absolute magnetic sensors for large diameter through-shaft ... · absolute magnetic sensors for...

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.


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.


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



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



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


linearity at -40°C


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 [°]


-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 [°]




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



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