ME 434 MEMS Tuning Fork Gyroscope Amanda Bristow Stephen Nary Travis Barton 12/9/10

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1 ME 434 MEMS Tuning Fork Gyroscope Amanda Bristow Stephen Nary Travis Barton 12/9/10 1

2 Abstract MEMS based gyroscopes have gained in popularity for use as rotation rate sensors in commercial products like cars and game consoles due to their cheap cost and small sized compared with traditional gyroscopes. The MEMS gyroscope consists of the basic mechanical structure an electronic transducer to excite the system as well as an electronic sensor to detect the change in the mechanical structures modal shape. The goal of this project is to refine the accuracy and detail given by the MEMS gyroscope to military grade standards enabling the expansion of their use, while maintaining the attraction of their low cost. To achieve this, the structure of the mechanical portion will be analyzed to generate the two modal shapes as close as possible within the 15-30kHz range so as to be differentiable from background noise yet still require minimal power to be detected. 2

3 Table of Contents Abstract...2 Introduction...4 Proposed Approach...4 Organization...5 Cost Consideration...5 Summary...5 3

4 Introduction Gyroscopes are becoming more desired for their ability to sense and report rotation. There are many industries using gyroscopes in their products. More precise gyroscopes are in demand and research is being done to accomplish that goal. Main considerations involved are how does it work, why type of design attributes need to be added, and what type of cost considerations are involved. Gyroscopes are designed to sense rotation. The way they do this is through their underlying principal, the Coriolis effect. The coriolis effect can be described by having a person stationary on the x axis and a particle moving along the positive y axis. Once a rotation is added about the z axis a force is caused which makes the stationary person believe that the particle is moving toward him or her. When used in a Gyroscope, the coriolis effect Is used to ultimately detect angular velocity. Two modes of vibrations are used to do this. The first mode of vibration is the drive mode. This mode is intentionally introduced to the system for the coriolis effect to work. This moved of vibration involves two proof masses moving horizontally in opposite directions from one another. The second mode of vibration is caused by introducing an angular velocity to the already moving device. The coriolis effect causes the proof masses to move in a different direction than the first mode. The proof masses then move vertically in directions opposite each other. Sense electrodes are placed above the proof masses and can sense the distance that the proof mass is moving. These sense electrodes are able to calculate capacitance which is a function of distance. This capacitance is directly proportional to the angular velocity, thus giving an accurate measurement of angular velocity in terms of capacitance. The design of a gyroscope is very important for the application of which it use for. The most common gyroscope is the mico electro mechanical system tuning for gyroscope. This gyroscope is designed with a tuning fork structure meaning that it is symmetrical on both sides. This design uses two proof masses. The structure is composed of long thin beams and anchored in the center that the proof masses are free to move. It is essential to adjust all of the dimensions so that a certain range of frequency is output. It is important so that data can be received and it is not being overwhelmed with noise rather than real measurements. Gyroscopes are used in many different industries, some new and some old. They are used for gaming in systems such as the wii motion plus attachement, telecommunications such as the iphone 4. Gyroscopes are also used in the automotive industry for many different purposes along with space exploration and ship building. Gyroscopes are desirable because they are very small, it does not take much energy to use them and because they are relatively inexpensive to mass produce. Most gyroscopes are now made from layered silicon wafers. These wafers are micro machined such that the gyroscope s form is exposed from the middle of the wafer. The purpose of this project is to design a precision gyroscope that is comparative to optical devices. If the team succeeds in yielding data with great precision it could be mass produced and added to various different devices to yield better accuracy for many different industries. Proposed Approach This project has three main tasks: 1. Model and analyze the mechanical structure 2. Model and analyze the electrical components 3. Couple the mechanical and electrical structures together and analyze the reaction of the system One member of the group will take the lead for each task while the other two serve as backup. The modeling and finite element analysis will be performed using COMSOL. Additionally, all members of the group have reviewed technical documents related to the project in order to gain a basic understanding of the project. Once modeling and simulation of the gyroscope design have been completed, the group will then learn about the

5 manufacture and testing of MEMS devices. However, because of time constraints, the group will be unable to manufacture and test the design. Further details of each task will be clarified during meetings with Dr. Julie Hao, the project supervisor, throughout the course of the Spring 2011 semester. Organization The duties for each section of the project have been tentatively assigned as follows Group member Travis Barton Amanda Bristow Stephen Nary Responsibility Mechanical Structure Electrical Structure Coupling of Mechanical and Electrical Additionally, all group members will contribute to the writing of the final technical report, as well as any progress updates or presentations that may be necessary. Cost Consideration The cost of this project is covered by a grant from the National Science Foundation and can be organized into the following categories: Design: no cost since the cost of all the software is covered by the university Fabrication: 1 mask: $6000, 2 SOI wafers: $400 Fabrication processes: $5000 Testing: 1 PCB: $200 Consumption: $800 Summary As of December 2010, the initial design of the mechanical structure has been successfully modeled in COMSOL. The next step is to modify the dimensions of the structure so that the two desired modal shapes fall within the specified range of khz. After that task is completed, the group will then begin modeling the electrical structure of the gyroscope based on specifications which will be provided by Dr. Hao. Finally, the two components of the gyroscope, mechanical and electrical, can be coupled together to simulate the behavior of the entire gyroscope system. This modeling will provide great insight into the working principles of a MEMS gyroscope. Furthermore, the hands-on design work will be supplemented by additional background research into the fabrication and testing of MEMS devices.