Abstract:
An improved vehicle yaw control that does not require a yaw sensor, wherein the validity of an estimate of vehicle yaw is determined and used to select an appropriate control methodology. The vehicle yaw is estimated based on the measured speeds of the un-driven wheels of the vehicle, and various other conditions are utilized to determine if the estimated yaw rate is valid for control purposes. When it is determined that the estimated yaw rate is valid, a closed-loop yaw rate feedback control strategy is employed, whereas in conditions under which it is determined that the estimated yaw rate is not valid, a different control strategy, such as an open-loop feed-forward control of vehicle yaw, is employed. The validity of the estimated yaw rate is judged based on a logical analysis of the measured wheel speed information, braking information, and steering wheel angle. The measured speeds of the un-driven wheels are used to compute an average un-driven wheel speed and an average un-driven wheel acceleration. The operator steering angle and the vehicle velocity are used to determine a desired yaw rate, which is compared to the yaw estimate to find a yaw rate error. Based on these variables, the control reliably determines whether the estimated yaw rate is valid, and selects an appropriate control methodology in accordance with the determination.

Description:
FIELD OF THE INVENTION 
     This invention relates to vehicle yaw control that does not require a yaw sensor, wherein the validity of an estimate of vehicle yaw is determined and used to select an appropriate control methodology. 
     BACKGROUND OF THE INVENTION 
     Chassis control technology has achieved noteworthy progress, thanks to advancements in sensing and computing technologies as well as advances in estimation and control theory. This has permitted the design of various control systems using active means to achieve a more maneuverable vehicle. One such enhancement is the control and adjustment of the tire forces through the braking force distribution control strategy, using a steering wheel angle sensor, a lateral accelerometer, and a yaw rate sensor to devise a yaw rate feedback control. Because the price of these different sensors, especially the yaw rate sensor, is still high, this technology is limited to a small number of vehicles. While the vehicle yaw rate can be computed as a function of the measured speeds of the un-driven wheels, the estimate fails to faithfully track the actual vehicle yaw during braking or when the vehicle exhibits an oversteer condition. What is desired is a yaw control that does not require a yaw sensor, but that can reliably control yaw even during conditions that degrade the validity of the estimated or computed yaw. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is directed to an improved vehicle yaw control that does not require a yaw sensor, wherein the validity of an estimate of vehicle yaw is determined and used to select an appropriate control methodology. According to this invention, the vehicle yaw is estimated based on the measured speeds of the un-driven wheels of the vehicle, and various other conditions are utilized to determine if the estimated yaw rate is valid for control purposes. When it is determined that the estimated yaw rate is valid, a closed-loop yaw rate feedback control strategy is employed, whereas in conditions under which it is determined that the estimated yaw rate is not valid, a different control strategy, such as an open-loop feed-forward control of vehicle yaw, is employed. 
     According to the invention, the validity of the estimated yaw rate is judged based on a logical analysis of the measured wheel speed information, braking information, and steering wheel angle. The measured speeds of the un-driven wheels are used to compute an average un-driven wheel speed and an average un-driven wheel acceleration. The operator steering angle and the vehicle velocity may be used to determine a desired yaw rate, which is compared to the yaw estimate to find a yaw rate error. Based on these variables, the control reliably determines whether the estimated yaw rate is valid, and selects an appropriate control methodology in accordance with the determination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a vehicle including an electronic controller and associated input and output devices constituting a control system for carrying out an active brake control of vehicle yaw. 
     FIG. 2 is a main flow diagram representative of computer instructions executed by the electronic controller of FIG. 1 in carrying out the control of this invention. 
     FIG. 3 is flow diagram of an interrupt service routine executed by the electronic controller of FIG. 1 to determine the validity of a yaw estimate according to this invention. 
     FIGS. 4-10 are flow diagrams setting forth further detail regarding the various flow diagram steps of FIG.  3 . FIG. 4 is directed to a Brake Disturbance status check; FIG. 5 is directed to a Straight-Line status check; FIG. 6 is directed to a Yaw Rate Error status check; FIG. 7 is directed to an Onset of Instability status check; FIG. 8 is directed to a Brake Disturbance History status check; FIG. 9 is directed to a Spin Detection status check; and FIG. 10 is directed to a Yaw Rate Valid status check. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a mechanization of an active brake control according to this invention on a vehicle  10 . The vehicle  10  includes a brake system having a micro-processor based controller  68  for controlling the brakes  20 ,  22 ,  24 ,  26  of the respective wheels  12 ,  14 ,  16 ,  18 . The controller  68  receives various inputs, including wheel speed signals on lines  36 ,  38 ,  40 ,  42  from respective wheel speed sensors  28 ,  30 ,  32 ,  34 ; a brake pedal travel signal on line  83  from pedal travel sensor  82 ; a steering wheel angle signal on line  62  from angle sensor  61 ; and a master cylinder pressure signal on line  96  from the pressure sensor  94 . The sensors  28 ,  30 ,  32 ,  34 ,  61 ,  82 ,  85 ,  94  may be implemented with conventional devices in a manner known to those skilled in the art. 
     Under certain conditions such as wheel lock-up or spinning, or lateral instability, the controller  68  modifies the normal braking of one or more wheel  12 ,  14 ,  16 ,  18  via the respective actuators  52 ,  54 ,  56 ,  58  in order to restore a desired overall operation of the vehicle. In an incipient lock-up condition, the controller  68  commands one or more of the respective actuator(s)  52 ,  54 ,  56 ,  58  to modulate the brake force developed at the wheel(s) experiencing the condition. In a wheel slip condition, the controller  68  commands one or more of the respective actuator(s)  52 ,  54 ,  56 ,  58  to develop brake force at the slipping wheel(s). In a case of lateral instability, the controller  68  commands one or more of the respective actuator(s)  52 ,  54 ,  56 ,  58  to selectively increase or decrease the brake forces generated at the various wheels  12 ,  14 ,  16 ,  18  to produce a commanded yaw or differentially brake the vehicle wheels to impart a yaw moment; the control may be carried in a two-channel system in which only the front brakes  20 ,  22  are controlled, or a four-channel system in which all four brakes  20 ,  22 ,  24 ,  26  are controlled. Exemplary actuators are shown and described in detail in the U.S. Pat. No. 5,366,291, assigned to the assignee of the present invention. 
     A main flow diagram for carrying out a control according to this invention is depicted in FIG.  2 . The block  100  designates a series of initialization instructions executed at the initiation of vehicle operation for appropriately setting the initial condition or state of the various terms and flags referred to below. After reading the various sensor inputs at block  102 , the block  104  is executed to determine the status of the YAW VALID FLAG. The status of the YAW VALID FLAG indicates whether the estimated yaw value is considered to be valid, and the status of the flag is periodically determined by the interrupt service routine of FIG. 3, described below. 
     If the estimated yaw rate is considered to be valid (i.e., YAW VALID FLAG=1), the blocks  106 - 112  are executed to determine a closed-loop yaw command. Block  106  determines a desired yaw value for the vehicle based on various inputs including the vehicle speed and the measured steering wheel angle. Block  108  determines the yaw error based on the deviation of the yaw estimate from the desired yaw rate. Block  110  determines a yaw rate command based on the yaw rate error and suitable gain factors, and block  112  decides if active brake control is warranted based on predefined entry and exit conditions. 
     If the estimated yaw rate is considered to be invalid (YAW VALID FLAG=0), the blocks  118 - 120  are executed to determine an open-loop or feed-forward yaw command based on various inputs including the vehicle speed and the measured steering wheel angle. For example, a desired yaw rate value Ω des  may be determined according to the expression: 
     
       
         Ω des   =V   x δ/( L+K   u   V   x   2 )   (1) 
       
     
     where L is wheel base of the vehicle, and K u  is an understeer coefficient, and the yaw command may be determined with conventional proportional and derivative control gains. Block  120  decides if active brake control is warranted based on predefined entry and exit conditions, which may differ from the entry and exit conditions designated at block  112 . 
     Finally, block  114  carries out an algorithm for distributing braking forces between the left and right vehicle wheels, and block  116  applies corresponding brake control signals to the brake actuators  152 - 158 . Preferably, braking is only applied to the driven wheels of the vehicle so as to not corrupt the yaw estimate. Various brake distribution strategies may be utilized, exemplary strategies being disclosed in the U.S. patent applications Ser. Nos. 08/654,982 and 08/732,582, both of which are assigned to the assignee of the present invention. 
     As indicated above, FIG. 3 is a flow diagram for an interrupt service routine which is executed in response to a periodic interrupt request to determine the status of the YAW VALID FLAG. Block  132  designates input signal processing such as filtering of the wheel speed signals to reject noise or unwanted information. The yaw rate is then estimated as a function of the left and right un-driven wheel speeds ω l  and ω r  and the track t of the vehicle, as indicated at block  134 . In the usual application, the vehicle has a front-wheel drive powertrain, and the un-driven wheels are the left and right rear wheels  16  and  18 . In the illustrated embodiment the estimated yaw rate {dot over (ψ)} e  is determined according to the expression:                  ψ   .     e     =         ω   l     -     ω   r       t             (   2   )                                
     Block  136  then determines the simple arithmetic average ω avg  of speeds of the un-driven wheels, and block  138  estimates the average acceleration a avg  of the un-driven wheels. The average acceleration a avg  may be estimated as a function of current and previous speed and acceleration values according to the following expression, where the designation (k) refers to the current control loop, and the designation (k−1) refers to the prior control loop: 
      {circumflex over (ω)} av ( k )=(1 −Tg   1 ){circumflex over (ω)} av ( k −1)+ Tg   1 ω av ( k )+ Tâ   av ( k −1)â av ( k )=â x ( k −1)+ Tg   2 (ω av ( k )−{circumflex over (ω)} av ( k ))   (3) 
     where T is the sampling time or period, and the gain terms g 1  and g 2  are given by the following expressions: 
     
       
           g   1 =2ξ(2 πf   n ), and  g   2 =(2π f   n ) 2    (4) 
       
     
     where ξ is the desired damping ratio, and f n  is the desired bandwidth. The damping ratio ξ governs the convergence of the estimated and true acceleration, while the bandwidth f n  determines the maximum frequency content of the acceleration to be included in the estimation. 
     Blocks  140 - 152  are then executed to determine the status of a number of flags which indicate the presence or absence of various predefined operating conditions relevant to the validity of the yaw rate determined at block  134 . Block  140  concerns the status of a Brake Disturbance flag (BK-DIST FLAG), described in detail below in reference to the flow diagram of FIG.  4 . Block  142  concerns the status of a Straight Line flag (ST-LINE FLAG), described in detail below in reference to the flow diagram of FIG.  5 . Block  146  concerns the status of a Yaw Error flag (YAW-ERR FLAG), described in detail below in reference to the flow diagram of FIG.  6 . Block  148  concerns the status of An Onset of Instability flag (INSTAB FLAG), described in detail below in reference to the flow diagram of FIG.  7 . Block  150  concerns the status of a Brake Disturbance History flag (BK-DIST HIS FLAG), described in detail below in reference to the flow diagram of FIG.  8 . Block  152  concerns the status of a Spin Detection flag (SPIN FLAG), described in detail below in reference to the flow diagram of FIG.  9 . Finally, block  154  concerns the status of the Yaw Rate Valid flag (YAW-VALID FLAG) referred to at block  104  of the main flow diagram of FIG. 2, and is detailed below in reference to the flow diagram of FIG.  10 . 
     The Brake Disturbance flag (BK-DIST FLAG) is intended to indicate the presence of a brake disturbance. This flag is used in determining the status of the Brake Disturbance History flag (BK-DIST HIS FLAG) described below in reference to the flow diagram of FIG.  8 . If the service brakes are applied, as determined at block  160  of FIG. 4, blocks  162 - 164  are executed to initialize a timer or counter referred to herein as the Brake Disturbance Timer (BK-DIST TMR) to a predetermined value, designated as BKTIME, and to set the BK-DIST FLAG=1. Once the brakes are released, the blocks  166 - 168  are executed to decrement the Brake Disturbance Timer at each interrupt until the timer value has been decremented to zero. At such point, block  166  will be answered in the affirmative, and block  170  is executed to set BK-DIST FLAG=0. Thus, the BK-DIST FLAG is set at the initiation of braking, and reset a predefined time after the brakes have been released. 
     The Straight Line flag (ST-LINE FLAG) is intended to indicate whether the vehicle  10  is heading straight; that is, not turning. This flag is used in determining the status of the SPIN flag (SPIN FLAG) described below in reference to the flow diagram of FIG.  9 . If the steering wheel angle (SWA) in either direction is less than a reference angle (SWA th1 ) and the estimated yaw rate {dot over (ψ)} e  in either direction is less than a reference rate {dot over (ψ)} th1 , as determined by blocks  180  and  182  of FIG. 5, the block  184  is executed to increment a timer or counter referred to herein as the Straight Line Timer (ST-LINE TMR). Otherwise, block  186  is executed to reset the Straight Line Timer to zero. So long as the value or count of ST-LINE TMR is less than a reference time designated as SLTIME, as determined at block  188 , the block  192  sets the ST-LINE FLAG=0. Once the value or count of ST-LINE TMR exceeds SLTIME, the block  190  sets the ST-LINE FLAG=1. Thus, the ST-LINE FLAG is maintained in a reset (0) condition until straight line driving conditions (steering wheel angle and yaw) have been established for a predefined period of time. 
     The Yaw Rate Error flag (YAW-ERR FLAG) is intended to indicate whether the vehicle  10  is in a linear operating region, based on the deviation of the estimated yaw value {dot over (ψ)} e  from the desired yaw value determined at blocks  106  or  118 . This deviation, referred to herein as the yaw error, or {dot over (ψ)} err , is determined at block  198  of FIG.  6 . If the yaw error {dot over (ψ)} err  in either direction is at least as great as a threshold error {dot over (ψ)} th2 , as determined by block  200 , the blocks  202  and  204  are executed to set the YAW ERROR FLAG=0, and to reset a timer or counter referred to herein as the Yaw Error Timer (YAW-ERR TMR). If the yaw error is within the threshold error, blocks  206 - 208  increment the Yaw Error Timer at each interrupt until the value or count reaches a predefined time designated as YETIME. At such point, the block  210  sets the YAW-ERR FLAG=1. Thus, the YAW-ERR FLAG is maintained in a reset (0) condition until a linear operating condition (based on yaw error) has been established for a predefined period of time. This flag is used in determining the status of the Brake Disturbance History and Spin flags described below in reference to the flow diagrams of FIGS. 8 and 9, respectively. 
     The Onset of Instability flag (INSTAB FLAG) is intended to indicate the presence of a condition in which the vehicle  10  has a tendency to become unstable, based on the average acceleration a avg  of the un-driven wheels. As noted below in reference to FIG. 10, the YAW VALID FLAG is reset to zero if INSTAB FLAG=1, indicating the presence of such a condition. If the average acceleration a avg  in either direction is within a threshold acceleration a th1 , as determined by block  220  in FIG. 7, the blocks  222  and  224  are executed to set the INSTAB FLAG=0, and to reset a timer or counter referred to herein as the Acceleration Timer (ACCEL TMR). If the average acceleration is outside the threshold, blocks  226 - 228  increment the Acceleration Timer at each interrupt until the value or count reaches a predefined time designated as ACTIME. At such point, the block  230  sets the INSTAB FLAG=1. Thus, the INSTAB FLAG is maintained in a reset (0) condition until the average acceleration of the un-driven wheels exceeds a threshold for a predefined period of time. 
     The Brake Disturbance History flag (BK-DIST HIS FLAG) is intended to indicate the presence of braking that would corrupt the yaw rate estimate of expression (2), above. As noted below in reference to FIG. 10, the YAW VALID FLAG is reset to zero if BK-DIST HIS FLAG=1, indicating the presence of such braking. If the BK-DIST FLAG described above in reference to the flow diagram of FIG. 4 is set, as determined at block  232  of FIG. 8, the block  234  is executed to set BK-DIST HIS FLAG=1. If the BK-DIST FLAG=0, indicating that the brakes have been released for a predefined time, the blocks  236 - 240  are executed to determine if the BK-DIST HIS FLAG should be reset, based on the average speed and acceleration of the un-driven wheels, and the status of the YAW-ERR FLAG, described above in reference to the flow diagram of FIG.  6 . If the average speed ω avg  is greater than a threshold speed ω th2 , and the average acceleration a avg  in either direction is less than a threshold acceleration a th2  or the YAW-ERR FLAG=1 (indicating the presence of a linear operating condition), the block  242  is executed to set BK-DIST HIS FLAG=0. Thus, the BK-DIST HIS FLAG is set as soon as the brakes are applied, and reset after the brakes have been released for a predefined period of time, and the average un-driven wheel speed is greater than a reference, and the average acceleration and yaw error are indicative of linear operation of the vehicle. 
     The Spin flag (SPIN FLAG) is intended to indicate the occurrence of a vehicle spin-out. As noted below in reference to FIG. 10, the YAW VALID FLAG is reset to zero if SPIN FLAG=1, indicating the occurrence of such a condition. If the average speed ω avg  of the un-driven wheels is less than a threshold speed ω th3 , and the average acceleration a avg  in either direction is greater than a threshold acceleration a th3 , as determined at blocks  250 - 252  in FIG. 9, the block  254  is executed to set SPIN FLAG=1. Blocks  256 - 262  then determine if the SPIN FLAG should be reset to zero. To clear or reset the flag, three conditions must be met. The first condition, determined by blocks  256 - 258 , requires that either the YAW-ERR FLAG=1 (low yaw error), or that the ST-LINE FLAG=1 (straight vehicle heading). The second and third conditions, determined by blocks  260 - 262  require that the average speed ω avg  of the un-driven wheels is greater than the threshold speed ω th3 , and that the average acceleration a avg  in either direction is less than the threshold acceleration a th3 —i.e., the opposite condition defined by blocks  250 - 252 . If all three conditions are met, the block  264  is executed to set SPIN FLAG=0. Thus, the SPIN FLAG is set if the average speed and acceleration values are indicative of a spin-out condition, and reset if the average speed and acceleration values and the states of the Yaw Error and Straight Line flags indicate stable linear operation. 
     The status of the Yaw Valid flag, determined by the flow diagram of FIG. 10, depends on the status of the Brake Disturbance History, Spin and Onset of Instability flags. If any of these flags are set, as determined at blocks  270 - 274 , the block  276  is executed to set the YAW-VALID FLAG=0, indicating that the yaw estimate determined at block  134  of FIG. 3 should not be used for yaw control; in this case, the open-loop control based on blocks  118 - 120  is utilized, as described above. If each of the Brake Disturbance History, Spin and Onset of Instability flags are reset to zero, the block  278  is executed to set the YAW-VALID FLAG=1, indicating that the yaw estimate determined at block  134  of FIG. 3 should be used for yaw control; in this case, the closed-loop control based on blocks  106 - 112  is utilized, as described above. 
     In summary, this invention provides a low-cost vehicle yaw control that does not require a yaw sensor. An estimate of yaw based on the speeds of the un-driven wheels of the vehicle is utilized for closed-loop control of yaw so long as a logical analysis of other parameters including the un-driven wheel speeds, estimated yaw error, braking and steering wheel angle indicates that the yaw estimate is valid. When the logical analysis indicates that the yaw estimate is no longer valid, an alternate control that does not require yaw feedback (such as the disclosed open-loop feed-forward control) is utilized for control of yaw. Obviously, various modifications of the illustrated embodiment will occur to those skilled in the art, and in this regard, it will be understood that the scope of this invention is not necessarily limited by the illustrated embodiment, but is defined by the appended claims.