TECHNICAL PAPER

Development of Auxiliary Power Supply System
for Electric Power Steering
F. SATO

M. HIGASHI T. SUGIYAMA

When employing an electric power steering (EPS) system on a large vehicle, issues include an increase in theelectric power load of the vehicle during stationary steering (steering while parked), and lowering of assist torqueresponsiveness due to lack of electric power.
An auxiliary electric power supply system for EPS is being developed to solve these issues.
The purpose of this system is to achieve a high power output EPS system which compensates momentarily
required electric power with an auxiliary power supply.
This report describes an outline of the higher power output system and vehicle test results of the auxiliaryelectric power supply system, as well as application development to lower the vehicle power supply load and ensure
safety within the redundant power supply system.
Key Words: electronics and control, electric power steering, auxiliary power supply system, high power output

1. Introduction
Motorization of automobiles has progressed in recent
years due to environmental concerns. In electronic
power steering (EPS) as well, demands are increasing
for installation on large size vehicles (SUVs, minivans,etc.) in addition to small size vehicles. As steering axial
force increases with increased vehicle weight, however,
the installation of EPS on large size vehicles requiresconsideration of increased load on the vehicle power
supply during stationary steering (steering maneuvering
when parking), as well as reduction in responsivenessof the assist torque during rapid steering action.
Furthermore, if for example the input electric powerof EPS were to be temporarily cut off in a large size
vehicle, it would cause a massive and immediate change
in steering force, and therefore safety assurance during
reduced maneuverability is considered as one of theissues regarding EPS.
Due to these reasons, we are currently engaged in thedevelopment of an auxiliary power supply system for EPS
that achieves the following three functions.
1) EPS with high power output (Ensures responsivenessof assist torque during rapid steering action)2) Peak shaving of electric power (Levels vehicle power
supply load during stationary steering)3) Power supply backup (Maintains EPS function throughredundant power supply)This paper first describes the details of the developmentof EPS with high power output using an auxiliary power

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)supply system, and the results of its evaluation. It thenintroduces the practical application of the developed
technology in an overview of peak shaving of electric
power and power supply backup.

2. Issues regarding EPS installation onlarge size vehicles
2. 1 Increased load on vehicle power supply
The configuration of the EPS system is shown in
Fig. 1. The electronic control unit (ECU) controls the
motor based on the values of steering torque and motor
angle that are detected by the sensor, and outputs theappropriate assist torque for steering in order to lessen
the steering burden on the driver. The electric power
necessary for EPS is, as for other electrical equipment
for automobiles, supplied by the vehicle power source(alternator and battery).
・12V battery
・Alternator
Vehiclepower supply

Other electricalcomponent

Steering torque assist

ECU

Motor
Angle
sensorSteering
torque sensor
EPS system

Fig. 1 EPS system block diagram39

−Development of Auxiliary Power Supply System for Electric Power Steering−

Power consumption(Normalization)

1.0
0.8

Large

comfortably. This is caused by a decrease in assist torqueresponsiveness to the fast steering maneuvers of thedriver. This phenomenon is highly likely to occur in large
size vehicles and it is therefore imperative that the designof EPS takes into account assist torque responsiveness inorder to reduce the steering burden on the driver.

Steering torqueThe results of measured maximum electric power
consumption of electronic equipment for automobiles
are shown in Fig. 2. These results show that among
the equipment, EPS has the smallest maximum power
consumption when driving in a straight line, but has thelargest during stationary steering. Power consumption
temporarily increases during stationary steering as moreassist torque is required for larger and heavier vehicles.
This causes increased load on the vehicle power supply,
and as such is an issue within EPS installation on large
size vehicles that must be resolved.

③
②
①

0.6
0

0.4

0

0.2

Fig. 4 Peak value plot of steering torque against steering
angle speed3)

(S

tat

ion

ar
ys E
t P
Ai eeri S
r c ng
on )
dit
ion
He er
Po adlig
we
h
rw t
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Ha R ndo
za adi wardwa tor f
rn anin
Re g la
m
ar
de p
fog
ge
r
W
i
p
Br
ak er
el
a
Ca mp
ra
ud
io
dr Strivi aing gh EPco t lin S
nd e
itio
n

0.0

Fast
Steering angle speed

0.03

Fig. 2 Comparison of maximum electric power
consumption1), 3)

3. 1 Principle of high power output

2. 2 Decrease of assist torque responsiveness+ Right

Fast←0→Fast

Steering angle
speed

To check the responsiveness of assist torque, we
conducted a test using an actual vehicle. The test involved
turning the steering wheel (SW) left and right during
stationary steering, and measuring steering angle speed
and steering torque while gradually increasing steering
angle speed. The results, displayed in Fig. 3, show that anincrease in steering angle speed causes the absolute value(for example, ①, ②, and ③) of the maximum amplitudeof steering torque to increase as well. The results of anevaluation of this relationship are shown in Fig. 4. Fromthese results, we can see that the SW becomes difficult
to maneuver if the steering angle speed of the driver
exceeds a certain value, inhibiting the ability to steer− LeftLarge←0→Large

Steering torque①
Time, s

0

②

③

60

+ Right− Left0①Time, s②③

3. Auxiliary power supply system
The following is an explanation on the phenomenonof steering torque increase due to decreased assist torqueresponsiveness, using characteristics diagrams (N-T
characteristics diagrams) of the rotational speed of the
EPS motor and the torque. In Fig. 5 (a), the point (rated
point A) where the N-T characteristics of the EPS motor
for midsize cars (line A) intersects with the maximum
electric power able to be supplied by the vehicle power
supply (dashed line) is the maximum rotational speed at
the rated torque. When rated torque is increased (line B)
in order to support larger vehicles, rated point A changes
to rated point B (arrow ①). As a result, assist torqueresponsiveness in large cars decreases in comparison to
midsize cars due to a reduction in maximum rotational
speed (arrow ②) at the rated torque. This in turn causes
steering torque to increase.
In general, motor rotational speed is dependent on
induced voltage, and it is therefore possible to create amethod to increase motor input voltage (higher voltage)in order to improve assist torque responsiveness. A
characteristics diagram with increased input voltage isshown in Fig. 5 (b). When voltage is increased, rated
point B changes to C (arrow ③), allowing rotational
speed to be increased while still maintaining rated torque.
Through this, we have enabled a countermeasure against
decreased assist torque responsiveness within fastersteering ranges.

60

Fig. 3 Steering angle speed and steering torque transition3)

40

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)

−Development of Auxiliary Power Supply System for Electric Power Steering−

3. 3 Selection of auxiliary power supply

Torque (rack force)

Electric power consumption

Stationary steering is performed mainly when parking
and entering a garage. These actions involve rapid
steering angle speed and often involve consecutive
steering maneuvers. To implement steering force assist
continuously in these situations, it is necessary to utilize
an electric power storage device that enables quick
charging on the auxiliary power supply. We conducted acomparative study on three types of electric power storage
devices with good results in automobiles, including a leadbattery, lithium-ion battery, and capacitor. Table 1 shows
the results of the study. Based on Table 1, we decided to
employ the capacitor, which has high output density, canbe charged quickly, and maintains a long battery cycle
life.

Rated point B

Line B
Large carLine A
Midsize car①
Rated point A②
Rotational speed (response)

Torque (rack force)(a) Basic characteristics3)

Large carRated point B
Rated point C③

Midsize car

1)

Table 1 Comparison of electric power storage devices

Rotational speed (response)

Capacitor(b) Characteristics with addition of electric voltage3)

Fig. 5 N-T characteristics of EPS assist motor

3. 2 System configuration
To achieve higher voltage, we may at first consider
increasing the voltage of the actual power supply of thevehicle. However, in consideration of demands for EPS
with high power output (high voltage) at the present 12
V vehicle power supply, we decided to install a separate
auxiliary power supply solely for EPS, without modifying
the vehicle power supply. A block diagram of this system
is shown in Fig. 6. The system is configured with the
ECU, which controls the auxiliary power supply as well
as charging and discharging, located on the power supply
line between the vehicle power supply and EPS.
The charge-discharge ECU monitors the power suppliesof EPS and the vehicle. If EPS power is insufficient, thedischarge circuit is activated, immediately supplementing
the EPS input power with the power of the auxiliary
power supply. To prevent insufficiency of the power fromthe auxiliary power supply, the charge circuit is activatedwhen the voltage of the auxiliary power supply decreases,
and charges the auxiliary power supply system using thevehicle power supply. This resolves the two issues of thepreviously stated increased load on vehicle power supply,and decreased assist torque responsiveness.
Auxiliary
power supply
ChargeChargeChargecircuit
Vehiclepower supply

Discharge

CPU

Discharge
Discharge
circuit

Energy density, Wh/kg

4〜5

Lead battery Li-ion battery

25〜35

40〜70

Output density, W/kg
(25℃)
1 800〜2 000

300〜400

1 500〜2 000(−15℃)

900〜1 100

200〜300

300〜500

Cycle life, cycles

100 000 <500 <1 000 <

3. 4 Charge-discharge control method
As stated previously, stationary steering maneuvers
are conducted consecutively; when steering with a fixedsteering angle speed, stationary steering force generally
increases in response to the rise in SW steering angle,
which in turn increases the necessary electric powerfor EPS. Therefore, as shown in Fig. 7, we constantly
monitored EPS energy consumption, within which weestablished a power threshold value. If EPS energy
consumption exceeds the threshold value, then power is
supplied by the capacitor. If EPS energy consumption
falls below the threshold value, then the capacitor ischarged by the vehicle power supply. In summary, we
have implemented controls to conduct charging anddischarging for each steering maneuver as necessary.
As a result, the effects of steering assist are able to be
maintained during consecutive steering maneuvers evenwhen using a capacitor, which has low energy density.
These results are visible in Fig. 8. Without charge
control, repeated stationary steering causes capacitor
voltage to rapidly decrease. In contrast, with charge
control, capacitor voltage is maintained even through 20
consecutive stationary steering maneuvers, indicating that
there is no power insufficiency in the capacitor.

EPS
Charge-discharge ECU

Fig. 6 Block diagram of auxiliary power supply system
for EPS3)

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)41

Discharge
Supply from
capacitor

1

threshold
Supply fromvehicle power
supplyCharge0TimeFig. 7 Transition of electric power consumption(Charge-discharge control)1)

As shown in the figure, we have confirmed that asudden rise in EPS input voltage causes variation insteering torque. Furthermore, this torque variation
caused discomfort to the driver when performing actual
steering maneuvers. As a countermeasure against this, weestimated EPS electric power according to steering angle
speed as shown in Fig. 10, and conducted a study on
control methods to enable a smooth change of discharge
voltage.
Discharge(Continuous steering in parking)12+
Capacitor Discharge
circuitWith charge control

Capacitor voltage, V

Discharge voltage map
EPS input
voltage

EPS power consumption(Normalization)−Development of Auxiliary Power Supply System for Electric Power Steering−

Vehiclepower supply

Without charge control

Steering angle
speed

EPS

Fig. 10 Proposed countermeasure for discharge control3)
0

0

2

4

6

8

10

12

14

16

18

20

Steering cycle, Time

Fig. 8 Capacitor voltage transition with/without charge
control1)

3. 5 Discharge control issues and countermeasures
The discharge control method involves supplementing
EPS input voltage with voltage from the capacitor and
immediately increasing voltage when the abovementioned
EPS power consumption exceeds the power threshold.
This poses concerns about what effect the sudden change
in voltage during steering will have on steering feeling.
Therefore, to confirm the influence on steering torque,
we installed the developed system on an actual vehicle
and conducted measurements of EPS input voltage andsteering torque during stationary steering. The results ofthese measurements are shown in Fig. 9.

4. Evaluation test on actual vehicle
4. 1 Confirmation of effect of countermeasuresagainst torque variation
We introduced, on an actual vehicle, the discharge
control with countermeasures against torque variation thatis described in the previous paragraph. Measurements
were conducted, the results of which are shown in
Fig. 11. This figure represents EPS input electric power
measured under the same testing conditions of the actual
vehicle measurement results shown in Fig. 9, and time
sequence data of steering torque. These results confirm
that the variation in steering torque visible in Fig. 9 has
been suppressed in Fig. 11. In addition, we have enabledsmooth steering which does not cause the driver to feel
discomfort when performing actual steering maneuvers.
Discharge voltage control

4

12

Steering torque, N·m

Voltage
TorqueEPS input voltage, V

Steering torque, N·m

Voltage
Torque

24

0
0

0
1.0

0
Time, sFig. 9 Steering torque fluctuation at change of control(before countermeasure)3)

42

12

4

EPS input voltage, V

Discharge
8

24

8

0
1.0

0
Time, sFig. 11 Steering torque fluctuation at change of control (after
countermeasure)3)

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)

−Development of Auxiliary Power Supply System for Electric Power Steering−

Large

Without discharge control

With discharge control

0

Steering torque

Fast

0
Steering angle speed

Fig. 12 Measurement results of steering angle speed against
steering torque3)

5. Practical application
5. 1 Peak shaving of electric power
The electric power taken from the vehicle power supply
increases temporarily during steering when parking, andconsequently peak shaving of electric power is utilized asa means of reducing the load on the vehicle power supply,
by supplying power from the capacitor. The evaluation
results of peak shaving on an actual vehicle are shownin Fig. 13. The results shown were obtained using thesame method as the actual vehicle evaluation test for highpower output, and aimed to reduce peak electric power
during steering when parking by 50% by connecting
multiple capacitors in sequence. Within the figure, thex-axis indicates time, and the y-axis indicates the electric
power supplied from the vehicle power source (solid line)
and the steering angle speed (dashed line). These results
confirm that, when compared with no electric powercompensation (conventional system), electric power
assisted by the capacitor (developed system) achieves
leveling of electric power supplied from the vehicle power

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)

Conventional system

100%

50%

threshold

Development system
0

Steering angle speed

Steering angle speed

Fast

supply, as well as approximately 50% peak shaving of
maximum electric power.

0.5

1.0

Slow

To confirm the effects of high power output, we
implemented an evaluation test on assist torqueresponsiveness in the same method as the abovementioned
Fig. 4. We also connected multiple capacitors in series
with the aim of doubling assist torque responsivenessduring steering when parking. The results of theevaluation test are shown in Fig. 12. In this figure, wehave plotted the maximum steering torque when steering
angle speed is changed. The results show that with
discharge control, steering torque does not increase untila higher steering angle speed is reached, in contrast to thecase without discharge control. These results confirm thatthe power assist provided by the capacitor improves assist
torque responsiveness by twice its original amount.
In the following paragraphs, we will introduce peakshaving of electric power and the power supply backup
function as examples of the practical application of thedeveloped system.

The power by thealternator and battery

4. 2 Confirmation of effect of high power output

1.5

2.0

Time, sFig. 13 Measurement results of power consumption1)

5. 2 Power supply backup
The purpose of the power supply backup function is, inthe event of a failure of the vehicle power supply system,to ensure a smooth transition to the auxiliary power
supply in order to temporarily maintain EPS function, sothat steering torque variation will not occur if the power
supplied to EPS is temporarily cut off. The method forevaluating this function involved deliberately cutting off
the input electric power of the EPS while keeping the
SW turned during circular driving. The results of thisevaluation are shown in Fig. 14.
In the conventional system shown in the figure to theleft, the EPS function ceases when electric power fromthe vehicle power supply is cut off (①), causing loss of
assist torque and increase in the steering torque burden on
the driver (②).
In contrast, as seen in the figure on the right, theinstallation of an auxiliary power supply system mitigates
the effect on the driver by switching the power supply
to the capacitor the moment electrical power is cut off,
which maintains electric power supply to EPS, thus
preserving EPS function and suppressing sudden variationin steering torque (④).

43

−Development of Auxiliary Power Supply System for Electric Power Steering−

Power backup system
Power-supply
voltage, V①↓Power-off
・Circle driving
・Speed：20km/h
020

Steering
torque, N・m

Steering
torque, N・m

Power-supply
voltage, V

Conventional EPS system
20

0
160

③↓Power-off

0
20⑤ Torque limit
④

0
160

Steering
angle, deg

Steering
angle, deg

②

100

20

0

100

30

0

30

Time, sTime, sFig. 14 Power-off test results

6. Conclusion

References

We have developed a prototype charge-discharge
ECU installed with a capacitor as an auxiliary power
supply, and conducted evaluations on the high output
function of this developed ECU. Through the resultsof the evaluation, we have confirmed an improvement
in steering assist responsiveness during rapid steering
action without increasing the electric power taken fromthe 12 V vehicle power supply, which has been achievedby temporarily supplying power from the capacitor. As
a result, we believe this system may serve as a proposal
for countermeasures against insufficient electric power,
which is an issue in the installation of EPS on large
size vehicles. We plan to improve the functions of thedeveloped system in order to expand the applicable rangeof EPS to include large size vehicles.
In addition, we introduced electric power peak shaving,which reduces maximum electric power during steering
when parking, as well as a power supply backup function,
which prevents sudden change in steering torque when
the power supply is cut off. We plan to continue studies
on both of these functions in order to optimize the entire
system we have developed.

1)

2)

3)

S. Shinoda, M. Higashi, T. Sugiyama: Developmentof the electric power steering with an auxiliary power
supply, JSAE Annual Congress (Spring) Proceedings, no.
4-14, 20145065, p. 1-3 (2014)
IEEJ Investigation committee of 42V power supply: 42V
technology of automobile power supply, Ohmsha, Ltd, p.
85-86
F. Sato, M. Higashi, T. Sugiyama: Development of
Auxiliary Power Supply System for Electric Power
Steering, JSAE Annual Congress (Spring) Proceedings,
no. S307, 20155307, p1625-1628 (2015)

F. SATO****

44

M. HIGASHI**

T. SUGIYAMA

A d v a n c e d D e v e l o p m e n t C e n t e r, E n g i n e e r i n g
Headquarters
S y s t e m s I n n o v a t i o n R & D D e p t . , R e s e a rch &Development Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1013E (2016)