MEMS sensors ensure vehicle safety

Modern sensors use a micro-electromechanical system (MEMS), which can be mass-produced at low cost and enable very small form factors. In this post I'll look at two types of MEMS sensors; accelerometers and gyroscopes, and explain how these parts work and how they are used in today’s vehicle safety systems.

Accelerometers
An accelerometer measures acceleration in one or more orthogonal directions. A typical way to visualise an accelerometer is a mass hanging from a spring, with the whole system subject to vertical acceleration. The deflection of the mass can be measured and used to calculate how much acceleration the system experienced.

A MEMS accelerometer typically has a silicon mass suspended at the corners by flexible beams, which allow the mass to move when it experiences acceleration. The mass usually has comb-like structures up the sides, interlocking with adjacent combs that don’t move. The lateral movement of the mass changes the capacitance between the teeth of the two combs, which is measured electrically and used to deduce the deflection, and therefore, the acceleration.

Accelerometer figures of merit include measurement range, accuracy and stability, physical size and operating temperature range. For example, the Murata SCA3100-D04 is a three-axis MEMS accelerometer with a ±2g range and ±30mg offset stability over the part’s full temperature range, which is -40 to 125°C. It’s stable over a wide range of humidity and vibration conditions, too. The SCA3100-D04 is qualified to AEC-Q100 and features a digital SPI interface and extensive self-diagnostic features (see the figure).

Murata's SCA3100-D04 MEMS accelerometer has extensive sel-diagnostic features

In modern vehicles, automotive accelerometers are used in a variety of safety systems. Sudden deceleration in the event of a crash triggers the airbags, for example. They are also used to determine when the car is travelling on an incline, supplying data to electronic handbrakes and hill start aid systems. Together with gyroscopes, they are also extensively used to detect the car’s motion in Electronic Stability Control (ESC) systems, roll over detection and electronically controlled suspension.

Gyroscopes
A gyroscope measures the orientation of an object, or its angular rate around one or more orthogonal axes. MEMS gyros use the Coriolis effect to measure angular rate; simply put, when a mass is subject to an angular rotation, it will experience a Coriolis (centripetal) force. The angular rate can be deduced from the size of the Coriolis force. In turn, the Coriolis force can be deduced by measuring the displacement of the mass due to the force. MEMS gyros measure the displacement of two masses relative to each other, which is deduced from the structure’s change in capacitance. This change in capacitance is easy to measure electrically.

The structure of a basic MEMS gyro might be shaped like a tuning fork; if the system is exposed to angular velocity, the Coriolis force on the two prongs acts in opposite directions, which changes the capacitance between them. If the system experiences linear acceleration, they move in the same direction and the capacitance doesn’t change. Today’s complex MEMS gyros, instead of two prongs on a fork, may have two complete accelerometer structures side by side, with the difference between their outputs used to determine angular rate.

Gyroscope figures of merit include the range of angular rate measurement, measurement precision and stability, low noise, small physical size and operating temperature range. For example, the Murata SCR1100-D04 is a single-axis MEMS gyro with an angular rate measurement range of ±300°/s. RMS Noise is typically just 0.14°/s, operating temperature range is -40 to 125°C and it measures 8.5x18x4.5mm. This part also features a sophisticated signal conditioning ASIC (see figure), temperature compensation and a digital SPI interface.

Murata's SCR1100-D04 MEMS gyroscope features a sophisticated signal processing ASIC

In a modern car, automotive gyroscopes serve several different purposes. They may be used to determine the direction a vehicle is travelling in as part of a dead-reckoning navigation system to complement a GPS. The dead-reckoning system kicks in when the GPS signal is lost, perhaps when travelling through an underground road tunnel.

Aside from navigation, a gyroscope is typically used in things like skid-prevention systems. The gyro is placed near the car’s centre of gravity to measure the yaw – yaw; beyond a certain level could indicate aquaplaning, for example. They also feature alongside accelerometers in Electronic Stability Control (ESC) systems, which help keep the car level when it goes round a corner at speed, and roll sensors, which indicate that the car has completely rolled over so the airbags can be triggered.

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