Safety & Security

MEMS accelerometer to tackle vehicle safety issues

The next generation of active safety systems will require a range of additional sensors in order to measure the vehicle dynamics. MEMS devices play a dominant role in this context. The article describes requirements, technology and applications of the latest MEMS sensor generation.

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End of 2010 should have seen European road fatalities reduced by half compared to 2001. While this very challenging goal set back in 2003 by the European Commission will not be reached, estimated average reduction in the EU-27 member states is around 40%. All fatalities and injuries are down, reflecting trends observed in other Western nations, including the recently published findings from the United-States.

The US Department of Transportation's National Highway Traffic Safety Administration (NTHSA) disclosed that 2009 have seen the lowest number of death since 1950. These achievements were possible by focusing on several factors like legislation, road infrastructure and driver behavior but more importantly on improving overall vehicle safety. Passive restraint systems like seatbelts and airbags did play an important role in the last decades helping to save thousands of lives. Nowadays, many governments are ready to go even further and mandate programs shifting the focus from passive to active safety. Between 2011 and 2020, a new range of active safety systems to prevent crash to happen will be introduced including vehicle's Electronic Stability Control (ESC).

The ESC is an additional improvement to the anti-lock braking system (ABS) and traction control system (TCS). Its basic function is to stabilize the vehicle when it starts to skid by applying differential braking force to individual wheels and reducing engine torque. This automatic reaction is engineered for improved vehicle stability, especially during severe cornering and on low-friction road surfaces, by helping to reduce over-steering and under-steering. Additional sensors must be added to the ABS system in order to implement ESP functionality which includes a steering wheel angle sensor, a yaw rate sensor and a low g acceleration sensor that measure the vehicle dynamic response. This, obviously, creates new opportunities for MEMS sensor manufacturers like Freescale, who was ranked by iSuppli as the first supplier of automotive MEMS accelerometers in 2009.

Market perspective

ESC gained traction in the last 15 years by proving its ability to have a huge safety benefit. Several international studies have demonstrated through significant data collection that ESC significantly reduce the risk of a crash and help save thousands of lives annually. As a matter of fact, several car manufacturers in Japan, Europe and in the US have already introduced this equipment as standard on some of their car lines, promoting its safety benefit to their customers. Toyota and Daimler estimated that it could decrease the risk of single vehicle accident between -35% and -42%, respectively.

NTHSA estimated in 2006 that risk could be slashed by more than 30% avoiding up to 9600 fatalities and 252 000 injuries, per year. For example, it was demonstrated that SUV, which are very popular in this country, are more subject to rollover or loose of steering control in difficult driving conditions due to their high center of gravity. Equipped with ESC, rollover situation for this type of vehicle can be drastically reduced by more than 80%. With such safety benefits, governments all over the word mandated ESC as a compulsory safety equipment for passenger car: All new vehicles up to 4.5 tons sold in the US will have to be equipped by 2012. The European commission adopted legislation on ESC for new cars by November 2014 following major countries like Brazil, Japan and South Korea which already announced their ESC mandates for 2012 and beyond. With the 2010 fit rate being around 35% on a worldwide basis, this gives an idea about the huge effort still to be accomplished to reach the full implementation.

All these mandates create a huge demand and this is without surprise that Strategy Analytics recently announced that Safety systems will provide one of the highest growth applications over the 2009 to 2014 period. It is mainly driven by the implementation of several active systems including ESC, which goes from 26 million today up to 44 million systems in 2014. iSuppli estimates that this would represent a market of 47.7 million MEMS accelerometers at that time, 66% being stand alone dual axis low-g sensors. Indeed, the system requirements have evolved and refined over the years to better take into account various vehicle types (like four-wheel drive cars) and roads under a variety of weather conditions. The use of two axis low-g sensor gives also the possibility to integrate new functionalities such as hill start assist & Electric Parking Brake (EPB) by measuring accurately the tilt of the vehicle while on a slope. The addition of such functions together with the already tight performance required by the ESC, is a challenge for the accelerometer.

Sensor requirements

In an ESC system, the various MEMS sensors are usually installed very close to the vehicle's center of gravity and their task is to continuously watch for the vehicle’s chassis movements. Together with a yaw rate sensor which measures the angular acceleration along the vertical axis, a low-g inertial sensor is used to detect the vehicle’s lateral acceleration and thus provide additional information to the system. During a loss of control when the vehicle starts to slide, this acceleration is less than 1g. So, the inertial sensor must have a high sensitivity to sense the low-g motion together with a high accuracy.

This translates into a device’s output need with very low noise and a small zero-g acceleration shift in temperature. Furthermore, the accelerometer needs to be immune to the parasitic high frequency content present in the car at the chassis level. Low energy signals with large frequency bandwidth can be found, from few hundred Hz during normal driving condition to few kHz due to shocks coming from the road. All frequencies above 1kHz must be filtered to avoid corrupting the sensor response. By definition, an inertial sensor is highly sensitive to acceleration of any origin, since the micromachined sensing element is based on a seismic mass moving relative to a fixed plate. The sensor output signal is typically cleaned of parasitic high frequencies via electronic low pass filtering.  A sensor with an overdamped transducer which can eliminate this unwanted higher frequency acceleration content mechanically provides additional benefit.

Freescale recently released the MMA6900Q, a new XY low-g accelerometer which tackles all these challenges. It brings interesting characteristic and features making it perfectly suitable for ESC systems. It offers a robust design with very good immunity to parasitic vibrations and a wide full scale range (+/- 3,5g) thus enabling the ESC application to remain operational above +/- 1.7g in case of vehicle roll-over conditions. It also provides low noise output with a +/- 50mg offset stability over the entire automotive range, from -40°c to 105°C.

MMA6900Q, dual axis XY low-g sensor targeting ESC application

Technologies

Like all Freescale accelerometers, the MMA6900Q includes a surface micromachined capacitive sensing element and a control ASIC for the signal conditioning (conversion, amplification and filtering) assembled in a small QFN 6x6mm plastic package.

One of the key elements in the device's performances is obtained thanks to the proven automotive High Aspect Ratio MEMS transducer (MMA6900Q). The term 'high aspect ratio' refers to the width of the key mechanical features in the transducer such as the spring portion of the mass-spring system or the gap between movable and fixed capacitor plates. The technology delivers this high aspect ratio by a combination of a 25 µm thick SOI layer and narrow trenches defined by deep reactive ion etching (DRIE).

The HARMEMS silicon on insulator (SOI) process uses a deposited polysilicon layer, with air bridges, to form the electrical interconnects for the MEMS die. The poly air bridges are formed on a sacrificial oxide layer after the DRIE is used to form the MEMS structures. A timed chemical etch is then used to release the MEMS structures. Single crystal SOI allows better control of the DRIE process, thus giving better consistency in the mechanical properties of the device. The thick SOI layer provides increased stiffness and greater mass for the moving mechanical element, plus increased electrical capacitance.

Benefits are an increased sensitivity with an enhanced noise performance compared to standard surface micromachined processes, together with a improved reliability since it better mitigates possible in-use stiction. Combined with higher-than-vacuum hermetic sealing possible with glass frit wafer bonding, the transducer experiences considerable air resistance as it moves providing an overdamped mechanical response with a natural cut-off frequency below 1 kHz. Finally, small error tolerance of the system is enabled, again by the high aspect ratio of the MEMS process. Thicker capacitor plates mean less out of plane deformation of the sensor structure due to package stress over temperature variation. And, the improved signal-to-noise ratios available in HARMEMS translate to lower gain of the transducer signal in the sensor system.  Errors in transducer, ASIC, or package are reduced, making for a tighter total error from the product system.

Closer look of an HARMEMS transducer, with thick SOI layers, narrow trenches and poly bridges

For signal conditioning, an automotive proven 0.25 µm analog mixed signal technology is used combining precision analog blocks and high speed CMOS logic. Its high density of 25K gates/mm2 allows the integration of complex digital signal processing blocks (DSP) with many parametric trimming options. Two independent 16 bit sigma delta converters for the X and Y channels provide the interface between the sensing element and DSP.

Their detection resolution has been improved thanks to a high over-sampling frequency of the ΣΔ conversion, increasing the signal/noise ratio and dynamic range. The device has a 4 ms maximum recovery time following acceleration overload. A full digital signal conditioning is implemented bringing advantages such as programmability (filters; acceleration range) and autodiagnostics. Data integrity features improve the system's fail safe strategy like a continuous parity determination of programmed data array and SPI commands, capable of detecting potential "bit flips" during operation. Should any of these integrity checks fail, the device will respond with an error message, ensuring that communications faults will not be misinterpreted for valid acceleration measurements.

The device temperature and all critical internal voltages are continuously monitored to ensure accuracy of acceleration measurements. The device resets if any voltage exceeds acceptable limits or sends an error message when temperature exceeds a certain threshold. It provides an 11 noise free bit data output thus reducing susceptibility to PCB routing effect. Furthermore, it offers flexibility to the system designer by proposing a dual power supply 3.3V or 5V capability.

For packaging, MMA6900Q comes in a 16 leads 6x6x1.98mm QFN. This Industry standard package enables smaller PCB designs and better immunity to parasitic frequency vibrations. Indeed, its first package drum mode resonance frequency is at around 160kHz (per FEA results), far above any potential parasitic frequencies found in a vehicle.

Stacked die configuration of the MMA6900Q in a QFN 6x6mm package

Moving forward

With more than 15 years experience in airbag sensors design, test, simulation and production, Freescale brings its proven processes and technologies to the overall vehicle safety debate. The sensor volume increase due to the various mandates around the world already brought new challenges, especially in terms of system cost. Automotive suppliers and car manufacturers have looked at new system partition by integrating various safety modules together, like airbag and ESC. While this merge of passive and active safety systems brings for sure some benefits, it also has new impacts on the sensor characteristics which will require a new level of integration. Multi-axis components will be needed, integrating different sensing elements like yaw rate + low-g or medium-g + low-g sensors, capable of addressing various module configurations. Combining its system expertise with newer technologies to develop such future solutions, Freescale is ready to tackle these new challenges.

About the author: Matthieu Rezé, Freescale Halbleiter Deutschland, is resposnible for Sensor Product Marketing activities for Automotive customers in Europe. He is particularly in charge of developing automotive MEMS inertial and pressure sensors business for Freescale. Part of this activity includes the definition of next generation Sensor Products in close relationship with automotive suppliers for active and passive safety applications like airbag, VSC and TPMS.

Matthieu holds an engineering degree in Microelectronics from the French “École Supérieure de Technologie Électrique” in Paris and a Master of Science degree in Marketing from Middlesex University, UK.

He can be contacted via Matthieu.reze@freescale.com

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