Modern cars are developing from a simple means of transportation to one that can meet human needs and is safe, comfortable, convenient and pollution-free.
The key to achieving these goals lies in the electronicization and intelligence of cars, and the prerequisite is the timely acquisition of various information, which will inevitably require the use of a large number of various sensors in cars. Traditional sensors are often large in size and weight, and high in cost, and their application in automobiles is greatly limited.
In recent years, microelectromechanical systems (MEMS) technology, which has evolved from semiconductor integrated circuit (IC) technology, has become increasingly mature. Microsensors are currently the most successful and practical microelectromechanical devices, mainly including micropressure sensors and microaccelerometers that use the mechanical deformation of microdiaphragms to generate electrical signal outputs; in addition, there are microtemperature sensors, magnetic field sensors, gas sensors, etc., and the area of these microsensors is mostly below 1mm2. With the further development of microelectronics processing technology, especially nanoprocessing technology, sensor technology will also evolve from microsensors to nanosensors. These micro sensors are small in size, can realize many new functions, are easy to produce in large quantities and with high precision, have low unit cost, and are easy to form large-scale and multifunctional arrays. These characteristics make them very suitable for automotive applications.
Classification of automotive sensors
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 are either used to enable drivers to understand the status of various parts of the car; or to control the status of various parts of the car. According to their role in the car, they 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; according to the output characteristics, they are divided into analog sensors and digital sensors; according to the composition principle, they are divided into structural, tough, and composite types. For convenience, automotive sensors are now classified according to the control objects.
Application of micro sensors in automobiles
There are many types of sensors used in automobiles, and their applications are very wide. The following introduces the application of sensors in automobile engine control, safety systems, vehicle monitoring, and self-diagnosis.
(I) Sensors for automobile engine control
Electronic control of the engine has always been considered one of the main application areas of MEMS technology in automobiles. Sensors for engine control systems are the core of the entire automobile sensor, and 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 electronic control unit with information on the engine's operating conditions, so that the electronic control unit can accurately control the engine's operating conditions to improve the engine's power, reduce fuel consumption, reduce exhaust emissions and perform fault detection.
1. Temperature sensor
Automobile 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 have their own characteristics and their application scenarios are slightly different. Thermistor temperature sensors have high sensitivity and good response characteristics, but poor linearity and low adaptability to temperature. Among them, the general type has a temperature measurement range of -50℃～30℃, an accuracy of 1.5%, and a response time of 10ms; the high temperature type has a temperature measurement range of 600℃～1000℃, an accuracy of 5%, and a response time of 10ms; the wire-wound resistance temperature sensor has high accuracy, but poor response characteristics; the thermocouple resistance temperature sensor has high accuracy and a wide temperature measurement range, but needs to be used with an amplifier and cold end 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%, and response time of about 20ms), etc.
2. Pressure sensor
Pressure sensor is the most commonly used sensor in automobiles, mainly used to detect airbag storage pressure, transmission system fluid pressure, injection fuel pressure, engine oil pressure, intake pipe pressure, air filtration system fluid pressure, etc. At present, the main companies dedicated to the development and production of automotive pressure sensors include Motorola, Delco Electronic Instruments, LucasNovasensor, HiStat, NipponDenzo, Siemens, Texas Instruments, etc.
The more 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~100kPa. Its characteristics are high input energy, good dynamic response characteristics, and good environmental adaptability; the performance of piezoresistive pressure sensors is greatly affected by temperature and requires a separate temperature compensation circuit, but it is suitable for mass production; differential transformer pressure sensors have a large output and are easy to digitally output, but have poor anti-interference; surface acoustic wave pressure sensors have the characteristics of small size, light weight, low power consumption, high reliability, high sensitivity, high resolution, digital output, etc. They are used for automotive intake valve pressure detection and can work stably at high temperatures.
The intelligent tire pressure sensor KP500 developed by Infineon of Germany integrates pressure and temperature sensor modules. It does not need to add acceleration sensors to the sensor module. It can automatically start up and enter self-test when the car starts, and can measure pressure, temperature and voltage, etc. All functions are integrated on 0.8μm bipolar complementary metal oxide semiconductor (BiCMOS) using surface micromachining technology. The electrically erasable programmable read-only memory in each sensor module stores a unique 32-bit chip identification code. The chip identification code can be read out by the synchronous serial interface, and can be used to identify the location 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 abandoned.
3. Flow sensor
The flow sensor is mainly used to measure the engine air flow and fuel flow. The intake volume is one of the basic parameters for calculating the fuel injection volume. The function of the air flow sensor: sense the size of the air flow and convert it into an electrical signal to transmit 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, start, ignition, etc. There are four types of air flow sensors: rotary vane type, Karman vortex type, hot wire type, and hot film type. The main technical indicators of air flow sensors: working range is 0.11m3/min~103m3/min, working temperature is -40℃~120℃, and accuracy is >1%. Fuel flow sensors are used to detect fuel flow, mainly water wheel type and circulating ball type, with a dynamic range of 0~60kg/h, working temperature is -40℃~120℃, accuracy is ±1%, and response time is <10ms.
Honeywell's subsidiary Microswitch Company uses thermal micromachining technology to produce a microbridge air flow sensor chip. It uses micromachining technology to process a cavity on a silicon wafer, and a platinum resistor is suspended above the cavity. When air flows through the device, heat transfer occurs from the bottom to the top of the air flow direction. Therefore, the lower resistor is cooled and the upper resistor is heated. The air flow rate can be measured by the change in bridge resistance.
4. Position and speed sensor
Crankshaft position and speed sensor is mainly used to detect engine crankshaft angle, engine speed, throttle opening, vehicle speed, etc., to provide reference point signals for ignition timing and injection timing, and at the same time, provide engine speed signal. At present, the position and speed sensors used in automobiles mainly include AC generator type, magnetoresistive type, Hall effect type, reed switch type, optical type, semiconductor magnetic transistor type, etc., with a measurement range of 0°~360°, an accuracy better than ±0.5°, and a bending angle of ±0.1°.
There are many types of vehicle speed sensors, some sensitive to wheel rotation, some sensitive to power transmission shaft rotation, and some 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 0.5km/h~250km/h, the repeatability is 0.1%, and the distance measurement error is better than 0.3%.
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5. Gas concentration sensor
Gas concentration sensors are 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 automobile exhaust, determines the air-fuel ratio according to the oxygen concentration in the exhaust, 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 (operating temperature is -40℃～900℃, accuracy is 1%), chromium oxide concentration cell type gas sensors (operating temperature is 300℃～800℃), solid electrolyte chromium oxide gas sensors (operating temperature is 0～400℃, accuracy is 0.5%), in addition, there are also titanium dioxide oxygen sensors and zirconium dioxide oxygen sensors. 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. Zirconium dioxide micro-ion sensor is composed of calcium oxide stabilized zirconium oxide ion body, porous platinum thick film working electrode, palladium/palladium oxide thick film parameter electrode, impermeable layer, electrode contact and protective layer. Among them, calcium oxide stabilized zirconium oxide is deposited by reactive sputtering. Both the working electrode and the 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 output voltage of the oxygen sensor increases. The electronic control unit recognizes this sudden change signal and corrects the injection amount, thereby adjusting the air-fuel ratio accordingly to change it near the ideal air-fuel ratio.
