Hall sensor targets safety-critical automotive systems

A Hall-effect sensor varies its output voltage in response to a magnetic field. Hall-effect devices are used as proximity sensors and for positioning, speed, and current detection. A Hall-effect sensor is a long-lasting solution because there are no mechanical parts to wear down over time.

Melexis has announced the MLX91377 ASIL-ready Linear Hall sensor IC. The device is intended for use in safety-critical automotive systems such as electric power-assisted steering (EPAS). Developed as a Safety Element Out of Context (SEooC), the MLX91377 complies with the ISO 26262 standard and is qualified to AEC Q-100 Grade 0.

Hall Effect Sensors

The Hall effect is the measurable voltage through a conductor (or semiconductor) when an electric current flowing through it is influenced by a magnetic field. In these conditions, a transverse voltage is generated perpendicularly to the applied current because of the balancing of the Lorentz (electromagnetic) and electric forces.

The design of any Hall-effect detection device requires a magnetic system capable of responding to the physical parameter detected through an electronic input interface. The Hall-effect sensor detects the magnetic field and produces an analog or digital signal suitably converted into a standard according to the requirements of the electronic system.

As this allows them to operate without the need for contact, Hall sensors, therefore, find a wide range of applications: they are used, for example as proximity, positioning, and speed sensors.

In their simplest form, Hall sensors operate as analog transducers, which return a voltage; in a known magnetic field, it is, therefore, possible to measure the distance to the Hall plate. It is also common to find Hall sensors combined with circuits that allow the device to act digitally – on/off – and thus as a switch.  Another typical application of Hall sensors is to measure the speed of shafts and wheels, such as in speedometers, combustion engine ignition systems, or anti-lock braking systems.

Automotive applications face a wide variety of operating conditions ranging from very cold (-40 degC) to very hot (160 degC). In addition, they are subjected to high vibration and potential contamination by dirt, dust, liquids, etc. Even in these conditions they are required to operate without failure for many years. A hall sensor needs to perform well even across this broad range.

Melexis Solution

With an ambient operating temperature up to 160°C and combining high linearity with excellent thermal stability, including low offset and sensitivity drift, the MLX91377 supports accurate, dependable torque sensing in EPAS systems to enable safe control in conventional and autonomous driving.

The MLX91377 satisfies a wide variety of automotive and industrial contactless position-sensing use cases including steering torque sensors, acceleration, brake, or clutch pedal sensors, absolute linear position sensors, float-level sensors, non-contacting potentiometers, small-angle position sensors and small stroke position sensors.

“The MLX91377 brings improved performance across the board to enable highly demanding safety critical applications like automotive torque sensing. Currently there is no development board available however the MLX91377 is supported by the standard Melexis programming tool the PTC-04,” said Nick Czarnecki, Global Marketing Manager, Position and Speed Sensors, at Melexis.

Figure 1: MLX91377 block diagram

Programmable measurement range and multi-point calibration improve flexibility for designers, and the variety of output protocols allows a single integrated circuit to be used in multiple applications, reducing requalification efforts and costs. The Short PWM Code (SPC) protocol allows measurements to be made and transmitted after a trigger pulse is detected. This allows up to four MLX91377 sensors up to 2 kHz to be synchronized, allowing multiple simultaneous measurements of magnetic parameters with deterministic latency to ensure high accuracy (Figure 1).

“Depending on the output type the MLX91377 could be triggered by the protocol or be triggered internally. The MLX91377 offers both methods. When using the SPC protocol the MLX91377 waits until a trigger pulse is received by the controlling microcontroller, “ said  Nick Czarnecki.

He continued, “When the pulse is detected the sensor quickly acquires the magnetic data, digitizes it and compensates for offset and sensitivity errors, linearizes it according to a programmable lookup table, and then transmits it to the master in a digital format according to the SENT format. When operating in analog output mode the MLX91377 autonomously acquires the data, performs the same compensation as in SPC mode, and then outputs the value via an analog ratiometric voltage for the system microcontroller to read. Both interfaces are widely used in automotive with SPC being newer and generally preferred for multi-IC configurations. The triggerable nature of SPC allows for all sensors to acquire the magnetic field at the same time thus minimizing time delay between the measurements across ICs. The time to receive the response depends on the protocol configuration but is typically <500us and considerably lower for the analog output.”

Figure 2: SPC timing illustration in 1.5μs tick time mode and H.2 format

In SPC mode, the MLX91377 starts data acquisition once the trigger pulse has been received, regardless of the configured mode. It will send the acquired data in the same SENT frame. This feature is available for any tick time greater than or equal to 1.5μs (figure 2 and 3).

Figure 3: SPC standard master-slave configuration

The MLX91377 supports ASIL-C functional safety level in digital mode (SENT or SPC) and ASIL-B in analog mode, providing a high level of diagnostics at the die level and is able to detect internal faults and place itself in a safe state to prevent unwanted vehicle behavior.

The next challenges will increasingly see an operating environment with increasingly critical operating temperatures. Likewise, immunity to leakage fields, with increasing vehicle electrification, will become increasingly widespread, as will functional safety requirements.

>> This article was originally published on our sister site, Power Electronics News.

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