The automotive electronic control system generally follows the workflow of perception → control → execution.
Sensors serve as sensing units to obtain the working status of the system, while the control unit processes sensor signals and calculates output control instructions, which are ultimately executed by the execution unit.
Taking the electric power steering system (EPS) as an example, during the operation of the vehicle, the steering wheel torque angle sensor monitors the steering wheel angle and torque information, the wheel speed sensor monitors the wheel speed, and the controller (ECU) obtains the sensor signal in real time through the CAN bus, processes the signal in real time according to specific logic, calculates an ideal assist torque, and finally controls the motor through MOSFET to achieve the assist effect.
In the four major systems of automotive power, chassis, body, and electrical, the vast majority of electronic controls have similar working principles. From perception, control to execution, semiconductor devices are everywhere, including sensors in the perception system, microcontrollers (MCU), communication chips (CAN/LIN, etc.), analog-to-digital converters (A/D) in the control process, and power devices (MOSFET, IGBT, DCDC) in the execution process. Among them, sensors are the opportunity for cars.
Automotive sensors can be divided into two categories: vehicle perception and environmental perception. Sensors in power, chassis, body, and electronic and electrical systems belong to the category of vehicle perception, while in ADAS and unmanned driving systems, onboard cameras, millimeter wave radar, laser radar, etc. belong to the category of environmental perception.
This article focuses on explaining car perception sensors.
According to the working principle, automotive sensors can be mainly divided into four categories: MEMS, magnetic, chemical, and temperature. We have calculated that there are over 50 MEMS sensors and over 30 magnetic sensors on traditional gasoline vehicles, accounting for about 90% of the total.
MEMS sensors
MEMS sensor (Micro Electro Mechanical System) is a microelectromechanical system that integrates micro mechanical structures, micro sensors, micro actuators, signal processing and control circuits, as well as interfaces, communication, and power modules on a chip. It is widely used in pressure and motion sensors in automobiles.
According to Bosch's estimation, there are currently over 50 MEMS sensors installed in a car, and we estimate the value of each vehicle to be between 500-1000 yuan. The most commonly used inertial navigation system sensors are pressure sensors, accelerometers, gyroscopes, and magnetometers. Although these products are packaged in microelectromechanical systems, their corresponding principles are different.
Pressure MEMS: Most of them are based on the piezoresistive effect of silicon. The pressure acting on the silicon film causes changes in the resistance of four resistance strain gauges. The Wheatstone bridge outputs a voltage signal proportional to the pressure, which is suitable for medium and low voltage scenarios such as engine intake manifold, tire pressure detection system TPMS, vacuum degree, fuel tank pressure, etc. Ceramic capacitors are commonly used in medium and high voltage applications.
Acceleration MEMS: Based on Newton's second law, the acceleration value is obtained by measuring the inertia force corresponding to the mass block during the acceleration process.
Using capacitive, piezoresistive, or convective principles, it is divided into two categories: low g (gravity acceleration) and high g, with the difference being that the measured acceleration range is different. Low/medium g sensors such as ± 2g~± 24g are used for non safety systems such as active suspension, ESP, rollover, navigation, etc., while high g sensors such as ± 200g are used for safety systems such as airbags.
Angular velocity MEMS/gyroscope: based on the Coriolis force principle: when an object moves in a straight line along the coordinate axis, assuming the coordinate system rotates, the object will experience a vertical force and vertical acceleration.
MEMS gyroscopes are typically equipped with movable capacitive plates in two directions. The radial capacitive plates are subjected to oscillating voltage to force the object to move radially. When rotating, the transverse capacitive plates can measure the capacitance changes caused by the transverse Coriolis motion, thereby calculating the angular velocity. Up to x/y/z three-axis angular velocities can be measured, used for rollover, vehicle stability control systems, inertial navigation IMU, etc.
Magnetometer: During motion, the geomagnetic field changes the direction of the main magnetic field of the magnetometer, causing a change in the angle between the magnetic field direction and the current inside the conductive film. The change in angle is linearly related to the resistance value, and the relative position with the geomagnetic field can be determined through conversion for positioning.
Magnetometers are mainly used in inertial navigation systems (Dead Reckoning) along with accelerometers and gyroscopes. They are used to determine the heading angle and attitude of a car by measuring the relative position to the geomagnetic field when GPS signals are missing. Magnetometers are based on magnetic effects and use MEMS technology. Due to the difficulty in achieving the required sensitivity of Hall effect, AMR is widely used to induce the geomagnetic field.
At present, there are four generations of magnetic sensor technologies, namely Hall effect, AMR (Anisotropic Magnetoresistance Effect), GMR (Giant Magnetoresistance Effect), TMR (Tunnel Magnetoresistance Effect), mainly used for measuring motion. The specific product forms include speed sensors, linear and angular position sensors, current sensors, etc.
Hall sensor: Currently, most of the magnetic sensors used in automobiles are based on the principle of Hall effect, abbreviated as Hall sensors. Mainly used to measure motion quantities such as position, angle, speed, current, etc., it is divided into Hall switches, position Hall (linear/angle/3D), speed Hall, current Hall, and navigation system magnetometer types.
The measurement principle of Hall sensor - Hall effect refers to the force exerted by the magnetic field on the electrons in the Hall element perpendicular to the direction of electron motion when current passes through the Hall element in the magnetic field, causing positive and negative charges to accumulate in the direction of the perpendicular conductor and magnetic induction line, forming a Hall voltage.
The measurement principle of a Hall sensor is to detect changes in the target's motion state based on changes in the magnetic field and induced current caused by cutting magnetic lines, ultimately resulting in changes in the Hall voltage.
XMR Magnetic Resistance: AMR, GMR, and TMR are all based on the principle of magnetic resistance. As the next generation of magnetic sensor technology, with their performance advantages, the permeability is increasing day by day. At present, AMR/GMR technology has been widely applied 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 materials. Compared with Hall elements, AMR, and GMR, it has outstanding advantages:
Firstly, the temperature performance is good, and the front-end module is electroplated with a nano thick oxide layer instead of a semiconductor;
Secondly, the current power consumption is low, reducing from 5-20mA in Hall to the μ A level;
Thirdly, it has strong sensitivity and lower cost in terms of scale and quantity. Hall elements require the use of strong magnets such as neodymium iron boron.
TMR sensors will replace Hall sensors in high demand application scenarios with outstanding 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 that require very high functional safety levels.
2. Liquid level sensor: TMR replaces reed switches, which are prone to breakage, poor consistency, and high cost. TMR has high sensitivity, low cost, and overcomes the problem of breakage.
Chemical sensors
Oxygen sensors: Generally, cars are equipped with two oxygen sensors, front oxygen and rear oxygen. The front oxygen sensor detects the oxygen content in the mixed exhaust and provides feedback to the engine ECU to correct the fuel injection amount, controlling the air-fuel ratio of the mixture near the theoretical value to achieve the highest efficiency of the three-way catalytic converter. The rear oxygen sensor detects the oxygen content in the mixed gas after catalytic conversion to determine whether the three-way catalytic converter has failed.
Nitrogen oxide sensor: Nitrogen oxide sensors are mainly used in the Selective Catalytic Reduction System (SCR) of diesel vehicles for detecting whether the NOx content after catalytic reduction of exhaust gas meets emission requirements.
Temperature sensor
Thermistors are commonly used in cars to measure temperature, which can be divided into two categories: PTC and NTC.
NTC: The resistance decreases with increasing temperature and is mainly used to measure the temperature of gases, liquids, and environments, including coolant, intake pipes, air conditioning evaporator outlets, and temperature detection inside and outside the vehicle.
PTC: When the temperature exceeds a certain level, the resistance increases significantly. It is mainly used for overcurrent protection, temperature limitation, heating, and other scenarios, such as motor protection sensors.
In the face of high temperature situations, such as engine exhaust manifolds and three-way catalytic converters with temperatures exceeding 800 ℃, traditional thermistors cannot meet the requirements, and platinum resistance temperature sensors are usually used for measurement.
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(Editor: Huilian Technology)
