A Study of an Electromagnetic Energy Harvester Device with Negative Magnetic Spring Characteristics

Transcription

1 APSAEM14 Journal of the Japan Society of Applied Electromagnetics and Mechanics Vol.3, No.3 (15) Regular Paper A Study of an Electromagnetic Energy Harvester Device with Negative Magnetic Spring Characteristics Yinggang BU *1, Kaname INOUE *1 and Tsutomu MIZUNO *1 In this paper, we proposed a compact electromagnetic generator structure with high instantaneous power output which can be used for energy harvester device. It used the negative magnetic spring characteristics to transform the human operation to the instantaneous electrical energy. With a radio module load, the induced maimal voltage of the prototype device is 4.8 V, the maimal current is 53 ma, and the instantaneous power is about 5mW. Under our eperiment, we successfully drove Enocean s PTM33 radio module. Keywords: energy harvester, electromagnetic, negative magnetic spring, back electromotive force. (Received: 4 July 14) 1. Introduction In our daily life, there are some unnoted weak environmental energy eist. For eample, electromagnetic waves, light, vibration, heat and so on. Recently, collecting and then transforming these weak energy to electrical energy is becoming broad research interests in Energy Harvesting Technology [1-4]. These weak energies from surrounding environment might provide power to drive devices such as sensors where only tiny power is needed. Harvested from environmental energies, these devices could be free from the echange of batteries, so it is epected to be longer life and more compact. Besides the application to sensor devices, the applications to wireless device are also published [5]. These wireless switches are similar to an ordinary wallmounted light switch, but it can generate electricity when the switch is pressed. The instantaneously generated electrical power further drive an integrated wireless module to emit radio signals, so as to trigger controlled devices, for eample, room lightings. To generate electricity in such wireless switch, piezoelectric type, dielectric type, magneto-striction type and electromagnetic type are widely discussed. Since electromagnetic type makes use of flu variations in coils to induce electrical energy, it can produce relatively large power and is easier to be realized. On the other hand, how to make compact structure but still keep large instantaneous power output needs to be solved [1-]. In this research, we proposed a compact electromagnetic generator structure with high instantaneous power output for a wireless switch. By using the negative magnetic spring characteristics, the mechanical energy from human s switching operation is transformed to the instantaneous electrical energy. And this electrical energy further becomes the power supply to a radio Correspondence: Y. BU, Faculty of Engineering, Shinshu University Wakasato, Nagano , Japan buyinggang@shinshu-u.ac.jp *1 Shinshu University transmitter of the wireless switch. In this paper, we will eplain the fundamental structure and the principle. Based on the eperimental sample, we further evaluate the power supply characteristics on both open and loaded status.. Basic Structure and Operating Principle.1 Basic Structure Fig. 1 shows the basic structure of the proposed electromagnetic generator. The dimension of this electromagnetic generator is (mm 3 ). In the perspective view of the generator, the mover which is located on the upper portion consists of a permanent magnet and a U-shaped yoke, the stator which is located on the lower portion consists of a coil and a fied yoke. The mover connected to the stator via pairs of leaf springs. These leaf springs etend from the plastic frame of the stator to the U-shaped yoke and are fied at their ends by the screws. The leaf springs are made by stain- Fig.1. Basic structure of the proposed generator (unit: mm). 557

2 日本 AEM 学会誌 Vol. 3, No.3 (15) less steel. As illustrated in Fig.1 (b), the magnetic force with divergent magnetic spring force along direction will be generated between the mover (the permanent magnet and the U-shaped yoke) and the stator (the coil and the fied yoke). As a result, the stationary position of the mover will not be on the mechanical center, but either on the left side of generator or on the right side. It has a negative magnetic spring characteristics. When the eternal force pushes the mover towards the opposite side from its stationary position, the eternal mechanical energy is first restored in the negative magnetic spring at the stage from the stationary position to the mechanical center. Once passing through the mechanical center, the divergent magnetic spring force will instantaneously accelerate the mover. This instantaneous acceleration will cause instantaneous change of magnetic flu in the coil and induce the electrical voltage. Since the magnetic force is divergent, even if releasing the eternal force after passing through the mechanical center, the mover will still keep accelerating until it is stopped by the stopper on the stator. During the movement from the mechanical center to the stopper, the magnetic flu is also changing, so the electrical energy can also be generated.. Back-EMF analysis We can analyse the operating principle from a movement block diagram. The movement block diagram of the mover is shown in Fig.. K ms is magnetic spring constant and K s is mechanical spring constant. The system spring constant K is represented by equation (1)-(3). We can use the finite element method (FEM) structural analysis software for calculation K ms and K s. The result is shown in Fig. 3. K = K ms + Ks (N/m) (1) Fm K ms = (N/m) () Fs Ks = (N/m) (3) where K ms (N/m) is the magnetic spring constant, K s (N/m) is the mechanical spring constant, and (m) is the Static thrust of -direction,f m,f s (N) dψ /d (Wb/m) Displacement (mm) displacement of the mover. As indicated in Fig. 3, F m is the magnetic force spring characteristics which is clearly characteristics of a diverging force we predicted and K ms is a negative value. At the displacement position of mm, the value of F m is 3 N. F s in Fig. 3 is the mechanical spring force of the leaf springs which has the positive spring characteristics and will reduce the divergence properties of the magnetic spring. As a result, the system spring constant is the combination of both F m and F s, and it will determine the movement of the mover. In order to increase the induced electrical energy, an effective way is to reduce the value of F s. But be noted, if there is no F s, it means the mover will lose the support. After estimating the strength of the leaf springs, we designed the value of K to be 384 N/m. The damping coefficient C is derived by substituting measured values into the equation (4) []. 4m C = T log e 1 F s F m First order approiamate line K = 384 N/m F m + F s Fig. 3. Static thrust characteristics. 1 1 Displacement (mm) 4. Fig.4. Calculated value of dψ/d-displacement characteristic. Fig.. Mechanical movement block diagram of the proposed electromagnetic generator. = 4m K (N s/m) log e π m 1 (4) 558

3 日本 AEM 学会誌 Vol. 3, No.3 (15) where m (kg) is the mass of mover, is the attenuation coefficient measured by the free vibration eperiment, and T (s) is period. From equation (5).we can obtain the output voltage in the coil induced by the back electromotive force [7]. dψ e = d t dψ d = (V) (5) d dt where ψ (Wb) is the magnetic flu interlinked with the coil and t (s) is time. In the above equations, dψ/d indicates the amount of change of the magnetic flu by the displacement. We carried out the analysis of the static magnetic field by FEM software Mawell 14. and obtained dψ/d characteristics based on the displacement scale in Fig. 4. As seen in Fig. 4, the change of the magnetic flu dψ/d is relatively larger when the magnet approaches to the fied yoke. The maimum value is 4. Wb/m at mm. Table 1 Basic performance of the prototype. Item Symbol Value (Unit) Diameter of conductor d.1 (mm) Number of turns N 5 (turn) Space factor ζ.39 DC resistance Rdc 1.8 (Ω) of coil Stroke s 4 (mm) Mass of mover m 1.91 (g) 3. Prototype and characteristics 3.1 Prototype device Based on the proposal structure in Fig.1, we made a prototype device and evaluated its performance. Table 1 shows the basic performance of the prototype. Both the U-shaped yoke in the mover and the fied yoke in the stator are made by SPCC material, and the permanent magnet is made by NdFeB material for generating large magnetic force. Fig. 5 is the photography of the prototype device. Because of the negative magnetic spring, the mover stopped on the left side and contacted to the fied yoke by their magnetic attraction force. First, we measured the static characteristics of the negative magnetic spring. Fig. shows measurement block diagram in the eperiment. We pushed the mover from its left stationary position with very small force. The mover first moved towards the mechanical center at a slow velocity close to zero and then instantaneously accelerated once it passed through the center. We measured the eternal thrust force by a loadcell and traced the displacement of the mover and its local velocity by a laser displacement meter. The output voltage is measured by a digital oscilloscope. Fig.. Measurement block diagram of the static thrust and the back electromotive force. Fig. 5. Photography of the prototype device. Fig. 7. Measured values of the displacement and the static thrust characteristics based on the time scale. 559

4 日本 AEM 学会誌 Vol. 3, No.3 (15) Fig. 7 shows the measurement results. The magnetic force at the starting position is.(n) and the direction is opposite to the movement direction because of the negative magnetic spring characteristics. This negative magnetic force gradually decreases when the mover moves closer to the mechanical center (the displacement is mm). Once passing through the mechanical center, the direction of the magnetic force changes to be positive and will instantaneously accelerate the mover. As shown in Fig. 7, the displacement value suddenly changed to be 1.94 mm. Fig. 8 is the enlarged view of this instantaneous change. As shown in Fig. 8, during the acceleration from the mechanical center to the right fied yoke (the stopper), it spent 9 ms to reach the maimal displacement of 1.94 mm where the maimum velocity is.7m/s and the induced maimum open voltage is 3.V. Fig. 9 shows the measurement results where the abscissa is the displacement of the mover. The curve in the third quadrant is the relationship of the thrust force to the displacement. To push the mover away from the starting position, the thrust force is at its maimum level of N, and then it decreased to be zero when arriving at the mechanical center. The curve in the first quadrant is the relationship of the induced voltage to the velocity of the mover. Fig. 1 gives the comparison of the measured back electromotive force and the calculation value based on the movement block diagram in Fig.. Compared with the calculated value of 3.97V, the measured result is 3.V which is very close to the theoretical prediction. 4. Actual operation performance By using the prototype device as a real switch, we analyzed its performance under manual switching operation without and with a load. Fig. 8 Enlarged view of the displacement, the velocity, the thrust force and the back electromotive force characteristics based on the time scale. Fig. 9. The velocity, the thrust force and the back electromotive force characteristics based on the displacement scale Calculated value 3.97 V 1.94 mm. 1 Displacement (mm) Measured value 3. V Fig. 1. Comparison between the measured back electromotive force and the calculated value. 4.1 Performance without load Suppose the prototype switch is operated by an adult male under a general force. The mover is moved by finger. Fig. 11 shows the measured data under this condition. Corresponding to the displacement from mm to mm, the velocity v of the mover reached to be.88 m/s, then from mm to mm, the velocity v reached up to be 1. m/s. Because the change of the magnetic flu became the maimum at the displacement of mm as indicated from Fig. 4, the maimum open voltage reached to be 5.4V at this position. Fig. 1 is the same measurement result but changing the abscissa to be the displacement. At the starting position where the finger started to push the mover away from the left fied yoke (the maimal negative displacement), the magnetic force is biggest and so the change of the velocity is biggest as well. 5

5 日本 AEM 学会誌 Vol. 3, No.3 (15) 3 3 Displacement (mm) m/s( = mm) 5.4 V.11 mm v e 1 1 Velocity v (m/s) Current I (ma) 3 3 Displacement (mm) 3 3 v e 53 ma 1.33 m/s 4.8 V.14 mm I 8 1 Velocity v (m/s) Time t (ms) Time t (ms) 1 8 Fig. 11. The displacement, the velocity and the back electromotive force characteristics based on the time scale (without load). Fig. 13. The displacement, the velocity and the back electromotive force characteristics based on the time scale (with PTM33 radio module). Velocity v (m/s) m/s ( = mm) e 5.4 V v 1 m/s 3.11 mm Displacement (mm) Fig. 1. The velocity and the back electromotive force characteristics based on the displacement scale (without load). 4. Performance with load We used a universal radio module of PTM33 from EnOcean Company as the load and measured the operation data again. The operation voltage of PTM33 is 3-5V, and it can send radio signal to switch on the corresponding lighting apparatus [8]. Fig. 13 shows the result based on the time scale and Fig. 14 is based on the displacement scale. Under the similar operation condition as mentioned in 4.1, the maimal induced voltage is 4.8 V, the maimum current of 53mA. The instantaneous power is about 5mW and the electricity energy by one stroke is about 1µJ. In the eperiment, we successfully drove PTM33 to send radio signals. With the radio receiver, we also successfully switched on the lights. Fatherly, we received the transmitted radio signal up to the range of 5 meters. Fig. 14. The velocity and the back electromotive force characteristics based on the displacement scale (with PTM33 radio module). 5. Conclusion In this paper, we proposed a compact electromagnetic generator structure with high instantaneous power output as an energy harvester. It changes human operation force to the instantaneous electrical energy based on the negative magnetic spring characteristics. On our prototype device, by manually move the mover under the general operation, we measured the velocity of the mover to be.88m/s at the center position, and it reached up to be 1m/s at the maimal displacement. Under the condition without load, the induced maimal voltage is 5.4V. When connecting to PTM33 raido module (EnOcean) as the load, the induced maimal voltage is 4.8V, the maimal current is 53mA, and the instantaneous power is about 5mW. Driven by the prototype device in the eperiment, PTM33 successfully transmitted the radio signals and we received the radio signal at the range up to 5 meters. We successfully verified our proposed structure is feasible to be applied as a switch by harvesting human operation energy and drive a radio 51

6 日本 AEM 学会誌 Vol. 3, No.3 (15) transmitter with the low power consumption such as Enocean s PTM33 radio module. References [1] A. Munaz, B.C. Lee, and G.S. Chung, A study of an electromagnetic energy harvester using multi-pole magnet, Sensors and Actuators A Physical, Vol.1, pp , 13. [] X. Wang, C. L. Pan,Y. B. Liu, and Z.H. Feng, Electromagnetic resonant cavity wind energy harvester with optimized reed design and effective magnetic loop, Sensors and Actuators A Physical, Vol.5, pp.3-71, 14. [3] G.A Lesieutre, G.K Ottman, and H.F Hofmann, Damping as a result of piezoelectric energy harvesting, Journal of Sound and Vibration, Vol.9, pp , 4. [4] T. Galchev, E. E. Aktakka, and K. Najafi, A Piezoelectric Parametric Frequency Increased Generator for Harvesting Low-Frequency Vibrations, Journal of Microelectromechanical Systems, Vol.1, pp , 1. [5] Y. Suzuki and S. Kawasaki, An autonomous wireless sensor powered by vibration-driven energy harvesting in a microwave wireless power transmission system, in Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference on, pp , 11. [] T. MizunoO, T. Horio and Y. Tearmae, Characteristics of Oscillatory Actuator Using Torsion Bar Spring for Optical Scanner, Journal of JSAEM, Vol.19, No.3, pp.54-59, 11. (in Japanese) [7] T. Yamaguchi, Y. Kawase, and T. Asano, Dynamic Analysis of Latch-in Relay Using 3-D Finite Element Method with Mesh Modification Method Employing Multi-Mesh and the Interpolation, Journal of JSAEM, Vol.1, No.3, pp , 13. [8] EnOceanGmbH: 5