The automotive industry is moving toward partial and fully autonomous driving, aligning with the trend toward vehicle electrification. In this evolution, systems like Electric Power Steering (EPS) and Electronic Braking Systems must meet stringent safety standards to ensure the safety and reliable control of driverless vehicles. This article introduces Anisotropic Magnetoresistance (AMR) position sensors and shunt-based current sense amplifiers, which enable high-performance commutation and safe operation of brushless motors in EPS and electronic braking systems. We will also discuss related solutions provided by Analog Devices, Inc. (ADI).
Meeting ASIL D standards in automotive electronic control units
In recent years, as vehicle safety has become a top priority, active Advanced Driver Assistance Systems (ADAS) have evolved and gained widespread adoption. ADAS complements traditional passive safety systems, like airbags, by actively assisting drivers in making safe decisions in critical situations. In the long term, these systems aim to replace driver decision-making entirely, leading the automotive industry towards semi- and fully autonomous driving.
By enabling Electronic Control Units (ECUs) to make driving decisions, actuators can manage vehicle steering and braking operations, effectively transferring the task of vehicle control to the sensors, the ECUs, and the electronic actuators. However, given the potential impact on human lives, these solutions must comply with the ISO 26262 functional safety standard. This risk-based safety standard assesses the qualitative risks associated with hazardous operations and incorporates safety measures into component and system designs to prevent or manage system malfunctions and detect or mitigate random hardware failures.
These actuator systems, which are commonly driven by brushless direct current (BLDC) motors, are critical to vehicle safety. As a result, designers must ensure that both hardware and software solutions meet the high standards of Automotive Safety Integrity Level (ASIL) D, the highest ASIL rating for automotive safety.
Motor position sensors ensure the safety of BLDC motor-driven systems
Actuators in automotive control systems are often powered by brushless direct current (BLDC) motors. Unlike brushed motors, BLDC motors no brush contacts and instead rely on motor position sensors (MPS) to measure the relative position between the stator and rotor, ensuring the stator coils are energized in the correct sequence. The role of motor position sensors is especially crucial during startup, as there is no back electromotive force (EMF) available for the microcontroller to determine the relative positions of the rotor and stator.
Traditionally, three Hall switches are used to detect the rotor position for block commutation in BLDC motors. However, due to increasing demands for enhanced performance in BLDC motor drivers (including EPS systems) —particularly in reducing noise, vibration, and harshness (NVH) and improving operational efficiency—block commutation is gradually being replaced by sine commutation control. The Hall switches can be substituted by an MR angle sensor placed in front of a bipole magnet mounted at the end of the motor shaft. In typical applications, the MPS is mounted within the ECU assembly, which is integrated into the motor casing and positioned at the motor shaft end.
In a standard EPS (Electric Power Steering) system topology, the EPS ECU calculates the required assist power based on the steering torque applied by the driver, the position of the steering wheel, and the vehicle's speed. The EPS motor exerts force to turn the steering wheel, reducing the torque required from the driver.
The motor shaft position (MPS) angle, combined with phase current measurement information, is used for the commutation and control of the EPS motor drive. The required level of torque assist varies based on driving conditions, determined by wheel speed sensors and a torque sensor, which measures the torque applied by the driver or motor actuator in driverless cars. The microcontroller then uses the MPS data and phase current data to control the current loads supplied to the motor, providing the necessary assist.
A failure in the MPS sensor can result in or exacerbate system malfunctions, such as steering lock or self-steering, making the MPS a critical component in EPS systems. Therefore, it is essential for the system to have comprehensive diagnostic and redundancy capabilities to ensure continued safe operation if an MPS sensor error or failure occurs. This allows the system to either shut down safely or continue operating without severe malfunctions.
Current sense amplifiers are commonly used for precise, indirect measurement of motor load, typically in two of the motor’s three phases, providing additional diagnostic information as part of the overall system safety measures.
Furthermore, highly precision motor position and phase current measurements can enhance EPS motor control performance at the system level, enabling highly efficient, quiet, and smooth steering feel. This improves the overall driving experience, making MPS a critical component in the system.
Functional safety of ASIL D compliant EPS motor control
In EPS (Electric Power Steering) or other safety-critical motor control applications, various methods can be used to achieve ASIL D compliance. For instance, integrating dual anisotropic magnetoresistive (AMR) motor position sensors with ADI’s current sense amplifiers within the system can provide the necessary performance level and redundancy, achieving ISO 26262 ASIL D compliance from a system-level perspective.
In EPS motor control, an additional sensor based on different technology (such as Hall, GMR, or TMR) can complement and enhance the dual AMR sensors. The dual AMR sensors serve as the primary (high precision) sensing channel, while the second channel, using the secondary alternate sensor technology, has three key functions. First, it enables a “two-out-of-three” (2oo3) comparison, allowing verification if one sensor channel fails when combined with other system inputs. Second, it provides positional feedback in the extremely rare event of a dual AMR channel failure. Lastly, in cases where the motor has an uneven number of poles, it provides 360˚ quadrature information for motor commutation to the microcontroller.
The precision angle measurement continues to be provided by the two channels of the dual AMR sensor. Additional system diagnostics, such as motor load and shaft position, can be indirectly inferred from the dynamic state (back EMF) of the precise phase current sense amplifier.
Among all potential sensor failure modes, there should always be two position sensor inputs available for a plausibility check. Even in the rare and extreme scenario where both AMR channels fail due to a common cause, degraded positional detection information from the auxiliary sensor channel, combined with the back EMF information from the current sensors in the dynamic state, can be cross check to ensure the basic functions of the system continue to operate safely.
Dual AMR sensors for enhanced automotive safety
With the introduction of ADAS aimed at enhancing vehicle safety and the emergence of fully and semi-autonomous vehicles, there is a growing demand for more reliable, intelligent, and high-performance redundant electronic actuator solutions that comply with ISO 26262 functional safety standards. ADI’s motor shaft position and phase current sensing products meet the high ASIL requirements for critical safety applications, such as EPS and braking systems, by providing the necessary redundancy and improved performance for smoother, more efficient motor control.
ADI's ADA4571-2 dual AMR sensor is designed specifically for such safety-critical applications requiring redundant and independent sensing channels. This dual-channel AMR sensor integrates signal conditioning amplifiers and ADC drivers. It includes two AMR (Sensitec AA745) sensors and two amplifier signal conditioning ASICs, providing very low angular error signals, typically within 0.1 degrees, with negligible hysteresis, high bandwidth, low latency, and excellent linearity. These features help reduce torque ripple and audible noise, allowing for smooth and efficient BLDC motor control. The AMR sensor operates in saturation above 30 mT without an upper magnetic field window limitation, and it functions well in high magnetic field conditions, making it resilient to stray magnetic fields in harsh environments.
The ADA4571-2 generates an analog output indicating the angular position of the surrounding magnetic field, with each channel integrating two dies within one package: an AMR sensor and a variable-gain instrumentation amplifier. The ADA4571-2 provides clean and amplified cosine and sine output signals for each channel, correlated with the rotation angle of the magnetic field. The output voltage range is ratio metric to the supply voltage. Each sensing channel contains two separate Wheatstone bridges at a relative 45° angle to each other. A rotating magnetic field parallel to the IC package plane provides two sinusoidal output signals, with the angle α of the sensor to the magnetic field direction doubled in frequency. In a homogeneous field parallel to the IC package plane, the output signals are independent of the airgap between the sensor and magnet. The ADA4571-2 comes in a 16-lead SOIC package.
Bidirectional current sense amplifier
On the other hand, ADI’s AD8410 current sense amplifier enables bidirectional current measurement across the shunt resistor in EPS and other BLDC motor control systems. This is a high-voltage, high-resolution, and high-bandwidth current-shunt amplifier, designed to deliver the required precision measurements in harsh environments and to provide diagnostics for safety-critical applications. It helps reduce torque ripple and audible noise, ensuring smooth, efficient BLDC motor control (such as in EPS or braking systems), thus enhancing the overall driving experience.
The AD8410 is a high-voltage, high-resolution, high-bandwidth current shunt amplifier with an initial gain of 20 V/V and a 2.0 MHz bandwidth, with a maximum gain error of ±0.3% over the entire temperature range. The buffered output voltage can connect directly to any typical converter. The AD8410 exhibits excellent input common-mode rejection performance with an input common-mode voltage range of −2 V to +70 V, allowing it to measure bidirectional current across the shunt resistor. It is suitable for a range of automotive and industrial applications, including motor control, power management, and solenoid control.
The AD8410 provides outstanding performance across a wide temperature range of −40°C to +150°C. It uses a package trim core, maintaining a typical offset drift of 1.0 µV/°C across its operating temperature and common-mode voltage range. The AD8410 is certified for automotive applications and integrates patented circuitry, ensuring high output accuracy even under pulse-width modulation (PWM) type input common-mode voltage conditions. The typical input offset voltage is ±200 µV. The AD8410 is available in both 8-lead MSOP or SOIC packages.
Conclusion
AMR motor position sensors have become an indispensable key technology in vehicle systems for automotive safety applications, thanks to their high precision, stability, and durability. These sensors not only provide accurate real-time position sensing to ensure precise control of power output and steering systems, but they also maintain excellent interference resistance in complex environments, thereby enhancing the safety performance of vehicles. The ADI AMR motor position sensors and current sense amplifiers introduced in this article will assist in achieving ISO 26262 ASIL D compliance for automotive control systems, making them an ideal solution for developing related applications.
