TECHNICAL REPORT

Plastic Sensor Housings for Hall IC Torque Sensor
N. SASAGUCHI T. OISHI T. IIDA T. YUKIMURA A. TAKEUCHI

Amid the growing demands for a lighter EPS system, we have developed a pinion type EPS (P-EPS) and a dual
pinion type EPS (DP-EPS) plastic sensor housing for Hall IC torque sensors. We have achieved housings that havethe capability to endure being installed in engine rooms. Moreover, when compared to the conventional aluminum
sensor housing, we have lightened the weight of the housings; 148 g (which is 52% reduction) for P-EPS, and 81 g
(which is 31% reduction) for DP-EPS.
Key Words: electric power steering, torque sensor, Hall IC, sensor housings, plastic, polyamide

1. Introduction
As part of the improvements to automobile environment
responsiveness, we are pursuing ever-increasing inquiries
1)for electric power steering (EPS) system. JTEKT beganmass production of Hall IC torque sensors for columntype EPS (C-EPS) in 2006, as a torque sensor for theEPS system. In 2007 we started mass production for awaterproof specification Hall IC torque sensor for piniontype EPS (P-EPS), and as such we currently support anEPS system corresponding to the needs of customers.
Nowadays there are also increasing demands forweight reduction within EPS systems. This has led to thedevelopment of plastic sensor housings for Hall IC torquesensors used in P-EPS and DP-EPS in particular, where
weight reduction demands are intensifying. We will
introduce this new technology below.

2. Explanation of Hall IC Torque Sensor
Mechanism2. 1 Explanation of Structure
The Hall IC torque sensor is structured from a board
assembly and ring assembly mounted to the sensor
housing, magnetic yoke assembly mounted to the pinion
shaft, and a magnet mounted to the input shaft (Fig. 1).

JTEKT ENGINEERING JOURNAL English Edition No. 1011E (2014)

Sensor housing

Board assembly

Input shaft (magnet)

Ring assembly
Pinion shaft(magnetic yoke assembly)

Fig. 1 Structure of Hall IC torque sensor

2. 2 Detection Principle
When steering is not being operated, the yoke coreof the magnetic yoke assembly (tooth) short circuits themagnetic flux, and therefore the magnetic flux cannot betransmitted from the ring assembly (ring core) to the HallIC.
When steering toque is inputted, the relative angle
difference is generated for the magnet and magnetic yokeassembly connected with the torsion bar, and the magnetic
flux of the magnet is transmitted to the ring assembly(ring core) from the magnetic yoke assembly (yokecore). Steering torque is detected when the magnetic flux
proportionate to the twisted angle of the torsion bar istransmitted to the Hall IC in between the protruding part1) 2)of the ring core . Detection principle is shown in Fig. 2.

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Magnet(S pole)

Magnet(N pole)

Ring core

Ring core(protruding part)

Unification

Ring assembly
Plastic sensor housingSensor housing(aluminum alloy)
Yoke core(outer edge part)

Yoke core(tooth part)

Conventional productHall IC(unification of ringassembly section)

Developed productFig. 3 Structure of sensor housing

3. 2 Demands for Environment-resistanceAs the sensor housing is equipped to the engine room
within a rough thermal environment outside of theautomobile interior, heat resistance and waterproof ability
must be taken into consideration. The environmentalresistance ability required for plastic sensor housings isshown in Table 1.

Torque input

Angle degree differenceis generated in yoke core
(tooth part) and magnet

Table 1 Demand for environment resistance

Magnetic flux passing
through the Hall IC

Fig. 2 Hall IC torque sensor detection principle

3. Plastic Sensor Housing
3. 1 Structural Changes
We have unified and created the conventional aluminumalloy sensor housing, and the ring assembly connected
to it, in a resin mold. The board assembly was utilized as
it is in present products. The structural changes from theconventional product are shown in Fig. 3.

Item

Requirement

Temperature andrelative humidity

Must be able to withstand the engine
room environment

Oil and chemicalresistanceMust have resistance to various typesof oil, grease, and battery ﬂuid, etc.

Airtightness

Must ensure the airtight ability
required by P-EPS and DP-EPS

3. 3 Material Selection
A semi-aromatic polyamide was selected as thematerial of use, based on its superior characteristics
within its environment of use, such as durability in hightemperatures, strength, heat resistance, and chemicalresistance. The resin utilized is the same as that for theprimary molded part and secondary molded part described
below in Item 3. 4. The characteristics of the material are
shown in Table 2.

Table 2 Material property
Present

Utilized

Aluminum alloy

Semi-aromaticpolyamide

PBT

PA612

[MPa]

228

190

114

168

Thermal[×10−6]
expansion coeﬃcient21

20 (parallel)
50 (Transverse)

20 (parallel)
90 (Transverse)

17 (parallel)
113 (Transverse)

Material
Tensile strength

Melting point[℃]

Density[kg/m3]

Reference−325

225

215

2.68×10−3

1.44×10−3

1.47×10−3

1.32×10−3*Numbers are typical values

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3. 4 Internal Composition

4. 2 Dimension Accuracy and Cracks

We have created a structure where we attach themagnetic shield and the ring core (a structural part of thering assembly) to the primary molded part, and conducted
an insert molding for this part to complete secondary
molding (Table 3).
By adopting the secondary mold structure, we wereable to secure position accuracy of the ring core, which
influenced the ability of the torque sensor, using a
relatively simple die structure.

Within the manufacturing of resin sensor housing,problems such as:• Decrease in dimension accuracy due to moldshrinkage• Crack after thermal shock test (welded part andmagnetic shield edge part)are anticipated to occur. Due to this, we reviewed
countermeasures based on FEM analysis, etc. beforehand,and performed the following countermeasures during
manufacturing.
• Optimization of gate position/rib position (Fig. 5)• Internal bending of magnetic shield edge part (Fig. 6)
Through the results of the above countermeasures, wewere able to satisfy the required dimension accuracy,
strength and durability (Table 4).

Table 3 Composition of Plastic Sensor Housing

Process

Primarymold

Magneticshield, ring coreattachment

SecondarymoldMagnetic shieldring core

ProductshapeRib addition

Fig. 5 Case example of rib addition

4. Topics in Development
4. 1 Waterproof Compatibility
Since primary molded part and secondary molded partare shaped from the same resin, immersion may occur
due to capillary action from the boundary face when theboundary is exposed to the outer part. On the other hand,with the Hall IC torque sensor, it is necessary to continue
the magnetic circuit by inserting the board assembly HallIC part into the protruding part of the ring core, which isembedded in the primary molded part.
By providing the seal face which connects the board
assembly and Hall IC to the secondary molded side, wehave achieved the connecting of the Hall IC and ring core,
without exposure of the primary molded part to the outerside to hinder waterproof ability (Fig. 4).
Primary molded part(further inward than the O ring seal face)

Ring core
Secondary molded part
protruding part (O ring seal face)

Shield edge madeinternally bent

Fig. 6 Structure of magnetic shield

Table 4 Results of reliability test
ResultNoTest Item

1

High temperature storage test○2

Low temperature storage test○

3Thermal shock test○

4High temperature and highhumidity test○

5Sea water acetic acid test(SWAAT)○

6Ice water test○

7

High pressure washing test○

8Oil and chemical resistance test○

Fig. 4 Structure of Hall IC insertion part

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5. Reduction in Mass
By unifying the shapes of the sensor housing and ringassembly, we have achieved weight reduction over thestructure of the conventional product (Item 3. 1), as well
as strength and durability (Table 5).
Table 5 Weight reduction of Development product for P-EPS
and DP-EPS

Reduced mass

For P-EPSFor DP-EPS

148 g(52% reduction)

81 g(31% reduction)

6. Conclusion
We have developed a plastic sensor housing forEPS, subsequently contributing to weight reduction inautomobiles. We plan to promote continuous development
for rising demands within these products.

References
1)

2)

Y. Nagahashi, A. Kawakubo, T. Tsujimoto, K. Kagei,
J. Hasegawa, S. Kakutani: JTEKT ENGINEERINGJOURNAL, no. 1003 (2007).
K . H o t t a , T. I s h i h a r a : J T E K T E N G I N E E R I N GJOURNAL, no. 1007 (2009).

N. SASAGUCHI*

T. OISHI**

T. IIDA***

T. YUKIMURA**** A. TAKEUCHI******Electronics Engineering Dept. 1, Automotive SystemsBusiness Headquarters** Experiment & Analysis Dept. 1, Automotive Systems
Business Headquarters*** Steering System Engineering Dept 1, Automotive
Systems Business Headquarters*** Steering Production Engineering Dept., Automotive*Systems Business Headquarters*** Casting & Forging Production Engineering Dept.,**Production Engineering Headquarters

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JTEKT ENGINEERING JOURNAL English Edition No. 1011E (2014)