Future Power Steering System

Titanium You will find the figures mentioned in this article in the German issue of ATZ 4/2004 beginningMATERIALS on page 310.

Zukünftige verbrauchsarme Servolenkungen für vollständige Steer-By-Wire-Funktionalität

Future PowerSteering Systems for Full Steer-By-Wire Functionality and Low Fuel Consumption Present hydraulic power-steering systems do not meet the requirements on future steering systems anymore. Increasing demands on agility, ride comfort and ride stability as well as stricter restrictions on the fuel consumption demand new approaches. This article describes the essential requirements on future power-steering systems from BMW’s point of view. An analysis shows for which vehicle classes and for which functionalities these requirements cannot be fulfilled by present systems. At the end of this article, concepts for new power-steering systems are proposed which could also cover these areas in the future.

1 Introduction

By Steffen Müller

ATZ worldwide 4/2004 Volume 106

In the past years, the increasing demands led to the development of electric (EPS) and electrohydraulic (EHPS) power-steering systems where steering power is provided by an electric motor, which drives a pump or directly by an electric motor and a gear at the steering column, at the steering pinion or at the rack. Both systems reduce the fuel consumption portion of a power-steering system compared to a conventional system. Moreover, an EPS offers new opportunities to modify the steering torque. But both systems stress the electrical power system and their field of application is

therefore limited. Conventional hydraulic power-steering systems have been improved and optimized over the past years, but their fuel consumption portion is still relatively high and their functionality is limited. Active steering systems, which control the wheel steering angle, have been successful on the market since a short time. These systems increase the driving stability and the steering effort at low speed is reduced while at the same time safe handling at high speed is ensured. Thus, the agility and the driving comfort can be significantly increased. However, in this connection the direct steering ratio during parking in-

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creases the power demand on the powersteering system. Based on this state-of-the-art, in the following the essential requirements on future power-steering systems are summarized. Then, an analysis shows the potentials and limits of present power-steering systems and new steering system concepts are proposed, which are in particular applicable to middle and upper class cars. 2 Essential Requirements on Future Power-steering Systems

The essential requirements on future power-steering systems from BMW’s point of view are: ■ enabling full steer-by-wire functionality and preserving or optimizing the present steering feel ■ Significant reduction of the fuel consumption portion compared to present hydraulic power-steering systems ■ Sufficient power capacity for all vehicle classes. 2.1 Full Steer-By-Wire Functionality

Full steer-by-wire functionality allows for free and independent variations of the front wheel steering angle and the driver’s hand torque, both in amplitude and sign. For a steering system with no mechanical connection between steering wheel and steering gear, this can be achieved by steering wheel and steering gear sided electric or hydraulic actuators. BMW has at first decided in favor of a mechanically continuous steering column. Thus, the mechanical connection can be utilized as a manual fallback system and the driver’s steering work is not lost. Then, full steer-by-wire functionality can be provided by a combination of an overriding drive [2] and a power-steering system, which allows for free and independent control of the servo force (4-quadrant operation of the servo force, Figure 1). 2.2 Significant Reduction of the Fuel Consumption Portion

Present conventional hydraulic powersteering systems are supplied by a pump, which is driven by the engine via belt drive and crankshaft. Thus, the steering system is always consuming energy although an average driver is only steering during approximately 15 % of the total driving time. The fuel consumption portion of the steering system is therefore unnecessarily high, since the average power consumption is basically power loss. A significant reduction of the fuel consumption portion of the steering system can particularly be achieved by a significant reduction of the

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power consumption during straight driving, Figure 2. 2.3 Sufficient Power Capacity

To ensure comfortable steering effort for the driver a power-steering system has to provide sufficiently fast adequate servo forces. This leads to requirements on the power capacity which are at the highest stage during braked parking. During this maneuver the rack forces are maximum. Neglecting the force portion of the driver the demanded steering power at the steering rack is proportional to the maximum steering wheel speed ωH and the rack force Fzs, Eq. (1). PZS = FZS · iZSLR · ωH

Eq. (1)

iZSLR is the steering gear ratio (ratio between rack velocity and rotational speed of the steering wheel). Eq. (1) shows that a high steering ratio iZSLR at low car speed additionally increases the power demand on a power-steering system during parking. 3 Analysis of Present Power-steering Systems

Present front wheel steering systems can be divided into systems, which modify the driver’s hand torque or the wheel steering angle in a hydraulic, electric or mechanical manner. The sources for these modifications are belt driven pumps, electric motor driven pumps or electric motors combined with a gear, Figure 3. In the following present conventional steering systems are described based on this kind of classification and it is evaluated to which extent those systems meet the above given essential requirements. 3.1 Hydraulic Modification of the Driver’s Hand Torque

At present the most common hydraulic power-steering system in passenger cars is the rack and pinion power steering gear. Here the relative rotational displacement between a valve rotor (5) and a valve sleeve (7) and against the torsion stiffness of a torsion bar (6) varies oil control edges such that hydraulic fluid flows to the left (ZL) or right (ZR) chamber of the power cylinder (1). The difference pressure in the cylinder acts on the piston (3) and causes a resulting force on the steering rack (2) and thus supports the driver while he is steering, Figure 4. Present hydraulic power-steering systems for passenger cars can be divided into systems with belt driven pumps and systems with pumps driven by an electric motor.

3.1.1 Belt Driven Pump

In present standard hydraulic passenger car steering systems mainly belt driven vane pumps are employed. They are designed such that for a given maximum rack velocity and the piston cross sectional area a specific volume flow at idle-running speed of the engine is produced. To avoid an engine speed dependent steering feel a passive flow control valve (18) cuts off the volume flow output for engine speeds higher than the idle-running speed. The unused fluid is circulated within the pump, Figure 5. An important disadvantage of belt driven vane pumps is the high power consumption during straight driving. Here, the consumed power of the pump is pure power loss PGVerlust, which depends on the pump speed proportional volume flow of the pump Qp, the pressure drop in the steering system ∆pNW and the efficiency of the pump ηP , Eq. (2). 1 PGVerlust = –– · QP · ∆pNW ηP

Eq. (2)

Speed-Dependent Power Assistance If a conventional hydraulic power-steering system is designed for low steering efforts during parking, the driver’s steering effort for high speeds is also very low, which leads to a nervous and sensitive steering system. On the other hand, a design which focuses on ride safety only, can solely achieved with the disadvantage of higher steering efforts when the car is maneuvered into or out of a parking space. Even though parking is in this case still much more comfortable compared to a car without steering support, the benefit of a power-steering system is clearly diminished. These conflicting goals led to technical solutions, where the power assistance varies with the driving speed. This can be realized through hydraulic reaction effects in the rotary valve, such that the amount of assistance can be speed-dependently varied. However, a 4-quadrant operation of the servo force is not possible. Control of Volume Flow Output by Controlled Flow Control Valves in the Pump There are two basic possibilities to control the volume flow output by a controlled flow control valve – a pressure controlled flow control valve, where the displacement of a control pin is regulated through hydraulic feedback, or an electrically controlled flow control valve with electromagnetic regulation of the pin displacement. Through the active regulation of the control pin displacement the operating points of the pump are no longer limited to

ATZ worldwide 4/2004 Volume 106

Titanium

3 Analysis of Present Power-steering Systems

MATERIALS

3.1.2 Electric Motor Driven Pump

A different type of hydraulic steering system is the electrohydraulic power-steering system (EHPS), where the pump is driven by an electric motor. An EHPS enables steering support even if the engine is not running and the fuel consumption portion can be reduced e.g. by regulating the motor speed with respect to the driving state. However, to ensure sufficient availability the pump has to be operated on a certain power level. Therefore, the potential for reducing the fuel consumption portion is limited. Moreover, the electrical loads on the electric power net of such a system can be expected to be as high as those of an electric power-steering system. Although the pump’s volume flow output can be regulated by the motor speed a free and independent control of the driver’s hand torque is not possible – if a servo force is applied to the steering rack or not still depends on the deflection of the torsion bar. 3.2 Electric Modification of the Driver’s Hand Torque

Figure 3: Classification of present passenger car front wheel steering systems by principles for modifying the hand torque and the wheel steering angle

the characteristic curve of a vane pump with a passive flow control valve, dashed line in Figure 6, but are variable. Thus, the volume flow output Qförder can be reduced for high engine speeds, which reduces the pressure drop within the steering system (tubes, valves, cylinder, etc.) and the power consumption decreases. However, test rig measurements show that by means of vane pumps with controlled flow control valves the potential for reducing the power loss of conventional belt driven vane pumps with passive flow control valves is limited, Figure 7. Variable Displacement Pump Regulating the relative displacement between the inner ring and the rotor of a vane pump changes the effective pump volume and thus the volume flow output for a given pump speed, Figure 8. In contrast to a pump with a flow control valve, where the characteristic of the volume flow curve remains the same, this kind of control varies the gradient of the linear volume flow curve. This reduces the power consumption

ATZ worldwide 4/2004 Volume 106

of the pump at higher engine speeds compared to a corresponding vane pump with a controlled flow control valve. The advantages of such a pump are more moderate temperature rises and lower engine loads at high speeds. From an energetic point of view, the control of a variable displacement pump demands the two conflicting requirements low displacement forces and high volumetric efficiency. Lower friction forces between chamber and rotor decrease frictional losses, while small variations between chamber and rotor reduce volumetric losses in the pump. Compared to a corresponding conventional vane pump, the power consumption is in general higher at lower pump speeds owing to the additional power which is needed to regulate the inner ring’s displacement. At present, it cannot be expected that for customer relevant driving cycles the potential of variable displacement pumps for fuel consumption reduction is much higher than the potential of vane pumps with controlled flow control valves, Figure 7.

The main disadvantage of a hydraulic power-steering system with a belt driven pump is the high power consumption during straight driving. An electric power-steering system (EPS) is only active while the driver is steering, which significantly reduces the average power consumption, Figure 7. Further, the driver’s hand torque can be easily modified without any additional actuators (4-quadrant operation of the servo force) and various steering parameters can be changed simply by changes in the software. Moreover, electric power-steering systems improve the manufacturing process and need less maintenance, since filling, oil-level control and leak tests are no longer necessary. An EPS, where the power assistance is applied through a separate pinion at the steering rack, is flexible with respect to the positioning place. The stiffness and efficiency of the EPS’s pinion can be optimized such that higher servo forces can be applied to the rack in comparison to less expensive EPS systems, which are attached to the steering pinion. The advantages of an EPS system, which is integrated into the steering column, are lower ambient temperatures and less dirt and corrosion. However, since the electric motor and the gear are located close to the driver the acoustical risks are higher. For rack forces higher than 9 to 10 Nm rack-assist EPS systems are in use, where the servo force is applied to the rack through a ball-and-nut-type gear or a worm gear. Since the power capacity of a 14 V-pow-

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er supply system is limited bigger cars cannot be equipped with EPS systems. A 42 V generator could be beneficial but the range of application of EPS systems for high steering power is also limited (for example owing to the moment of inertia of the rotor or the size of the motor).

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WORLDWIDE is a publisher’s supplement to the journal ATZ Automobiltechnische Zeitschrift. Internet: www.all4engineers.com

4/2004, April 2004, Volume 106 Vieweg Verlag / GWV Fachverlage GmbH

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4 Summary and Outlook

The analysis above shows, that full steer-by-wire functionality is not possible with present hydraulic power-steering systems. Although speed-dependent power assistance varies the driver’s hand torque, a sign change is not possible with currently available technology. A significant reduction of the fuel consumption portion of hydraulic steering systems by means of energy saving pumps (e.g. vane pumps with a controlled flow control valve or variable displacement pumps) can presently not be expected. EHPS systems have a higher potential for reducing the power consumption, but do not meet the power demands of heavier cars with high rack forces and a 14 V power supply system. Electric power-steering systems enable 4-quadrant operation of the servo force, which is necessary for full steer-bywire functionality, and the power consumption is significantly lower compared to present hydraulic steering systems. However, like for an EHPS system, the power demands of heavier cars are too high and their application is still restricted to smaller and light-weighted cars. Figure 9 summarizes the results of the analysis of present power-steering systems. It is illustrates that a power-steering system, which enables full steer-by-wire technology for bigger cars with a 14 V power supply system, does not exist. Moreover, a significant reduction of the fuel consumption portion compared to a conventional hydraulic system is only possible with an EPS and for smaller cars. Middle class cars with speed dependent steering gear ratio or upper class cars still need hydraulic power-steering systems with a belt driven pump due to the high steering power demand. But even the power capacity of such systems is limited. Promising concepts, which have the potential to fulfill the above given requirements better than present systems, are for example ■ EPS system combined with a 42 V power supply system or electric storage elements ■ closed-center hydraulic power-steering system with a controlled steering valve ■ open-center hydraulic power-steering system with controlled volume flow and controlled steering valve ■ open-center hydraulic power-steering system with controlled volume flow combined with a hydraulic or electric hand torque simulator. In order to meet the requirements on future power-steering systems, not only EPS and present hydraulic systems must be advanced and optimized – it is also necessary that new electrohydraulic or electric concepts will be developed. Literature [1]

[2]

[3]

Breitweg, W.: Hat eine hydraulische Lenkung noch Chancen für die Zukunft? In: Proceedings der Tagung Pkw-Lenksysteme – Vorbereitung auf die Technik von morgen, Haus der Technik, Essen, 02./03.04.2003 Fleck, R.: Aktivlenkung – ein wichtiger erster Schritt zum Steer-byWire. In: Proceedings der Tagung Pkw-Lenksysteme – Vorbereitung auf die Technik von morgen, Haus der Technik, Essen, 02.03.04.2003 Sonderdruck zum 7. LuK Kolloquium. 11./12. April 2002, Hrsg. LuK GmbH & Co

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