Energy Harvesters
Piezoelectric Vibratory Energy Harvester Design and Simulation
MEMS Energy harvesters have the potential to provide “perpetual” power to small systems. Applications span the markets for medical, consumer, automotive and environmental devices. Examples include tire pressure monitors, ID tags and wireless sensor networks (WSN). 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)
Successful design depends on optimizing the energy creation within the proper environment while simultaneously avoiding time-consuming build and test methodologies. Experts believe the piezoelectric vibratory energy harvester is one of the most promising technologies in this field.
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 obtained running a standard finite element analysis (FEA). However, the power generated and the power transferred is highly dependent on the power management circuit and hence simulations must be run in a closed loop environment. Thus, the design platform chosen must be able to solve for the coupled piezo-mechanics and electronics in order to be considered practical for energy harvesting applications.
New Hybrid-Design Methodology
- Rapid MEMS Design Exploration
Coventor’s design flow starts by constructing a parametric model of the piezoelectric vibratory harvester and assigning foundry materials and process data in MEMS+. The MEMS designer works in a 3D graphical environment to assemble a parametric model using high-order MEMS-specific finite elements (piezo-mechanical shells). Each element is linked to the process description and material database so that piezoelectric 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 Simulink and Matlab, and 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 creating hand-crafted reduced order models from FEA and/or from analytical equations. Second, because MEMS+ models include non-linear effects and hand-crafted models are often linear only, critical information that may affect the design is included, decreasing the possibility of redesign during the final stages in the design process.
Figure 2 Coventor Design Flow for Energy Harvesters
- Power Management and Circuit Design with CadenceModeling the harvester together with the conditioning circuit requires the MEMS+ to be placed into Cadence Virtuoso using a simple mouse-click. Electronic components can then be added from the Virtuoso libraries to complete the design. 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 best match the performance required.
Figure 3 Cadence Virtuoso circuit and Spectre analysis with a graph showing power transferred to the bridge load-resistor with resistor value
- 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 derived 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. Simulating piezoelectric harmonic analysis with a linear resistive load is an example where a user may wish to compare the results between both tools.
Figure 4 Simulation in CoventorWare Analyzer showing air 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 traditional FEA analysis only. This hybrid methodology has many advantages. It allows the co-design and co-simulation of harvesting device together with the conditioning circuit. The models are parametric and solve extremely quickly, enabling rapid exploration of design and process changes practical. The time consuming process to create reduced-order models from FEA data and/or analytical expressions is eliminated and the resulting models from this methodology are non-linear and highly accurate.
In addition, using a platform that integrates the best-in-class simulators like Cadence Spectre and/or Matlab/Simulink with MEMS specific high order 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.
