Mechanical Energy Harvester With Ultralow Threshold Rectification ...
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Mechanical Energy Harvester With Ultralow Threshold Rectification Based on SSHI Nonlinear Technique Lauric Garbuio, Mickaël Lallart, Daniel Guyomar, Claude Richard, and David Audigier
Abstract—Harvesting energy from ambient sources has become of great importance these last few years. This can be explained not only by advances in microlectronics and energy harvesting technologies, but also by a growing industrial demand in wireless autonomous devices. In this field, piezoelectric elements offer outstanding performances, thanks to their high power density that makes them suitable for integrated microgenerators. However, such a domain still offers challenges to the research community. Particularly, embedding piezoelectric inserts as MEMS components raises the issue of low voltage output. Classical energy harvesting interfaces that feature bridge rectifier suffer from threshold voltage introduced by such discrete components, therefore compromising their use in real-life applications. In this paper is presented a new energy harvesting circuit that operates with ultralow voltage output, by the use of a magnetic voltage rectifier that does not present significant voltage gap. Experimental measurements performed on a simple transducer confirm theoretical predictions, and show that the proposed architecture operates well even for low-level vibrations, outperforming all known energy interfaces. Particularly, it is theoretically and experimentally shown that such an interface provides a gain greater than 50 compared to classical energy harvesting structures. Index Terms—Energy harvesting, integrated devices, microgenerators, nonlinear processing, piezoelectric.
I. I NTRODUCTION
T
HE CONCEPT of mobile, wireless, and autonomous devices is no longer chimerical thanks to combined advances in ultralow power electronics and energy harvesting technologies. Such a research interest has also been greatly influenced by the increasing demand in autonomous sensors from the industrial and biomedical fields. In this domain, the concept of “Smart Dust” presented in [1] has been a first step for the conception of autonomous systems that can operate without any connections. Nowadays, the future of sensing is about to experience a technological rupture due to the convergence of the energy requirements of electronic devices and the power outputs of microgenerators [2], [3]. Although first prototypes of such devices that are fully powered up by harvested environmental energy have already been presented [4]–[9], a further Manuscript received April 30, 2008; revised January 14, 2009. First published February 6, 2009; current version published April 1, 2009. The authors are with the Laboratoire de Génie électrique et Ferroélectricité EA 682, Institut National des Sciences Appliquées de Lyon, Université de Lyon, 69621 Villeurbanne, France (e-mail: [email protected]; [email protected]; [email protected]; claude.richard@ insa-lyon.fr; [email protected]). Color versions of one or more figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2014673
step still needs to be taken to make such systems fully operating when integrated into a structure. Environmental sources for ambient energy harvesting are numerous: magnetism [10], solar [11]–[13], thermal [14], mechanical [15]–[19]. . ., but the actual needs in terms of embedded devices require the active material to have high power densities. In this domain, piezoelectric materials are good candidates for energy scavenging from ambient vibrations that are one of the most commonly sources available [20], [21]. Particularly, it has been shown that applying original nonlinear processing can lead to a significative improvement of the output power of PEGs1 [22]–[25]. Such treatments consist in switching intermittently the piezoelement on a resonating electrical network for a very short time, therefore artificially increasing the voltage output and the coupling coefficient of the piezomaterial device. Among these interfaces, the so-called series Synchronized Switch Harvesting on Inductor (SSHI) allows a typical gain of eight in terms of harvested energy compared to the classical energy harvesting technique [24]–[26]. However, whatever the type of interface considered, the output voltage of the piezoelectric element requires to be rectified in order to ensure a proper charge flow in the storage battery or capacitor. Such a process is typically done using a diode bridge rectifier (or synchronous rectifier) that presents threshold voltage to be operative. This element is a particular issue for integrated systems [27] that has typical output voltages below this threshold voltage, making the energy scavenging process inefficient [9]. In order to tackle this drawback, this paper introduces a new technique for rectifying the piezoelectric voltage without significative voltage gap. Based on the so-called “series SSHI,” the proposed scheme features a magnetic rectifier that does not induce significative voltage gap when conducting, therefore allowing operations even for low voltage levels, and leading to the concept of SSHI with Magnetic Rectifier (SSHI-MR). It is shown both theoretically and experimentally that such an approach provides a significant gain (up to 50) when the electromechanical structure is excited with low-level vibrations (which is often the case for high-frequency systems for example). This paper is organized as follows. Section II briefly recalls standard and series SSHI technique operations and introduces the concept of the SSHI-MR. Section III provides theoretical developments for estimating the power outputs from such microgenerators. Section IV discusses the self-powering
1 Piezoelectric
Electrical Generators (PEGs).
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Fig. 1.
Standard energy harvesting circuit and typical waveforms.
issues of the proposed technique for real-life applications. Section V aims at validating the previously exposed analysis through experimental measurements. This section also points out specificities of the energy harvesting circuit, particularly for the SSHI-MR, whose performances will be discussed in Section VI. Finally, Section VII briefly concludes this paper. II. E NERGY H ARVESTING B ASICS AND SSHI-MR P RINCIPLES This section aims at qualitatively exposing the general principles that lie behind three energy harvesting processes from the piezoelectric effect. The first one is the widely used standard energy harvesting interface that simply consists in directly connecting the piezoelement to a diode bridge rectifier connected to a storage capacitor and the load. The second includes a digital switch in series with the piezoelement and the rectifier, and performs a nonlinear treatment in order to increase the energy output of the microgenerator. Finally, the concept of the SSHI-MR is introduced and explained. A. Standard Interface The standard energy scavenging circuit as well as associated waveforms are shown in Fig. 1. When the absolute value of the piezoelectric voltage is less than the rectified voltage VDC + 2VD , the diode bridge rectifier is blocked and the piezoelectric element is let in open circuit, making the piezoelectric voltage to vary with the displacement. When the absolute value of the piezoelectric voltage equals the rectified voltage VDC + 2VD , the bridge rectifier conducts and a charge flow appears from the piezoelectric element to the storage capacitor CS . This energy extraction process terminates when the current cancels, which appears on displacement extrema.
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Fig. 2. Series SSHI energy harvesting circuit and typical waveforms.
B. Series SSHI Interface In the case of the series SSHI, two unipolar digital switches in series with an inductor are placed in series with the piezoelement and the rectifier as shown in Fig. 2. The switching event is synchronized with the electrostatic charges that are available on the piezoelectric element. When these charges are maximal (which actually occurs when the displacement or the piezoelectric voltage reaches an extremum), the piezoelectric element is briefly connected to the circuit, therefore providing energy to the storage capacitor CS through the inductor L and the bridge rectifier. The switching time period ti corresponds to the half-period (1) of the electrical circuit shaped by the blocking capacitance C0 of the piezoelectric element and the inductor (the voltage VDC is assumed to be constant and CS C0 ). Such a process leads to an inversion of the piezoelectric voltage with respect to the voltage ±(VDC + 2VD ) (+VDC + 2VD for a positive switching and −VDC − 2VD for a negative switching). However, this inversion is not perfect and characterized by the coefficient γ defined as (2) where Qi is the electrical quality factor of the circuit ti = π LC0 (1) (2) γ = e−π/(2Qi ) . C. SSHI-MR Interface The schematic of the SSHI-MR is shown in Fig. 3. The associated waveforms of this technique are very similar to those obtained with the series SSHI. In the SSHI-MR technique, the inductive element L is replaced by a transformer T with two primary windings and one secondary winding. The coupling factor m is chosen to be far greater than one. Each primary coil is mounted in series with one unidirectional switch (S1 or S1 )
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A. Standard Interface
Fig. 3. SSHI-MR energy harvesting circuit.
but with inverse polarity. The secondary winding is connected to the smoothing capacitance CS in parallel with the load RL through a diode D that ensures a proper charge flow. When the voltage or the displacement is maximal (respectively, minimal), the switch S1 (respectively, S1 ) is closed. In effect, a resonance occurs between the clamped capacitance C0 and the primary leakage inductance of transformer through the load. Like the SSHI technique, the switch is opened after one resonant halfperiod for the inversion of the piezoelectric voltage. The inversion is characterized by the inversion factor γ in the same way as the series SSHI inversion coefficient. This combination of the transformer T and the one diode rectifier D aims at converting the ac voltage to dc voltage, and also offers significant advantages. The diode threshold voltage is reduced to VD /m compared with the 2VD threshold of the standard and series SSHI cases. This allows energy harvesting with lower voltage levels. The rectifier power losses are in the standard and series SSHI techniques about 2VD IPZT whereas in SSHI-MR they are given by IPZT (VD /m). Moreover, as the current is very low, the losses in the transformer are negligible. As well, the transformer allows an increase of the output voltage, which is often desirable for the compatibility with electronic circuits. III. T HEORETICAL D EVELOPMENT In this section are presented the theoretical developments taking into account the effect of the threshold voltage of each diode (denoted VD ) for the previously exposed techniques. It is considered that the device is excited by a monochromatic driving force and exhibits a constant displacement magnitude uM . Such a consideration is well adapted to MEMS devices due to their frequency response band. For this purpose, a simple electric modeling of the electromechanical structure is used. This modeling consists in a current source proportional to the displacement velocity in parallel with the blocking capacitance C0 . It gives a simple but realistic model when the electromechanical system is excited near one of its resonance frequencies [28]. The piezoelectric microgenerator behavior can be described by (3) [29], where u is the flexural displacement of the structure, VC0 the piezoelectric voltage, and IPZT the current flowing out of the piezoelement. α and C0 are given as the force factor and the blocking capacitance of the piezoelectric element, respectively. Furthermore, it is supposed that the time constant RL CS is far greater than the vibrational period T0 (corresponding to the frequency f0 ), such as VDC could be considered as constant in steady state IPZT = αu˙ − C0 V˙ C0 .
(3)
In the case of energy harvesting using the standard technique, the expression of the output power yields (4), with uM the magnitude of vibration and u1 the value of the displacement when the energy transfer process starts. The value of u1 is derived considering the end of the last conduction period, yielding (5) considering that the last conduction period is for negative values, with VD the threshold voltage for a single diode. Therefore, the expression of the power (4) leads to (6). Considering the expression of the power (7) as a function of the rectified voltage VDC and the load resistance RL leads to (8) T0
Pstandard
2 = T0
2
VDC IPZT
uM du
2 = αVDC T0
(4)
u1
0
C0 (VDC + 2VD ) + K u1 = α C0 VDC + 2VD ) − uM with K = α 4 Pstandard = VDC (αuM − C0 (VDC + 2VD ) T0 V2 Pstandard = DC RL 16f02 RL (αuM − 2C0 VD )2 . Pstandard = (1 + 4f0 RL C0 )2
(5) (6) (7) (8)
B. Series SSHI Interface In the case of the series SSHI, the expression of the power gives (9), with VM and Vm the absolute values of the piezovoltage just before and after the inversion process, respectively. The expression of these two voltages can be obtained considering the switching process and the open-circuit condition between two consecutive inversion processes that leads to the relationships (10) and, therefore, to the values of the voltages VM and Vm as (11). Moreover, considering the expression of the output power (12) therefore yields the expression of the power as a function of the load resistance (13) T0
Pseries
2 = T0
2
VDC IPZT
2 = − C0 VDC T0
0
−V m
dV VM
= 2f0 C0 VDC (VM + Vm ) Vm + (VDC + 2VD ) = γ (VM (VDC + 2VD )) VM Vm = 2 Cα0 uM ⎧ ⎨ VM = 1 2 α uM − (1 + γ)(VDC + 2VD ) 1−γ C0 ⎩ Vm = 1 2γ α uM − (1 + γ)(VDC + 2VD ) 1−γ C0 Pseries = Pseries =
2 VDC RL
(9) (10)
(11)
(12)
16(1 + γ)2 f02 RL (αuM − 2C0 VD )2 . [(1 − γ) + 4(1 + γ)f0 RL C0 ]2 (13)
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TABLE I OPTIMAL VALUES
Compared to theoretical developments that do not take into account diode threshold voltages [25], it can be seen that the value of αuM is changed to αuM − C0 VD in the case of the standard harvesting technique and the series SSHI.
C. SSHI-MR Interface In the case of the SSHI-MR technique, the harvested energy into the load can therefore be written as (14), with VM and Vm the absolute values of the voltage just before and after the inversion process, respectively, T0
PSSHI−MR =
2 T0
2
VDC IPZT dt m
0 −V m
VDC = −2f0 C0 m
dV
Fig. 4. Experimental test structure.
VM
= 2f0 C0
VDC (VM + Vm ). m
(14)
The considerations of the switching event and the opencircuit condition that arises between two switching processes lead to the relationships on VM and Vm given by (15) (with uM the vibration magnitude) that therefore gives the expressions of VM and Vm as (16)
Vm + VDCm+VD = γ VM − VM − Vm = 2 Cα0 uM
VDC +VD m
VDC +VD 2α 1 +C UM VM = − 1+γ 1−γ m 0 1−γ 1+γ VDC +VD 2α γ Vm = − 1−γ + U m C0 1−γ M .
(15)
(16)
Considering (14) and (16), as well as the output power (17) therefore leads to the expression of the output power as a function of the load resistance RL as (18). It can be seen from this expression that the voltage gap VD of the diode is divided by a factor m, making the threshold voltage to tend to zero for high values of m PSSHI−MR PSSHI−MR
V2 = DC RL
(17)
2 RL 2 2 αuM − C0 VmD 16 m 2 f0 (1 + γ) =
2 . (18) RL (1 − γ) + 4f0 C0 m 2 (1 + γ)
D. Optimal Performance By derivation of (8), (13), and (18), it can be seen that the output power of each technique reaches a maximal value for an optimal load resistance Ropt which corresponds to an optimal output voltage VDCopt , whose values are included in Table I.
IV. S ELF -P OWERING I SSUES In the proposed method, the nonlinear treatment performed by the SSHI-MR necessitates digital switches to operate. For this paper, both unidirectional switch command is generated using a DSP (DSpace RTI 1104 connected to a computer) which turns on S1 when piezovoltage reaches a maximum and during a specific time corresponding to a half-period of resonant inversion circuit (and, respectively, S1 on each minimum). Stand-alone operations of microgenerators are a key point for real applications and good operations of these switches require a given energy amount. However, this energy is limited and can be supplied by the piezotransducer. Particularly, the selfpowered version of the SSHI-MR can use, for example, the autonomous breaker that is described in [30]. This device typically consumes less than 5% of the electrostatic energy available on the piezoelectric material, therefore allowing the conception of a truly stand-alone SSHI-MR. V. E XPERIMENTAL V ALIDATION AND D ISCUSSION A. Experimental Setup This section aims at validating the previous theoretical development through measurements carried out on a commercial disk bender (Buzzer KPSG100, KINGSTATE) with an additional seismic mass as shown in Fig. 4. This bimorph is made vibrating at one of its resonance frequencies f0 by the use of an electromagnet, and is connected to one of the three presented energy harvesting interfaces. The load value RL is also made varying in order to change the power output of the microgenerator. Preliminary measurements carried out on the structure for identification led to the parameters consigned in Table II according to the electromechanical equivalent circuit [31] shown in Fig. 5. The differences in the inversion coefficient between series SSHI and SSHI-MR can be explained by the fact that the extremum detection is better performed in the case of the SSHI-MR due to a higher piezoelectric voltage.
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TABLE II PARAMETER IDENTIFICATION
Fig. 6. SSHI-MR energy harvesting circuit with magnetization current recovery.
Fig. 5. Equivalent electromechanical circuit of piezoelement near a resonance frequency [31].
The magnetic core used for this paper, is an unoptimized common Fe-Si transformer whose parameters are given in Table II. In MEMS applications, development of power systems on chip is hindered by the difficulty of integration of magnetic passive components for frequency below 100 kHz because of their size [32], [33]. However, Lu et al. [34], [35] have shown a crafty process for on-chip inductors and transformers which consists in mounting on surface several parallel bondwires embedded in magnetic ferrite drop. They realized experimentally bondwire inductors of hundred nanohenrys with quality factor around 40. The hybridization of micro-PEGs with this technique is therefore promising for energy scavenging. The minimal inductance properties, respectively, defined by its value Linv and its magnetic surface S required for inversion can be estimated from energetic (19) and magnetic (20) considerations. For example, with a maximal induction Bmax = 0.5 T and a peak current Imax = 1 A, the minimal value of Linv is 0.9 μH corresponding to a required surface S of 1.8 mm2 and an inversion frequency of 300 kHz. Furthermore, multiplexing magnetic cores between several energy harvesters mounted in parallel would reduce the required magnetic material volume 1 1 2 C0 VC20 max = Linv Imax 2 2 Bmax S = Linv Imax .
(19) (20)
In order to improve the SSHI-MR efficiency, the part of energy extracted from the piezoelectric material for the magnetization of the magnetic core could be partly recovered instead of being dissipated. This could done by replacing the single diode D by a full rectifier bridge after the secondary windings
Fig. 7. Experimental results and theoretical predictions for a constant vibration magnitude of ∼23 μm.
as shown in Fig. 6. Therefore, the demagnetization current that appears just after the harvesting process, flows through the storage capacitor CS thus bringing additional charges to the storage stage. The use of a high transformer ratio m allows one to reduce the impact of the supplementary diode threshold voltage. B. Experimental Results The first set of measurements consists in making the test structure vibrating at its resonance frequency f0 with a constant vibration magnitude of approximately 23 μm and measuring the output power for several load resistance values. Results are shown in Fig. 7. These results show good agreement with theoretical predictions, and point out the performances of the SSHI-MR over the other methods when the structure vibration levels are very low. Particularly, the gain obtained with the SSHI-MR, in this case, is greater than 56 compared to the standard approach and 30 compared to the series SSHI. Furthermore, it can be noted that although the open-circuit voltage magnitude without any process is 598 mV, the SSHI-MR allows a voltage output of 4.51 V at the optimal load. The second set of measurements shown in Fig. 8 consists in measuring the maximal power output for each method considering several vibration magnitudes at the resonance frequency. Here also, a very good agreement between theoretical predictions and experimental results can be observed, validating the performances of the SSHI-MR, that can harvest up to
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Fig. 9. Experimental waveforms of the SSHI-MR technique for a 6-μm displacement. Fig. 8. Experimental results and theoretical predictions for the maximal harvested power as a function of the vibration magnitude.
400 μW by dramatically reducing the energy losses in the discrete components. Therefore, the technique starts harvesting energy efficiently for very low vibration magnitudes such as 1 μm, which corresponds to an open-circuit voltage of 26.6 mV (the equivalent voltage gap given by VD /m is equal to 10.5 mV), while the other techniques need at least 2VD (460 mV) before starting the harvesting process. Typical experimental waveforms are shown in Fig. 9 for a 6-μm displacement. In this case, the magnitude of the piezovoltage in open circuit is about 0.32 V (corresponding to VM − Vm ) which is lower than the threshold voltage 2VD of rectifier bridge and therefore prevents the use of the standard technique. With nonlinear treatment, the output voltage is increased to 0.79 V and allows energy harvesting. The energy distribution within the system is shown in Fig. 10. The output electrical energy EDC is measured experimentally and extracted energy from piezoelement EC0 is calculated according to (21) for one vibration cycle. The mechanical input energy Emech is estimated using (22)
EC0 = C0 VM2 − Vm2 T u˙ dt + α 0
(21)
T 2
Emech = c
Fig. 10. Experimental distribution of energy per vibration cycle.
VC0 udt ˙ Fig. 11. (Solid line) Experimental efficiency and (dashed line) output voltage for SSHI-MR method for a constant vibration magnitude of 23 μm.
0
2 = cπωUM + 2αUM (VM + Vm ).
(22)
The harvested energy EDC reaches a maximum for the optimal load resistance Ropt = 1.2 × 105 Ω as predicted by theoretical developments. It corresponds to the combining of two behaviors. First, the energy extracted from the piezoelement EC0 decreases with the load or equivalently with the output voltage VDC as it can be shown from (16) and (21). Second, for high values of the load resistance (i.e., high value of VDC as shown in Fig. 11), the threshold voltage of the rectifier bridge and the inversion losses become less significant and the extracted energy is fully transferred to the load. Therefore, an optimal load exists that maximizes the output power of the
harvester. The input mechanical energy Emech is constant for low harvested energy but with the increase of load resistance a feedback force appears on the piezoelectric element. This force is opposite to the mechanical displacement and acts as dry friction force. In this paper, a constant displacement magnitude is considered but if the system is excited by a constant force, the harvesting system is inclined to muffle mechanical vibrations. In this case, a new optimum set can be defined for maximal energy scavenging as described in [25]. The global efficiency of system (PDC /Pmech ) is shown in Fig. 11 and presents a maximal value of 20.2% for the optimal load.
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Fig. 12. Comparison of main electronics for harvesting energy considering the same magnetic element and strong vibrations.
VI. D ISCUSSION In MEMS applications, harvesting energy is promising for the development of autonomous systems (sensors, transmitter, etc.). Due to their size, the natural vibration frequency is naturally high, and one needs bulky modal mass and compliant mechanism to limit the frequency [36], [37]. As well, the energy per cycle is weak and with actual coupling factors (which represents the ability of the system to convert mechanical energy to electrical energy), the output voltage is often lower than 1 V particularly with microscale active material. For power harvesting, the energy per cycle and the frequency are both important because their product defines the available power within the system. For low-level vibrations, energy harvesting requires the reduction of electronic threshold voltage impact. Only in the case of large and strong vibrations and with good coupled systems, the voltage generated by PEGs can afford neglecting the effect of threshold voltage of semiconductor and does not require a transformer. On Fig. 12, ambient energy scavenging ability is compared for several main methods (SECE,2 series and parallel SSHI, Standard method described in [25]). The output power was computed for each of them considering strong vibrations and the same experimental conditions (inversion factor, piezoelectric coefficients, and vibration magnitude) as in this paper. Results are normalized with the power value obtained with the standard method. The SSHIMR technique appears to be equivalent to SSHI but allows tuning the optimal load trough the transformer ratio. The results highlight that efficient electronics need a magnetic element to extract and convert energy. Without such an element, a part of the generated current is used every half-period to reverse the clamped capacitance voltage and, therefore, cannot be scavenged. Both series SSHI and SSHI-MR could extract three times more energy than standard method. SECE methods present lower performances but are independent of the load. 2 Synchronous Electric Charge Extraction (SECE) method consists of a flyback or buck–boost converter which is synchronized with the mechanical vibration. On each maximum or minimum of vibration, the available energy on the clamped capacitance is first transferred into a magnetic element and then to the load.
Parallel SSHI is the most efficient method but does not allow one to replace the inductor by a transformer. Moreover, when using nonlinear energy scavenging methods, the output power of the microgenerator strongly depends on the inductor quality factor. When dealing with strong vibration magnitude, the value is typically [0.6; 0.8], allowing a typical power gain of nine compared to the classical method. The SSHI-MR method thus allows scavenging energy with low piezovoltage. In the perspective of integration on a single substrate, the standard method is more easy to integrate than SSHI or SSHI-MR methods. However, it needs very low threshold voltages compared to the available piezovoltage which is tightly related to the intrinsic coupling factor. Nevertheless, high coupling coefficients are rather difficult to obtain when integrating the piezoelement. In opposite, nonlinear methods (SSHI and SECE) increase significantly the effective coupling factor and reduce the necessary active material volume but require hybrid magnetic components. Moreover, SSHI-MR method can be a suitable method in order to compensate the intrinsic low coupling factor of materials like AlN piezomaterials which are easily deposited by sputtering and present less dielectric losses compared to PZT thin films. VII. C ONCLUSION This paper provides new insights in energy harvesting using integrated microgenerators such as MEMS devices. Taking advantage of magnetic coupling through an electrical transformer, the proposed method, called SSHI-MR, allows a dramatic reduction of the energy losses in discrete components such as diodes by the use of an original nonlinear processing. Experimental measurements carried out on a simple disk bender structure excited at low vibration levels confirm theoretical predictions and demonstrate the outstanding performances of the SSHI-MR under low output voltage levels, allowing a gain greater than 50 compared to classical energy harvesting methods. As well, this method is very promising for energy harvesting at high mechanical frequencies, where displacements are very low but present high power capabilities. R EFERENCES [1] J. M. Kahn, R. H. Katz, and K. S. J. Pister, “Next century challenges: Mobile networking for smart dust,” in Proc. Mobicom, 1999, pp. 483–492. [2] J. Krikke, “Sunrise for energy harvesting products,” Pervasive Comput., vol. 4, no. 1, pp. 4–8, Jan.–Mar. 2005. [3] J. A. Paradiso and T. Starner, “Energy scavenging for mobile and wireless electronics,” Pervasive Comput., vol. 4, no. 1, pp. 18–27, Jan.–Mar. 2005. [4] N. G. Elvin, A. A. Elvin, and M. Spector, “A self-powered mechanical strain energy sensor,” Smart Mater. Struct., vol. 10, no. 2, pp. 293–299, Apr. 2001. [5] N. Elvin, A. Elvin, and D. H. Choi, “A self-powered damage detection sensor,” J. Strain Anal., vol. 38, no. 2, pp. 115–124, 2003. [6] T. H. Ng and W. H. Liao, “Sensitivity analysis and energy harvesting for a self-powered piezoelectric sensor,” J. Intell. Mater. Syst. Struct., vol. 16, no. 10, pp. 785–797, 2005. [7] D. Guyomar, Y. Jayet, L. Petit, E. Lefeuvre, T. Monnier, C. Richard, and M. Lallart, “Synchronized switch harvesting applied to selfpowered smart systems: Piezoactive microgenerators for autonomous wireless transmitters,” Sens. Actuators A, Phys., vol. 138, no. 1, pp. 151–160, Jul. 2007. [8] M. Lallart, D. Guyomar, Y. Jayet, L. Petit, E. Lefeuvre, T. Monnier, P. Guy, and C. Richard, “Synchronized switch harvesting applied to selfpowered smart systems: Piezoactive microgenerators for autonomous
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[32] S. C. O. Mathuna, T. O’Donnell, W. Ningning, and K. Rinne, “Magnetics on silicon: An enabling technology for power supply on chip,” IEEE Trans. Power Electron., vol. 20, no. 3, pp. 585–592, May 2005. [33] Y. Fukuda, T. Inoue, T. Mizoguchi, S. Yatabe, and Y. Tachi, “Planar inductor with ferrite layers for DC–DC converter,” IEEE Trans. Magn., vol. 39, no. 4, pp. 2057–2061, Jul. 2003. [34] J. Lu, H. Jia, A. Arias, X. Gong, and Z. J. Shen, “On-chip bondwire transformers for power SOC applications,” in Proc. 23rd Annu. IEEE APEC, Feb. 24–28, 2008, pp. 199–204. [35] Z. J. Shen, J. Lu, X. Cheng, H. Jia, and X. Gong, “On-chip bondwire inductor with ferrite-epoxy coating: A cost-effective approach to realize power systems on chip,” in Proc. IEEE PESC, 2007, pp. 1599–1604. [36] D. Shen, J. H. Park, J. Ajitsaria, S. Y. Choe, and H. C. Wikle, “The design, fabrication and evaluation of a MEMS PZT cantilever with an integrated Si proof mass for vibration energy harvesting,” J. Micromech. Microeng., vol. 18, no. 5, p. 055 017, May 2008. [37] M. Marzencki, Y. Ammar, and S. Basrour, “Design, fabrication and characterization of a piezoelectric microgenerator including a power management circuit,” in Proc. DTIP MEMS NOEMS, 2005, pp. 299–302.
Lauric Garbuio was born in France in 1976. He received the B.S. and M.S. degrees from the Ecole Normale Supérieure de Cachan, Cachan, France, and the Ph.D. degree in electrical engineering from the Institut National Polytechnique de Toulouse, France, in 2006. Since 2007, he has been a Lecturer with the Electrical Engineering Department, Institut National des Sciences Appliquées de Lyon and a member of the Laboratoire de Génie Électrique et Ferroélectricité, Villeurbanne, France. His research interests are energy harvesting, piezoelectric actuators, and multiphysic interactions.
Mickaël Lallart was born in France, in 1983. He received the M.S. degree in electrical engineering from the Institut National des Sciences Appliquées de Lyon, Lyon, France, in 2006, where he is currently working toward the Ph.D. degree in electrical engineering in the Laboratoire de Génie Électrique et Ferroélectricité. He also spent one year as an exchange student at Trinity College Dublin, Dublin, Ireland, where he was with the Centre for Telecommunication Valuechain Research. His current field of interest focuses on vibration damping, energy harvesting, and Structural Health Monitoring using piezoelectric devices, as well as autonomous wireless systems.
Daniel Guyomar received the Master’s degree in mechanical engineering and the Ph.D. degree in acoustics and vibration from Compiègne University, Compiègne, France, and the Ph.D. degree in physics from Paris VII University, Paris, France. In 1982–1983, he was a Research Associate in fluid dynamics with the University of Southern California, Los Angeles. In 1985, he was hired by the Schlumberger group to lead several projects dealing with ultrasonic imaging. He then moved to Thomson Submarine Activities in 1987 to manage research activities in the field of physical underwater acoustics. He cocreated two startups involved in ultrasonic devices. He is currently a full-time Professor with the Institut National des Sciences Appliquées de Lyon, France, where he manages the Electrical Engineering and Ferroelectricity Laboratory (Laboratoire de Génie Électrique et Ferroélectricité). His current research interests are in the field of smart materials and systems: semiactive vibration control, wave control, energy harvesting, piezotransformers, electroactive materials, and nonlinear/hysteretic modeling of these materials. Dr. Guyomar was a National Research Council Awardee from 1983 to 1984 at the Monterey Naval Postgraduate School, Monterey, CA, to develop transient wave radiation modeling.
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Claude Richard was born in 1965. He received the M.S. degree in electrical engineering and the Ph.D. degree in acoustics, focusing on piezoelectric composite hydrophones, from the Institut National des Sciences Appliquées (INSA) de Lyon, Lyon, France, in 1988 and 1992, respectively. After postdoctoral study on 1-3 piezocomposite materials with the Underwater Sound Reference Detachment, Naval Research Laboratory, Orlando, FL, in 1993, he took a position at INSA where he has been working on applications of piezoelectric ceramics since 1994. He is currently the Head Director of the Electrical Engineering Department, INSA, where he also manages the Electroactive Systems Team of the Laboratoire de Génie Électrique et Ferroélectricité. His research interests include nonlinear switched piezodevices for vibration damping and energy harvesting, 1-3 piezocomposites (dice and fill, and piezofiber composite types), power applications of piezoelectrics, and material characterization.
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David Audigier was born in France in 1966. He received the B.S. degree in electrical engineering from Lyon I University, Lyon, France, in 1988, and the M.S. and Ph.D. degrees in electrical engineering from the Institut National des Sciences Appliquées de Lyon (INSA de Lyon), Lyon, in 1990 and 1996, respectively. Since 1997, he has been an Assistant Professor with the INSA de Lyon, where he is a member of the Laboratoire de Génie Electrique et Ferroélectricité. His current research activities include piezoelectric systems, energy harvesting, vibration control and noise reduction, and characterization and power applications of piezoelectric materials.
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Mechanical Energy Harvester With Ultralow Threshold Rectification Based on SSHI Nonlinear Technique Lauric Garbuio, Mickaël Lallart, Daniel Guyomar, Claude Richard, and David Audigier
Abstract—Harvesting energy from ambient sources has become of great importance these last few years. This can be explained not only by advances in microlectronics and energy harvesting technologies, but also by a growing industrial demand in wireless autonomous devices. In this field, piezoelectric elements offer outstanding performances, thanks to their high power density that makes them suitable for integrated microgenerators. However, such a domain still offers challenges to the research community. Particularly, embedding piezoelectric inserts as MEMS components raises the issue of low voltage output. Classical energy harvesting interfaces that feature bridge rectifier suffer from threshold voltage introduced by such discrete components, therefore compromising their use in real-life applications. In this paper is presented a new energy harvesting circuit that operates with ultralow voltage output, by the use of a magnetic voltage rectifier that does not present significant voltage gap. Experimental measurements performed on a simple transducer confirm theoretical predictions, and show that the proposed architecture operates well even for low-level vibrations, outperforming all known energy interfaces. Particularly, it is theoretically and experimentally shown that such an interface provides a gain greater than 50 compared to classical energy harvesting structures. Index Terms—Energy harvesting, integrated devices, microgenerators, nonlinear processing, piezoelectric.
I. I NTRODUCTION
T
HE CONCEPT of mobile, wireless, and autonomous devices is no longer chimerical thanks to combined advances in ultralow power electronics and energy harvesting technologies. Such a research interest has also been greatly influenced by the increasing demand in autonomous sensors from the industrial and biomedical fields. In this domain, the concept of “Smart Dust” presented in [1] has been a first step for the conception of autonomous systems that can operate without any connections. Nowadays, the future of sensing is about to experience a technological rupture due to the convergence of the energy requirements of electronic devices and the power outputs of microgenerators [2], [3]. Although first prototypes of such devices that are fully powered up by harvested environmental energy have already been presented [4]–[9], a further Manuscript received April 30, 2008; revised January 14, 2009. First published February 6, 2009; current version published April 1, 2009. The authors are with the Laboratoire de Génie électrique et Ferroélectricité EA 682, Institut National des Sciences Appliquées de Lyon, Université de Lyon, 69621 Villeurbanne, France (e-mail: [email protected]; [email protected]; [email protected]; claude.richard@ insa-lyon.fr; [email protected]). Color versions of one or more figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2009.2014673
step still needs to be taken to make such systems fully operating when integrated into a structure. Environmental sources for ambient energy harvesting are numerous: magnetism [10], solar [11]–[13], thermal [14], mechanical [15]–[19]. . ., but the actual needs in terms of embedded devices require the active material to have high power densities. In this domain, piezoelectric materials are good candidates for energy scavenging from ambient vibrations that are one of the most commonly sources available [20], [21]. Particularly, it has been shown that applying original nonlinear processing can lead to a significative improvement of the output power of PEGs1 [22]–[25]. Such treatments consist in switching intermittently the piezoelement on a resonating electrical network for a very short time, therefore artificially increasing the voltage output and the coupling coefficient of the piezomaterial device. Among these interfaces, the so-called series Synchronized Switch Harvesting on Inductor (SSHI) allows a typical gain of eight in terms of harvested energy compared to the classical energy harvesting technique [24]–[26]. However, whatever the type of interface considered, the output voltage of the piezoelectric element requires to be rectified in order to ensure a proper charge flow in the storage battery or capacitor. Such a process is typically done using a diode bridge rectifier (or synchronous rectifier) that presents threshold voltage to be operative. This element is a particular issue for integrated systems [27] that has typical output voltages below this threshold voltage, making the energy scavenging process inefficient [9]. In order to tackle this drawback, this paper introduces a new technique for rectifying the piezoelectric voltage without significative voltage gap. Based on the so-called “series SSHI,” the proposed scheme features a magnetic rectifier that does not induce significative voltage gap when conducting, therefore allowing operations even for low voltage levels, and leading to the concept of SSHI with Magnetic Rectifier (SSHI-MR). It is shown both theoretically and experimentally that such an approach provides a significant gain (up to 50) when the electromechanical structure is excited with low-level vibrations (which is often the case for high-frequency systems for example). This paper is organized as follows. Section II briefly recalls standard and series SSHI technique operations and introduces the concept of the SSHI-MR. Section III provides theoretical developments for estimating the power outputs from such microgenerators. Section IV discusses the self-powering
1 Piezoelectric
Electrical Generators (PEGs).
0278-0046/$25.00 © 2009 IEEE
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GARBUIO et al.: MECHANICAL ENERGY HARVESTER WITH ULTRALOW THRESHOLD RECTIFICATION
Fig. 1.
Standard energy harvesting circuit and typical waveforms.
issues of the proposed technique for real-life applications. Section V aims at validating the previously exposed analysis through experimental measurements. This section also points out specificities of the energy harvesting circuit, particularly for the SSHI-MR, whose performances will be discussed in Section VI. Finally, Section VII briefly concludes this paper. II. E NERGY H ARVESTING B ASICS AND SSHI-MR P RINCIPLES This section aims at qualitatively exposing the general principles that lie behind three energy harvesting processes from the piezoelectric effect. The first one is the widely used standard energy harvesting interface that simply consists in directly connecting the piezoelement to a diode bridge rectifier connected to a storage capacitor and the load. The second includes a digital switch in series with the piezoelement and the rectifier, and performs a nonlinear treatment in order to increase the energy output of the microgenerator. Finally, the concept of the SSHI-MR is introduced and explained. A. Standard Interface The standard energy scavenging circuit as well as associated waveforms are shown in Fig. 1. When the absolute value of the piezoelectric voltage is less than the rectified voltage VDC + 2VD , the diode bridge rectifier is blocked and the piezoelectric element is let in open circuit, making the piezoelectric voltage to vary with the displacement. When the absolute value of the piezoelectric voltage equals the rectified voltage VDC + 2VD , the bridge rectifier conducts and a charge flow appears from the piezoelectric element to the storage capacitor CS . This energy extraction process terminates when the current cancels, which appears on displacement extrema.
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Fig. 2. Series SSHI energy harvesting circuit and typical waveforms.
B. Series SSHI Interface In the case of the series SSHI, two unipolar digital switches in series with an inductor are placed in series with the piezoelement and the rectifier as shown in Fig. 2. The switching event is synchronized with the electrostatic charges that are available on the piezoelectric element. When these charges are maximal (which actually occurs when the displacement or the piezoelectric voltage reaches an extremum), the piezoelectric element is briefly connected to the circuit, therefore providing energy to the storage capacitor CS through the inductor L and the bridge rectifier. The switching time period ti corresponds to the half-period (1) of the electrical circuit shaped by the blocking capacitance C0 of the piezoelectric element and the inductor (the voltage VDC is assumed to be constant and CS C0 ). Such a process leads to an inversion of the piezoelectric voltage with respect to the voltage ±(VDC + 2VD ) (+VDC + 2VD for a positive switching and −VDC − 2VD for a negative switching). However, this inversion is not perfect and characterized by the coefficient γ defined as (2) where Qi is the electrical quality factor of the circuit ti = π LC0 (1) (2) γ = e−π/(2Qi ) . C. SSHI-MR Interface The schematic of the SSHI-MR is shown in Fig. 3. The associated waveforms of this technique are very similar to those obtained with the series SSHI. In the SSHI-MR technique, the inductive element L is replaced by a transformer T with two primary windings and one secondary winding. The coupling factor m is chosen to be far greater than one. Each primary coil is mounted in series with one unidirectional switch (S1 or S1 )
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A. Standard Interface
Fig. 3. SSHI-MR energy harvesting circuit.
but with inverse polarity. The secondary winding is connected to the smoothing capacitance CS in parallel with the load RL through a diode D that ensures a proper charge flow. When the voltage or the displacement is maximal (respectively, minimal), the switch S1 (respectively, S1 ) is closed. In effect, a resonance occurs between the clamped capacitance C0 and the primary leakage inductance of transformer through the load. Like the SSHI technique, the switch is opened after one resonant halfperiod for the inversion of the piezoelectric voltage. The inversion is characterized by the inversion factor γ in the same way as the series SSHI inversion coefficient. This combination of the transformer T and the one diode rectifier D aims at converting the ac voltage to dc voltage, and also offers significant advantages. The diode threshold voltage is reduced to VD /m compared with the 2VD threshold of the standard and series SSHI cases. This allows energy harvesting with lower voltage levels. The rectifier power losses are in the standard and series SSHI techniques about 2VD IPZT whereas in SSHI-MR they are given by IPZT (VD /m). Moreover, as the current is very low, the losses in the transformer are negligible. As well, the transformer allows an increase of the output voltage, which is often desirable for the compatibility with electronic circuits. III. T HEORETICAL D EVELOPMENT In this section are presented the theoretical developments taking into account the effect of the threshold voltage of each diode (denoted VD ) for the previously exposed techniques. It is considered that the device is excited by a monochromatic driving force and exhibits a constant displacement magnitude uM . Such a consideration is well adapted to MEMS devices due to their frequency response band. For this purpose, a simple electric modeling of the electromechanical structure is used. This modeling consists in a current source proportional to the displacement velocity in parallel with the blocking capacitance C0 . It gives a simple but realistic model when the electromechanical system is excited near one of its resonance frequencies [28]. The piezoelectric microgenerator behavior can be described by (3) [29], where u is the flexural displacement of the structure, VC0 the piezoelectric voltage, and IPZT the current flowing out of the piezoelement. α and C0 are given as the force factor and the blocking capacitance of the piezoelectric element, respectively. Furthermore, it is supposed that the time constant RL CS is far greater than the vibrational period T0 (corresponding to the frequency f0 ), such as VDC could be considered as constant in steady state IPZT = αu˙ − C0 V˙ C0 .
(3)
In the case of energy harvesting using the standard technique, the expression of the output power yields (4), with uM the magnitude of vibration and u1 the value of the displacement when the energy transfer process starts. The value of u1 is derived considering the end of the last conduction period, yielding (5) considering that the last conduction period is for negative values, with VD the threshold voltage for a single diode. Therefore, the expression of the power (4) leads to (6). Considering the expression of the power (7) as a function of the rectified voltage VDC and the load resistance RL leads to (8) T0
Pstandard
2 = T0
2
VDC IPZT
uM du
2 = αVDC T0
(4)
u1
0
C0 (VDC + 2VD ) + K u1 = α C0 VDC + 2VD ) − uM with K = α 4 Pstandard = VDC (αuM − C0 (VDC + 2VD ) T0 V2 Pstandard = DC RL 16f02 RL (αuM − 2C0 VD )2 . Pstandard = (1 + 4f0 RL C0 )2
(5) (6) (7) (8)
B. Series SSHI Interface In the case of the series SSHI, the expression of the power gives (9), with VM and Vm the absolute values of the piezovoltage just before and after the inversion process, respectively. The expression of these two voltages can be obtained considering the switching process and the open-circuit condition between two consecutive inversion processes that leads to the relationships (10) and, therefore, to the values of the voltages VM and Vm as (11). Moreover, considering the expression of the output power (12) therefore yields the expression of the power as a function of the load resistance (13) T0
Pseries
2 = T0
2
VDC IPZT
2 = − C0 VDC T0
0
−V m
dV VM
= 2f0 C0 VDC (VM + Vm ) Vm + (VDC + 2VD ) = γ (VM (VDC + 2VD )) VM Vm = 2 Cα0 uM ⎧ ⎨ VM = 1 2 α uM − (1 + γ)(VDC + 2VD ) 1−γ C0 ⎩ Vm = 1 2γ α uM − (1 + γ)(VDC + 2VD ) 1−γ C0 Pseries = Pseries =
2 VDC RL
(9) (10)
(11)
(12)
16(1 + γ)2 f02 RL (αuM − 2C0 VD )2 . [(1 − γ) + 4(1 + γ)f0 RL C0 ]2 (13)
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TABLE I OPTIMAL VALUES
Compared to theoretical developments that do not take into account diode threshold voltages [25], it can be seen that the value of αuM is changed to αuM − C0 VD in the case of the standard harvesting technique and the series SSHI.
C. SSHI-MR Interface In the case of the SSHI-MR technique, the harvested energy into the load can therefore be written as (14), with VM and Vm the absolute values of the voltage just before and after the inversion process, respectively, T0
PSSHI−MR =
2 T0
2
VDC IPZT dt m
0 −V m
VDC = −2f0 C0 m
dV
Fig. 4. Experimental test structure.
VM
= 2f0 C0
VDC (VM + Vm ). m
(14)
The considerations of the switching event and the opencircuit condition that arises between two switching processes lead to the relationships on VM and Vm given by (15) (with uM the vibration magnitude) that therefore gives the expressions of VM and Vm as (16)
Vm + VDCm+VD = γ VM − VM − Vm = 2 Cα0 uM
VDC +VD m
VDC +VD 2α 1 +C UM VM = − 1+γ 1−γ m 0 1−γ 1+γ VDC +VD 2α γ Vm = − 1−γ + U m C0 1−γ M .
(15)
(16)
Considering (14) and (16), as well as the output power (17) therefore leads to the expression of the output power as a function of the load resistance RL as (18). It can be seen from this expression that the voltage gap VD of the diode is divided by a factor m, making the threshold voltage to tend to zero for high values of m PSSHI−MR PSSHI−MR
V2 = DC RL
(17)
2 RL 2 2 αuM − C0 VmD 16 m 2 f0 (1 + γ) =
2 . (18) RL (1 − γ) + 4f0 C0 m 2 (1 + γ)
D. Optimal Performance By derivation of (8), (13), and (18), it can be seen that the output power of each technique reaches a maximal value for an optimal load resistance Ropt which corresponds to an optimal output voltage VDCopt , whose values are included in Table I.
IV. S ELF -P OWERING I SSUES In the proposed method, the nonlinear treatment performed by the SSHI-MR necessitates digital switches to operate. For this paper, both unidirectional switch command is generated using a DSP (DSpace RTI 1104 connected to a computer) which turns on S1 when piezovoltage reaches a maximum and during a specific time corresponding to a half-period of resonant inversion circuit (and, respectively, S1 on each minimum). Stand-alone operations of microgenerators are a key point for real applications and good operations of these switches require a given energy amount. However, this energy is limited and can be supplied by the piezotransducer. Particularly, the selfpowered version of the SSHI-MR can use, for example, the autonomous breaker that is described in [30]. This device typically consumes less than 5% of the electrostatic energy available on the piezoelectric material, therefore allowing the conception of a truly stand-alone SSHI-MR. V. E XPERIMENTAL V ALIDATION AND D ISCUSSION A. Experimental Setup This section aims at validating the previous theoretical development through measurements carried out on a commercial disk bender (Buzzer KPSG100, KINGSTATE) with an additional seismic mass as shown in Fig. 4. This bimorph is made vibrating at one of its resonance frequencies f0 by the use of an electromagnet, and is connected to one of the three presented energy harvesting interfaces. The load value RL is also made varying in order to change the power output of the microgenerator. Preliminary measurements carried out on the structure for identification led to the parameters consigned in Table II according to the electromechanical equivalent circuit [31] shown in Fig. 5. The differences in the inversion coefficient between series SSHI and SSHI-MR can be explained by the fact that the extremum detection is better performed in the case of the SSHI-MR due to a higher piezoelectric voltage.
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TABLE II PARAMETER IDENTIFICATION
Fig. 6. SSHI-MR energy harvesting circuit with magnetization current recovery.
Fig. 5. Equivalent electromechanical circuit of piezoelement near a resonance frequency [31].
The magnetic core used for this paper, is an unoptimized common Fe-Si transformer whose parameters are given in Table II. In MEMS applications, development of power systems on chip is hindered by the difficulty of integration of magnetic passive components for frequency below 100 kHz because of their size [32], [33]. However, Lu et al. [34], [35] have shown a crafty process for on-chip inductors and transformers which consists in mounting on surface several parallel bondwires embedded in magnetic ferrite drop. They realized experimentally bondwire inductors of hundred nanohenrys with quality factor around 40. The hybridization of micro-PEGs with this technique is therefore promising for energy scavenging. The minimal inductance properties, respectively, defined by its value Linv and its magnetic surface S required for inversion can be estimated from energetic (19) and magnetic (20) considerations. For example, with a maximal induction Bmax = 0.5 T and a peak current Imax = 1 A, the minimal value of Linv is 0.9 μH corresponding to a required surface S of 1.8 mm2 and an inversion frequency of 300 kHz. Furthermore, multiplexing magnetic cores between several energy harvesters mounted in parallel would reduce the required magnetic material volume 1 1 2 C0 VC20 max = Linv Imax 2 2 Bmax S = Linv Imax .
(19) (20)
In order to improve the SSHI-MR efficiency, the part of energy extracted from the piezoelectric material for the magnetization of the magnetic core could be partly recovered instead of being dissipated. This could done by replacing the single diode D by a full rectifier bridge after the secondary windings
Fig. 7. Experimental results and theoretical predictions for a constant vibration magnitude of ∼23 μm.
as shown in Fig. 6. Therefore, the demagnetization current that appears just after the harvesting process, flows through the storage capacitor CS thus bringing additional charges to the storage stage. The use of a high transformer ratio m allows one to reduce the impact of the supplementary diode threshold voltage. B. Experimental Results The first set of measurements consists in making the test structure vibrating at its resonance frequency f0 with a constant vibration magnitude of approximately 23 μm and measuring the output power for several load resistance values. Results are shown in Fig. 7. These results show good agreement with theoretical predictions, and point out the performances of the SSHI-MR over the other methods when the structure vibration levels are very low. Particularly, the gain obtained with the SSHI-MR, in this case, is greater than 56 compared to the standard approach and 30 compared to the series SSHI. Furthermore, it can be noted that although the open-circuit voltage magnitude without any process is 598 mV, the SSHI-MR allows a voltage output of 4.51 V at the optimal load. The second set of measurements shown in Fig. 8 consists in measuring the maximal power output for each method considering several vibration magnitudes at the resonance frequency. Here also, a very good agreement between theoretical predictions and experimental results can be observed, validating the performances of the SSHI-MR, that can harvest up to
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Fig. 9. Experimental waveforms of the SSHI-MR technique for a 6-μm displacement. Fig. 8. Experimental results and theoretical predictions for the maximal harvested power as a function of the vibration magnitude.
400 μW by dramatically reducing the energy losses in the discrete components. Therefore, the technique starts harvesting energy efficiently for very low vibration magnitudes such as 1 μm, which corresponds to an open-circuit voltage of 26.6 mV (the equivalent voltage gap given by VD /m is equal to 10.5 mV), while the other techniques need at least 2VD (460 mV) before starting the harvesting process. Typical experimental waveforms are shown in Fig. 9 for a 6-μm displacement. In this case, the magnitude of the piezovoltage in open circuit is about 0.32 V (corresponding to VM − Vm ) which is lower than the threshold voltage 2VD of rectifier bridge and therefore prevents the use of the standard technique. With nonlinear treatment, the output voltage is increased to 0.79 V and allows energy harvesting. The energy distribution within the system is shown in Fig. 10. The output electrical energy EDC is measured experimentally and extracted energy from piezoelement EC0 is calculated according to (21) for one vibration cycle. The mechanical input energy Emech is estimated using (22)
EC0 = C0 VM2 − Vm2 T u˙ dt + α 0
(21)
T 2
Emech = c
Fig. 10. Experimental distribution of energy per vibration cycle.
VC0 udt ˙ Fig. 11. (Solid line) Experimental efficiency and (dashed line) output voltage for SSHI-MR method for a constant vibration magnitude of 23 μm.
0
2 = cπωUM + 2αUM (VM + Vm ).
(22)
The harvested energy EDC reaches a maximum for the optimal load resistance Ropt = 1.2 × 105 Ω as predicted by theoretical developments. It corresponds to the combining of two behaviors. First, the energy extracted from the piezoelement EC0 decreases with the load or equivalently with the output voltage VDC as it can be shown from (16) and (21). Second, for high values of the load resistance (i.e., high value of VDC as shown in Fig. 11), the threshold voltage of the rectifier bridge and the inversion losses become less significant and the extracted energy is fully transferred to the load. Therefore, an optimal load exists that maximizes the output power of the
harvester. The input mechanical energy Emech is constant for low harvested energy but with the increase of load resistance a feedback force appears on the piezoelectric element. This force is opposite to the mechanical displacement and acts as dry friction force. In this paper, a constant displacement magnitude is considered but if the system is excited by a constant force, the harvesting system is inclined to muffle mechanical vibrations. In this case, a new optimum set can be defined for maximal energy scavenging as described in [25]. The global efficiency of system (PDC /Pmech ) is shown in Fig. 11 and presents a maximal value of 20.2% for the optimal load.
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Fig. 12. Comparison of main electronics for harvesting energy considering the same magnetic element and strong vibrations.
VI. D ISCUSSION In MEMS applications, harvesting energy is promising for the development of autonomous systems (sensors, transmitter, etc.). Due to their size, the natural vibration frequency is naturally high, and one needs bulky modal mass and compliant mechanism to limit the frequency [36], [37]. As well, the energy per cycle is weak and with actual coupling factors (which represents the ability of the system to convert mechanical energy to electrical energy), the output voltage is often lower than 1 V particularly with microscale active material. For power harvesting, the energy per cycle and the frequency are both important because their product defines the available power within the system. For low-level vibrations, energy harvesting requires the reduction of electronic threshold voltage impact. Only in the case of large and strong vibrations and with good coupled systems, the voltage generated by PEGs can afford neglecting the effect of threshold voltage of semiconductor and does not require a transformer. On Fig. 12, ambient energy scavenging ability is compared for several main methods (SECE,2 series and parallel SSHI, Standard method described in [25]). The output power was computed for each of them considering strong vibrations and the same experimental conditions (inversion factor, piezoelectric coefficients, and vibration magnitude) as in this paper. Results are normalized with the power value obtained with the standard method. The SSHIMR technique appears to be equivalent to SSHI but allows tuning the optimal load trough the transformer ratio. The results highlight that efficient electronics need a magnetic element to extract and convert energy. Without such an element, a part of the generated current is used every half-period to reverse the clamped capacitance voltage and, therefore, cannot be scavenged. Both series SSHI and SSHI-MR could extract three times more energy than standard method. SECE methods present lower performances but are independent of the load. 2 Synchronous Electric Charge Extraction (SECE) method consists of a flyback or buck–boost converter which is synchronized with the mechanical vibration. On each maximum or minimum of vibration, the available energy on the clamped capacitance is first transferred into a magnetic element and then to the load.
Parallel SSHI is the most efficient method but does not allow one to replace the inductor by a transformer. Moreover, when using nonlinear energy scavenging methods, the output power of the microgenerator strongly depends on the inductor quality factor. When dealing with strong vibration magnitude, the value is typically [0.6; 0.8], allowing a typical power gain of nine compared to the classical method. The SSHI-MR method thus allows scavenging energy with low piezovoltage. In the perspective of integration on a single substrate, the standard method is more easy to integrate than SSHI or SSHI-MR methods. However, it needs very low threshold voltages compared to the available piezovoltage which is tightly related to the intrinsic coupling factor. Nevertheless, high coupling coefficients are rather difficult to obtain when integrating the piezoelement. In opposite, nonlinear methods (SSHI and SECE) increase significantly the effective coupling factor and reduce the necessary active material volume but require hybrid magnetic components. Moreover, SSHI-MR method can be a suitable method in order to compensate the intrinsic low coupling factor of materials like AlN piezomaterials which are easily deposited by sputtering and present less dielectric losses compared to PZT thin films. VII. C ONCLUSION This paper provides new insights in energy harvesting using integrated microgenerators such as MEMS devices. Taking advantage of magnetic coupling through an electrical transformer, the proposed method, called SSHI-MR, allows a dramatic reduction of the energy losses in discrete components such as diodes by the use of an original nonlinear processing. Experimental measurements carried out on a simple disk bender structure excited at low vibration levels confirm theoretical predictions and demonstrate the outstanding performances of the SSHI-MR under low output voltage levels, allowing a gain greater than 50 compared to classical energy harvesting methods. As well, this method is very promising for energy harvesting at high mechanical frequencies, where displacements are very low but present high power capabilities. R EFERENCES [1] J. M. Kahn, R. H. Katz, and K. S. J. Pister, “Next century challenges: Mobile networking for smart dust,” in Proc. Mobicom, 1999, pp. 483–492. [2] J. Krikke, “Sunrise for energy harvesting products,” Pervasive Comput., vol. 4, no. 1, pp. 4–8, Jan.–Mar. 2005. [3] J. A. Paradiso and T. Starner, “Energy scavenging for mobile and wireless electronics,” Pervasive Comput., vol. 4, no. 1, pp. 18–27, Jan.–Mar. 2005. [4] N. G. Elvin, A. A. Elvin, and M. Spector, “A self-powered mechanical strain energy sensor,” Smart Mater. Struct., vol. 10, no. 2, pp. 293–299, Apr. 2001. [5] N. Elvin, A. Elvin, and D. H. Choi, “A self-powered damage detection sensor,” J. Strain Anal., vol. 38, no. 2, pp. 115–124, 2003. [6] T. H. Ng and W. H. Liao, “Sensitivity analysis and energy harvesting for a self-powered piezoelectric sensor,” J. Intell. Mater. Syst. Struct., vol. 16, no. 10, pp. 785–797, 2005. [7] D. Guyomar, Y. Jayet, L. Petit, E. Lefeuvre, T. Monnier, C. Richard, and M. Lallart, “Synchronized switch harvesting applied to selfpowered smart systems: Piezoactive microgenerators for autonomous wireless transmitters,” Sens. Actuators A, Phys., vol. 138, no. 1, pp. 151–160, Jul. 2007. [8] M. Lallart, D. Guyomar, Y. Jayet, L. Petit, E. Lefeuvre, T. Monnier, P. Guy, and C. Richard, “Synchronized switch harvesting applied to selfpowered smart systems: Piezoactive microgenerators for autonomous
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GARBUIO et al.: MECHANICAL ENERGY HARVESTER WITH ULTRALOW THRESHOLD RECTIFICATION
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Lauric Garbuio was born in France in 1976. He received the B.S. and M.S. degrees from the Ecole Normale Supérieure de Cachan, Cachan, France, and the Ph.D. degree in electrical engineering from the Institut National Polytechnique de Toulouse, France, in 2006. Since 2007, he has been a Lecturer with the Electrical Engineering Department, Institut National des Sciences Appliquées de Lyon and a member of the Laboratoire de Génie Électrique et Ferroélectricité, Villeurbanne, France. His research interests are energy harvesting, piezoelectric actuators, and multiphysic interactions.
Mickaël Lallart was born in France, in 1983. He received the M.S. degree in electrical engineering from the Institut National des Sciences Appliquées de Lyon, Lyon, France, in 2006, where he is currently working toward the Ph.D. degree in electrical engineering in the Laboratoire de Génie Électrique et Ferroélectricité. He also spent one year as an exchange student at Trinity College Dublin, Dublin, Ireland, where he was with the Centre for Telecommunication Valuechain Research. His current field of interest focuses on vibration damping, energy harvesting, and Structural Health Monitoring using piezoelectric devices, as well as autonomous wireless systems.
Daniel Guyomar received the Master’s degree in mechanical engineering and the Ph.D. degree in acoustics and vibration from Compiègne University, Compiègne, France, and the Ph.D. degree in physics from Paris VII University, Paris, France. In 1982–1983, he was a Research Associate in fluid dynamics with the University of Southern California, Los Angeles. In 1985, he was hired by the Schlumberger group to lead several projects dealing with ultrasonic imaging. He then moved to Thomson Submarine Activities in 1987 to manage research activities in the field of physical underwater acoustics. He cocreated two startups involved in ultrasonic devices. He is currently a full-time Professor with the Institut National des Sciences Appliquées de Lyon, France, where he manages the Electrical Engineering and Ferroelectricity Laboratory (Laboratoire de Génie Électrique et Ferroélectricité). His current research interests are in the field of smart materials and systems: semiactive vibration control, wave control, energy harvesting, piezotransformers, electroactive materials, and nonlinear/hysteretic modeling of these materials. Dr. Guyomar was a National Research Council Awardee from 1983 to 1984 at the Monterey Naval Postgraduate School, Monterey, CA, to develop transient wave radiation modeling.
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Claude Richard was born in 1965. He received the M.S. degree in electrical engineering and the Ph.D. degree in acoustics, focusing on piezoelectric composite hydrophones, from the Institut National des Sciences Appliquées (INSA) de Lyon, Lyon, France, in 1988 and 1992, respectively. After postdoctoral study on 1-3 piezocomposite materials with the Underwater Sound Reference Detachment, Naval Research Laboratory, Orlando, FL, in 1993, he took a position at INSA where he has been working on applications of piezoelectric ceramics since 1994. He is currently the Head Director of the Electrical Engineering Department, INSA, where he also manages the Electroactive Systems Team of the Laboratoire de Génie Électrique et Ferroélectricité. His research interests include nonlinear switched piezodevices for vibration damping and energy harvesting, 1-3 piezocomposites (dice and fill, and piezofiber composite types), power applications of piezoelectrics, and material characterization.
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David Audigier was born in France in 1966. He received the B.S. degree in electrical engineering from Lyon I University, Lyon, France, in 1988, and the M.S. and Ph.D. degrees in electrical engineering from the Institut National des Sciences Appliquées de Lyon (INSA de Lyon), Lyon, in 1990 and 1996, respectively. Since 1997, he has been an Assistant Professor with the INSA de Lyon, where he is a member of the Laboratoire de Génie Electrique et Ferroélectricité. His current research activities include piezoelectric systems, energy harvesting, vibration control and noise reduction, and characterization and power applications of piezoelectric materials.
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