype of such generator has been designed and analyzed in detail. Moreover, the performance of the electromagnetic hip-mounted generator (EHG) has been tested by both rotary motor and human motion which demonstrates its feasibility. Furthermore, improvements have been discussed, with various possible applications suggested.

Generator prototype

The structure of the EHG prototype is depicted in Fig. 1. From Fig. 1(a), it can be seen that the generator is mounted on the waist using a belt. On the generator there is a swing link, one end of which is fixed on the generator while the other end is tied to the outside of the upper thigh via another belt. The structure of this generator prototype is shown in Fig. 1(b). It can be seen that the generator is mainly comprised of a rotor and a stator. The stator can be fixed on the waist using a belt. The rotor, which is concentric to the stator, is fixed on one end of the swing link. The detailed structure is illustrated in Fig. 1(c). The rotor can rotate around the axis fixed on the stator. In detail, the rotor contains one magnet seat and eight magnets, while the stator contains one coil seat and eight coils. In addition, eight magnets and the eight coils are fixed in the magnet seat and the coil seat respectively.

Figure 2 presents the cross-sectional view of this generator prototype. From Fig. 2(b), it can be seen that the magnets and coils are inlayed in the magnet seat and coil seat along the direction of circumference respectively. Meanwhile, the magnet seat has the same radius as the coil seat. Besides, the diameter of the magnet is chosen to be the same as the bobbin for the coil. The relative movement between the rotor and the stator is illustrated in Fig. 2(a). When the magnet seat rotates, inlaid in the magnet seat, the eight magnets begin to revolve with the magnet seat synchronously. The rotation of these magnets produces a time-varying magnetic field around the coil seat. So the induced voltage will be produced in the coils fixed in the coil seat. It is worthy to note that every two adjacent magnets must be arranged with opposite magnetic poles on one side. Therefore, when the magnet seat rotates, the magnetic field direction changes four times in a cycle.

During walking, the thigh swings around the hip like a simple pendulum, as shown in Fig. 3. From Fig. 3, it can be seen that after a walking cycle, the thigh swings a cycle around the hip joint. Assuming that a swing link is fixed on the thigh, it also swings a cycle together with the thigh. In addition, the maximum swing angel is defined as α.

The working principle of the EHG is revealed in Fig. 4. From Fig. 4(a) it can be seen that when it is at the start, the swing link stays vertical, which makes the poles of magnets and the coils aligned. At that moment, the coils have the maximum magnetic flux. Then with the thigh starting to swing, the swing link swings round the hip joint following the thigh. When the thigh rotates to its limit, the intersection angle between the thigh and the vertical direction is α. Driven by the thigh, the swing link starts to rotate from the start to a quarter of a cycle in this process. The movement of the swing link makes the rotor of the generator rotate an angle of α clockwise. At this moment, each of the magnets is rotated to the middle between two adjacent coils, as shown in Fig. 4(b), and, the magnetic flux of the coils is at its minimum. Following the walking, a second step is made. Then the thigh swings to the opposite limit around the hip joint, where there is also an intersection angle of α between the thigh and the vertical direction. Following the rotation of the thigh, the swing link swings an angle of 2α from a quarter of a cycle to half of a cycle along the left. This movement of the swing link makes the rotor rotate an angle of 2α counterclockwise, as shown in Fig. 4(c). At the half cycle, each magnet is rotated to the middle between two adjacent coils where they produce a minimum magnetic flux for the coils. Then the walking cycle is closed, the thigh returns to the vertical position. During this process, the swing link swings an angle of α from the right limit to the vertical position clockwise. The movement of the swing link makes the rotor rotate an angle of α round the central shaft of the stator from a half cycle to the end, as revealed in Fig. 4(d). Now, the central lines of each magnet and coil come back to be aligned again. By this time, the generator works a cycle, and the rotor rotates round the stator a total angle of 4α while the human takes two steps forward.

Based on the law of Faraday’s electromagnetic induction, the induced voltage of the generator is mathematically expressed by Lenz’s Law as

(1)E=N(\mathrm{\Delta }\phi /\mathrm{\Delta }t),

Where E is the induced voltage of the coil in the receiver; \phi, the magnetic flux through the coil; N, the number of coil turns; and \mathrm{\Delta }\phi /\mathrm{\Delta }t, the changing rate of the magnetic flux. Assuming the magnetic field is normal to the face of the coil, the maximum value of the induced voltage of generator prototype can be written as

(2)E_{\max }=N\times B\times S\times C\times {\omega }_{elec},

where B is the magnetic induction of magnetic field; S, the area of each coil; C, the number of the coils; and {\omega }_{elec}, the angular velocity of induced electromotive force which is related to the mechanical angular velocity by the number of the magnetic pole pairs, as expressed by

(3){\omega }_{elec}={\omega }_{mech}\times p,

where {\omega }_{mech} is the angular velocity of the generator and P is the number of the magnetic pole pairs. The total peak voltage produced by the generator can be changed to

(4)E_{\max }=N\times B\times S\times C\times {\omega }_{mech}\times p.

For the EHG, the angular velocity {\omega }_{mech} of the generator depends on the maximum angle and the swing period of the swing link. Equation (4) can be transformed as

(5){\omega }_{mech}=\mathrm{4}(\pi \alpha /\mathrm{180}{T}_G),

where T_G is the swing period of the swing link; α, the maximum swing angle of the swing link which is also the maximum swing angle of the thigh when walking. Moreover, according to the relationship between the rotation frequency of the generator and walking frequency shown in Fig. 4, T_G can be rewritten as

(6)T_G=\mathrm{2}{T}_{walk},

where T_{walk} is the time required for taking one step, which depends on the walking speed v and the length of one step l. So Eq. (6) can be expressed as

(7)T_G=\mathrm{2}{T}_{walk}=\mathrm{2}l/v.

Putting Eq. (7) into Eq. (5), the angular velocity {\omega }_{mech}of the generator can be changed into

(8){\omega }_{mech}=\pi \alpha v/\mathrm{90}l.

Combining Eq. (4) and Eq. (8), the maximum output voltage of the EHG can be defined as

(9)E_{\max }=N\times B\times S\times C\times (\pi \alpha V/\mathrm{90}l)\times p.

From Eq. (9) it can be noticed that the maximum value of output voltage depends on the number of coil turns, the magnetic induction of magnetic field, the area and the number of the coil, the maximum swing angle of the swing link, the walking speed, the length of a step and the pole number of magnet. When the magnets and coils are fixed, the performance of the EHG depends on the gait and the speed of walking. However, for a person, the walking speed is the only factor to influence the property of the EHG. Therefore, in the following experiment, the influences of walking speed on the EHG will be presented.

Demonstration experiments

To demonstrate the feasibility of the EHG, a prototype is developed. The experimental setup, as demonstrated in Fig. 5, consists of a generator, two belts, a swing link and a rectifier doubler. The two belts are used to fix the generator on the human body. In detail, one belt is employed to tie the generator on the waist, while the other to tie one end of the swing link to the thigh, as shown in Fig. 5(a). The detailed structure of the generator is revealed in Fig. 5(b) which shows that the generator is mainly comprised of a rotor and a stator.

Rotor

The rotor, containing a magnetic seat and eight magnets and, looking like a cake, is made of nylon with a diameter of 80 mm and a height of 12 mm. Thirty millimeters from the center of the rotor, there are eight holes with a diameter of 10 mm and a depth of 10 mm uniformly distributed along the circumference for the eight magnets. Moreover, there is a through-hole with a diameter of 10 mm at the center of the rotor to accommodate to the axle. For the magnet, the eight cylindrical magnets are made of NdFeB (neodymium iron boron) with a diameter of 10 mm and a height of 8 mm. The magnetic field formed by the eight magnets is along the axial direction. Furthermore, every two adjacent magnets must be arranged in an alternating north-south pole fashion.

Stator

The stator consists of a coil seat and eight coils. The coil seat is also made of nylon with a diameter of 80 mm and a height of 10 mm. Besides, there is a shaft with a diameter of 10 mm fixed to the center of the stator. Thirty millimeters from the center of the stator, there are eight holes with a diameter of 18 mm and a depth of 8 mm distributed along the circumference for the eight coils. The eight coils are connected in series and winded by copper wire with a diameter of 0.1 mm. For each coil, the turn number is 1200 with an external diameter of 18 mm, an inner diameter of 10 mm and a height of 5 mm. Therefore, the total turn number of the generator is 9600 with a total resistance of 880 Ω. The rotor is mounted on the shaft of the stator using washers and nuts. Furthermore, a spacer made of polytetrafluoroethylene with an inner diameter of 12 mm, an external diameter of 20 mm and a thickness of 1 mm is employed to reduce the friction between the rotor and the stator. Therefore, the air gap between the lower surface of the magnets and the top of the coils is 1 mm.

The swing link is made of stainless steel with a dimension of 270 mm × 30 mm × 2 mm along each axis. One end of the swing link is fixed on the rotor via nuts and screws, and the other end is fixed on the belt using a slider. In addition, a rectifier doubler is used to convert the single phase AC voltage into DC voltage. The circuit diagram of the rectifier doubler is displayed in Fig. 6. From Fig. 6, it can be observed that this rectifier doubler is comprised of two filter capacitors (22 μF/50 V) C1, C2 and two rectifier germanium diodes (2AP9) D1, D2. As an electric equipment, a light emitting diode (LED) is connected to each end of the second filter capacitor.

The total weight of the generator is 0.48 kg. The experimental voltage data was measured by oscilloscope module of Agilent 54624A, USA.

In this experiment, to demonstrate the feasibility of the EHG, the prototype is both tested by a rotary motor and human motion respectively.

Test by a rotary motor

A rotary motor is employed to test the performance of the generator when it rotates at a constant rotation speed. The rotor of the EHG is fixed on the shaft of the rotary motor which allows the rotor to rotate with the motor at the same speed. To make the rotation frequency of the generator as low as human walking, the generator has been tested at a rotation speed of 60 r/min, 80 r/min and 100 r/min respectively. The results are shown in Fig. 7. From Fig. 7, it can be seen that the output voltage of the EHG appears to be a sinusoidal wave. In addition, when the rotor works at the rotation frequency of 0.33 Hz, 0.43 Hz and 0.55 Hz respectively, the maximum output voltage of the EHG is 1.5 V, 2 V and 2.5 V accordingly. Furthermore, the output voltage of the EHG increases with the rotation speed of the rotary motor while the period decreases, which corresponds with the conclusion of Eq. (4).

In addition, a rectifier doubler, as shown in Fig. 6, is employed to convert the AC voltage to DC voltage. Figure 8 exhibits the output voltage of the rectifier doubler when it is connected to the generator. It can be noticed that, transformed by the rectifier doubler, the output voltage of the generator is changed to direct voltage of 1.6 V which can lighten a LED.

Test by human body motion

Except being tested by rotary motor, the experimental setup is also tested by human body motion. The output voltage of the EHG prototype is illustrated in Fig. 9. From Fig. 9, it can be observed that the wave form of the output voltage produced by this prototype is pulse when it is driven by human body. The maximum voltage produced by the EHG prototype is 0.39 V, 0.47 V and 0.59 V at a walking speed of 0.98 m/s, 1.18 m/s and 1.47 m/s, respectively. Furthermore, the output voltage of the prototype increases with the walking speed, which corresponds with the conclusion of Eq. (9).

Discussion

According to Eq. (4), when the EHG prototype is driven by a rotary motor, the maximum output voltage of the experimental setup can be calculated as E_{\max }=N\times B\times S\times C\times {\omega }_{mech}\times p. For the rotary motor, the relationship between rotation speed n_{mech} and angular velocity {\omega }_{mech} can be expressed as

(10){\omega }_{mech}=\mathrm{2}\pi \times n_{mech},

So, Eq. (4) can be rewritten as

(11)E_{\max }=N\times B\times S\times C\times \mathrm{2}\pi \times n_{mech}\times p.

For the experimental setup of the EHG, the maximum output voltage can be calculated by

(12)$$\begin{gathered} E_{\max }=N\times B\times S\times C\times \mathrm{2}\pi \times n_{mech}\times p \\ =\mathrm{1200}\times \mathrm{0.5}\times \mathrm{3.14}\times [{(\mathrm{18}/\mathrm{2})}^{\mathrm{2}}-{(\mathrm{10}/\mathrm{2})}^{\mathrm{2}}]\times {\mathrm{10}}^{-\mathrm{6}}\times \mathrm{8}\times \mathrm{2}\times \mathrm{3.14}\times n_{mech}\times \mathrm{4} \\ =\mathrm{21.2}\times n_{mech}. \end{gathered}$$

Moreover, when the generator is at the same resistance of 880 Ω as the total resistance of the coils, the maximum output power of the EHG can be calculated as

(13)P_{\max }={\displaystyle \frac{{(E_{\max }/\mathrm{2})}^{\mathrm{2}}}{\mathrm{880}}}.

According to Eqs. (12) and (13), the maximum theoretical output voltage and power of the EHG prototype can be calculated, as listed in Table 1. In addition, the maximum measured value of open-circuit voltage and output power can be gained from Fig. 7. When the size of the EHG is fixed, it can be seen that either the measured or calculated maximum value of the output voltage and power increase with the rotation speed of the rotary motor. Besides, an efficiency of 21% for the generator in the EHG prototype is gained. Hence, enhancing the rotation speed is a significant method to improve the output performance of the EHG.

In addition, according to Eq. (9), when walking with this generator prototype, its output voltage can be calculated as

(14)$$\begin{gathered} E_{\max }=N\times B\times S\times C\times 2\pi \times n_{mech}\times p \\ =1200\times 0.5\times 3.14\times \left[{\left(18/2\right)}^2-{\left(10/2\right)}^2\right]\times {10}^{-6}\times 8\times 2\times 3.14\times n_{mech}\times 4 \\ =21.2\times n_{mech}. \end{gathered}$$

Based on Eqs. (13) and (14), the maximum theoretical output voltage and power of the EHG prototype can be calculated, as tabulated in Table 2. It can be observed that both the maximum values of open-circuit voltage and output power increase with the walking speed when the structure of the generator is fixed. Meanwhile, a generator efficiency of 21% is also gained. This indicates that the rotation speed is a significant factor to increase the output of the EHG.

Except for raising the rotation speed of the rotor so as to improve the performance of the EHG, its structure can be optimized as well. To decrease the weight of such a generator and make it more comfortable to wear, the EHG and hip prosthesis need to be fused together. For example, the stator and the rotor can be implanted into the hip prosthesis, while the artificial limb can act as a swing link. In this way, the generator would help improve the design of the hip prosthesis which can flexibly power the sensors installed inside.

Conclusions

This paper presents a kind of electromagnetic hip-mounted generator (EHG) which can automatically capture the power produced by human walking and supply electricity for the autonomous sensors installed in the smart hip prosthesis. The feasibility of the system was experimentally demonstrated using a designed EHG prototype. The structure of the device was described and its working behavior was theoretically interpreted. The practical performance of the prototype was evaluated with promising features disclosed. Further approaches to improve the EHG were discussed. Overall, with pervasive virtues, the EHG is expected to be of great significance for human wearable sensor and portable low power electronic devices. Besides, it can also be used in emergency or in those areas where electrical energy is absent.

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