Micromachined Vertical Vibrating Gyroscopes

Tu Xiang Zheng

As well known, micromachined vibrating gyroscopes have gained popularity in recent years. A main application of these sensors is to determine the yaw (vehicle rotation about its vertical axis) or roll (vehicle rotation about its lengthwise horizontal axis) angle of the vehicle. These sensors also play a key role in consumer electronics for platform stabilization in camcorders and video-game headsets.

The above mentioned US patent describes a micromachined vertical vibrating gyroscope that can be used to counteract the rolling effect on a vehicle, and thus, are a preferred stabilization tool for vehicles such as airplanes, ships, and cars. The emergence of micromachining technology has generated the possibility to produce gyroscopes that present many advantages over their counterparts, such as lower cost, small size, higher performance, and lower power consumption. These advantages explain why the gyroscopes have been widely used in smart phones.

The present gyroscope is implemented by vibrating the proof mass in a direction parallel to the substrate, either lateral or rotational, and sensing vertical displacement or torsional motion due to Coriolis effect.

The gyroscope consists of three single crystal silicon assemblies: an outer single crystal silicon assembly, an intermediate single crystal silicon assembly, and an inner single crystal silicon assembly. The outer assembly includes a plurality of arc-shaped anchors arranged in a circle and extending from a single crystal silicon substrate coated with an insulating annulus thereon. The intermediate assembly is a suspended wheel concentric with the arc-shaped anchors. The inner assembly is a suspended hub concentric with the circle formed by the anchors and having no axle at its center. The three assemblies are connected to each other through several flexures. The intermediate suspended wheel is driven into rotational vibration by lateral comb capacitors. Input angular rates are measured by two vertical capacitors. The gyroscope is fabricated utilizing a bipolar-compatible process comprising steps of buried layer diffusion, selective epitaxial growth and lateral overgrowth, deep reactive ion etching, and porous silicon processing.

In operation of the gyroscope a voltage is applied to the lateral driving capacitors. The intermediate wheel is then stimulated into rotational vibration about the coordinate z-axis that is set to be vertical to the substrate plane. For the rotational vibration of the wheel, the flexures provide flexible mechanical support. As the rotational angular becomes too large the stops begin to abate the vibration so as to prevent the flexures from damaging. The lateral monitoring capacitors is used to measure the frequency and amplitude of the rotational vibration of the wheel. When the substrate experiences an angular rate about the coordinate x-axis that is set to be perpendicular to the flexures a Coriolis force is induced. The Coriolis force exerts on the inner vibrating hub and causes the hub to be rotationally vibrated about the coordinate y-axis.

In the balance state the two vertical capacitors and are designed to be completely equal. When the hub rotates about the coordinate y-axis, the two vertical capacitors are no longer equal. If the hub rotates counterclockwise, the capacitance of the vertical capacitor will increase and the capacitance of the vertical capacitor will decrease. If the rotation direction reverses, the difference of the capacitance also reverses. Since the difference of the capacitance of the two vertical capacitors depends upon the input angular rate, the input angular rate can be determined by measuring the difference of the capacitance of the two vertical capacitors.

The measurement circuit of the gyroscope can be adopted in open loop or in close loop. In open loop the amplitude of a carrier signal can be modulated by the difference of the capacitance of the two vertical capacitors. After demodulation with the carrier frequency and the driving signal frequency a DC voltage proportional to the input angular rate can be yielded as the output of the measurement circuit. In close loop the yielded signal is first fed to a rebalance circuit. The rebalance circuit then provides a rebalance voltage applying to the vertical capacitors to null the rotation of the inner vibrating hub about the coordinate y-axis. The rebalance voltage is proportional to the input angular rate. So the measurement circuit can be implemented as a Σ∆ interface circuit.