Re-configurable control of steer-by-wire systems

1. Technical Field

The present invention relates generally to steer-by-wire systems, and more particularly relates to re-configurable control of steer-by-wire systems.

2. Background Information

Vehicle steering control systems, such as electric power steering control systems and steer-by-wire control systems, are electro-mechanical motion control systems with sensors, actuators, and other electrical and mechanical components. Such systems have become increasingly complex in recent years. It is important for such steering control systems to deal with faults in sensors, actuators, power electronics, and mechanical components. The fault tolerant operation and ability to deal with malfunctions is, therefore, a key issue for vehicle steering control systems, especially for full steer-by-wire systems.

For safe operation of the steer-by-wire control system, hardware redundancy for critical components can be used. However, it is limited by weight and package, cost, space, and similar installation environment for the hardware.

It is possible to eliminate or reduce redundant hardware and to enhance safety by using the analytic redundancy and re-configuration for controller structures and/or parameters of the steer-by-wire systems to maintain the system stability, integrity and certain performance requirements when faults occur in the steer-by-wire control systems.

The present invention provides a re-configurable control for a steer-by-wire system. The re-configurable control maintains the steer-by-wire system to be stable and controllable despite faults that occur in the sensors, actuators, and other mechanical or electrical components.

The steer-by-wire control system has an adaptive re-configurable control structure that is implemented in the steer-by-wire system in accordance with the present invention. This allows a steer-by-wire controller to re-configure its structure and/or parameters to satisfy requirements of stability, integrity and certain performance requirements for the steer-by-wire system.

An adaptive re-configurable control unit is provided with fault detection and isolation, fault decision, parameter identification for the steer-by-wire controlled plant, and a re-configurable controller with automatic adjustable structure and/or parameters.

A residual generator generates a series of residual signals based on the output and input signals of the steer-by-wire controlled plant. The residual signals are evaluated in a fault decision unit to determine whether or not the fault occurs. In the event of a failure, a fault decision signal is given to indicate the fault.

A parameter identifier estimates the model parameters of the steer-by-wire controlled plant in real time. Based on the result of the fault decision signal and the parameter identification, the re-configurable controller reconfigures its structure and/or parameters to compensate for the effects of the fault according to the stability and certain performance requirements of the overall steer-by-wire control system.

The present invention also provides a method of re-configurable control of fault tolerant steer-by-wire systems.

The present invention may include one or more of the following advantages and benefits. The re-configurable control of a steer-by-wire system provides enhanced fault tolerance and safety. Because redundant hardware is eliminated or minimized, the implementation of the re-configurable control results in weight reduction of the vehicle. The system is also cost efficient and has improved system performance and product quality compared to conventional steering systems when the re-configurable control of steer-by-wire system is used. Moreover, the system is easily implemented in embedded systems.

The foregoing discussion has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.

The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings:

FIG. 1 is a schematic diagram of a steer-by-wire system with a reconfigurable control unit in accordance with the invention;

FIG. 2 is a schematic block diagram of a re-configurable steer-by-wire control system having a steering wheel control system and a road wheel control system;

FIG. 3 is a block diagram of a re-configurable steer-by-wire control system in accordance with the invention; and

FIG. 4 is a flow diagram of a sequence of steps performed by the re-configurable steer-by-wire control system of FIG. 2 in accordance with the invention.

FIG. 1 illustrates an embodiment of a steer-by-wire system 5 in accordance with the present invention. The steer-by-wire system 5 includes a steering wheel controlled plant 16 , a road wheel controlled plant 18 , and a re-configurable control unit 10 . The steering wheel controlled plant 16 and the road wheel controlled plant 18 are electro-mechanical systems that are controlled by the re-configurable control unit 10 . The re-configurable control unit 10 receives the measured sensor signals from the steering wheel controlled plant 16 and road wheel controlled plant 18 , and provides control commands to the steering wheel controlled plant 16 and road wheel controlled plant 18 to perform the required steering functions while maintaining system stability and certain performance requirements even if one or more faults occurs in the steer-by-wire system 5 .

The steering wheel controlled plant 16 includes a steering wheel mechanism, such as a joystick or steering wheel 20 , a steering wheel angle sensor 22 , an electrical steering wheel motor actuator 24 , a steering wheel motor amplifier 26 , and a steering shaft 28 that connects the steering wheel 20 , the sensor 22 , and the steering wheel actuator 24 together.

The road wheel controlled plant 18 includes one or more road wheels 30 , an electrical road wheel motor actuator 32 , a road wheel motor amplifier 34 , a road wheel angle sensor 36 , a road wheel mechanism 38 (such as a gear and rack), and a pair of tie rods 40 that connect the road wheels 30 and road wheel mechanism 38 together. In some implementations, the road wheel controlled plant 18 includes two front road wheels and two rear road wheels, each having its own road wheel actuator and road wheel angle sensor.

The re-configurable control unit 10 includes a steering wheel re-configurable controller 12 , which controls the operation of the steering wheel controlled plant 16 , and a road wheel re-configurable controller 14 , which controls the operation of the road wheel controlled plant 18 .

The steering wheel re-configurable controller 12 receives the steering wheel angle θ s from the sensor 22 and provides a steering wheel control command u s to the motor amplifier 26 of the steering wheel controlled plant 16 . The road wheel re-configurable controller 14 receives the road wheel angle Or from sensor 36 and provides a road wheel control command u r to the motor amplifier 34 of the road wheel controlled plant 18 .

In the illustrated embodiment, the steering wheel controlled plant 16 and its associated re-configurable controller 12 provide a steering control system, and the road wheel controlled plant 18 and its re-configurable controller 14 provide a road wheel control system. The steering control system and the road wheel control system are integrated as the steer-by-wire system 5 through signal transactions between the two controllers 12 and 14 . As shown in FIG. 1, the steering wheel angle θ s is sent to the road wheel re-configurable controller 14 , and the road wheel angle θ r and equivalent road wheel torque τ r are sent to the steering re-configurable controller 12 .

FIG. 2 illustrates a schematic block diagram of a re-configurable steer-by-wire control system 41 having a steering wheel control system 42 and a road wheel control system 43 . Therefore, in essence, FIG. 2 is a control system block diagram representation of the steer-by-wire system 5 shown in FIG. 1 . As shown in FIG. 2, the steering wheel control system 42 includes the steering wheel controlled plant 16 and its associated re-configurable controller 12 shown in FIG. 1, and the road wheel control system 43 includes the road wheel controlled plant 18 and its re-configurable controller 14 . The closed-loop feedback control for the steer-by-wire system 5 provides the expected steering functions.

In the steer-by-wire system shown in FIG. 2, the steering control system 42 provides the steering feel for the vehicle driver, the steering wheel reference command to the road wheel control system, and control functions for the steering wheel return rate. The road wheel control system 43 provides the control of the tracking of the road wheel angle to the steering wheel reference angle.

In accordance with the invention, the re-configurable control is implemented in either or both the road wheel control system 43 or the steering wheel control system 42 . Both systems 42 , 43 maintain stability and certain performance requirements even if one or more faults occur in the steering wheel controlled plant 16 and the road wheel controlled plant 18 .

Since the steering wheel control system 42 and the road wheel control system 43 are similar in their hardware architectures including sensors, motor actuators, wheels and their assemblies, there is the same general structure for the re-configuration control of the steering wheel control system and road wheel control system shown in FIG. 2 .

FIG. 3 shows a block diagram of an adaptive re-configurable steer-by-wire control system generally identified as 50 , corresponding to either the steering control system 42 or the road wheel control system 43 of FIG. 2 . As shown, the steering wheel re-configurable controller 12 or the road wheel re-configurable controller 14 in FIG. 2 are represented generally by a control unit 59 , and the steering wheel controlled plant 16 or the road wheel controlled plant 18 in FIG. 2 are represented generally by a controlled plant 52 .

The re-configurable steer-by-wire control system 50 shown in FIG. 3 provides a general re-configuration control structure and includes the controller 51 that has functions of adjustable structure and parameters, a parameter identifier 54 that identifies the parameters of the controlled plant 52 in real time, a residual signal generator 56 that generates a residual signal in the presence of faults, and a fault decision unit 58 that determines faults.

The re-configurable steer-by-wire control system 50 shown in FIG. 3 performs the following tasks:

Detects and diagnoses sensor, actuator, and system component failures in the controlled plant 52 ;

Evaluates the severity of the failures;

Identifies the parameters of the controlled plant 52 ; and

Restructures the control laws and/or reschedules the control gain for the controller 51 .

As shown in FIG. 3, the residual generator 56 produces residual signals r(t), using the inputs u(t) and outputs θ(t) of the controlled plant 52 . A mathematical model of the controlled plant is used in the residual generator 56 . The residual generator 56 then performs a consistency check between the controlled plant model and variables of the real controlled plant to produce the non-zero residual signal r(t) when any fault occur. The controlled plant models can be any kind of model, such as the state-space model discussed below.

Consider the following steer-by-wire controlled plant in the fault-free case described by the following state space equations:

{dot over (x)} ( t )= Ax ( t )+ Bu ( t )  (1)

y ( k )= Cx ( k )  (2)

where y(t), u(t) and x(t) are the input, output, and state, respectively, at time t, and A, B and C are constant matrices of the appropriate dimensions that describe the steer-by-wire controlled plant. The equations (1) and (2) describe the actual steer-by-wire controlled plant without faults.

The steer-by-wire controlled plant described in (1) and (2) will vary in presence of faults. The general description for the steer-by-wire controlled plant with faults is given as follows:

{dot over (x)} ( t )= Ax ( t )+ Bu ( t )+ R 1 f ( t )  (3)

y ( t )= Cx ( t )+ R 2 f ( t )  (4)

where R 1 and R 2 are known distribution constant matrices of faults, and f(t) is a fault vector, with each element representing a specific fault.

The residual r(t) is a signal that carries information on the time and location of the faults. It is near zero in the fault-free case and deviates from zero when a fault has occurred. Generally, the residual generator 56 which generates the residual signal r(t) is a type of state observer. The following gives a typical description for such a residual generator:

r ( t )= x ( t )− {circumflex over (x)} ( t )  (5)

{dot over ({circumflex over (x)})} ( t )= A{circumflex over (x)} ( t )+ Bu ( t )+ K ( y ( t )− ŷ ( t ))  (6)

where K is a constant matrix for the state observer (6).

The isolation of a specific fault f i (t) can be implemented in the residual generator 56 . It can be represented as an element of the residual vector, r i (t), where i=1,2,3, . . . , q.

As shown in FIG. 3, the residual signals r(t) from the residual generator 56 is sent to the fault decision unit 58 . The fault decision unit 58 evaluates the residuals and monitors if and where a fault has occurred. The decision function is represented by J(r(t)) and the following test is performed generally to determine a fault:

J ( r ( t ))≦ T ( t ) for f ( t )=0  (7)

J ( r ( t ))> T ( t ) for f ( t )≠0  (8)

where T(t) represents the threshold.

The isolation of a specific fault, such as the i th of possible faults, can be tested by:

J ( r i ( t ))≦ T i ( t ) for f i ( t )=0  (9)

J ( r i ( t ))> T i ( t ) for f i ( t )≠0  (10)

where i=1,2,3, . . . , q.

The model parameters of the controlled plant 52 are estimated by using a parameter identifier 54 in FIG. 3 . The general state-space model of the parameter identifier is given by:

{dot over (x)} ( t )= Âx ( t )+ {circumflex over (B)}u ( t )  (11)

y ( t )= Ĉx ( t )  (12)

where Â, {circumflex over (B)}, and Ĉ are constant matrices with the identified parameters. The signal α(t) of the parameter identifier 54 in FIG. 3 represents the set of identified parameters.

As shown in FIG. 3, the controller 51 is an adaptive controller with the capability to update controller parameters or structure in real time according to the fault information f(t) from the fault decision unit 58 and the identified parameters α(t) from the parameter identifier 54 . The controller parameters may be the simple gain or any parameters in the controller. As a result of the adaptive re-configuration of the controller, the effects of the fault occurring in sensors, actuators, and system components are compensated. Therefore, the steer-by-wire system shown in FIG. 3 will maintain stability and certain performance requirements.

The controller 51 described in the form of a state-space model is given as follows:

u ( t )=− f ( Â, {circumflex over (B)}, Ĉ ) x ( t )  (13)

where the function f(Â,{circumflex over (B)},Ĉ) is the parameter and structure adjustable.

The controller 51 shown in equation (13) can be implemented with different control algorithms. For example, in some implementations, the controller 51 is a simple proportional controller in the following form:

u ( t )=K p ( Â,{circumflex over (B)},Ĉ ) e ( t )  (14)

where K p (Â,{circumflex over (B)},Ĉ) is a proportional parameter which varies with the controlled plant parameters (A, B,C), and e(t) is the error signal between the reference signal and the output signal generally in the control system. Under a conventional proportional control, the value of the parameter K p remains constant. However, in the re-configurable steer-by-wire control system 50 , the parameter identifier 54 provides the controlled plant parameters such that the controller parameter K p varies with the identified plant parameters Â,{circumflex over (B)},Ĉin real time when the fault occurs to cause the plant parameters to vary.

The adaptive re-configurable control for the steer-by-wire system 50 shown in FIG. 3 minimizes the need for redundant hardware and enhances safety by reconfiguring the control strategy. Rather than using hardware redundancy, the steer-by-wire control system 50 uses analytical redundancy and a re-configurable control algorithm to be tolerant of faults of sensors, actuators, and system components in the steer-by-wire system.

Referring now to FIG. 4, there is shown a process 100 that depicts a general procedure performed during the operation of the adaptive re-configurable control for the steer-by-wire system 50 of FIG. 3 . The process 100 initializes in step 102 . The input signal u(t) and the output signal θ(t) of the controlled plant 52 are captured in step 103 . In step 104 , the failure residual generator 56 generates the residual signal r(t) to indicate the fault-free with near zero residual or a deviate from zero when a fault occurs. For example, the residual signal is 0.1° for a road wheel angle sensor measurement without fault and varies to 2° with a fault at a time t.

Subsequently, in step 106 , the fault decision unit 58 generates the fault isolation and decision signal f(t) that is based on the residual signal r(t) and the decision algorithm. The fault signal f(t) indicates that the residual signal r(t) is larger than the threshold which represents the acceptable range. The fault signal f(t) carries information on the time and location of the faults.

Accordingly, if the process 100 decides in step 108 that there is no fault in the steer-by-wire control system, then the process proceeds to step 110 , indicating that the steer-by-wire control system 50 is operating in its normal mode, and returns to step 103 to capture the input signal and the output signal of the controlled plant 52 and step 104 to repeat the fault detection, isolation and decision in steps 104 and 106 . If, on the other hand, the process 100 decides that there is a fault, then the fault signal f(t) is sent to step 116 which is the re-configurable control algorithm.

In the meantime, the parameter identifier 54 identifies the appropriate parameters of the controlled plant 52 in step 114 . The identified parameters α(t) in step 114 and the fault signal f(t) in steps 106 and 108 are sent together to step 116 . In step 116 , the controller 51 re-configures its structure and/or adjusts its parameters according to the fault signal f(t) and the identified parameters α(t). Subsequently, the process 100 ends in step 118 .

In sum, the steer-by-wire control system is an adaptive re-configurable control system that compensates for faults occurring in one or more sensor, actuator, and other components that could compromise the operation of the system. The adaptive re-configurable steer-by-wire control system has the ability to maintain system stability and certain performance requirements through a methodology where the controller automatically re-configures its structure and/or parameters when the fault occurs.

It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.