MEMS gyroscopes, or strictly speaking MEMS angular rate sensors, are used whenever rate of turn (°/s) sensing is required without a fixed point of reference. This separates gyros from any other means of measuring rotation, such as a tachometer or potentiometer.
Silicon Sensing’s MEMS gyros all use the same unique patented VSG resonating ring technology to sense rotation rate through a phenomenon known as coriolis. The basic construction and principles of operation are explained briefly below.
Silicon Sensing produced the first MEMS VSG in the late 1990s, since then well over 20,000,000 MEMS gyros have been delivered to thousands of satisfied customers. To the best of our knowledge, almost all of these gyros are still operating satisfactorily after 20 years of continuous service, testament to the reliability of VSG. There are now three generations of MEMS VSG; Inductive, Capacitive and PZT, allowing Silicon Sensing to produce a wide range of MEMS VSG gyros from low-cost, precision, chipscale sensors (e.g.PinPoint®) up to FOG-grade high performance MEMS Gyro modules (e.g. CRH01).
A brief history of the Gyro
Gyros, and their useful application for stabilising things, first emerged at the start of the 20th Century and Silicon Sensing’s family tree dates back to these pioneering years. Gyros were originally mechanical devices which used a spinning mass supported such that its position in inertial space remains fixed thus allowing rotation of its support structure to be measured. Mechanical gyros, such as DTGs (Dynamically Tuned Gyros) are still around today where high precision is needed.
In the 1970s optical gyros emerged. RLGs (Ring Laser Gyro) and FOGs (Fibre Optic Gyro) use the phase shift of light travelling in opposite directions around a fixed path length to detect angular velocity. RLGs and FOGs are very accurate but are quite complex and so are relatively large and costly to manufacture.
Over the last twenty years, the world of inertial sensors has turned on its head with the emergence of ’solid state’ non-rotating rate sensors, incorrectly called gyros. Silicon Sensing was one of the first companies to commercially exploit the potential for solid state gyroscopes back in the 1980s, with the launch of VSG – Vibrating Structure Gyroscope.
Click here to see a complete timeline of Silicon Sensing’s pedigree of making gyros over the last 100 years.
MEMS VSG gyros - a short description of their basic construction and operation
All Silicon Sensing’s MEMS VSG gyros use a vibrating or resonating ring fabricated using a DRIE (Deep Reactive Ion Etch) bulk silicon process. The annular ring is supported in free-space by eight pairs of ‘dog-leg’ shaped, symmetrical spokes. The bulk silicon etch process and unique patented ring design enable close tolerance geometrical properties for precise balance and thermal stability and, unlike other MEMS gyros, there are no small gaps to create problems of interference and stiction. These features contribute significantly to VSG’s benchmark bias and scale factor stability over temperature and vibration and shock immunity. Another advantage of the design is its inherent immunity to acceleration induced rate error, or ‘g-sensitivity’.
Actuators/transducers are attached to the upper surface of the silicon ring perimeter and are electrically connected to bond pads on silicon via tracks on the spokes. These actuate or ‘drive’ the ring into its Cos2θ mode of vibration at resonance (like rubbing a wet finger on a wine glass causing it to ‘ring’) or detect radial motion of the ring perimeter either caused by the primary drive actuator or by the Coriolis force effect when the gyro is rotating about its sensing axis, which is through the centre of the ring. The combination of transducer technology and secondary pick-off transducers improves VSG’s signal-to-noise ratio, the benefit of which is a very low-noise device with excellent bias instability and ARW (Angular Random Walk).
Below are two videos, diagrammatically showing the resonance of the silicon MEMS ring. The first show the gyro powered, but not rotating (i.e. zero angular rate input). You can see that every point on the ring moves radially – in a straight line from the centre of the ring - except for the ‘nodes’ at +/- 45deg, shown as blue dots, which remain stationary.
In the second video, the gyro is now subjected to an angular rate input. The Coriolis force comes into play, causing each point on the ring moving outwards to ‘bend’ in one direction, whilst the points moving inward ‘bend’ the other way. The net effect of this is to move the vibrating mode around the ring, to an angle which is proportional to the rotational velocity.
The rotational velocity can be measured in two ways (i) by detecting the amount by which the previously nodal points now move – termed open loop measurement or (ii) by establishing a restoring force which keep the ring vibration mode in the original place on the ring – termed closed loop measurement.
Finally, the next illustration below shows a typical application. The gyro is assumed to be attached to the car such that it is sensitive to the car’s turning rate.
Powered up, and with the car travelling in a straight line, the silicon ring resonates in its Cos2θ mode, in the 0°-90° axis. The nodes at the 45° points (green and red dots) are essentially stationary. When the car turns a corner, coriolis forces (proportional to turning rate) try to move the mode of vibration round the axis, meaning that the original nodal points are no longer stationary. In the real gyro implementation, the controlling electronics operate in a closed loop configuration to maintain the resonating position with respect to its original axis. The ‘force’ necessary to achieve this is translated into an analogue (or digital) signal which is proportional to rotation rate.