The microelectromechanical systems (MEMS) sensors used in electric vehicles (EVs) include inertial sensors, such as:
- Accelerometers and gyroscopes
- Magnetometers
- Pressure sensors
- Thermal sensors
- Gas sensors
- Optical MEMS
MEMS are used to improve safety for drivetrain monitoring and battery management. These sensors also support driver comfort, improving convenience and security, as well as other functions (Figure 1). This FAQ discusses the operation and applications for several types of MEMS sensors in EVs.
Figure 1. MEMS sensors perform a variety of functions and are found in many locations in EVs. (Image: Atomica)
Inertial sensors
Inertial sensors like accelerometers and gyroscopes can be used individually or together. Accelerometers can measure static acceleration, gravity, or dynamic acceleration. These MEMS sensors generally work based on the piezoelectric effect or the movement of a weight attached to a spring. In piezoelectric-based MEMS accelerometers, the piezoelectric material is stressed by the acceleration and produces a corresponding voltage. In spring-based designs, the spring generates an electric signal when the weight moves.
MEMS gyroscopes use vibrations to measure changes in angles. A common design uses a pair of vibrating tuning forks. While the tuning forks are moving in the same direction, there is no difference between their relative motions. If the tuning fork assembly is rotated, the force on the two forks is in opposite directions, and the resulting force differential is converted to a voltage corresponding to the rate or angular change.
MEMS accelerometers and gyroscopes can be co-packaged into a single inertial measurement device (Figure 2). Inertial sensors are used in various advanced driver assistance (ADAS) systems. They can be used to measure lateral, longitudinal, vertical acceleration, yaw rate, and inclination. For example, a MEMS gyroscope can detect a rolling vehicle in a crash detection system.
Figure 2. MEMS inertial sensor including the gyroscope and accelerometer elements. (Image: STMicroelectronics)
Magnetometers
A MEMS magnetometer is a magnetic field sensor that detects and measures magnetic fields like those produced by the Earth. The most common technology operates by measuring the effects of the Lorentz force, whereby a change in resonant frequency or voltage can be measured electronically, or optical techniques can be used to measure a mechanical displacement.
Typical applications of MEMS magnetometers include miniaturized compasses for navigation or detecting nearby metal objects like another automobile or truck coming close to the front or rear of a car.
Pressure sensors
MEMS pressure sensors use piezo resistors on a thin silicon diaphragm to convert varying pressures into electrical signals. There are three common types of MEMS pressure sensors:
- Absolute pressure sensors measure pressure relative to a vacuum. In an EV, this type of sensor can detect a pressure rise inside the battery pack, indicating a thermal runaway event of battery cell venting.
- Differential pressure sensors measure the pressure difference between two points. They’re used in applications like measuring airflow in an HVAC system or monitoring the pressure drop across air filters.
- Gauge pressure sensors measure pressure relative to atmospheric pressure. They’re not generally used in EVs.
Thermal sensors
MEMS thermal sensors are available in two designs: contact and non-contact. A contact thermal sensor uses a resistance temperature detector (RTD) element like a thin film of platinum, nickel, or copper to measure changes in electric resistance in response to changes in temperature. Non-contact infrared (IR) MEMS thermal sensors are available based on the Seeback effect that generates a thermoelectric output produced by the junction of two dissimilar materials.
Thermal sensors are used in a wide variety of EV systems, including the battery pack, battery cells, traction motor, traction inverter, dc/dc converter, on-board charger, and external charging systems — including the connecting cables and handles, in the cabin and under the hood.
Gas sensors
MEMS gas sensors measure changes in the interaction between gas molecules and the sensor surface. Gas sensors are available based on various technologies, including electrochemical, pellistor (a solid state device based on a Wheatstone bridge), and photoionization. In an EV, they can be used to monitor the atmosphere in the cabin, but their most common application is to detect the presence of hydrogen in battery packs.
Some of these sensors are available as complete subsystems with analog and I2C outputs housed in an IP 69K enclosure with a wiring harness (Figure 3).
Figure 3. This MEMS gas sensor assembly is designed for use in EV battery packs. (Image: Posifa Technologies)
Optical MEMS
Optical MEMS (MOEMS) are available that operate from infrared to visible wavelengths and use micromirrors or microlenses to direct and detect light. The active elements are often photodiodes or photoresistors. MOEMS are used for steering lasers in LIDAR or heads-up displays and for directing smart adaptive headlights.
Summary
As new autonomous driving features are added, the number of MEMS sensors used in EVs is expected to grow. Here’s a wide array of MEMS sensor technologies that enhance EV operation, safety, comfort, and convenience.
References
- MEMS automotive sensors drive the future of mobility, Atomica
- MEMS, Micro-Electro-Mechanical Systems, STMicroelectronics
- MEMS Pressure Sensor, Electricity-Magnetism
- MEMS sensors are (literally) driving the electric vehicles industry, Escatec
- MEMS Thermal Sensors, Omron
Images
Filed Under: Componentry, FAQs, Sensors