Ziyang Wang

A batch process micromachined thermoelectric energy harvester: fabrication and characterization

Micromachined thermopiles are considered as a cost-effective solution for energy harvesters working at a small temperature difference and weak heat flows typical for, e.g., the human body. They can be used for powering autonomous wireless sensor nodes in a body area network. In this paper, a micromachined thermoelectric energy harvester with 6 μm high polycrystalline silicon germanium (poly-SiGe) thermocouples fabricated on a 6 inch wafer is presented. An open circuit voltage of 1.49 V and an output power of 0.4 μW can be generated with 3.5 K temperature difference in a model of a wearable micromachined energy harvester of the discussed design, which has a die size of 1.0 mm × 2.5 mm inside a watch-size generator.

Optimum power and efficiency of piezoelectric vibration energy harvesters with sinusoidal and random vibrations

Assuming a sinusoidal vibration as input, an inertial piezoelectric harvester designed for maximum efficiency of the electromechanical energy conversion does not always lead to maximum power generation. In this case, what can be gained by optimizing the efficiency of the device? Detailing an answer to this question is the backbone of this paper. It is shown that, while the maximum efficiency operating condition does not always lead to maximum power generation, it corresponds always to maximum power per square unit deflection of the piezoelectric harvester. This understanding allows better optimization of the generated power when the deflection of the device is limited by hard stops. This is illustrated by experimental measurements on vacuum-packaged MEMS harvesters based on AlN as piezoelectric material. The results obtained for a sinusoidal vibration are extended to random vibrations. In this case, we demonstrate that the optimum generated power is directly proportional to the efficiency of the harvester, thus answering the initial question. For both types of studied vibrations, simple closed-form formulas describing the generated power and efficiency in optimum operating conditions are elaborated. These formulas are based on parameters that are easily measured or modeled. Therefore, they are useful performance metrics for existing piezoelectric harvesters.

Shock Reliability of Vacuum-Packaged Piezoelectric Vibration Harvester for Automotive Application

This paper reports for the first time the shock reliability of vacuum-packaged piezoelectric vibration harvesters for automotive application. Experimental study on >80 devices shows that the failure is induced by the impact between seismic mass and glass package. The statistical results obtained for various types of harvesters have shown a trend of higher shock reliability for a higher resonance frequency. The influence of beam thickness on the shock reliability is then discussed. The trade-off between the power output and shock reliability is elaborated. To further improve the reliability to the level of automotive application, the idea of stepped cavity is discussed and implemented. The experimental results confirmed a significantly enhanced reliability increased by ~50%, up to maximum 2000 g, for the same type of device under shock excitations.

A MEMS vibration energy harvester for automotive applications

The objective of this work is to develop MEMS vibration energy harvesters for tire pressure monitoring systems (TPMS), they can be located on the rim or on the inner-liner of the car tire. Nowadays TPMS modules are powered by batteries with a limited lifetime. A large effort is ongoing to replace batteries with small and long lasting power sources like energy harvesters [1]. The operation principle of vibration harvesters is mechanical resonance of a seismic mass, where mechanical energy is converted into electrical energy. In general, vibration energy harvesters are of specific interest for machine environments where random noise or repetitive shock vibrations are present. In this work we present the results for MEMS based vibration energy harvesting for applying on the rim or inner-liner. The vibrations on the rim correspond to random noise. A vibration energy harvester can be described as an under damped mass-spring system acting like a mechanical band-pass filter, and will resonate at its natural frequency [2]. At 0.01 g2/Hz noise amplitude the average power can reach the level that is required to power a simple wireless sensor node, approximately 10 μW [3]. The dominant vibrations on the inner-liner consist mainly of repetitive high amplitude shocks. With a shock, the seismic mass is displaced, after which the mass will “ring-down” at its natural resonance frequency. During the ring-down period, part of the mechanical energy is harvested. On the inner-liner of the tire repetitive (one per rotation) high amplitude (few hundred g) shocks occur. The harvester enables an average power of a few tens of μW [4], sufficient to power a more sophisticated wireless sensor node that can measure additional tire-parameters besides pressure. In this work we characterized MEMS vibration energy harvesters for noise and shock excitation. We validated their potential for TPMS modules by measurements and simulation.

A piezoelectric vibration harvester based on clamped-guided beams

The paper addresses the design, modeling, fabrication and experimental results of a piezoelectric energy harvester based on clamped-guided beams. The design is featured by shorter mass displacement and higher reliability than cantilever beams. Two separate sets of capacitors allow exploiting both tensile and compressive stress at the same time. The maximum output power reaches about 20 μW at an input acceleration of 1.2 g. A tuning range of 3 Hz is demonstrated by varying the voltage across the capacitors. Nonlinear behavior observed at larger mass displacement is also discussed.

Improved mechanical reliability of MEMS piezoelectric vibration energy harvesters for automotive applications

This paper addresses the issue of the mechanical reliability of MEMS piezoelectric vibration harvesters aimed at powering tire pressure monitoring systems. These harvesters generate sufficient power for the targeted application. However, for bringing them to the automotive market, their mechanical reliability has to be optimized, particularly in terms of shock resilience. Experimentally verified methods for improving the mechanical reliability of such devices are showcased in this article. These methods concern both the design of the harvesters (introduction of stoppers in the package) and their manufacturing process (release method of the MEMS structure).

Large power amplification of a piezoelectric energy harvester excited by random vibrations

This paper reports the method and results of amplifying the power output of a piezoelectric energy harvester excited by random vibrations. The amplification is achieved by applying a dual-mass-spring system. A maximum power amplification of 80 times has been experimentally demonstrated. The generated power output from a piezoelectric energy harvester, when excited by as-measured random vibrations, amounts to 28.9 μW. This is sufficient to operate a battery-free Tire Pressure Monitoring System (TPMS) for wireless sensing of pressure and temperature in car tires. Thus, the piezoelectric energy harvester with power amplification is proved to be a viable solution to replace batteries in the TPMS application.

Energy-autonomous wireless vibration sensor for condition-based maintenance of machinery

This paper addresses the development of an energy-autonomous wireless vibration sensor for condition-based monitoring of machinery. Such technology plays an increasingly important role in modern manufacturing industry. In this work, energy harvesting is realized by resorting to a custom designed thermoelectric generator. The developed wireless vibration sensor has a remotely tunable sampling rate, which caters to the different needs of various operating conditions. The two key features, energy autonomy and wireless measurement, are demonstrated successfully by the experimental results obtained on the thermoelectric generator and the wireless sensor.

Material optimization of phosphorus-doped polycrystalline silicon germanium for miniaturized thermoelectric generator

This paper reports the results of the material optimization of phosphorus-doped polycrystalline silicon germanium (poly-SiGe) for use in thermoelectric energy harvesting. The problem of high specific contact resistance is tackled by optimizing the microfabrication process and choosing a new metal stack for interconnect. Other material properties relevant to the thermoelectric energy harvesting have also been characterized and reported. Calculations show that once the optimized process flow is adopted, the output power of a micromachined high-topography thermopile can be improved by a factor of at least 6.

Process challenges of MEMS harvesters and their effect on harvester performance

In this paper we present process challenges for the fabrication of energy harvesters and how they affect the performance. For the piezo harvesters, improved etching recipes and packaging enables to reduce the variation in resonance frequency from 8 to 1.5%. Similar results were obtained for electrostatic harvesters. For thermal harvesters, we find that when reducing dimensions, the output voltage will increase, but also the contact resistance Rc. By optimizing the material stack, Rc has been reduced by two orders of magnitude, tripling the output power.