Application of micro sensors in automobiles

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Safety, comfort, pollution-free and economical have always been the goals pursued by the automotive industry and users. The key to achieving these goals lies in the electronic and intelligentization of automobiles, and the prerequisite is the timely acquisition of various information, which inevitably requires the use of a large number of various sensors in automobiles. Traditional sensors are often large in size and weight, and high in cost, which greatly limits their application in automobiles.

In recent years, MEMS (Microelectromechnical System ) technology , which has evolved from semiconductor integrated circuit (IC) technology , has become increasingly mature. This technology can be used to produce a variety of micro sensors that can sense and detect mechanical quantities, magnetic quantities, thermal quantities, chemical quantities, and biological quantities. These sensors are small in size and energy consumption, 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.

In the early 1980s , miniature piezoresistive multi-channel absolute pressure ( Manifold Absolute Pressure ) sensors began to be mass-produced, replacing the early pressure sensors using LVDT technology. In the mid -1980s , miniature acceleration sensors began to be used in automobile airbags . They are the miniature sensors that have been mass-produced and widely used in automobiles so far. However, the large-scale application of microsensors will inevitably not be limited to engine combustion control and airbags . In the next 5 to 7 years, applications including engine operation management, exhaust gas and air quality control, ABS (antilock brake system ), vehicle dynamics control, adaptive navigation, and vehicle driving safety systems (such as airbags and obstacle detection and collision avoidance, etc.) will provide a broad market for MEMS technology.

The world's automobile production will increase from 47.5 million vehicles ( including passenger cars, special vehicles and light trucks) in 2000 to 54 million vehicles in 2005. The market for automotive electronics was $ 22.7 billion in 2000 and is expected to reach $ 30.9 billion in 2005 , with an average annual growth rate of 6.3% . Correspondingly, the cost of electronics per car increased from $ 477 in 2000 to $ 572 in 2005 .

According to literature reports, the automotive sensor market was $ 6.17 billion (904 million units) in 2000 and will reach $ 8.45 billion (1.268 billion units) in 2005 , with a growth rate of 6.5% ( in US dollars ) and 7.0% ( in units). In 2000 , North America had the largest share of the automotive sensor market, at 47% , followed by Europe (26%), Japan (22%), and South Korea (5%) .

Speed and position sensors accounted for 38% of the total automotive sensor market ( in US dollars) in 2000 , followed by oxygen sensors ( 20%) , mass air flow sensors ( 13% ), accelerometers (11% ), pressure sensors (10% ), temperature sensors (5%) , and others (3% ).

The major growth areas for automotive sensors from 2000 to 2005 include the following: accelerometers for vehicle dynamics control and airbags; pressure sensors for transmission, brakes, cooling, tires, fuel, etc.; yaw rate sensors for vehicle dynamics control, rollover alarm and GPS backup; position sensors sensitive to wheel speed and camshaft, crankshaft and pedal position; humidity sensors for cabin environment monitoring; daylight, rain and humidity sensors; and ranging sensors for close-range obstacle detection and collision avoidance.

Auto parts need to meet multiple environmental, reliability and cost requirements. Specifically, they must be able to withstand various adverse factors such as high and low temperatures, vibration, impact, humidity, corrosive atmosphere, electromagnetic interference, etc. On the other hand, they must also be suitable for mass production, generally reaching more than one million pieces per year, which is not only a requirement for mass production of automobiles, but also necessary to recover the huge investment in design and manufacturing. In addition, their reliability should also be consistent with the service life of the car of up to 10 years /150,000 miles. In short, auto parts are " military quality, civilian price " .

Most micro sensors are made of silicon. As we all know, silicon is easy to obtain high purity, has good mechanical properties and is light in weight. It has multiple sensing properties such as photoelectric effect, piezoresistive effect and Hall effect, and is easy to make sensors with integrated signal sensitivity and processing circuits. The mainstream process of micro sensors is silicon-based micromachining technology, which is derived from mature semiconductor technology and can simultaneously process a large number of almost identical mechanical structures. Therefore, in addition to the advantages of small size, light weight and low energy consumption, micro sensors are highly reliable and their supply price can be much lower than that of sensors using traditional electromechanical technology and processes. MEMS technology is bound to become the mainstream technology for automotive sensors. A recent Roger Grace Associates/Nexus report estimates that sales of automotive MEMS products will increase from US$ 1.75 billion in 2000 to US$ 2.27 billion in 2005 , with an average annual growth rate of 16.9% .

Due to the advantages of micro sensors based on MEMS technology in reducing the cost of automotive electronic systems and improving their performance, they have begun to gradually replace sensors based on traditional electromechanical technology. Early multi-channel absolute pressure sensors have been basically replaced by miniaturized sensors. At present, the sensitive elements and signal processing circuits of such sensors can be integrated on the same chip, which greatly reduces the size and can improve reliability and reduce interference.

In other pressure-sensitive applications, especially in harsh environments (such as engine oil and radiator coolant), ceramic capacitive pressure switches made of discrete components are now gradually replaced by silicon strain gauges made by bonding methods (usually fixed in low-cost and rugged packages) or varistor chips (mounted in silicone housings filled with silicone oil with stainless steel diaphragm end caps ) . Manufacturers that can currently provide such products include Keller, Measurement Specialties , SSI Technologies , Fasco , and Integrated Sensor Solutions (ISS)/Texas Instruments . Mass flow sensors are usually made of discrete hot wires, but Bosch 's miniaturized sensors made by surface micromachining methods have shown significant advantages. Impact sensors for airbags have evolved from simple " ball and tube " sensors to miniaturized and integrated accelerometers. The current major manufacturers include ADI, Motorola, SensoNor, and Nippondenso . Among them, Motorola 's miniature accelerometer products are shown in Figure 2 . In addition, the wheel speed sensor (used for ABS and vehicle dynamics control) which is widely used will also develop from the traditional variable reluctance type to the Hall type, anisotropic magnetoresistance ratio (AMR) and giant magnetoresistance ratio (GMR) . The above analysis and examples show that miniaturization is the main direction of the development of existing automotive sensors.

This section will analyze the main application directions of micro sensors in automobiles, which include safety systems, improving comfort / convenience / anti- theft, engine / drive chain, and vehicle monitoring and self-diagnosis.

Airbags are and will continue to be a major application for MEMS . The range of the silicon-based accelerometers used is generally 50g . Except for ADI , general accelerometers are still composed of multi-chip components to provide conditioned signals. ADI uses a more advanced monolithic integration method to produce acceleration sensors and corresponding signal conditioning circuits on the same chip, as shown in Figure 3 .

Many manufacturers have studied the use of compressed air to replace or supplement the sodium-based explosives commonly used when opening air bags. For this purpose, sensors for detecting the pressure of compressed air cylinders have been studied, but they have not yet been put into practical use.

The suspension system should be able to keep the vehicle in good driving performance during high-speed cornering, driving on uneven roads, and sudden acceleration and braking. Many systems use overall closed-loop control. Fully active systems are very expensive ( $ 2,500 to $ 4,000 ), and the hydraulic transmission they use will consume a large part of the power and greatly increase the weight. The introduction of these systems does improve the performance of the vehicle, but it also greatly increases the cost. Therefore, their application in mass-produced cars is limited. However, many manufacturers have introduced semi-active control suspension systems. Some of them are equipped with displacement sensors on the shock absorbers and use a number of linear accelerometers. These applications require acceleration ranges of only ±2g , which provides a good opportunity for the application of micro sensors.

Silicon-based pressure sensors have been used to detect the pressure of the master brake cylinder since the 1995 S-Class Mercedes. In addition to angular rate sensors, acceleration sensors, steering wheel angle and wheel speed sensors are gradually being used in vehicle dynamics control. However, most vehicles only use wheel speed sensors (ABS, traction control). Currently, angular rate sensors such as Bosch's surface micromechanical gyroscopes or Systron Donner and Matsushita's tuning fork gyroscopes are sold at a price of $25 each, and their application is limited to high-end cars such as Mercedes (S-Class), BMW, and Cadillac. Several well-known automotive and parts suppliers are working on designing more cost-effective systems. Cheaper angular rate sensors based on MEMS technology will help these systems be used in lower-priced cars.

Current car navigation systems combine GPS (Global Positioning System) and electronic maps stored in CD-ROM with wheel rotation sensors and rate gyros or magnetic compasses. However, the price of such systems is very high, about $1,800, and can only be used as optional accessories. MEMS sensors will play an increasingly important role in this application, and their application will reduce the price of such systems to less than $900 in the near future.

Table 2 lists the applications of micro sensors in improving comfort/convenience/theft prevention.

Pressure measurement in automotive air conditioning compressors provides a good opportunity for MEMS technology. Currently, other technologies (such as TI's ceramic capacitor pressure sensor) are used. However, many MEMS technology companies are actively developing corresponding micro sensors to compete for this large market.

Table 3 lists the applications of micro sensors in engines/drive chains.

Electronic control of engines has long been considered one of the main applications of MEMS technology in automobiles. Millions of multi-channel pressure sensors are produced by Delco, Motorola and Bosch. The multi-channel intake pressures measured by these devices can be used to calculate the air/fuel ratio. People are trying to replace such devices with mass airflow sensors. The products currently on the market use discrete hot wire wind speed measurement methods, which are large and expensive. Bosch introduced a new mass flow sensor based on thin film technology in 1995. Many organizations are now evaluating this miniaturized sensor. In addition to multi-channel pressure and mass flow parameters, the engine controller also needs to obtain atmospheric pressure parameters in order to determine the appropriate air/fuel ratio based on the inferred altitude information. MEMS technology will play an increasingly important role in these applications.

Cylinder pressure measurement is critical for optimizing engine performance. Piezoelectric and fiber optic technologies are practical due to the high temperatures in the cylinder, but their high price limits their application. Exhaust gas recirculation (EGR) systems are currently used in Ford and Chrysler vehicles. In these systems, ceramic capacitive pressure sensors are being replaced by miniature silicon piezoresistive sensors. Continuously variable transmission systems require the measurement of hydraulic oil pressure. Here, miniature pressure sensors that are isolated from the working medium by various methods will have broad application prospects. Fasco, Measurement Specialties, Integrated Sensor Solutions (recently acquired by TI) and SSI Technologies are developing miniature sensors for such applications. They all integrate silicon-based CMOS ASIC hybrid circuits programmed via EEPROM.

Table 4 lists the applications of micro sensors in vehicle monitoring and self-diagnosis.

In this regard, a major application of MEMS technology will be tire pressure monitoring. Whether from the perspective of safe driving or fuel saving, it is necessary to maintain proper tire inflation. There are already many real-time tire pressure monitoring systems. Micro pressure sensors based on MEMS technology are ideal sensitive devices. With the introduction of run-flat tires, these measurement systems have been widely installed in various new models. The use of run-flat tires eliminates the need for spare tires and jacks. The Clinton administration in the United States has enacted a decree requiring that passenger cars used in the United States must be equipped with such devices by 2004. This provides a huge market for micro pressure sensors, and manufacturers such as Lucas NovaSensor, Motorola and Sensonor are actively competing for this market.

Engine oil monitoring also provides an important application opportunity for MEMS technology. The biggest obstacle to the application of these systems is their price. This type of micro pressure sensor must be able to work properly in the engine oil at a very high temperature, and its silicon sensitive chip part must be isolated from the working medium. The price of the sensor with all signal conditioning circuits and packaged should be between $5 and $7. Pressure sensors can also be used to monitor the vapor pressure in volatile fuel tanks to ensure that there is no fuel vapor leakage.

In the near future, automotive applications will continue to constitute an important market for MEMS technology. Today, traditional multi-channel absolute pressure sensors and airbag accelerometers have been almost completely replaced by miniaturized sensors. In terms of wheel speed measurement, cooling system pressure, engine oil pressure and brake pressure measurement, people are considering or have already started to replace existing products with sensors based on MEMS technology. In addition, people are also considering using micro sensors in many newly developed systems. Advantages in price, reliability and size will make micro sensors the preferred product for measuring various parameters in automobiles. These advantages have been confirmed in practice. Automotive micro sensors have been highly valued by scientific research institutions and manufacturers. It can be predicted that their development, production and application will achieve rapid growth in the next 5 to 7 years.