MEMS vs. FOG: Which you should choose

When selecting gyroscopes for a range of applications there is a growing need to choose between Si MEMS (Silicon Micro-Electro-Mechanical Systems) and Fibre Optic Gyroscopes (FOG). Both technologies have their strengths, with FOG technology having replaced mechanical gyroscopes and set the performance benchmark for many emerging technologies. 

MEMS VSG vs. Fibre Optic Gyroscopes (FOG) – the technology

MEMS gyroscopes come in various design formats. However, one of the most established designs delivering high performance is the Coriolis Vibratory Gyro (CVG).  This is a type of vibrating structure gyroscope, ‘VSG’, that uses a vibrating or resonating ring supported in free space by small spokes. Miniature actuators drive the ring into Cos2ϑ vibration, allowing radial motion to be measured.

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 calculates the direction and amount of rotation.

Comparison - CSWaP

In assessing FOG and MEMS gyroscopes it is crucial to consider their cost, size, weight, power (CSWaP) alongside their performance. Core project constraints such as budget, available space, platform weight, and power consumption will frequently dictate whether FOG or MEMS gyroscopes would be more appropriate.

FOGs are known for their precision and stability making them ideal for high-end, navigation-grade applications – for example, for commercial aviation and military operations. They provide exceptional bias stability over extended periods. However, the need to house long optical fibres mean FOGs typically have a much higher CSWaP than MEMS. The optical fibres increase the size and weight of the gyroscope and consume more power to maintain the light beams within them. Additionally, their complex, labour-intensive manufacturing process means they are more expensive to produce. These factors limit their use to applications where the need for sustained accuracy and precise performance outweighs higher cost, size and weight and power consumption.

Typically, Si MEMS gyroscopes are viewed as less accurate than the highest-performing FOGs when it comes to meeting bias stability requirements for some navigation environments.  However, they offer a noticeably lower CSWaP compared to FOG and so are particularly suitable wherever compact size and low weight are critical factors. Platforms such as autonomous vehicles and small satellites, applications such as aerospace systems, and operations such as down-hole drilling are just a few examples.

The extremely small size of MEMS gyros, often in single-digit millimetres, contrasts with FOGs which can be orders of magnitudes larger and heavier. Additionally, MEMS devices have no moving parts so consume significantly less power - a key factor for battery-operated and other energy-sensitive applications. 

Turning to cost - the manufacturing process of MEMS devices allows for mass production and economies of scale, making them a highly affordable option wherever budgets are constrained. 

In terms of performance, the latest MEMS technologies are closing the gap. MEMS gyroscopes today are achieving tactical-grade accuracy, with a bias instability as low as 0.03°/hr and angle random walk (ARW) of 0.004°√hr.  This is significantly challenging FOG performance – and for some applications, can match the performance of FOGs in a much smaller and lighter package. In addition, Si MEMS technology can enable innovative approaches, such as integrating multiple sensors, enhancing error detection, and incorporating advanced sensor fusion techniques. 

And improvements already well-underway in sensor design, manufacturing processes and materials will soon unlock performance enhancements that will allow Si MEMS technology to deliver navigation-grade performance for the first time, while maintaining their low CSWaP. 

MEMS vs FOG: The Smart Choice for Your Application

Choosing between MEMS and FOG gyroscopes 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 makes them unsuitable for many applications with constraints on these parameters.

Si 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. And as technology advances towards increased autonomy and connectivity, it is likely that MEMS gyroscopes will emerge not only as viable alternatives to FOGs but also as transformative solutions for the evolving challenges in inertial sensing.