TECHNICAL REPORT

Development of Torque Sensor Integrated
with Plastic Sensor Housing
S. KOYAMA N. MATSUI

H. KUTSUMI

M. KAJITANI Y. TOYAMA

Demands for energy saving in automobiles due to environmental concerns have led to the necessity of downsizing
automotive parts. As such, there is a demand for lighter weight electric power steering (EPS). By integrating thetorque sensor and torque sensor housing used in EPS, we have achieved a 20% reduction in the weight of the torquesensor housing assembly compared with the conventional type. This report highlights the results of the developmentof this product.
Key Words: electric power steering, torque sensor, Hall IC, sensor housing, resin, integrate

1. Introduction
From the perspective of reducing the burden placed on
the environment, there has been an ongoing movementin the industry to make automobiles more energy-saving.
As part of this, power steering systems are shifting fromhydraulic power steering to electric power steering (EPS).
Since 2006, JTEKT has mass produced the Hall IC torque1)
sensor for use on column-type EPS as an EPS system
torque sensor and has continuously engaged in the below
initiatives.
・ Mass production of the waterproof Hall IC torque2)
sensor for use on pinion EPS since 2007 .
・ In response to a strong demand in recent years toconsider the environment, switched to mass productionof plastic torque sensor housing instead of aluminum3)
since 2013 .
・Most recently, development of an integrated production
comprising the plastic torque sensor housing andtorque sensor in response to a demand for further3)
weight reduction .
This paper will introduce the results of developing thisproduct; the torque sensor integrated with plastic sensor
housing.

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

2. Explanation of the Hall IC TorqueSensor Mechanism
2. 1 EPS System Structure
Figure 1 shows the structure of the EPS system. The
key electrical components of an EPS system are thetorque sensor, control unit and motor.
Steering wheel
Input shaft

Input torque
Torque signal Control unit

Torque sensor
Current
control

Reduction gear

Vehiclespeed
signalMotorTire
PinionTie rod

RackFig. 1 Structure of EPS system

The steering force from the steering wheel is detectedby the torque sensor and that torque signal is sent tothe control unit. The control unit controls the amountof electrical current that flows through the motor in
response to the torque signal and vehicle speed signal
from the vehicle, and then assists steering by applying the2)appropriate assisting force .

43

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2. 2 Structure of the Hall IC Torque Sensor
The Hall IC torque sensor comprised of a ring coremounted on the sensor housing, a magnetic yokeassembly mounted on the pinion shaft and a magnet3)
mounted on the input shaft (Fig. 2) .

Torsion barHall IC

MagnetYoke
Ring core

3. Structure of the Integrated PlasticSensor Housing
The conventional structure is to have the plastic torquesensor housing and the torque sensor separated, howeverin such a case, as Fig. 4 shows, there was a need to use
two bolts to assemble the housing portion comprising aring core as the magnetic circuit and collar to mount thetorque sensor integrated with a plastic mold to the torquesensor integrating a sensor for detecting a magnetic fieldfrom the ring core and plate for assembly. Meanwhile, inthe case of the structure of the integrated plastic sensor
housing developed on this occasion, as Fig. 5 shows, theterminal combining a Hall IC and circuit is covered with aholder and the housing is molded based on this holder tointegrate the housing with the torque sensor. As a result,the plate, collar and bolts have been eliminated.

Fig. 2 Structure of Hall IC torque sensor
HousingCollar
Plate

2. 3 Detection Principle of the Hall IC TorqueSensor
When the steering wheel is not being operated, theyoke core (teeth) of the magnetic yoke assembly shortcircuits the magnetic flux of the magnet, therefore
magnetic flux is not transmitted from the ring assembly(ring core) to the Hall IC.
When steering torque is input, a relative angle
difference emerges between the magnet and magnetic
yoke assembly and the magnetic flux of the magnet istransmitted to the ring assembly (ring core) from themagnetic yoke assembly (yoke core). Steering torqueis detected when the magnetic flux proportional to thetwisting angle of the torsion bar is transmitted to the HallIC in between the protruding parts of the ring core. The3)
detection principle is shown in Fig. 3 .

Torque sensor

Housing and torque sensor are separated

Fig. 4 Structure of conventional product

Holder(Hall IC, circuit)

Housing
Torque sensor

Integrated molding

Housing and torque sensor are integrated

Fig. 5 Structure of developed product

MagnetN pole

Yoke outer edge

Hall ICS pole

Ring core

Magnetic flux passing through the Hall IC
Input torque

Yoke toothAngle difference emerges
between yoke and magnet

Fig. 3 Principle of detection in Hall IC torque sensor44

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

−Development of Torque Sensor Integrated with Plastic Sensor Housing−

Holder cross-section

4. Merits of Integration
By developing an integrated design and eliminating a
number of parts, we succeeded in reducing the weight ofthis product. In addition, the following two merits wereobtained.
①Reduction of the tolerance for the positions of the Hall
IC and magnetic circuit due to the reduction of partsin the product. This makes it possible to supply a morestable magnetic flux to the Hall IC and improve torquesensor output accuracy.
②Conventionally, water resistance was secured by using
an O ring to assemble the housing and torque sensor,
however there was concern about the deteriorationof the O ring. By integrating the housing and torquesensor, water resistance has been improved and semipermanent water resistance has been secured (Fig. 6).

Assembly of the housing and torque sensor ismade water resistant with an O ring
Conventional productNo assembly
DevelopedproductFig. 6 Improved water resistance

RibHall IC

Fig. 7 Hall IC preservation countermeasure

5. 2 Prevention of the Cracks that Form in theHousing
The magnetic shield (steel) and housing plastic (PA6T/6I), which pass through a process of insert molding,have differing linear expansion coefficients, therefore
when the temperature changes during the hot and cold
shock tests, the edge of the magnetic shield is subjected
to excessive stress, causing cracks to form. One of thelimitations on molding conditions from the perspective ofprotecting Hall IC, as mentioned in 5. 1 above, included
these cracks that form in housing, as shown in Fig. 8,
which were discovered from the initial development stage
when conducting these tests. As a countermeasure, theportion where stress becomes concentrated was covered
with a holder and the design was changed to prevent
plastic flowing inside the housing during integration moldformation to prevent plastic from contacting the edgeof the magnetic shield during molding, thus preventing
concentration of stress. This countermeasure made it
possible to prevent cracks.

5. Integration Issues and theirCountermeasures

Magnetic shield edge

5. 1 Protection of Hall IC
The design of the developed product is such that theholder comprising of the Hall IC, etc. is molded together
with the housing and, due to this, the Hall IC may beaffected by the plastic forming pressure and forming
temperature when the mold is created. The Hall IC has
low resistance against external force, therefore the design
must prevent the Hall IC from being subjected to pressure
when the mold is being formed. Moreover, the forming
temperature must also be below the temperature thatthe Hall IC can withstand. To resolve this, as shown inFig. 7, the Hall IC was covered with a holder to prevent
it from being affected by the pressure and temperatureduring integration molding. In specific terms, a rib waspositioned around the periphery of the Hall IC storage
area which had the effects of increasing rigidity andcreating clearance with the Hall IC, thus avoiding contact
with the Hall IC during mold forming.

Before crack countermeasure

Developed crack from edge

HolderAvoid stressconcentration
through creatinga cavity

After crack countermeasure

Fig. 8 Countermeasures against cracking

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−Development of Torque Sensor Integrated with Plastic Sensor Housing−

5. 3 Securing Dimensional Accuracy
An FEM analysis elucidated that dimensional accuracydrops as a result of shrinkage after the product ismolded due to a high number of parts that are integrated
together through molding. To resolve this, we optimized
the rib position and molding conditions, thus securing
dimensional accuracy (Fig. 9).

Rib

7. Conclusion
This initiative involved the development of a torquesensor integrated with a plastic sensor housing for EPS
systems.
Through doing so, we further reduced the weightof vehicles and improved vehicle reliability. Moving
forward, we will promote the horizontal deploymentof this product, advance this technology and develop
products which are lightweight and reliable, more thanever.

References
1)Fig. 9 Rib position2)

6. Development Results3)

6. 1 Weight Reduction
By integrating the plastic sensor housing and torquesensor, weight was reduced by 20%.

Y. Nagahashi, A. Kawakubo, T. Tsujimoto, K. Kagei,
J. Hasegawa, S. Kakutani: JTEKT ENGINEERINGJOURNAL, No. 1003E (2007) 35.
K . H o tta , T. I s h ih a r a : J TEK T EN G I N EER ING
JOURNAL, No. 1007E (2010) 60.
N. Sasaguchi, T. Oishi, T. Iida, T. Yukimura, A. Takeuchi:
JTEKT ENGINEERING JOURNAL, No. 1011E (2014)31.

6. 2 Protection of the Hall IC Portion AfterMolding the Housing
A Hall IC with the worst dimensional accuracy wasused to mold the product using molding conditions
such as molding pressure and temperature below theirrespective upper limits then a cross-section was observed
to confirm whether or not the protruding parts of the ringcore were making contact with the Hall IC. As a result, itwas confirmed that there was no contact, therefore there
were no problems.

Y. TOYAMA*N. MATSUI**

M. KAJITANI***

6. 3 Results of Reliability Tests
Table 1 shows the results of the major reliability tests
performed. It was confirmed that there were no problems
for all reliability evaluations.
Table 1 Reliability check results
Test item

ResultHigh temperature storage test

OKLow temperature storage test

OK

Thermal shock test

OKHigh temperature and high humidity test

OKVibration test

OKShock test

OK46

H. KUTSUMI**** S. KOYAMA******

Electronics System Planning Dept., Steering Systems
Business Headquarters**Steering System Engineering Dept. 1, SteeringSystems Business Headquarters***Experiment & Analysis Dept.1, Steering Systems
Business Headquarters**** Casting & Forging Production Engineering Dept.,
Production Engineering Headquarters***** Steering Production Engineering Dept., ProductionEngineering Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)