In the process of autonomous driving, cars are equipped with an increasing number of sensors. Compared to sensors that use other measurement principles and technologies, Hall effect-based magnetic field sensors have many advantages. Current models also meet the stricter requirements of the ISO 26262:2018 standard.
When a magnetic field is applied to a semiconductor that is perpendicular to the direction of the current, the Hall sensor can detect the voltage difference generated within it. Since the Hall voltage is proportional to the magnetic field strength, these sensors can be used with permanent magnets to indirectly measure many variables such as rotation, speed, distance, pressure, angle, and fill level. Because the sensor measures the magnetic field strength around the conductor through a stable current, it can also measure current non-contactly. The same principle allows for the non-contact detection of other parameters, meaning that Hall sensors experience no wear during operation and can provide precise and reliable measurements even under long-term operation.
Simple Hall sensors are often used as switches, for example, in automotive seatbelt buckles, windshield wipers, or engine switch systems. In these cases, the threshold for magnetic field strength is stored in the sensor. If the detected value reaches this threshold, the switch state of the sensor changes. For example, Melexis’s MLX92241 has a built-in isolation capacitor that can be operated directly with cable harnesses, allowing for near-end designs without circuit boards for detecting whether a seatbelt is fastened, among other things. The EEPROM memory of the MLX92241 can store customer-specific switch point thresholds, output polarity, Ioff current, and temperature compensation coefficients for magnetic materials. A programmable negative temperature coefficient can be used to compensate for the weakening behavior of permanent magnets at high temperatures. Hall sensor elements are equipped with safety mechanisms to prevent electrostatic discharge, reverse polarity, and thermal overload. They meet ASIL A safety level requirements.
Linear Hall Sensors Measure Distance and Rotation
Hall sensors with linear output signals are essential devices for measuring distance or rotational motion. They can not only recognize “on” and “off” states but also output an analog signal proportional to the magnetic field strength. An A/D converter integrated into the MCU or Hall sensor converts the analog signal into a digital signal. Subsequently, the MCU outputs a proportional pulse-width modulation (PWM) signal or data stream compatible with the SENT automotive sensor bus system to control other system components. Some of the latest generation Hall sensors feature built-in PWM and SENT interfaces.
If a Hall sensor that only detects magnetic fields perpendicular to the chip plane is used, it typically requires larger and more expensive additional components. More advanced solutions integrate an increasing number of sensors along with signal processing and computing capabilities, often eliminating the need for additional components while being able to measure other additional parameters.
For example, vertical Hall sensors can detect not only magnetic fields perpendicular to the direction of current but also magnetic fields parallel to the direction of current or the chip plane. In addition to the amplitude of the magnetic field, 2D sensors can also detect its direction. For instance, this can be used to determine the rotation direction of an engine. Infineon’s Hall effect-based Xensiv TLE4988C can quickly measure the position of the camshaft. A significant benefit for module manufacturers is the reduced reliance on rare earth reverse-bias magnets; for example, this sensor is optimized for Fe, SmCo, and NdFe. Automotive onboard calibration applications take into account the tolerances of ferromagnetic wheels and magnetic encoders, as well as the mounting tolerances of the sensor, ensuring extremely precise sensing performance in actual application environments. The TLE4988C comes in a PG-SSO-3-52 camshaft sensor package with tin-plated leads, featuring a 3-wire voltage I/F and increased 220/1.8nF power/output capacitance for enhanced EMC robustness.
Infineon’s Xensiv TLE5501 analog angle sensor is based on tunnel magnetoresistance (TMR) technology, offering high detection sensitivity and high output voltage without the need for an internal amplifier, allowing the sensor to connect directly to a microcontroller. Additionally, TMR technology has minimal temperature drift, reducing the need for external calibration and compensation. The TLE5501 uses 360° angle measurement, detecting the direction of the magnetic field by measuring the sine and cosine angle components with TMR elements. The sensor outputs the raw signal in the form of a differential output signal. Due to the higher bridge output voltage, no additional signal amplification is required. The TLE5501 is available in AEC-Q100 and automotive ASIL variants, suitable for angle position sensing, steering angle sensing, safety applications, and BLDC motor commutation.
Image Source: Infineon Technologies
Third Dimension
3D Hall sensor technology combines lateral and vertical Hall sensors, allowing for the detection of magnetic field strength in all three dimensions. Such sensors can detect the absolute rotational or linear position of each moving magnet. For example, the MLX92256 is equipped with lateral sensing, specifically designed for use in window lift systems, integrating a voltage regulator, two Hall sensors (one with an integrated magnetic concentrator (IMC), both featuring advanced offset elimination systems), and two open-drain output drivers in a single package. This device is available in two variants. The MLX92256LSE-AAA-000 variant switches the pulse signal when the lateral or vertical magnetic field component changes, while the direction pin changes only when the direction changes. The MLX92256LSE-ABA-000 is equipped with two speed outputs for vertical and lateral magnetic fields, respectively.
Melexis’s MLX90371/MLX90372 Triaxis position sensors have now launched their third generation, which combines a magnetic Triaxis Hall front end, an analog/digital signal regulator, a DSP (digital signal processor) for signal processing, and output stage drivers. The increasing electrification of vehicles (especially electric and hybrid vehicles) can generate stray magnetic fields of up to 4kA/m (or 5mT), yet these sensors remain unaffected. These devices can also operate in weak magnetic fields, and smaller, cheaper magnets are sufficient, which not only brings cost benefits but also saves space. The MLX90371 complies with ISO 26262 standards as ASILB SEooC (not safety-related) and offers analog or PWM output; the MLX90372 complies with ISO 26262 standards as ASIL-C SEooC and features SENT or PWM output. Both sensors meet the EMC requirements of automotive OEM manufacturers and are rated for operating temperatures up to 160°C. For applications with particularly strict safety requirements, such as accelerator pedal position sensing, the MLX90372 also offers a dual-chip (fully redundant) TSSOP-16 package for redundancy sensing functionality.
TDK-Micronas offers a programmable 3D sensor series for position sensing, which includes three models: HAL 3900 with SPI interface, HAL 3930 with PWM/SENT interface and switch output (configurable as high/low side switch), and HAL 3980 with PSI5 interface. These sensors can use a Hall plate array to suppress external stray magnetic fields. Only a simple two-pole magnet is needed to measure rotational angles, preferably placed in the sensitive area configured at the end of the shaft. This sensor series can also be used for off-axis measurements, capable of measuring a 360° angle range, linear motion, and 3D positions, making it ideal for steering angle position sensing. Depending on the product, temperature compensation values for BX, BY, BZ or up to two calculated angles can be transmitted. HAL-39xy sensors operate in an ambient temperature range of -40° to +160°C; they are designed according to automotive application ISO 26262 standards, comply with ASILB requirements, and are designed as SEooC products.
Image Source: TDK-Micronas
Robust and Reliable
These examples illustrate that sensor technology has made significant advancements to meet the growing demands of the automotive industry, and their designs are continually evolving. Many of the latest models also comply with the stricter ISO 26262:2018 standards, some of which are ASIL-C SEooC. Redundancy and safety features, as well as improved EMC measures, contribute significantly to this. Appropriate packaging ensures that sensors are moisture-proof, dust-proof, and dirt-proof. For use in high-temperature environments, Hall sensors can operate at temperatures up to 170°C. Due to these characteristics, they are crucial technologies for achieving autonomous driving.
(This article is authored by Edgar Schaefer, Automotive Product Application Engineer at Ruzhuoli)