Automotive electronic control systems generally follow a workflow of perception → control → execution.
Sensors, acting as sensing units, acquire the operating status of the system. The control unit processes the sensor signals and calculates and outputs control commands, which are then executed by the execution unit.
(Sensing sensor)
Taking the electric power steering (EPS) system as an example, during vehicle operation, the steering wheel torque angle sensor monitors the steering wheel angle and torque information, the wheel speed sensor monitors the wheel speed, the controller (ECU) acquires the sensor signals in real time through the CAN bus, processes the signals in real time according to specific logic, calculates an ideal assist torque, and controls the motor through MOSFET to achieve the assist effect.
In the four major systems of automobiles—powertrain, chassis, body, and electrical—most electronic controls share similar working principles. From sensing and control to execution, semiconductor devices are ubiquitous, including sensors in the sensing system, microcontrollers (MCUs), communication chips (CAN/LIN, etc.), and analog-to-digital converters (A/D) in the control system, and power devices (MOSFETs, IGBTs, DC-DC converters) in the execution system. Among these, sensors represent a significant opportunity for automobiles.
Automotive sensors can be broadly categorized into vehicle perception and environmental perception. Sensors in the powertrain, chassis, body, and electronic and electrical systems fall under the category of vehicle perception, while onboard cameras, millimeter-wave radar, and lidar introduced into ADAS and autonomous driving systems fall under the category of environmental perception.
This article focuses on automotive perception sensors.
Based on their working principles, automotive sensors can be mainly divided into four categories: MEMS, magnetic, chemical, and temperature. According to our statistics, there are more than 50 MEMS sensors and more than 30 magnetic sensors in traditional gasoline cars, accounting for about 90% in total.
MEMS sensor
MEMS (Micro-Electro-Mechanical System) sensors are micro-electro-mechanical systems that integrate micro-mechanical structures, micro-sensors, micro-actuators, signal processing and control circuits, as well as interfaces, communication and power modules on a single chip. They are widely used in automobiles for pressure and motion sensors.
According to Bosch's estimates, a typical car currently contains over 50 MEMS sensors, with a combined value of 500-1000 yuan per vehicle. Commonly used sensors include pressure sensors, accelerometers, gyroscopes, and magnetometers, which are part of inertial navigation systems. While all these products utilize MEMS packaging, their underlying principles differ.
Pressure MEMS: Most are based on the piezoresistive effect of silicon. Pressure acts on a silicon thin film, causing changes in the resistance of four strain gauges. The Wheatstone bridge outputs a voltage signal proportional to the pressure. It is suitable for low- to medium-pressure applications, such as engine intake manifolds, tire pressure monitoring systems (TPMS), vacuum levels, and fuel tank pressure. Ceramic capacitors are often used in medium- and high-pressure applications.
Acceleration MEMS: Based on Newton's second law, acceleration values are obtained by measuring the inertial force corresponding to the mass during acceleration. Employing capacitive, piezoresistive, or thermal convection principles, they are divided into two main categories: low-g (gravitational acceleration) and high-g. The difference lies in the range of acceleration measured. Low/medium-g sensors (±2g~±24g) are used in non-safety systems such as active suspension, ESP, rollover protection, and navigation, while high-g sensors (±200g) are used in safety systems such as airbags.
Angular velocity MEMS/Gyroscope: Based on the Coriolis force principle: When an object moves linearly along a coordinate system, assuming the coordinate system rotates, the object experiences a perpendicular force and a perpendicular acceleration. MEMS gyroscopes typically mount movable capacitor plates in two directions. An oscillating voltage is applied to the radial capacitor plate, forcing the object to move radially. During rotation, the lateral capacitor plate measures the capacitance change caused by the lateral Coriolis motion, thus calculating the angular velocity. It can measure the x/y/z-axis angular velocity and is used in rollover systems, vehicle stability control systems, and inertial navigation IMUs, among others.
Magnetometer: During movement, the Earth's magnetic field changes the direction of the magnetometer's main magnetic field, which in turn causes a change in the angle between the direction of the magnetic field and the current in the conductive film. The change in the angle is linearly related to the resistance value. By conversion, the relative position with respect to the Earth's magnetic field can be determined for positioning.
Magnetometers, along with accelerometers and gyroscopes, are primarily used in inertial navigation systems (dead reckoning) to determine a vehicle's heading angle and attitude by measuring its relative position to the Earth's magnetic field when GPS signals are unavailable. Magnetometers are based on the magnetic effect and employ MEMS technology. Because the sensitivity of the Hall effect is difficult to achieve, AMRs (Automatic Magnetic Resonators) are commonly used to sense the Earth's magnetic field.
magnetic sensor
Currently, there are four generations of magnetic sensor technology: Hall effect, AMR (Anisotropic magnetoresistance effect), GMR (Giant magnetoresistance effect), and TMR (Tunnel magnetoresistance effect). They are mainly used to measure motion, and specific product forms include speed sensors, linear and angular position sensors, and current sensors.
Hall effect sensors: Most magnetic sensors used in automobiles are based on the Hall effect principle and are simply called Hall sensors. They are mainly used to measure motion quantities such as position, angle, speed, and current, and are divided into Hall switches, position Hall sensors (linear/angle/3D), speed Hall sensors, current Hall sensors, and magnetometers for navigation systems, among other types.
The measurement principle of Hall sensors—the Hall effect—is that when an electric current passes through a Hall element in a magnetic field, the magnetic field exerts a force on the electrons in the Hall element perpendicular to the direction of electron motion, causing positive and negative charges to accumulate in the direction perpendicular to the conductor and the magnetic field lines, thus forming a Hall voltage.
The Hall sensor measures by the change in magnetic field and induced current caused by the movement cutting magnetic field lines, which in turn causes a change in Hall voltage. This change is then used to detect changes in the motion state of the target.
xMR magnetoresistive: AMR, GMR, and TMR are all based on the magnetoresistive principle. As the next generation of magnetic sensor technology, their penetration rate is increasing due to their performance advantages. Currently, AMR/GMR technology has been widely used in sensor fields such as wheel speed, steering wheel angle/torque, electronic throttle position, crankshaft and camshaft speed.
The performance improvement of TMR sensors is significant. Utilizing the tunneling magnetoresistance effect of magnetic multilayer film materials, they offer outstanding advantages compared to Hall elements, AMR, and GMR.
Firstly, it has good temperature performance; the front-end module is electroplated with a nanometer-thick oxide layer, rather than a semiconductor.
Secondly, the current consumption is low, reduced from 5-20mA of Hall effect current to the μA level;
Third, it is highly sensitive, and the cost is lower after scaling up production. Hall elements require strong magnets such as neodymium iron boron.
TMR sensors will replace Hall sensors in demanding applications due to their superior product performance.
1. Angle, speed, and position sensors: including BLDC rotor position, steering wheel angle, wheel speed, throttle position, crankshaft/camshaft angle, and other applications with very high functional safety requirements.
2. Liquid level sensor: TMR replaces reed switch. Reed switches are prone to breakage, have poor consistency, and are expensive. TMR has high sensitivity, low cost, and overcomes the breakage problem.
Chemical sensors
Oxygen Sensors: Automobiles typically have two oxygen sensors: a front oxygen sensor and a rear oxygen sensor. The front oxygen sensor detects the oxygen content in the exhaust mixture and feeds this information back to the engine ECU to adjust the fuel injection quantity, controlling the air-fuel ratio near its theoretical value to ensure high efficiency of the three-way catalytic converter. The rear oxygen sensor detects the oxygen content in the mixture after catalytic conversion and is used to determine if the three-way catalytic converter has failed. (Figure: Working principle of an oxygen sensor)
Nitrogen oxide sensor: The nitrogen oxide sensor is mainly used in the diesel vehicle after-treatment SCR (Selective Catalytic Reduction System) to detect whether the NOx content after the exhaust gas catalytic reduction meets the emission requirements.
Temperature sensor
Thermistors are commonly used in automobiles to measure temperature, and they can be divided into two categories: PTC and NTC.
NTC: Resistance decreases as temperature increases. It is mainly used to measure the temperature of gases, liquids, and the environment, including the temperature of coolant, intake manifold, air conditioning evaporator outlet, and inside and outside of vehicles.
PTC: When the temperature exceeds a certain level, the resistance increases significantly. It is mainly used in overcurrent protection, temperature limiting, heating and other scenarios, such as motor protection sensors.
In high-temperature environments, such as engine exhaust manifolds and three-way catalytic converters where temperatures can reach over 800°C, traditional thermistors are insufficient, and platinum resistance temperature sensors are typically used for measurement.
