Galileo meets MEMS

Nils Forsblom
Nils Forsblom wrote this on

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Galileo, father of modern physics, meets MEMS

What does the father of modern physics have to do with the tiny MEMS motion sensors in your smartphone? Plenty. A mathematician known for his pioneering observations of nature, Galileo (1564-1642) was the forefather of much of what we know about motion today.

The ultimate disrupter of his time, Galileo challenged Aristotelian theories people held true for centuries. For example, Aristotle claimed a rock fell to the ground because the two were made of the same element: earth. In contrast, Galileo studied quantifiable entities like time, distance and acceleration to explain what makes objects fall, break, and bend.

Galileo showed that force causes acceleration. On the basis of the law of parabolic fall, Galileo found that bodies fall at a constant acceleration, and that gravity is a constant force. In other words, a constant force does not lead to constant speed but to constant acceleration. He also developed the concept of inertia, which states an object in motion only stops due to friction. A hundred years later, Isaac Newton (1642-1726) built on these ideas to develop his first law of motion.

Later researchers expanded on the discoveries of Galileo—and Newton— to develop mechanical devices like the gyroscope and the accelerometer, instruments that play a critical role in navigation systems. The problem was these early mechanical devices were bulky and expensive. So scientist continued working to make them smaller and smaller.

Today gyroscopes and accelerometers have evolved into tiny devices called MEMS (micro-electro-mechanical systems). Built on silicon wafers alongside the circuits that control them, these are the smallest machines ever made.

It is pretty amazing to think that some of the MEMs motion sensors today are rooted in the principles of physics discovered centuries ago. Let’s take a look at four motion devices on the smartphone, how they work, and their history.

Gyroscope

If you’ve ever played with a top, you know that a spinning top somehow has the power to stand upright. Without torque to change direction, a spinning wheel will always remain pointed in the same direction. A gyroscope is essentially a spinning top mounted on a gimbal, so the top’s axis is free to orient itself anyway it wants.

The first gyroscope was invented in 1852 by French physicist Leon Foucault to demonstrate the Earth’s rotation. Since our planet rotates, the gyroscope’s axis completes a full rotation once every 24 hours.

Electric motors in the 1800s made it possible for gyroscopes to spin indefinitely. And today, gyroscopes play an essential role in inertial guidance systems on ships and aircraft.

Many types of MEMS gyroscopes exist. Each type has some form of oscillating component for detecting directional change. All MEMs gyroscopes take advantage of the (Coriolis effect)[https://en.wikipedia.org/wiki/Coriolis_effect. And typically, the one on your smartphone is a three-axis gyroscope for measuring roll, pitch and yaw.

Accelerometer

Have you ever wondered how your smartphone knows how you are holding it? It uses a device called an accelerometer, which measures g-force. The first accelerometer was invented by George Atwood in 1783.

If you want to know how an accelerometer works, picture a box with a metal ball inside suspended by a spring. If you move the box up, the ball lags behind, stretching the spring. By measuring the force on the spring, you can measure acceleration.

While the MEMs accelerometer in your phone is more complex than the simple ball and spring model, it uses the same fundamental principles. Inside the chip, engineers have created a tiny triple-axis accelerometer out of silicon that measures acceleration in the x, y, and z dimensions, so your phone always knows which way is down.

Magnetometer compass

Used for navigation, the traditional compass has a magnetic needle that points to the North Pole. The earliest compasses were most likely invented by the Chinese in 206 BC and used for fortune telling. Later, compasses were used for navigation so sailing vessels could set their direction without having to rely on the stars.

The magnetometer sensor in your smartphone doesn’t use a magnetic needle. Instead, it uses an analog transducer to create a miniature Hall-effect sensor that detects the Earth’s magnetic field along the x, y and z axis.

The Hall-effect sensor produces voltage proportional to the strength and polarity of the magnetic field along the axis each sensor is directed. The voltage is converted to a digital signal representing the strength of the magnetic field.

The magnetometer is enclosed in a small electronic chip that often incorporates a three-axis accelerometer to determine which way is down.

GPS

The GPS receiver on your smartphone is a more modern invention—and technically it is not a sensor—but it works with the motion sensors in your device to more accurately determine movement and location.

Originally created by the US for military purposes, GPS (global positioning system) is a space-based navigation system made up of 24 satellites that circle the earth twice a day in a precise orbit. The satellites are spaced so that at least four are visible from any point on Earth at a given time.

Your GPS receiver requires data from at least four satellites to fix a position. It uses three satellites for trilateration to reduce your possible location to two points. The timing code from a fourth satellite is used to narrow down your location to one of those two points. (GPS satellites, by the way, tell very accurate time.)

Working together, these three MEMS sensors (gyroscope, accelerometer, compass) and the GPS receiver in your smartphone provide accurate data to determine navigation and a data on range of different user motions. Next time, you use your mobile device, you can thank Galileo, who opened up humankind to a new age of discovery and ultimately contributed to much of the smartphone’s innate intelligence.

Introducing Project Galileo.

The Adtile Technologies team is planning to release a cutting-edge motion processing dev kit (Project Galileo) that will allow developers to create new interactions and applications. Please sign up for more updates.

Nils Forsblom

About the Author

Nils is the CEO & Founder of Adtile. You can reach him at nils@adtile.me.

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