Nowadays, developers have access to an array of sensor solutions for detecting positions in electrohydraulic and electromechanical braking systems. Two of the most established ones are Hall and TMR-based sensors. Both offer specific technical advantages when it comes to their mounting flexibility, interference immunity and signal quality. Which principle is best for the job depends largely on the system requirements and the planned security architectures. The key properties of both technologies to facilitate targeted selection for demanding braking applications are compared below.
Hall, TMR – and their combination
Since Hall and TMR sensors are based on varying physical principles, their functionalities complement one another. Their combination opens up additional possibilities for reliably and precisely detecting positions in electrical brake systems.
- Hall sensors ensure accurate absolute position detection and sufficient dynamics for rotating measurement tasks, such as BLDC motors (approx. 70 kHz).
- TMR sensors deliver very high angular resolution and dynamic performance, especially for periodically moving or rotating components, such as those in BLDC and AC motors as well as actuators. Depending on their setup, TMR sensors provide mechanically absolute or electrically absolute positions. The latter are also referred to as incremental positions, i.e., 2-pole magnet for absolute or multi-pole magnet for incremental measurement.
- Combined solutions enable heterogeneous redundancy for safety-critical systems, thereby offering a solid foundation for applications like brake-by-wire or redundant steer-by-wire steering systems.
Table 1 provides an overview of the typical properties, areas of application and differences between the two types of sensors.
Table 1: Comparison of Hall and TMR sensors in terms of their application
| Criterion | Hall sensors | TMR sensors |
|---|---|---|
| Measuring principle | Detection of the absolute position (e.g., angle, linear movement) in a setup with multiple horizontal and vertical Hall plates On-axis (2-pole magnet) for dynamic absolute positions / mechanical domains (use of one HAL 302x sensor with up to six Z-sensitive Hall plates) | On-axis (2-pole magnet) for absolute positions / mechanical domains (use of saturated XY-TMR or linear TMR) Off-axis (multi-pole magnet) for incremental measurement / electrical domains, XY saturated or linear TMR |
| Typical applications | BLDC/AC motor rotor position (for HAL 302x), pedal position, encoder cylinders, limit stops, absolute reference points | BLDC/AC motor rotor position, actuators, dynamic movements |
| Type of mounting | On-axis layouts, also available as linear and 3D variants | Preferably radial measurement with multi-pole magnet rings |
| Magnetic stray field behavior | Compensation through differential structures (e.g., 6ZD) possible | Compensation through the application of stronger magnets or magnetic differential linear TMR sensors |
| Sensitivity | Low-level, integrated amplifiers amplify the sine/cosine signals to 4 Vpp | Very high, enables a direct µC connection without amplification (3 Vpp); linear TMR sensors require an external amplifier |
| Type of signal | Sine/cosine signals or output of the calculated angle | Sine/cosine signals or output of the calculated angle |
| Signal processing | Integrated stray field compensation reduces external effort; dynamic error compensation required for very high accuracy of < 0.1° | Direct application of sine/cosine outputs without the need for amplification. For stray field compensated systems, an amplifier that enables intrinsic stray field compensation is recommended. |
| Strengths | Absolute position, high dynamics, very robust against mechanical tolerances, compact on-axis integration, suitable for remote and on-board applications | High resolution, extensive dynamic range, robust off-axis measurement, high signal stability throughout the life cycle |
| Weaknesses | Radial mounting demands precise magnet guidance | Does not provide an absolute position without additional reference for off-axis integration |
A combination of both technologies is usually advisable in safety-critical systems. Hall and TMR sensors basically measure the same magnetic signal but rely on differing physical principles. This level of diversity increases system robustness and enables heterogeneous redundancy in accordance with functional safety (e.g., ASIL D). If a homogeneous component, such as a Hall sensor system, should fail completely, the TMR sensor system remains fully functional, thus ensuring that ASIL D can still be achieved even in common cause emergency operation. Since technology-related failure mechanisms vary, they can be monitored in a highly differentiated way, providing extensive diagnostic coverage. This supports fail-operational concepts in applications, thereby enabling very high system availability. Developers need to check which sensor configuration best suits the requirements for accuracy, immunity and safety.
Position detection in electrohydraulic brake actuators
The various integration options for Hall and TMR sensors and their respective properties are shown using the example of electrohydraulic brake actuators. The implementation variants differ significantly in terms of accuracy, stray field tolerance and system setup:
- On-axis integration with Hall sensors (Figure 1):
Hall sensors like the HAL 302x series are installed directly in line with the axis. They are compact, resistant to stray fields and enable PCB-free and redundant designs with single-sided PCB assembly. Their absolute accuracy in terms of temperature and lifespan is approximately 0.5 to 0.6° in real-world practical conditions, which is sufficient for most brake and actuator applications. For higher accuracy requirements, an external microcontroller can perform dynamic error compensation and achieve accuracy levels of up to 0.1°. - On-axis integration with TMR sensors (Figure 2):
TMR sensors (e.g., TAS224x or TAS214x) achieve an accuracy of 0.3° in a stack configuration and even up to 0.1° with dynamic compensation. The direct connection to the microcontroller without any external amplification simplifies the circuit architecture and enables compact designs. That said, stray field compensation is usually required via software or at least a second TMR sensor. When using two linear TMR sensors, stray field-robust position determination can occur like in Figure 1; in this case, external amplifiers are required. As such, single-sided PCB assembly is possible. - Off axis integration with TMR sensors (Figure 3):
TMR sensors detect the magnetic period in a radial arrangement with multi-pole magnetic rings. This architecture offers exceptional stray field tolerance already at hardware level and is particularly suitable for dynamic applications, such as BLDC motors, where there is no space at the end of the shaft. The short signal processing latency and high modularity of the sensor modules help ease implementation noticeably. However, it must be noted that they do not provide absolute positions but rather relative positions with an accuracy of 0.5 to 1°. This is often sufficient for electric motor commutation and actuator control.
Technology meets application requirement
The choice of sensor system depends not only on its technological features but, above all, on the specific application: What position needs to be detected? What requirements apply to accuracy, dynamics, EMC and functional safety?
On-axis Hall sensors offer robust, simple integration and good stray field tolerance. On-axis TMR sensors provide excellent accuracy but demand compensation against magnetic field interference. Off-axis TMR architectures are inherently insensitive to interference effects and are ideal for dynamic applications, such as the “electrical position” of motors.
The best solution depends on the respective safety, accuracy and stability demands. The clear trend shows: The targeted combination of both technologies unites their strengths and ensures a balanced combination of performance, safety and efficiency.
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