DESIGN OF A MODULAR STEERING SYSTEM TEST BENCH FOR DURABILITY, PERFORMANCE AND CHARACTERIZATION TESTS

ELİF KILINÇ, ORHAN ATABAY, ŞEREF SERVER ERSOLMAZ, HATİCE ERDOĞAN

ABSTRACT Steering systems take a significant role on safety and comfort in road

vehicles. Driving pleasure is highly associated with the forces on the steering wheel, which are depending on road conditions, speed of the vehicle, hydraulic-electric power steering assistance and the angle of steering wheel. These forces are transmitted over the tires and the steering system to the steering wheel. Steering system test benches are used for checking durability, evaluating performance and investigating characteristics of these systems. Steering system tests are mostly done as subjective testing which depend on test drivers and vehicle level field testing. Extensive subjective testing results in a waste of valuable development time and introduce high test and development budgets with the actual increase of vehicle variants especially those with electric steering assist functions. Due to these facts steering system testing and development moves to the laboratories and Hardware-In-The-Loop testing devices to overcome these situations.

Within the scope of this work, to simulate the real kinetic and kinematic relations on a steering system, 3 axis and 5 axis steering system test benches are designed and constructed in OTAM Laboratories as a modular structure in order to research and develop various steering systems independently from vehicles. In addition to the durability, performance and characterization tests for a steering system, a driving simulator can also be coupled with the whole system to perform driver-in-the-loop tests. The driving simulator setup enables to check the subjective feeling of different setups with respect to different hardware components and steering-ECU software evaluation steps.

INTRODUCTION Steering systems are among the most significant subsystems for road

vehicles in terms of a controllable, safe and comfortable driving. Although this importance, it is obvious that there is not enough effort in the industry and academia for steering system evaluation in the laboratory. According to benchmark surveys, there are only a few companies which design steering system test benches and they have mostly systems with a few specific aims. The presented test benches in this study have a modular and flexible design which is helpful for analyzing durability, evaluating performance and investigating characteristics of a steering system together.

At the present time, automotive steering systems are assisted by power systems in order to share the necessary force for steering wheel between driver and the assistant system. As an assistant system, electric power steering systems have gained attention rather than conventional hydraulic power systems because of their environmental friendliness, functionality and energy efficiency. Corresponding to development of electric power steering systems, problems which are related to adaptation, characterization and reliability emerge and laboratory testing system requirements increase. Additionally, increment of vehicle variety day by day, limited development time and budgets make essential the movement of steering system testing to the laboratories and Hardware-In-The-Loop testing devices instead of carrying out the steering system characteristic optimization by experienced test drivers being subjective. It is known that steering reaction torque is an important information for drivers since it has significant influence on vehicle maneuverability. Therefore, one of the most important point is to simulate steering feeling precisely while designing the test bench. Youchun Xu et all (2009) talked about a test-platform which is designed on the basis of the steering system of the vehicle can manipulate directly the steering mechanism in their studies. They used a DC motor for steering wheel rotation in the Hardware in the Loop Simulation Test Platform and provide a tool to research and analyze the steering control algorithm. Besides the steering feeling simulation significance, by increasing the features such as safety, importance of environmental protection, and fuel efficiency, electric power steering (EPS) has been pervaded. Segewa et all (2005) mentioned a steering Hardware in the Loop (HIL) simulator development for the steering system in their paper. An AC servomotor and a ball screw are used to transmit force to the steering system ball joints. And also Chih-Jung Yeh et all (2007) talked about a test bench for electric power steering system’s tuning and validation in their article in 2007. They have developed a test bench by using force feedback motors as rack force generating elements. Although researches recently carried by Yongsheng IIu et all (2004), Masahiko Kurishigc et all (2000) and A.T. Zaremba et all (1998) found that the EPS test bench use springs for the steering load setting generally, in this study direct drive electric motors are used instead of springs or small DC motors with gearboxes.

In order to overcome these problems related with steering system tests, an original steering system test bench is designed for characterization, durability, optimization and performance testing for both current and future vehicles. The test bench is controlled and automated also by an in-house software and a comprehensive vehicle dynamics module is coupled with the system in order to control steering tests. Another remarkable point for the designed test bench is to provide testing independent of full-vehicles. Due to its original design, steering system test bench enables development and validation testing for both OEM and suppliers on their steering systems and sub-components. The test bench is designed as a 3 axis and a 5 axis steering system test bench for all passenger cars and light commercial vehicles.

3 Axis Steering System Test Bench 5 Axis Steering System Test Bench

The power and force ratings of the 3-axis steering system test bench are foreseen for passenger cars. In addition to mentioned tests above for a steering system, a driving simulator can be also coupled with the whole system to perform driver-in-the-loop tests.

MARKET SITUATION All companies which give usable information about their systems in the

market are analyzed in order to observe necessities about steering system testing. According to these analyses it is founded that, each company has specialized on specific setups. One of the companies has presented a fully hydraulic 5 axis steering system test bench. The most distinctive property of this test bench is to use a hydraulic steering wheel motor and situate all actuators into a compact construction. Due to the valves’ high oil transfer rate, it is estimated that the system has notably great size of tank and pump.

Another company has used only 3 direct drive rotary elements for the test bench. Two of them are used for rack forces and the other is used for steering wheel motor. The most distinctive property of this test bench is to have Hardware in the Loop tests in a 3 axis test system configuration. The test bench of the steering system comes in the forefront with testing capabilities rather than multi axes variety.

One more company has produced a test bench in order to test mainly electric power steering (EPS) systems. The test bench consists of only electrical linear actuators. Due to these linear actuators, the test system bench is more dynamic than the other systems with hydraulic actuators.

A different company has designed again a column type and a rack type electric power steering (EPS) test system bench to realize performance, durability and road simulations of steering systems at variable humidity and temperature. The test bench is able to simulate driver behavior, road vibration, and rack forces at the right and left ball joints. For this purpose, pinion driver robot, hydraulic servo actuators for right and left wheel’s load and a distinct feature compared to other systems, a Wobble actuator are used in the test bench.

A company else has designed a 3 axis steering system test bench which has a steering wheel robot simulates driver behavior on the steering wheel while 2 hydraulic cylinders apply the variance of load and displacement to the steering rack.

In addition to these companies, one company has designed a fully hydraulic test system bench which allows different tests as 3, 4 and 5 axis. The main difference between 3 and 4 axis system test bench is wobble actuator which allows shock tests on the tower module in order to simulate front crashing. 5 axis system test bench is designed for durability and simulation tests for mechanical and hydraulic steering boxes.

DEVELOPMENT APPROACH The steering system test bench is designed as a 3 axis and a 5 axis steering

system test bench with a modular and flexible structure. The 3 axis steering system test bench has a very accurate positioning capacity because of the torque motors which are used as system’s rack force actuators. Moreover, it is a faster and more dynamic system than the 5 axis system test bench. In this way, getting sensitive steering characterization with desired accuracy is allowed. The 3 axis system test bench is composed of one direct drive motor behind the steering wheel and two torque motors for the rack forces.

The 5 axis steering system test bench serves from passenger cars up to light commercial vehicles with intend of testing durability. The test bench provides a wider force range than the 3 axis system test bench. In addition to 3 degrees of freedom, the test bench has additional 2 degrees of freedom with jounce and rebound effects. Servo hydraulic actuators for vertical and lateral forces have hydrostatic bearings and 25 kN and 15 kN maximum force levels, respectively. A torque motor which is assembled on steering shaft constitutes the fifth axis of the test bench.

Development approach is started with assembling trials between vertical and horizontal pistons for a 5 axis steering system test bench. Primarily, wheel’s (wheel carrier) movements have to be taken into account while designing the wheel side of the test bench.

Wheel Movements

Over 20 trials are examined and elected due to the technical and budgetary reasons; like number of parts, high precision or high durable parts, complexity, kinematic locking, system’s volume, etc.

Design Trials

The design team is then left with the most appropriate 3 design options.

Within these options, the aim is to obtain the most appropriate hydraulic actuator positioning which has the minimum hydrostatic bearing forces. To briefly summarize 3 design trials; in the first trial, horizontal cylinder is positioned vertically to the test bench, and cylinder is connected to fixing unit with cardan joint, at the end of the cylinder there is a force transmission element which transfers the force with 180 degrees angle. In the second trial, cylinder is positioned parallel to the test bench and fixed to its plate. To provide freedom of movement, a rod attached to cylinder’s rod with cardan joint. This type of assembly makes the unit two times longer than the other 2 trials. Its force transmission element transfers the force with 90 degrees angle. The third and the last trial are very similar to first trial, but it is positioned parallel to the test bench and its force transmission element is same with second trial. These 3 design trials are analyzed by using kinematic software in more detail. According to these analyses, it is obviously seen that those 3 design trials kinematic results are similar to each other. Also, bearing forces on horizontal cylinders are as low as not to damage cylinder hydrostatic bearings. One of the layouts is preferred among these 3 options due to position of horizontal cylinders. Nevertheless, the third design is distinguished with its smaller volume Thus, third design is preferred for all critical points.

ADAMS Designs

3-D DESIGN of the MODULAR TEST SYSTEM

3 Axis 3-D Design The steering system test bench modules which are designed as a modular

structure in order to provide a flexible system are explained below:

Rack motor module applies rack force (lateral force) in the 3 axis steering system test bench.

Rack Motor Module

Rack module provides fixing of steering system’s gear box.

Rack Module

Tower module provides adjustment of steering system’s height, angle and position.

Tower Module

Hanger module is used for representing the assembling point between steering system and vehicle body.

Hanger Module

Middle module plate is designed to provide movement of hanger module and tower module as assembled together. The reason of using the middle module plate instead of middle module is to to prevent unnecessary length of chassis which is assembled to torque motors. Because the torque motors are shorter than hydraulic cylinders.

Middle Module Plate

5 Axis 3-D Design The most critical point for the steering system test bench design is to create,

manufacture and configure the 5 axis steering system test bench. Cooperative axes in the test bench have not to create load on each other. This setup is succeeded:

1. Choosing appropriate cardan joints which allow piston movements in appropriate directions

2. Designing suitable assembling parts. 3. Using hydrostatic bearing cylinders (hydraulic actuators) and increasing

cylinder rod thickness.

The steering system test bench modules which are designed as a modular structure in order to provide a flexible system are explained below:

Rack module is a common module for both the 3 axis and 5 axis steering system test benches. It provides fixing of steering system’s gear box.

Rack Module

Tower module provides adjustment of steering system’s height, angle and position.

Tower Module

Hanger module is used for representing the assembling point between steering system and vehicle body.

Hanger Module

Middle module is designed to provide movement of hanger module and tower module as assembled together.

Middle Module

Cylinder module provides transmitting vertical forces and horizontal positions to the ball joints of tested steering system.

Cylinder Module

DESIGN VERIFICATION Designing of all the mechanical parts and system’s support structure is

completed in computer environment and after the manufacturing process comes the assembling of parts. The modules and their assembling details are explained below for both the 3 axis test system bench and 5 axis test system bench:

3 Axis Design Verification 1. Rack Module

Rack Module Assembly for 3 Axis Test Bench

2. Tower Module

Tower Module Assembly

3. Hanger Module

Hanger Module Assembly

4. Middle Module Plate

Middle Module Plate Assembly

5. Rack Motor Module

Rack Motor Module Assembly

Rack module, tower module and hanger module are common for both 3 axis and 5 axis system test benches. Rack module, tower module and hanger module are assembled on the middle module plate. Rack motor module simulates lateral forces in the test bench.

One of the most important modules in the test bench is the tower module. It is designed as a highly flexible structure, thereby each part of the module is almost movable or knockdown. In addition to fix of steering system’s tower with the position and angle on the vehicle, tower module undertakes fixing of steering motor and torque sensor which measures torque on the steering motor.

As a result of functional testing in the prototype phase, it is observed that the system gives similar results according to tests and trials on virtual environment (MSC/ADAMS).

The working conditions are summarized below

Number of Axes: 3 (2 lateral forces, 1 steering wheel motor moment)

Dimensions: 2000 x 3000 mm

Stand Control: NI CRIO

Steering Wheel Actuator: Electric Motor

Steering Wheel Rotation: +/- 1080 degree

Steering Wheel Velocity & Acceleration: 2000 degree /s (35 rad/s)

Steering Wheel Motor Moment: 100 Nm (continuous)

Rack Forces (Lateral) Actuators: 2 torque motors

Rack Forces (Lateral) Capacity: 3350 N (continuous), 11825 N (peak)

Dynamic Rack Displacement Working Range: +/- 125 mm

Rack Force Actuator Velocity: 100 rpm

Communication: CAN & Ethernet

5 Axis Design Verification 1. Rack Module

Rack Module Assembly for 5 Axis Test Bench

2. Tower Module

Tower Module Assembly

3. Hanger Module

Hanger Module Assembly

4. Middle Module

Middle Module Assembly

5. Cylinder Module

Cylinder Module Assembly

Rack module, tower module and hanger module are common for both 3 axis

and 5 axis system test benches. Rack module, tower module and hanger module are assembled on the middle module.

A cylinder module is used in the 5 axis system test bench. Horizontal and vertical hydraulic cylinders are assembled on each of two cylinder module. Rack motor module which is situated both at the right side and the left side simulates horizontal forces and stands in the 3 axis system test bench.

One of the most important modules in the system is the tower module. It is designed as a highly flexible structure; thereby each part of the module is almost movable or knockdown. In addition to fix of steering system’s tower with the position and angle on the vehicle, tower module undertakes fixing of steering motor and torque sensor which measures torque on the steering motor.

The cylinder module which provides 5 axis function of the steering system test bench distinguishes the system from other existing steering system test benches. The cylinder module is formed by assembling of two distinct cylinder and ball joints of testing steering system with a special design.

As a result of functional testing in the prototype phase, it is observed that the system gives similar results according to tests and trials on virtual environment (MSC/ADAMS).

The working conditions are summarized below

Number of Axes: 5 (2 lateral forces, 2 vertical forces, 1 steering wheel motor moment)

Dimensions: 2000 x 3000 mm

Stand Control: NI CRIO

Steering Wheel Actuator: Electric Motor

Steering Wheel Rotation: +/- 1080 degree

Steering Wheel Velocity & Acceleration: 2000 degree /s (35 rad/s)

Steering Wheel Motor Moment: 100 Nm (continuous)

Rack Forces Lateral Actuators (Max): 15000 N

Rack Forces Vertical Actuators (Max): 25000 N

Rack Displacement Working Range (Lateral): +/- 60 mm (max +/- 125 mm)

Rack Displacement Working Range (Vertical):125 mm

Communication: CAN & Ethernet

DRIVING SIMULATOR

A driving simulator software is also cooperating with the steering system test bench in order to perform characterization tests. This Hardware in the Loop (HIL) system setup incorporates a two track vehicle dynamics model in a programmed virtual environment. The communication between the test system and the simulator software is setup with TCP protocol. A suitable and ergonomic driver cabin is also designed for subjective testing with real drivers.

Whole System

CONCLUSION

Consequently, an original steering system test bench is designed as both 3 axis and 5 axis steering system test benches with a modular structure. This design allows life-cycle, durability, performance and characterization tests of steering systems of passenger cars up to light commercial vehicles.

The steering system test bench is designed as an original structure. Correspondingly, a patent application process continues for this design.

REFERENCES Chih-Jung Yeh, Shih-Rung Ho, Ming-Chih Lin, Tsung-Hsien Hu and Tsung-Hua Hsu, 2007, Development of a Test Bench for Tuning and Validating Electric Power Steering Control Method, IEEE, O-7803-9761-4/07 pp.618-622

M.Segawa M.Hıgashi, S.Nakano, M.Shino, M.Nagai, 2005, Development of Simulator for Evaluation of Steering Systems, Koyo Engineering Journal English Edition, No.168E

Youchun Xu Xiao Wang Xianbing Zeng Yongsheng Peng Yongjin Zhang Yi Yuan Hongquan Liu, 2009, A Hardware-in-loop Simulation Test-platform For the Intelligent Vehicle's Steering Control System, IEEE, 978-1-4244-3504-3/09