TDK Front Line

Vol.7 3D Hall Sensor Solutions in Automotive Electronics

With current developments in automotive electronics, the demand for automotive sensors is growing rapidly. In powertrain systems, bodies, and chassis in particular, there is an increasing need for high-precision and small-sized position sensors for pedals, valves, and gearshifts. In addition, the expanding hybrid electric vehicle / electric vehicle (HEV/EV) market and trends toward improved fuel efficiency, reduced emissions, and downsizings require position sensors that are highly functional and flexible, low power consumption, and be capable of 2D/3D detection. They also need to be compliant with ISO 26262 (functional safety) and other standards. To meet these latest market needs, TDK-Micronas, a TDK group company and a global Hall sensor manufacturer, has developed a new family of 3D Hall sensors: HAL 39xy.

Principles and features of Hall sensors

Hall sensors, a type of magnetic sensor, are widely used in a variety of fields, with application in automobiles, industrial equipment, household appliances, and information and communications technology (ICT) devices. The principles and features of Hall sensors are described below.

When a magnetic field is applied perpendicular to a current-carrying conductor (e.g., metal, semiconductor), voltage is induced in the direction perpendicular to that of both the current and the magnetic field. This phenomenon was discovered by the American physicist Edwin H. Hall in 1879 and is called Hall effect.
Although the existence of electrons was not yet known at the time of its discovery, it was later described that the Lorentz force exerted on electrons that move through a magnetic field causes the electron path to curve, thereby producing a potential gradient within the conductor, observed as the Hall voltage. The Lorentz force represents an electromagnetic force applied in the direction of the thumb in the well-known Fleming's left-hand rule.

Principles and features of Hall sensors

Hall sensors are magnetic sensors that use the principle of the Hall effect in semiconductors. The primary materials comprising Hall elements include compound semiconductors (e.g., InSb, GaAs, InAs) and silicon. Each has its advantages and disadvantages. For example, InSb is highly sensitive but it has inferior thermal properties, whereas GaAs is the other way around. Compound semiconductor Hall elements exhibit output voltages of around ten to hundreds of millivolts, which are typically amplified for use. In this case, silicon-based integrated circuits (ICs) such as amplifier circuits are required, which makes the Hall sensor a two-chip configuration.

Silicon Hall sensors, however, are less sensitive than sensors based on compound semiconductors, but they have a major advantage in that the sensor elements, amplifier circuit, signal processing circuit, and memory can all be fit onto the same silicon wafer and built into a single chip through semiconductor manufacturing technology such as large-scale integration (LSI). Small in size, highly reliable, and price competitive, silicone Hall sensors are user friendly in that it allows for the adjustment of sensitivity and offset, as well as the compensation of thermal properties, making it a common choice for use for a variety of Hall ICs.

Types of TDK-Micronas Hall sensors

Based in the southwestern German city of Freiburg, TDK-Micronas is a pioneer in CMOS Hall sensors. The CMOS technology has become a mainstream in the LSI manufacturing process. It has cumulatively shipped more than 4 billion units, with automotive sensors mainly provided to, and well regarded by, automotive electronics manufacturers around the world. The types of Hall sensors TDK offers under the Micronas brand include the following:

• Hall switches
A Hall sensor is typically combined with a magnet. In a Hall switch application, when a magnet is close by, the sensor detects a magnetic field, and the output of the sensor alternates between high and low based on a predetermined threshold. According to the switching behavior, a Hall switch can be unipolar (i.e., unipolar detection), bipolar (i.e., bipolar detection), or latching (detecting magnetic pole shifts). Primary applications: Open/close switches, commutation of brushless DC motors, and detection of rotational position.

• 1D Hall sensors
1D Hall sensors are used to detect an angle of rotation up to 90 or 180°, for stroke sensing, and for current measurements.
Primary applications: Detection of the angle of rotation of pedals and steering-torque sensing in automobiles.

• 2D/3D Hall sensors
Although a 1D Hall sensor measures the magnetic flux density in terms of a scalar quantity, a 2D or 3D Hall sensor detects the density and direction of a magnetic flux in terms of a vector quantity, which is then signal processed inside the sensor to output high-precision angle and position information. These types of sensors are used for 360° rotation-angle detection and stroke sensing.
Primary applications: Detection of the angle of rotation of pedals, stroke position, and gearshift position in automobiles.

There are a wide variety of Hall sensors available, each of which is utilized at a suited place according to a purpose. This article provides an overview of HAL 39xy, a new family of 3D Hall sensors marketed under the trade mark master HAL®. The sensors are developed based on TDK-Micronas' proprietary 3D-HAL® technology, and mainly outlines its applications as an automotive sensor.

Growing demand for automotive 2D/3D position sensors

There is a growing demand for 2D/3D position sensors that are compatible with ISO 26262 (functional safety) and other standards and that are also highly functional and flexible, with the goal of manufacturing automobiles that run in a safer, more energy-saving and comfortable manner. HAL 39xy is a new family of 3D Hall sensors developed by TDK-Micronas in response to such market demand.

In principle, a Hall sensor basically detects a magnetic field perpendicular to the chip surface (i.e., the z-axis). For a position sensor to make a 3D measurement of the density and direction of the magnetic flux, it needs to detect magnetic fields that are parallel to the chip surface (i.e., the x- and y-axes). TDK-Micronas made it possible to make triaxial (3D) magnetic field measurements of the x-, y-, and z-axes using an array of Hall plates comprised of multiple Hall elements.
In addition to 3D magnetic field measurements, HAL 39xy sensors are capable of four types of measurements, including stroke-position sensing and 360° rotation-angle detection based on different combinations of Hall elements. A single device that is capable of making four types of measurement with a superior level of precision brings major advantages to designers.

A Hall-plate array for 3D magnetic field detection. Three A/D converters, a DSP, micro-converter, and memory are all placed on a single chip.

Perfect for X-By-Wire application

In recent years, automotive control systems have been seeing the introduction of "by-wire" technology (shift-by-wire, brake-by-wire, steer-by-wire), which replaces conventional mechanical control with electronic control. In a throttle-by-wire system, for instance, a position sensor detects how far the gas pedal is pressed while the electronic control unit (ECU) sends an electrical signal to the actuator to control the opening and closing of the throttle valve.
Shift-by-wire is a system in which a position sensor detects the position of the gearshift and sends an electrical signal to control the transmission system. The introduction of shift-by-wire has made mechanical parts linked to the gearshift unnecessary, and it has made the gearshift itself as small as a joystick. However, a smaller gearshift means a smaller angle of operation, which calls for a position sensor with higher precision. TDK-Micronas' HAL 39xy family of 3D Hall sensors is capable of high-precision measurement, even of displacement caused by the gearshift's small angle of operation.

In response to the gearshift maneuver, the magnets move back and forth
and from side to side. The resulting changes to the magnetic field
(i.e., magnitude and direction) are detected three-dimensionally,
and electrical signals are sent to the ECU.
Even a small angle of operation can be measured three-dimensionally
with high precision, making it possible to reduce the size of the shift system.

Loaded with stray-field compensation function

Apart from 3D magnetic field measurements, another major feature of the HAL 39xy family of 3D Hall sensors is the installation of a function to compensate for stray fields.

With automobiles becoming increasingly more computerized, automotive electronic devices generate a host of magnetic fields. With HEVs and EVs, in particular, a large current passes through the traction motor, making magnetic fields generated by the wires prone to interfere with the magnetic field of the magnets used in a Hall sensor.
To ensure that an automobile runs safely, a Hall sensor must be able to suppress any interference from these stray fields. In the past, this was done by using magnetic shielding or a magnet with a strong magnetic force, or by switching from a 1D to a 2D Hall sensor. The HAL 39xy family of 3D Hall sensors, on the other hand, has overcome this technical challenge by rendering Hall sensors themselves capable of compensating for stray fields. This makes it possible to do away with magnetic shield members and to use inexpensive magnets, resulting in a large cost advantage.

TDK-Micronas is improving and expanding the HAL 39xy Line-up of 3D Hall sensors equipped with various digital output interfaces while simultaneously developing other types of sensors such as redundant dual-die sensors and built-in capacitor types.

Primary features of HAL 39xy family of 3D Hall-effect position sensors

• Programmable 3D Hall sensors with stray-field compensation
• The register selects from pre-determined multiple combinations of Hall elements, making it possible to perform the following four types of measurements:
・Linear position detection with stray-field compensation
・360° rotation-angle detection with stray-field compensation
・180° angle detection with stray-field compensation, including gradient in the magnetic field amplitude
・3D magnetic field measurements using the x-, y-, and z-axes