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Examining automotive MEMS inertial sensors

17 Nov 2014  | Michael Baus, Peter Spoden

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SMI7 sensors are designed for use in integrated airbag ECUs (ABplus) or for sensor clusters (e.g. MM7), which allow to locate the sensor close to the centre-of-gravity of the car. In order to achieve the necessary performance with respect to mission profile or vibration loads, engineers need to reflect corresponding requirements in each part of the sensor design. Figure 1 displays the offset stability of the low-g sensor output of the integrated accelerometers [ref: MM7].


 Offset stability of low-g sensor output

Figure 1: Offset stability of the low-g sensor output.


The laboratory measurements were verified in a real-world scenario: A vehicle test-drive on a gravel road. Driving maneuvers under bad road conditions or on gravel roads constitute the hardest test for an inertial sensor in vehicle dynamics applications.

Figure 2 shows an example of such a test drive: Despite high-frequency signals of up to 4000 m/s², the deviations between SMI7 in an MM7 sensor cluster and an industrial reference sensor were negligible [ref: MM7]. These measurements show that SMI7 is able to fulfil the expectations for an automotive inertial sensor, such that you can trust the equilibrium sense of your car in every situation.


 Test drive results

Figure 2: Sensor response during drive under bad road conditions.


MEMS sensors contain the finest silicon structures. As the casing moves, these structures shift a fraction of a thousandth of a millimeter – and their electrical properties change in the process. These properties can be measured and converted into a data stream. The dimensions are incredibly small; while a human hair has a diameter of 70 thousandths of a millimeter (70µm), some components measure only four micrometers – that is 17 times smaller than the diameter of a single human hair. Since the micromechanical sensor produces only weak electrical signals, the developers built in another electronic component – sometimes in the casing beside the sensor, sometimes even directly on the same chip. This second component processes, amplifies, and converts the weak signal into digital data. In this way, MEMS sensors can supply control units directly with readings.


References
1. [FS] P. Spoden, Current aspects of functional safety of SMI7 inertial sensors for vehicle dynamics functions", IPQC conference "The road to automated drive", 30 June 2014.

2. [MM7] M. Baus "MM7: a high performance inertial sensor cluster for future automotive safety and assistance functions", IPQC conference "The road to automated drive", 30 June 2014.


About the authors
Michael Baus studied Electrical Engineering and Information Technology at the RWTH Aachen. He joined Bosch in 2005 as sensor application engineer. In 2009, he took over the project responsibility for a new platform generation of inertial sensor at the division Chassis Systems Control. Since 2012 he is the responsible group manager for the development of inertial sensor clusters.

Peter Spoden works for Bosch as a product manager for acceleration and inertial sensors in the division Automotive Electronics. He holds a PhD in physics and joined Bosch in 2005. Since then he worked as an application engineer and as project manager in the field of MEMS sensors.


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