Gyroscopes are devices that measure rotational changes. On first mention, most people picture the beautifully rotating classical mechanical gyroscope. Although widely used till the late 1900s, optical gyroscopes have largely replaced as the new industry standard due to having no moving parts, thus being far more reliable and accurate.

Above is an animation of a mechanical gyroscope. Notice how the inner spinning disk and axis maintain their rotation direction despite the outer ring’s random rotational movement. A mechanical gyroscope can thus be used to detect rotations by using the inner gyroscope’s axis as a reference point. However, all the moving parts make it a complex device to maintain. An optical gyroscope, shown below, is a compact piece of technology consisting of a laser inside a ring interferometer. As previously mentioned, the main advantage of optical gyroscopes is they have zero moving parts. Mostly used to stabilize and control motion, optical gyroscopes can be found in aircrafts, robots, smartphones and all such devices that need to constantly determine their current orientation in space.

human gyroscopes – vestibular systems

At first, this might not seem like it is so important. After all, we have a good sense of direction and can sense rotations. Why is it vital for pilots and aircrafts to have gyroscopes telling them their orientation?

Our brain primarily uses a combination of sight and internal pressure sensors (our vestibular systems) to gauge the orientation. Standing upright without swaying from side to side is hard when we close our eyes. But it is not impossible. Our eyes only assist. Our brain can independently detect gravity with the help of our vestibular systems. Consisting mainly of sensory hair cells in our inner ear that are activated by the moving of local fluids when our entire head or body moves, it is this vital system which fails in individuals suffering from the Meniere’s disease, causing vertigo and dizziness.

necessity of external gyroscopes?

When a pilot is in a plane, both the brain and eyes can provide a ‘false’ orientation of which way is down. As a plane banks while turning, our eyes fail to detect the turn since the entire cabin is also inclined. Similarly, the centripetal force causing the turn increases the downward pressure on our legs. This combined force is not in the direction of gravity, as can be seen in the figure, but in fact parallel to the plane cabin’s orientation. So the brain has no way of knowing its real orientation. All those who have looked out of a plane while it is turning know that while they can see they are leaning, sometimes more than 60 degrees, they do not feel an instability.

Fighter planes are exposed to much rougher banks during rolls and dives. No pilot’s vestibular system can help him maintain his orientation with respect to gravity, as his inner ear fluids will be moving in all directions. Thus having a reliable external gyroscope to keep track of the orientation of the aircraft at all times is essential.

Detecting Rotations

Each type of gyro has its share of advantages and drawbacks, and different requirements necessitate different forms of gyros.

Optical Gyroscopes Optical gyroscopes use the constancy of the speed of light as their governing principle to detect rotational changes. There are two different variations of optical gyroscopes, both of whose designs and comparisons we discuss in the next article. Advantages over mechanical gyros Optical gyroscopes have largely replaced their mechanical counterparts. Firstly, optical gyroscopes are far more accurate. They can measure rotational changes with an uncertainty of 0.001 degree/hour. As a reference the most accurate mechanical gyroscope provide only 0.1 degree/hour precision. Secondly, their internal laser and sensor technology are more reliable. Additionally, they have no moving parts and therefore require no maintenance. This results in a mean time of failures in excess of 60,000 hours. Optical gyroscopes also directly generate soft data, unlike the mechanical gyros. This is essential in our modern age. From autopilot functioning in aircrafts to guided missiles and autonomous rovers in Mars, modern systems only use sensors if they reliably generate computer-friendly data. Governing Principle of Optical GyrosCopes As mentioned above, there are two kinds of optical gyroscopes: the Ring Laser gyro and the Fiber Optics Gyro. Both rely on the same principle – the Sagnac effect – which is an outcome of the constancy of the speed of light.

The Sagnac effect is a shift of interference fringe position in a ring interferometer depending on the angular velocity of the set-up.

In 1913, Georges Sagnac conducted his experiment to prove the existence of the illusive aether. The existence of aether was a widely believed theoretical idea due to light’s constant speed, regardless of the source’s velocity. We now know that this is wrong, and that the constancy of the speed of light is actually due to the relativistic nature of our universe, as Einstein discovered. Interferometers Like most scientists working at relativistic scales in the physics of light, Sagnac used an interferometer – a laser device that splits its beam in 2 with the help of a half-silvered mirror. The light beams travel in different directions, with one of both often encountering different optical obstacles. The light beams merge back, and detector captures the resultant beam. Depending on the differences in their individual paths, the beams are out of phase, producing an interference pattern. A detector analyses the interference pattern to determine various properties of the experiment and the obstacles the light beam encountered. Scientific researchers and industry use interferometers extensively to measure small displacements, refractive indexes and surface irregularities. If you are more interested in the functioning of an interferometer, definitely do check out our previous article here

The Sagnac Effect The diagram of a ring setup interferometer is provided below. Once the silvered mirror splits the light beams, they both travel an equal distance in opposite directions around a loop. When the beams join back, a regular interference occurs since the light beams are in phase.