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This article describes the sensor electronics needed in airbag applications. Over time, new features in the domain of passive safety have been added to overall automotive safety. On top of airbag sensing, electrically activated seat belt pre-tensioners that firmly hold passengers prior to the crash and rollover detection/mitigation are becoming more and more popular.

Combining with active safety systems (i.e. electronic stability control) can protect the passenger even better and will become the first step towards crash free driving—and sensors are the key building blocks.

Integration of active and passive safety functions in application specific ICs (ASICs) is vital. There are only a few companies having all function blocks and necessary semiconductor processes available to create synergies and to implement cost-effective, high-volume silicon strategies.

Once upon a time…
The idea of airbags is to reduce the risks of injuries or death for the passengers of a vehicle during a crash. Already in the 1960s, researchers were looking for an enhancement of the seat belt, which had been introduced in the 1950’s by Ford [1]. To further improve the safety of passengers, a new system had to be introduced to optimize the life saving effect of the seat belt—the airbag system.

The airbag system was originally an inflatable bag for the driver only, and in the case of an accident, the airbag electronics initiated the inflation of the airbag. Airbag electronics contained a bulky mechanical mass/spring contact sensor and many discrete components.

In the late 1980s, the electronics got more and more integrated, and the suppliers started to work on ASIC components. With the integration of the functionality into ASICs, the system cost could be significantly reduced, and a wider distribution of airbag systems was possible.

Have you got a bag?
A standard airbag system today consists of:

• Sensors that measure the reduction of velocity during a crash (negative acceleration sensors) and/or deformation of doors (pressure sensors)
• The cable/communication between the sensors and the electronic control unit (ECU)
• The ECU itself
• pyrotechnic inflators
• The airbag

The number of bags has increased significantly, and today, cars with more than 10 airbags can be found. With the increasing number of airbags, the number of sensors also increased—thus it is important to understand precisely the direction of impact and severity of the crash (fired bags, when not needed, usually lead to high repair costs and thus higher insurance rates).

Today it is common to have at least three mono-axial acceleration sensors in the periphery of the car body and one bi-axial acceleration sensor in the electronic control unit (ECU). To improve the performance and reaction time of the system, pressure sensors are also used to sense side impacts. The interpretation of a pressure sensor signal is quicker than the signal of an accelerometer, but the mounting location of the pressure sensor is inside the door. Side impacts that don't deform the door need acceleration sensors to get detected.

Electronic control unit (ECU) <br>The function of the ECU is to quickly read the data from the satellite sensors (those distributed throughout the car body) and to process the data in the airbag algorithm. The algorithm analyses the signals and does some validation by comparing the incoming signals with each other.

Once the algorithm detects a strong deceleration, it determines the angle of impact and calculates the severity of the crash. In modern multistage airbag systems, the algorithm also uses the signals coming from the capacitive seat sensors. They measure if there is a person sitting (passenger & rear seats) and whether it is a full size person or a child. The force of the airbag deployment will then be adapted accordingly.

The ECU receives the data from the satellite sensors (accelerometers and pressure sensors) via a serial 2-wire current bus, applies the algorithm in a microcontroller, and verifies the functionality of the main microcontroller with a second microcontroller.

Additionally, the ECU drives the actuators by immediately providing a high current (1.2/1.75A, 40V, 2 ms) to ignite the pyrotechnic inflators of the airbags and the seatbelt pre-tensioners. These tasks require different semiconductor technologies: Microcontrollers need memory and are low voltage devices, the communication with the various buses needs higher voltages (5-40V) and the actuator drivers (squib drivers) need to provide significant energy in a short time.

Data from the sensors and the status of the airbag system has to be forwarded to other systems in the car (i.e. emergency call) via a CAN or LIN bus. And all systems have to be safe and work reliably over the lifetime of the car. And all this must happen in a matter of milliseconds.

Satellites
Satellites are peripheral sensors linked to the ECU via a cost/size/weight efficient cable connection. A Manchester encoded transmission was considered ideal for robustness, efficiency, speed, and EMR/EMI. The sensors inside the satellite and the signal conditioning need to be powered over the wires. Today's buses are based on two wires only, and the power is also transmitted to the satellite on the same wires as the communication. The market requirements and commoditization require standards and now there are two major buses left: PSI5 and DSI.

Tricky applications
One of the most critical scenarios is a side crash in which the large thorax/side bags have to be inflated within 10 ms and the decision to inflate has to be done within 3-5ms [2 ].

The typical communication speed on the buses from the satellites is about 100 kbit/s—a satellite with 16-bit data output can send a theoretical maximum of 18 samples in 3 ms—but in reality, it is less than that. The system has to decide, if it is an abuse case or a crash, and then immediately open the side airbags based on a few measured samples. To make it as safe as possible, various cross checks are needed in such a situation.

New types of almost ultrasonic sensors (measuring the structure borne sound during a crash) promise to be faster and therefore allow the ECU to calculate the crash event more precisely. Those sensors are now seen in the first car platforms and need to prove their benefits.

MEMS satellite sensors
In today's cars, MEMS-based sensors are the standard solution for sensing inertial motions. Accelerometers in airbag applications have been available since 1994 (Volvo). One of the major market opportunities in car applications for accelerometers is sensing crashes [3]. The increasing number of airbags required to boost passenger safety is accompanied by an increased number of satellites, which partially counteracts the price drops over time.

Fig.1. Satellite sensors are mounted in the front, side, and sometimes side/rear and/or rear. The number and mounting location of the satellites depend on the size and construction/stiffness of the car body and available mounting space.

Airbag sensing is a high-volume market that is driven quickly towards standardization imposed by the two existing consortia (DSI and PSI5), where also production capability and efficiency will play a decisive role on the road to success. STMicroelectronics, already a major semiconductor supplier to the automotive industry, leverages consolidated and efficient 8-inch-wafer-based MEMS manufacturing machines and is directing efforts and resources to rapidly become a reference in the market.

(Part 2 of this feature gives an overview of automotive MEMS sensor technology and future applications.)

References

[1] Ching-Yao Chan, Fundamentals of Crash Sensing in Automotive Air Bag Systems, SAE, 2000

[2] Horst Bauer, Bosch, Kraftfahrtechnisches Taschenbuch, 1999

[3] Hubert Geitner, M. Ferraresi, F. Sindaco; Airbag Electronics: from single building blocks to integrated solutions, AMAA 2009 Berlin