Study On Key Technologies Of Improvement In Bias Stability Of The Micromachined Butterfly Gyroscope

Gyroscope is the sensor that measures the rate of rotation of a carrier. It is one ofthe basical core devices in inertial navigation system and has very important militaryand civil applications. Because of its small size, low cost, low power consumption andbatch fabrication compared to traditional mechanical spinning gyroscope, electrostaticgyroscope, laser gyroscope and fiber optical gyroscope, micromachined gyroscope hasan extensive application prospect in the weapons such as guided munitions andunmanned combat vehicles. The related products and technologies of high performancegyroscopes are embargoed against China by foreign countries while most domesticmicromachined gyroscopes can not meet the demands. Therefore, improvement in biasstability which is one of the core performance specifications is the key technology fordeveloping high performance micromachined gyroscope.Although it has a small volume and simple structure, the micromachined gyroscopeis a complex electromechanical coupling system. Its structural precision is easilyrestricted by design level, fabrication imperfections, etc. Meanwhile, it is faced withtemperature variations, vibrations and shocks in engineering applications. So theformation mechanism of bias is complicated. Aiming at the urgent demands of weaponequipment, we developed a micromachined butterfly gyroscope. The basical theoriesand key technologies of structure designing, micromachining and signal processingwere studied. This dissertation focuses on the methods of improving the bias stability ofmicromachined gyroscopes, which takes the micromachined butterfly gyroscope as theobject. Breakthroughs were made on the key technologies such as modal coupling error,structural stress and fabrication imperfection. The research results will provide a noveldesign theory and method for developing high performance micromachined gyroscopes.The main research contents of this dissertation are as follows:1. The structure and operation principle of the micromachined butterfly gyroscopewere presented, and its dynamic characteristics and readout technologies weresystematically studied. Emphatically, the bending and torsional stiffnesses of thesuspension beam, which has a parallelogram cross section, were derived considering theanisotropic properties of monocrystalline silicon. The damp, driving and Coriolismoments of the gyroscope were derived. The operation modal frequencies and thedynamic equation of the gyroscope were theoretically modeled. Then, expressions ofthe mechanical sensitivity and bandwidth of the gyroscope were obtained. According toits capacitive output characteristics, capacitance detection using double sinusoidalcarriers and closed loop excitation using PID controller were adopted in the readoutcircuit. The driving singal and sensing signal were successfully extracted from eachother. Finally, the theoretical model of the output signal of the gyroscope due to angular rate inputs was obtained.2. The formation mechanism and major sources of bias stability of the gyroscopewere studied. The model of the modal coupling errors which contain electrostatic forcecoupling error, Coriolis force coupling error, vibration coupling between drivingoscillator and sensing oscillator was established. Finally, the systematic errors such asstructural error, circuit error and environmental error and their influence on the biasstability were studied.3. Structural stresses induced during the fabrication process and their effectmechanisms were studied, and the mechanical structure of the gyroscope was optimized.Structure deformation and dynamic characterictics affected by the pull-in effect, axialthermal stress and internal stresses induced by the die attachment and package weretheoretically analyzed. In order to theoretically model the modal frequency of thegyroscope under action of the axial thermal stress, two types of structures whichadopted a clamped-free suspension beam and a clamped-clamped suspension beam weredesigned and characterized. Structures adopting stress release groove, flexural supportframe and stress balancing were designed and characterized. The temperatureperformance of the capacitance output, modal frequency, quality factor and bias outputwere evidently improved.4. Fabrication imperfections of the micromachined gyroscope and their influenceson the bias stability were studied. The structural trimming method was experimentallycharacterized by UV laser micromachining. Emphatically, geometric errors of thematerials and fabrication imperfections due to misalignments and etching defect andtheir influences on the structure precision and bias stability were analyzed. Theoperation principle of structure trimming was studied and experimentally characterizedby UV laser micromachining. The experimental results showed that the modal couplingerror of the gyroscope was decreased significantly after laser trimming.5. Test and comparison of the major performance specifications including scalefactor, nonlinearity, bias stability, temperature sensitivities of scale factor and biasstability were carried out for the micromachined butterfly gyroscopes before and afterimproving. The results showed that the nonlinearity, temperature sensitivities and biasstability under short in-run time were improved significantly.