1 Introduction

Automotive sensors are automotive electronics, intelligence foundation and key, which use more, is the fastest growing pressure sensor. Automotive pressure sensors in the automotive many systems, such as electronic detection system, security, anti-collision systems. Wherein the pressure applied to the tire's objective is to minimize or eliminate the high and low rolling flat tire damage caused by the earlier of the tire, the tire keep the standard pressure, extend tire life, reduce the consumption of the tire, increase economic efficiency. It is reported that the micro pressure sensor embedded in the tire, the measurement of which pressure, to control the amount of the pneumatic tire, to avoid excessive and insufficient, thereby saving ten percent gasoline.

2 Car Pressure Sensor

2.1 Pressure sensor principle and application classification

Sensor is a variety of non-electricity (including physical, chemical, biomass, etc.) according to certain rules to facilitate handling and transport into another physical quantity (usually electricity) devices. Sensors are generally sensitive components, conversion elements and measurement circuit composed of three parts, and sometimes need additional auxiliary power supply. Sensor block diagram shown in Figure 1.

Block diagram of Figure 1 Sensor 
Manufacturing a semiconductor pressure sensor basic principle is to use silicon piezoresistive effect. Monocrystalline material subjected to stress, its resistivity change significantly, a phenomenon known as the piezoresistive effect.

A pressure sensor element is a piezoresistive effect material silicon, polycrystalline silicon and diffusion doping. According to the crystal orientation without stress, the conductivity is of the same sex, and only when the stress exhibited by the directional anisotropy of the band due to stress can cause changes in energy valleys move, resulting in changes in resistivity, so there is resistance changes, resulting in piezoresistive effect. Monocrystalline effects include n-type and p-type silicon piezoresistive effect. Optional object of diffusion silicon pressure sensor in the design and manufacture different temperatures according to the diffusion layer of silicon piezoresistive select the appropriate conditions for dispersion characteristics, and strive to make the pressure sensor has a good performance. Silicon in the sensor in a wide range of uses, can be used as filling material microstructure and sensitive material.

Pressure sensor by use classification is mainly pressure monitoring, pressure measurement and pressure control and conversion into other volume measurements. Press the power supply into the piezoresistive type and the piezoelectric type sensor, the former is a passive power supply, the need for external power. The latter is the sensor itself generates an electric charge, does not require external power, depending on the measurement accuracy of the pressure field divided into low-precision and high-precision pressure sensor.

2.2 Pressure Sensors

A) energy and principle: mainly used to detect the pressure sensor. In the middle of the wafer, etching from the back of the diaphragm forming a square, the use of the diaphragm into the pressure stress, the surface of the diaphragm, is formed by diffusion of the four p-type impurity measuring resistors are connected by a bridge circuit, will be added using the piezoresistive effect of the stress on the diaphragm is converted into changes in resistance, the change in resistance through the bridge circuit, the bridge circuit in between the two outputs in the form of external output potential difference. Sensor schematic diagram shown in Figure 2.

Figure 2 Schematic sensor

2) pressure output characteristics: pressure and output relationship shown in Figure 3, is a nearly linear relationship.

　　                                                                                                            Figure 3 Pressure and output curves

3 tire pressure measurement and circuit design

Below to measure tire pressure for example detail car tire pressure sensor in the application. This design can be made ​​into a portable device, measuring the tire pressure sensor sheet placed on the valve stem, then the tire pressure acting on the sensor diaphragm, and a series of transformation through the piezoresistive effect output voltage signal weak the corresponding voltage signal processing and display voltage value. It can be seen different pressure corresponding to different voltage values, namely pressure and voltage values ​​is one to one, and thus indirectly measured pressure values. Tire pressure measuring circuit block diagram shown in Figure 4.

　　                                                                                                             Tire pressure measuring circuit block diagram of Figure 4

3.1 Pressure and voltage values ​​of each conversion

Any one type of tire has its standard atmospheric pressure, a type of tire pressure than standard pressure is P0 P1 pressure, but lower than P2 is low. Then the standard pressure! After such a device displays a certain voltage value V0 of the voltage is displayed when the pressure is P1 V1, to achieve the low pressure P2 is displayed when the V2, the strength of pressure through such means embodied in the voltage value, when the voltage value V1 -V2 range of changes, the pressure is standard. If the voltage is higher than V1 compared to high pressure, low pressure was lower than V2, then you should take a series of measures to change the tire pressure values ​​within the standard, thus helping protect the tire effect.

3.2 voltage signal amplification circuit

We use high-precision low-noise instrumentation amplifier AD620, can be used in small sensor output signal amplifier, such as photocell sensors, strain gauge sensors and pressure sensors. Because of its low noise, high gain accuracy, gain temperature coefficient and high linearity and excellent performance. For this system is ideal. AD620 is able to use only one external resistor to set magnification 1-1000 instrumentation amplifier with good DC performance and ac performance, AD620's small size, low power consumption, the application of pressure to become an important factor in the sensor, the sensor signal amplification circuit shown in Figure 5.

　　                                                                                                   Sensor signal amplification circuit 5

3.3 voltage signal digitization and display

Amplified analog voltage after A / D converter for analog-to-digital conversion. A / D conversion result by counting the decoding circuit is converted into 7-segment decoder, the last driver shows corresponding values, the sensor drive circuit block diagram shown in Figure 6.

　　                                                                                                         Figure 6 shows the sensor driver circuit

Optional double integral A / D converter MC14433 constitute 31/2 digital voltage display, using dynamic scanning display a number of road modulated BCD code output and overrange output, easy to achieve automatic control. MC1413 driver integrated circuit, which contains seven inverter drive unit, each unit uses a Darlington transistor circuit. CC4511 to 7-segment decoder driver. MC14433 provides an adjustable reference voltage output, when the reference voltage is a certain value, the output voltage is a fixed value.

4 Conclusion

Car tire pressure sensors in the application is constantly improving, which has a variety of ways, such as the use of radio transmitting and receiving means to measure air pressure level. Automotive Pressure sensor types is constantly increasing, the function is constantly improving. Generally, switch gradually being linear sensor pressure sensor replaced gradually by ordinary sexual performance, versatile sensor replaced analog pressure sensors have been gradually digital sensors, intelligent sensor replaced.