Modelling and Simulation of MEMS Based Automotive Systems

Abstract

In addition to pressure sensors for engine control and accelerometers for airbag systems, yaw rate sensors and gyros are foreseen as a recent automotive challenge for MEMS based components. Such devices, when using coriolis forces as a sensing principle, lead to extremely complex system design. The moving structure has to be rotationally excited and tightly controlled. Then, the structure tilt, induced by the coriolis force, has to be simultaneously measured. The simultaneous sensing and actuation can, in theory quite easily be created with capacitive electro-mechanical structures and switched-capacitor based signal conditioning circuits. But in practice, the strong interactions between the two parts of the device makes such a system extremely difficult to understand, analyse and ultimately, to successfully design. Also, products using these devices may combine numerous analog/mixed signal microelectronics blocks and MEMS functions on a single chip or on two or more chips assembled within an integrated package, complicating the analysis. This paper will demonstrate a design methodology and tools that enable mechanical and electronic engineers to efficiently model, simulate and design such systems. This integrated solution for the development of Micro Electro Mechanical Systems (MEMS) combines aspects of electronic design automation with mechanical, thermal, and fluidic computeraided design, and allows the complex interactions and multiphysical behaviour of automotive sensors to be captured and analyzed.

In the tools, system designers create a mixed technology schematic using building blocks. Each of these bricks encapsulates a behavioural model, written in HDL-A or in VHDL-AMS. Full system functionality can be verified, through a multilevel, mixed-mode, multi-domain behavioural simulation. The system can be a MEMS component or a full system including the read-out electronics, whether it is integrated monolithically or in a hybrid assembly. When simulation is done, the user can perform a schematic driven layout generation (SDL) to create mask layout. To further enhance design capability by facilitating “what if” analysis, optimization and synthesis, our optimization tools have recently been extended to MEMS technology and coupled to our MEMS model library so that design tradeoffs may be made at both the system level (between sensor, electronics and packaging) and the device design level. The tools also encompass statistical analysis for design centering and yield analysis based on data from MEMS manufacturing processes. Our MEMS design system also enables IP usage in system level products by providing mechanisms for designers to protect their IP and create and manage versions of their IP libraries. The paper will describe how these design tools are applied to the creation of yaw rate sensors and gyros utilising the coriolis sensing principle by providing an example of simulation, optimisation, statistical analysis and finally an IP library based on the sensor design.