Measuring air pressure pulses quickly and reliably

Author : Mustafa Abu Wishah, Infineon Technologies

29 May 2013

Fig 1: The capacitive pressure sensor KP200 is the heart in innovative systems that increase protection for pedestrians and vehicle occupants in collisions.

The KP200 pressure sensor from Infineon Technologies is the key component in innovative systems that increase protection for pedestrians and vehicle occupants in collisions.
The decisive capability of these pressure sensors is that they are able to measure air pressure pulses very reliably, quickly and independently of variations in ambient air pressure. This they have already proven for many years in a similar safety-related automobile application as side airbag sensors.

The KP200 is integrated monolithically, i.e. the sensor unit to measure air pressure and the evaluation ASIC are both found on the same chip. For the pedestrian protection system developed by Continental together with an automobile manufacturer, an air hose is built into the front bumper of the vehicle. A KP200 pressure sensor is fixed at both ends of the hose for reasons of redundancy.
In a collision it measures the change in pressure in the air hose, evaluates the data in real time and sends the pressure values to a central controller every 500 microseconds. The sensor needs less than 40 microseconds on average to measure and process the data. The controller then activates a mechanism to raise the bonnet, which softens the impact for the pedestrian. During data processing the pressure sensor independently equalises changes in the ambient air pressure. This is necessary to detect a collision reliably regardless of weather conditions or altitude above sea level.

Fig 2: With its 84 sensor cells and 84 reference cells, the KP200 enables not only verification of the signal path, but also reliable self-testing of the sensor cells (Diag 2).

Principle of measurement
Compared to conventional pressure sensors, which work by a piezo-resistive measurement principle (resistance measurement), the KP200 is based on a capacitive solution. The absolute dynamic pressure is measured by a monolithically integrated sensor cell manufactured by surface micro-machining. A sigma-delta converter digitalises the signal.
The KP200 consists of two sensor arrays and two reference arrays. Every array has 42 cells (arranged in three columns and 14 rows). The four arrays form a Wheatstone measurement bridge. The sensor detects changes in the air pressure through deformation of the membrane. This deformation causes a change in the capacitance between the membrane and an electrode on a fixed plate. The measurement signal is independent of the ambient air pressure, but proportional to relative pressure changes. This is achieved by on-chip division of the change in absolute air pressure by the ambient pressure. The ambient pressure is determined from the absolute pressure via a low-pass filter. The sensor then outputs a digital Manchester code as a linear function of the normalised relative pressure. The cost-optimised configuration allows autonomous operation of the sensor without further logic ICs in the sensor unit. Further, an internal range check is performed continuously in normal operation.
Advantages over piezo-resistive sensors
The KP200 sensors offer a number of significant benefits over conventional, piezo-resistive sensors. Whereas piezo-resistive sensors are manufactured by bulk micro-machining (relevant structures are etched out of the substrate), the capacitive sensors are manufactured by surface micro-machining, which enables much finer structures.
The structures are deposited on the wafer as additive films (deposition of thin layers). The bulk micro-machining process is not compatible with CMOS, which leads to higher manufacturing complexity. Apart from that, the lateral structures of the piezo-resistive sensors are relatively large and sensitive to burst-like pressure changes. By contrast, the sensors, which are manufactured by surface micro-machining and which are CMOS/BiCMOS-compatible, enable very small lateral structures to be built and offer high robustness vis-à-vis burst-like changes in pressure.
Thanks to the small and precise structures, it is possible to implement multiple sensor cells in one component, which in turn makes effective self-test functions possible. With its total of 84 sensor cells and a further 84 reference cells, the KP200 therefore offers not only self-diagnosis for the signal path (figure 1), but also integrated verification of the individual sensor cells by way of a patented diagnosis mode (figure 2). In testing of the signal path, a constant voltage is fed into the sigma-delta AD converter and the system’s reply compared with the expected value. The complete path including filter is tested.
If you want to perform a self-test for a pressure sensor with just one membrane, you need at least one other sensor as a reference. With the many sensor cells of the KP200, there are by contrast a sufficient number of references available on one chip to check the sensor cells. Thanks to this high number, it is possible to clearly identify defective sensor cells.
Main features of the KP200
The KP200 comes in a PG-DSOF-8 SMD case with dimensions of 7 mm by 7 mm. The sensor chip is designed for a standardised pressure range of 50.9 kPa to 126.5 kPa (509 mbar to 1265 mbar or 7.4 psi to 18.3 psi) and an operating temperature between -40°C and 85°C. Its operating voltage lies between 4.5 V and 11 V. Its intrinsic power consumption lies at an average of 8 mA and its sensitivity is 20.48 LSB per percent pressure change. The sensor communicates with the central controller by PSI5 protocol and needs only two physical lines for this, with the data being modulated on the power supply line. The KP200 is thus compatible with the automotive standard AK-LV 29.
To meet market demands for a higher operating temperature, Infineon is currently working on developing the KP201 variant. It will cover applications up to 125°C.
Fig 1: The capacitive pressure sensor KP200 is the heart in innovative systems that increase protection for pedestrians and vehicle occupants in collisions.
Fig 2: With its 84 sensor cells and 84 reference cells, the KP200 enables not only verification of the signal path, but also reliable self-testing of the sensor cells (Diag 2).


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