BOSCH: Automotive Equipment – New generation of ESP® sensors

Bosch sensor now measures yaw rate and acceleration simultaneously SMI540: Two sensor elements in one housing

· Monoaxial yaw sensor combined with biaxial, linear acceleration sensor

· First micromechanical sensor that simultaneously measures yaw rate and acceleration in SOIC16x housing

· Tilt measurement along the vehicle’s longitudinal axis makes new applications possible

The Bosch SMI540 is the world’s first micromechanical inertial measurement unit for vehicle dynamics control in low-cost SOIC16w housing. The unit opens up new development possibilities for the ESP® electronic stability program. The 3-D sensor simultaneously monitors three of a vehicle’s movement axes – two acceleration or inclination axes (ax, ay), and one axis of rotation (Ωz). Until now, at least two separate sensors were required for this. The SMI540 has a digital serial peripheral interface (SPI) with two standard protocols. Thanks to a constant internal self-test of the yaw-rate sensor element, this interface meets the highest operating safety standards.

Samples of the new sensor are already available at Bosch Automotive Electronics. The sensor is set to go into large-scale series production from the second quarter of 2010. Bosch also plans to launch the SMG540, a version of the sensor that contains only the yaw sensor element.

Basis for new control unit concepts

Two of SMI540’s three sensor signals provide information that is relevant for the ESP® system: the yaw rate and lateral acceleration. The remaining third sensor signal is not generally used by ESP®. It measures a vehicle’s acceleration or inclination in the direction of travel. Developers can use the information this sensor delivers for other applications, such as the hill hold function or for fuel-saving functions in cars with automatic transmissions (N control). The car automatically shifts into the fuel-saving “N” gear, provided that – and this is where the inclination sensor comes into play – the car is on level ground and cannot begin to move by itself.

The SMI540 serves conventional ESP® concepts as with a separate sensor device in the vehicle interior as well as new, more cost-effective approaches. As only one example, the Bosch ABplus airbag concept no longer contains the sensor device for ESP® control. In this case, the inertial sensor is now integrated directly into the airbag control unit. These are ideal conditions for the SMI540, with its small SOIC16w housing (10.3 x 10.3 x 2.65 millimeters) and multiaxial sensors, to show its strengths.

The new sensor is also well suited to the alternative concept of a central domain control unit (DCU). This unit integrates several of a vehicle’s sensor functions and forwards measurements for safety and comfort and well as driver information to the relevant on-board electronics.

Prize-winning Bosch MEMS sensors

When ESP® was launched in 1995, precision-engineered mechanical sensors were still used. When demand for the electronic stability program skyrocketed, in 1998 the switch was made to lower-cost micromechanical sensors (MEMS). With more than 800 MEMS patents and over a billion MEMS sensors produced to date, Bosch is the global market leader for this technology. In 2008 alone, 200 million units were sold. Its MEMS technology has won Bosch prestigious innovation awards, such as the European Patent Office’s “European Inventor of the Year” prize and the German president’s Future Prize 2008 (Deutscher Zukunftspreis).

SMI540 3-D sensor: Key technical data

Sensitivity axes Ωz ax, ay
Measurement range ±160 °/s ±2 g
Sensitivity 175 LSB/(°/s) 6667 LSB/g
Sensitivity error ±3 % ±3 %
Balance error ±3 °/s ±50 milli-g
Resolution 16 Bit 16 Bit
Nonlinearity < ±1 °/s < ±40 milli-g
Noise (@13 Hz) 0,1 °/s (rms) 3 milli-g (rms)
Output filter 13 Hz oder 57 Hz
Temperature range -40 °C / +105 °C