Binh Duc Truong

Power optimization and effective stiffness for a vibration energy harvester with displacement constraints

This paper presents experiments on how to approach the physical limits on power from vibration energy harvesting under displacement-constrained operation. A MEMS electrostatic vibration energy harvester with voltage-control of the system stiffness is used for this purpose. The power saturation problem, when the proof-mass displacement reaches a maximum amplitude for sufficient acceleration amplitude, is shifted to higher accelerations by use of load optimization. In addition, we demonstrate the effect of varying the electromechanical coupling k 2. Measurement results show that harvested power can also be made to follow the optimal power of the velocity-damped generator for a range of accelerations, which implies displacement constraints. Compared to the saturated power, the power increases 1.5 times with the optimal load for electromechanical coupling at k 2 = 8.7%. This is improved 2.3 times for a higher coupling of . The obtained system effectiveness exceeds 60%. This work shows a first demonstration of reaching optimal power in the intermediate acceleration-range between the two extremes of maximum efficiency and maximum power transfer. The experimental results follow the theoretical results for a device with both load and stiffness tuning surprisingly well, despite only optimizing the load here. We compared a linearized lumped-model of the device with the same augmented by end-stop nonlinearities. The comparison shows that an effective stiffness due to end-stop impacts in the latter model closely matches the optimal stiffness for the former model, and therefore can explain why the experimental output power is close to optimal despite the lack of deliberate stiffness tuning.

Experiments on power optimization for displacement-constrained operation of a vibration energy harvester

This paper presents experiments on how to approach the physical limits on power from vibration energy harvesting under displacement-constrained operation. A MEMS electrostatic vibration energy harvester with voltage-control of the system stiffness is used for this purpose. The power saturation problem, when the proof mass displacement reaches maximum amplitude for sufficient acceleration amplitude, is shifted to higher accelerations by use of load optimization and tunable electromechanical coupling k2. Measurement results show that harvested power can be made to follow the optimal velocity-damped generator also for a range of accelerations that implies displacement constraints. Comparing to the saturated power, the power increases 1.5 times with the optimal load and an electromechanical coupling k2=8.7%. This value is 2.3 times for a higher coupling k2=17.9%. The obtained system effectiveness is beyond 60% under the optimization. This work also shows a first demonstration of reaching optimal power in the intermediate acceleration-range between the two extremes of maximum efficiency and maximum power transfer.

Electric control of power extracting end-stop for MEMS vibration energy harvesting

This experimental work investigates a technique to further improve performance of vibration energy harvesters under displacement-constrained operation. Previously, a device concept based on end-stops acting as additional transducers was developed so that the harvested power can be increased beyond the power obtained from a conventional harvester of the same size. However, there is a range of tested acceleration amplitudes in which the transducing end- stop device performs worse than the conventional device. In this paper, an approach using electric control is used to optimize the end-stop transducer performance and thereby further improve the system effectiveness under displacement constrained operation. For example, the maximum power increases by a factor of 2.4 compared to that of a conventional prototype under the same operating conditions and constrained displacement amplitude, while this value was about 1.3 for the previous technique.

Experimentally verified model of electrostatic energy harvester with internal impacts

This paper presents experimentally verified progress on modeling of MEMS electrostatic energy harvesters with internal impacts on transducing end-stops. The two-mechanical-degrees-of-freedom device dynamics are described by a set of ordinary differential equations which can be represented by an equivalent circuit and solved numerically in the time domain using a circuit simulator. The model accounts for the electromechanical nonlinearities, nonlinear damping upon impact at strong accelerations and the nonlinear squeezed-film damping force of the in-plane gap-closing transducer functioning as end-stop. The comparison between simulation and experimental results shows that these effects are crucial and gives good agreement for phenomenological damping parameters. This is a significant step towards accurate modeling of this complex system and is an important prerequisite to improve performance under displacement-limited operation.

Comparative performance of voltage multipliers for MEMS vibration-based energy harvesters

This paper investigates by numerical simulation the performance of an electrostatic vibration energy harvester when it is electrically configured in two different diode-capacitor multiplier topologies. The complete lumped-model of an overlap-varying generator along with power electronic interface circuit is constructed for analysis using a crcuit simulator. Parasitic capacitance of the transducers and nonideal diode traits such as leakage current and junction capacitance are incorporated. We find that both configurations are able to efficiently operate with a ratio of capacitance variation much lower than 2, which overcomes a challenging obstacle of MEMS-based devices. Other advantages and disadvantages of the two topologies are compared and discussed.

Power-electronic-interface topology for MEMS energy harvesting with multiple transducers

Based on circuit simulations, this paper investigates a concept for a power electronic interface circuit for MEMS electrostatic energy harvesters. Two ordinary overlapvarying transducers are first electrically configured as a symmetric voltage doublers which enables the device to self-start from an initially low bias. The harvesting system is then reconfigured to couple with a buck-boost DC-DC converter in order to maximize the power delivered to an electronic load. The losses of electronic components due to diode voltage drop and parasitic resistance of inductors are taken into account for a feasibility investigation. Dependence of the maximum output power on inductance and switching frequency is explored.

Analysis of Power Electronic Interface for Capacitive MEMS Energy Harvesters

This paper presents an alternative to electrical configuration of capacitive energy harvesters with multiple transducers. The configuration is constructed to combine all the transducer outputs into a single output and performs identically to the linear two-port model. Further development of a power electronic interface circuit is investigated by circuit simulation. The circuit is used to convert the AC outputs of the harvester to a DC voltage and has a tunable resistive input impedance. The introduced topology shows a significant improvement in comparison with the conventional circuit using a full bridge rectifier. The maximum output power is two times higher when the losses of electronic components are taken into account.