Dynamic Switching Conversion for Piezoelectric Energy Harvesting Systems Aldo Romani, Cinzia Tamburini, Alessandro Golfarelli, Rossano Codeluppi, Enrico Sangiorgi, Marco Tartagni
Rudi Paolo Paganelli IEIIT-CNR National Research Council Bologna, Italy
[email protected]
ARCES-LYRAS, II School of Engineering University of Bologna Forlì, Italy {aromani, mtartagni}@arces.unibo.it
non-linear circuital elements (such as, for example, diodes, switches, etc.), which are normally used in power converters. This problem is overcome by the use of a circuital approach. In addition, time and frequency domain analyses of the whole system can be performed as well. The equivalent electromechanical circuit shown in Fig. 2 was used for simulating the piezoelectric bender. The mechanical part is described in terms of the mechanical stress σ and the derivative of the strain dδ/dt. Each lumped element takes into account different physical quantities: σin, Lm, Rm, Cm are respectively related through (σ, δ) to input vibrations strength, kinetic energy, mechanical losses, elastic energy whereas CP is the electrical capacitance of the piezoelectric element. The transformer models the piezoelectric effect. The lumped parameters values were characterized experimentally.
Abstract—The current advances in ultra-low power design let foresee great opportunities in energy harvesting platforms for self-powered systems. This paper presents a switching conversion scheme based on active control for harvesting energy with a higher efficiency than traditional approaches. The approach has been validated for piezoelectric energy harvesters with mixedsignal circuital simulations of non-linear equivalent electromechanical systems and a prototype has been developed. The proposed switching converter may increase harvested power of about 30% with respect to solutions based on traditional rectifiers and is expected to achieve up to 755μW in a train passenger car running at about 100km/h with a 32×6mm2 cantilever with a 35g mass attached at the free end.
I. INTRODUCTION Electronics is deeply penetrating into human life: pervasive computing and wireless sensor networks are introducing their potential while power consumption has been greatly reduced thanks to energy-aware design techniques. Even though, power supplies still mainly rely on electrochemical cells with limited stored charge and often unpractical to replace. However, the environment is an intrinsic source of low-density highlyavailable energy [1] in either steady or intermittent and irregular forms such as, for example, vibrations. Traditional converters based on rectifiers do not operate efficiently with such kind of sources: threshold voltages are often comparable with those to be converted and losses are usually not neglectable. For these reasons the use of analog switches and an active digital control can provide better performance. This paper presents a digitally controlled switch-based conversion scheme and compares it in terms of harvested power to traditional solutions.
II.
In order to provide reliable power estimates, the amplitude of input vibrations was recorded in specific cases of interest
Figure 1. Structure of a conventional 2-layers piezoelectric mass-cantilever energy harvester. A commercial 28×6×0.5mm3 PZT-5H bender was considered for validating the presented approach.
SYSTEM MODELLING
Without lack of generality, a piezoelectric mass-cantilever harvester was considered as a case study, as shown in Fig. 1. Physical modelling of piezoelectric materials is usually achieved with finite-element methods by coupling together the constitutive equations of piezoelectricity, Maxwell’s equations, and the dynamics of the structure. However, this approach makes it difficult to evaluate the operation of the system with
Figure 2. Equivalent electro-mechanical circuit model of a piezoelectric mass-cantilever system. σ is the average mechanical stress and δ the average strain on the cantilever system. Vp and I are the developed voltage and current at the electrical port.
This research is funded by Eurotech Group (http://www.eurotech.com)
1-4244-2581-5/08/$20.00 ©2008 IEEE
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IEEE SENSORS 2008 Conference
Figure 3. Amplitude spectrum of vibrations measured on the wall of a train passenger car during motion at about 100km/h. Accelerations were recorded for about 20s with a Kionix KXP-74 accelerometer and 1kHz sampling rate. aRMS = 0.24g = 2.37 m/s2, g = 9.81 m/s2
Figure 5. Simulation results of available output power from vibrations shown in Fig. 3 with a diode rectifier bridge. The load is an ideal resistor while the diode bridge is composed of 1N4148 devices and cantilever mass is 35g.
a specific resistance value. This scheme is not efficient with real systems, where the load operates discountinously (e.g. depending on its state, such as for example a microprocessor based system or a wireless sensor network node) and vibrations occur randomly. For this reason the maximum value of the curve only represents an upper bound which can be achieved by the system.
Figure 4. A conventional scheme based on a diode rectifier bridge for energy harvesting.
Inductors can be efficiently used to convey energy [2]. Furthermore, analog switches overcome the main limitations of a diode: there is no threshold voltage and on-resistance is usually very low. Fig. 6 shows the proposed switch-based solution for harvesting piezoelectricity. It consists of three pairs of switches, an inductor and a storage capacitor. A custom control circuit, which is required to drive the switches, can operate in real-time because of the low frequency of vibrations.
with a Kionix KXP-74 accelerometer. Fig. 3 shows the results obtained when the accelerometer was bound to the wall of a train passenger car and acquired data during motion at about 100 km/h.
The operation of the circuit is shown in Fig. 6 and described herein. During normal operation the piezo element is disconnected from the electrical part of the circuit (configuration t1) and Vp is a time-varying signal directly related to the amplitude of vibrations. Vp is monitored by the control system in real time, due to the relatively low frequency of vibrations. The energy stored on the piezo input capacitor is 1/2 CpVp2. When Vp>0 and a local maximum [Vp Cp, it is also mandatory, for achieving better energy efficiency, that ω0,3 τ >> 1. This results in a constraint for properly sizing the inductor value depending on the parasitic resistance values and the storage capacitor required by the application: L >> RPAR2 Cstore
Figure 7. Simulation results of available output power from vibrations in Fig.3 with the proposed scheme. Simulations were performed with the equivalent circuit of the piezo transducer and different mass values, Cstore = 1μF, L = 10mH and models of ADG836 switches from Analog Devices.
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Figure 9. Oscilloscope waveforms during operation of the power converter. When a maximum of charge is detected (Vp), it is transferred to the storage capacitor (Vo). Switch pairs are activated by Vt2, Vt3 according to the timings Δt2 and Δt3 computed by the control unit.
Figure 8. The developed prototype. A Microchip PIC18F1330 microcontroller, Maxim MAX4590 and MAX4662 analog switches and a 10mH Schaffner inductor were used.
jointly optimize mechanical and electrical parameters. Measurements were taken in both time and frequency domains in order to obtain realistic values of the lumped parameters used in the equivalent electro-mechanical circuit as well as of the amplitude of vibrations in specific cases of interest. This approach allowed to use a circuital simulator for reliably estimating the available output power with different conversion circuits.
TABLE I. AVAILABLE OUTPUT POWER FOR THE PROPOSED CONVERSION SCHEME AS A FUNCTION OF CANTILEVER MASS Cantilever mass value [g] 15
Average output power* [μW] 279
25 35 45
701 755 685
55 65
631 603
75
The proposed circuit, which tracks low frequency vibrations and activates conversion when maxima of energy are detected offers about 30% more available power with respect to traditional solutions based on diode rectifiers. This available power budget is adequate for supplying the active control unit. This scheme is suitable for storing energy in either capacitors or electrochemical cells (e.g. common button batteries) and for being implemented in CMOS technology.
585 *
after 3s of operation
optimal load resistance, which is a non-realistic best case. On the other hand, our scheme can extract 755μW (about 32% more) from the same input vibrations with no further load specifications. However, part of this power is required to run the control unit for monitoring voltages and driving switches. A low power CMOS design of the control unit is expected to consume just a small part of the available power budget. The operating frequency would be low, as it is correlated to the frequency spectrum of vibrations. Furthermore, detecting maxima and minima and activating switches would require circuits composed of few transistors. Recently, power management circuits consuming just few μW have been implemented and reported [3].
ACKNOWLEDGMENT This work was supported by Eurotech Group as part of the research project “Self-Powered Portable and Wireless Electronic Systems” carried out jointly with University of Bologna. The authors also thank M. Magi of ARCESUniversity of Bologna for his support in prototype fabrication and testing. REFERENCES
A prototype has been developed and preliminary test results proved functionality of the proposed approach, as shown in Fig. 8 and Fig. 9.
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[2]
V.
CONCLUSIONS [3]
A switch-based power conversion technique based on active control suitable for use with piezoelectric energy harvesters and other power sources has been presented. The circuital approach allowed to model the whole system and to
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