2004-21-0073 Concept and Functionality of the Active Front Steering System
Willy Klier, Gerd Reimann and Wolfgang Reinelt
ZF Lenksysteme GmbH, Schwäbisch Gmünd, Germany Copyright © 2003 SAE International
ABSTRACT
Active Front Steering (AFS) provides an electronically controlled superposition of an angle to the steering wheel angle. This additional degree of freedom enables a continuous and driving-situation dependent adaptation of the steering characteristics. Features like steering comfort, effort and steering dynamics are optimized and stabilizing steering interventions can be performed. After the successful introduction of AFS (or active steering) together with the new BMW 5-ries into the international market, ZF Lenksysteme focus on aspects like system modularization and integration. For that reason the system bounds, its functionality, and the required system interface are defined to provide a compatibility to veral overall chassis control concepts. This paper focus on a modular system concept and its respective advantages and requirements.
4格漫画
1. INTRODUCTION
土豆英文怎么说This steering system developed by ZF Lenksysteme and BMW AG enables driver dependent as well as automatic steering interventions without loss of the mechanical connection between steering wheel and road wheels [1,2,3] (e Figure 1).
This fact together with current definitions for steering systems imply that AFS is not a steer by wire system. The AFS system provides (compare [3,4,5,6]):
•an improved steering comfort (reduced steering effort),
•an enhanced dynamic behavior of the steering system (quick respon to driver’s input) and •vehicle stabilization (active safety).南通市公积金
After a short description of the steering system and respective components in Section 2, the modular concept, its functionality and the required system interface will be illustrated in Section 3. Some conclusions and an outlook will be prented in Section 4.2. COMPONENTS AND FUNCTIONALITY
The electrical and mechanical components as well as the functionality of the AFS system will be briefly described in this ction. Figure 2 shows the following AFS components and subsystems:
•Rack and pinion power steering system including (e Figure 2) the main gear (1), a Servotronic val
ve
(2), a steering pump (9), an oil rervoir with filter
(10) and the respective hos (11),
•AFS actuator including the synchronous motor (3) with its respective electrical connections, the superposition gear system (4) and the
electromagnetic locking unit (7),
Figure 1: Principle of the angle superposition Figure 2 : Schematic reprentation of the AFS-system components
•
AFS electronic control system with the AFS ECU (5),the pinion angle nsor (8), the motor angle n
sor (6), the respective electrical connections of the ECU and the required software modules.
COMPONENTS
The electric motor (e Figure 3) generates the required electrical torque for the desired motion of the AFS actuator. This synchronous motor has a wound stator, a permanent magnet rotor asmbly and a nsor to determine the rotor position. The motor torque is controlled by a field oriented control. This control strategy transform the stator currents into the torque-and rotor-flux-producing components. The current components can be controlled parately and do not depend on the rotor angle. The motor angle nsor is bad on a magneto-resistive principle and includes a signal amplification and a temperature compensation.This nsor signal is ud for control and monitoring purpos.
In analogy to the motor angle nsor, the pinion angle nsor is also bad on a magneto-resistive principle and includes a signal amplification and a temperature compensation. This nsor also includes a CAN-interface which enables other control systems like ESP to directly u the raw signal. The pinion angle is ud as an input to the steering assistance functions and for monitoring purpos.
The metal stud of the electromagnetic locking unit (ELU)is presd towards the worm-locking gear by a spring.This mechanism is unlocked by a specific current supplied by the ECU. The ELU locks the worm (Figure 3) if the system is shut down and in ca of a safety relevant malfunction (compare [7,8,9]). In this ca the driver is able to further steer with a constant steering ratio (i.e. the mechanical ratio).
The electronic control unit developed for the AFS system establishes the connection between the electrical system of the vehicle, the vehicle CAN – bus, the AFS
nsors and the electric motor.
Figure 3 : Electric Motor and Electromagnetic Locking Unit
The core components of the ECU are two microprocessors. They perform the computations required for control, monitoring and safety purpos. Via the integrated power output stages, the electric motor,the ELU, the ECO–pump and the Servotronic subsystem are controlled. The microprocessors also perform redundant computations and monitoring.
The basis of the AFS system is the well-tried and reliable rack and pinion power steering system of ZF Lenksysteme.
The core subsystem of AFS is the mechatronic actuator which is placed between the steering valve and the steering gear (e Figure 4). The actuator includes the planetary gear t with two mechanical inputs and a single mechanical output. The rvo-valve connects the input shaft of the planetary gear with the steering column and the steering wheel. The cond input shaft is driven by the electric motor and is connected to the planetary gear by the worm and worm wheel. The pinion angle nsor is mounted on the output shaft, which is the mechanical input for the steering gear. The relation between the input of the steering gear (pinion) and the road wheel angle is a nonlinear kinematic relation.FUNCTIONALITY
舒适近义词The functionality of AFS is defined by the so-called hardware oriented (low level) and the ur oriented (high level) functions. The functions can also be classified
into application and safety functions (e Figure 5).
Figure 4 : AFS actuator
Application functions are tho functions, that are required for the normal operation of the system. All other functions are part of the safety system. High level application functions can be classified into kinematic and kinetic functions (e Figure 6).
Figure 7 shows the signal flow of the AFS system in the vehicle-driver overall clod loop. With the vehicle signals as input, the stabilization (e.g. yaw rate control)and the assistance functions (e.g. variable steering ratio)compute a desired superposition angle. This angle rves as command input signal to the controlled actuator. A safety system monitors the function and the components of the steering system (compare [7] and [8]). Every failure or error, that may lead to a safety relevant situation, is identified and suitable actions are initiated in order to keep the system in a well defined
state.
Figure 5 : Structure of the AFS Functionality
Figure 6 : Structure of high level functions
Figure 7 : Block diagram including the overall signal flow in the AFS system
The actions reach from partial deactivations of single functions to shutting off the AFS system (fail silent behavior).
In the next subctions, some high level functions of the AFS system will be described.
KINEMATIC STEERING ASSISTANCE FUNCTIONS Kinematic steering assistance functions are feedforward controllers which adapt the static and dynamic steering characteristics to the current driving/vehicle situation as functions of the steering activity. This functionality is restricted by the actuator dynamics and the steering feel.The functions are part of the steering system (e Section 3). They are developed and implemented by ZF Lenksysteme.
Currently, the variable steering ratio (VSR) provides the most noticeable benefit for the driver. This kinematic function adapts the steering ratio i V (1), between the steering wheel angle and an average road wheel angle,to the driving situation as a function the vehicle velocity (e Figure 8). Under normal road conditions at low and medium speeds, the steering becomes more direct, requiring less steering effort (e Figure 9) of the driver which increas the agility of the vehicle in city traffic or when parking. At high speeds the steering becomes less direct, offering improved directional stability. Additional to the velocity dependency, the variable steering ratio developed by ZF Lenksysteme includes a dependency of the pinion rack displacement. This feature provides a reduced steering effort for large steering angles and a more preci steering for small steering angles.
The principle of this function is bad on the definition of the steering ratio
Fm
S
V :i δδ=
.
(1)
Figure 8 : Example of the variable steering ratio as function of vehicle velocity
Inrting the nonlinear kinematic relation ()()
G sk Fm f δ=δ between pinion angle δG , average road
wheel angle δFm and the linear kinematic relation ()
S S M M G k k δ⋅+δ⋅=δ between pinion angle,
steering wheel angle δS and motor angle δM into (1)yields the relation
()
最大的蚯蚓S S M M sk S
V k k f i δ⋅+δ⋅δ=
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为什么喜欢一个人
(2)
The core algorithm of the VSR function computes a motor angle VSR
d M δthat fulfils (2) for a predefined desired steering ratio i V and a measured steering wheel angl
e δS .Another steering assistance function that is evident for the driver in usual driving conditions is the so-called steering lead function (SLD). This kinematic function adapts the steer respon to the driving/vehicle situation as a function o
f suitable vehicle and steerin
g measured signals. The ZF Lenksysteme approac
h includes a differentiating prefilter for the steering wheel angle (e Figure 10). The weighted steering wheel angular velocity
S
SLD T δ⋅& defines then the desired motor angle (output
小猫钓鱼故事
of the SLD function) for the controlled AFS actuator.
Figure 9 : Slalom ride (cones distance: 16m and
vehicle velocity approx. 50 kph) with AFS/VSR and
with a conventional mechanical ratio
Figure 10 : Overall block diagram of the steering lead function
This algorithm reprents an inrtion of a zero 1 in the transfer function between steering wheel angle and average front wheel angle. This additional zero is placed so that the delay due to the dynamic of the steering system is reduced, partially compensated or if desired incread. Figure 11
shows the results of a double lane change manouver on asphalt at a vehicle speed of approx. 85 [km/h]. The incread steering dynamic reduces the required steering interventions in order to perform the driving task.
KINETIC STEERING ASSISTANCE FUNCTIONS Kinetic steering assistance functions also include feedforward controllers. Besides the primary task of providing the usual steering torque assistance like in conventional steering systems, the functions the additional task is providing a reduction/compensation of the reaction torque caud by the AFS actuator motion.The functions are restricted by the steering feel and the dynamics of the steering system. They are part of the steering system (e Section 3) and are developed and implemented by ZF Lenksysteme.
The first kinetic function is the rvotronic control function (SVT). The function algorithms include the computation of the desired current for the electro-hydraulic converter of the Sercotronic 2 component. The torque assistance is adapted to the driving/vehicle situation as a function of the vehicle velocity and the pinion angle velocity (actuator activity) (e Figure 12).The first dependency is the well-known vehicle-velocity dependent assistance torque, that provides the highest assistance torques for low velocities (i.e. steer comfort)and low assistance torques at high velocities in order to
improve the lateral stability of the vehicle.
Figure 11 : Double lane change with and without the SLD function
1
in terms of control engineering
The cond dependency is AFS specific and ts a reduction/compensation of the reaction torque.
Due to the possible high rack-displacement velocities, a higher 2 flow rate is required in order to take fully advantage of the AFS functionality. On the other hand thermal strains and a high fuel consumption have to be avoided. For that reason an electronic controlled orifice pump that modifies the flow rate in the hydraulic system has been included into the steering system. Another important kinetic function includes the control of the electronic controlled orifice pump (ECO). The main task of this function is to compute a desired current for the ECO-pump as a function of the vehicle velocity and the pinion angle velocity (actuator activity). The dependencies have been chon in analogy to dependencies for the Servotronic control.KINEMATIC STABILIZATION FUNCTIONS
The stabilization functions reprent another kind of consumer value increment. The functions include clod loop control algorithms that generate automatic 3steering interventions to stabilize the vehicle (e Figure
13).
Figure 12 : Example of the dependencies of the
desired current for the rvotronic control
Figure 13 : Lane change / ABS-braking with different steering functions (µ≈0.2)
2
higher than the required flow rate for similar vehicles with conventional steering systems 3
Automatic in a n of an explicit independency from the steering wheel angle defined by the driver
They are not part of the steering system (e Section 3),they are developed and implemented by the car manufacturer. Some examples of this kind of functions are (e [4,6]):
• yaw rate control,
• yaw torque control and
• disturbance rejection function.
SAFETY AND MONITORING FUNCTIONS
The above described functions imply high requirements for the safety integrity of the system [8,9]. For this reason ZF Lenksysteme has developed a suitable safety concept for the steering system that includes veral safety and monitoring functions on high and low level (e [7]).
3. MODULAR CONCEPT
In the first pha of the market introduction of AFS, ZF Lenksysteme developed the rack and pinion steering components, the mechatronic actuator as well as the electronic control unit which includes the low level software (e Figure 14). BMW developed the safety concept, the application and associated safety high level functions and also took the system responsibility [4,9](e Figure 14). In the cond pha of the AFS development ZF Lenksysteme focus on a modular concept that simplify the combination and integration of the AFS system with other chassis control systems and in different vehicle platforms [10]. The modular concept implies a clear distribution of responsibilities and the associated functionality and safety distribution (e Figure 15). Hereby, the steering system has to be autonomous and keep the complete steer functionality even in ca of failure or abnce of veral vehicle dynamic control systems (including the kinematic stability functions). The simplest approach to achieve this autonomy is a paration of vehicle and steering
functionality and safety in a hardware level.
Figure 14 : Overall block diagram of the first system concept
