Pressure-Sensing Elements

Local Static Pressure In a moving fluid, the local static pressure is equal to the pressure on a surface which moves with the fluid or to the normal pressure (for newtonian fluids) on a stationary surface which parallels the flow. The pressure on such a surface is measured by maldng a small hole perpendicular to the surface and connecting the opening to a pressure-sensing element (Fig. The hole is... 

The inductance-type transducer consists of three parts a coil, a movable magnetic core, and a pressure sensing element. The element is attached to the core, and, as pressure varies, the element causes the core to move inside the coil. An AC voltage is applied to the coil, and, as the core moves, the inductance of the coil changes. The current through the coil will increase as the inductance decreases. For increased sensitivity, the coil can be separated into two coils by utilizing a center tap, as shown in Figure 7. As the core moves within the coils, the inductance of one coil will increase, while the other will decrease. 

The silicon piezoresistive sensor detects distortion. Therefore diffused resistor (gage) layout and diaphragm geometry design have a profound effect on the sensor characteristics. We utilize simulation technology for pressure-sensing element design. 

The signal from the pressure-sensing element must be processed in some way into a desired electric output in order to be transferred to the vehicle ECU. The signal-processing circuit design can be divided into the following three parts. 

The signal-processing circuit susceptible to contamination is placed in the mold package and only the pressure-sensing element is exposed outside the package. 

The pressure-sensing element is protected by a three-layer protection structure comprising a SiN passivation film with superior moisture resistance, a parylene film for protecting the bonding wire, and gel for protecting the whole element. This structure makes it possible to achieve the creation of a surface-detection intake pressure sensor with high contamination immunity. 

Figure 11A.2 Variable capacitance differential pressure transducer, (a) Differential pressure sensing element, (b) Pressure signal transmission system.

Pressure-sensing elements (a) flat diaphragm (b) corrugated diaphragm (c) capsule (d) bellows (e) straight tube (f) Cshaped Bourdon tube (g) twisted Bourdon tube (h) helical Bourdon tube (i) spiral Bourdon tube. (From Norton, H.N., Handbook of Transducers, Englewood Cliffe, NI Prentice-Hall, 1989, 294-330. Reprinted with permission.) Previously published in Chau, K.H.L., Pressure and sound measurement, in The Measurement, Instrumentation, and Sensors Handbook, Webster, I.G., Ed., CRC Press, Boca Raton, FL, 1999, p. 26-3.)... 

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