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A MEMS GENERATOR AND A POWER MANAGEMENT CIRCUIT M. Marzencki, Y. Ammar and S. Basrour MNS Group, TIMA Laboratory, Grenoble, FRANCE Tel: +33-4-76-57-43-04; E-mail: [email protected] TRANSDUCERS & EUROSENSORS ’07 The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007 Abstract: This paper presents a novel ambient energy scavenging system for powering wireless sensor nodes. It uses a MEMS generator and an ASIC power management circuit. The system is realised as a System On a Package (SoP) with all components fabricated entirely using the microfabrication techniques. The electromechanical transduction is performed using the piezoelectric effect of Aluminium Nitride thin films. The reported experimental results prove the possibility of exploiting very low amplitude signals delivered by the generator for charging a storage capacitor. It is also shown that the proposed system of 5mm3 can endlessly power a simple wireless sensor node; while a Lithium-Polymer thin film battery of the same volume can do so only for less than two months. Keywords: ambient energy scavenging, piezoelectric MEMS, ultra low power, autonomous micro system. efficiency, their application is limited to systems with relatively high power outputs because of the need of powering the extraction system itself and the use of standard diodes for rectification. Systems adapted for very low powers are inexistent partly because of the absence of micro vibration power harvesters. In this work we present a system that incorporates a MEMS power generator delivering very small powers (in the nW range) at voltages often inferior to 200mV. The custom power management circuit is used to charge a storage capacitor from which power is delivered for one cycle of operation of a very low power wireless sensor node. All the components of the system are created using CMOS compatible batch microfabrication techniques. We propose to implement the device as a System On a Package (SoP) in order to reduce its size and cost. 1. INTRODUCTION AND MOTIVATION The future development of wireless sensor networks is conditioned by the availability of small but long lasting power sources for its nodes. Ambient power harvesting is a possible breakthrough in this domain [1]. We explore the possibility of converting the energy of environmental mechanical vibrations into useful electrical energy by the means of the piezoelectric effect. The Fig. 1 presents a schematic of the energy scavenging system. Fig. 1 Schematic of the energy scavenging system with the MEMS piezoelectric generator, the custom ASIC containing the voltage multiplier and an energy storage unit. 2. MEMS GENERATOR The presented system incorporates a MEMS micro power generator (µPG) that uses the piezoelectric effect for converting the energy of ambient mechanical vibrations into useful electrical energy. The active piezoelectric material is a thin layer (1µm) of Aluminium Nitride (AlN), deposited on an SOI substrate. This material is inferior to PZT in the means of coupling coefficient but its deposition is relatively simple, compatible with microelectronics and It contains a piezoelectric generator (in our case a MEMS structure), a rectification and voltage elevation circuits and finally an energy storage unit. In recent years a number of researchers have proposed innovative solutions for power extraction from piezoelectric elements. These were either adaptive methods [2] or active synchronised methods [3]. In spite of their 887 1-4244-0842-3/07/$20.00©2007 IEEE 2EG7.P INTEGRATED POWER HARVESTING SYSTEM INCLUDING rectification. For this reason there is a need of using a special power management circuit. 3. POWER MANAGEMENT CIRCUIT TRANSDUCERS & EUROSENSORS ’07 The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007 The power management circuit consists of two elements: an AC/DC converter which rectifies the alternative signal delivered by the generator and a DC/DC converter adapting the levels of voltage to the storage element characteristics. The output voltage of the MEMS generator is often smaller than the threshold voltage of diodes used in power management circuits. To overcome this obstacle we propose novel low threshold diodes based on MOS transistors. These diodes are then used in the design of a voltage multiplier circuit (VM). Fig. 2 SEM image of the fabricated piezoelectric MEMS vibration energy scavenger. The proposed structures are the outcome of an optimisation process using the previously presented FEM and analytical models [4]. These are cantilever beams with seismic masses that measure 800µm by 1200µm of an SOI plate (525µm thick) and resonate at 1300Hz. In order to explore the performance of these devices, the output power was analysed on a matched resistive load at resonance for different acceleration levels. The results presented in the Fig. 3 show that power of 2µW at 1.6V can be obtained from one MEMS device at 4g excitation. 3.1. Low threshold voltage diodes The proposed diodes use DTMOS transistors (Dynamic Threshold voltage MOSFET). The idea of operation is based on the connection of the gate, the drain and the bulk of the transistor together in order to obtain diodes with low threshold voltage. PMOS transistors were chosen because of the facility of separating their bulks and therefore a possibility of varying their potentials. We used the 0.12µ HCMOS9 technology from STMicroelectronics to realize the circuit. The Fig. 4 presents the results of characterisation of such device, being an optimised transistor of W=5µm and L=300nm. For very low currents the value of the threshold voltage is inferior to 200mV. Fig. 3 Experimental voltage and power generated at resonance by one MEMS µPG dissipated on a matched resistive load versus the input acceleration amplitude. This output power is sufficiently high for many applications, but in reality the excitation levels are much lower and there is a necessity of signal Fig. 4 I(V) characteristic of the ultra low threshold voltage diode. 888 1-4244-0842-3/07/$20.00©2007 IEEE 2EG7.P does not require polarisation. The device, presented in the Fig. 2, is entirely made using microfabrication techniques. 4. EXPERIMENTAL RESULTS To characterize the created system, we used a controlled vibration source, a VM20 shaker from DataPhysics. The output voltage was observed through a very high impedance instrumentation amplifier (BurrBrown INA116) and recorded using a custom LabVIEW application. The Fig. 7 presents the open circuit output voltage of the system for different input acceleration amplitudes. TRANSDUCERS & EUROSENSORS ’07 The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007 Fig. 5 Conventional structure of 4 stage voltage multiplier (Villard structure). If we replace the standard diodes with the proposed low threshold voltage ones we obtain a voltage multiplier that can accept very low amplitude signals at the input. The efficiency of this multiplier is related to the amplitude of voltage provided by the microgenerator and the number of stages. We were limited to 1mm2 of the circuit surface and the fact that the maximum value of a single capacitor that can be realized in the technology used is equal to 40pF. We decided to implement 6 stages of the VM. Fig. 7 Experimental voltage output of the SoP system versus the input acceleration level. This experimental results show that a 1V output voltage can be obtained for very low excitation amplitude of about 50mg applied on the generator. The Fig. 8 presents the process of charging a 1µF capacitor connected to the output of our system for different input acceleration levels. The Fig. 6 presents the realised SoP containing the piezoelectric MEMS micro power generator and the voltage multiplier circuit. The total volume occupied by the assembly can be estimated at 1mm3. Fig. 6 Photograph of the proposed system containing the voltage multiplier circuit and the piezoelectric MEMS power generator. Fig. 8 Curves of charge of 1µF capacitor for different values of input acceleration amplitude. 889 1-4244-0842-3/07/$20.00©2007 IEEE 2EG7.P 3.2. Voltage Multiplier The Fig. 5 presents the structure of a voltage multiplier based on cascaded Villard voltage doublers [5]. It is used to rectify and raise the generated voltage at the same time. compatible with microelectronics and a novel power conditioning electronics adapted for ultra low voltage input. The experimental results show that one such system can produce power of almost 30nW at 3V from acceleration of 0.4g. We have shown that a storage capacitor can be efficiently charged even in case of very low input acceleration levels. It proves the usefulness of the proposed device as an ecological and very long lasting alternative for powering wireless sensor nodes. TRANSDUCERS & EUROSENSORS ’07 The 14th International Conference on Solid-State Sensors, Actuators and Microsystems, Lyon, France, June 10-14, 2007 The future work consists in using other piezoelectric materials and improving the effectiveness of the AC/DC and DC/DC converters for charging micro batteries or supercapacitors. REFERENCES [1] S. Roundy et al., “Energy Scavenging for Wireless Sensor Networks with Special Focus on Vibrations”, Kulwer Academic Publishers, 2004, I-4020-7663-0. Fig. 9 Instantaneous power transferred to a 1µF capacitor versus the capacitor voltage. [2] G.K. Ottman, H.F. Hoffman, G.A. Lesieutre, “Optimized Piezoelectric Energy Harvesting Circuit Using Step-Down Converter in Discontinuous mode”, IEEE Transactions of Power Electronics, Vol. 18, No. 2, pp. 696-703, 2003. We have analysed power consumption of a simple wireless sensor node containing a 4bit RISC microcontroller, a wireless transmitter as well as temperature and acceleration sensors. In case of a very low duty cycle operation (one action each 10 minutes), the average power needed to power the device is equal to 150nW. It means that with a very low input acceleration of 0.4g, five devices occupying a total volume of less than 5mm3 are sufficient. For comparison, a best thin film Lithium Polymer battery of the same volume [6] would be capable of producing this power only for less than two months. Our solution will provide this power as long as the ambient source exists. [3] E. Lefeuvre et al., “A comparison between several vibration-powered piezoelectric generators for standalone systems”, Sensors and Actuators A, Vol. 126, pp. 405-416, 2006. [4] M. Marzencki, S. Basrour, “Enhanced Models for Power Output Prediction from Resonant Piezoelectric Micro Power Generators”, in Proceedings of the Eurosensors XX Conference, Göteborg, pp. 130-131, 2006. [5] H. Yan et al., “An Integrated Scheme for RF Power Harvesting”, in Proceedings of SAFE 2005 Conference, Utrecht, pp. 64-66, 2005. 5. CONCLUSIONS AND FUTURE WORK This paper presents an innovative, totally integrated, ambient mechanical energy scavenging system. It contains piezoelectric MEMS devices manufactured using microfabrication techniques [6] R.Hahn et al., “Assembly of Wafer Level Secondary Batteries”, in Proceedings of PowerMEMS 2005 Conference, Tokyo, pp. 17-20, 2005. 890 1-4244-0842-3/07/$20.00©2007 IEEE 2EG7.P Finally the Fig. 9 shows the instantaneous power with which the system is charging a 1µF capacitor. It can be seen that the optimal point of operation changes with the input acceleration/voltage. For 0.4g the optimal region is situated between 1.5V and 3V in which power of over 26nW can be generated. The maximum output powers are similar to the ones obtained on a matched resistive load.