Magnet Selection Guide - Rotary Position Sensors

Application Note
DN[Document ID]
AN5000
Rotary Magnetic
Position Sensors
Magnet Selection Guide
ams Application Note
[v1-03] 2015-Oct-16
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Content Guide
1
Introduction .......................................................................................................................... 3
1.1
Purpose ................................................................................................................................ 3
1.2
Measurement principle ......................................................................................................... 3
1.3
Magnetic input range............................................................................................................ 5
1.4
Magnetic field measurement location .................................................................................. 6
1.5
Non linearity definition .......................................................................................................... 7
1.6
Mechanical orientation and misalignment ............................................................................ 9
1.6.1
Vertical distance change ...................................................................................................... 9
1.6.2
Horizontal distance change ................................................................................................ 11
2
Magnets ............................................................................................................................. 13
2.1
Magnet materials................................................................................................................ 13
2.2
Magnet dimensions ............................................................................................................ 13
2.2.1
Thickness increase of magnets ......................................................................................... 14
2.2.2
Diameter increase of magnets ........................................................................................... 15
2.3
Magnetic grades ................................................................................................................. 16
2.4
Magnetization types ........................................................................................................... 18
2.5
Magnetization errors .......................................................................................................... 18
2.6
Temperature effects on magnets ....................................................................................... 19
2.7
Mounting the magnet ......................................................................................................... 20
3
Magnet suppliers ................................................................................................................ 22
3.1
Magnets on AMS web shop ............................................................................................... 22
4
Contact Information ............................................................................................................ 24
5
Copyrights & Disclaimer ..................................................................................................... 25
6
Revision Information .......................................................................................................... 26
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1
Introduction
1.1
Purpose
The purpose of this Application Note is to explain the fundamental principles of ams AG magnetic
position sensors. In addition the selection of proper magnets is highlighted. This application note
covers all on axis single or dual magnetic position sensors products. Important aspects for magnet
selection e.g. temperature effects are described.
1.2
Measurement principle
ams’ magnetic position sensor products uses a patented differential measurement principle. These
circuits are using integrated lateral Hall sensors in standard CMOS technology. Lateral Hall elements
are sensitive to the magnetic field component perpendicular to their surface. This means they are
only sensitive to magnetic fields vertical to the IC surface. The magnetic flux density in z–direction Bz
is measured and horizontal Bx and By components are not measured at all.
Figure 1: On-axis magnetic position sensor IC + magnet
The magnetic position sensor circuits are a system-on-chip, they contain all components required to
create a non-contact rotation angle position measurement system. Basically, the only external
component required is a magnet rotating over the surface of the IC. Depending on the use case
(target accuracy, vertical air gap, temperature range and mounting possibilities), different magnets
are used.
In this type of measurement, a magnet rotates over the chip such that



the center of the magnet,
the center of rotation
and the center of the chip
are in one vertical line (see Figure 1).
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The integrated Hall sensors of the sensor IC are arranged in a circle using different diameters
depending on the product (see Table 1). The principle for rotation angle measurement requires that
the Hall elements on the IC can sense a full magnetic period as the magnet rotates. This requirement
is obtained by using a diametrically magnetized magnet.
HS1
HS2
r
HS3
HS4
Figure 2: Example Hall sensor locations and measurement radius
Figure 2 shows the circular arrangement of the Hall sensors HS1 – HS4. The rotary position sensor
model can be mathematically described as following:
𝑆𝑖𝑔𝑛𝑎𝑙1 = +𝑉𝐻𝑆1 + 𝑉𝐻𝑆2 − 𝑉𝐻𝑆3 − 𝑉𝐻𝑆4
𝑆𝑖𝑔𝑛𝑎𝑙2 = +𝑉𝐻𝑆1 − 𝑉𝐻𝑆2 − 𝑉𝐻𝑆3 + 𝑉𝐻𝑆4
∝ = 𝐴𝑇𝐴𝑁2(𝑆𝑖𝑔𝑛𝑎𝑙1 , 𝑆𝑖𝑔𝑛𝑎𝑙2 )
Note: The purpose of using ATAN2 instead of ATAN is to gather information on the signs of the inputs in order to return the
appropriate quadrant of the computed angle. ATAN2 provides an angle output over the full range 0-360 degrees.
Figure 3 Internal signals of Hall sensors HS1-HS4 and resulting signals
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As the magnet rotates over the chip, the Hall sensors create sinusoidal signals. The four individual
Hall sensor output signals are subtracted and summed according to the formulas. The resulting
signals are 90° phase shifted and represent sine and cosine signals. The ATAN2 algorithm is used
to calculate the angle over the complete measurement range of 360 degrees. This method is capable
of measuring absolute angle information.
Figure 4 3D Graph of magnetic flux density Bz
Magnetic scanning of a diametric magnetized magnet with a given z-distance (air gap) will lead to
Figure 4. The yellow track indicates the projection of the circle of the Hall element array on the 3D
scan. This given linear area makes the sensor system tolerant against mechanical misalignments
over a certain mechanical range.
1.3
Magnetic input range
Magnetic position sensor datasheets specifies the required magnetic flux density B z. This refers to
the best mechanical alignment case. Figure 5 shows the sinusoidal distribution of the flux density.
Figure 9 shows the green zone of required input range. This zone varies between different magnetic
position sensor products. Mechanical displacements will cause a magnetic offset shift in the
measured individual signals. Therefore a relative extraction according the formula is recommended.
The sensor system operates also in case of exceeding the absolute magnetic flux density.
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Figure 5 Magnetic flux density at the circular measurement track
Formula for relative extraction of the magnetic flux density. Static magnetic offset shift is ignored.
𝐵𝑚𝑖𝑛 ≤
1.4
𝐵𝑃𝑒𝑎𝑘𝑃𝑒𝑎𝑘
≤ 𝐵𝑚𝑎𝑥
2
Magnetic field measurement location
Magnetic position sensor datasheets specify the required magnetic flux density on the sensor die
surface and not on the package surface. Cross sections of the different packages show the
mechanical distance. Table 1 summarizes these parameters.
Figure 6 Air gap and distance package surface to die surface
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Table 1 Magnetic Position Sensor Product Matrix – Overview single die sensor
AS5047D
AS5115
AS5040
AS5132
AS5145
AS5134
AS5045B
Sensor
Radius [mm]
1.0
1.1
Magnetic
Input Range
[mT]
20-80
Product
DiePackage
Surface [mm]
45-751
22-842
AS5162
AS5050A
AS5048A
AS5147
AS5600
AS5161
AS5055A
AS5048B
AS5047P
AS5601
AS5147P
1.25
1.0
1.1
1.1
1.0
10-903
30-90
30-70
35-70
30-904
0.576
0.576
0.459
0.383
0.306
0.306
0.459
SSOP
SSOP
SOIC8
QFN
TSSOP
TSSOP
SOIC8
Table 1 summarizes the three import parameters required for simulation and selection of magnets.
Table 2 Magnetic Position Sensor Product Matrix – Overview dual die sensor
Product
AS5215
AS5245
Sensor Radius
[mm]
1.0
1.1
Magnetic Input
Range [mT]
20-80
DiePackage
Surface [mm]
45-751
22-842
AS5262
AS5261
AS5247
1.25
1.1
10-903
35-70
0.234 Top Die
0.234 Top Die
0.234 Top Die
0.234 Top Die
0.607 Bottom Die
0.607 Bottom Die
0.607 Bottom Die
0.611 Bottom Die
MLF
MLF
MLF
MLF
Table 2 summarizes the three import parameters required for simulation and selection of magnets
1.5
Non linearity definition
The integral non linearity (INL) is one of the important parameters for position sensors in general.
This parameter specifies the effective angle error from the total system. The magnetic position sensor
system performance is mainly dependent on magnetic and mechanical constraints. Electrical errors
from position sensor IC play mostly a minor role.
1
Magnetic input range for green range
Magnetic input range for yellow range
3 Extended mode selected
4 Lost magnet diagnostic at 8 mT
2
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Figure 7 Non Linearity of the angle output
𝐼𝑁𝐿 𝐸𝑟𝑟𝑜𝑟 =
𝐿𝑖𝑛𝑒𝑎𝑟𝑖𝑡𝑦 𝐸𝑟𝑟𝑜𝑟 max −𝐿𝑖𝑛𝑒𝑎𝑟𝑖𝑡𝑦 𝐸𝑟𝑟𝑜𝑟 𝑚𝑖𝑛
2
The non-linearity parameter represents the difference between the measured and the ideal line. The
formula above extracts the relative angle error. Offset angle components are not considered in this
calculation. (Best-Line-Fit method).
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1.6
Mechanical orientation and misalignment
Figure 8 Mechanical misalignments in vertical and horizontal direction
Two mechanical parameters and tolerances are important. The magnetic flux density changes with
bigger air-gaps. The linearity changes with mechanical displacements in x and y direction.
1.6.1 Vertical distance change
Figure 9 shows the difference between 6 and 8 mm diameter magnet (N35H).
The vertical distance from IC package surface to the magnet surface (air gap) is in addition an
important parameter for the linearity parameter of the system. Due to magnetic properties an
optimum can be chosen.
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Figure 10 Nonlinearity and Automatic Gain Control (AGC) value over air gap. D6H2.5 magnet.
Figure 11 Nonlinearity and Automatic Gain Control (AGC) value over air gap. D8H2.5 magnet.
Figure 10 and Figure 11 show the tendency of the non-linearity choosing different air gaps. Both
settings have their best operating points. In addition the automatic gain control value is shown. This
value is increasing with increasing distance with reaching the limit at to large air gaps. The magnetic
position sensor is still operating in this area with slightly increased noise output. Magnetic field
warning flags can be set by the position sensor in this region.
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1.6.2 Horizontal distance change
Figure 12 Non-Linearity change over horizontal misalignment
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4,5
4
Linearity Error in [°]
3,5
3
2,5
D6H2.5
2
D8H3.0
D10H3.0
1,5
1
0,5
0
0
500
1000
1500
Displacement in [µm]
Figure 13 Non-Linearity error over displacement
Figure 13 shows the improvement by selecting 8 mm or 10 mm magnets. The error at best aligned
case is improved as well.
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2
2.1
Magnets
Magnet materials
Table 3 Magnet materials and properties
2.2
Magnet dimensions
Table 4 Possible magnet dimensions
Shape
Size
Cylinder
Diameter = 6 mm
Thickness = 2.5 mm
Diameter = 8 mm
Recommended
Thickness = 3 mm
Diameter = 8 mm
Thickness = 4 mm
Diameter = 10 mm
Thickness = 5 mm
Square
Length/Width = 6 mm
Thickness = 2.5 mm
Length/Width = 8 mm
Thickness = 3 mm
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2.2.1 Thickness increase of magnets
N35H
Figure 14 Magnetic Flux Density increases with increasing the magnet thickness (different magnets)
Figure 15 shows the relationship of the peak amplitude in a rotating system (essentially the
magnetic field strength of the Bz field component) in relation to the thickness of the magnet. The Xaxis shows the ratio of magnet thickness (or height) [H] to magnet diameter [D] and the Y-axis
shows the relative peak amplitude with reference to the recommended magnet (D=6mm,H=2.5mm).
The recommended magnet has H/D ratio of 0.42.
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Bz amplitude vs. magnet Thickness
of a cylindrical diametric Magnet with 6mm Diameter
relative peak amplitude [%]
160%
140%
120%
100%
80%
60%
Diameter = 6mm x Thickness =
2.5mm
H/D = 0.42
rel. amplitude = 100%
40%
20%
0%
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
Thickness to Diameter [H/D] ratio
Figure 15 Thickness/Diameter Ratio
As the graph shows, the amplitude drops significantly at H/D ratios below this value and remains
relatively flat at ratios above 1.3.
Therefore, the recommended thickness of 2.5mm (@6mm diameter) should be considered as the low
limit with regards to magnet thickness.
It is possible to get 40% or more signal amplitude by using thicker magnets. However, the gain in
signal amplitude becomes less significant for H/D ratios >~1.3. Therefore, the recommended magnet
thickness for a 6mm diameter magnet is between 2.5 and ~8 mm.
2.2.2 Diameter increase of magnets
Table 5 Comparison of different magnet diameters 6 mm, 8mm and 10 mm
Small diameter magnet (6mm):
+++ stronger differential signal =
good signal / noise ratio,
larger air gaps
--- shorter linear range =
smaller horizontal misalignment area
ams Application Note
[v1-03] 2015-Oct-16
Large diameter magnet (8 mm, 10 mm):
+++ wider linear range =
larger horizontal misalignment area
-- weaker differential signal =
poorer signal / noise ratio,
smaller air gaps
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2.3
Magnetic grades
Both SmCo and NdFeB magnets are available in different grades, mainly determined by the
remanence, essentially the strength of the magnet.
The recommended magnet grade for the magnetic position sensor when used for on-axis angle
measurement is N35H for NdFeB magnets.
Note that NdFeB magnets have a lower operating temperature than SmCo magnets. A grade N35H
has a maximum operating temperature of 120°C. If the magnet is to be operated at higher ambient
temperatures, it is recommended to use a N35SH grade, which can operate up to 150°C
Table 6: SmCo magnet grades (www.bomatec.ch)
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Table 7: NdFeB magnet grades (www.bomatec.ch)
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2.4
Magnetization types
Table 8 Magnetization types
2.5
Magnetization errors
S
ams Application Note
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N
Magnetization Angle
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Figure 16 Magnetization angle
Linearity degradation with increasing
magnetization tilt angle and displacement
1
0,9
Nonlinearity INL [°]
0,8
0,7
0°
0,6
1°
0,5
2°
0,4
3°
0,3
0,2
0,1
0
0
100
200
300
400
Displacement in [µm]
Figure 17 Magnetization tilt and impact to the INL parameter over displacement
2.6
Temperature effects on magnets
N35H
Figure 18: Magnetic flux density Bz of N35H magnet at different temperature (same magnet)
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2.7
Mounting the magnet
Generally, for on-axis rotation angle measurement, the magnet must be mounted centred over the IC
package. However, the material of the shaft on which the magnet is mounted, is also of utmost
important.
Magnetic materials in the vicinity of the magnet will distort or weaken the magnetic field being picked
up by the Hall elements and cause additional errors in the angular output of the sensor.
Figure 19 Magnetic field lines in air
Figure 19 shows the ideal case with the magnet in air. No magnetic materials are nearby.
Figure 20 Magnetic field lines in plastic or copper shaft
If the magnet is mounted in non-magnetic material, such as plastic or diamagnetic material, such as
copper, the magnetic field distribution is not disturbed.
Even paramagnetic material, such as aluminum may be used. The magnet may be mounted directly
in the shaft.
Note: stainless steel may also be used, but some grades are magnetic, they should be avoided.
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Figure 21 Magnetic field lines in iron shaft
If the magnet is mounted in a ferromagnetic material, such as iron, most of the field lines are attracted
by the iron and flow inside the metal shaft (see Figure 21). The magnet is weakened substantially.
This configuration should be avoided !!
Figure 22 Magnetic field lines with spacer between magnet and iron shaft
If the magnet has to be mounted inside a magnetic shaft, a possible solution is to place a nonmagnetic spacer between shaft and magnet, as shown in Figure 22. While the magnetic field is rather
distorted towards the shaft, there are still adequate field lines available towards the sensor IC. The
distortion remains reasonably low.
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3
Magnet suppliers
Table 9 Magnet supplier for Position Sensor Products
3.1
Preferred
Suppliers
Link
Contact
AIC
Engineering
Limited
www.aicengineering.com
www.aicengineering.com
Alliance LLC
www.allianceorg.com
www.allianceorg.com/contactus.html
Arnold
Magnetic
Technologies
www.arnoldmagnetics.com
www.arnoldmagnetics.com/Contact.aspx
Bomatec AG
www.bomatec.ch
www.bomatec.ch/standorte.html
Dexter
Magnetic
Technologies
www.dextermag.com
www.dextermag.com/Offices
Magnetfabrik
Bonn
www.magnetfabrik.de
www.magnetfabrik.de/kontakt.php
Mittelland
Magnets
http://www.mittellandmagnets.com
www.mittelland-magnets.com/contact.html
MSSchramberg
GmbH & Co
KG
www.magnete.de
www.magnete.de/kontakt.html
Zhejiang
Innuovo
Magnetics
Co., Ltd.
http://en.innuovomag.com/
http://en.innuovomag.com/comcontent_contact.html
Magnets on AMS web shop
Table 10 Available magnets on AMS web shop
Part No.
AS5000-MD6H-1
AS5000-MD6H-2
AS5000-MD6H-3
AS5000-MD6H-4
AS5000-MD6H-5
AS5000-MD6H-6
Description
Diametric Magnet,
D6x2.5mm, Arnold
Magnetic
Diametric Magnet,
D6x2.5mm, Bomatec AG
Diametric Magnet,
D6x2.5mm, Dexter
Magnetics
Diametric Magnet,
D6x2.5mm, Mittelland
Magnets
Diametric Magnet,
D6x2.5mm, AIC
Engineering Limited
Diametric Magnet,
D6x2.5mm, Zhejiang
Innuovo Magnetics Co.,
Ltd.
ams Application Note
[v1-03] 2015-Oct-16
Magnetization
Size
Material
max
temp.
Diametric Magnet
D6x2.5mm
NdFeB
120°C
Diametric Magnet
D6x2.5mm
NdFeB
120°C
Diametric Magnet
D6x2.5mm
NdFeB
120°C
Diametric Magnet
D6x2.5mm
NdFeB
120°C
Diametric Magnet
D6x2.5mm
NdFeB
120°C
Diametric Magnet
D6x2.5mm
NdFeB
Others
< 3° Tilt
magnetizatio
n error
< 3° Tilt
magnetizatio
n error
< 3° Tilt
magnetization
error
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Part No.
Description
AS5000-MD6SH1
Diametric Magnet,
D6x2.5mm, Alliance LLC
Diametric Magnet,
D8x2.5mm, Bomatec AG
AS5000-MD8H-1
AS5000-MD8H-2
Diametric Magnet,
D6x2.5mm, AIC
Engineering Limited
AS5000-MD8H-3
Diametric Magnet,
D6x2.5mm, Zhejiang
Innuovo Magnetics Co.,
Ltd.
ams Application Note
[v1-03] 2015-Oct-16
Magnetization
Size
Material
max
temp.
Diametric Magnet
D6x2.5mm
NdFeB
150°C
Diametric Magnet
D8x2.5mm
NdFeB
120°C
Diametric Magnet
D8x2.5mm
NdFeB
120°C
Diametric Magnet
D8x2.5mm
NdFeB
Others
< 3° Tilt
magnetization
error
< 3° Tilt
magnetization
error
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4
Contact Information
Buy our products or get free samples online at:
www.ams.com/ICdirect
Technical Support is available at:
www.ams.com/Technical-Support
Provide feedback about this document at:
www.ams.com/Document-Feedback
For further information and requests, e-mail us at:
[email protected]
For sales offices, distributors and representatives, please visit:
www.ams.com/contact
Headquarters
ams AG
Tobelbaderstrasse 30
8141 Unterpremstaetten
Austria, Europe
Tel: +43 (0) 3136 500 0
Website: www.ams.com
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Copyrights & Disclaimer
Copyright ams AG, Tobelbader Strasse 30, 8141 Unterpremstaetten, Austria-Europe. Trademarks
Registered. All rights reserved. The material herein may not be reproduced, adapted, merged,
translated, stored, or used without the prior written consent of the copyright owner.
Information in this document is believed to be accurate and reliable. However, ams AG does not
give any representations or warranties, expressed or implied, as to the accuracy or completeness of
such information and shall have no liability for the consequences of use of such information.
Applications that are described herein are for illustrative purposes only. ams AG makes no
representation or warranty that such applications will be appropriate for the specified use without
further testing or modification. ams AG takes no responsibility for the design, operation and testing
of the applications and end-products as well as assistance with the applications or end-product
designs when using ams AG products. ams AG is not liable for the suitability and fit of ams AG
products in applications and end-products planned.
ams AG shall not be liable to recipient or any third party for any damages, including but not limited
to personal injury, property damage, loss of profits, loss of use, interruption of business or indirect,
special, incidental or consequential damages, of any kind, in connection with or arising out of the
furnishing, performance or use of the technical data or applications described herein. No obligation
or liability to recipient or any third party shall arise or flow out of ams AG rendering of technical or
other services.
ams AG reserves the right to change information in this document at any time and without notice.
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Revision Information
Changes from previous version to current revision 1-03 (2015-Oct-16)
V1.02: new magnets added: AS5000-MD8H-2; AS5000-MD8H-3; AS5000-MD6H-5;
AS5000-MD6H-6
Page
23
V1.01: Additional information about INL over z-distance AS5600, AS5601, AS5047P,
AS5147P included
Initial version 1-00
Note: Page numbers for the previous version may differ from page numbers in the current revision.
Correction of typographical errors is not explicitly mentioned.
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