Disc Resonating Gyroscopes: A Summary of a Recent Development in MEMS Technology

Transcription

1 Disc Resonating Gyroscopes: A Summary of a Recent Development in MEMS Technology Final Project, ME 381: Introduction to MEMS Instructor: Professor Horacio D. Espinosa Table of Contents

2 INTRODUCTION.. 3 Background. 3 Recent Developments..3 Overview of Disc Resonating Gyroscopes..3 MOTIVATION...6 Resistance to Liner Acceleration.6 Space...6 MICROFABRICATION PROCESS...7 DESIGN FOR PERFORMANCE...8 Advantages of Geometry.8 Vibration Control.9 Noise Analysis...10 Quality Factor 10 DESIGN ISSUES..11 FUTURE DEVELOPMENTS...12 Conclusion.13 REFERENCES.. 14 INTRODUCTION Background 2

3 Since its birth in 1817, the gyroscope has been transformed from a heavy, large, spinning apparatus to a miniscule, vibrating disc [1]. The purpose of both, however, remains the same. Gyroscopes provide a reference frame from which angular velocity can be determined, such as the speed at which a car is turning (or spinning in circles also increased as fabrication precision improves. Today, gyroscopes have entered the field of MEMS and are now being manufactured on the scale of millimeters [2]. Earlier MEMS designs, such as tuning fork, hemispherical, and piezoelectric plate gyroscopes proved that scaling down the technology is possible. on ice). Although gyroscopes have traditionally been most widely used in navigation to simply sense 2D direction change, they have become far more useful, in 3 dimensions even, within the last halfcentury. Measuring yaw, pitch, and roll on spacecraft is possible, and when coupled with accelerometers, gyroscopes can even sense and track motion in 3D space [2] [3]. Overview of Disc Resonating Gyroscope This report aims to discuss a more recent advancement, the Disc Resonating Gyroscope (DRG). It incorporates design aspects from previous gyros while also providing for higher performances and new applications. The DRG is most similar to the Ring Gyroscope [5][6], but usually Recent Developments Modern microfabrication and circuitry are the major contributors in reducing gyroscope size. In addition, incorporates concentric rings linked by sort offsetting radial beams, allowing for much higher sensing volume [2] (see Figure 4) than just a single ring. gyroscope resolution and performance has 3

4 Gyroscopes sense angular velocity by measuring the Coriolis acceleration. When rotation occurs normal to the sensing and actuating plane, the direction in which the disc was being driven is offset. The direction of the Coriolis acceleration is governed by the relationship in the following equation [4]: modes, resulting in cyclical deformation into an oval shape. The actual deformation is quite small, as it cannot exceed the clearance between the internal electrodes and the disc (Figure 2). This deformation occurs in the driving direction until rotation occurs, whereby the disc begins to deform at an angle to its driving direction (Figure 3) governed by the above equation. Where a is the Coriolis acceleration, ω is the angular velocity, and v is the driving velocity, as shown in the following figure: Figure 2: Cross sectional view of electrodes within disc ring matrix. Note small clearance between electrode and disc. Deformation cannot exceed this tolerance. [4] Figure 1: Examples of Coriolis acceleration with respect to rotation (ω) and driving (v) directions [4] The driving voltage in the DRG causes a squishing effect, otherwise known as having two degenerate in-plane vibration 4

5 microfabrication steps will also be discussed accordingly, as well as current problems with the design and future improvements. Figure 3: Car rotation example demonstrating Coriolis effect on resonating disc. [4] The electrodes used for sensing and actuation of this device are positioned in a somewhat novel fashion. They are placed inside the slots that are formed between the concentric rings and the support segments. At least one electrode pair, one positive and Figure 4: Disc Resonator Gyro Design one negative, fits inside each slot, but two or more pairs are sometimes used to enhance sensitivity [2]. It is of interest that the application and motivation for such a design is determined. The remainder of this report aims to do just that, as well as summarize its performance characteristics and the advantages this design has to offer. The Figure 5: Ring Gyroscope [5][6] 5

6 MOTIVATION In the past, companies have been somewhat limited by the gyroscope technology of the time. Inertial measurement is one of the areas that has been largely been held in check. In these systems, configurations typically contain three accelerometers along with three gyroscopes to effectively track the position and direction of a body in 3D space. The earlier systems were able to hold a GPS route when signal is lost, as well as other, less precise functions. While the old designs were enough for uses in inertial containing a solid body mass, developers learned that the stresses due to high accelerations of ballistic launch caused gyroscopes to be damaged or deform slightly. This slight drift, as it is called, causes the sensors to read incorrectly. While all gyros are subject to these errors as well, this design limits that by doing away with a large, centralized proof mass. Instead, the mass of the disc is more evenly distributed throughout a matrix of concentric rings and connecting segments so localized stresses and strains do not occur as easily [2]. measurement systems for cars and some small planes, they were insufficient for use Space in larger planes that flew longer routes, as well as in many military applications. Another area for gyroscope improvement is in the field of outer space. Current satellites operate primarily with Resistance to Linear Acceleration One problem military researchers have been looking to solve concerns the original use of the proof mass. In gyros larger gyroscopes, which are also heavier and must be manufactured in three dimensions [2]. These factors result in undesirable manufacturing and launching 6

7 costs. In addition, these gyroscopes also need to very accurate, as well as be less susceptible to drift from the heavy forces of the launch [3]. Because of these and other problems, there is a new market for improved gyroscopes. Presently, companies and even other countries [3] are seeking gyro designs that are lighter, smaller, and more accurate than previously possible. With smaller guidance and control systems for satellites, along with other scaled-down technologies available, scientists believe they can construct MICROFABRICATION PROCESS Planar microfabrication of DRG s results in cheaper and faster production than larger gyroscopes. DRG s utilize the now well-understood science of microfabrication, specifically through the processes of etching and modifying a silicon substrate. This explains another part of the motivation for this design, as production costs are drastically cut when only basic microfabrication is utilized. The general process for fabricating the disc is as follows [4]: Substrate Fabrication: satellites weighing less than 10kg and costing mere fractions of their bigger brothers. That would make it possible for smaller countries to launch their own minisatellites (dubbed nano- or picosatelllites ) from aircraft or submarines [3]. 1) 2) 3) 4) 7

8 2) Pattern with photoresist, etch for center 1) First begin with 500micron thick, single crystal silicon. 2) Pattern with photoresist and etch for center support and electrodes 3) Oxidize silicon to a depth of 150 nm 4) Pattern with photoresist and apply a 10nm layer of Ti, a 20nm layer of Pt, support and electrodes, and dope 3) Bind two sections together 4) First pattern and etch the disc only to expose electrodes and then etch them out. Next pattern and etch the rest of the disc to open the slots for the electrodes to move freely in. and a 350nm layer of gold with e- beam deposition and lift-off. Disc Fabrication: 1) 2) An example of a mask used for negatively etching out the slots in the disc would look exactly like the top view of the disc, as shown in figure 4. DESIGN FOR PERFORMANCE 3) Advantages of Geometry The key factor of the disc resonator gyroscope (DRG) design is its geometry, 4) which is unique and advantageous in several ways. The concentric rings allow for a large sensing area, compared to the ring 1) Again begin with 500 micron thick single crystal silicon. gyroscope designs where actuating and sensing electrodes are placed on the 8

9 periphery. The older ring designs, such as Figure 5, would operate much the same as the disc resonator, but would not be as sensitive. With the concentric circles in the DRG, there are electrodes implanted between each ring, resulting in a much higher sensitivity to changes in vibration direction (due to angular velocity and thus the Coriolis effect) [2]. The ring gyroscopes also require flexible support beams to operate properly, which makes its accuracy quite dependent on the consistency of beam construction and dimensions (low tolerance). The DRG, on the other hand, only requires high accuracy in the z-direction (x and y forming the disc plane), which is appropriate for masking, photolithography, and etching processes [2]. Another issue with some other gyroscopes is their reliance on bending out of the plane. This bending is dependant on the thickness of a very thin mass and is susceptible to failure. In-plane deformation is therefore preferred, as in the DRG. Vibration Control Because the thickness has a tolerance of 0.1%, there is added error. For the disc resonator, the vibration is in two degenerative planes. One embodiment of this design includes several cuts to be made, essentially breaking the rings into smaller pieces. This reduces the in-plane stiffness, allowing the system to move more and, therefore, be more sensitive. The axial symmetry of the design allows for much simpler operation and calculation of the sensors, eliminating vibration rectification. With the concentric rings, the electrodes are set up such that the electrodes in the slots between the inside rings are primarily actuating the system, while the electrodes in the slots of the outside rings are used for sensing. This allows for isolation of the individual capacitances and separate optimization [2]. 9

10 With this design, the multiple rings allow for Noise Analysis a high mass M, as well as a great Q [2]. Noise is one area to watch for in gyros, and it is caused by a number of factors in a MEMS device. Smaller massed gyroscopes are subject to high levels of noise. The sensor, the readout, mechanical damping, and all the system s electrical Quality Factor Quality, or Q, is a design factor that essentially represents the amount of damping in system and is governed by the following equation: resistances contribute to the noise. Johnson noise, the thermal agitation between electrons, is also a contributor [5]. This motion causes a force, which can be represented by the equation: There are many sources for such energy dissipation some of which are more significant than others, depending on the design of the gyroscope. The DRG is again This can then be manipulated to solve for what is known as the Browning equivalent acceleration noise: advantageous because it minimizes some of these effects compared to previous gyroscopes. Most significantly, the anchor geometry, as discussed before, supports the Given this equation, we can see that a higher mass and higher Q (Quality) help limit the noise, allowing for a more accurate signal. disc from the center and is the only attachment point to the substrate. Because sensing and actuation is in a plane normal to the support anchor, so called anchor loss 10

11 is minimized. In other gyroscopes, such as the Piezoelectric Plate Gyroscope, elastic waves can more easily radiate away from the structure because the sensing direction is in-plane with the support structure. between high and low frequency actuation. High frequency may lead to more thermoelastic dissipation while low frequency could lead to higher fluid damping [4]. Thermoelastic damping is another source for energy dissipation that is minimized to a smaller extent with this gyro. Thermoelastic damping occurs when local deformation occurs in a material and subsequent heat is dissipated. This process converts detectable kenetic/mechanical energy into undetectable thermal energy. The DRG design is less susceptible to this effect if resonance frequency is low because it does not deform very much during a cycle. One other source of energy dissipation with an even smaller influence is fluid damping of ambient air. There is very DESIGN ISSUES Although this design has many great improvements to its performance and ease of use, this gyroscope still has a number of problems. Inherently, many of the benefits cause problems in some way or another. The concentric rings are great for being able to sense and actuate the gyro; however, you can run into what is called sticktion. This is when a ring deforms past the pull-in distance and sticks to an electrode, as shown in Figure 7. little, if any, affect at high frequencies, but at low frequencies it can become a concern if the gyro is not packaged in a high vacuum. There exists a tradeoff, then, 11

12 material interfaces, with materials of different thermal expansion rates, stresses form, which can add to the drift. Vibration frequency contributes to some high thermoelastic damping, which will produce Figure 6: "Sticktion" [4] high drift. Because of the complicated material structure needed to run this design, there are also other issues to deal with. In several different locations, there are possibilities of FUTURE DEVELOPMENTS The future of MEMS is quite bright, with many areas for increased improvement and utilization. There will always be some material diffusion. This leaking of areas that can always be looked at for material (see Figure 8) can cause shortcircuiting of electrodes. improved quality and performance, such as noise. Because noise is always a factor in the micro scale, noise reduction will always have room to grow. Sean Neylon, a CEO of an international MEMS supplier believes that structural improvements only account Figure 7: Material Diffusion [4] There can be issues of chemical reactions, or oxidation. In addition, thermal changes can cause large problems. At for 25% percent of the area for increased performance. He also believes that the other 75% can be found in the electronics. While the electronics of this design have not been covered in detail, it is important to note that 12

13 there is also room for improvement from that standpoint. In addition, there are many new concepts that are being looked at for the future of MEMS [3]. One idea is to examine new materials. Silicon has been the primary material throughout the entire short history of MEMS, because of its easy manufacturability and strength, but there are other materials that could be suitable for aerospace fields, which was previously not possible. This new design should begin to bridge the gap between the idea and the production process. In all, this MEMS gyroscope design makes numerous significant improvements on past gyroscope designs and types, although there is still much room for increased performance and operation. particular situations. A material that has begun to be suggested is titanium. This could be used to increase the strength of the system, although other problems arise with manufacturing and cost. Conclusion In the near future, MEMS should start to take hold in the military and 13

14 REFERENCES [1] "Gyroscope." Wikipedia. 20 Nov < [2] Shcheglov Et Al. Isolated Planar Gyroscope with Internal Radial Sensing and Actuation (Patent No 7,040,163). U.S. Patent Office [3] Brown, Allan S. "MEMS Across the Valley of Death." Mechanical Engineering April (2006): [4] Liu, Wing K. A Microsystem to Drive Research and Demonstrate the Predictive Science. Department of Mechanical Engineering, Northwestern University. [5] Fell Et Al. Angular Rate Sensor (Patent No 6,282,958). U.S. Patent Office Nov < [6] Ayazi Et Al. A High Aspect-Ratio Polysilicon Vibrating Ring Gyroscope. Center for Integrated MicroSystems, University of Michigan Nov < [7] Bernstein, Jonathan. "An Overview of MEMS Inertial Sensing Technology." Sensors. 1 Feb Cornin-Intellisense Corp. 30 Nov < 14

15 [8] "Johnson Nyquist Noise." Wikipedia. 30 Nov < 15