Abstract:
Diagnostic monitoring of electrical signals from a quadrature sensor used for vehicle steering angle detection includes comparing the signals with a fault threshold (i.e., out-of-range voltages). A controller distinguishes between events of short enough duration that they do not cause loss of position accuracy and those where signal transitions were potentially missed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates in general to a relative steering angle sensor for detecting rotation of a steering wheel in a vehicle wherein a relative position offset is determined according to a dynamically determined steering wheel center position, and, more specifically, to sensor diagnostics for detecting short circuits or open circuits and for modifying the process for maintaining the relative position offset during fault conditions. 
     Steering angle sensors are employed on vehicles for a variety of purposes, such as yaw stability control (YSC) systems which sense vehicle operation and automatically apply braking forces to improve vehicle stability and control (e.g., reduce understeer or oversteer). To address certain drawbacks of steering angle sensors that incorporated an absolute position angle reference (e.g., inaccuracy due to steering mechanism wear, influence of uneven road surface, etc.), techniques were developed for using steering angle sensors that sense rotation without having a fixed reference. A “soft” position reference is determined by dynamically adjusting a relative position offset for each driving cycle beginning at “key on” of the vehicle in response to predetermined vehicle parameters from other sensors or actuators in the vehicle. These parameters include vehicle speed and yaw rate, for example. 
     Angle sensors employed for monitoring steering angle changes are typically comprised of quadrature pulse generators wherein the relative phase between two pulse trains identifies the direction of steering wheel rotation. The pulses are typically square waves produced by optical sensors wherein light beams are interrupted by a slotted disc that rotates with the steering wheel. Wiring from the sensor to a control module includes a connector which may typically be integral with a sensor housing. 
     The pulse signals from the sensor transition between a high signal state and a low signal state. Because of voltage drops across devices in the sensor circuitry, the high signal state has a voltage slightly less than the supply voltage V ss  and the low signal state has a voltage slightly higher than ground. The wiring to a sensor typically includes separate wires for V ss , ground, and each of the quadrature signals (e.g., phase A and phase B). 
     Electronic control modules typically include diagnostic routines for detecting faulted sensors. Potential failures associated with an angle sensor include intermittent connections caused by a loose connector, shorts to ground, and shorts to battery. These faults can be detected by monitoring voltage on the quadrature signal lines. Each voltage is compared to acceptable voltage ranges for the high signal state and the low signal state (i.e., voltages other than V ss  and ground), and a fault is detected whenever the voltages fall outside these ranges. 
     In order to provide noise immunity, a fault is not detected until an unacceptable voltage has persisted for a predetermined length of time (e.g., 100 milliseconds). However, intermittent faults can appear and then disappear on a shorter time scale. It has been discovered that such intermittent faults can go undetected and that they can cause the loss of rotation pulses from the sensor resulting in accumulated error in the determined steering angle. Incorrect steering angle can lead to false activation of the YSC system. 
     SUMMARY OF THE INVENTION 
     The present invention has the advantage of detecting intermittent sensor signal faults very quickly without complete shutdown of the sensor unless a fault persists for a sufficient amount of time. 
     In one aspect, the present invention provides a method of determining a relative position offset for a steering angle sensor in a vehicle. The relative position offset is iteratively adjusted in response to predetermined vehicle parameters. A voltage from the angle sensor is compared to a fault threshold. A duration of time is measured during which the comparing step indicates a fault. If a fault is indicated for a duration of time greater that a first predetermined duration, then the adjusting step is suspended. If the fault ceases to be indicated prior to reaching a second predetermined duration, then the adjusting step is resumed. If the fault continues to be indicated until reaching the second predetermined duration, then a faulted sensor signal is generated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic, block diagram showing elements of a yaw stability control system employing the present invention. 
     FIG. 2 is a waveform diagram showing output signals from a shaft rotation sensor. 
     FIG. 3 is a schematic, block diagram showing elements of FIG. 1 in greater detail. 
     FIG. 4 is a waveform diagram showing diagnostic voltage ranges for evaluating a sensor signal. 
     FIG. 5 is a schematic representation of the comparison of a sensor signal to the diagnostic voltage ranges. 
     FIG. 6 is a plot showing the adjustment of a relative position offset during fast-zeroing and fine-tuning sequences. 
     FIG. 7 is a state diagram showing a preferred embodiment of a controller implementation of the present invention. 
     FIGS. 8A and 8B are plots showing a first case of a type of fault handled by the present invention. 
     FIGS. 9A and 9B are plots showing a second case of a type of fault handled by the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a shaft  10 , such as a steering wheel shaft, is rotatable in both a clockwise and counterclockwise direction. A slotted disc  11  is mounted for rotation with shaft  10 . A rotation sensor  12  is mounted adjacent to slotted wheel  11  and provides position pulse signals to a controller module  13 . In one preferred application of the present invention, control module  13  is part of a yaw stability control for a motor vehicle. Thus, control module  13  receives signals from other vehicle sensors to detect vehicle stability performance (e.g., understeer or oversteer) and send the appropriate command signals to brake actuators  14  to correct for the understeer or oversteer. Control module  14  operates using DC power at a supply voltage V ss  referenced to ground. 
     In a preferred embodiment of shaft rotation sensing, slotted disc  11  has light transmitting openings equally spaced and having a width equal to the width of the light blocking areas between the openings. Optical transducers sense disc rotation as follows. A light source (e.g., an LED) is arranged on one side of slotted disc  11  and a pair of light sensors (e.g., phototransistors) are arranged on the other side of slotted disc  11  with an angular separation corresponding to one-half the width of the openings. This sensor arrangement provides what is known in the art as an optical quadrature signal. By using a pair of staggered light sensors, not only the amount of rotation but also the direction of rotation can be detected in response to the relative phase of the sensor signals. 
     The pair of light sensors, referred to as phase A and phase B, produce sensor signals as shown for example in FIG.  2 . The sensor signals are square-wave voltages that transition between a high signal state and a low signal state. Since the light sensors are spaced one-fourth of a cycle apart, direction of rotation is found in response to whichever sensor signal has transitions leading by 90°. As shown in FIG. 2, a change in the direction of rotation occurred at the dashed line since the sensor signal leading in phase changes from phase A to phase B. In a preferred embodiment, slot widths and spacings may be equal to about 9° and the light sensors spaces by about 4.5°, giving an angular resolution of about 4.5°. 
     FIG. 3 shows the electronic hardware for monitoring shaft position in greater detail. An electrical connector  15  is mated to connector pins  16  of position sensor  12 . Connector  15  is mounted to a wiring harness  17  including at least individual wire conductors carrying supply voltage V ss , phase signal A, phase signal B, and ground. 
     In a preferred embodiment, phases A and B are coupled to analog-to-digital (A/D) converters  20  and  21 , respectively, which provide digitized phase signals to a microprocessor  22 . A voltage range comparison is performed in microprocessor  22  to determine the high or low state of each of the phase signals and transitions are identified to track the amount and direction of rotation of the steering shaft. Alternatively, the voltage range comparison could be performed using analog comparison circuits and separate inputs provided to microprocessor  22  according to the comparison results. In any event, microprocessor  22  preferably also executes control algorithms for yaw stability control based in part on the steering wheel rotation signals and sends the appropriate commands to the brake actuators to improve yaw stability during understeer or oversteer conditions. 
     The relative voltage levels for signal phase detection and signal diagnostics are shown in FIG.  4 . Due to the driver circuitry in the rotation sensor, the actual voltages of the high signal state and the low signal state of the square-wave position pulses are offset from supply voltage V ss  and ground by a small voltage, typically corresponding to a diode drop (e.g., about 0.6 volts). For a supply voltage V ss  of about 5.0 volts, the high and low signal states would nominally be about 4.4 and 0.6 volts, respectively. Voltage ranges approximately centered on these values are used to distinguish the high and low signal states whenever measured voltage is within the ranges and to detect fault conditions when measured voltage is outside the ranges. The ranges allow for component variations and noise that may cause actual voltage to vary from nominal. 
     In FIG. 4, a voltage range from a voltage V 1  to a voltage V 2  corresponds to the low signal state and a voltage range from a voltage V 3  to a voltage V 4  corresponds to the high signal state. A pulse signal  24  corresponds to either one of the phase signals A or B and has the form of a square-wave with noise (e.g., electromagnetic interference). The controller samples signal  24  periodically (e.g., every 5 milliseconds) and compares the value of the signal with the predetermined ranges. Edge transitions of the pulse signal are detected when the range comparisons indicate a change between the “valid low” range (from V 1  to V 2 ) and the “valid high” range (from V 3  to V 4 ). 
     When the range comparisons indicate a value of pulse signal  24  other than valid low or valid high (i.e., the value is less than V 1 , between V 2  and V 3 , or greater than V 4 ), then a fault condition may exist and a diagnostic routine is executed. The most likely cause of a fault is typically an intermittent connection from a loose connector during-vibrations while driving the vehicle. However, as shown at point  25 , a very short excursion into the fault range might be caused by random noise. Thus, a lower limit on the duration of such an excursion may be employed below which a potential fault condition is not recognized. For example, a minimum duration of 100 milliseconds has been used for detecting a true loss of signal due to a fault. At point  26 , a fault clearly has occurred due to a complete loss of the sensor signal since the signal value has fallen below V 1  for a sufficiently long duration. 
     FIG. 5 shows the logic employed by a range comparator  29  in the controller (preferably performed in software but which can alternatively be implemented in analog hardware). A phase signal A or B is coupled to the noninverting inputs of a series of comparators  30 - 33 . Constants V 4 , V 3 , V 2 , and V 1  are coupled to the inverting inputs of comparators  30 - 33 , respectively. Inverters  34 - 37  invert the outputs of comparators  30 - 33 , respectively. The outputs of inverters  34 - 36  are coupled to respective first inputs of AND-gates  40 - 42 . The second inputs of AND-gates  40 - 42  are coupled to the outputs of comparators  31 - 33 , respectively. 
     The output of AND-gate  40  goes high to indicate a “valid high” signal value when comparator  31  is turned on and comparator  30  is turned off (i.e., the value is in the range from V 3  to V 4 ). The output of AND-gate  42  goes high to indicate a “valid low” signal value when comparator  33  is turned on and comparator  32  is turned off (i.e., the value is in the range from V 1  to V 2 ). A fault indication is provided by one of the outputs from comparator  30 , AND-gate  41 , or inverter  37  if the signal value is outside the valid ranges. Depending upon which invalid voltage range is detected, the fault indication has a sub-state of high, intermediate, or low. The sub-state provides additional troubleshooting information for the diagnostic code which is stored to facilitate servicing of a faulted system. 
     Since the steering position sensor in the preferred embodiment only provides signals based on rotation and does not give a fixed reference position, the controller must dynamically derive the steering wheel center position. A relative position offset is a variable that is maintained as an estimate of the difference between the initial steering angle upon power-up at the beginning of an ignition cycle and the steering wheel center position once straight driving is underway. FIG. 6 shows a beginning portion of a driving cycle and an actual value  43  of the steering wheel position angle and the adjusted value  44  of the relative position offset. Initially, the relative position offset is zero and the steering wheel angle is at some arbitrary position from the last driving cycle and is shown to be not at the center position. The prior art employs a fast zeroing technique during the early driving cycle in order to quickly obtain a relative position offset that is close to the correct value. The fast zeroing sequence is characterized by allowing the relative position offset to change at a fast rate. Thereafter, a fine-tuning sequence varies the value of the relative position offset at a slower rate but uses a more accurate algorithm. 
     In a presently preferred embodiment of the invention, the controller is programmed to implement the states shown in FIG. 7, including a power-up state  50 , a tentative state  51 , a recovery state  52 , a valid state  53 , and a faulted state  54 . 
     Upon power-up at the beginning of an ignition cycle (i.e., when a car is started up by turning a key in the ignition lock), the controller automatically proceeds from power-up state  50  to tentative state  51 . The purpose of the initial entry into tentative state  51  is to allow the controller to determine whether sensor signals within the proper range are being received. 
     At each entry into tentative state  51  (except for the first entry from power-up state  50 ), a “Tentative Occurrences” counter is incremented by one and the previous steering wheel angle (SWA) relative position offset is stored. Then a flag for indicating the validity of the relative position offset is cleared. While in tentative state  51 , the attempt to find a zero angle relative position offset is suspended. 
     In connection with the states, the controller maintains a series of SWA flags or fault codes to indicate the diagnostic condition of SWA operation. While in tentative state  51 , an “SWA Uncertain” flag is set or latched. When the SWA Uncertain flag is set, operation of YSC control is disabled. 
     During the time that the controller is in tentative state  51 , an “open/short” timer is continuously incremented (starting at zero when state  51  is first entered) in order to count the duration of the current out-of-range condition. A “tentative” timer is also incremented but is not reset to zero each time tentative state  51  is entered so that the total time during which a fault is indicated for all occurrences is integrated. 
     While in tentative state  51 , the sensor signals continue to be checked for any open/short (i.e., out-of-range) conditions. If an open/short is not being detected, then a transition is made to recovery state  52 . In a preferred embodiment, the determination of “not detecting open/short” is made when the sensor signals have been in range for a predetermined number of samples (e.g., five consecutive samples while sampling at 1 millisecond intervals). 
     For as long as the controller is not able to make a determination that an open/short is not detected, then the open/short timer and the tentative timer continue to increment. In the event that the open/short timer exceeds a time duration equal to a predetermined constant C 2  (e.g., 100 milliseconds) or the tentative timer exceeds a time duration equal to a predetermined constant C 5  (e.g., 10 seconds), then a transition is made to faulted state  54 . A transition is also made to faulted state  54  in the event that the tentative occurrences counter exceeds a predetermined number C 4  (e.g., 1000 entries into tentative state  51 ). The open/short timer detects a wiring harness fault or a sensor electrical fault. The tentative timer and the tentative occurrences counter are intended to determine that although the sensor has not lost significant counts during intermittent faults, it has been out of the valid range quite often and has a significant risk of losing counts. 
     Recovery state  52  is only entered from tentative state  51 . Upon entry into recovery state  52 , the open/short timer is zeroed and the adjustment process to find a zero angle relative position offset is enabled. In recovery state  52 , the fast-zeroing sequence described above is performed wherein the relative position offset can be adjusted to change value at a fast rate to reach a validated value as soon as possible during an ignition cycle. 
     While in recovery state  52 , the SWA Uncertain flag remains set and YSC control operation continues to be inhibited. 
     Once a validated value of the relative position offset is reached in recovery state  52 , a state transition is made. If this is the first time through recovery state  52  (i.e., there was not a previous valid value for the SWA relative position offset) or if the difference between the current SWA relative position offset and the previous SWA relative position offset is less than or equal to a predetermined difference C 6  (e.g., 15 degrees), then a transition is made to valid state  53 . If, instead, the difference between the current SWA relative position offset and the previous SWA relative position offset is greater than predetermined difference C 6 , then a transition is made to faulted state  54 . 
     Valid state  53  can only be entered from recovery state  52  and is the normal operating state of a healthy system. SWA relative position offset adjustment continues at the fine-tuning rate, the SWA Uncertain flag is cleared, and YSC control operation is allowed. 
     In both valid state  53  and recovery state  52 , the sensor signals continue to be compared to the fault threshold(s) to identify a fault condition. If either state detects that the sensor signals violate the thresholds (i.e., are out of range), then a transition is made back to tentative state  51 . A test for making the determination of “detecting open/short” is preferably comprised of checking for the sensor signal being out of range for more than 5 milliseconds (5 samples at 1millisecond per sample). 
     In faulted state  54 , a fault flag is set in order to disable further operation of the YSC control. Preferably, several fault flags are used in order to identify the type of fault that occurred. Specifically, an “SWA Open/Short” flag is set if the open/short timer caused the fault (a sub-state flag showing if the fault condition was high, intermediate, or low can also be set and recorded in a diagnostic code). An “SWA Intermittent” flag is set if either the tentative occurrences or the tentative timer caused the fault. An “SWA Shift” flag is set if the difference between successive offset values caused the fault. 
     The controller may respond to different fault flags differently. For example, when the open/short timer caused the fault but the tentative occurrences and tentative timer have not exceeded their maximums, it may be desirable to reset the fault flag at the beginning of the next ignition cycle and allow monitoring of the sensor signals and possible adjustment of a relative position offset. On the other hand, where the SWA Intermittent or SWA Shift flags are set, it may be preferable to maintain YSC control disabled until a vehicle can be serviced. 
     The following specific examples show how the present invention reacts to a momentary open circuit on a phase line. In a first case, FIGS. 8A and 8B show the phase A and phase B signals as received during an ignition cycle. Phase B shows nominal valid transitions during the entire time. Phase A shows a noise glitch at  60  having a sufficiently short duration of its departure from the valid range that it is ignored by the present invention. A momentary open at  61 , however, has a long enough duration to cause the diagnostic method to go from the valid state to the tentative state. The duration is short enough that when it ends the diagnostic method goes to the recovery state. 
     Dashed line  62  shows the actual signal that would have been present from phase A if not for the momentary open circuit. Since transitions were lost during the tentative state, the re-zeroing during the recovery state will produce a new relative offset value that is different from its previous value. If this difference is large enough (i.e., if enough transitions were missed) then the diagnostic method goes to the faulted state. 
     In a second example, FIGS. 9A and 9B show the phase A and phase B signals as received during another ignition cycle. Phase B shows nominal valid transitions during the entire time. Phase A shows a momentary open at  63  having a long enough duration to cause the diagnostic method to go from the valid state to the tentative state. The duration is short enough that when it ends the diagnostic method goes to the recovery state. Dashed line  64  shows the actual signal that would have been present from phase A if not for the momentary open circuit. Since transitions were not lost during the tentative state, the re-zeroing during the recovery state will produce a new relative offset value that is substantially the same as its previous value and a return can be made to the valid state. However, the tentative occurrences counter is incremented and the tentative timer grows larger as a result of the momentary fault. If momentary faults occur repeatedly, the sensor will eventually be faulted because data is not being reliably received.