Sensors and sensibility from Bosch

  • Image: Bosch12-10MEMS airbag sensor.jpg
  • Image: Bosch12-10SMP13X.jpg
Image: Bosch12-10Marek.jpg

Bosch's Jiri Marek, Senior Vice President Engineering Sensors, Bosch Automotive Electronics Division, underlined the space- and power-saving benefits of the SMA560 sensor for airbags.

Bosch has moved into its fifth generation of micromechanical sensors for airbag control unit applications, which save space and have increased functions. The dual-axis SMA560 in an SOIC8 housing offers space savings of some 40% compared to the previous generation example, enabling circuit designers to assign one of four measuring ranges to each sensing axis independently. Using the serial peripheral interface, the sensor outputs measuring values in Bosch-SPI or Open-SPI format.

A proven, dual-chip solution within the sensor is used. With two sensing axes, the SMA560 offers central sensor capability for front- and side-impact detection. To record the different deceleration forces with enhanced accuracy, application engineers may set the sensor’s measuring ranges for the two axes as required to ±35 g; ±48 g; ±70 g; or ±96 g. Operating voltage of the sensor can be 3.3 or 5.0 V.

The new generation consumes what Jiri Marek, Senior Vice President Engineering Sensors, Bosch Automotive Electronics Division, quantifies as “significantly less” power than its previous generation equivalent. Operating temperature range is -40ºC to +105ºC (-40ºF to 220ºF). A self-test capability is integrated. A bidirectional 16-bit serial peripheral interface provides the measuring signal in 10-bit resolution.

Micromachined microscopic springs and weights form the capacitive readings recorder of the sensor. An application specific integrated circuit (in the sensor housing) looks after signal conditioning, together with further electronic functions.

Bosch states that, depending on the measurement values of the acceleration sensor, a control unit decides whether to activate the airbag and seatbelt tensioner restraint systems.

Another significant Bosch sensor development, this time for automatic transmission and air-conditioning applications, is the SMP137 (SMP13x series) because it marks a first for the company in the integrated medium-pressure sensor sector, initially with 70-bar (1020-psi) maximum pressure and high media resistance.

Previously sensors for low pressures, up to 10 bar (145 psi), and high pressure, from 140 bar (2030 psi) to over 2000 bar (29 ksi), were offered. The SMP137, measuring up to 70 bar, is described by the company as offering a range of special design features including an hermetically sealed metal housing, high bursting strength measuring membrane, 150ºC (302ºF) maximum ambient temperature capability, and integrated electromagnet compatibility capacitors.

The sensor is ex-works aligned and encapsulated within a housing, facilitating its direct installation into control units. There is no necessity of a second level package, which therefore saves cost and space. Marek regards the SMP137’s main application as being automatic transmissions in which its less than 1-ms response time measures the pressure for clutch activation.

Giving details of the sensor, he stated that its disc-shaped housing measures some 19 mm (0.75 in) in diameter with an installation height of slightly less than 10 mm (0.4 in). The new SMP13x series housing has been designed to withstand pressures in excess of 100 bar (1450 psi). The sensor’s electrical and hydraulic connections are positioned on opposite surfaces, facilitating easy installation.

The hydraulic fluid presses against the monocrystaline measuring membrane, which bends under the pressure while sealing the sensor’s interior against the hydraulic fluid.

The sensor’s element includes a measuring bridge with piezoresistors, produced using microelectromechanical systems technology. The level of the electric bridge signal changes in proportion to the bend of the membrane. The signal is then processed by an analog application specific integrated circuit (integrated on the chip on which the membrane is located), before being routed to the exterior through the sensor’s electrical connections. The sensor can also be applied to pressure measurement within air-conditioning systems.

Bosch plans to put the SMP137 into high-volume production in the fourth quarter of 2011, and another sensor in the SMP13x range, designated SMP132, having a maximum pressure capability of 22 bar (320 psi), will enter production in 2012.

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