I. INTRODUCTION Modern vehicles are being developed from a simple vehicle in a direction that meets human needs and is safe, comfortable, convenient and pollution-free.
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The key to achieving these goals is the electronic and intelligent nature of the car. The prerequisite is the timely acquisition of all kinds of information, which is bound to require a large number of sensors in the car. Traditional sensors tend to be bulky and heavy, and costly, and their application in automobiles is greatly limited.
In recent years, microelectromechanical systems (MEMS) technology developed from semiconductor integrated circuit (IC) technology has matured. Micro-sensors are the most successful and most practical micro-electromechanical devices, including miniature pressure sensors and micro-acceleration sensors that use electrical deformation of micro-diaphragms to generate electrical signals. In addition, miniature temperature sensors, magnetic field sensors, and gases Sensors, etc., the area of â€‹â€‹these miniature sensors is mostly below 1mm2. With the further development of microelectronic processing technology, especially nanofabrication technology, sensor technology will also evolve from microsensors to nanosensors. These miniature sensors are small in size and offer many new features for high volume and high precision production, low cost per part, and easy to form large-scale and multi-function arrays, making them ideal for automotive applications.
Second, automotive sensor classification
Automotive sensors are a general term for various sensors used in automotive displays and electronic control systems. It involves many physical quantity sensors and chemical quantity sensors. These sensors either let the driver know the status of the various parts of the car; or they are used to control the state of the various parts of the car. According to the role of the car, it can be divided into control engine, control chassis and sensors for providing various information to the driver. The materials constituting these sensors are fine ceramics, semiconductor materials, optical fibers and polymer films; There are analog sensors and digital sensors; according to the principle of composition, there are structural, tough and composite. For convenience, it is now classified according to the control object of the car sensor.
Third, the application of miniature sensors in automobiles
There are many types of sensors used in automobiles, and the applications are wide. The following describes the application of sensors in automotive engine control, safety systems, vehicle monitoring and self-diagnosis.
(1) Sensors for automobile engine control
Electronic control of the engine has long been recognized as one of the main applications of MEMS technology in automobiles. Sensors for engine control systems are at the heart of the entire automotive sensor, including temperature sensors, pressure sensors, position and speed sensors, flow sensors, gas concentration sensors, and knock sensors. These sensors provide engine operating status information to the engine's electronic control unit for the electronic control unit to accurately control engine operating conditions to improve engine power, reduce fuel consumption, reduce exhaust emissions, and detect faults.
1. Temperature Sensor
Automotive temperature sensors are mainly used to detect engine temperature, intake gas temperature, cooling water temperature, fuel temperature, and catalytic temperature. Temperature sensors are available in three main types: thermistor type, wirewound resistor type and thermocouple type. These three types of sensors each have their own characteristics, and their applications are slightly different. The thermistor temperature sensor has high sensitivity and good response characteristics, but the linearity is poor and the temperature is low. Among them, the general-purpose temperature range is -50 Â° C ~ 30 Â° C, the accuracy is 1.5%, the response time is 10ms; the high temperature type is 600 Â° C ~ 1000 Â° C, the accuracy is 5%, the response time is 10ms; wirewound resistance temperature The sensor has high accuracy but poor response characteristics; the thermocouple resistance temperature sensor has high accuracy and a wide measurement temperature range, but it needs to be used together with the amplifier and cold junction processing.
Other practical products include ferrite temperature sensor (temperature range -40 Â° C ~ 120 Â° C, accuracy of 2.0%), metal or semiconductor film air temperature sensor (temperature range -40 Â° C ~ 150 Â° C, accuracy 2.0%, 5%, response time is about 20ms) and so on.
2. Pressure Sensor
The pressure sensor is the most used sensor in the automobile, and is mainly used to detect the air bag storage pressure, the transmission system fluid pressure, the injection fuel pressure, the engine oil pressure, the intake pipe pressure, the fluid pressure of the air filtration system, and the like. At present, the main companies dedicated to the development and production of automotive pressure sensors are Motorola, Deco Electronic Instruments, LucasNovasensor, HiStat, NipponDenzo, Siemens, Texas Instruments and so on.
The more commonly used automotive pressure sensors are capacitive, piezoresistive, differential transformer, and surface acoustic wave. Capacitive pressure sensor is mainly used to detect negative pressure, hydraulic pressure and air pressure. The measuring range is from 20kPa to 100kPa. It is characterized by high input energy, good dynamic response and good environmental adaptability. The performance of piezoresistive pressure sensor is affected by temperature. Larger, need to set up a temperature compensation circuit, but suitable for mass production; differential transformer type pressure sensor has a large output, easy to digital output, but poor anti-interference; surface acoustic wave pressure sensor has small size and light weight Low power consumption, high reliability, high sensitivity, high resolution, digital output, etc. It is used for pressure detection of automotive suction valves and can work stably at high temperatures.
The intelligent tire pressure sensor KP500 developed by Infineon of Germany integrates a pressure and temperature sensing module. It does not need to add an acceleration sensor to the sensor module. It can automatically start the self-test when the car starts, and can measure pressure, temperature and voltage. Wait. All functions are integrated on a 0.8Î¼m 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, check the chip identification code. If the chip identification code is found to be inconsistent, the received data frame is discarded.
3. Flow Sensors
Flow sensors are primarily used for the measurement of engine air flow and fuel flow. The intake air amount is one of the basic parameters for calculating the fuel injection amount. The function of the air flow sensor: the amount of air flow is sensed and converted into an electrical signal that is transmitted to the engine's electronic control unit. The measurement of air flow is used by the engine control system to determine combustion conditions, control air/fuel ratio, starting, ignition, and the like. The air flow sensor has four types: rotary vane type, Karman scroll type, hot line 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 Â° C ~ 120 Â° C, accuracy > 1%. The fuel flow sensor is used to detect fuel flow. It mainly has a water wheel type and a recirculating ball type. Its dynamic range is 0-60kg/h, the working temperature is -40Â°C-120Â°C, the accuracy is Â±1%, and the response time is <10ms.
Honeywell's subordinate microswitch (microswitch) has fabricated a microbridged airflow sensor chip using thermal microfabrication technology, which uses microfabrication techniques to machine a cavity on a silicon wafer with a platinum resistor suspended above the cavity. When air flows through the device, heat transfer from below to above the air flow direction occurs, so that the lower resistance is cooled, the upper resistance is heated, and the air flow is measured by the bridge resistance change.
4. Position and speed sensor
The crankshaft position and speed sensor is mainly used to detect the crank angle of the engine, the engine speed, the opening degree of the throttle valve, the vehicle speed, etc., and provides a reference point signal for the ignition timing and the injection timing, and at the same time, provides an engine speed signal.
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 0Â° to 360Â°. It is better than Â±0.5Â° and the bending angle is Â±0.1Â°.
There are many types of speed sensors, such as rotating the sensitive wheel, rotating the sensitive power transmission shaft, and rotating the sensitive differential driven shaft. When the vehicle speed is higher than 100km/h, the general measurement method has a large error. The non-contact photoelectric speed sensor is required. The speed measurement range is from 0.5km/h to 250km/h, the repeatability 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 automobile, determines 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 yttria sensor (use temperature is -40 Â° C ~ 900 Â° C, accuracy of 1%), chromium oxide concentration battery type gas sensor (use temperature is 300 Â° C ~ 800 Â° C), solid electrolyte chrome oxide gas sensor ( The use temperature is 0 to 400 Â° C, the accuracy is 0.5%), in addition, there is a dioxide oxidation sensor and a dioxide oxidation error sensor. Compared with the yttrium oxide sensor, the titanium dioxide oxygen sensor has the characteristics of simple structure, light weight, low cost, and strong resistance to lead pollution. The zirconium dioxide microion sensor is composed of a calcium oxide stabilized oxidation fault, a porous platinum thick film working electrode, a palladium/oxidation thick film parameter electrode, a water impermeable layer, an electrode contact and a protective layer. Among them, the calcium oxide stable oxidation error is accumulated by a reactive sputtering method. Both the working electrode and the reference electrode are fabricated by a thick film process. The output voltage near the ideal A/F point suddenly changes. 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, the oxygen concentration in the exhaust gas decreases. The output voltage of the sensor increases. The electronic control unit recognizes this abrupt signal and corrects the amount of fuel injected to adjust the air-fuel ratio accordingly to vary around the ideal air-fuel ratio.
6. Knock sensor
The knock sensor is used to detect the vibration of the engine, by adjusting the ignition advance angle control and avoiding knocking of the engine. In order to maximize engine power without causing knocking, the ignition advance angle should be controlled at a critical value generated by knocking. When the engine produces knocking, the sensor converts the vibration caused by the knocking into an electrical signal and transmits it to the electronic control unit. There are three methods for detecting knocking, such as detecting cylinder pressure, engine body vibration and combustion noise. The knock sensor is magnetostrictive and piezoelectric. The magnetostrictive knock sensor has a temperature range of -40 Â° C to 125 Â° C and a frequency range of 5 kHz to 10 kHz. The piezoelectric knock sensor has a sensitivity of up to 200 mV/gn at a center frequency of 5.417 kHz and an amplitude of 0.1. Good linearity in the range of -10gn.
7. Throttle position sensor
The throttle position sensor is mounted on the throttle valve and functions to convert the engine throttle opening signal into an electrical signal and transmit it to the electronic control unit for sensing the engine load and acceleration and deceleration conditions. The most commonly used is a variable resistance throttle position sensor. The sensor is a typical throttle sensor consisting mainly of a linear positioner and an idle contact. The resistance positioner is made of a ceramic film resistor, and the sliding contact is controlled by a return spring and rotates coaxially with the throttle. During operation, the contacts of the linear positioner slide over the resistor body, and a linear output voltage signal proportional to the throttle opening can be measured based on the varying resistance value. According to the output voltage value, the electronic control unit can know the opening degree of the throttle valve and the rate of change of the opening degree, thereby accurately determining the operating condition of the engine and improving the control precision and effect. The idle signal sliding contact is a normally open contact, which is closed only when the throttle is fully closed, and generates an idle contact signal, which is mainly used for the idle speed control, the oil cut control and the ignition advance angle correction.
(2) Sensors for safety systems
Safety is the primary factor in car considerations, and there are many sensors for safety, such as miniature accelerometers for automotive airbags, surface micromachined gyroscopes for angular rate measurement, and so on.
1. Micro acceleration sensor
At present, airbags are and will be a major application of MEMS technology in the future. The range of silicon accelerometers used is typically 50 gn. Earlier, such as silicon acceleration sensors made by Motorola's micro-machining technology.
Sweden's Henrik et al. reported a new type of silicon micro-three-axis accelerometer with an external structural parameter of 6mm Ã— 4mm Ã— l.4mm, which has 4 sensitive masses, 4 independent signal readout electrodes and 4 references. electrode. It cleverly utilizes the structural features of sensitive beams that have very small stiffness in their thickness direction and are sensitive to acceleration, and that are relatively stiff in other directions and are not sensitive to acceleration. On the cross section of the accelerometer, the thickness direction of the sensitive beam is 35.26Â° (tan35.26Â°=0.707) from the normal direction of the accelerometer (z axis) as a result of anisotropic corrosion.
2. Surface micromachined gyroscope
The conventional gyroscope is composed of a high-speed rotating rotor, an inner ring, an outer ring and a base. The inner and outer rings of the gyroscope are usually supported by ball bearings, which are usually made by mechanical processing and require high machining precision. It is difficult and the gyroscope made is large in size and heavy in weight. Micromachined gyros are MEMS devices with complex detection and control circuits. SaidEmreA1per et al. reported a surface micromachined gyroscope with structural symmetry and decoupling properties. The sensitive structure is provided with support "anchors" at its outermost four corners, which is different from the conventional direct support on the "anchor", which is supported on the connecting beam by a symmetrical structural sensitive mass, and The drive electrode and the sensitive electrode are organically connected together by a beam. After simulation analysis using the micro device simulation software package (MEMCAD), the vibrations in the two directions do not affect each other, so such a connection method does not need to consider mechanical coupling.
The planar outer profile of the micromachined gyroscope has a structural parameter of 1 mm 2 and a thickness of only 2 Î¼m. The working principle is that when a DC bias voltage is applied to the sensitive mass, the sensitive mass will generate natural vibration in the y-axis direction when a suitable AC excitation voltage is applied between the movable finger and the fixed finger. When the gyro senses the angular velocity about the z-axis, the sensitive mass will produce additional vibration along the x-axis due to the Coriolis effect. The measured angular velocity can be obtained by measuring the amplitude of the vibration of the additional vibration. In the case of conventional atmospheric conditions, the sensitive structure has a resolution better than 0.37Â°/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; secondly, sensors for cooling, braking and other systems. In addition, there are also optical sensors used in brightness control systems; magnetic sensors and airflow speed sensors in electronic driving systems; indoor temperature sensors, inhalation temperature sensors, air volume sensors, sunlight sensors, and humidity sensors in automatic air conditioning systems. Azimuth sensor, vehicle speed sensor, etc. are used in the guided driving system.
(4) Application of high temperature microelectronics in automobiles
High-temperature microelectronics play a very important role in the monitoring of automotive engine control, cylinders and exhaust pipes, electronic suspension and brakes, power management and distribution. For example, high-temperature microelectronic sensors and controllers for engine control will help to better monitor and control combustion, which will make combustion more thorough and improve combustion efficiency.
However, microelectronic devices fabricated using conventional silicon semiconductor technology are no longer capable of operating at very high temperatures. In order to solve the temperature measurement problem in 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 anti-irradiation ability, and is especially suitable for manufacturing high-temperature, high-voltage, high-power, radiation-resistant semiconductors. Device. The integrated sic sensor can be in direct contact with the high temperature fuel tank and exhaust pipe, which provides further information on fuel combustion efficiency and reduced exhaust emissions. Research shows that once sic semiconductor technology can solve the problems of 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.
Fourth, the 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 future development trend of automotive sensor technology is miniaturization, multi-functionality, integration and intelligence.
MEMS-based microsensors have begun to gradually replace sensors based on traditional electromechanical technology in terms of reducing the cost of automotive electronic systems and improving their performance. With the advancement of nanotechnology, miniature sensors with smaller size, lower cost and more functions will be widely used in all aspects of automobiles. In the next few years, applications including engine operation management, exhaust and air quality control, brake anti-lock braking systems, vehicle dynamics control, adaptive navigation, and vehicle driving safety systems will provide a broad market for MEMS technology.
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