1. Field of the Invention
The invention relates to silicon bulk micromachined transducers and in particular vibratory symmetric transducers usable as gyroscopes, sensors and accelerometers.
2. Description of the Prior Art
The applications for gyroscopes in guidance and control devices is notorious and has been realized in more recent technologies by means of optical gyroscopes such as ring laser gyroscopes, and fiberoptic gyroscopes as well as vibratory gyroscopes such as hemispherical resonator gyroscopes, tuning fork gyroscopes, and silicon micromachined vibratory gyroscopes.
The advantages of optical gyroscopes include high dynamic range, high band width, rapid start up, little acceleration sensitivity at least to the first order, good day-to-day drift stability and a stable linear scale factor. The ring laser gyroscope and fiberoptic gyroscopes in particular have excellent stability over periods of hours, but are noisy during short term intervals of the duration of several seconds, where they act as white noise generators. High performance optical gyroscopes also require complex electronics support, a path link controller, a high voltage power supply, mechanical dithering, optical modulators, thermoelectric coolers, signal processing, high order modeling, precision optical components and complicated and delicate assembly and manufacturing processes. Each of these requirements increases the cost and power requirements of high performance optical gyroscopes. Moreover, the performance of an optical gyroscope depends directly with the area enclosed within its optical path. Apart from various technical difficulties encountered in attempting to reduce fiberoptic gyroscopes and ring laser gyroscopes to millimeter dimensions, this fundamental scaling law makes it effectively impossible to realize small, high performance optical gyroscopes.
In a vibratory gyroscope, the Coriolis effect induces energy transfer from the driver input vibratory mode to another mode which is sensed or output during rotation of the gyroscope. An example of such a high performance vibratory gyroscope is the hemispherical resonator gyroscope. The hemispherical resonator gyroscope is made of quartz and has a shell resonator design. It is immune to external vibration and is capable of standing high g shocks. However, it requires precise machining of the resonator and housing, high vacuum sealing and gettering, and computer support. The hemispherical resonator gyroscope is very large, difficult to manufacture, expensive and consumes a large amount of power.
Another type of vibratory gyroscope is the quartz tuning fork vibratory gyroscope. This type of gyroscope is insensitive to linear vibration, has low mechanical loss, and is capable of standing high g shock. However, the quartz tuning fork microscope has low responsitivity, high temperature sensitivity, and is not integratable with silicon electronics.
Silicon micromachined vibratory gyroscopes are integratable with silicon electronics. These devices are capable of achieving high Q factors, can withstand high g shocks due to their small masses, are insensitive to linear vibration and consume little power. However, most of these micromachined gyroscopes have a very small rotation response, since their input and output vibration modes have different mode shapes and resonant frequencies. The use of different resonant modes also makes these devices very temperature sensitive due to the different temperature dependency of each of the modes. These devices usually have very high resonant frequencies resulting in low responsitivity, since the Coriolis induced response is inversely proportional to the resonant frequency of the structure. Finally, due to the small mass of the structure, thermal noise limits the ultimate performance and use of microgyroscopes. For these reasons, micromachined vibratory gyroscopes have not been used for spacecraft navigation and attitude control applications, but have been employed primarily for automotive applications in which extreme low cost is a major driving factor and performance is set at a lower premium.
Therefore, what is needed is some type of silicon micromachined vibratory gyroscope, accelerometer or sensor which overcomes each of the shortcomings of the prior art. In particular, what is needed is a silicon micromachined vibratory microgyroscope that:
(1) has a much larger mass than other micromachined microgyroscopes therefore providing a much lower mechanical resonance and lower thermal noise limit; PA1 (2) has a design which reduces fabrication complexity, is adaptable for batch production and avoids temperature sensitivity while providing a high rotational response; PA1 (3) has an inherently high Q design; PA1 (4) provides for simple and rugged detection and actuation; PA1 (5) operates to provide increased sensitivity and to reject common mode signals such as environmental vibrations and bias offsets; and PA1 (6) can be fabricated using silicon bulk micromachined techniques to reduce cost of fabrication complexity, to provide high reproducibility in the mechanical characteristics while retaining low cost, overall low mass, and low power.