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TRANSCRIPT
Absolute magnetic sensors for large diameter through-shaft applica-
tions
Dr. Didier Frachon, Dr.-Ing. Gerald Masson, Thierry Dorge, Dipl.-Ing. Michaël Delbaere, Dr.-Ing.
Stephan Biwersi Moving Magnet Technologies SA, 1 Rue Christiaan Huygens, 25000 Besancon, France
Contact : [email protected]
Abstract This paper introduces application of a through shaft 360° magnetic position sensor technology developed by MMT to the
case of large diameter shafts (typically > 40 mm and up to 100 mm ore more) and the specific solutions that are required
to get an efficient signal processing and an accurate output (typically < +/- 0.5% of the full stroke).
1 Introduction
Through shaft rotary position sensors are a strong re-
quirement for certain applications where a location of the
sensor’s moving element at the end of a shaft is not possi-
ble.
Examples are automotive steering angle sensors or ab-
solute detection of shaft position of various electric mo-
tors over 360°.
Such a requirement corresponds to specific challenges,
especially in the demanding field of automotive applica-
tions: simple structures, high reliability, high accuracy,
compactness…
In that scope, MMT has developed a contactless 360°
through shaft absolute angular position sensor using a
probe which measures the angle of the magnetic field
generated by a diametrically magnetized magnet [1, 2].
However, this solution meets some limits when the
shaft diameter is getting large (typically > 40 mm and up
to 100 mm ore more), because of significant differences
between the two magnetic field components used by the
sensor, and therefore can’t be used as it is. Such cases are
currently getting frequent in applications like absolute po-
sition sensing of high power electric drives dedicated to
electric or hybrid electric vehicles. The current state of the
art for such sensors is the resolver [3]. However, magnetic
position sensors based on Hall or GMR ICs may offer an
attractive solution due to their performances and cost.
In this paper, after some reminder on the basic principle
of our through shaft sensor, we will explain the solution
proposed by MMT to adapt this technology to the con-
straints of large diameter shafts, while retaining the merits
of a simple and accurate structure. Then, we will also dis-
cuss possibilities to reach higher accuracies in this confi-
guration. Prototype measurement will be provided.
2 360° through shaft sensor solu-
tion
2.1.1 Overall principle
The through shaft 360° position sensor developed by
MMT relies on a sensitive device placed on an outer di-
ameter of a diametrically magnetized magnet ring and
able to measure at least two components of the magnetic
field at a single point (Figure 1).
Figure 1 : Through shaft 360° position sensor principle
In opposition to standard end-of-shaft configurations
[4], in that case the angle of the magnetic field does not
follow the rotational angle of the shaft due to the fact that
although magnetic field components theoretically have a
sine and cosine shape, they do not have the same ampli-
tude. It has been shown however [2] that through the use
of an adjustment parameter before the computation of the
field angle, this difference can be balanced and that an
accurate output can be reached for various sizes and
strokes of sensor.
Figure 2 provides the example of the two magnetic field
components in a steering angle sensor prototype having
an inner shaft diameter of 25 mm. The ratio between the
amplitudes is of approximately 3.
Sensoren und Messsysteme 2010 ∙ 18. – 19.05.2010 in Nürnberg Paper 38
ISBN 978-3-8007-3260-9 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach 1
Figure 3 shows its measurement curve over the -
40/+150°C temperature range. As one can see, the lineari-
ty is below +/- 0.5% of the full stroke.
Figure 2 : Magnetic field components over 360° of a
through shaft position sensor (shaft diameter 25 mm )
Figure 3 : Non-linearity on -40°C/+150°C of the 25
mm shaft diameter position sensor
2.1.2 Case of a larger diameter shaft
When the shaft diameter is getting higher, the differ-
ence between the components is also increasing, with am-
plitude ratio that will typically be comprised between 5
and 10 for shaft diameters in the range of 30 to 100 mm,
if one doesn’t want to significantly increase the magnet
height. Figure 4 illustrates the case of a 40 mm shaft di-
ameter.
This is especially due to the tangential component getting
critically low, which makes it very difficult to process for
the Hall ICs that are typically used for such sensors. Also,
this low component will be more likely to be affected by
external magnetic fields.
Figure 4 : Magnetic field components over 360° of a
through shaft position sensor (shaft diameter 40 mm)
3 Use of flux concentrators to ad-
just field components
3.1.1 Principle
In order to allow a proper signal processing, it is re-
quired to significantly lower the differences between both
field components or if possible to make them equal, while
keeping the signal amplitude within a useful range (for
example 200 to 700 G for a MLX90316).
Considering the challenge to retain the simple design of
the basic 360° through shaft sensor (one magnet, one Hall
IC), MMT has developed a very simple solution based on
flux concentrators [5], as depicted for example hereafter
(Figure 5). The small ferromagnetic parts are modifying
the field lines so that it drastically reduces the ratio be-
tween the field components.
Figure 5 : Through-shaft sensor with flux concentrators
As an example, Figure 6 shows the modification of the
field components provided in Figure 4 thanks to the use
of flux concentrators.
Figure 6 : Magnetic field components over 360° of a
large through shaft position sensor (shaft diameter 40 mm
mm) with flux concentrators.
We have to notice from Figure 6 that even if in the
scope of the large diameter application we are looking for
bringing both components within a reasonable range from
each other, this solution even enables to get equal ampli-
tudes for both field components.
3.1.2 Influence of geometrical tolerances
One could be slightly concerned by the increase of sen-
sitivity due to positioning tolerances of the collectors and
probe.
If one considers the distance between the two collectors
as an important parameter, one can see on Figure 7 that
-450
-350
-250
-150
-50
50
150
250
350
450
0 40 80 120 160 200 240 280 320 360
Ind
uct
ion
[G]
Magnet position [°]
Br Bt
-450
-350
-250
-150
-50
50
150
250
350
450
0 40 80 120 160 200 240 280 320 360
Indu
ctio
n [G
]
Magnet position [°]
Br Bt
Sensoren und Messsysteme 2010 ∙ 18. – 19.05.2010 in Nürnberg Paper 38
ISBN 978-3-8007-3260-9 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach 2
for a given angle between collectors there is a value for
which the influence of this parameter on the radial and
tangential is minimal even for typical tolerances on posi-
tioning of such parts on a PCB.
Figure 7 : Influence of the distance between collector
On the figure 8 below, we show the influence of the
angular width of the collector. As one can see the radial
component of the induction is quite insensitive to this pa-
rameter but the tangential component increases almost
linearly with this parameter.
Figure 8 : Influence of the angular width of the collec-
tor
If we consider the sensitivity to the distance between
the probe and the collector and the sensitivity to the radial
position of the probe, one can see in the 2 figures below
that they are two important parameters.
If one considers the influence of the axial distance be-
tween the collector and the probe, one can see that for the
tangential component, the closer the probe the larger the
induction but for the radial component the closer the
probe the smaller the induction.
If one considers the radial position of the probe, one
can see that the tangential component is relatively insensi-
tive and the radial component increases as the probe gets
closer to the magnet.
Figure 9 : Influence of the axial and radial position of
the probe on radial and tangential field components
As a summary, these effects can be minimized by de-
sign, but also relate mostly to assembly tolerances that
can be handled by programming.
3.1.3 Prototype measurement
This principle has been successfully validated through
prototype measurement as depicted in Figure 9 using a 92
mm shaft diameter.
It has to be noted that it is typical of the target applica-
tions like position control of an electric machine that the
sensor only has to provide sine and cosine signals to the
ECU that handles the motor control.
Therefore, linearity plot provided here is obtained
through post-processing on a computer after acquiring the
raw values of the both magnetic field components.
One can see that the base non-linearity is in the range of
+/- 0.25% of the full scale at 25°C.
Figure 9 : Prototype sensor measurement at 25°C (shaft
diameter : 92 mm)
4 Higher accuracy solutions
4.1.1 Principle
A general concern for through shaft sensors, especially
under the requirements of automotive steering sensor ap-
plication, is to increase the accuracy.
While a typical of our through shaft sensor features +/-
0.5% of the full-stroke of non-linearity, requirements
down to +/- 0.2 or 0.1% of the full stroke can be met.
In that scope, MMT has developed a solution based on
signal combination of the components of two Hall ICs
placed at 90° (Figure 10) as described in [6, 7]
0
20
40
60
80
100
120
140
160
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Am
plit
ud
e o
f fi
eld
co
mp
on
en
ts [
G]
Angle between collector [°]
Bradial
Btan
0
50
100
150
200
250
300
0 10 20 30 40 50
Am
plit
ud
e o
f fi
eld
co
mp
on
en
ts [
G]
Collector angle [°]
Btan
Bradial
0
20
40
60
80
100
120
140
160
180
200
-2.5 -2 -1.5 -1 -0.5 0
Bta
nge
nti
al [G
]
Axial position [mm]
R = 56.0 mm
R = 56.5 mm
R = 57.0 mm
R = 57.5 mm
R = 58.0 mm
0
20
40
60
80
100
120
140
160
180
200
-2.5 -2 -1.5 -1 -0.5 0
Bra
dia
l [G
]
Axial position [mm]
R = 56.0 mmR = 56.5 mmR = 57.0 mmR = 57.5 mmR = 58.0 mm
-1.5
-1
-0.5
0
0.5
1
1.5
-1.5
-1
-0.5
0
0.5
1
1.5
0 30 60 90 120 150 180 210 240 270 300 330 360
No
n li
ne
arit
y [%
of
FS]
Ind
uct
ion
[Vo
lts]
Position [°]
Br1Bt1Linearity
Sensoren und Messsysteme 2010 ∙ 18. – 19.05.2010 in Nürnberg Paper 38
ISBN 978-3-8007-3260-9 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach 3
Figure 10 : Principle of higher accuracy through shaft sen-
sor
Using this configuration and the following signal
combination :
21
21
ntt
tnn
BBB
BBB
it is then possible to get proper sine and cosine signals
having same amplitude that enable to deduce (after
processing) the position with a non-linearity below +/-
0.2% of the full stroke.
An other feature of this solution is to drastically reduce
effects of an homogeneous external magnetic field, which
can be very interesting in the case where the sensor is
closely coupled to an electric motor in order to reduce the
effect of the field generated by coils.
4.1.2 Application to large shaft diameter sensors
In a large shaft diameter sensor, this principle can be
combined with the flux concentrators as shown on Figure
11.
The flux concentrators will help providing base field
components (Bn1, Bt1, Bn1, Bt2) within a reasonable
window before measurement by the magnetosensitive
elements.
Figure 11 : Principle of higher accuracy through shaft
sensor
4.1.3 Prototype results
In this paragraph we will detail measurement results on
a prototype using the same magnet and shaft diameter as
described in paragraph 3.1.2. and with the same process-
ing conditions, but under principle described in 4.1.1.
Figures 12 to 14 display Bn and Bt deduced from meas-
urement of Bn1, Bt1, Bn2 and Bt2
Figure 12 : Measured Bn and Bt signals as well as non
linearity
Figure 13 : Measured Bn and Bt signals as well as non
linearity at -40°C
Figure 14 : Measured Bn and Bt signals as well as non
linearity at 150°C.
As a summary, one can notice that the non-linearity is
remarkably low at 25°C and -40°C (typically +/- 0.1% of
the full stroke), and is still very good at very high tem-
perature.
4.1.4 Hysteresis
In such a structure using ferromagnetic concentrators, it
is important to check the hysteresis.
Figure 15 shows linearity measurement on the sensor
for 360° displacement in the clockwise and counter-
clockwise directions with very small difference which il-
lustrates the hysteresis is extremely small.
Bn1
Bt1
Bn2
Bt2
-1.5
-1
-0.5
0
0.5
1
1.5
-3
-2
-1
0
1
2
3
0 30 60 90 120 150 180 210 240 270 300 330 360
No
n li
ne
arit
y [%
of
FS]
Ind
uct
ion
[V
olt
s]
Position [°]
Br1+Bt2Bt1-Br2Linearity
-1.5
-1
-0.5
0
0.5
1
1.5
-3
-2
-1
0
1
2
3
0 30 60 90 120 150 180 210 240 270 300 330 360
No
n li
ne
arit
y [%
of
FS]
Ind
uct
ion
[V
olt
s]
Position [°]
Br1+Bt2Bt1-Br2Linearity
-1.5
-1
-0.5
0
0.5
1
1.5
-3
-2
-1
0
1
2
3
0 30 60 90 120 150 180 210 240 270 300 330 360
No
n li
ne
arit
y [%
of
FS]
Ind
uct
ion
[V
olt
s]
Position [°]
Br1+Bt2Bt1-Br2Linearity
Sensoren und Messsysteme 2010 ∙ 18. – 19.05.2010 in Nürnberg Paper 38
ISBN 978-3-8007-3260-9 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach 4
Figure 15 : CW and CCW linearity measurement
4.1.5 Effect of an external magnetic field
One of the feature of the signal combination solution
described here above is to theoretically provide a direct
cancellation of an homogeneous external field (see [7] for
a complete description), keeping in mind however that in
reality this field may be more or less homogeneous.
It is therefore interesting to look at the realistic case of
a field generated by the coils of an electric machine and
driven to the sensor through its shaft, which in general
will be somehow ferromagnetic.
We have therefore built up an experimental set-up with
a large coil axially placed over the sensor to simulate such
an axial perturbating field.
Figure 16 shows results obtained with an equivalent
field of 120 Gauss which is a significant value because it
represents approximately a large part of the useful
magnetic field of the sensor (for example amplitude of Br1
+ Bt2 is of 500G)
We can notice that the overall linearity of the sensor
stays however well below +/- 1% of the full stroke.
Figure 16 : Effect of an axial external magnetic field of
120 G.
5 Conclusion
In this paper, we have described a simple way to adapt
a through shaft magnetic position sensor to the conditions
of very large shaft diameter applications, by adjusting
magnetic field components and enabling therefore a
proper signal processing. Possibilities to improve accu-
racy have also been discussed and prototype results have
illustrated the good potential accuracy of such sensors, as
well as low hysteresis and capability to withstand external
disruptive fields.
A direct application of such principle is absolute posi-
tion of the shaft of large electric drives for electric or hy-
brid electric vehicles.
It may also be used to insure compatibility of a through
shaft sensor of any size with specific ICs or magnetic
field measuring principles that are not able to cope with
an adjustment of the magnetic field components, through
direct matching of both field components before they are
measured.
6 Literature
[1] Magnetic angular position sensor for a course up to
360°, European patent application EP1949036
[2] Jerance, N. et al.: Through-shaft contactless magnetic
sensor with a stroke up to 360, Proc. of SENSOR+TEST
Conference 2007
[3] Kitazawa, K., Principle and application of resolvers
for hybrid electric vehicles, Proc. of Innovative Automo-
tive Transmissions Conference 2009
[4]http:///www.melexis.com/Assets/MLX90316_Datashe
et_4834.aspx
[5] Capteur de position magnétique à mesure de direction
de champ et à collecteur de flux, French patent applica-
tion, not published yet
[6] Capteur de position magnétique angulaire ou linéaire
présentant une insensibilité aux champs extérieurs, French
patent application FR2923903
[7] Masson G. et al., Multi-turn and high precision
through-shaft magnetic sensors, Proc. of SENSOR+TEST
Conference 2009, pp. 41-46
Sensoren und Messsysteme 2010 ∙ 18. – 19.05.2010 in Nürnberg Paper 38
ISBN 978-3-8007-3260-9 © VDE VERLAG GMBH ∙ Berlin ∙ Offenbach 5