Putting cars in to motion: Page 2 of 5

April 03, 2014 //By Jari Nieminen, Murata
Putting cars in to motion
MEMS sensors are increasingly providing a crucial element in closed-loop automotive control systems.
performance, being overall more demanding than an airbag which only requires a relatively simple 'switch-like' response.

Fundamentally, however, both employ accelerometers implemented in MEMS technology, typically sensing the movement of a suspended mass, detected in the form of a capacitive change. As well as measuring an increase/decrease in acceleration, accelerometers use the same principle to detect a range of effects, including tilt/inclination, shock/vibration and centrifugal force. Sensors, such as gyroscopes, are typically used to detect pitch, roll and yaw, while accelerometers detect linear movement in the X, Y and Z-axes.

The use of multiple sensors allows more sophisticated control, while many safety systems require just one sensor, often in a single plane. One such application is electronically controlled suspension systems, where the important axis of interest is essentially only the Z axis, given that a car’s movement, predominantly in the X/Y axes, can be improved under all road conditions by dynamically adjusting the suspension.

MEMS Machining

Unlike passive systems, active safety relies more heavily on degrees of change. This demands sensors that offer an output calibrated to represent a window of inertial change (effectively the gravitational mass), which in turn requires greater sensitivity when measuring the degrees of change in capacitance caused by the mass moving. The speed and accuracy with which any resolvable change can be detected therefore dictates the efficiency of the sensor in a given application.

Sensitivity is directly influenced by the manufacturing process used to create the micro-structures; most MEMS processes use either bulk or surface micro-machining to create structures. Bulk micro-machining techniques employ selective etching (normally KOH-wet etching) of a silicon wafers, which allow features to be created inside the structure by etching through the wafer; surface micro-machining, on the other hand, uses deposition (and etching) to build structures layer-by-layer on a substrate, or etching structures on top of the wafer.

Ostensibly, bulk micro-machining offers greater stability and scaleability, while surface micro-machining is more compatible

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