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Hall sensors in automotive applications - Suitable for a variety of parameters

  Knowledge

On the road to autonomous driving, cars are being equipped with an in-creasing number of sensors. Magnetic field sensors based on the Hall effect offer several advantages over other measuring principles and technolo-gies. Current models also meet the more stringent requirements of ISO 26262:2018.

Hall sensors detect the voltage differ-ence that results when a magnetic field is applied to a semiconductor perpendicular to the direction of flow of the electric current. Since this Hall voltage is di-rectly proportional to the strength of the magnetic field, the sensors, together with a permanent magnet, can indirectly measure numerous variables, e.g. rotation, speed, dis-tance, pressure, angle, and fill level. Since the sensor measures the strength of a magnetic field around a conductor with a steady flow of current, it can also measure current with-out contact. Similarly, it also detects the other parameters without contact, meaning that it operates without any wear and tear and therefore reliably delivers precise measure-ments, even in long-term operation. Simple Hall sensors are often used as switch-es, e.g. in seat belt buckles, windshield wipers, or engine switching systems. In this context, a threshold value for the magnetic field strength is saved in the sensor. If the detect-ed value reaches this threshold, the switching state of the sensor changes. For example, Me-lexis' MLX92241, with its built-in blocking ca-pacitors, makes a local board-less design pos-sible with operation directly on a cable harness, e.g. for detecting whether the seat belt is fastened. Its EEPROM memory can store customer-specific threshold values for the switching points, output polarity, Ioff current, and temperature compensation coefficient for the magnetic material. The programmable negative temperature coefficient can be used to compensate for the behavior of permanent magnets that become weaker at high tem-peratures. The Hall sensor elements are equipped with safety mechanisms to protect against electrostatic discharge, reverse polar-ity, and thermal overload. They meet the re-quirements of ASIL A.

Linear hall sensors also measure distances and rotation

Hall sensors with linear output signals are necessary to measure distances or rotational movements. They not only recognize the states "on" and "off," but also output an ana-log signal proportional to the magnetic field strength. The A/D converter, which is either integrated into the MCU or the Hall sensor, converts the analog signal into a digital sig-nal. To control other system components, the MCU then outputs a proportional pulse-width modulated (PWM) signal or a data stream compatible with the SENT automotive sensor bus system. Some of the latest-generation Hall sensors have the PWM interface and the SENT interface built right into the sensor it-self.When using Hall sensors that merely detect the magnetic field perpendicular to the chip plane, large and cost-intensive additional as-semblies are often required. More advanced solutions integrate an increasing number of sensors as well as signal processing and cal-culation functions, often eliminating the need for additional components and allowing ad-ditional parameters to be measured.Figure: TDK-Micronas

Automotivewww.rutronik.com99Powered by2020Committed to ExcellenceFor example, vertical Hall sensors detect not only the magnetic field perpendicular to the direction of the current, but also that paral-lel to the direction of the current or to the chip plane. In addition to the amplitude of the magnetic field, 2D sensors also detect its direction. This can be used to determine an engine's direction of rotation, for example. For example, the Hall-based Xensiv TLE4988C from Infineon makes it possible to quickly measure the position of the camshaft. One major benefit for module manufacturers is the reduced dependence on rare-earth back-biased magnets; the sensor is optimized for Fe, SmCo, and NdFe, for example. Automat-ic in-vehicle calibration takes the tolerances of ferromagnetic wheels and magnetic en-coders as well as the mounting tolerances of the sensor takes into account, thereby ensur-ing extremely accurate sensing in real appli-cation environments. The TLE4988C comes inside a PG-SSO-3-52 camshaft sensor pack-age with Sn plating as well as a 3-wire volt-age I/F and increased supply/output capaci-tance of 220/1.8nF for higher EMC robustness. The Xensiv TLE5501 analog angle sensor from Infineon is based on TMR (tunneling magne-toresistive) technology. It features high detec-tion sensitivity with a high output voltage, eliminating the need for an internal amplifier and allowing the sensor to be connected di-rectly to the microcontroller. In addition, TMR technology features minimal temperature drift, which reduces the need for external cal-ibration and compensation. The TLE5501 uses 360° angle measurement to detect the orien-tation of a magnetic field by measuring the sine and cosine angle components using TMR elements. It outputs the raw signals as a dif-ferential output signal. Due to the high bridge output voltage, additional signal amplification is not necessary. The TLE5501 is available in AEC-Q100 and automotive ASIL versions and is suitable for angular position sensing, steer-ing angle sensing, safety applications, and BLDC motor commutation.

The third dimension

3D Hall sensor technology combines lateral and vertical Hall sensors and as a result, can detect the strength of the magnetic field in all three dimensions. This type of sensor makes it possible to detect the absolute rotary or lin-ear position of each moving magnet. The MLX92256, for example, is equipped with lat-eral sensing. Designed specifically for use in window lift systems, it integrates a voltage regulator, two Hall sensors - one with IMC (integrated magnetic concentrator) and both with an advanced offset cancellation system - and two open-drain output drivers all in a single package. It is available in two versions. The MLX92256LSE-AAA-000 toggles the pulse signal when there is a change in the lateral or vertical component, while the direction pin only changes in the event of a change in di-rection. The MLX92256LSE-ABA-000 is equipped with two speed outputs, one for the perpendicular field and one for the lateral field. The MLX90371/MLX90372 Triaxis position sensors from Melexis are now available in the third generation. They combine a magnetic Triaxis Hall front-end, an analog/digital signal conditioner, a DSP (digital signal processor) for signal processing, and an output stage driver. They are immune to stray fields up to 4 kA/m (or 5 mT) as they occur as a result of the increasing electrification of vehicles, es-pecially electric and hybrid vehicles. Since they can also work with a weak magnetic field, smaller and cheaper magnets are suffi-cient. This not only results in cost benefits, but also space savings. The MLX90371 is an ASIL-B SEooC (safety element out of context) to ISO 26262 and offers an analog or PWM out-put. The MLX90372 is an ASIL-C SEooC to ISO 26262 and has a SENT or PWM output. Both meet the EMC requirements of automotive OEMs and are specified for operating tem-peratures up to 160°C. For applications with particularly stringent safety requirements such as accelerator pedal position sensing, the MLX90372 is also available in a "dual die" (fully redundant) TSSOP-16 package for re-dundant sensing. TDK-Micronas offers a programmable 3D sen-sor family for position sensing. It consists of three members: the HAL 3900 with an SPI in-terface, the HAL 3930 with a PWM/SENT in-terface and switch output (configurable as a high/low-side switch), and the HAL 3980 with a PSI5 interface. The sensors can suppress ex-ternal magnetic stray fields by using an array of Hall plates. Only a simple two-pole magnet is required to measure a rotation angle, ide-ally placed over the sensitive area in an end-of-shaft configuration. ́The sensors can also be used for off-axis measurements. They can measure an angular range of 360°, linear movements, and a 3D position, making them ideal for steering angle position sensing. De-pending on the device, it is possible to trans-mit temperature-compensated values of BX, BY, BZ, or up to two calculated angles. The HAL-39xy sensors operate in an ambient tem-perature range from -40°C to +160°C; they are ASIL B-ready and designed as SEooC ac-cording to ISO 26262 for automotive applica-tions.

Rugged and reliable

The examples show that not only have tre-mendous advancements been made in sensor technology to meet the increasing demands in cars, but that their design has also evolved. Many of the latest models also meet the more stringent guidelines of ISO 26262:2018, some as ASIL-C SEooCs. Redundancy and safety functions contribute to this as well as mea-sures to improve EMC. Corresponding pack-ages ensure the sensors are resistant to mois-ture, dust, and dirt. For use in high-temperature environments, Hall sensors are specified for operating temperatures of up to 170°C. As a result of these features, they play a key role on the road to autonomous driving.

 

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