MEMS inertial sensors provide the equilibrium sense to your car
MEMS sensors for acceleration and rotation entered the automotive market about 15 years ago. Acceleration sensors were providing high-g signals of crashes in airbag systems, whereas gyroscopes and low-g sensors were providing signals for roll-over detection and ESP® systems. Typically, systems for active and passive safety demand for a high degree of functional safety (addressed by a redundant system architecture) and for a moderate performance of sensors with respect to offset stability (±100 mg or ±3.5 °/s) and to noise (<15 mgRMS or 0.4 (°/s)RMS).
The ongoing introduction of advanced driver assistance systems and comfort features to modern cars relies on improved sensing capabilities of inertial sensors: The low-g offset stability needs to be narrowed down to ±50 mg (and an impressive 20 mg for short-term drifts) and the noise being as quiet as 6 mgRMS or 0.2 (°/s)RMS, respectively [ref: MM7].
Reflecting the different sets of requirements (summarized in Table 1), Bosch developed two product categories for its SMI7 sensor family (see Figure 1): SMI700 and SMI710 support advanced comfort and driver assistance functions (ADAS) such as adaptive cruise control, hill-hold control, roll-stability control, active damping systems or vehicle motion observers. Active and passive safety functions are targeted by the second product category, constituted by SMI740 for ESP® and SMI720 for roll-over sensing.
Table 1: Measurement characteristics of the acceleration channels
If a car is equipped with SMI7 sensors, the integrated gyroscopes will notice whether the chassis is rolling or pitching, or whether the car is skidding in a curve (the ESP® system will react). The low-g accelerometers will assist when starting at a steep hill. They will notice whether the chassis is jumping (the damper control will react) or whether the driver is accelerating or braking. Considering all this information, the SMI7 components truly provide the equilibrium sense to the car. They could even be used to improve the performance of a GNSS based navigation system on the road to automated driving capabilities.
Fig. 1: The SMI7 MEMS sensors
Unlike the signals of inertial sensors in a smart phone, the signals of automotive sensors are connected in the safe vehicle communication infrastructure. In combination with several other car internal sensors and the aid of appropriate algorithms, SMI7 signals are the basis for high reliable life saving functions. Therefore these sensors are designed in such a way that one can trust the signals they provide under the demanding conditions a car might be exposed to. Bosch addressed these topics with special focuse on functional safety, thermal drifts and on vibration robustness to provide highest levels of safety and availability.
SMI7 sensors can be used in applications with a rating up to ASIL D according to ISO26262, which is the international standard for automotive functional safety. In order to fulfill the requirements of ASIL D systems, Bosch relied on its system competence, its expertise in sensor integration as well as in semiconductor and MEMS design.
System specialists shaped the hardware safety requirements of SMI7 components by deriving them from safety goals of car systems for airbag, vehicle dynamics, electronic parking brake, or active front steering. Semiconductor specialists designed SMI7 such that these demanding requirements can be fulfilled, by introducing able monitors into the signal path and by systematically evaluating the sensor performance by Fault Tree Analysis (FTA) and the Failure Modes Effects and Diagnostic (FMEDA) method [ref: FS]. As a result, the applicability of SMI7 sensors in ASIL D vehicle dynamics systems was confirmed by a functional safety assessment according to ISO26262.
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 center-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. The choice of package, the design of MEMS elements and read-out ASICs need to mate in order to result in an excellent offset performance at hardest driving conditions. Figure 2 displays the offset stability of the low-g sensor output of the integrated accelerometers indicating the excellent performance of SMI7 [ref: MM7].
Fig. 2: Offset stability of the low-g sensor output
The promising 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 3 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 prove impressively, that SMI7 clearly fulfills the expectations for an automotive inertial sensor, such that you can trust the equilibrium sense of your car in every situation.
Fig. 3: 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 (70 micrometers), 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.
Bosch is one of the companies that has been shaping the micro-electro-mechanical systems (MEMS) technology since it first emerged in the 1980s. Until now, Bosch has manufactured well in excess of around five billion MEMS sensors. More than one billion sensors are shipped each year from its wafer fab in Reutlingen – or around three million each day.
References:
- [SMI7] „New generation of Bosch inertial sensors“, www.bosch-sensors.com, 10 March 2014.
- [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.
- [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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