Download INTEGRATED POWER HARVESTING SYSTEM INCLUDING MNS Group, TIMA Laboratory, Grenoble, FRANCE

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
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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.
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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.
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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.
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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.