Research & Development Causes and consequences of electromagnetic interference
The Hall sensors sketched here are immune to electromagnetic interference fields. Depending on the measuring mode, they determine linear positions and rotary movements via the X/Y component of the magnetic field detected.
A mid-range vehicle now has more than 20 control units that communicate with each other via bus systems. In the luxury class segment, there are quickly 80 control units. Just as many servo motors for seats, headlights, windows, air conditioning, exterior mirrors, seatbelt delivery systems, sunroofs, infotainment, and others can be wired.
This complexity of today's vehicles is always prone to malfunctions. The invisible enemy is called the electromagnetic interference field. In this inhospitable environment, hundreds of sensors for position, speed, acceleration, pressure, temperature, force, flow and much more must function reliably, depending on the vehicle type and equipment, because they permanently determine the actual condition of the vehicle and transmit their values to the control units by radio or as a current signal.
Safe sensor technology through active interference field compensation
Immune to magnetic interference fields are the sensors of the HAL-39xy family, which are described below in their mode of operation. They create a significant simplification in the development of magnetic systems. The HAL-39xy sensors detect linear and rotary movements (up to 360°) and measure vertical and horizontal components of the magnetic field.
First of all, it should be noted that magnetic interference fields are distinguished between homogeneous and inhomogeneous interference fields (gradient fields). Homogeneous interference fields exist if their field lines point in the same direction and have the same distance from each other. Consequently, the magnetic flux density is spatially the same over a specific range. Helmholtz coils are used to generate homogeneous interference fields. These coils are also used for robustness tests (Fig. 2). In contrast, a current-carrying conductor (such as power supply lines) creates an inhomogeneous interference field. The smaller the distance to the conductor, the stronger the effect of the interference field. In addition, inhomogeneous interference fields can also be caused by adjacent sensor systems that use a permanent magnet as an encoder.
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Currently, a Hall effect sensor according to the latest ISO standard 11452-8 [ISO 11452-8, Helmholtz Coil Test Level 4] and corresponding OEM requirements must have a high immunity to these interference fields. In the low-frequency range, this strength is 4000 A/m (≈5 mT). In the previous version of ISO 11452-8, only interference fields in the 1.2 mT range were used as interference variables. With such weak interference fields, it was sufficient to adapt the strength of the magnet used for the application to the interference field or to implement a complex shielding. For cost reasons, smaller magnets are used today and shielding is not required, so that active interference field compensation is indispensable for magnetic field sensors.
The HAL 39xy Hall Effect position sensors discussed here guarantee high-precision measurement of magnetic fields and are at the same time insensitive to magnetic interference fields. For this purpose, the chip integrates interference field compensation as an array of vertical and horizontal Hall plates. The special feature of the sensor is the patented 3D-HAL pixel cell. This highly flexible sensor array allows development engineers to select the most suitable concept for interference field compensation for the respective measurement task.
The sensors offer the following four measurement modes:
- Linear position sensing with interference field compensation according to ISO-11452-8 requirements
- 360° rotation angle detection with interference field compensation in accordance with ISO-11452-8 requirements
- 1180° rotation angle detection with interference field compensation according to ISO-11452-8 requirements, including gradient fields
- and full 3D magnetic field measurement (BX, BY, BZ) without interference field compensation.
Each measurement mode uses a different combination of Hall plates to achieve optimum performance. The Micronas HAL 39xy is the only sensor on the market that combines all four measurement modes in the same device. This feature gives designers the advantage of qualifying only one compact sensor instead of multiple hardware versions. In addition, the suppression of homogeneous interference fields according to ISO 11452-8 does not require a complex magnet shape, but only a two-pole magnet for linear position detection and 360° angle of rotation detection.
A four-pole magnet is used for a 180° angle of rotation detection, which enables additional suppression of gradient fields. The integrated digital signal processor of the sensor calculates the interference fields and then determines the angle. The angle error caused by the interference fields is thus reduced to less than 0.1°. Due to their flexible architecture, the HAL-39xy sensors offer a wide range of configuration options. In addition to the powerful DSP, they also integrate an embedded microcontroller that is responsible for time sequences, interface configuration and monitoring of functional safety.
Customer-specific hardware can be developed for both blocks. In combination with the flexible Hall front end, new applications are possible, for example, that include application-specific signal processing or support new interface standards. The architecture of the HAL 39xy makes it easier to develop new solutions using rapid prototyping. Furthermore, a fast and effortless adaptation to changed interface standards is possible, such as SENT or PSI5.
Example of electrohydraulic brake systems
As the requirements for interference field compensation become stricter, so do the requirements for the safety of electronic vehicle systems. For example, strict safety requirements according to ISO 26262 apply to autonomous driving so that the sensor continues to operate in a safe state even in the event of a fault. The new Micronas sensor family is designed for applications with ASIL certification so that HAL-39xy sensors can be used for a wide range of applications in automotive electronics. This also includes brake pedal position detection.
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As a result of increasing electrification and driving automation, passenger car braking systems are also changing. On the one hand, regenerative braking (brake recuperation) is intended to recover energy and thereby increase efficiency. On the other hand, it should be possible to intervene in the brakes independently of the driver. These requirements can be met by electrohydraulic braking systems. Instead of the usual vacuum brake booster, an electric motor drives a hydraulic piston via a transmission. This combines the function of the brake booster when the pedal is actuated with that of the slip control system for active pressure generation.
The brake pedal must be monitored by means of position, pressure and speed measurement to ensure that the driver's braking request is recognized. For this purpose, the brake pedal is decoupled from the pressure generator. The angle information is transmitted electronically to the integrated brake control system. This is used to calculate the desired braking deceleration and the brake pressure is generated by the electric motor. The brake control system ensures that in hybrid and electric vehicles the transition between the regenerative brake and mechanical friction brake is not perceptible to the driver.
The position detection in the brake pedal must meet the highest safety requirements according to the ASIL level and function correctly even under the influence of interference fields at all times. Especially in the new generation of braking systems, the magnetic interference fields generated by the electric currents of the electric motor must be compensated so that position detection is not impaired. The angle of rotation of brake pedals is in a range of only 15° to 20°; the angle error resulting from interference fields in conventional magnetic sensors can be up to 5°. As a result, the brake control system could then no longer be able to process the position information correctly.
For this reason, it is extremely important that the sensor itself is insensitive to magnetic interference fields due to active interference field compensation. For this reason, the HAL 39xy can be installed in the immediate vicinity of live conductors. The sensor has a second signal output, configurable as a high or low-side switch, which can be used as a switching output to switch on the brake light and deactivate the speed control system when the brake pedal is pressed. Its switching point is derived from the calculated position information and enables a switching point along the entire measuring range.
In addition to this application, the sensors are also suitable for all types of valves and actuators such as coolant valves, exhaust gas recirculation (EGR) and turbocharger actuators, selectors and shift levers, position detection in transmission systems, steering angle detection, and chassis position detection.
This article was first published in German by Elektronikpraxis.
* Frederik Berstecher is Product Marketing Manager for 3D position sensors at TDK-Micronas, Freiburg.