Have you ever marveled at how your smartphone knows how you are holding it?
How does it know that you are holding it sideways and that the landscape view is the best way to look at that website? What in your phone enables it to be used as a race car’s steering wheel in a game? And what does that have to do with the fact that hurricanes rotate clockwise in the northern hemisphere and counter-clockwise in the southern?
To find the answers to these questions, let’s look inside of a nuclear submarine.
Submarines used to navigate by sighting stars and landmarks through their periscope. With the advent of nuclear power, submarines could stay underwater for weeks at a time. Some submarine missions (like sailing under the Arctic ice cap) prohibit the use of periscopes, so how can the crew avoid hitting rocks and running aground? Clearly, a new means of navigation was needed and inertial navigation was developed in response.
Inertial navigation uses gyroscopes to measure the motion (acceleration) and orientation of the submarine.This information is then used in the ancient technique called dead reckoning to determine the submarine’s position. The first gyroscopes were precision-machined rotors spinning in a gimbal and mounted in a heavily-instrumented frame. An inertial navigation unit used three of these gyroscopes, was about the size of a table, and only governments could afford them. In the past 60 years, gyroscope design has made tremendous strides: mechanical gyroscopes were replaced by laser gyroscopes in submarines and the microelectricalmechanical system (MEMS) gyroscope has been developed. More on this later.
According to Wikipedia, the underlying physical principle of gyroscopes is that a rotating (or vibrating) object tends to continue rotating (or vibrating) in the same plane because of it’s inertia, even if its support rotates.If a mechanical gyroscope’s support frame is rotated, the gyroscope’s rotor exerts a force on that frame due to the Coriolis effect. This force can be measured very precisely. Put simply, a gyroscope knows its orientation in space and measures the forces used to change that orientation. Since gravity is a force, a gyroscope detects it and always knows up from down.
So, a gyroscope can be used in your smartphone to orient it to gravity, but how do you fit one in there?This is where the MEMS gyroscope comes in. These are extremely tiny vibrational gyroscopes that are made using many of the same techniques used to make computer chips plus some micromachining processes. The vibrating object at the heart of the MEMS gyroscope is a plate of silicon (called the “proof mass”) suspended by bridges of silicon and surrounded by interlocking fingers of conductive silicon. The interlocking fingers form the plates of a capacitor that changes its capacitance as the fingers get closer to each other. These capacitance changes are converted by an integrated circuit into electrical signals that your smartphone uses to orient the image on its display screen.
Let’s take a look at a typical MEMS gyroscope. The old iPhone 4 uses the STMicroelectronics L3G4200D MEMS gyroscope chip pictured here (outlined in red):
Here is a scanning electron microscope image of the L3G4200D gyroscope chip found inside that little black case:
In the center of this die is the proof mass – a suspended square plate. And here is a close-up view of that proof mass:
Note the scale marker. If you lay 8 human hairs (each of which are about 50 µm wide) side-by-side they would barely cover this little plate.
See this excellent article for a more detailed exploration – and more pictures - of MEMS gyroscopes.
So a teeny-tiny micro-machined plate of silicon vibrating inside a quarter-inch square chip is what your smartphone relies upon to keep it oriented. But what’s that got to do with hurricanes?
Gyroscopes - like this microscopic one - rely upon the Coriolis effect and a force (like gravity) to work. Applied on a much larger scale, the Coriolis effect and the rotation of the earth is what determines whether hurricanes rotate clockwise or counter-clockwise.
Which all goes to show just how important scale and perspective are when trying to understand how our wonderful world works.
BTW, Forget what you might have heard about Coriolis effects and toilets. Toilet bowls drain however they’re designed to – it’s irrelevant whether you are flushing north or south of the equator. Toilet bowls are just too small to be slaves to the Coriolis effect. Another myth down the ... ahem … drain.