MEMS vs FOG: Which one should you choose?

MEMS VSG vs. Fibre Optic Gyroscopes (FOG): A comparative analysis

When selecting gyroscopes for various applications, the need to choose between Si MEMS (Silicon Micro-Electro-Mechanical Systems) and Fibre Optic Gyroscopes (FOG) is becoming more common. Both technologies have their strengths, and FOG technology has matured to the point where it has replaced mechanical gyroscopes and set the benchmark for many emerging technologies.

Because MEMS inertial sensors are proliferated in low-end, consumer applications, many people still view MEMS as an immature technology for high-end, high-integrity inertial applications. However, the reality is very different from this viewpoint. For over a decade, MEMS inertial sensors have been able to offer incredible performance in high-integrity applications, in some cases becoming a viable alternative to FOG.

MEMS gyroscopes come in various forms, but one of the most proven designs for high performance is the Coriolis Vibratory Gyro (CVG), a type of vibrating structure gyroscope (VSG). CVGs utilise a vibrating or resonating ring, supported in free space by small spokes. Miniature actuators drive the ring into Cos2ϑ vibration, allowing the radial motion to be measured. In contrast, Fibre Optic Gyroscopes (FOGs) use continuous or pulsed light to calculate motion. Identical beams of light are sent in opposite directions through a coil of optical fibre towards a detector. By measuring either the time difference or phase shift between them, the FOG can calculate the direction and amount of rotation.

 

Comparison

The suitability of FOG and MEMS gyroscopes cannot be assessed without considering their cost, size, weight, power (CSWaP), and performance. These factors are crucial because they directly impact the viability of integrating inertial sensors into the chosen application. Project constraints such as budget limits, space, weight, and power restrictions dictate if FOG or MEMS gyroscopes would be more appropriate. Broadly speaking, MEMS offer a lower CSWaP compared to FOG due to their design and manufacturing process.

MEMS gyroscopes excel in applications where compact size and low weight are essential, such as autonomous vehicles, down-hole drilling, and aerospace systems. Their incredibly small size, often in single-digit millimeters, provides a significant advantage over FOGs which are typically orders of magnitudes larger and heavier. Additionally, MEMS gyroscopes are highly power-efficient. Their design, which operates without moving parts, significantly reduces power consumption, making them ideal for battery-operated and energy-sensitive applications. Furthermore, the manufacturing process of MEMS devices allows for mass production and economies of scale, dramatically reducing their cost. This affordability makes MEMS gyroscopes ideal for a broad range of applications, particularly where budget constraints are a concern.

More importantly, MEMS gyroscopes are already available in the market today that can achieve tactical-grade accuracy, with a Bias Instability as low as 0.03°/hr and Angle Random Walk of 0.004°√hr.  This significantly closes the gap, and in some cases exceed the performance of some FOGs, but in a much smaller, lighter package. It can be stated that Si MEMS gyroscopes are generally less accurate than the highest-performing FOGs when it comes to the bias stability required for navigation in certain environments.  However, due to the maturity, further improvements are expected in terms of sensor design, manufacturing processes, and materials that will unlock significant performance improvements, allowing Si MEMS to achieve navigation grade performance in the coming years while maintaining low CSWaP.

FOGs are known for their precision and stability, making them ideal for high-end applications such as navigation systems in commercial aviation and military operations. Their ability to provide exceptional bias stability over extended periods of time makes them suitable for these navigation-grade applications. Despite the perception that FOGs are far removed from MEMS gyroscopes in terms of performance, modern MEMS technologies are closing the gap. However, FOGs still hold an edge in applications requiring extreme accuracy. Due to the need to house long optical fibres, FOGs typically have a much higher CSWaP than MEMS. These optical fibres increase the size and weight of the gyroscope and consume more power to maintain the light beams within them, making the system less energy efficient. Finally, their complex and labour-intensive manufacturing process also makes them far more expensive to produce, which limits their use to applications where accuracy and performance justify the higher cost.

 

MEMS vs FOG: The Smart Choice for Your Application

Choosing between MEMS and FOG gyroscopes heavily depends on the specific requirements of your application. FOGs offer exceptional bias stability, making them ideal for navigation-grade applications where inertial data is needed for extended periods. However, their high CSWaP make them unsuitable for many applications with constraints on these parameters such as unmanned autonomous vessels, robotics SmallSat space applications.

On the other hand, MEMS gyroscopes deliver high-accuracy data at a more affordable price and in a smaller, lighter form factor, making them optimal for industrial and tactical applications. MEMS technology also enables innovative approaches, such as integrating multiple sensors, enhancing error detection, and incorporating advanced sensor fusion techniques. This not only improves performance but also keeps the CSWaP (Cost, Size, Weight, and Power) lower than that of FOGs.

As technology advances towards increased autonomy and connectivity, MEMS gyroscopes are emerging not only as viable alternatives to FOGs but also as transformative solutions for the evolving challenges in inertial sensing. Their cost-efficiency, compactness, and reliability make them suitable for a wide range of applications.

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