Development of a Steering Angle and Torque Sensor of Contact-type
by Akira Noguchi * , Kosuke Yamawaki * , Toshiro Yamamoto * 2 and Tomoaki Toratani * 2

With the growing concern over the global environment in recent years,
environmental preservation technologies are required to be developed in thefield of automotive industries also. Accordingly, there is an increasing need for electric power
steering system (EPS) of low energy consumption to replace hydraulic power steering system
conventionally used. Because steering angle and torque sensors used in the EPS are basedon contact-type sensing, they tend to have such problems as delayed response caused by thefriction of the sensing brush against conductive resin in addition to friction noise. The authors
have developed, by studying structures of the rotor and the sliding portions, a low friction noise
sensor with outstanding electrical performance.

ABSTRACT1.

INTRODUCTION

In modern automobiles in which comfortability andsafety of steering are highly required, power steeringsystem has become an essential component enabling
adjustment of steering assisting force whenever needed
during stop time and low- or high-speed driving. The
hydraulic power steering system conventionally used
for steering control consumes, since a hydraulic pump
has to be driven full-time, 4~5 HP constantly, whichcorresponds to a loss of about 3 % in terms of fuelconsumption. The electric power steering system (EPS),
in contrast, consumes less energy since its assist motor
is driven at the time of steering operation only, and also
has an advantage of space saving due to its simplified
structure. Thus the demand for this system is expected to
rapidly grow in future.
In EPS a steering angle and torque sensor is needed,which detects rotation torques applied on the torsion
bar of the steering shaft together with steering angles(rotation angle) at the time of steering, and subsequently
inputs these torque and angle signals, in order todetermine the control output, into electronic control unit
(ECU) that controls the rotation force of the assist motor.
The steering angle and torque sensor reported here has
been designed and developed against this background.

2.

REQUIRED SPECIFICATIONS

Figures 1 and 2 show the mechanism for torque andangle sensing of the sensor developed here. A torque-sensing pattern is printed on one side of the resistive
element substrate that rotates combined with the columnside rotor, and an angle-sensing pattern is printed on
the other side. The torque-sensing side is in contact
with and slides on brushes provided on the steering������������� �����
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Figure 1

Torque and angle sensing mechanism (I).

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* 2 Advanced Production Systems Center, Plant and Facilities
Div.

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Development of a Steering Angle and Torque Sensor of Contact-typeside rotor, while the steering-angle side is in contact
with other brushes provided on the output substrate in
the sensor housing. Thus the contact positions of thebrushes with the substrates change according to the twistof torsion bar and the rotation of steering wheel during
car driving, so that torque and angle signals are output in
conformity with the required specifications. Indispensable
performance for this sensor includes:
1) Steerability: To supply signal outputs of fast
response and high accuracy so as to offer acomfortable steering feeling during car driving.
2) Durability: To supply noiseless output signals over
several hundreds thousands of kilometers of distancedriven.
3) Quietness: To reduce friction noise generated by thesensor.
This report describes the development as well as the
improvement of steerability and quietness.

3.

SOLUTIONS TO THE REQUIREDSPECIFICATIONS AND THE RESULTS

3.1 Steerability
3.1.1 Objectives in Steerability Improvement
Because in EPS the steering assisting force is adjusted
based on the output signals from its sensor, the sensing
function of an EPS sensor is very influential over theentire system. If the sensor has poor responsiveness of
output signals to shaft rotation and oscillation during car
driving, the smoothness of steering feeling will be lost
sometimes resulting in a scratchy feeling of steering.
The responsiveness of output signals depends on the
tracking performance of the sensing brushes that slide,
as the shaft rotates and the torsion bar twists according
to steering, on the slip rings of the rotating substrate. The
responsiveness tends to deteriorate as the frequency ofsteering shaft oscillation increases, making it an essential
task for development how to maintain the signal response
fast at high frequencies whereby the shaft rotates. Factors
in degradation of signal response include:1) Backlash in the shaft bearing portion comprising a
number of constituting parts
2) Sliding friction at the bearing portions for thesteering-side rotor and for the column-side rotor3) Inertia due to rotation
4) Deteriorated stiffness of constituting parts such as
the steering-side rotor
Based on these standpoints, we proceeded to fabricate
prototypes and evaluate their performance so as tooptimize the design.
3.1.2 Measures for Improving Signal ResponsivenessIf the backlash and the sliding friction mentioned above
could be eliminated, signal delay might not be generated.
However, it is impossible to completely remove thesedegradation factors since both the sensor and thesteering shaft are mechanical elements. In practice,signal responsiveness of the sensor itself was improved������������� �����
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Figure 3

Coupling structure connecting the sensing portion
with the shaft.

to a certain degree by countermeasures such that thebacklash in the shaft bearing portion is reduced as muchas possible by fine tuning the molding dies to improve thedimensional accuracy and tolerance of constituting parts,and friction is decreased by finely polishing the molding
dies for the bearing portion to minimize the sliding
surface roughness of the bearing portion.
The inertia due to rotation was dealt with by reducing
the weight and volume of the steering-side rotor as muchas possible, but we had to be satisfied with a certain
degree of reduction.
With respect to the stiffness degradation of constituting
parts, the signal responsiveness would significantly
improve if all the structures from the fitting portion of thesteering-side shaft to the actual sensing portion wereconnected using a rigid body without elasticity. In theshaft structure shown in Figure 1, however, axial andangular misalignment in the X-Y plane readily occurs
between the steering-side and column-side shafts, so
that, if the steering-side rotor is composed of a rigid bodyand axial misalignment occurs between the shafts, asignificant load will be imposed on the bearing portionof the sensor, thereby increasing the sliding friction andadversely affecting the durability of the bearing portion.
We have decided, therefore, to set up a coupling of
metal spring between the rotor fitted on the shaft andthe sensing portion, as shown in Figure 3. Although thecoupling was intended to absorb the possible eccentricityof the shaft in the X-Y plane so as not to increase
the sliding friction, it was found that the result of anexperiment was unsatisfactory, in which a prototype using
this coupling structure was fabricated and integrated
with the shaft to confirm the signal responsiveness.
Accordingly, we immediately investigated the matter totake measures.
3.1.3 Design of Steering-side Rotor with Coupling
There were two factors to be considered in designing
the optimized structure of the steering-side rotor to meetthe requirements for signal responsiveness from thestandpoint of stiffness. One is to make allowance for

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Development of a Steering Angle and Torque Sensor of Contact-type������������ �� ������

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Figure 4

Result of FEM analysis.

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Steering-side rotor after design change.

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Design changes of coupling.

flexures of the steering-side and column-side shafts in
the X-Y plane in order to absorb the axial misalignmentof these shafts. The other is to provide the rotor with
sufficient stiffness in the direction of rotation to prevent
delayed response.
Accordingly finite element method (FEM) analysis was
carried out in terms of strain and stress to obtain the
optimized sensor configuration that could remove these
two factors simultaneously. After setting up material
properties and sensor configurations, torque and stressapplied on the steering-side rotor from the shaft were
input to analyze the stress and strain over the entire rotor,
and subsequently investigations were made based on the
result of the analysis to find out suitable changes of thecurrent design.
Figure 4 shows the result of an FEM analysis, andFigure 5 and Photo 1 illustrate the design changes in the
coupling structures effected based on this analysis. With
respect to the flexure in the X-Y plane, the wall thicknessof the R-shaped portion was greatly reduced to permit
flexures of this portion. As a result, it was proved that
this structure could absorb the backlash, suppressing
the growth of sliding friction between the parts in case ofoccurrence of eccentricity between the two shafts.
On the other hand, beads were built up to increase thestiffness of the entire coupling in the direction of rotation,and the wall thickness of the coupling as well as therelated rotor was increased reviewing the design of theComparison of the frequency characteristics ofsignal amplitude before and after design changes.

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Comparison of the frequency characteristics ofsignal phase delay before and after design changes.

entire steering-side rotor, and the result was that the shaft
rotation tracking performance of the brush provided at
the sensing portion was much improved. Thus even whenhigh frequency oscillation was imposed on the shafts, thesignal responsiveness was seen to be enhanced. Figures
6 and 7 show the improvements in signal responsivenessin terms of amplitude and phase versus rotation
frequency at the time of shaft oscillation.
Moreover, prototypes of the modified sensor were
installed on a test car to verify the steering feeling. The
experimental results confirmed that the prototype gave
no scratchy feeling during steering rotation, satisfactorily
meeting the requirement for steerability.

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Development of a Steering Angle and Torque Sensor of Contact-type

3.2 Quietness
3.2.1 Objectives in Quietness Improvement
Recently, car manufacturers are emphasizing quietness
improvement of vehicles as a whole, so that requirements
for individual parts constituting a vehicle are growing inseverity. With regard to the steering angle and torquesensor of contact-type also, reduction of the noise
generated by the sensor was stringently required from the
beginning of development. In particular the peak soundpressure at 3150 Hz, specified by JIS C 1513 as a center
frequency with third-octave-band, had been considered
problematic, necessitating an immediate investigation.
Noise generated by the sensor include the sliding
noise of the contact point between the brushes andthe substrate at the time of steering rotation and thatbetween plastic housings for the bearing portion of
the sensor. Specifically the noise between the brushes
and the substrate sounds very stridently since both of
the two surfaces are metallic. Accordingly investigation
was focused on identifying the cause of, and devising acountermeasure against, the noise between the brushes
and the substrate, under the suspicion that the peaksound pressure at 3150 Hz mentioned above coincidedwith this noise.
3.2.2 Brush Design Aimed at Sliding Noise Reduction
Design of the brush was investigated first.
In the first place, in order to measure the natural
frequency of the brush sliding over the substrate while
the steering shaft rotates, an experimental sensor was
fabricated which permits external observation of a brushin contact with a substrate, and vibration frequency of thebrush during sensor rotation was measured using a laser
vibrograph. The schematic of the measurement is shownin Figure 8.
As a result, it was confirmed that all the brushes to beinstalled had a natural frequency very close to 3150 Hz.
Simultaneously with this, FEM analysis was performed
to verify several vibration modes that are likely to appear
during brush sliding. Figure 9 shows a vibration modethat is most likely to appear. It corresponds to one of
the lateral vibration modes of the contact wire, which isprovided on the tip of the brush to constitute an actual
sliding interface, while the wire slides over the substratesurface, and its frequency of 3157 Hz is very close to
the frequency of 3150 Hz above mentioned. Whereas
vibration of the brush arm with large amplitudes was not����� ����������

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Figure 8

Schematic of vibration measurement for brush.

Figure 9Vibration mode of sliding brush (by FEM analysis).

observed in this mode, other modes were confirmed inwhich the brush vibrates vertically.
It was thus confirmed from the results and analysis ofvibration mentioned above, that the sound in question at
3150 Hz is generated by the following mechanism:• The contact wire on the brush tip vibrates reciprocally,
while it slides over the substrate surface, in the
direction lateral to the wire length.
• This vibration propagates to the brush arm, making
the wire and the brush arm as a whole vibrate at 3150
Hz.
• The brush arm amplifies the vibration to generate ahigh-pitched sound.
Based on this result of verification, we proceeded
to develop countermeasures against noise that are
realizable in consideration of mass production.
3.2.3

Substrate Plating Aimed at Sliding Noise
Reduction
Verification of substrate design was performed in parallelwith the verification of the brush design.
In addition to the fact that a peak sound pressure
was generated at 3150 Hz due to the sliding action of
the brush installed on the output substrate over thesurface of the resistive element substrate, it was foundthat variations of plating conditions of the substratesurface existed between the lots, resulting in a soundpressure difference of as much as 10 dB dependingon the lot. Accordingly comparisons in terms of plating
compositions and plating conditions were made to verify
the differences between the substrates with high soundpressure and those with low sound pressure.
Photos 2 and 3 show the plated surface conditions ofhigh sound pressure and those of low sound pressure,respectively. It could be confirmed that while the sampleof high sound pressure shown in Photo 2 had a roughsurface, that of low sound pressure shown in Photo 3 hada finely plated surface. An attempt was made, therefore,
to correlate the averaged surface roughness (R a) or
the maximum surface roughness (Rmax) with the soundpressure, but this idea was soon abandoned because nocorrelation was found. In an effort to find out a suitableindicator that is correlated with the sound pressure, we
paid attention to the sliding friction force with the brush.

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Development of a Steering Angle and Torque Sensor of Contact-type������ ������

Photo 2

Plated surface of high soundpressure substrate (× 250).

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Figure 11 Design change of brush.

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Photo 3

Plated surface of low soundpressure substrate (× 250).

Photo 4

Resistive element substrate coatedwith conductive resin.

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Figure 10 Measurement results of friction forces
between plated surface and brush.

The friction force between the brush and the plated
surface of the resistive element substrate was measured
using an exclusive instrument incorporating a force
gauge with a turntable, and the results are shown in
Figure 10. It was shown that the plated surface withlow sound pressure had a low sliding friction force withthe brush, so that we proceeded to investigate surface
treatment methods for low sliding friction force to achievesound pressure reduction.
3.2.4 Implementation of Countermeasures
We have investigated countermeasures against the soundpressure peak at 3150 Hz based on the verification ofbrush design and plated surface treatment, and decided
to take the countermeasures described below.

Firstly, based on the verification of the brush design,changing the configurations of the contact wire andthe brush arm was studied, and it became clear that
increasing the mass of the brush was most effective,
and this was included as one of the design changes.
Figure 11 shows changes in the brush configuration. In
view of the mounting space for the brush in addition tomaintaining the proven reliability of brush contact, an
increment of the brush weight was given by making thebrush arm configuration asymmetry.
What is more, to improve the surface treatment for
reduction of the sliding friction of the brush, the plated
surface was coated with conductive resin, and thetreatment method for the plated surface was changed. As
shown in Photo 4, coating the resistive element substratewith conductive resin can make the sliding surface
smooth because, even if the plated surface should berough, the resin will permeate the surface. Moreover, forplated surfaces where resin coating is not very effective, a
new plating method for low friction was adopted, whereby
chemical polishing treatment was carried out thoroughly
as a pre-process of the Ni and Au plating. This enabled
suppression of variations in surface roughness andsurface irregularities, resulting in the smooth surface of
plating as shown in Photo 5.
These measures brought about a considerable
reduction of the peak sound pressure of significant
magnitude at 3150 Hz. Figure 12 shows the results ofthird-octave-band frequency measurements before andafter the implementation of these measures.

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Development of a Steering Angle and Torque Sensor of Contact-type

4.

FUTURE DEVELOPMENT

The sensor developed here has achieved theimprovements in terms of steerability, durability andquietness as were required by our customers, and has
been in mass production since April, 2003. Photo 6
shows an appearance of the sensor. In the future, we plan
to reduce the cost as well as to improve its steerability
and quietness further.
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5.
Photo 5

Au plated surface of substrate after
countermeasure implementation (× 250).

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CONCLUSION

In the development of this steering angle and torquesensor of contact-type, which was required to provide
higher performance than conventional sensors, wehave been successful in overcoming many problems
and in launching into mass production through theconcentration of efforts of people involved. In view of theincreasing demand for EPS that is predicted, the demand
for this sensor is expected to grow also.
We would like to express our sincere gratitude to Tokyo
Cosmos Electric Co., Ltd. and concerned parties for theircooperation in developing this sensor.

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��������� ����Figure 12 Sliding friction noise of substrates before andafter countermeasure implementation.

Photo 6

Appearance of steering torque and angle sensor.

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