MEMS Sensors Enable Precise, Efficient Automotive Systems

The internet of things and industrial IoT applications aim to obtain useful information from the data flows acquired by sensors. The IoT and IIoT are both based on a multilayered approach in which peripheral devices communicate with a high-level application directly or through intermediate resources. Microelectromechanical-system (MEMS) sensors have the potential to provide significant benefits in cost and performance for IoT and IIoT applications.

Figure 1: Block diagram for an intelligent system using sensors (Image: EE Times Europe)

Multi-sensor systems are fundamental to the success of Industry 4.0 applications. They record, process, and transmit measurement parameters such as pressure, acceleration, and temperature, all in an extremely compact space. Workpieces are increasingly equipped with intelligent sensors so that each product can report the production status.
Sensor systems are also critical to vehicle safety systems, ranging from basic airbags to highly complex advanced driver-assistance systems (ADAS) with next-generation accident-prevention features.

MEMS DESIGN

MEMS technology derives from the combination of mechanical devices, sensors, and electronic components placed on a single common silicon substrate using micro-fabrication technology. The technology allows designers to realize an entire system on a single electronic circuit, leveraging advantages offered by the robust technology.
Designing MEMS sensors into intelligent systems nonetheless poses several key challenges:

  • Energy consumption. For large-scale deployments, battery replacement is a difficult task. Energy-harvesting methods must be used to make the sensors and related devices self-sufficient.
  • Maintenance. A failure in a node should not explicitly affect the operation of other sensors in the network. Debugging of errors should be facilitated by the support of the other nodes. Backup control should avoid losing valuable data.
  • Robustness. Sensor nodes are sometimes deployed in severe environments and thus must be able to work accurately and problem-free for long periods under harsh conditions.
  • Form factor. The size of the sensor is determined by a series of application and energy parameters. Downsizing affects both mechanical and electronic performance.

Some segments of the MEMS market require higher precision than others. An automotive accelerometer used in an ADAS, for example, must be much more precise than one used in a smartphone. The amount of engineering work and complexity required to achieve a higher-precision MEMS sensor solution translates into added value and higher selling prices. Noise reduction and higher resolution are part of the design equation for high-performing sensor systems. Some of these design tasks are performed on the ASIC side, using noise-reduction techniques with active components.

Integration of intelligent MEMS sensors can reduce operating costs, increase resource efficiency, improve demand, and provide critical information about the system. As communications platforms and networks continue to evolve for IoT purposes, companies have a variety of intelligent sensors to choose from to improve their solutions’ features.

APPLICATIONS

MEMS sensors used in automotive systems include accelerometers, inclinometers, and gyroscopes; sensors for measuring flow and pressure; and even sensors for innovative on-board infotainment apps.

In automotive systems, MEMS sensors enable a more extensive range of control solutions that provide greater efficiency and vehicle safety. MEMS sensors are present in airbags, anti-lock braking systems (ABS), electronic stability program (ESP) systems, and electronically controlled suspensions, as well as in numerous driver-assistance systems — uphill starts, electric parking brakes, lane departure warnings, cruise speed control, and many more.

STMicroelectronics offers a portfolio of automotive MEMS sensors qualified according to the AEC-Q100 standard. ST’s low-G three-axis accelerometers have advanced energy-saving features and a wide range of operating temperatures that suit the devices for non-critical automotive applications. The company’s high-G acceleration sensors are suitable for applications such as vehicle airbag systems. The three-axis gyroscopes offer high stability over time and as a function of temperature and guarantee a level of precision required by the most advanced navigation systems.

Figure 2: Block diagram of the A3G4250D sensor (Image: STMicroelectronics)

ST’s AIS328DQ, AIS3624DQ, AIS1120SX/AIS2120SX, and AIS1200PS three-axis accelerometers offer high measurement resolution and low noise levels, with different working modes to save energy and with intelligent functions for wake-up options. The company’s high-G acceleration sensors enable a wide signal amplitude detection range, operate over an extended temperature range, and are suitable for precise airbag deployment. The three-axis A3G4250D gyroscope exhibits high stability over time and as a function of temperature and guarantees a level of precision required by the most advanced navigation systems.

Bosch has released a new generation of MEMS sensors with significantly faster response times compared with previous-generation devices. The SMA7xy series of accelerometers increases safety when deployed in airbag systems. Bosch has doubled the bandwidth and improved the signal-processing speed for a more effective response upon impact. The different sensors in the family can also be used in complementary ways. For example, the SMA760 detects frontal and side impacts, whereas the SMA720 has an x and a z channel to measure the acceleration along the vertical axis.

Both sensors support the SafeSPI communications standard for automotive systems. All sensors in the SMA7xy family comply with Automotive Safety Integrity Level (ASIL) D of the ISO 26262 standard and meet German Association of the Automotive Industry (VDA) AK-LV 27 specifications (Figure 3).

Figure 3: SMA7xy MEMS sensor (Image: Bosch)

Vehicle dynamic control (VDC), also known as electronic stability control, is a growth application that is critical to ADAS design. VDC’s primary function is to keep a skidding vehicle on course. When a VDC system is functioning correctly, the driver should not even notice its intervention. A VDC system is composed of a gyroscope, an accelerometer, and a speed sensor placed on each wheel (possibly shared with the adaptive braking system).

The speed sensors continuously measure the speed of the wheel rotation and compare the speed of rotation predicted along the yaw axis (yaw rate) with that measured by the gyroscope. A low-G accelerometer detects whether the car is slipping sideways. If the measured yaw rate differs from the predicted rate, or if the vehicle is determined to be slipping sideways, the system intervenes, acting on the individual brakes or on the engine torque to keep the car on track.

As simple passive measurement systems evolve to add built-in control functions and even enable entirely autonomous operation, sensors are playing an increasingly demanding support role. The automotive industry will see an exponential rise in demand for increasingly efficient technology within vehicles to support further ADAS development.
The new frontier for advanced MEMS development is high accuracy — with, of course, low power consumption and small system dimensions. High precision is required to enable demanding applications such as virtual-reality or augmented-reality navigation. The ultimate aim is autonomous driving, which requires highly precise systems to achieve efficient positioning and control of the vehicle in traffic.

The extraction of accurate and stable information from a MEMS device requires a robust signal-to-noise ratio driven by the silicon’s geometric characteristics. The approaches to integration, packaging, and test/calibration can be optimized to maintain native performance and minimize costs even in complex environments like automotive systems. ■