Abstract:
A speed sensor instability detection and correction system for use with a vehicle includes a speed sensor output variation extractor monitoring and recording change in output of a speed sensor over time. A speed sensor output variation comparison module detects speed sensor instability based on a comparison between the speed sensor output variation and a predetermined threshold. An output speed determination module selectively determines output speed in alternative ways based on whether speed sensor instability is detected.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to acceleration based traction control systems, and more particularly to a control system and method for detecting wheel slip during output speed instability due to speed sensor instability. 
     BACKGROUND OF THE INVENTION 
     As a motor vehicle is being driven, it is not uncommon for one of the driven wheels to lose its grip on the road. To promote safety, traction control systems have been developed for detecting wheel spin and reducing wheel torque in response to wheel spin. In one traction control system, described in U.S. Pat. No. 6,182,002 B1, vehicle acceleration is measured and used to detect wheel spin. Another traction control system is described in U.S. patent application Ser. No. 10/601,063, filed Jun. 20, 2003, entitled “Wheel Slip Detection and Torque Management”, which is commonly assigned and hereby incorporated by reference in its entirety. This traction control system uses pulley speed of a transmission, such as a continuously variable transmission (CVT), to detect wheel slippage. 
     In other systems, wheel speeds are monitored to detect wheel spin. For example, in vehicles having anti-lock brake systems (ABS), wheel speed sensors are used to provide information to a traction control system. As a result, wheel slippage can be detected directly. 
     Speed sensors also play other important roles in a motor vehicle. For example, a CVT ratio is determined as a function of throttle pedal signal, pulley speed signal, and torque converter turbine speed signal. As a result, various output speeds are combined to affect control of the vehicle. Further, some output speeds, such as pulley speed, may play multiple roles. 
     A problem therefore arises in each of the forgoing systems and methods when a speed sensor exhibits instability resulting, for example, from a missing tooth or electrical instability. For example, when instability occurs in the output pulley speed or turbine speed, the ratio of the CVT is perturbed. Also, when instability occurs in output pulley speed or output wheel speed, engine flare may result due to incorrect detection of wheel slippage. 
     SUMMARY OF THE INVENTION 
     In one embodiment in accordance with the present invention, a speed sensor instability detection and correction system for use with a vehicle includes a speed sensor output variation extractor that monitors and records change in output of a speed sensor over time. A speed sensor output variation comparison module detects speed sensor instability based on a comparison between the speed sensor output variation and a predetermined threshold. An output speed determination module selectively determines output speed in alternative ways based on whether speed sensor instability is detected. 
     In another embodiment in accordance with the present invention, a vehicle includes a speed sensor that detects a speed of a vehicle. A speed sensor instability detector detects speed sensor instability based on a change in output of the speed sensor over time. An output speed determination module employs a median filter to filter output of the speed sensor when the speed sensor instability detector indicates speed sensor instability. 
     In a further embodiment in accordance with the present invention, a method for detecting wheel slip based on output speed of a vehicle includes determining whether output speed instability is detected. The method also includes generating a median filtered output speed when output speed instability is detected. The method further includes determining whether wheel slip is present based on the median filtered output speed when output speed instability is detected. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a vehicle including a traction control system according to one embodiment of the present invention; 
         FIG. 2  is a flow diagram depicting wheel speed slippage detection in accordance with the present invention; 
         FIG. 3  is a flow diagram depicting speed sensor instability detection in accordance with the present invention; and 
         FIG. 4  is a flow diagram depicting output speed determination in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar components. 
     A vehicle having a traction control system in accordance with one embodiment is referred to generally in  FIG. 1  by reference number  10 . The vehicle  10  includes an engine  12  having a plurality of cylinders  14 , each with one or more intake valves and/or exhaust valves (not shown). The engine  12  is configured with a fuel injection system  16  and an ignition system  18 . An output of the engine  12  is coupled by a torque converter  22  having a torque converter clutch  23 , a continuously variable transmission  24 , a driveshaft  26  and a differential  28  to driven front wheels  30   a.  Rear wheels  30   b  are non-driven in the present embodiment. The transmission  24  is controlled by a controller  20 . 
     One or more wheel speed sensors  32  transmit speed signal(s) to the controller  20 . In the embodiment shown in  FIG. 1 , driven wheels  30   a  and non-driven wheels  30   b  have speed sensors  32   a  and  32   b  respectively. In other embodiments, fewer than four wheel speed sensors may be provided. For example, one sensor is used on one of the non-driven wheels. Shaft speed sensors  34  transmit input signals to the controller  20 . 
     An electronic throttle controller (ETC)  36  adjusts a throttle  38  in an intake manifold  40  based upon a position of an accelerator pedal  42  and a throttle control algorithm that is executed by the controller  20 . The throttle  38  adjusts an output torque that drives the wheels  30   a.  One or more sensors (not shown), for example a manifold pressure sensor and a manifold air temperature sensor, may be provided in the intake manifold  40 . 
     An accelerator pedal sensor  44  generates a pedal position signal that is output to the controller  20  based on a position of an accelerator pedal  42 . A position of a brake pedal  46  is sensed by a brake pedal sensor  48 , which generates a brake pedal position signal that is output to the controller  20 . Other sensor inputs collectively indicated by reference number  50  are used by the controller  20  for traction control. Such inputs include a signal  52  indicating engine speed, a vehicle speed signal  54 , an accelerator position signal  56  and a throttle position signal  58 . One such signal to the controller  20 , a trans throttle signal  60 , indicates throttle pressure in the transmission  24 . Signals  62  and  64  indicate transmission input and output pulley speeds. 
     The present invention operates in the above-mentioned context to detect output speed sensor instability and take appropriate corrective action. It should be readily understood that instability in output pulley speed, output turbine speed, and output wheel speed sensor signals may be utilized in accordance with the present invention. 
       FIG. 2  illustrates wheel slippage detection in accordance with the present invention. Accordingly, when output speed instability is detected as at  70 , a median filter is used to filter the output speed at step  72 . In contrast, when output speed instability is not detected at  70 , then a Kalman filter is used to filter the output speed at  74 . It is envisioned that an output pulley speed sensor instability is detected at  70 , and that the output pulley speed sensor signal is filtered in steps  72  and  74 . Then, actual speed ratio is calculated at step  76  and continued detection of output speed instability is enabled at step  78 . The actual speed ratio is used at step  80  to calculate wheel axle torque, and this wheel axle torque is used at step  82  with the output speed filter value of step  72  or  74  to determine an expected output speed derivative. In parallel, the same output speed filter value is used at step  84  to calculate an actual output speed derivative. Then, the actual output speed derivative of step  84  is compared at  86  to the expected output speed derivative of step  82 . Thus, if the expected output speed derivative is greater than the actual output speed derivative, then wheel slippage is undetected at step  88 . Otherwise, wheel slippage is detected at step  90 . 
       FIG. 3  illustrates detection of output speed instability based on speed sensor instability. Accordingly, if the sensor is not in a default state and the instability detection is currently enabled as at  94 , then instability detection is allowed to take place. In order to have flexibility on different applications, a selectable instability bit, or flag, is added to govern whether detection is enabled. The detection is preferably performed when the bit is set to true. However, the detection is disabled when the speed sensor has failed, and any previously detected instability is undetected at step  96  if detection is disabled for any reason. 
     Speed sensor instability detection according to the present invention includes comparing a median sensor speed to a predetermined threshold at  98  to ensure that instability can be differentiated from a low speed output at low speeds. Thus, if the median output speed is not too low at  98 , then the absolute value of the speed sensor variation for one loop is compared to a predetermined threshold at  100 . If the variation is high enough for the loop, then an instability counter is incremented at  102 . Each loop represents a small amount of time, so this counter allows loop to loop instability to be tracked over time to improve accuracy. Thus, if the counter reaches a predetermined threshold during a predetermined amount of time as at  104 , then instability is deemed reliably detected and the counter is reset to zero at  106 . Instability then remains detected until undetected, for example, at step  96 . However, instability can also be undetected at step  108  if instability has remained detected for a predetermined amount of time and the instability counter is zero as at  110 . These conditions ensure that other software processes will have time to appropriately react to detected instability before it is undetected, and also that detected instability is not immediately undetected as a result of concurrently resetting the instability counter at step  106 . Additionally, the counter is reset at step  112  if the counter does not exceed the threshold within the predetermined amount of time as at  114 . This reset results in undetection of instability on a subsequent loop at  108  if enough time has passed at  110 . It is envisioned that the detection may be employed with respect to an output pulley speed sensor, a turbine speed sensor, a wheel speed sensor, or additional types of sensors detecting speed of vehicle component or the vehicle itself. 
       FIG. 4  illustrates alternative procedures for determining output speed based on output speed instability detection in accordance with the present invention. Accordingly, a Kalman filter is employed at step  116  to filter the output of the speed sensor if sensor instability is not detected at  118 . If an antilock brake system employing wheel speed sensors is not present at  120 , then a median filter is employed to filter output of the speed sensor at  122 . This filter takes a point sample and rejects the extreme value. However, the present invention is also capable of recalculating output speed based on a maximum output speed of the wheel sensors and a transfer gear of the CVT at step  124 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the current invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.