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You Spin Me Right Round: Measuring Movement with MEMS

The industry keeps growing, as the use of microscopic inertial sensors finds its way into new industries.

When Bosch introduced its “electronic control unit for passive restraint” with the Mercedes-Benz S-Class in 1981, the sensor that triggered its airbag and seatbelt tightener was a steel ball held by a magnet at one end of a tube. If an impact was strong enough to dislodge the ball, it rolled toward a switch that signaled the  car’s airbag control unit.

Since then, airbags are estimated to have saved between 50,000 and 90,000 lives. Their impact was huge in the 1980s, even though that original steel ball was smaller than a pea. Today, inertial sensors which use micro-electromechanical systems (MEMS) to measure acceleration and motion in space are the new standard. They are much more accurate and they are much, much smaller.

Meet the marvelous MEMS in motion

Not only is it easy to overlook the importance of MEMS, it’s almost physically impossible not to: MEMS are microscopic systems (measured in units between micrometers and millimeters) with both electrical and mechanical components. They are the three-dimensional counterpart to the two-dimensional integrated circuit: etched silicon wafers with additional layers that can include springs, gears, and electrodes that can act as inertial sensors, better known to those of us who own smart phones (i.e. everyone) as accelerometers and gyroscopes.

This MEMS devices measures acceleration with this center mass, which moves between fixed outer plates. Acceleration in one direction causes the mass to move between the plates, changing the capacitance which is measured and processed to correspond to a specific acceleration value.

Every iPhone since the original version has contained an accelerometer, and Apple added the ability to detect motion into six axes by adding a gyroscope with the release of the iPhone 4 in 2010. Together, these inertial sensors enable a variety of functions, from stabilizing the camera to translating motion into inputs for games (just as Nintendo did in 2006 when it introduced the Nintendo Wii).

Today, the steel ball of the airbag’s proto-accelerometer has been replaced by a “proof mass” of semiconductive material, like the silicon it’s usually built on, whose movement is measured by piezoresistive sensors that quantify force by sensing the corresponding change in electrical resistance in the material. And while most of us have seen gyroscopes in the self-stabilizing spin of a bike wheel, in MEMS the wheel is replaced by a microscopic proof mass whose angular velocity is found by using its vibrations to measure Coriolis force (the same force that keeps the bike wheel level).

The market for MEMS

To find their place in phones and IoT devices, the devices’ increasing sophistication and complexity can’t increase their size.

But these foundries’ MEMS production need to feed high-volume markets like cell phone and automotive manufacturers, as well as low-volume, high-margin markets like disposable medical devices and the inertial sensors in missiles and rockets. So despite all this complexity, technology manufacturers need MEMS foundries to be able to produce at scale.

The total modern MEMS market was estimated at $12.1 billion USD in 2020. By 2025, total revenue could grow $17.7 billion.

Motion-sensing MEMS are making massive market movements

Pedometers were one of the earliest uses for a simple, wearable accelerometer whose single axis of motion allowed the device to register each step. Now, more flexible electronics are allowing these sensors to be integrated into an even wider range of products, including clothing, moving inertial sensors and their related health monitors from our wrists onto our bodies.

As wearable devices like the Apple Watch have already shown, integrating health sensors that can measure blood oxygen and heart rate makes it simpler to track fitness goals and even literally save lives, all because the devices are already on hand (or on wrist). And none of it would be possible without the miniaturization of MEMS. In 2017, the global wearable market was worth about $30 billion USD (5.1MB download). By 2026, it could grow to as much as $150 billion.

And inertial sensor technology continues to improve, offering the same degree of innovation that once transformed automotive safety and rocket flight alike. In space, the James Webb space telescope, designed to see far enough into space to see the earliest galaxies, will include “wine glass” gyroscopes with no moving parts, instead measuring the flexing vibration of a bowl-shaped crystal. Meanwhile, at the University of Michigan, scientists have developed a tiny wine glass gyroscope that could be is 10,000 times more accurate than conventional phone gyroscopes (albeit 10 times more expensive). With increasing demand for autonomous drones and vehicles that use driver-assist technologies, these more accurate sensors could increase the accuracy of navigation in areas where a GPS signal is weak or nonexistent.

From building inertial sensors into our cars, to integrating them into our clothing or aiming them at space, the forward momentum of inertial MEMS is undeniable. And as for their future: just consider what Isaac Newton what told us about a body already in motion.