Valores, algoritmos e imágenes de la Política de Defensa y Seguridad (2015-2018) Valores

Application

Intake-Manifold or Boost-Pressure Sensor This sensor measures the absolute pressure in the intake manifold between the super-charger and the engine (typically 250 kPa or 2.5 bar) against a reference vacuum and not against the ambient pressure. This enables the air mass to be precisely determined and the boost pressure to be regulated in accor-dance with the engine demand.

Ambient-Pressure Sensor

This sensor (also called the atmospheric-pressure sensor) is located in the ECU or in the engine compartment. Its signal is used for magnitude-dependent correction of the setpoint values for the closed control loops, e.g. exhaust-gas recirculation and boost-pressure control. This allows the varying atmospheric density to be taken into consid-eration. The ambient-pressure sensor mea-sures the absolute pressure (60 … 115 kPa or 0.6 … 1.15 bar).

Oil and Fuel-Pressure Sensor

Oil-pressure sensors are installed on the oil filter and measure the absolute oil pressure so that the engine load can be determined for the service display. Their pressure range is 50 … 1000 kPa or 0.5 … 10.0 bar. On account of its high media resistance, the measurement cell is also used to measure pressure in the fuel low-pressure stage. It is installed in or on the fuel filter. Its signal is used to monitor the level of filter contami-nation/clogging (measurement range 20 … 400 kPa or 0.2 … 4 bar).

Type with Reference Vacuum on the Structure Side

Design

The measurement cell is the heart of the micromechanical pressure sensor. It consists of a silicon chip (Figure 1, item 2), into which a thin diaphragm is micromechanically

56 Pressure Sensors Micromechanical Pressure Sensors

Fig. 1 1 Diaphragm 2 Silicon chip 3 Reference vacuum 4 Glass (Pyrex) 5 Bridge circuit p Measurement

pressure U0 Supply voltage UMMeasurement

voltage R₁ Strain-gauge

resistor (deflected) R2 Strain-gauge

resistor (expanded)

Fig. 2

1, 3 Electrical connec-tions with glass-enclosed bushing 2 Reference vacuum 4 Measurement

cell (chip) with electronic evaluation circuitry 5 Glass pedestal 6 Cap

7 Feed for measure-ment pressure p

p

Measurement cell of pressure sensor with reference vacuum on the structure side (schematic)

1

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Measurement cell of pressure sensor with cap and reference vacuum on the structure side (view)

3

Measurement cell of pressure sensor with cap and reference vacuum on the structure side (layout)

2

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etched (1). The diaphragm incorporates four diffused strain-gauge resistors (R1, R2) whose electrical resistance changes under mechanical stress. A cap, under which the reference vac-uum is enclosed, covers the measurement cell on its structure side and acts as a seal (Figures 2 and 3). The pressure-sensor housing can also accommodate a temperature sensor (Fig-ure 4, item 1), whose signals can be evaluated independently. Just one sensor housing is therefore sufficient for measuring both tem-perature and pressure at a single point.

Operating Concept

The diaphragm of the sensor cell is deflected to varying degrees (10 … 1000 µm), de-pending on the magnitude of the measure-ment pressure. The four strain-gauge resis-tors on the diaphragm alter their electrical resistance under the mechanical stresses generated (piezoresistive effect).

The measuring resistors are arranged on the silicon chip in such a way that when the diaphragm is deflected the resistance of two of the measuring resistors increases while it decreases in the other two resistors. The measuring resistors are arranged in a Wheat-stone bridge circuit (Figure 1, item 5). As the resistances change, so too does the ratio of the voltages to the measuring resistors. This causes the measurement voltage UMto change. This as yet unamplified measure-ment voltage is thus a measure of the pressure acting on the diaphragm.

The Wheatstone bridge circuit produces a higher measurement voltage than when an individual resistor is evaluated, thereby increasing the sensitivity of the sensor.

The structure side of the diaphragm which is not subjected to the measurement pres-sure is exposed to a reference vacuum (Fig-ure 2, item 2) with the result that the sensor measures the absolute pressure value.

The electronic circuitry for signal condition-ing is integrated on the chip and serves to amplify the bridge voltage, compensate temperature influences, and linearize the

pressure characteristic. The output voltage ranges between 0 and 5 V and is supplied to the engine control unit via electrical connec-tions (Figure 4, item 5). The control unit calculates the pressure from this output voltage (Figure 5).

Pressure Sensors Micromechanical Pressure Sensors 57

Fig. 4

1 Temperature sensor (NTC)

2 Lower housing section 3 Intake-manifold wall 4 Sealing rings 5 Electrical

connec-tion (connector) 6 Housing cover 7 Measurement cell

1 2 3 4 5

6 7

1 cm Micromechanical pressure sensor with reference vacuum on the structure side (layout)

4

æ

Pressure

kPa 250 100

Output voltage

V

4.65

1.87

Micromechanical boost-pressure sensor (characteristic, example)

5

æ

Type with Reference Vacuum in a Cavern

Design

The pressure sensor with the reference vacuum in a cavern (Figures 6 and 7) for use as an intake-manifold or boost-pressure sensor is simpler in design than its counter-part with the reference vacuum on the struc-ture side: A silicon chip with an etched dia-phragm and four strain-gauge resistors in a bridge circuit is located – like the pressure sensor with cap and reference vacuum on the structure side – in the form of a

measure-ment cell on a glass pedestal. But, unlike that sensor, the glass pedestal does not have a hole through which the measurement pressure acts from the rear side on the measurement cell. Instead, the silicon chip is pressurized from the side on which the electronic evalua-tion circuitry is situated. This side must therefore be protected by a special gel against environmental influences (Figure 8, item 1).

The reference vacuum is located in the cavity (cavern) between the silicon chip (6) and the glass pedestal (3). The entire measuring ele-ment is supported on a ceramic hybrid (4), which has soldered surfaces for further con-tacting in the sensor.

The pressure-sensor housing can also accommodate a temperature sensor. The temperature sensor projects openly into the air flow and reacts at great speed to temper-ature changes (Figure 6, item 4).

Operating Concept

Its operating concept, and thus the signal conditioning and amplification and the characteristic, are identical to that of the pressure sensor with cap and reference vacuum on the structure side. The only difference is that the diaphragm of the measurement cell is deflected in the opposite direction and therefore the strain-gauge resistors undergo a deflection in the opposite direction.

58 Pressure Sensors Micromechanical Pressure Sensors

Fig. 6

1 Intake-manifold wall 2 Housing 3 Sealing ring 4 Temperature sensor

(NTC)

5 Electrical connection (connector) 6 Housing cover 7 Measurement cell

Fig. 8 1 Protective gel 2 Gel frame 3 Glass pedestal 4 Ceramic hybrid 5 Cavern with

reference vacuum 6 Measurement

cell (chip) with electronic evaluation circuitry 7 Bonded connection p Measurement Micromechanical pressure sensor with reference vacuum in a cavern (layout)

6 Measurement cell of pressure sensor with reference vacuum in a cavern (layout)

8

æ

Micromechanical pressure sensor with reference vacuum in a cavern and integrated temperature sensor (view)

7

æ