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Explore MEMS uses in automotives

Posted: 23 Apr 2014     Print Version  Bookmark and Share

Keywords:Control systems  micro electro-mechanical systems  MEMS  active safety  SCA720 

Control systems of all types utilise closed-loops that rely on feedback in a bid to assess cause and effect. Once the effect has been assessed, the cause (or stimulus) can be modified, a reiterative process that improves the time to result exponentially. Not surprisingly this closed-loop approach to control enables many applications, but it is also now being applied extensively to improve the efficiency of a growing number of others.

A key element to this trend is the availability of reliable, responsive and robust sensors, which form a critical link in the feedback chain. Sensors, in general but particularly those that rely on a measurable, physical change in the sensor material (caused by exposure to the condition/property being measured) are commonplace, often passive and have well defined limitations. The introduction of micro electro-mechanical systems, or MEMS, however, is breaking down these barriers to how, where and for what sensors can now be used.

MEMS in automotives
Probably the most widely reported, researched and developed application for MEMS sensors is the measurement of inertial change in the form of acceleration and orientation (accelerometers and gyroscopes, respectively). Their tiny dimensions and robust construction mean MEMS sensors can be used in harsh environments where space is limited, to create use-cases that would not be possible without them.

In automotive systems, MEMS sensors now enable a growing range of control solutions that deliver greater efficiency and safety in vehicles; MEMS now enable safety features such as airbags, antilock braking systems, electronic stability programs and electronically controlled suspension, as well as advanced driver-assist systems such as hill start assist and electric parking brake features, lane departure warning, automatic cruise control and more.

Broadly speaking, MEMS sensors in automotive are used to enable or enhance safety; typically, in passive safety systems, such as airbags, the sensor acts as a trigger. Conversely, active systems, such as antilock braking, active suspension or stability control, require more sensitivity and 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 cars 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 with traditional IC fabrication, allowing simpler active circuit integration. However both also have their associated drawbacks and the difference between the two has, in recent times, become less distinct with the evolution of MEMS manufacturing technology.

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