Nick Czarnecki, Melexis
In the past few years, Electric Vehicles (EV) and Hybrid Electric Vehicles (HEV) have started to become a reality. Figures from the International Council on Clean Transportation (ICCT) confirm this; whereas in 2010 the worldwide EV market was negligible, by 2015 over 500,000 electric cars had been sold - and sales have continued to increase. Even with traditional internal combustion engine (ICE) vehicles, electrification is having an impact as many previously mechanical and hydraulic systems are being replaced with cleaner, lighter and more efficient and reliable electric versions.
As vehicles become more sophisticated, computer systems such as the engine control unit (ECU) control more of the functionality. However, in order to be able to work accurately they need to be able to sense key operating parameters of the vehicle. For many of the main electrical systems (such as the battery stack, motors and pumps), knowing the current flowing is required. The ECU also needs to know the position of mechanical devices such as the throttle body, the gear selection lever or the parking brake to be able to manage the vehicle correctly.
While many methods exist to measure current and position, using magnetism and the principles of the Hall Effect provides a reliable and simple contactless approach to this engineering challenge.
Current sensing technology
Conventional methods for sensing current are relatively cumbersome and often involve a current transformer where the circuit has to be broken so that the measuring transformer can be inserted. Even conventional Hall Effect sensing is not straightforward since traditional Hall Effect sensors measure current perpendicular to the device’s surface. This requires a gapped ferromagnetic core to provide magnetic gain and align the magnetic field so that the sensor can measure it.
However, the core also limits the performance of this approach. The selected geometry and material can impact the saturation and hysteresis as well as the frequency response and thermal drift of the solution.
Semiconductor-based IMC-Hall sensors can measure current directly due to the integrated magnetic concentrator (IMC). The IMC is made from an amorphous magnetic material featuring very high permeability and very low hysteresis.
It converts the external magnetic field parallel with the chip surface locally into a perpendicular field, which is sensed by conventional Hall elements. Combining the IMC with Hall sensors gives higher magnetic sensitivity when compared to conventional approaches.
Position sensing technology
Many traditional Hall Effect sensors are only sensitive to magnetic flux perpendicular to the IC. While these devices can be used for applications such as accelerator pedal position detection, gear selector detection and other proximity applications in an automotive environment, it can be necessary to develop complex (and therefore expensive) custom magnetic structures to achieve the desired measurement.
Triaxis sensors are an innovative magnetic technology that allows the measurement of three magnetic flux components in a single IC through the use of an IMC. Using the three magnetic components, two-dimensional or three-dimensional sensors that detect rotary (angle), linear (stroke) or joystick-type motion can be created.
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Figure 2. Ball and socket joystick
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Figure 3. Linear motion
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Figure 4. Through shaft
Magnetic sensing technology
The Melexis MLX91208 is a monolithic current sensor IC that employs automotive-qualified Triaxis Hall technology and, due to the integrated IMC, offers very high sensitivity. Housed in a tiny SOIC8 package, the MLX91208 can measure currents from a few Amperes to 1000 Amperes. The highly programmable and configurable device offers a short response time of 3μs and a bandwidth from DC to 250kHz. The linear transfer characteristic can be programmed and an analog ratiometric output can be selected. The device is suitable for many automotive current sensing applications and can form the heart of a smart fuse.
The MLX90371 is another monolithic sensor which provides absolute position sensing based upon Triaxis Hall technology. The device includes a magnetic front end, an analog to digital signal converter, a DSP for advanced signal processing and an output stage driver. The three-dimensional Triaxis technology facilitates non-contact sensing of the absolute rotary or linear position of any moving magnet with an analog or PWM output. A second version (the MLX90372) offers a SENT (SAE J2716) or PWM output.
The MLX90371 is ASIL-B capable and the MLX90372 is ASIL-C capable. Both devices are available in a PCB-less dual-mold package (DMP) which allows them to be directly integrated into small sensing assemblies without the need for a PCB.
One of the critical challenges in modern vehicle applications is the ability to accurately detect magnet positions with the high levels of stray magnetic fields that are present due to the high level of electrification, especially in EV and HEV. By measuring a magnetic field gradient, the MLX90371 and MLX90372 are intrinsically insensitive to stray fields up to 4kA/m (or 5mT), which meets the needs of most automotive applications. A further benefit is that this allows smaller and lower-cost magnets to be used, which makes for an elegant solution in space- and cost-constrained automotive applications.
The MLX90372 also includes an extra pin, which allows external sensors (such as temperature or pressure) to be integrated into the SENT bus, reducing system complexity.