Commentary

Development of Multi Track Magnetic Encoder Integrated
Rolling Bearing

Hiroshi OKUMURA*
Hiroyoshi ITO*
Yasuyuki HAMAKITA*
The robot industry has been expanding on the back of market demand for
improved productivity and labor saving as the countermeasures of diminishingworking population. Especially, the demand of small robots such as collaborative
robots that work with human labors and service robots has been increasing.
Through the recent trend, market requirements for the robots have been getting
higher and more diversified.
NTN has developed the “Multi Track Magnetic Encoder Integrated Rolling Bearing” based on the technologyof the “Multi Track Magnetic Ring” for rotary encoders. The bearing is suitable for robot joints and performs
high-precision angle detection as well as the shaft support. This report introduces the features, structure,and performance of the bearing.

1. Introduction
The use of robots to perform work instead ofhumans is on the increase throughout the world in
all industrial manufacturing sectors, including forautomobile manufacture1) 2). In recent years we haveexperienced significant social problems such as thedecline in the working-age population ratio due tothe decreasing birthrate and aging population as
well as slow production activity due to the increase
in COVID-19 infections, prompting the increased
introduction of robots. Industrial robots installed in
production lines, robots that work in locations awayfrom humans, and small robots such as collaborative
robots and service robots are on the increase, while
the structure of robots and the work they do has
become increasingly diverse.
Vertical articulated robots (Fig. 1) that move similar
to the arms of a human are typical, and such robots
must transport a workpiece with high accuracy andspeed to a determined position. To achieve this, thejoint mechanism must be able to support the motorʼsrotating shaft while detecting its rotational speed,
rotational direction, and absolute angle with a highlevel of accuracy to achieve the correct position. In
addition to servo motors and reducers, robots use
bearings to support rotation and rotary encoders todetect rotational conditions3).
Furthermore, recent joint mechanisms must support
the diverse trends as mentioned above, and to becompact, lightweight, be assembled from as fewer
parts as possible to reduce man-hours to build themand have a high environmental resistance so they canbe used in various operating conditions.
So far NTN has commercialized the highly accurate
and compact Multi Track Magnetic Ring for use indetecting the absolute angle and rotational speed
for the robot joint mechanisms4). The Multi Track
Magnetic Ring, in combination with the magnetic
sensor IC, forms a magnetic rotary encoder thatprovides high environmental resistance.
The application of this technology contributes
to greater improved performance for robot joint
mechanisms, so NTN has developed a new “Multi
Track Magnetic Encoder Integrated Rolling Bearing”(hereafter, developed product) that brings together
the bearing, the Multi Track Magnetic Ring, andmagnetic sensor IC in a single unit. The developed
product supports the rotation of the shaft while
detecting the rotational speed, rotational direction
and absolute angle as a rotary encoder. Bringing
together the bearing and magnetic rotary encoder
in a single unit can make the robot joint mechanism
more compact and reduce the man-hours required
for assembly and setup in the robot manufacturing
process. The developed productʼs features, structure
and test results are introduced below.
Axis 5: Wrist bend

Axis 6: Wrist rotation

Axis 2: Lower arm

Axis 4: Wrist turn

Axis 3: Upper arm

Axis 1: Turn

Fig. 1 6 axes vertical articulated robot* Robotics & Sensing Engineering Dept., Industrial Business Headquarters

NTN TECHNICAL REVIEW No.89

41

Development of Multi Track Magnetic Encoder Integrated Rolling Bearing
2. Developed product structure and features
The developed product is a unit composed of adeep groove ball bearing, Multi Track Magnetic Ring,and magnetic sensor IC (Fig. 2).
The Multi Track Magnetic Ring is attached tothe inner ring of the deep groove ball bearing. The
magnetic sensor IC is attached to the outer ring of
the deep groove ball bearing through the sensor
housing. The developed product can be used for joint
mechanisms on the robot as shown in Fig. 3. The
Multi Track Magnetic Ring rotates as the axis rotates,
and to correspond to the rotation the magnetic sensor
IC reads the change in magnetic poles. This enables
the developed product to detect the rotational
speed, rotational direction, and absolute angle. Data
detected by the magnetic sensor IC is transmitted
to a motor driver prepared by the user, then used for
positioning the robot joint mechanisms (Fig. 4).

Deep Groove Ball
Bearing

Multi Track
Magnetic Ring

Magnetic
sensor IC

Rotary encoder unit

Deep Groove Ball
Bearing

Rotary encoder unit

Multi Track Magnetic Encoder
Integrated Rolling Bearing

Fig. 2 Configuration of major components for the Multi Track
Magnetic Encoder Integrated Rolling Bearing

The developed product is a compact unit that uses
technology to integrate the rotation detection unit
and bearing that NTN has developed for conventionalrotation sensor bearings (Fig. 5), which are wellestablished for use in transportation machinery5). It
has the same bearing bore diameter, outer diameter,
width, and load rating as a standard bearing without
the rotation detection unit, making it compatible withstandard bearings. This unitʼs features are shown in (1)to (4) below. Table 1 also shows the specifications forthe developed product using deep groove ball bearing
6907.
(1) Can support shaft rotation and provide highaccuracy angle detection.
The developed product detects rotational speed,
rotational direction, and absolute angle while supporting
shaft rotation. Configuring parameters in the magnetic
sensor IC enables a maximum high accuracy output of
20 bits (resolution of approximately 0.00034°).
(2) Contributes towards making the robot joint
mechanism compact and lightweight.
Integrating the deep groove ball bearing, Multi Track
Magnetic Ring and magnetic sensor IC means that acoupling to connect the joint mechanism and rotary
encoder is not needed. This enables a reduction in
the joint mechanism shaft length, the use of fewer
components, and results in producing a more compact
and lighter joint mechanism (Fig. 6).
(3) Can reduce man-hours for robot assembly and setup.
As mentioned above in (2), integrating components
can reduce the number of robot components, which
reduces man-hours required for assembly. It alsomeans that the user does not have to perform setup
such as adjusting output signals required for whenassembling the magnetic ring and magnetic sensor, so
man-hours can be reduced.
(4) Affected less by the surrounding environment andhigh environmental resistance.
The developed product is a magnetic rotary
encoder that detects the rotational speed, rotational
direction and absolute angle using magnetism. In
comparison with optical rotary encoders that use lightfor detection, it is affected less by the surrounding
environment such as temperature changes, dust, andoil, and has a high environmental resistance6).
Host controller
Motor driver
Robot joint mechanisms
Servo motor
ReducerAngle information

Rotaryencoder unit

Operation instructions

Rotational shaft
Robot arm

Fig. 3 Positions where the Multi Track Magnetic Encoder
Integrated Rolling Bearing can be used42

NTN TECHNICAL REVIEW No.89

Fig. 4 Schematic drawing of positioning for robot
joint mechanisms using the Multi Track
Magnetic Encoder Integrated Rolling Bearing3. Absolute angle detection principles
and resolution

Fig. 5 Rotation sensor bearing(NTN conventional product)
Bearing

Joint
Couplingmechanism

Bearing

Rotary encoder(a) Conventional

Multi Track
Magnetic Encoder
Integrated Rolling
Bearing(b)Developed
product

3.1 Multi Track Magnetic Ring and magnetic sensor IC
The Multi Track Magnetic Ring (Fig. 7, Fig. 8) is composedof a core press-formed from thin steel plates and rubbermaterial that contains a magnetic material (hereafter, rubber
magnetic material). The rubber magnetic material is adhered
to the core by vulcanization. Two magnetic tracks withdifferent number of polar pairs for the N-pole and S-pole
are formed on the outer diameter circumferential surface ofthe rubber magnetic material for the radial magnetization
type, and the width surface for the axial magnetization type.
For a 64/63 polar pair Multi Track Magnetic Ring, the main
track is magnetized with N-pole S-pole 64 polar pairs andthe sub track is magnetized with 63 polar pairs.
The magnetic sensor IC arranged opposite the Multi
Track Magnetic Ring is equipped with two detection
units that face the main track and sub track, and an
absolute angle calculation unit. For example, IC (iC-MU)
manufactured by iC-Haus can be used (Fig. 9).
Sub track (63 polar pairs)
Main track (64 polar pairs)
Rubber magnetic
material

Using the developed product enables the rotary encoder to be
integrated into the joint mechanism

Fig. 6 Adopting the Multi Track Magnetic Encoder
Integrated Rolling Bearing reduces
the number of components required
for the robot joint mechanism,and makes it more compact
Table 1 Specifications for the Multi Track
Magnetic Encoder Integrated Rolling Bearing(deep groove ball bearing 6907)
Item

Specifications

Deep Groove Ball Bearing

6907（ID35×OD55×10）

Wiring connector
height

6 mm (protrusion amount frombearing outside diameter surface)

Encoder unit width

10.5 mm

Multi Track Magnetic Ring 64/63 polar pairs, 1.28 mm pitch
Magnetic sensor IC

iC-MU

Allowable rotational speed

6 000 min-1

Angle resolution

Max. 20 bits

Detection information

Rotational speed, rotational
direction, absolute angle

Angle detection error± 0.1゜

Power supply voltage

DC 5 V

Current consumption

60 mA

Signal format

ABZ，SPI，BiSS，SSI

Operating temperaturerange

ー25 to ＋110 ℃

Magnetization pattern view
through magnetic sheet(b)Magnetization patternCore(a)External
appearance(c)Illustrated magnetization patternFig. 7 Multi Track Magnetic Ring:
Radial magnetization typeCore

Rubber magnetic(a)Externalmaterial
appearance(c)Illustrated magnetization
patternFig. 8 Multi Track Magnetic Ring: Axial magnetization typeFig. 9 Illustration of combining the Multi Track
Magnetic Ring with iC-MU series(images provided by iC-Haus GmbH)NTN TECHNICAL REVIEW No.89

43

Development of Multi Track Magnetic Encoder Integrated Rolling Bearing
3.2 Absolute angle detection principles and resolution
This section explains the absolute angle detection
principles (Fig. 10) and resolution with the 64/63
polar pair Multi Track Magnetic Ring as an example.
For the 64/63 polar pair Multi Track Magnetic
Ring, at each rotation the output signal of the period
corresponding to magnetization polar pairs 64 fromdetection unit 1 of the magnetic sensor IC, andmagnetization polar pairs 63 from detection unit
2 are obtained. These output signals have a phasedifference of one polar pair for each rotation so
calculating this phase difference enables the absolute
angle to be detected. Thus, on a magnetic pair with64 polar pairs, it is possible to detect at what polar
pair the position lies.
By accurately reading the magnetic strength
between polar pairs with the magnetic sensor IC,
12 bit multiplication is possible. Therefore, angle
information for one polar pair can be divided into
212 sections, and together with 64 polar pairs (26),
18 bit (resolution of approximately 0.0014°) angle
information can be output8). Magnetic sensor IC
parameters can be configured to also enable up to 20
bit (resolution of approximately 0.00034°) output.
Angle information output from the sensor isdependent upon the magnetic pitch accuracy. To beable to detect the absolute angle and achieve this kindof high angle accuracy, it is necessary to sufficiently
magnetize two magnetic tracks with a different
number of magnetized polar pairs. NTN has developed
original magnetization technology to provide highly
precise control of the magnetic pitch for eachmagnetic pole on the Multi Track Magnetic Ring9).
Magnetic sensor
Detection Unit 1
Detection Unit 2

4. Evaluation test
4.1 Absolute angle detection error
The absolute angle detection error was measured forthe developed product using deep groove ball bearing
6907. Table 2 shows the measurement conditions
and Fig. 11 shows the measurement results. The
developed product uses both a Multi Track Magnetic
Ring and magnetic sensor IC, and these items areattached to a deep groove ball bearing with good
accuracy to achieve high accuracy detection of theabsolute angle. Under the measurement conditions
shown in Table 2, an absolute angle detection error of± 0.1°or less was obtained.
4.2 Resistance to electromagnetic noise
The developed product achieves a compact unit
composed of few elements as possible, including
a deep groove ball bearing, Multi Track Magnetic
Ring, magnetic sensor IC and circuit substrate, andcontributes towards a more compact and lightweightrobot joint mechanism. To investigate its resistance tovarious noise assumed at the robot joint mechanism,
an electromagnetic noise test was conducted. This
test results show that the developed product has
resistance to EMC Standards (IEC 61000-6-2) for
industrial environments10) (Table 3, Fig. 12).
Table 2 Absolute angle detection error measurement
conditions for Multi Track Magnetic Encoder
Integrated Rolling Bearing(deep groove ball bearing 6907)
Item

Conditions

Power supply voltage

DC 5 V

Rotational speed

5 min-1

Measurement angle

360°

Measurement temperature

Room temperature

Number of polar pairs

64/63 polar pairs

Multi Track Magnetic Ring

S

N

S

N

S

N

S

N

S

N

S

N

S

N
S

S
N

N
S

S
N

N
S

64 polar pairs
63 polar pairs

S N
Output Voltage of
detection unit 1
Output Voltage of
detection unit 2
Linear Conversion of output
Voltage of detection unit 1
Linear Conversion of output
Voltage of detection unit 2
Phase difference between
detection unit 1 and 2

Rotation angle

0°

360°

Angle detection error deg.

Fig. 10 Absolute angle detection principles

0.3
0.2
0.1
0
-0.1
-0.2
-0.3

0

30

60

90

120

150

180

210

240

270

300

330

360

Rotation angle (position) deg.

Fig. 11 Absolute angle detection error measurement results of Multi Track Magnetic Encoder Integrated Rolling Bearing

44

NTN TECHNICAL REVIEW No.89

A. Static electricity discharge caused
by electrification frompeople and materials, etc.

5. Developed product examples of application
The developed product can be used for compact
robot joint mechanisms, such as collaborative robots
and service robots using the features introduced in thispaper. Additionally, it can also be used in general servo
motors by replacing the analog rotary encoders, suchas optical rotary encoders and resolvers (Fig. 13).
For example, optical rotary encoders requirebarriers to prevent the intrusion of dust and oil to thelight emitter and light receiver, as well as the internalareas of the encoder so it is difficult to make themcompact. Furthermore, resolvers require high accuracy
arrangement of many coils in the coil unit which is a
key component to achieving high resolution, so they
have a complex shape, and it is difficult to make themcompact. Additionally, resolvers must have an R/Dconverter to convert the output signal to digital.
In contrast, the developed product only requires a
Multi Track Magnetic Ring and magnetic sensor IC tocreate the magnetic rotary encoder. This produces a
simple structure in comparison with an optical rotary
encoder or resolver, which can contribute towards
making robot joint mechanisms and servo motors
more compact and lighter in weight. The developed
product also outputs digital signals, so an R/Dconverter or similar device is not necessary, so it iseasy to make the system compact and lightweight.

Circuitsubstrate

B. Magnetic field
caused by power
supply line,transformer,etc.

Deep Groove Ball Rotary encoder unit
Bearing
Electromagnetic noise charged directly to circuit substrate
C. Electromagnetic waves
caused by wireless device
antenna, household appliances,
etc.
Electromagnetic noise

D. Electromagnetic waves
caused by wireless device
antenna, household
appliances, etc.

Electromagnetic noise
charged directly tocircuit substrate &Electromagnetic noise
charged to circuit
substrate via signal line
and power supply line

Deep Groove Rotary
Ball Bearing encoder
unit

Electromagnetic noisecaused by electricmotor turning on/off
and lightning strike,etc.

Deep Groove
Ball Bearing

Rotaryencoderunit

Signal processing
equipment such as
decoder

E. Electromagnetic field fluctuation
due to lightning surge
Signal processing
equipment such as
decoder

Electromagnetic noise
charged to circuit substrate
via signal line and power
F. Reverse electromotive
supply line from otherforce generated bymachinery or outdoor
switching of machinery
power line

Fig. 13 Servo motorthat uses coils, etc.

Fig. 12 Types of electromagnetic noise
Table 3 Resistance evaluation test to electromagnetic noise
Test
No.
1
2
3
4
5
6

Test standards and name
IEC 61000-4-2 : 2008
Electrostatic discharge immunity test
IEC 61000-4-4 : 2012
Burst immunity test
IEC 61000-4-5 : 2014
Surge immunity test
IEC 61000-4-8 : 2009
Power frequency magnetic field immunity test
IEC 61000-4-6 : 2013
Radio frequency conducted disturbances immunity test
IEC 61000-4-3 : 2006
Radiated, radio-frequency electromagnetic field immunity test

Assumedelectromagnetic
noise

Test conditions

A in Fig. 12

4 kV

F in Fig. 12

1 kV

E in Fig. 12

1 kV

B in Fig. 12
D in Fig. 12
C and Din Fig.12

Results

No damage possible
to conform with
EMC standards(IEC 61000-6-2)

30 A/m(50 Hz/60 Hz)
10 V
0.15 to 80 MHz
10 V/m（80 to 1 000 MHz）
3 V/m (1.4 to 6.0 GHz)NTN TECHNICAL REVIEW No.89

45

Development of Multi Track Magnetic Encoder Integrated Rolling Bearing
6. Expanding the developed product series
The developed product can be applied to compact
robot joint mechanisms such as collaborative robots
and service robots. In addition to using the deep
groove ball bearing 6907 introduced in this paper,
NTN plans to expand the series from bearing bore
diameter 15 mm to 45 mm.

7. Summary
The demand for robots, including collaborative
robots and service robots, is on the increase, andmarket demands are becoming more sophisticatedand diverse.
To respond to the above needs, NTN has developed
a Multi Track Magnetic Encoder Integrated Rolling
Bearing that integrates a deep groove ball bearing andmagnetic rotary encoder using Multi Track Magnetic
Ring technology. This enables robots that are morecompact and lightweight while also reducing manhours to assemble and setup the robots.
NTN will continue to work on developing
products that contribute to further improvementsin performance of industrial machinery, including
robots, and support the advancement of technology
for further enriching peopleʼs lives and technology forprotecting the global environment.

Photo of authors

46

Hiroshi OKUMURA

Hiroyoshi ITO

Yasuyuki HAMAKITA

Robotics & Sensing
Engineering Dept.,
Industrial Business
Headquarters

Robotics & Sensing
Engineering Dept.,
Industrial Business
Headquarters

Robotics & Sensing
Engineering Dept.,
Industrial Business
Headquarters

NTN TECHNICAL REVIEW No.89

References
１） Keiichi Ueda, Technical Trend of the Precision
Bearings for Machine Tools, Tool Engineer, Vol. 60
No. 16, (2019) 41-43.
２） Makoto Oebisu, Hiromichi Kokumai, Yasuyoshi
Hayashi, Masato Tsujihashi, Technical Trend
and Features of the Bearing for Robot, NTN
TECHNICAL REVIEW, No.86, (2018) 34-39.
３） Hiromichi Kokumai, Hideaki Tanaka, Kosuke
Suzuki, Yuichiro Kawakami, Approach to
Development of Robot Joint-related Products,
NTN TECHNICAL REVIEW, No.88, (2020) 27-32.
４） Takashi Koike, Yasuyuki Fukushima, Yusuke
Shibuya, Hiroyoshi Itou, Development of Multi
Track Magnetic Ring for High Accuracy Absolute
Angle Detection, NTN TECHNICAL REVIEW,
No.86, (2018) 45-49.
５） Hiroyoshi Itou, Takashi Koike, Bearings with
Rotational Sensors, NTN TECHNICAL REVIEW,
No. 69, (2001) 108-116.
６） Yasuyoshi Hayashi, Development of Robotic
Joint-Related Products, Bearing & Motion-TechMagazine, No.035, (2022) 25-27.
７） iC-Haus GmbH，iC-MU off-axis nonius encoderwith integrated hall sensors８） Shoji Itomi, Hiroyoshi Itou, NTN Sensor Units forConstruction Machine, NTN TECHNICAL REVIEW,
No. 76, (2008) 118-125.
９） Takashi Koike, Yasuyuki Fukushima, Yusuke
Shibuya, High Accuracy Absolute Angle Sensor
for Use in Absolute Angle Detection on Robot
Joints, Vol. 62 No. 3, (2018) 55.
10）Electromagnetic compatibility (EMC) - Part
6-2，Generic standards – Immunity standard for（2016）．
industrial environments IEC 61000-6-2，