Electrohydraulic power steering pursued for heavy-duty vehicles

  • 07-May-2015 10:42 EDT
Figure 1.jpg

The most noticeable features of the new electrohydraulic power steering system are the proportional solenoid valve and the accumulator. (Click arrow at top right of image to view additional images.)

Energy savings is one of the most significant concerns in the development of new vehicles, particularly power-steering systems, as over 70% of the fuel consumed by conventional hydraulic power steering (HPS) systems is unnecessary and can be avoided. Therefore, the application of a more advanced power steering systems like electric power steering (EPS) and EHPS could save a lot of energy. New types of power steering systems are particularly needed for commercial vehicles, since conventional HPS systems are still predominantly used.

EHPS systems have been widely used in passenger cars since the 1980s, and their energy-saving capacities have been acknowledged worldwide. Therefore, many companies have tried to develop a similar EHPS system for heavy-duty commercial vehicles.

TRW Automotive has made a significant effort to improve the maximum power assist of the current EHPS system so that it can be used on vehicles with heavier steering loads. A demonstration vehicle with a 3.5-ton (3.2-t) gross weight and 15.3-kN (3440-lbf) rack load, and equipped with an EHPS product from TRW, has achieved fuel savings of 0.2 L/100 km in comparison to the conventional HPS system. In addition, a fully developed commercial product of TRW was launched several years ago for vehicles whose maximum rack load was no more than 18 kN (4050 lbf). Furthermore, the company tested a system on a 10.6-ton (9.6-t) medium-duty truck, but the oil supply decreased significantly when the pressure was over 8 MPa (1160 psi) as the output power of the electric motor was limited, and the max current was over 130 A.

Many other strategies for improving the fuel economy of power steering systems have been put forward or are being researched. For instance, a hydraulic power assist steering system proposed by Tata Motors added a magnetic clutch between the engine and the steering pump so that the steering pump could work on-demand, which reduces energy consumption. This simple strategy may also be suitable on heavy-duty vehicles.

EHPS systems offer the advantages of pressure-flow characteristic programmability, energy savings, and a great steering feeling; however, EHPS systems for heavy-duty vehicles continue to face significant challenges, especially the electric power limitations of the 24-V electrical systems. As a solution to this problem, researchers from Tsinghua University propose a new type of EHPS system with the purpose of reducing the power demand of an electric motor while guaranteeing sufficient power assist. The motor is only activated on-demand, which also helps to improve fuel economy.

System’s basic parts and principles

The most noticeable features of the new system are the proportional solenoid valve and the accumulator. These are different from the traditional EHPS system with a center-closed steering valve, in which a small accumulator is also usually included to compensate for the oil supply shortage caused by the dormancy of the motor before a steering signal is detected.

The gas envelope-type accumulator in the proposed system is the main resource of hydraulic flow. It has a nominal volume—around 4 L—and its oil pressure is monitored by the electronic control unit (ECU). A 500-W permanent magnetic brushless dc motor drives the gear pump to charge the accumulator until oil pressure reaches the motor-stop threshold; then the motor can move into a dormant state. The check valve is used to prevent backflow. The outlet of the accumulator is connected to the proportional solenoid valve, which is equipped with a pressure compensator that keeps the flow rate stable while the pressure difference between outlet and inlet varies.

A steering angle and torque sensor are installed between the steering wheel and hydraulic power steering gear. The ECU regulates the opening of the solenoid valve according to the steering torque, steering angle, and vehicle speed. Therefore, the power assist is speed sensitive and highly adjustable. When the oil flows out of the accumulator while steering, the oil pressure drops and the motor is triggered as soon as the pressure drops under the motor-on threshold.

In this study, an 8 degree-of-freedom (DOF) vehicle model was used, including longitudinal movement, lateral movement, yaw, the rolling of four wheels, and the steering of the front wheels. Based on a J6 heavy-duty truck from FAW Jiefang Automotive, the 8-DOF model was built in Matlab from MathWorks, and the corresponding tire, 9.00R20, was built based on the test data using the company’s Magic Formula.

Simulation results, prototype, and static steer test

To investigate the performance of the new system, a 5-min simulation was carried out. From 0 to 200 seconds, the vehicle was driven at a relatively low speed, 18 km/h (11 mph). It was then accelerated to around 76 km/h (47 mph), a relatively high speed.

In the simulation, the average power consumption was around 350 W for the 4x2 heavy-duty truck with a front load of approximately 2.7 ton (2.5 t), driven for 5 min in which the non-steering time was around 70% of the total time. The power consumption at high speed is much less than 350 W. The system can work normally in this scenario; therefore, it is clear that the proposed EHPS system works efficiently under specific situations.

However, the working load of the power steering system was still relatively light in this simulation. In certain critical situations, such as parking and driving on winding roads, the steering operation lasts longer and the steering angle is larger. Therefore, the nominal volume of accumulator still needs to be selected, and the strategy to discharge/charge the accumulator should be designed more comprehensively to prevent the shortage of oil in the accumulator happen. Otherwise, a lack of power assist may occur when the oil in the accumulator is used up, which would result in a severe failure and create an accident risk.

A prototype of the new EHPS system was built, and a static steer test bench for feasibility verification was constructed. Several key parameters for the EHPS system and the test bench are shown in the table above (click arrow next to image to view). The front axle of the truck was put on a steel panel with its rear end articulated on a settled support, and a hydraulic cylinder was equipped to produce a vertical front load on the axle.

To verify the feasibility of the proposed EHPS system, static steer sweep tests were carried out on the test bench under a vertical front load of 2.7 ton. Test parameters included the steering angle, steering torque, pressure of the accumulator, and pressure of the solenoid valve outlet.

In the static steer sweep test, the oil pressure, Paccu, dropped slowly until the accumulator was nearly empty. The pressure of the outlet of the solenoid valve, Pdfsv, increased over time according to the torque applied to the steering wheel, Tsw. The steering torque was kept around 5 N·m during the steering, and the angle of the steering wheel, θsw, was changed smoothly from side to side from second 2 to second 14 of the test. The feasibility of the EHPS system was preliminarily verified.

After second 14, the oil in the accumulator had completely flowed out, so the steering torque increased and the rotating speed of the steering wheel slowed down because the motor and the gear pump were not powerful enough to offer sufficient assistant force directly under static steering conditions. It was also found that using an accumulator to store hydraulic energy to increase the transient power is a key feature of the proposed EHPS system.

In future work, the whole EHPS system should be integrated into a real heavy-duty commercial vehicle, and more road tests under various vehicle speeds and vertical loads should be carried out.

This article is based on SAE International technical paper 2015-01-1502 by Liangyao Yu, Wenwei Xuan, Liangxu Ma, Jian Song, Xianmin Zhu, and Shuai Cheng of Tsinghua University.

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