EMC/EMI & magnetic sensing in modern automotive applications
01 October 2019
Efficiency is the ‘watchword’ in automotive circles these days, driven by environmentally aware consumers, legislation & rising fuel costs. The industry is looking to technology to deliver the required advances, whether this is in the development of the latest hybrid or electric vehicles (HEVs/EVs), or finding ways to make internal combustion engine (ICE) vehicles more efficient.
This article was originally featured in the October 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
As Nick Czarnecki, Marketing Manager – Position & Speed Sensors at micro-electronic semiconductor & automotive sensor specialist, Melexis observes, while many things are uncertain in the automotive world, it’s a sure bet that such technologies are here to stay.
One significant area of change is that many in-vehicle applications are becoming electrical or electronic where, in the past, they would have been mechanical or hydraulic. Included in this list are electric throttle bodies, electric fluid pumps, electric turbochargers, and, especially, the electrical drivetrain in battery-powered EVs and HEVs. This move to electric is not only making vehicles more efficient, it is also making them more reliable – and lighter. As the vehicles become lighter, so efficiency is boosted even further.
However, all of these systems contain moving parts, which require accurate and reliable sensing of the position of those moving parts, so that the control units can manage the system. Given the desire to wring every ounce of efficiency out of the vehicle, high levels of accuracy are required.
Position sensing of moving parts is not new, and resistive or optical sensors have been used to measure both linear and rotary position for many years. While both these methods can work well, they are susceptible to dust, dirt and vibration, which is often present in vehicles, and being mechanical devices themselves, they are prone to wear. Meanwhile, magnetic sensing is coming to the fore: this highly accurate technology does not wear and is not impacted by environmental effects such as dirt, dust and vibration.
While magnetic sensing represents a significant step forward in terms of reliability, it is typically susceptible to stray magnetic fields, which presents vehicle designers with a challenge – especially as electronic content in vehicles proliferates.
Electric motors that drive EVs and HEVs require significant amounts of electric current, and therefore produce strong magnetic fields around the cables delivering the electric current from the battery or alternator to the motor. Also relevant are the lower currents necessary to drive electronic power steering (EPS) pumps, open or close windows or sunroofs, and operate other electrically actuated devices in the vehicle, as these too will create magnetic fields.
Any stray magnetic fields near a magnetic sensor will affect the accuracy of the sensors, and they can potentially cause significant output errors – even catastrophic results.
A sunroof that fails to close correctly is annoying. However, a brake pedal, accelerator pedal or steering system that is sensed inaccurately has the potential to be life threatening to vehicleoccupants and other road users or pedestrians in the vicinity.
As the automotive industry takes safety very seriously, it has developed standards to determine the impact of stray magnetic fields. Important standards include ISO81452-8 that covers testing for magnetic field immunity. Additionally ISO26262 defines processes and procedures to ensure the IC itself performs correctly and safety during normal operation, and when a failure occurs.
They are designed to measure the magnetic field produced by a nearby magnet that is connected to the item being measured, and traditional planar and vertical Hall effect and magnetoresistive (MR) sensors are sensitive to the stray fields that are found in vehicles. Since the stray magnetic fields created by electric currents (especially the huge currents for the main drive motors) can be large, traditional sensing cannot be relied upon without measures to mitigate stray fields. With rotary sensors, errors can be more than 10 degrees – significant as systems such as valves or throttle bodies have a maximum rotation of 90 degrees and pedals much less, with sometimes only 15 degrees of rotation. Given this, notwithstanding significant safety issues in steering and braking, the engine control unit (ECU) would not be able to effectively manage the engine functions and many other functions would be significantly impaired.
There are two options for design engineers that need (or want) to use magnetic sensing in modern vehicles. Firstly, the magnetic sensor and associated magnet can be shielded from stray fields. This requires the use of materials with high magnetic permeability, making it complex and expensive. As the shielding partially absorbs magnetic fields, as well as changing the path, it can also have a negative impact on the field produced by the magnet used to measure the position. This can be avoided, but physical spacing is required to achieve this – increasing size, weight and cost – all of which are to be avoided in modern vehicle design.
The other approach is to use a magnetic sensor that is intrinsically immune to stray magnetic fields. One such sensor is the MLX90372 Triaxis position processor from Melexis. This is a monolithic IC consisting of a Triaxis Hall magnetic front end, an analogue to digital signal conditioner, a digital signal processor (DSP) and an output stage driver.
The MLX90372 is sensitive to three components of the magnetic flux applied to the IC (Bx, By and Bz) so, with the correct magnetic circuit, the absolute position of any moving magnet can be detected – either rotary or linear. The sensing is completely non-contact and as such there is no wear mechanism; it is also unaffected by dirt, dust and liquids.
The device includes an in-built stray field immune mode that allows for a substantial reduction or elimination of any error caused by stray magnetic fields up to 4kA/m (or 5mT). As such, it can be used in close proximity to current carrying conductors or other magnets in the vehicle. The stray field immune mode requires only a simple 4-pole magnet for rotary motion and a simple 2-pole magnet for linear motion.
With almost trivial magnetic design, the error due to stray fields is reduced to below 0.4 degrees of angular error, which is an acceptable value for most major vehicle manufacturers. Additionally, any shielding that may have been necessary in the past can be eliminated or substantially reduced, resulting in significant system size, weight and cost savings.
The flexible MLX90372 offers a programmable measurement range, as well as a programmable linear transfer characteristic (based on 4 / 8 multi-points or 16 / 32 piece-wise linear points) that improves overall accuracy where necessary.
Communication with a host processor is achieved via SENT frames that are encoded according to a secure sensor format and these enhanced serial messages can include error codes and user-defined values. A pulse width modulated (PWM) output can be configured, if desired. The sister product, MLX90371, offers a ratiometric analogue voltage output.
The MLX90372 has high levels of EMC robustness and is available in a single die version as well as a fully redundant dual die version, for the most safety-critical applications. The single die version is housed in an SOIC-8 package, while the dual die version comes in a TSSOP-16 package. A single die DMP-4 package is also available for PCB-less installation in the tightest spaces, or for direct fitment into housings. This, along with its ruggedness and ability to meet ISO26262 ASIL-C with a single die, make it ideal for demanding modern automotive applications.
Technology is at the forefront of making modern automobiles more efficient and reliable, whether they are electrically powered or still rely on the internal combustion engine. As more and more parts of the vehicle become electric to support this goal, so sensing is an increasingly important function – yet, the strong stray magnetic fields in modern vehicles have presented challenges to designers who want to use magnetic position sensing.
Until recently, using traditional magnetic sensors has required shielding which is complex, cumbersome and expensive. Fortunately, the Gen III Triaxis devices from Melexis, and the MLX90371 and MLX90372 position processors in particular, are intrinsically insensitive to stray magnetic fields. These rugged devices are ideal for automotive applications and remove the need for shielding, making the design task much simpler, as well as reducing vehicle weight, size and cost – while also increasing efficiency and reliability.
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