How autonomous vehicles turn safely

Conventional sensors are sensitive to magnetic field interference found in electric vehicles (EVs), mainly caused by electrification systems. This article introduces a 3D position Hall-effect sensor that is immune to this interference.

Electric power steering development

A car’s steering system is a mechanism that changes the direction of the wheels based on the angle at which the steering wheel is rotated. Power steering systems assist the driver by enabling the tyres to be turned with a light touch to the steering wheel. In the past, these power steering systems were driven hydraulically but have now been replaced by electric power steering (EPS) systems, eliminating the need for a hydraulic pump, hoses, and a drive belt to the internal combustion engine (ICE). They also offer the cost advantages of increased fuel efficiency.

A typical EPS system uses electric motors, sensors, and an electronic controller to provide the steering assist. The motor drives a gear that is either connected to the steering column or rack, and the sensors measure the steering torque (steering effort) and the steering wheel’s speed of rotation and angular position. The electronic control module interprets these torque, speed, and position inputs, along with the vehicle’s speed and other chassis control inputs, to turn the tyres accordingly.

Since EPS systems can be controlled electronically, they can steer vehicles without driver input and are already used for parking assistance systems that feature automated steering and electronic stability control (ESC) systems. Also, EPS systems support the migration from ICE to battery-driven vehicles. However, the steering angle sensors used within these EPS systems must be highly accurate to implement autonomous driving effectively and safely.

Figure 1: Applications of HAL 39xy include position detection at various points throughout the vehicle, such as pedals, valves and gear changers, in addition to the steering angle.

Steering sensor advancements

Generally, Hall-effect sensors are used for steering angle detection by measuring the strengths or changes in magnetic fields. However, as vehicles have become more computerised and electrified, the stray magnetic fields generated by the e-motors or power lines in hybrid or electric vehicles interfere with the sensor’s function. Mitigating this interference, therefore, has become a rising concern.

TDK has overcome this challenge by introducing the HAL 39xy sensor that implements stray-field compensation functions. At the heart of the sensor is TDK’s patented 3D HAL pixel cell technology that helps measure magnetic fields accurately while also being insensitive to stray fields.

The compact 4-in-1 sensor chip features measurement modes for linear, two rotational positions of up to 180- and 360-degrees and 3D magnetic field measurements in the x-, y-, and z-axes. The sensor uses a different combination of the array of Hall plates within the chip to optimise performance in each mode. Also, as the sensor integrates all four modes in a single device, it offers a clear benefit to design engineers as they only have to qualify one part instead of four.

The low power consumption part supports various digital interface standards for in-vehicle networks, including SENT/PSI5 and Short PWM Code (SPC). SPC is an enhancement of the standard SENT protocol, according to SAE J2716. It enables data to be transmitted based on a trigger pulse sent by an external ECU and supports point-to-point connections as well as a single-wire bus mode with up to four sensors.

The HAL 39xy sensor is an ASIL B ready SEooC (Safety Element out of Context) part, developed according to ISO 26262 to support functional-safety applications. The sensor has already been adopted in steering angle detection systems for autonomous vehicles.

Further sensor applications

Additionally, solutions such as steering angle detection in steer-by-wire systems are currently under development for practical use and are a natural fit with advanced autonomous driving. Steer-by-wire removes the mechanical connection between the driver-controlled steering wheel and the turning of the tyres, relying instead on electric signals to achieve steering. Furthermore, the HAL 39xy can contribute to even more by-wire systems, ranging from brake-by-wire to shift-by-wire (changing gear).

The 3D sensors are also ideal for a wide range of other applications, including all kinds of valves and actuators, selectors and gear shifters, pedal-position detection, position detection in transmission systems, or chassis-position detection.