Energy Harvesters

Piezoelectric Vibratory Energy Harvester Design and Simulation

MEMS-based energy harvesters have the potential to provide “perpetual” power to small systems. Applications include medical, consumer, automotive and environmental devices. Wireless sensor networks (WSN) such as the one shown below are of particular interest for in-situ environmental, health and habit monitoring, where batteries are difficult or impractical to replace.

Figure 1: A typical WSN node (Yole Development, dMEMS Conference April 2012)

Experts believe the piezo-electric vibratory energy harvester is one of the most promising approaches. Successful design depends on maximizing energy capture for target environmental conditions.

Design Challenges

The criteria that need to be considered when designing a piezoelectric energy harvester are the frequency of operation, the power generated and the power transferred to the management circuit. The frequency of operation can be determined by running a modal analysis in a conventional finite element analysis (FEA) tool. The power generated and transferred, however, are highly dependent on the power management circuit and hence must be simulated in a closed-loop circuit. Thus, the design platform chosen must be able to simulate the coupled piezo-mechanics and the electronics together in order to be useful for energy harvester design.

New Hybrid Methodology

  1. Rapid MEMS Design Exploration
    Coventor’s design flow starts by constructing a parametric model of the piezoelectric vibratory harvester in MEMS+. The designer works in a 3D graphical environment to create a parametric model by assembling high-order, MEMS-specific, finite elements (piezo-mechanical shells). Each element is linked to the MEMS process description and material database so that piezo-electric material properties and electrodes are assigned automatically. The high-order elements give a precise mathematical description of the device physics and include the non-linear mechanics that are inherent in these devices. Furthermore, these high order components have been specifically crafted to simulate extremely quickly within MATLAB®, Simulink® and Cadence® Spectre®. We’ve found that engineers working at the device level prefer MATLAB and Simulink, while engineers working on designing the IC and system prefer to work in Cadence. The use of MEMS+ high-order finite elements provides several benefits. First, design teams no longer no need to spend valuable time hand crafting reduced-order models from FEA and/or analytical expressions. Second, because MEMS+ models include non-linear effects while hand-crafted models are often linear only. Non-linear effects that may affect device performance are included, reducing the chances of having to redesign the device during the final stages of development.

    Figure 2: Coventor design flow for energy harvesters

  2. Power Management and Circuit Design with Cadence
    The MEMS+ model can be imported to the Cadence Virtuoso environment with a few mouse clicks, and placed in a schematic that includes the conditioning circuit. The combine harvester and circuit can then be simulated in Cadence Spectre. Both the harvester parameters and circuit parameters can then be varied simultaneously to optimize device performance. For example, the designer can tune the harvester dimensions and resistive load to obtain the maximum power transfer from the harvester to the conditioning circuit. Different circuits can be also tested to achieve the performance requirements.

    Figure 3: Cadence Virtuoso circuit schematic and Spectre analysis with a graph showing power transferred to the bridge load resistor with resistor value

  3. Verification and final analysis with FEA
    Further detailed modeling can be done using CoventorWare’s field solvers. For example, the design can be checked for high stress areas that may lead to failures when the device is overloaded due to shock. Gas damping coefficients can also be obtained with CoventorWare and included in the MEMS+ model to more accurately predict Q-factor. When necessary, simulation results from MEMS+ and CoventorWare can be verified against each other to provide increased confidence prior to tape-out. For instance, the closed-loop harmonic response with a linear resistive load can be simulated in both tools and compared.

    Figure 4: Simulation in CoventorWare of gas damping forces acting on the vibrating harvester

A Complete Platform

Coventor’s platform for piezoelectric energy harvesting combines MEMS+ and CoventorWare to provide a complete solution which solves the coupled and multi-domain physics not addressed with conventional FEA analysis alone. This hybrid methodology has many advantages. It allows the co-design and co-simulation of an energy harvesting device together with the conditioning circuit. The models are parametric and solve extremely fast, making rapid exploration of design and process changes practical. The time-consuming process of creating reduced-order models from FEA data and/or analytical expressions is eliminated and the resulting models from this methodology are highly accurate.

In addition, using a platform that integrates the best-in-class simulators like Cadence Spectre and/or Matlab/Simulink with MEMS-specific models provides the best combination of accuracy and capacity. Going forward, as Cadence and the Mathworks continue to make speed and capacity improvements to their algorithms and Coventor continues to refine and add high-order elements to our MEMS+ library, the benefits multiply. Thus, the individual achievements from each company multiply and result in speed and capacity improvements that increase exponentially for the user.

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