Application of miniature sensors in automotive electronic and intelligent engineering
1. Introduction Modern cars are developing from a pure transportation to meet human needs and safety, comfort, convenience and pollution-free direction.
The key to achieving these goals lies in the electronic and intelligentization of the car. The prerequisite is the timely acquisition of various information. This inevitably requires a large number of sensors in the car. Traditional sensors are often bulky, heavy, and costly. Their applications in automobiles are greatly restricted.
In recent years, the microelectromechnical system (MEMS) technology developed from semiconductor integrated circuit (IC) technology has gradually matured. Miniature sensors are currently the most successful and practical miniature electromechanical devices, mainly including miniature pressure sensors and miniature acceleration sensors that use the mechanical deformation of miniature diaphragms to generate electrical signal outputs; in addition, miniature temperature sensors, magnetic field sensors, and gas For sensors, etc., the area of ​​these miniature sensors is mostly below 1 mm2. With the further development of microelectronic processing technology, especially nano-processing technology, sensor technology will also evolve from micro-sensors to nano-sensors. These miniature sensors are small in size, can realize many brand-new functions, are convenient for mass production and high-precision production, have low unit cost, and easily form large-scale and multi-functional arrays. These characteristics make them very suitable for automotive applications.
2. Classification of automotive sensors
Automotive sensors are a general term for various sensors used in automotive display and electronic control systems. It involves many physical quantity sensors and chemical quantity sensors. These sensors either let the driver know the status of each part of the car; or they are used to control the status of each part of the car. According to the role in the car, it can be divided into sensors for controlling the engine, controlling the chassis, and providing various information to the driver. The materials that constitute these sensors include fine ceramics, semiconductor materials, optical fibers, and polymer films; they are divided according to output characteristics There are analog sensors and digital sensors; according to the principle of composition, there are structural type, toughness type and composite type. For the sake of convenience, it is classified according to the control object of the car sensor.
3. Application of miniature sensors in automobiles
There are many types of sensors used in automobiles, and they have a wide range of applications. The following describes the application of sensors in automotive engine control, safety systems, vehicle monitoring, and self-diagnosis.
(1) Sensors for automobile engine control
The electronic control of the engine has always been regarded as one of the main application fields of MEMS technology in automobiles. Sensors for engine control systems are the core of the entire automotive sensor. There are many types, including temperature sensors, pressure sensors, position and speed sensors, flow sensors, gas concentration sensors, and knock sensors. These sensors provide the engine's working condition information to the engine's electronic control unit. The power supply control unit precisely controls the engine's working condition to improve the engine's power, reduce fuel consumption, reduce exhaust emissions, and perform fault detection.
1. Temperature Sensor
Automotive temperature sensors are mainly used to detect engine temperature, suction gas temperature, cooling water temperature, fuel temperature and catalytic temperature. There are three main types of temperature sensors: thermistor type, wire wound resistance type and thermocouple resistance type. These three types of sensors have their own characteristics, and their applications are slightly different. The thermistor temperature sensor has high sensitivity and good response characteristics, but has poor linearity and low temperature adaptation. Among them, the general temperature range is -50 ℃ ~ 30 ℃, the accuracy is 1.5%, the response time is 10ms; the high temperature type is 600 ℃ ~ 1000 ℃, the accuracy is 5%, the response time is 10ms; wire wound resistance temperature The sensor has high accuracy but poor response characteristics; the thermocouple resistance temperature sensor has high accuracy and wide measurement temperature range, but it needs to be used together with the amplifier and cold junction processing.
Other products that have been put into practical use are ferrite temperature sensors (temperature range is -40 ℃ ～ 120 ℃, accuracy is 2.0%), metal or semiconductor film air temperature sensors (temperature range is -40 ℃ ～ 150 ℃, accuracy) 2.0%, 5%, response time about 20ms) etc.
2. Pressure Sensor
Pressure sensors are the most commonly used sensors in automobiles, and are mainly used to detect airbag air storage pressure, transmission system fluid pressure, fuel injection pressure, engine oil pressure, intake pipe pressure, fluid pressure of air filtration systems, etc. At present, the main companies devoted to the development and production of pressure sensors for automobiles are Motorola, Deco Electronic Instruments, LucasNovasensor, HiStat, Nippon Denzo, Siemens, Texas Instruments, etc.
Commonly used automotive pressure sensors are capacitive, piezoresistive, differential transformer, and surface acoustic wave. Capacitive pressure sensors are mainly used to detect negative pressure, hydraulic pressure, and air pressure. The measurement range is 20kPa to 100kPa. It is characterized by high input energy, good dynamic response characteristics, and good environmental adaptability. The performance of piezoresistive pressure sensors is affected by temperature Larger, need to set up a temperature compensation circuit, but suitable for mass production; differential transformer pressure sensor has a large output, easy digital output, but poor anti-interference; surface acoustic wave pressure sensor has a small size and light weight , Low power consumption, high reliability, high sensitivity, high resolution, digital output and other characteristics, used for pressure detection of automobile intake valve, can work stably under high temperature.
The intelligent tire pressure sensor KP500 developed by the German Infineon company integrates a pressure and temperature sensing module. It does not require an acceleration sensor to be added to the sensor module. It can automatically boot into the self-test when the car is started, and can measure pressure, temperature and voltage. Wait. All functions are integrated on the 0.8μm bipolar complementary metal oxide semiconductor (BiCMOS) using surface micromachining technology. A unique 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 by the synchronous serial interface, and can be used to identify the position of each tire pressure sensor. When receiving data, first of all, we must check the chip identification code, if found that the chip identification code does not match, then discard the received data frame.
3. Flow Sensors
The flow sensor is mainly used for the measurement of engine air flow and fuel flow. The intake air quantity is one of the basic parameters for the fuel injection quantity calculation. The function of the air flow sensor: to sense the size of the air flow and convert it into an electrical signal to be transmitted to the electronic control unit of the engine. Air flow measurement is used by the engine control system to determine combustion conditions, control air-fuel ratio, start, ignition, etc. There are 4 types of air flow sensors: rotary vane type, Karman scroll type, hot wire type, and hot film type. The main technical indicators of the air flow sensor: working range is 0.11m3 / min ～ 103m3 / min, working temperature is -40 ℃ ～ 120 ℃, accuracy> 1%. The fuel flow sensor is used to detect the fuel flow. It mainly has a water wheel type and a circulating ball type. Its dynamic range is 0 to 60 kg / h, the operating temperature is -40 ° C to 120 ° C, the accuracy is ± 1%, and the response time is <10ms.
Microwell, a subsidiary of Honeywell, has made micro-bridge air flow sensor chips using thermal microfabrication technology. It uses microfabrication technology to produce cavities on silicon wafers, and platinum resistors are 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 resistance is cooled and the upper resistance is heated. The air flow can be measured by the change of the bridge resistance.
4. Position and speed sensors
Crankshaft position and speed sensors are mainly used to detect engine crankshaft angle, engine speed, throttle opening, vehicle speed, etc., provide reference point signals for ignition timing and fuel injection timing, and at the same time provide engine speed signals.
At present, the position and speed sensors used in automobiles mainly include alternator type, magnetoresistive type, Hall effect type, reed switch type, optical type, semiconductor magnetic transistor type, etc., and the measurement range is 0 ° ～ 360 °, and the accuracy Better than ± 0.5 °, measuring bending angle up to ± 0.1 °.
There are many types of vehicle speed sensors, including those with sensitive wheel rotation, those with sensitive power transmission shaft rotation, and those with sensitive differential speed 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 range is 0.5km / h ～ 250km / h, the repeat accuracy is 0.1%, and the distance measurement error is better than 0.3 %.
5. Gas concentration sensor
The gas concentration sensor is mainly used to detect gas and exhaust emissions in the vehicle body. Among them, the most important is the oxygen sensor, which detects the oxygen content in the exhaust gas of the car, measures the air-fuel ratio according to the oxygen concentration in the exhaust gas, and sends a feedback signal to the microcomputer control device to control the air-fuel ratio to converge to the theoretical value. Commonly used are germanium oxide sensors (using temperature is -40 ℃ ~ 900 ℃, accuracy is 1%), chromium oxide concentration battery gas sensor (using temperature is 300 ℃ ~ 800 ℃), solid electrolyte chromium oxide gas sensor ( The use temperature is 0 ~ 400 ℃, the accuracy is 0.5%), in addition, there are oxygen dioxide sensor and oxygen dioxide sensor. Compared with germanium oxide sensors, titanium dioxide oxygen sensors have the characteristics of simple structure, light weight, low cost, and strong resistance to lead pollution. The zirconium dioxide micro-ion sensor is composed of calcium oxide stabilized oxidized ion body, porous platinum thick-film working electrode, palladium / oxidized thick-film parameter electrode, water-impermeable layer, electrode contact and protective layer. Among them, the stable oxidation of calcium oxide is deposited by reactive sputtering. Both working electrode and reference electrode are made by thick film process. The output voltage near the ideal A / F point changes suddenly. When the air-fuel ratio becomes higher and the oxygen concentration in the exhaust gas increases, the output voltage of the oxygen sensor decreases; when the air-fuel ratio becomes lower and the oxygen concentration in the exhaust gas decreases, the oxygen The output voltage of the sensor increases. The electronic control unit recognizes this abrupt signal and corrects the fuel injection amount, thereby adjusting the air-fuel ratio accordingly to make it fluctuate around the ideal air-fuel ratio.
6. Knock sensor
The knock sensor is used to detect engine vibration, control the ignition advance angle and prevent engine knock. In order to maximize the engine power without knocking, the ignition advance angle should be controlled at the critical value of knocking. When the engine produces knocking, the sensor converts the vibration caused by knocking into an electrical signal and transmits it to the electronic control unit. There are three methods for detecting knock, such as cylinder pressure, engine block vibration and combustion noise. Knock sensors are magnetostrictive and piezoelectric. The use temperature of the magnetostrictive detonation sensor is -40 ℃ ～ 125 ℃, the frequency range is 5kHz ～ 10kHz; the piezoelectric detonation sensor is at the center frequency 5.417kHz, its sensitivity can reach 200mV / gn, and the amplitude is 0.1 Good linearity in the range of -10gn.
7. Throttle position sensor
The throttle position sensor is installed on the throttle valve. Its function is to convert the opening signal of the engine throttle valve into an electrical signal and transmit it to the electronic control unit to sense the engine load and acceleration / deceleration conditions. The most commonly used is a variable resistance throttle position sensor. The sensor is a typical throttle sensor, mainly composed of a linear positioner and an idle speed contact. The resistance positioner is made of ceramic thin film resistors, and the sliding contact is controlled by a return spring, which rotates coaxially with the throttle valve. During operation, the contact of the linear positioner slides on the resistor body. According to the changed resistance value, a linear output voltage signal proportional to the throttle opening can be measured. According to the output voltage value, the electronic control unit can know the opening degree of the throttle valve and the opening rate change rate, so as to accurately judge the operating conditions of the engine and improve the control accuracy and effect. The idle signal sliding contact is a normally open contact, which is closed only when the throttle is fully closed, and generates the idle contact signal, which is mainly used for idle speed control, fuel cut control and ignition advance angle correction.
(2) Sensors for safety systems
Safety is the primary consideration for automobiles, and there are many sensors for safety, such as miniature accelerometers for automobile airbags, surface micromechanical gyros for measuring angular velocity, etc.
1. Micro acceleration sensor
At present, airbags are and will be a major application of MEMS technology in the future. The range of the silicon accelerometer used is generally 50gn. Earlier such as the silicon acceleration sensor made by Motorola using micro-fabrication technology.
Sweden Henrik et al. Reported a new type of silicon micro triaxial accelerometer, whose shape and structure parameters are 6mm × 4mm × 1.4mm, it has 4 sensitive masses, 4 independent signal readout electrodes and 4 references electrode. It cleverly utilizes the structural characteristics that the sensitive beam has a very small stiffness in its thickness direction and can be sensitive to acceleration, and the stiffness in other directions is relatively large and cannot be sensitive to acceleration. On the cross section of the accelerometer, due to the anisotropic corrosion, the thickness direction of the sensitive beam is 35.26 ° (tan35.26 ° = 0.707) with the normal direction (z axis) of the accelerometer.
2. Surface micromachined gyroscope
The traditional gyroscope is composed of a rotor, an inner ring, an outer ring and a base that rotate at high speed. The inner and outer rings of this gyroscope are usually supported by ball bearings. These are usually made by mechanical processing methods and require high processing accuracy. The difficulty is large, and the gyroscope is large and heavy. Micromechanical gyros are MEMS devices with complex detection and control circuits. SaidEmreA1per et al. Reported a surface micromachined gyroscope with symmetrical structure and decoupling properties. The sensitive structure is provided with supporting "anchors" at the four outermost corners, which is different from the traditional direct support on the "anchor". It uses a symmetric structure sensitive mass to support the connecting beam, and The driving electrode and the sensitive electrode are organically connected by a beam. After simulation analysis with the micro device simulation software package (MEMCAD), it can be seen that the vibrations in the two directions do not affect each other, so this connection does not need to consider mechanical coupling.
The structure parameter of the plane outer contour of the micromachined gyroscope is 1 mm2, and the thickness is only 2 μm. Its working principle is: when a DC bias voltage is applied to the sensitive mass, and an appropriate AC excitation voltage is applied between the movable and fixed fingers, the sensitive mass will generate natural vibration in the y-axis direction. When the gyro senses the angular velocity around the z-axis, due to the Coriolis effect, the sensitive mass will generate additional vibration along the x-axis. The measured angular velocity can be obtained by measuring the amplitude of the additional vibration. Under normal atmospheric conditions, the sensitive structure has a resolution better than 0.37 ° / s.
(3) Sensors for vehicle monitoring and self-diagnosis
In vehicle monitoring and self-diagnosis, one of the main applications of MEMS technology will be tire pressure monitoring; followed by sensors used in cooling, braking and other systems. In addition, there are, for example, the use of light sensors in brightness control systems; the use of magnetic sensors and airflow speed sensors in electronic driving systems; the use of indoor temperature sensors, intake temperature sensors, air volume sensors, sunlight sensors, and humidity sensors in automatic air conditioning systems ; Use orientation sensors, vehicle speed sensors, etc. in guided travel systems.
(4) Application of high-temperature microelectronics in automobiles
High-temperature microelectronics plays a very important role in the monitoring of automobile engine control, cylinders and exhaust pipes, electronic suspensions and brakes, power management and distribution. For example: high-temperature microelectronic sensors and controllers for engine control will help better monitor and control combustion, it will make combustion more thorough and improve combustion efficiency.
However, microelectronic devices made with traditional silicon semiconductor technology are no longer competent because they cannot work at very high temperatures. In order to solve the problem of temperature measurement in a high-temperature environment, a new material must be developed to replace the traditional semiconductor material. The third-generation wide-band 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, and is particularly suitable for making semiconductors with high temperature, high pressure, high power, and radiation resistance. Device. The integrated sic sensor can directly contact the high-temperature fuel tank and exhaust pipe, so that you can get more information about fuel combustion efficiency and reduce exhaust emissions. Research shows that once the sic semiconductor technology can be further developed by solving materials, packaging and other technologies, the working range of SIC power devices will exceed that of traditional silicon power devices, and its volume is smaller than that of Si power devices.
4. 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. In the future, the general development trend of automotive sensor technology is miniaturization, multifunctionalization, integration and intelligence.
The advantages of micro-sensors based on MEMS technology in reducing the cost of automotive electronic systems and improving their performance have begun to gradually replace sensors based on traditional electromechanical technology. With the advancement of nanotechnology, miniature sensors with smaller size, lower cost and stronger functions will be widely used in various aspects of automobiles. In the next few years, applications including engine operation management, exhaust and air quality control, brake anti-lock brake systems, vehicle dynamics control, adaptive navigation, and vehicle driving safety systems will provide a broad market for MEMS technology.
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