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Side airbag sensor

A new generation of side airbag sensors that use a linear accelerometer has been developed by researchers (D. Ullmann, G. Bischopink, M. Schofthaler, R. Schellin, B. Maihofer, J. Seibold, and J. Marek) at Robert Bosch GmbH. For side airbag systems it is necessary to measure the acceleration within a time frame of less than 3 ms to inflate the side airbag in time. The valuation circuit includes amplification, temperature coefficient compensation, two-wire unidirectional current interface, and a zero-offset compensation. The sensing element for the measurement of acceleration is a surface micromachined accelerometer. To minimize production costs, the surface micromachined sensor element and the corresponding evaluation ASIC are packaged into a standard PLCC28 housing. For the entire function only a few external components are necessary. During the power-on cycle an internal self-test is performed and the result is transmitted to the airbag control unit.

The peripheral acceleration sensor (PAS) is used in PC-board technology. The main part is a PLCC28 housing that contains the micromachined sensor element and the evaluation IC, which is also used for the calibration of the sensitivity. The PC-board is mounted inside a customer-specific plastic housing for direct assembly into the car. To obtain a waterproof outer housing, a lid with a sealing ring is used. This results in a high sensitivity against environmental conditions.

The sensor measures the real acceleration value. The amplified and filtered analog signal (three-pole Bessel filter at 400 Hz) is converted into a digital signal. This value is transmitted via a two-wire current interface to the airbag control unit. The sensor supply voltage is 6 V and current consumption is less than 40 mA. The full-scale acceleration range is normally ± 100 g and can be adapted to ± 50 g by using an external resistor. The accelerometer is tested by a real deflection of the sensing element with electrostatic forces. The interface from the airbag control unit to the PAS can be realized by using the peripheral integrated circuit (PIC). The PIC can provide the power supply for the control unit and PAS as well as the communication interface for two PAS.

Structure of the microsafing sensor being developed by Akebono Brake Industry Co., Ltd.

Micro-safing sensor

Control systems such as ABS and airbags are dependent on sensors. Many of today's vehicles have both frontal and side airbag systems. Safing sensors, whose purpose is the prevention of malfunction, are incorporated into airbag systems to detect an impact from collision simultaneoulsy with the main acceleration sensor in the deployment of the airbag. However, there is a problem concerning the acceleration sensor for the side airbag.

In a side collision the crushable zone is smaller than in a head-on collision, so in a side collision, the deployment of the airbag must be quicker. Therefore, analog sensors are used for the side airbags because mechanical lead-switch type safing sensors, used mainly for front collision sensors, have a slow response speed. However, this has resulted in such sensors being expensive and subject to electromagnetic disturbance. Researcher Masatomo Mori from Akebono Brake Industry Co., Ltd. has developed a new sensor to meet these challenges.

The sensor is composed of an upper electrode, cantilever, and pedestal formed by silicon micromachining. The upper electrode and cantilever contact mechanically when acceleration during a collision is applied and the "on" signal is input. To achieve a satisfactory solution, an optimal shape was designed using an FEM simulation. The frequency characteristic of 100 Hz and threshold acceleration of 2-4 g are obtained by electrostatic force and chattering measure at the contact point. Other characteristics include high-speed response, compactness, and the low cost.

Sensing system discrimination

The absolute severity of a vehicle crash cannot be determined until the crash event is complete. However, airbag-sensing systems are required to discriminate the severity of the crash event in the first milliseconds of an impact. Future airbag systems will require even more discrimination capability than current systems to provide separate deployment thresholds for advanced technologies such as multi-staged airbags and pretensioners and threshold shifting for belted and unbelted occupants. A prototype advanced sensing system has been tested by researchers (Colm Boran, Canice Boran, and Douglas McConnell) at Visteon Automotive Systems. This prototype may improve the severity measurement of an impact and offer multiple deployment thresholds without increasing the time required for event discrimination.

Sensing systems today often are packaged in the vehicle's interior on the center tunnel or other internal location. The sensing system is typically a single electronic control unit (ECU) which uses internal accelerometers to measure the severity of the impact and provide deployment signals to the appropriate squibs. The vehicle body structure must transfer the force from the crush zone to the ECU mounting location in time for the ECU to make the deployment decisions.

The acceleration signal at the ECU monitoring location in moderate-severity impacts may be similar to the signal in a severe impact due to the deployment decision. The ECU must predict the severity of the impact prior to deployment based on the acceleration data acquired. Increased separation between moderately severe and severe event signals will yield improved discrimination between the events.

The method chosen by the researchers to increase the separation between the signals was to use the combined measurements of the vehicle acceleration in the crush zone and the occupant compartment. The prototype system simultaneously uses accelerometer data from the engine and occupant compartments to provide more advanced discrimination than current sensing systems. This method allowed the sensor system to measure a significantly larger percentage of the overall vehicle velocity change prior to the deployment decision time, required little change in the vehicle structure, and also did not increase the time required for the deployment decision. In this way, deployment events may be more easily separated into various levels of severity, and non-deploy events may be more easily detected.

Magnetically distributed VR sensor configuration from GM.

Variable reluctance sensors

A novel variable reluctance speed sensor with distributed magnetic circuit configuration is being investigated by Andrzej M. Pawlak at the GM Global Research and Development Center. When compared with a lumped element sensor, it shows improved performance and simplicity because of its magnetically distributed configuration and reduced number of parts.

Variable reluctance (VR) speed and position sensors are widely used in industry and, in particular, the automotive industry because of their low cost and high reliability. Unlike other types, these sensors are self-excited. They do not require an external voltage source to operate.

Two factors led to the development of the magnetically distributed sensor configuration. One is concerned with the shape of the wheel bearing grease cover where the sensor would be integrated. The second is reduced sensitivity of the distributed sensor to the radial tolerances which offset the exciter wheel with respect to the sensor. A ring-shaped sensor would fit best and could be integrated within the bearing grease cover while its magnetic distribution would help to compensate for the radial tolerances. Unlike existing lumped element VR sensors, the distributed ones use fully 3D magnetic field variation for the signal development. This sensor signal is a function of the number of magnetic pole pairs.

Polymer PTC sensors

There has been increasing interest in developing a reliable sensor to guard automotive interior motors from an unexpected current surge. Researchers (Liren Zhao, Jeff West, and Prasad Khaadkikar) at Therm-O-Disc, Inc. have developed self-resettable positive temperature coefficient (PTC) sensors from two polymer PTC material systems. One is a polyolefin-based conductive material that provides a switch temperature of around 125°C. The other is a polyamide-based material with a higher switch temperature of 165°C for applications where the ambient temperature is typically higher. A wide range of motors in the automotive interior requires over-current or over-temperature protection. These motors include ones for power seats, power door locks, and window lifts. There is recent interest in protecting underhood cooling fan motors, which can get blocked with snow in the winter.

Three important material characteristics are required for these sensors—low material resistivity (10 Ù/cm or less) or low device resistance with a specified geometric design (typically 50 mÙ or less; relatively high PTC effect (5 X 10³ or higher), where the PTC effect is defined as the ratio of peak resistance, or resistivity, at high temperature to that at room temperature; and ability to self-reset. A polymer material with both low resistivity and high PTC effect is practically difficult to achieve since the low material resistivity normally detriments the PTC effect according to polymer PTC theory.

A polymer PTC material typically consists of a crystalline or semicrystalline thermoplastic polymer with a dispersion of conductive fillers. At a low temperature, the material has contiguous structure which provides a conducting path through direct contact of conductive aggregrates, thus providing low resistivity. Electron hopping or tunneling in the insulating polymer matrix is also proposed to explain low resistivity state of a PTC material. When the PTC material or sensor device is heated to the polymer melting temperature TM, or the fault overcurrent causes the material to self-heat to TM, the large thermal expansion of the polymer interrupts the conducting path, resulting in the high resistivity (latched) state. Thus, a dramatic increase in resistance is observed at a temperature close to TM. However, the low resistivity state can be restored upon cooling the material to a temperature below TM removing power from the circuit ad correcting the fault condition which caused the current surge. It is this behavior that provides the material the self-resettability.