How MEMS sensors improve the performance of automotive electronics

Micro-electromechanical systems (MEMS) technologies are achieved by deploying mechanical devices, sensors and electronic components on a single silicon substrate using advanced micro- fabrication techniques . Today, these technologies have gradually replaced most of the sensing devices used in various models of the automotive market, bringing considerable design advantages in terms of system cost reduction, continued reliability (due to inherent robustness), space utilization (due to its compact size) and operating performance.

　　Generally speaking, MEMS sensors used in automobiles can be divided into four categories:

　　Accelerometer and gyroscope .

　　Sensors for measuring flow and pressure.

　　Sensors for in-vehicle applications include infrared sensors to improve and monitor road vision , sensors for in-vehicle temperature and air quality measurement, and microscanners for head-up displays .

RF sensors 　　for automotive radar .

　　Figure 1: Illustration of automotive sensors.

　　In automotive systems, MEMS sensors can be used to obtain many key parameters that affect the operation of the car and create a wider range of control solutions. MEMS sensors can be used in airbags, anti-lock braking systems (ABS) and electronic stability programs (ESP) control systems, electronically controlled suspension systems, and numerous driver assistance functions.

　　With particular regard to automotive applications, it should be noted that contemporary cars have greatly increased in complexity, including the integration of an increasing number of safety devices, reduced fuel consumption and suppression of harmful gas emissions, and improved driving comfort.

　　Accelerometer and gyroscope

　　The purpose of an accelerometer is to measure acceleration in units of gravity (G). MEMS devices can provide high accuracy in applications with high noise sources. Some devices use the piezoelectric effect to determine acceleration. These devices contain microcrystalline structures that generate stress when there is an acceleration force , which in turn generates a corresponding AC/DC voltage. In most cases, design engineers tend to choose capacitive or thermal micro-electromechanical accelerometers  . However, to choose the right accelerometer for their application, several important variables need to be considered, including sensor structure, resonance, reliability, stability, bandwidth , and power consumption.

　　Unlike accelerometers, gyroscope sensors measure angular velocity in degrees per second (°/s) or revolutions per second (rps). Angular velocity is simply a measurement of rotational speed. When selecting a gyroscope, reliability, operating temperature range, and potential sensitivity to electromagnetic interference must be considered. Errors caused by noise sources may affect measurement accuracy and therefore affect system design.

　　Crash sensors for airbag control are the most classic application in automotive systems. They are mainly composed of MEMS inertial sensors (accelerometers and gyroscopes). The accelerometer continuously measures the acceleration of the car. When this parameter exceeds a predetermined threshold, the microcontroller unit (MCU ) can calculate the integral value of the acceleration to determine whether a significant speed change has occurred. Single/dual-axis acceleration sensors are usually used in airbags. In some designs, angular velocity sensors can also be used.

　　STMicroelectronics' AI S1120SX/AIS2120SX three-axis accelerometers have high measurement resolution and low noise levels, and can provide different operating modes to achieve smart functions such as energy saving and system wake-up. These high-G acceleration sensors have a full signal amplitude detection range and an extended operating temperature range (Figure 3), which are suitable for accurate deployment of airbags in automotive safety systems. STMicroelectronics ' product portfolio also includes a 6-axis iNEMO system, where both accelerometers and gyroscope sensors are packaged in the same chip.

　　ADX RS 910 is a MEMS-based gyroscope from ADI, designed for automotive rollover detection applications. The device contains an internal temperature sensor to compensate for offset and sensitivity performance, and has very high stability over a temperature range of -40°C to +105°C. The gyroscope provides a full range of ±300°/s, and data can be read via SPI communication (up to 10MHz). It uses a SOIC package, operates at 3.3V and 5V, and has an operating current of less than 20mA (see Figure 4).

　　Pressure and ultrasound

　　In automotive systems, there are different types of fluids (fuel, engine oil, coolant, washer fluid, etc.) that require non-invasive, safe and reliable methods to monitor their current state or consumption level. The Melexis MLX90819 MEMS pressure sensor, powered by a standard 5V power supply, can be used to accurately determine fluid pressure levels in a variety of applications, including monitoring engine oil, transmission oil and automotive fuel levels, coolant in air conditioning systems, and air pressure in heavy-duty vehicle brakes (see Figure 5).

　　High-frequency ultrasound waves are imperceptible to the human ear. In level measurement applications, the unit that reflects ultrasound waves floats on the surface of the liquid, the sensor is installed at the bottom of the container, and the signal is continuously transmitted. Therefore, by measuring the time required for the sound wave to reach the target, be reflected and return, the term time of flight (ToF), the liquid level can be determined. In order to improve the accuracy of ultrasonic sensors, temperature sensors are usually introduced so that the speed of ultrasound waves can be accurately calculated when the temperature changes. Ultrasonic waves can also be used to perform precise measurements of fluid velocity. SoC solutions for automobiles have an integrated analog front end ( AFE ) that can replace traditional discrete solutions and achieve 1mm-level detection accuracy in the range of 10mm to 1m. Texas Instruments ' ( TI ) TDC1000 is a fully integrated AFE for ultrasonic level measurement, fluid/concentration identification and proximity applications in the automotive market. Combined with an MCU, it can form a complete ultrasonic detection solution.

　　in conclusion

　　Many of the most important applications for MEMS technology are in the more demanding automotive market, where the technology can reduce costs and improve vehicle performance. It is clear that MEMS-based pressure sensors , accelerometers, and other devices are essential to improving road safety and will play a major role in the upcoming autonomous driving .