Magnetic position sensors are a favored component among automotive design engineers. Here's how to exploit the magnetic position sensor to meet the needs of one particular application: the electronic throttle body.
Sensor requirements in ETB designs
Position sensors in ETB applications differ from general purpose position sensors; they have additional features demanded in the safety-critical automotive environment.
The first requirement is redundancy. A dual (redundant) sensor is essential to the functional safety of an ETB throttle valve system. Figure 2 shows how redundancy can be provided for with a sensor tailored to throttle valve and pedal position sensing. This sensor can be made with either a single die or dual stacked dies; the dual-die variant is fully electrically isolated with a dielectric spacer between the dies.
In a stacked-die structure, the two dies measure almost exactly the same magnetic field values, so it's easy to compare them and detect malfunction in either die.
The AS5262 from ams has a dual-die structure that enables the provision of redundancy in a single package.
ETB applications also require precise analog output. The AS5262 from ams, for instance, provides a voltage output ranging from 10% to 90% of VDD over the 90° LMS-to-UMS span. Its 12-bit output is linearly proportional to the angle.
The dual-die version of the AS5262 produces a voltage output linearly proportional to the angle.
Also required are features supporting functional safety, including compliance with the ISO26262 standard, such as diagnostic features to alert sensor failure, and protection against over-voltage reverse polarity, permanent short circuits, and stray magnetic fields.
Defining the maximum allowable error
A vehicle manufacturer's ETB specifications always include an allowed INL (integral non-linearity) error band either side of the ideal curve; often the requirement is for INL of <1% of VDD. An EOL programming procedure is needed in defining the range of the voltage signal across the span of the valve's rotation. Some sensors, such as the AS5162, allow for programming that provides the user the opportunity to meet the target INL specified by the manufacturer.
Flexibility in choice of magnet
In ETB applications, there are important choices for the system developer to make in relation to the magnet.
Magnets with diametric magnetization (see Figure 4) cannot be put in direct contact with a ferromagnetic (iron) shaft, because the magnetic field would be weakened and distorted. This means a non-magnetic holder providing separation of at least 3 mm between magnet and shaft is required. Diametric magnets are typically made of SmCo (which has a very low temperature coefficient) or NeFeB. The nominal air gap distance between magnet and sensor is typically 1 to 2 mm.
Magnets with single-face magnetization, by contrast, can be fixed directly on an iron shaft. Since the field is concentrated on one side, it supports a large air gap of as much as 3 mm between the magnet and the sensor. Single-face magnets are typically made of NeFe with a plastic compound, such as NeoFer 48/60p.
Magnets of two types can be used with a magnetic position sensor such as the AS5262.
The choice of the magnet and its dimensions depends on the mechanical design tolerances. For wide tolerance of lateral displacement and a lower INL, use a larger magnet. Increased intensity of magnetic field is available from thicker magnets.
Throttling up with magnetic position sensors
The ETB is a particularly demanding application of contactless position sensing technology, requiring high performance, robustness, and functional safety attributes. By choosing a magnetic position sensor such as the ams AS5262 with features tailored to applications like automotive throttles, systems designers can more easily meet the specifications for accuracy, precision, and reliability of measurement performance.
— Roberto Scotti, is a field applications engineer at ams.