TECHNICAL REPORT

Development of Column Coaxial Steering Actuator forCommercial Vehicles
K. YOSHIOKA T. YAMAUCHI Y. NONOGUCHIThe field of commercial vehicles is seeing expanding use of automatic operation systems and practicalapplication of advanced driver assistance systems. We are developing a Column Coaxial Steering Actuator tomeet increasing needs in this area. This actuator provides ADAS functions for large commercial vehicles, can beinstalled with minimal vehicle modifications, and contributes to both safety and comfort.
Key Words: steering actuators, commercial vehicles, ADAS, coaxial motor

1. Introduction
In recent years, the pace of development for expanding
the use of advanced driver assistance systems (hereafter“ADAS”) and commercializing automated driving has
been accelerating. However because the required output
when steering in a large-size commercial vehicle is large,
hydraulic power steering (HPS) is installed and support
for ADAS is difficult with the functions of HPS alone.
For this reason, development has proceeded of a column
coaxial steering actuator (hereafter “coaxial steeringactuator”) that can actively control the angle for large-size
trucks and buses that are equipped with hydraulic powerrecirculating-ball steering (RBS)1). Because conventional
column type electric power steering (C-EPS) uses a worm
reduction gear structure, the motor protrudes from thesteering column shaft, making it difficult to ensure space
for the driver’s feet. With this system, a hollow-shaft
motor that does not require a reduction gear is installed
coaxially with the steering column shaft, and a directdrive-type electric actuator is installed on the steeringcolumn. This allows ADAS functions to be added toa commercial vehicle with minimal vehicle designmodifications in the steering column mounting part,
achieving further improved quietness and good steeringfeel. This report introduces the development of a steeringsystem that includes the world’s first hollow-shaft motor
installed coaxially with the steering column shaft.

2. Development Objectives and Issues
2. 1. Development Objectives

The following points were considered in thedevelopment of a system for commercial vehicles.

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)(1) Because commercial vehicles have a longer service
life than passenger cars, the hardware has a redundant
structure that allows software updates from Lane
Keeping Assist (LKA) to Lv4 automated driving
so that the vehicle can support future advances in
automated driving without modification.
(2) A direct drive motor system was used in order toachieve high installation performance that can support
large-size trucks and buses with wide variations in
cabin and body types, including existing vehicles.
(3) Control technologies were constructed to reduce tradeoffs resulting from system additions and improvesteering performance (such as steering wheel return)in a system connecting a coaxial steering actuator andHPS.

2. 2 Development Issues

The difference between a coaxial steering actuatorand conventional steering actuator (C-EPS) is that theformer can be installed as an addition to a commercialvehicle equipped with EPS without major changes to thecockpit layout. With the C-EPS type, it is necessary tosecure space for installing the motor. However because
the coaxial steering actuator is installed coaxially with themotor, it can be installed without major layout changes.
In order to achieve this, it is necessary to resolve thefollowing issues.
(1) Increasing the torque of the hollow-shaft direct drivemotor(2) E nsuring space for installation of the electric actuator(3) Improvement of control stability(1) Because the system does not use a reduction gear,
it is necessary to design a low-speed, high-torquemotor.

89

−Development of Column Coaxial Steering Actuator for Commercial Vehicles−(2) S hape design is necessary which allows installationin the space where the manual steering column
is installed with minimal vehicle (design)modifications.
(3) B ecause of the structure with two torsion bars
where the HPS and coaxial steering actuator eachhas a torsion bar, there is the possibility of vibration
occurring in the steering torque. Because the HPS
output status from the coaxial steering actuator
is not known, there is the possibility of load
fluctuations caused by changes in HPS output, andit is necessary to stabilize control of the steering
actuator.

Vehicle CAN

Vehiclepower
supplyMotor

4. 2 ECU

The ECU system configuration is shown in Fig. 2. A
new ECU supporting 24 V power supply voltage wasdeveloped for installation in a commercial vehicle. A
redundant structure is used for the ECU microcontroller,
motor driver, and CAN, dividing it into an independentsystem 1 and system 2. This enables normal system use
and continued steering function when a malfunction
occurs.
Batterypowersource (1)Vehicle CAN (1)Vehicle CAN (2)Digital output type

Motordriver 1

IG signalCANIC

Torque sensor
circuit 1
Torque sensor
circuit 2

Steeringsystem
microcontroller 1
Communications

MotorCommunications
Steering
CANsystemIC
microcontroller 2

4. 1 System Overview90Explanatory note
Signal line
Power line
External device

Fig. 1 System configuration

4. System Configuration
The system configuration is shown in Fig. 1. The
primary component parts are the ECU, electric actuator,and connected power/signal harness.
The coaxial steering actuator controls the steering
based on commands from the ADAS ECU, and transmits
torque output via the intermediate shaft2) to the HPS.
The angle and torque information required for control
is calculated from the values detected by the electric
actuator motor and torque sensor.

ECU

Electric actuator

3. Development Concept
The development concept for this system is to enable
the installation of the electric actuator while ensuring
space around the manual steering column as muchas possible in order to flexibly meet the demands for
installing ADAS functions in commercial vehicles as
described above. This development uses a hollow-shaft
motor which does not require a reduction gear, andinstalls it coaxially with the steering column shaft, fora system configuration with superior mountability. The
ECU is also a separate type that can be flexibly adapted tothe installation location. In order to support 24 V power
supply voltage and achieve degeneracy operation when
the ECU or motor malfunctions, the ECU is designed toincorporate redundant functions.

ADASECU

Torque sensor

Batterypowersource (2)Rotorrotation
anglesensor

Motordriver 2

Fig. 2 ECU system configuration

4. 3 Electric Actuator

The system positions the hollow-shaft motor
coaxially with the column shaft, and the torque sensor isincorporated in it. The motor has a redundant structure in
the windings and rotation angle sensor, and in the samemanner as the ECU is divided into an independent system
1 and system 2. The motor drives a single rotor using
the magnetomotive force generated by each of the motor
windings of both systems. While a conventional EPS
uses a reduction gear to amplify motor torque, torque is
output from the hollow-shaft motor, allowing improved
mountability of the coaxial steering actuator. This results
in a system with high response, low inertia, and highquietness.

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

−Development of Column Coaxial Steering Actuator for Commercial Vehicles−

The torque sensor is also redundant, and in additionto detecting driver torque, it is also used for purposes
including steering feel control and hands-off detection.

5. Electric Actuator Control
5. 1 Control Outline

The control configuration is shown in Fig. 3. Control
is composed of steering feel control, steering angle
command control, and hands-off detection.
Steering torque
Steering angle

Vehiclespeed
ADAS commands• Target steering angle• Control request

Steering feel control
Target• Basic assist control
torque• Steering return control• System stabilization
Add-on torquecontrol
Steering angle
command control

Control state
EstimatedHands-off detection steering• Estimated steering torquetorque calculation Hands-offflag• Hands-off
determination

Fig. 3 Control configuration

5. 2 Steering Feel Control

Steering feel control is composed of basic assist
control, steering return control, and system stabilization
control. The output of the coaxial steering actuator iscontrolled using the total value of each control output andthe steering angle command control output as the target
torque.
Basic assist control performs assist based on the vehicle
speed and driver steering torque in order to achieve bothvehicle handling at low vehicle speeds and response athigh vehicle speeds, which were previously difficult withHPS. Special assist is performed during steering angle
command control to easily allow the driver to performoverride steering.
Steering return control adds torque according to thedifference between the target steering speed found fromthe steering angle and the actual steering speed in order toachieve smooth steering return.
For system stabilization control, the coaxial steering
actuator is installed on the column shaft, and is connectedto the HPS via the intermediate shaft. This reduces thevibration caused by the structure which has two torsion
bars on the same axis, and improves control stability.

5. 3 Steering Angle Command Control

Steering angle command control calculates the torquerequired to track the actual steering angle by feedback
calculation in response to the target steering angle
received from the ADAS ECU, and outputs the result

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

as add-on torque. It also judges control start and end
according to the control request. Although the steeringload changes depending on the HPS output, the electric
actuator ensures stability regardless of the load conditionand sets feedback gain that can provide sufficient
response under the conditions of actual use.

5. 4 Hands-Off Detection

Hands-off detection is composed of estimated steering
torque calculation and hands-off determination. Based on
the steering torque, estimated steering torque calculation
calculates the estimated steering torque that compensates
for the torque deviation caused by inertia between thetorque sensor and steering wheel, and also by viscosity
and the difference between the rotational center and masscenter.
Hands-off determination determines that the driver’s
hands are off the steering wheel and notifies the ADAS
ECU when the estimated steering torque is at or below
the threshold value for the designated length of time or
more.
Figure 4 shows the coaxial steering actuator anda hydraulic RBS integrated actuator that has beencommercialized by another company. In the coaxialsteering actuator, the torque sensor is positioned directly
below the steering wheel. However in the hydraulic RBS
integrated actuator that installs the actuator directly on theHPS, the torque sensor and steering wheel are connected
via the manual steering column and intermediate shaft.
For this reason, the coaxial steering actuator is little
affected by inertia and friction, and enables highly
accurate estimation of steering torque, providing superiorhands-off detection performance.
Electric actuator
Torque sensor

Intermediate shaft
Hydraulicpower steering

Manual steering column
Torque sensor

Coaxial steering
actuator

Intermediate shaft
Electric actuator

Hydraulicpower steering
Hydraulic RBS integrated
actuator

Fig. 4 Difference in torque sensor position

6. Conclusion
By installing a coaxial steering actuator on the columnof a large-size commercial vehicle such as a truck or abus, where the front axle load is large and changing HPS
to EPS is difficult, we have created ADAS functions thatutilize the HPS. The hardware was made redundant inorder to improve safety and support future automated91−Development of Column Coaxial Steering Actuator for Commercial Vehicles−

driving. Unlike C-EPS, because the structure of thecoaxial steering actuator does not use a reduction gear,the steering actuator ensures high quietness and highresponse. In the future, we will expand sales of thecoaxial steering actuator as an ADAS-supporting solutionthat can replace a manual steering column with minimalvehicle design modifications in commercial vehicles
where the change to EPS is difficult.

References1)

2)

K. SAKATA: Outlook for Steering System Technology
Aimed at Self-Driving, JTEKT ENGINEERINGJOURNAL, No. 1017E (2020) 9.
Y. KAWADA: Energy Saving Related Technologies
Special Edition Development of Compact and LightWeight Column Type Electric Power Steering, No.
1010E (2013) 40.

K. YOSHIOKA* T. YAMAUCHI* Y. NONOGUCHI**92

Advanced System Development Dept., Automotive
Business Unit

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)