Technical
Article
Contactless position sensors: how to meet the safety and performance requirements of throttle valve systems
Roberto Scotti
Magnetic position sensors have become a favoured component among automotive design engineers. Over years of fault-free operation in many types of vehicle, this component type has proved
to be robust and resistant to vibration and contamination, while providing precise and accurate
measurements of angular displacement.
This success has prompted suppliers of magnetic position sensors to introduce many new variants,
in an attempt to provide more application-oriented devices to meet the needs of specific automotive
functions.
This article illustrates this trend, describing how features of the magnetic position sensor can be
modified to meet the needs of one particular application: the electronic throttle body.
How the electronic throttle operates
Inside a vehicle with a gasoline (petrol) engine, the amount of air entering the engine is regulated by
means of a throttle valve, typically a butterfly valve. A balanced mixture of fuel and air is necessary
to control the combustion at each cycle of the engine in such a way as to generate power while
producing low emissions. The throttle valve is located at the input of the intake manifold, or in more
advanced systems is housed in the Electronic Throttle Body (ETB).
In the case of older diesel engines, the fuel is injected into the cylinder without air flow control.
Modern diesel engines, on the other hand, have a throttle valve on the intake manifold to support
Exhaust Gas Recirculation (EGR). The use of exhaust gas serves to lower the combustion temperature, and this reduces the amount of nitrogen oxide (NOx) emissions, as required by recent air quality regulations.
The driver of a car does not have direct control over the throttle valve (see Figure 1): pressing the
accelerator (or gas) pedal sends a signal via a mechanical or electronic link to an Electronic Control
Unit (ECU). In a motorbike, a sensor measures the position of a linkage that tracks the rotation of
the accelerator handle. The ECU then precisely regulates the angle of the valve by means of a motorized actuator, in order to optimize performance or emissions. The valve is normally held closed
by a strong retaining spring when the ignition is off. This closed position is often called the Lower
Mechanical Stop (LMS).
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Fig. 1: the gas pedal (accelerator) does not directly control the throttle valve
The widest angle of opening is called the Upper Mechanical Stop (UMS). The span from LMS to
UMS is normally around 90°. The function of the throttle position sensor is to detect the absolute
angle of the valve and provide a stable and accurate signal to the ECU.
Migration from contacting to contactless solutions
Traditional angle-measurement systems used a potentiometer with three terminals (VDD, OUT,
GND) to measure the position of the throttle valve. The main drawback of the potentiometer arises
from its mode of operation: it produces an analogue voltage proportional to the rotation of the shaft
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by sliding a wiper over a resistive circular substrate. This makes it sensitive to dust and wear. For
safety-critical automotive applications, the potentiometer’s relatively low reliability and endurance
counts against it.
Automotive system manufacturers have now therefore turned to contactless sensors based on Halleffect (magnetic) sensing technology. In a magnetic position sensor, a two-pole (SN) magnetic disk
is fixed on the shaft of the valve. Its angular position is detected by an IC sensor aligned to it, with a
small air gap between magnet and sensor. Unlike potentiometers, magnetic position sensors do not
suffer mechanical wear, and are immune to contamination by dust or grease.
In order to maintain compatibility with legacy ECUs that interface to potentiometers, magnetic sensors for ETB applications also need to have a three-terminal topology and to generate a ratiometric
analogue signal.
In production, the position sensor must be end-of-line programmed, to configure the required output
voltage ramp (for instance, from 10% to 90% of VDD across the LMS-to-UMS span).
Sensor requirements in ETB designs
The position sensor used in ETB applications has several special requirements. General-purpose
position sensors aimed at a broad range of industrial and consumer applications can provide the
precision and accuracy required in an ETB, but not the additional features demanded in this safetycritical automotive environment. This is why a new generation of application-optimised position sensors is now appearing on the automotive components market.
The first of these special requirements is redundancy. A dual (redundant) sensor is essential to the
functional safety of an ETB throttle valve system. In a motorbike’s throttle system, triple redundancy
might even be specified. Figure 2 shows how redundancy can be provided for with the AS5262, a
position 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.
The advantage of the stacked-die structure is that the two dies measure almost exactly the same
magnetic field values. This means they can be easily compared and any malfunction in one of the
dies detected.
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Fig. 2: the AS5262’s dual-die structure enables the provision of redundancy in a single package
The IC is often soldered to a PCB affixed to the plastic cover of the throttle body; the plastic cover
carries the connector for the external cable linking the sensor to the ECU. The magnet is fixed to
the shaft of the valve. The effect of mounting the sensor on the cover of the throttle is that the
magnet is aligned with the two or three dies, at a distance dictated by the system’s mechanical design.
The second requirement of ETB applications is a precise analogue output. The AS5262, 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 actual angle measurement inside the IC has a 14-bit resolution over a full turn. For measuring
the 90° rotation of the throttle valve, this allows a resolution of 12 bits, which is sufficient for the ETB
application. It also provides for 10-bit resolution over a sector of 22.5°, which is the maximum angle
to be measured in an accelerator pedal (car) or handle (motorbike).
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sensors output
DIAGNOSTIC BAND
90%VDD
allowed
band
Output 1
Output 2
Output 1
Output 2
10%VDD
DIAGNOSTIC BAND
LMS
(0deg)
angle
UMS
(90deg)
Fig. 3: the dual-die version of the AS5262 produces a voltage output linearly proportional to angle
The high resolution of the AS5262’s output enables the ECU to precisely regulate the opening of
the throttle valve, and thus respond accurately to the driver’s action on the accelerator pedal. A
linear output is also easy for the ECU to process, requiring no complex compensation algorithms.
The third requirement in an ETB application is for extra features that support functional safety, and
in particular compliance with the automotive industry’s ISO26262 functional safety standard. These
requirements include:
• diagnostic features, to alert the system controller to the failure of the sensor. In the AS5262,
for instance, diagnostic features include magnet detection, broken wire detection, and the
provision of diagnostic signal bands (0-4% or 96-100% of VDD)
• protection against over-voltage, reverse polarity, and permanent short circuits
• protection against external stray magnetic fields. In many magnetic position sensors, this is
achieved by mounting a shield around the sensor. This is unnecessary for users of the
AS5262, which employs patented differential sensing technology. This uses two differential
pairs of Hall sensors inside the device: one pair for the x (cosine) component of the SN
magnetic vector field and one pair for the y (sine) component of the SN magnetic field. An
internal DSP compares these x and y values to calculate either the angle of the SN field of
the magnet or its magnitude. Because the sensor uses comparative rather than absolute
values, it is immune to the effect of stray magnetism.
Defining the maximum allowable error
A vehicle manufacturer’s ETB specifications always include an allowed INL (Integral Non-Linearity)
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error band either side of the ideal curve; often the requirement is for INL of <1% of VDD.
An EOL programming procedure is necessary to define the range of the voltage signal across the
span of the valve’s rotation. The method for programming the AS5162 gives the user the opportunity to meet the target INL specified by the manufacturer.
The normal method is to set the throttle body in the LMS and UMS positions and read the angles
measured by the IC, and to set the voltage signal range accordingly in software. This enables the
system to achieve an INL of <±1% of VDD.
For even tighter accuracy, the device also supports multi-point calibration at LMS, UMS and intermediate points. While this takes more time, it also provides for INL error of <±0.5% of VDD.
By the same token, single-point calibration at the LMS position only is quicker, but is only suitable
when the target INL error is >±1% of VDD.
The AS5262 also supports pre-programming with a pre-defined slope. Here, the ECU calibrates
itself by learning the output voltage at the LMS position. Again, this is only suitable when the target
INL error is >±1% of VDD and when the ECU has a learning capability.
Flexibility in choice of magnet
In a contactless position sensing system, the magnet is as important a component as the Hall effect
sensor. And in ETB applications, there are important choices for the system developer to make in
relation to the magnet.
Magnets with diametric magnetisation (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
that a non-magnetic holder (such as plastic, copper, brass or aluminium) providing separation of at
least 3mm 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 1-2mm.
Magnets with single-face magnetisation, by contrast, can be fixed directly on an iron shaft. These
magnets are intrinsically large (with a typical diameter of 16mm and thickness of 2.5mm); their field
lines have an asymmetric character. Since the field is concentrated on one side, it supports a large
air gap of as much as 3mm between the magnet and the sensor. Single-face magnets are typically
made of NeFe with a plastic compound, such as NeoFer 48/60p.
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Fig. 4: magnets of two types can be used with a magnetic position sensor such as the AS5262
The AS5262 sensor works with either type of magnet. It only requires a vertical magnetic field (Bz)
within the range 30-70mT over the 1.25mm radius of the circle in which the Hall sensors are located
inside the IC.
The choice of the magnet and its dimensions depend on the mechanical design tolerances. To provide for wide tolerance of lateral displacement and a lower INL, use a larger magnet. Increased
intensity of magnetic field is available from thicker magnets. This supports tight tolerances, such as
±0.5mm for both lateral centering and the air gap. For this, a diametric SmCo magnet with diameter
of 8mm and thickness of 3mm at a nominal air gap of 1.5mm works well, and provides for INL of
<1% of VDD.
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Summary
The ETB is a particularly demanding application of contactless position sensing technology, calling
for a combination of high performance and robustness and functional safety attributes. By choosing
a magnetic position sensor such as the AS5262 that provides features tailored to this automotive
application, the system designer can more easily meet the specifications for accuracy, precision
and reliability of measurement performance.
[ENDS]
[1.900 words]
For further information
ams AG
Roberto Scotti
Field Application Engineer
roberto.scotti@ams.com
www.ams.com
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