High-level modelling and performance optimisation of mixed-technology

energy harvester systems

Tom J Kazmierski, Leran Wang, Bashir M Al-Hashimi

University of Southampton, UK

MOS-AK, Edinburgh 19 September 2008

2

Energy harvesting in electronics

• State-of-the-art

• Energy harvesting from the environment

• Pervasive computing

3

Vibration-based energy harvester system

AC

Voltage Voltage

booster

• System consisting various components from different physical domains

– Reported equivalent circuit models are inadequate

– VHDL-AMS as modelling language

• Performance loss due to close mechanical-electrical interaction

– Systematic EH optimisation based on HDL model

EH modelling approachesmacro-generator models

4

- Ideal voltage source

- Equivalent circuit

Proposed accurate HDL model

5

- Use VHDL-AMS as modelling language- Describe micro-generator as a series of analytical equations- Mixed-technology (mechanical, magnetic, electrical etc.)

Case study design

• State-of-the-art EH (R. Torah, et.al, Development of a cantilever beam generator employing vibration energy harvesting. In Proceedings of The 6th Int. Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, 2006.)

•• VibrationVibration--basedbased

•• Electromagnetic microElectromagnetic micro--generatorgenerator

•• Voltage multiplier as boosterVoltage multiplier as booster

•• Super capacitor as storageSuper capacitor as storage

6

Experimental micro-generator

ElectromagneticCantilever basedFixed coil, moving magnetsSmall volume: 150 mm3

7

Micro-generator model

8

• (a)

• (b)

• (c)

B

-B

R

r

(b) Small displacement

z(t)

B

-B

R

r

z(t)

(c) Large displacement

NBtzrtzR **2*))()(( 2222 −+−=Φ

NBtzHrtzHR **)))(())((( 2222 −−+−−−=Φ

NBrR **2*)( +=Φ

Voltage booster

9

• Voltage multiplier as the voltage booster

– 6-stage Villard configuration

– Schottky diodes (BAT760)

0.22FD1 D2 D3 D4 D5 D6

C1

C2

C3

C4 C6

C5

Micro-

generator

Experimental setup

10

• Micro-generator sitting on vibration generator

– Frequency: 50Hz, acceleration level: 0.6 m/s2

• Data collected by LabView

Simulation and verification

11

0

0.5

1

1.5

2

2.5

3

0 30 60 90 120 150

Time (min)

Vo

ltag

e (V

)

Ideal voltage source

Experimental measurementProposed HDL model

Equivalent circuit model

• Different energy harvester models compared with experimental measurements

– Ideal voltage source fails: voltage booster can greatly affect the behaviour of micro-generator

Simulation and verification

12

• Equivalent circuit model is inaccurate:

– L=m, C=1/k, R=b is over simplification

0.8

0.0

-0.81.0

0.0

-1.01.0

0.0

-1.0

Proposed HDL model

Equivalent circuit model

Experimental measurement

Voltage (V)

1200 1220 1240 1260Time (ms)

EH performance optimisation

13

• Performance loss due to mechanical-electrical interaction

– ηLoss=(EHarvested-EDelivered) / EHarvested

• Integrated performance optimisation in VHDL-AMS testbench. (L. Wang and T.J. Kazmierski, VHDL-AMS based genetic optimization of a fuzzy logic controller for automotive active suspension systems, BMAS 2005)

Genetic optimisation in VHDL-AMS testbench

14

• GA parameters:

– 7 genes: 3 from micro-generator, 4 from transformer

– Fitness: super capacitor charging rate v’dot

– Tournament selection

– Elitism

– Arithmetic crossover

– Gene mutation

• Parallel GA: genes in one generation are evaluated simultaneously

• VHDL-AMS finite state machine

EH performance optimisation

15

• Use a voltage transformer as booster

– Optimisation algorithm found that the transformer exhibits better performance than voltage multiplier

Optimisation results

16

Voltage (V)

0 30 60 90 120 150Time (min)

2.0

1.6

1.2

0.8

0.4

0.0

Optimised

Un-optimised

• Super capacitor charging simulation waveforms

Optimisation results

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Micro-generatorOuter radius of coil (R) 1.2mm

Coil turns (N) 2300Internal resistance (Rc) 1600ΩΩΩΩ

Voltage transformer

Resistance (ΩΩΩΩ) No. of turnsPrimary winding 400 2000

Secondary winding 1000 5000

Micro-generatorOuter radius of coil (R) 1.1mm

Coil turns (N) 2100Internal resistance (Rc) 1400ΩΩΩΩ

Voltage transformerResistance (ΩΩΩΩ) No. of turns

Primary winding 340 1900Secondary winding 690 3800

Un-optimised

Optimised

Further work: automated design flow

18

GA-based performance optimisation

Architecturalsynthesis

Conclusion

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• Integrated approach to EH modelling and performance optimisation has been developed

• Existing electrical equivalent circuit models of micro-generator cannot predict the voltage booster’s performance accurately

• HDL model based on analytical equations can describe the actual shape and size of an EH

• Through performance optimisation it was possible to increase the energy harvesting rate by 30%