MEMS Energy Harvester

Presentation On

MEMS BASED ENERGY HARVESTING DEVICE

Presented by

Goutam RanaReg. No. 210710011

Exam Roll. 161007011

Under the guidance of

Dr.C.Roychaudhuri &

Prof. S.K.LahiriDept. of Electronics and Telecommunication Engg.

Bengal Engineering Science University, Shibpur

OUTLINE Introduction Background Limitations of the existing designs Objectives The device design Results & discussions Analytical estimation of voltage and current Estimated expression for power Figure of merit of the device Fabrication proposal of the device Flow diagram for sol-gel PZT synthesis Conclusion Scope for future work List of Publications

INTRODUCTION Research in the field of energy system has been

intensified over the last few decades mostly because the consumption has increased manifolds and the resources are drying up.

The later cause is popularizing the use of non-conventional energy sources alongside the conventional ones’.

Among many non-conventional energy sources MEMS(Micro Electro Mechanical Systems) harvesters are microstructures that taps mechanical energy from ambient vibration and convert it to equivalent electrical one.

Why MEMS energy harvester?

Self powered wireless sensors.

Small amplitude vibrations from environment can be converted to electrical energy.

Pollution free environment friendly energy source.

BACKGROUND The work thus far is mainly centered on different

structures like cantilever, doubly clamped beams, and tuning forks etc.

o Ref.1 “Fabrication and characterization of free-standing thick-film piezoelectric cantilevers for energy harvesting”, Meas. Sci. Technol. 20 (2009) 124010 (13pp).

These structures usually generates power in the order of few micro Watts, with resonating frequency around 100Hz to few KHz.

Most of these models are high Q small bandwidth structures, therefore not able to convert the practical vibration energy into electrical energy optimally.

oRef.2. “MEMS power generator with transverse mode thin film PZT”,

Sensors and Actuators A 122 (2005) 16–22. oRef.3“Comparison of piezoelectric energy harvesting devices for

recharging batteries” J. Mater. Sci., Mater. Electron. 16 799– 807 (2005)o Ref.4“Energy harvesting MEMS device based on thin film

piezoelectric cantilevers.”, J Electroceram (2006) 17:543–548

NONLINEARITY IN BENDING Hajati A. et al. showed that for doubly clamped

beams an amplitude dependent nonlinearity in stiffness causes the resonant frequency to stretch over a wide range.

Stretching vs. bending in doubly clamped beam section

Contd.

The presence of proof-mass in middle of the doubly clamped beam structure induces two strains viz. stretching and bending when subjected to a large deflection.

The pure bending require some lateral movement of the proof-mass, while the geometrical symmetry prevents such movements. This in turn results in a amplitude dependence stiffness nonlinearity.

Thus stretching strain predominates the bending strain.

Therefore, the devices resonant frequency tracks a wide range of frequency band.

Further, the paper has combined four such beams to limit the operating frequency.

Contd.

frequency response of the wide band resonator by Hajaiti A. et al

Ref. “Design and fabrication of nonlinear resonator for ultra wide band energy harvesting application, IEEE 24 conf. on MEMS, Cancun, MEXICO, January 23-27,pp 1301-1304 (2011).

LIMITATIONS OF THE EXSISTING

DESIGNS

The free standing cantilever though resonating on low frequencies are high Q low bandwidth and require large external force for vibration.

The micro-cantilevers usually offer high resonating frequency.

The doubly clamped micro-beams relies on nonlinearity therefore repeatability cannot be assured.

OBJECTIVE To reduce the resonating frequency as low as

possible.

To make the device responds to very low amplitude of force.

Maximize the area of vibration.

Introducing higher modes in close proximity by inserting number of proof-masses to widen the operating frequency range without entering into nonlinear region.

To find out a suitable piezoelectric material.

We designed and simulated a square membrane based resonator with different scheme of internal proof-mass.

The membrane is analyzed with 1,2,3,4 and 5 proof-masses.

A single external proof-mass arrangement is deployed over the membrane to achieve an initial loading or pre-stretched condition of the membrane. This will enable extraction of voltage even with small amplitude of vibration.

To avoid edge effect, the external loading is made cylindrical.

THE DEVICE DESIGN

Contd.

a) Cross-Sectional view of the proposed device.b) Top view of the lateral dimensions

o The square membrane has dimensions 10mm×10mm.

o Complete device with clamping region has dimensions 16mm×16mm.

o Different scheme of rectangular internal proof-mass dimension is analyzed.

o Tu supporting pillars each of dimension 0.5mm×0.5mm placed at center of each internal proof-mass

o External proof-mass is a Tu cylinder of radius 8mm

THE THICKNESS OF DIFFERENT

LAYERS

Name of the layer Name of the material Thickness of the layer (in μm)

Substrate (Base) Si 500

Membrane Thermal oxide 1.67

Si3N4 1.56

Piezo-electric PZT 0.53

Proof-mass(internal) Si 100

Proof-mass(external) Tu 3000

Pillars Tu 300

THE 3D SCHEMATIC OF THE

DEVICE IN COVENTORWARE10

(a)A 3D schematic back view of the device after design in COVENTORWARE10

(b) A 3D schematic view of the External proof-mass layer with supporting pillars after design in COVENTORWARE10

RESULTS & DISCUSSION Modal analysis:Modal analysis gives an idea about the resonating frequency of the device.The resonating frequency of the device decreases with

increase in membrane thickness, decrease in internal proof-mass thickness, increase in external proof-mass thickness, increase in membrane length.

o The results except the 2nd one can quite easily be explained with theory of vibration, based on the relation of resonant frequency with mass and stiffness of the vibrating system.

Contd.

o The reason behind the second phenomenon can probably be the dominance tension effect over the mass effect for distributing the proof masses in such way that the effect of one proof-mass is reduced in the presence of neighboring proof-masses due to the orthogonal placement.

Harmonic Analysis:Harmonic analysis is performed to find out an optimized response of the device with different scheme of internal proof-mass.

The harmonic response of the device with different scheme of internal proof masses

Contd.

Clearly, the device with four internal proof-masses resonates at lowest frequency, the displacement is also found to be maximum among all.

The harmonic response also depicts that the frequency is stretched with multiple closely placed peaks.

4 RECTANGULAR INTERNAL

PROOFMASS EACH OF 4×1mm2







Displacement Analysis:The displacement contour of device is found uniformly stretched over the entire membrane area.

(a)Displacement contour (membrane back with internal proof-mass)

(b)Displacement contour (membrane top face)

(a) (b)

Stress and Voltage Analysis: The stress contour found also having uniform

nature over the membrane. Therefore, the voltage is expected to be generated over the PZT surface uniformly.

Stress contour (on top PZT surface)

ANALYTICAL ESTIMATION OF VOLTAGE

AND CURRENTThe basic two piezoelectric equations

, and, , can used to find out the open circuit voltage and short circuit current of a piezoelectric device.

Here, S = Mechanical Strain (dimensionless) T= Mechanical Stress (Nm-2) E= Electric Field (Vm-1) D= Displacement Current Density (Cm-2) = Elastic Compliance at constant electric field (m2N-1) = Piezo-electric Charge Coefficient (CN-1) and,= Electric Permittivity at constant Stress (Fm-1).Contd.

With application of proper boundary conditions open circuit voltage can be calculated

Here, t=thickness of PZT layer (m), ==Piezo-electric voltage coefficient (VNm-1),and =constant with 0< <1 Similarly the short circuit current can be

estimated from the equation

where, = area under stress of the PZT,=conductivity of the PZT(mho-m), =radial frequency.

3D VOLTAGE CONTOUR PLOT

ESTIMATED EXPRESSION OF

POWER

Maximum estimated power can be found from the short circuit current and open circuit voltage expressions from maximum power transfer theorem

3D POWER CONTOUR PLOT

FIGURE OF MERIT OF THE

DEVICE

To find out an optimized response of the device a figure of merit is developed incorporating three parameters viz. maximum power estimated analytically, no of peaks in working frequency range and lower cutoff frequency.

FIGURE OF MERIT FOR 12 DEVICESInternal

proof-mass dimensions

in mm2

Gap between two consecutive proof-masses in mm

Resonating frequency

(in Hz)

No of peaks within 2

KHz

Estimated Power/area

(in Watts/cm2)

Figures of merit

2×4 0.5 370.04 3 0.301 2.44×10-3

1 463.96 3 0.417 2.70×10-3

2 720.5 3 0.814 3.25×10-3

1×4 0.5 420.3 4 0.360 2.57×10-3

1 344.17 4 0.274 3.16×10-3

2 527.29 3 0.509 2.90×10-3

1×3 0.5 273.59 4 0.209 3.06×10-3

1 333.35 4 0.263 3.16×10-3

2 399.35 3 0.334 2.51×10-3

3x2 0.5 378 4 0.598 3.28×10-3

1 372.8 3 0.304 2.45×10-3

2 563.24 3 0.567 3.02×10-3

FABRICATION PROPOSAL OF THE

DEVICE

Thermal oxidation to grow 1st layer of the membrane over

Si

Si3N4 growth as 2nd layer of membrane

1st layer of electrode grown

PZT layer grown over the membrane

2nd layer of electrode grown

Electrode patterning

Fresh oxidation

Lithography 1

Oxide removal

Fresh oxidation

Lithography 2

Oxide removal

Die bonding of external proof-mass

FLOW DIAGRAM FOR SOL-GEL PZT

SYNTHESIS

CONCLUSION

The resonating frequency of our devices lies in the range of 270-720Hz which satisfies the basic requirement of an energy harvester of resonating in low frequency.

With thin membrane the device will response to very small amplitude vibrations.

Different schemes of internal proof-mass orientation help to widen the bandwidth without entering into nonlinear region.

Using square membrane the effective area of PZT under stress increases also the internal impedance decreases.

SCOPE FOR FUTURE WORK

Once all the dimensions of the device is finalized the device can be fabricated and steps could be Sol gel PZT synthesis PZT layer deposition using spin on technique Fabrication of the membrane Polling of PZT layer Characterization of the PZT layer Electrical characterization of the device.

G. Rana, S. K. Lahiri, C. Roychaudhuri; “Design and analysis of a membrane based efficient wide band resonator for energy harvesting” International Conference on Smart material Structure & Systems, Jan 2012 Bangalore, India

G. Rana, S. K. Lahiri, C. Roychaudhuri; “Design Optimization of a wide band MEMS Resonator for Efficient Energy Harvesting” accepted in international symposium of VDAT-2012, to be published in LNCS,Springer

LIST OF PUBLICATIONS

THANK YOU

Documents

MEMS Energy Harvester