Modern cars are evolving from a simple means of transportation towards meeting human needs and being safe, comfortable, convenient, and pollution-free. The key to achieving these goals lies in the electronicization and intelligence of automobiles, and the prerequisite is the timely acquisition of various information, which inevitably requires the extensive use of various sensors in automobiles. Traditional sensors are often bulky, heavy, and costly, which greatly limits their application in automobiles.

In recent years, microelectromechanical system (MESM) technology, which has evolved from semiconductor integrated circuit (IC) technology, has become increasingly mature. Micro sensors are currently successful and practical micro electromechanical devices, mainly including micro pressure sensors and micro acceleration sensors that use the mechanical deformation of micro membranes to generate telecommunication signal outputs; In addition, there are micro temperature sensors, magnetic field sensors, gas sensors, etc., most of which have an area of less than 1mm2. With the further development of microelectronic processing technology, especially nanofabrication technology, sensor technology will evolve from microsensors to nanosensors. These miniature sensors have a small volume, can achieve many new functions, are easy to produce in large quantities and with high precision, have low unit costs, and are easy to form large-scale and multifunctional arrays. These characteristics make them very suitable for automotive applications.

　　1. Classification of automotive sensors

Automotive sensors are a collective term for various sensors used in automotive displays and electronic control systems. It involves many physical and chemical quantity sensors. These sensors either enable drivers to understand the status of various parts of the car; Or it is used to control the status of various parts of the car. According to their functions in automobiles, sensors can be used to control the engine, control the chassis, and provide various information to the driver. The materials that make up these sensors include fine ceramics, semiconductor materials, optical fibers, and polymer films; According to output characteristics, there are analog sensors and digital sensors; According to the principle of composition, there are structural, ductile, and composite types. For convenience, it is now classified according to the control objects of automotive sensors, as shown in Table 1.

　　2. The application of microsensors in automobiles

There are many types of sensors used in cars, with a wide range of applications. The following introduces the applications of sensors in automotive engine control, safety systems, vehicle monitoring, and self diagnosis.

2.1 Sensors for Automotive Engine Control

The electronic control of engines has always been considered one of the main application areas of MEMS technology in automobiles. The sensors used in the engine control system are the core of the entire automotive sensor system, with many types including temperature sensors, pressure sensors, position and speed sensors, flow sensors, gas concentration sensors, and knock sensors. These sensors provide the electronic control unit of the engine with information about the engine's working condition, allowing the electronic control unit to control the engine's working condition, improve engine performance, reduce fuel consumption, reduce exhaust emissions, and perform fault detection.

(1) Temperature replacement sensor

Automotive temperature sensors are mainly used to detect engine temperature, intake gas temperature, cooling water temperature, fuel temperature, and catalytic temperature. There are three main types of temperature sensors: thermistor type, wire wound resistor type, and thermocouple resistor type. These three types of sensors each have their own characteristics, and their application scenarios are also slightly different. Thermistor temperature sensors have high sensitivity and good response characteristics, but poor linearity and low temperature adaptability. Among them, the universal temperature measurement range is -50~30 ℃, with an accuracy of 1.5% and a response time of 10ms; the high-temperature type is 600~1000cC, with an accuracy of 5% and a response time of 10ms; the wire wound resistance temperature sensor has high accuracy but poor response characteristics; Thermocouple resistance temperature sensors have high accuracy and a wide temperature measurement range, but they need to be used in conjunction with amplifiers and cold junction processing. Other practical products include ferrite temperature sensors (temperature measurement range of -40~120 ℃, accuracy of 2.0%), metal or semiconductor film air temperature sensors (temperature measurement range of -40~150 ℃, accuracy of 2.0%, 5%, response time of about 20ms), etc.

(2) Pressure sensor

Pressure sensors are widely used sensors in automobiles, mainly used to detect airbag storage pressure, transmission system fluid pressure, fuel injection pressure, engine oil pressure, intake pipe pressure, air filtration system fluid pressure, etc. Currently. The main companies dedicated to the development and production of pressure sensors for automobiles include Motorola and Deke Electronic Instruments, LucasNovasensor，HiStat，NipponDenzo， Siemens, Texas Instruments, etc.

The commonly used automotive pressure sensors include capacitive, piezoresistive, differential transformer, and surface acoustic wave sensors. Capacitive pressure sensors are mainly used to detect negative pressure, hydraulic pressure, and air pressure, with a measurement range of 20-100kPa. They are characterized by high input energy, good dynamic response characteristics, and good environmental adaptability; The performance of piezoresistive pressure sensors is greatly affected by temperature and requires an additional temperature compensation circuit, but it is suitable for large-scale production; The differential transformer pressure sensor has a large output and is easy to output digitally, but has poor anti-interference performance; Surface acoustic wave pressure sensors have the characteristics of small size, light weight, low power consumption, high reliability, high sensitivity, high resolution, and digital output. They are used for pressure detection of automotive intake valves and can work stably at high temperatures.

The intelligent tire pressure sensor KPS00 developed by Infineon in Germany integrates pressure and temperature sensing modules internally. It does not require the addition of acceleration sensors in the sensor module and can automatically start up and enter self check when the car is started. It can measure pressure, temperature, and voltage, etc. All functions are integrated on a 0.8 μ m bipolar complementary metal oxide semiconductor (BiCMOS) using surface micro machining technology. The 32-bit chip identification code is stored in the electrically erasable programmable read-only memory in each sensor module. The chip identification code can be read out by a synchronous serial interface and can be used to identify the position of various tire pressure sensors. When receiving data, the first step is to check the chip identification code. If it is found that the chip identification code does not match, the received data frame will be discarded.

(3) Flow sensor

Flow sensors are mainly used for measuring engine air flow and fuel flow. The intake air quantity is one of the basic parameters for calculating fuel injection quantity. The function of an air flow sensor is to sense the magnitude of air flow and convert it into an electrical signal to be transmitted to the electronic control unit of the engine. The measurement of air flow is used in the engine control system to determine combustion conditions, control air-fuel ratio, start, ignition, etc. There are four types of air flow sensors: rotary wing type, Karman vortex type, type, and hot film type. The main technical specifications of the air flow sensor are: working range of 0.11-103m3/min, working temperature of -40-120 ℃, accuracy ≤ 1%. Fuel flow sensors are used to detect fuel flow, mainly including water wheel type and circulating ball type. Their dynamic range is 0-60kg/h, working temperature is -40-12 ℃, accuracy is ± 1%, and response time is 10ms.

Honeywell's subsidiary Microswitch has developed a micro bridge air flow sensor chip using thermal microfabrication technology. It uses microfabrication technology to create cavities on silicon wafers, with platinum resistors suspended above the cavities. When air flows through the device, heat transfer occurs from below to above in the direction of air flow. Therefore, the lower resistor is cooled and the upper resistor is heated. The air flow rate can be measured by the change in the bridge resistance.

(4) Position and speed sensors

The crankshaft position and speed sensor is mainly used to detect engine crankshaft angle, engine speed, throttle opening, vehicle speed, etc., providing reference point signals for ignition timing and injection timing, as well as engine speed signals. At present, the position and speed sensors used in automobiles mainly include AC generator type, reluctance type, Hall effect type, chip switch type, optical type, semiconductor magnetic transistor type, etc. Their measurement range is 0 °~360 °, with an accuracy better than ± 0.5 ° and a bending angle measurement of ± 0.1 °.

There are various types of speed sensors, including those that are sensitive to wheel rotation, those that are sensitive to power transmission shaft rotation, and those that are sensitive to differential driven shaft rotation. When the vehicle speed is higher than 100km/h, the general measurement method has a large error, and a non-contact photoelectric speed sensor is required. The speed measurement range is 0.5-250km/h, the repeatability is 0.1%, and the distance measurement error is better than 0.3%.

(5) Gas concentration sensor

Gas concentration sensors are mainly used to detect gas and exhaust emissions inside the vehicle body. Among them, the main one is the oxygen sensor, which detects the oxygen content in car exhaust, determines the air-fuel ratio based on the oxygen concentration in the exhaust, and sends feedback signals to the microcomputer control device to control the air-fuel ratio to converge to the theoretical value. Commonly used sensors include germanium oxide sensors (operating temperature range of -40~900 ℃, accuracy of 1%), chromium oxide concentration battery gas sensors (operating temperature range of 300~800 ℃), solid electrolyte chromium oxide gas sensors (operating temperature range of 0~400 ℃, accuracy of 0.5%), as well as titanium dioxide oxygen sensors and zirconium dioxide oxygen sensors. Compared with germanium oxide sensors, titanium dioxide oxygen sensors have the characteristics of simple structure, lightweight, low cost, and strong resistance to lead pollution. Zirconia micro ion sensors are composed of calcium oxide stabilized zirconia ions, porous platinum thick film working electrodes, palladium/palladium oxide thick film parameter electrodes, impermeable layers, electrode contacts, and protective layers. Among them, calcium oxide stabilized zirconia is deposited by reactive sputtering method. Both the working electrode and the reference electrode are made using thick film technology. The output voltage near the ideal A/F point undergoes a sudden change. As the air-fuel ratio increases and the oxygen concentration in the exhaust gas increases, the output voltage of the oxygen sensor decreases; When the air-fuel ratio decreases and the oxygen concentration in the exhaust gas decreases, the output voltage of the oxygen sensor increases. The electronic control unit recognizes this sudden change signal, corrects the fuel injection quantity, and accordingly adjusts the air-fuel ratio to vary around the ideal air-fuel ratio.

(6) Knock sensor

The knock sensor is used to detect engine vibration, control and avoid engine knock by adjusting the ignition advance angle. In order to maximize engine power without causing detonation, the ignition advance angle should be controlled at the critical value of detonation. When the engine produces detonation, the sensor converts the vibration caused by detonation into an electrical signal and transmits it to the electronic control unit. There are three methods for detecting detonation, including detecting cylinder pressure, engine body vibration, and combustion noise. There are magnetostrictive and piezoelectric types of knock sensors. The operating temperature of magnetostrictive detonation sensor is -40~125 ℃, and the frequency range is 5~10kHz; The piezoelectric detonation sensor has a sensitivity of up to 200mV/gn at a center frequency of 5.417kHz and an amplitude of 0.1-10gn. There is good linearity within the range.

(7) Throttle position sensor

The throttle position sensor is installed on the throttle valve, and its function is to convert the opening signal of the engine throttle valve into an electrical signal, which is transmitted to the electronic control unit to sense the load size and acceleration/deceleration conditions of the engine. The commonly used is a variable resistance throttle position sensor. This sensor is a typical throttle sensor, mainly composed of a linear positioner and an idle contact. The resistance positioner is made of ceramic film resistors, and the sliding contacts are controlled by reset springs and rotate coaxially with the throttle valve. During operation, the contacts of the linear positioner slide on the resistor, and based on the changing resistance value, a linear output voltage signal proportional to the throttle opening can be measured. Based on the output voltage value, the electronic control unit can determine the opening and rate of change of the throttle valve, thereby judging the operating conditions of the engine and improving control accuracy and effectiveness. The idle signal sliding contact is a normally open contact that only closes when the throttle valve is fully closed, generating an idle contact signal. It is mainly used for idle control, fuel cut-off control, and correction of ignition advance angle.

2.2 Sensors used in security systems

Safety is the primary consideration for cars, and there are many sensors used for safety, such as micro accelerometers for car airbags and surface micro mechanical gyroscopes for measuring angular velocity.

(1) Micro accelerometer sensor

Airbags are currently and will continue to be a major application of MEMSj in the future. The range of the silicon accelerometer used is generally 50gn. Earlier silicon accelerometers made using microfabrication technology, such as those used by Motorola, have a range of 50gn.

Henrik et al. from Sweden reported a novel silicon micro three-axis accelerometer with external structural parameters of 6mm × 4mm × 1.4mm. It has four sensitive mass blocks, four independent signal readout electrodes, and four reference electrodes. It cleverly utilizes the structural feature that sensitive beams have very low stiffness in their thickness direction and can be sensitive to acceleration, while their stiffness in other directions is relatively high and cannot be sensitive to acceleration. On the cross-section of the accelerometer, due to anisotropic corrosion, the thickness direction of the sensitive beam is at an angle of 35.26 ° (tan35.26 °=0.707) to the normal direction of the accelerometer (z-axis).

(2) Surface micro mechanical gyroscope

The traditional gyroscope is composed of a high-speed rotating rotor, inner ring, outer ring, and base. The inner and outer rings of this gyroscope are usually supported by ball bearings, which are usually made by mechanical processing methods, requiring high processing accuracy, difficulty, and large volume and weight. Micro mechanical gyroscope is a MEMS device with complex detection and control circuits. Said Emre Alper et al. reported a surface micro mechanical gyroscope with structural symmetry and decoupling properties. The sensitive structure is equipped with supporting "anchors" at its four outer corners, which is different from the traditional implementation method of directly supporting on the "anchors". It uses a symmetrical structure to support the sensitive mass block on the connecting beam, and organically connects the driving electrode and the sensitive electrode together through the beam. After simulation analysis using the MEMCAD software package, it was found that the vibrations in the two directions do not affect each other, so this connection method does not need to consider mechanical coupling.

The structural parameters of the planar outer contour of the micro mechanical gyroscope are 1mm2, with a thickness of only 2/μ m. The working principle is that when a DC bias voltage is applied to the sensitive mass block and an appropriate AC excitation voltage is applied between the movable and fixed fingers, the sensitive mass block will generate inherent vibration in the y-axis direction. When the gyroscope senses the angular velocity around the Z-axis, due to the Coriolis effect, the sensitive mass will produce additional vibrations along the X-axis. The measured angular velocity can be obtained by measuring the amplitude of the additional vibration. Under normal atmospheric conditions, this sensitive result has a resolution of 0.37 °/s.

2.3 Sensors for vehicle monitoring and self diagnosis

A major application of MEMS technology in vehicle monitoring and self diagnosis will be tire pressure monitoring; Next are sensors applied to cooling, braking, and other systems. In addition, there are also methods such as using light sensors in brightness control systems; Using magnetic sensors and airflow velocity sensors in electronic driving systems; Using indoor temperature sensors, suction temperature sensors, air volume sensors, sunlight sensors, and humidity sensors in automatic air conditioning systems; Use orientation sensors, speed sensors, etc. in guided driving systems.

2.4 Application of High Temperature Microelectronics in Automobiles

High temperature microelectronics play a very important role in monitoring aspects such as automotive engine control, cylinders and exhaust pipes, electronic suspension and brakes, power management and distribution. For example, high-temperature microelectronic sensors and controllers used for engine control will help to better monitor and control combustion, making combustion more efficient and improving combustion efficiency. However, microelectronic devices made using traditional silicon semiconductor technology are no longer capable of operating at very high temperatures. In order to solve the problem of temperature measurement in high-temperature environments, it is necessary to develop a new material to replace traditional semiconductor materials. The third-generation wide bandgap semiconductor material SiC has a series of advantages such as high breakdown electric field, high saturation electron drift rate, high thermal conductivity, and strong radiation resistance, making it particularly suitable for the production of high-temperature, high-voltage, high-power, and radiation resistant semiconductor devices. The integrated SiC sensor can directly contact the high-temperature fuel tank and exhaust pipe, which can further obtain more information about fuel combustion efficiency and reduce exhaust emissions. Research has shown that once SiC semiconductor technology can solve material, packaging and other technologies and achieve further development, the working range of SiC power devices will exceed that of traditional silicon power devices, and their volume will also be smaller than Si power devices.

　　3. Conclusion

Due to the important role of automotive sensors in automotive electronic control systems and the rapidly growing market demand, countries around the world attach great importance to their theoretical research, new material applications, and new product development. The overall development trend of future automotive sensor technology is miniaturization, multifunctionality, integration, and intelligence.

The advantages of micro sensors based on MEMS technology in reducing the cost and improving the performance of automotive electronic systems have gradually replaced sensors based on traditional electromechanical technology. With the advancement of nanotechnology, miniature sensors with smaller size, lower cost, and stronger functionality will be widely used in various aspects of automobiles. In the coming years, applications including engine operation management, exhaust and air quality control, anti lock braking systems, vehicle dynamics control, adaptive navigation, and vehicle driving safety systems will provide a broad market for MEMS technology.