High performance MEMS sensors for smarter vehicles

By Mark Patrick, Mouser

Micro Electro-Mechanical System (MEMS) technology is derived from the combination of mechanical devices, sensors and electronic components placed upon a single silicon common substrate by using advanced micro-fabrication technology. Nowadays, such technologies are progressively replacing a large proportion of the sensing apparatus used in automobile models. This is bringing considerable design advantages in relation to system cost reduction, ongoing reliability (thanks to the inherent robustness), space utilisation (due to their compact size) and also operational performance.

Generally speaking, MEMS sensors used in automotive can be classified into four categories:

  • Accelerometers and gyroscopes.
  • Sensors for measuring flow and pressure.
  • Sensors for on-board applications – include IR sensors to improve and monitor road vision, sensors for temperature and air quality measuring in the passenger compartment, micro scanners for head-up display, etc.
  • RF sensors for automotive radars.

In the automotive systems, MEMS sensors make it possible to access a greater number of key parameters that effect the running of the vehicle, and create a wider array of control solutions. They can be found in airbags, anti-lock braking system (ABS) implementations and electronic stability program (ESP) systems, as well as electronically controlled suspension systems, as well plus numerous driver assistance functions.

With particular reference to automotive applications, it should be noted that modern cars have evolved considerably regarding complexity. By integrating an ever-increasing number of safety-related devices, reducing fuel consumption and curbing harmful emissions, as well as improving driving comfort.

Figure 1: Diagram of a typical MEMS-based sensor with its accompanying conditioning circuitry

Accelerometers and gyroscopes

An accelerometer has the aim of measuring the acceleration in force units (G). MEMS devices offer high accuracy in applications with high noise sources. Some devices use the piezoelectric effect to determine G levels. These contain microscopic crystalline structures which are stressed by accelerating forces with the relative generation of an AC/DC voltage. In most cases, design engineers will tend to choose between either a capacitive or a thermal micro-electromechanical accelerometer option. To select the right accelerometer for their application, they need to consider several important variables. These include sensor structure, resonance, reliability, stability, bandwidth and power consumption.

In contrast to accelerometers, gyroscope sensors measures the angular velocity expressed in degrees per second (°/s) or revolutions per second (rps). The angular velocity is merely a measure of the rotation speed. When choosing a gyroscope, it is necessary to consider reliability, the temperature range allowed and potential susceptibility to electro-magnetic interference. The error caused by the noise sources can compromise acquired measurement precision and therefore the design of the system.

The crash sensor for airbag control is the classic application found in automotive systems. It mainly consists of MEMS inertial sensors (accelerometers and gyroscopes). The accelerometer continuously measures the acceleration of the car. When this parameter exceeds a predetermined threshold, a microcontroller unit (MCU) calculates the integral of the acceleration to establish if a considerable speed variation has occurred. Single/dual-axis acceleration sensors are commonly used in airbags. Alternatively, angular speed sensors can be implemented into the design.

The AIS1120SX/AIS2120SX3-axis accelerometers from STMicroelectronics, offer high measurement resolution and low noise levels, with different work modes to save energy and intelligent features such as a wake-up function. Suitable for ensuring accurate deployment of airbags in vehicle safety systems, these high-G acceleration sensors have a full signal amplitude detection range, along with an extended operational temperature range (Figure 2). The STMicroelectronics portfolio also includes 6-axis iNEMOsystems with both accelerometer and a gyroscope sensors housed within the same chip.

Figure 2: Block diagram for STMicroelectronics AIS1120SX

The ADXRS910is a MEMS-based gyroscope from Analog Devices designed for automotive rollover detection applications. The device also has an internal temperature sensor that is used to compensate for offset and sensitivity performance, providing strong stability across the temperature range of -40°C to + 105°C. The gyroscope provides a complete range of ± 300°/sec, with provision for making SPI communications (up to 10MHz) for data reading. It is available in a SOIC package and is intended to operate at 3.3V and 5V, drawing less than 20mA of current (Figure 3).

Figure 3: Block diagram for the ADXRS910

Pressure and ultrasound

In automotive systems, there are different types of fluids present (fuel, engine oil, coolant, washer fluid, etc.). These require the monitoring of the current state or consumption levels using non-invasive, safe and reliable methods. The MLX90819MEMS pressure sensor from Melexis operates with a standard 5V power supply. It can be used to accurately determine fluid pressure levels in a wide variety of applications. These include the monitoring engine oil, transmission oil and vehicle fuel levels, plus coolant from the air conditioning system, and air pressure in heavy-vehicle brakes (see Figure 4).

High frequency ultrasound waves are not perceptible by the human ear. In a level measurement application, an element that reflects ultrasound floats on the surface of the liquid. A transducer is installed on the bottom of the tank, from which a signal is continuously transmitted. The fluid level can thus be determined by measuring the time required by the sound wave to reach the target, be reflected, and go back. This period is indicated via use of the term time-of-flight (ToF). To improve the accuracy of an ultrasonic sensor, a temperature sensor is usually introduced, so as to be able to accurately calculate the speed of the ultrasound waves as the temperature changes. Ultrasound can also be used to perform an accurate measurement of the speed of a fluid. SoC solutions for vehicles offer an integrated analogue front end (AFE) as an alternative to discrete solutions, with 1mm level detection accuracy within a range of 10mm to 1m. The TDC1000from Texas Instruments is a fully integrated AFE for ultrasonic level measurement, fluid/concentration identification and proximity applications within the automotive market. When associated with an MCU, it can provide the complete ultrasonic detection solution.

Figure 4: Block diagram for the MLX90819

Conclusion

MEMS technology is finding many of its most important applications situated within the ever more demanding automotive business, allowing reductions in costs and improvements in vehicle performance. It is clear that MEMS-based pressure sensors, accelerometers and other devices can be pivotal in improving safety on our roads and are certain to have a major role to play in supporting the onset of autonomous driving.

Figure 5

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