Abstract:
A system and method of discriminating faults in an error signal, such as that from a control input sensor, comprises a system for and method of determining allowable limits of an error signal versus time, passing the error signal through a low-pass filter, comparing output of the low-pass filter with a threshold and outputting a fault condition when the output of the low-pass filter exceeds the threshold. The time constant or constants of the low-pass filter and the threshold are selected so that a fault condition is detected when the error signal approaches the allowable limits before exceeding them.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a robust fault detection method applicable to electronic steering systems for automobiles. 
     Steering equipment for assisting a driver to steer an automobile is well known in the art. In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is mechanically connected, usually through a gear assembly to the road wheels. To aid the operator, many systems utilize an auxiliary system to generate a force that is transmitted to a steering gear assembly. The additional force reduces the effort required by the operator in changing the direction of the vehicle. Typically, this auxiliary force is generated by either a hydraulic drive or an electric motor. 
     Control systems are known that provides a vehicle operator with an electric steering assist or electronic steering (or “steer-by-wire”) control for a vehicle. In an electric steering assist system, a control input is measured by a control input sensor, e.g., torque sensor. The output of the control input sensor is input into a control unit, which then drives a motor for assisting the driver in turning the steering column and thus turning the front wheels. It has been known to use sensors having a diagnostic output that provides an error signal for purposes of determining whether a sensor failure or fault condition exists. However, prior to the present invention, the control system has not been capable of determining whether a fault condition exists without occasionally generating false-positives. As will be made more clear, the approach taken by the prior art necessarily generated false positives in order to avoid false negatives. 
     The steer-by-wire control system comprises a steering wheel unit, a control unit, and a steering motor drive that operate together to provide steering control for the vehicle operator. The steering motor drive includes an electric motor for each road wheel steering mechanism and has several sensors including road wheel position sensors and steering wheel sensors. For the same reason mentioned above with respect to an electric steering assist system, the control system has not been capable of determining whether a fault condition exists in these control input sensors without generating false positives. 
     It would be desirable to provide a system that can accurately distinguish a fault condition from an ordinary transient control output without generating false positives. 
     SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by providing electronic steer-by-wire and electric steering assist systems with a robust fault detection scheme to determine if a control input sensor in the steering system is faulty. To determine the tolerable error, an error signal is applied at varying levels to determine how long the error can exist without violating pre-established system or vehicle deviation threshold. This testing results in a number of data points of error versus time which can be graphed and which will form a requirements curve. 
     Once the requirements curve is known, a time-constant and threshold are selected for a low-pass filter and comparator, respectively. The error signal is passed through the low-pass filter and comparator so that the fault discrimination curve approximates the requirements curve without exceeding it. 
     The above-discussed and other features and advantages will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the exemplary drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 shows a schematic overview of a steering control system; 
     FIG. 2 shows a schematic diagram of a electric power steering system; 
     FIG. 3 shows a control input error versus time graph depicting a fault detection strategy; 
     FIG. 4 shows a fault detection process diagram of a first exemplary embodiment; 
     FIG. 5 shows the fault detection process diagram of FIG. 4 with a bias correction filter; 
     FIG. 6 shows a fault detection process diagram of a second exemplary embodiment; 
     FIG. 7 shows a control input error versus time graph depicting another fault detection strategy; 
     FIG. 8 shows a fault detection process diagram of a third exemplary embodiment; and 
     FIG. 9 shows a control input error versus time graph depicting another exemplary fault detection strategy. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An exemplary system for detecting faults in a steer-by-wire control system  10  is shown in FIG.  1  and an exemplary electric steering assist system is shown in FIG.  2 . The steer-by-wire control system  10  includes a steering wheel unit  12  that accepts input from a steering wheel (not shown) or other input means from a driver or operator of the vehicle. The steering wheel unit determines the angle of the steering wheel or desired steering angle and provides this information via data signal  18  to control unit  20 . Control unit  20  accepts the steering wheel position data signal  18  along with other sensor inputs such as speed signal  16  from vehicle speed sensor  14 . 
     These inputs are used to determine a desired steering position for each wheel having steering capability (not shown). 
     The desired steering position for each wheel is compared with the actual steering position for each wheel, which is determined from feedback signal  26  containing the steering position for each wheel having steering capability. Steering torque or strain on the control rod may be measured to provide feedback to the driver. Control unit  20  calculates or otherwise determines (e.g., via a look-up table) a new position for steering motor based on the actual steering position for each wheel. The new output for each steering motor is sent to steering motor drive  28 . Note that there may be a single steering motor drive as shown in FIG. 1 for a four-wheel vehicle wherein the left and right wheels turn in unison, or multiple steering motor drives—one for each wheel having steering capability. For example, each front wheel may have an independent steering motor drive and each back wheel may also have a steering motor drive. For vehicles having more than 2 axles, each wheel may be independently controlled for maximum control and reduced wear on the tires. 
     FIG. 2 shows a schematic diagram of an electric steering assist system  30 . In this system, steering wheel  32  is mechanically connected to steering mechanism  48  which rotates front wheels  49  (only one shown). To assist the driver, steering wheel position and torque sensor  34  senses torque applied to steering column  33  by a driver and detects the angle of steering wheel  32 . The torque and angle information are provided to controller  36 . Speed sensor  38  also provides vehicle speed information to controller  36  via signal  39 . Controller  36  is powered by battery  35  and power line  37 . Controller  36  provides a drive voltage to motor  40  via line  41 . Motor  40  in turn applies a torque to output shaft  42 , thereby reducing the share of torque against steering mechanism  48  applied by a driver or operator at steering wheel  32 . 
     Electronic steering systems  10  and  30  both generate control input signals which are sent to a control unit that produces output for rotating, or assist in rotating, wheels having steering capability. In the case of a faulty control input sensor, the control unit will be operating with faulty inputs and a danger exists of generating faulty output. The control system disclosed herein allows for such a steering system to accurately distinguish between normal and abnormal operation of control input sensors. Although the description that follows is directed to torque sensors by way of example, the system described can be used to discriminate between normal and faulty operation using error signals from other control input sensors, as well as error signals in general. For example, the system may be applied to internal error signals generated within a control system. 
     Torque sensors are available that generate two separate signals. A first torque signal T 1  is a voltage signal, e.g., from 0 to 5 volts and a second torque signal T 2  is a voltage signal from, e.g., 5 to 0 volts. Adding the two signals together should always equal 5 volts (T 1 +T 2 =5). The actual torque is obtained by subtracting T 2  from T 1  (T 2 −T 1  input sensors that directly generate digital outputs, including a diagnostic error signal output are also available. The fault detection method described herein is applicable to either type of control input sensor. 
     FIG. 3 shows a graph  50  in which the vertical axis represents torque error signal in volts and the horizontal axis represents time in milliseconds. Requirements curve  60  is plotted connecting points representing the maximum safe or allowable time that a given amount of torque error can be tolerated based on the steering system characteristics. These points are determined empirically by direct observation, by extrapolation of such empirical studies, by a computer or mathematical modeling of a real-world steering system, or some combination thereof. For example, to empirically determine data points in the requirements curve, specific error amounts are applied to a control input, and the system is monitored to see how long such error amounts can be tolerated before they violate pre-established system or vehicle deviation thresholds. 
     When approximated by requirements curve  60 , the test or analysis results represent a requirement that the torque output remain below and left of the requirements curve  60  for any given interval of time. If a torque error signal can be plotted above requirements curve  60 , then the torque sensor is faulty. 
     One way of determining whether a fault condition exists is to test whether the torque error signal exceeds a threshold value for a given amount of time. Boundary  90  represents an example of this strategy. For any point above and to the right of boundary  90 , a fault condition is determined, while if the point is below or to the left of boundary  90 , then it is assumed that no fault condition exists. The use of a threshold value suffers from the drawback that a large percentage of possible good error signal-time values are excluded from boundary  90  and would erroneously be judged a fault condition. For example, point  80  which represents a sensor voltage signal of about 0.75 volts for approximately 80 milliseconds, is well on the safe side of requirements curve  60 , but outside boundary  90  and therefore would erroneously be considered a fault condition. It would be desirable to improve this result by finding a simple way of distinguishing fault conditions that closely approach requirements curve  60 , thereby avoiding false-positives for faults. 
     Referring to FIG. 4, an exemplary embodiment is shown in the form of a process diagram  100  comprising a low-pass filter  104  to modify incoming torque error signal  102 . The signal output from low-pass filter  104  is converted to a positive value using absolute value function  106 . Then it is compared with a fault threshold value in comparator  108 . If the filtered value exceeds the threshold value, then a fault signal is generated at  110 . 
     The usefulness of this method is best seen by the example shown in FIG.  3 . In this example, a time constant of 0.05 is applied to incoming torque signal voltages. A first curve  62  represents an input voltage of 620 millivolts. Curve  64  represents the input of 1.2 volts, curve  66  represents an input of 1.8 volts and curve  68  represents an input of 4 volts. Looking by way of example to curves  64  and  66 , it can be seen that they pass a threshold of 600 millivolts at about 25 and 35 milliseconds, respectively. This shows that an input voltage of 1.2 volts and 1.8 volts registers a fault at 25 and 35 milliseconds, respectively, and so these points are plotted at  74  and  72 . Remaining points  76 ,  78 ,  80 ,  82 , and  84  are similarly generated, though the curves for each of the represented inputs are not shown. As can be clearly seen, points  74 ,  76 ,  78 ,  80 ,  82 , and  84  are significantly closer to requirements curve  60  than is threshold boundary  90 . 
     In the above example, low-pass filter  104  is a first-order filter. To even more closely approximate requirements curve  60 , low-pass filter  104  may be a higher-order low-pass filters (having more than one time constant). Such a higher-order low-pass filter may in fact be multiple cascaded first-order low-pass filters, as is generally known in the art. 
     FIG. 5 shows a process diagram  120  similar to  100  in FIG. 4, but with the addition of a error signal bias correction function. Incoming error signal  122  is passed to a long-time low-pass filter  124 . Output from long-time low-pass filter  124  should approximate a bias of the torque error signal. This value is passed to limit function  126  so that a fault will be detected if the bias is too great. Output from limit function  126  is subtracted from the original incoming error signal  122 , resulting in a bias-compensated torque error signal, which is then passed to low-pass filter  130 , absolute value function  132 , comparator  134  as described above with respect to process diagram  100  above. If comparator  134  finds that the absolute value of the output of low-pass filter  130  is above a predetermined threshold, then a fault is determined. 
     A single low-pass filter  130  may be replaced with multiple low-pass filters having different time constants and multipliers may be employed with a maximum function to produce more complex voltage-time fault lines. An example of this technique is shown by process diagram  150  in FIG. 6 where an incoming signal  152  is directed to multiple low-pass filters  154 ,  156 . These are directed through gain multipliers  158 ,  160  and the output thereof is directed to absolute functions  159 ,  162  and then to maximum function  164 . Any number of filter-multiplier combinations are contemplated, as suggested in FIG. 5 by dashed lines leading to n th  low-pass filter  157 , n th  multiplier  161 , and n th  absolute function  163 . It should be understood that the multiplication constants for the gain multipliers may be normalized so that one of the multipliers, e.g., K 1 , has a multiplication constant of 1 or may be removed entirely. The output of maximum function  164  is the greatest of the two inputs from multipliers  158 ,  160 . This value is passed to comparator  166  which compares the signal to a fault threshold. If the signal is greater than the threshold value, then fault  168  is generated. Each low-pass filter  154 ,  156  has a distinct time constant and each multiplier  158 ,  160  has a distinct multiplication constant K x . Therefore, the filter and multiplier combination having the maximum value will vary depending on the strength and interval of the incoming signal. Additionally, the incoming signal may be sinusoidal in nature, so that it is possible that a low-pass filter having a slower time constant will exceed a low-pass filter having a faster time constant. While process diagram  150  does not include a bias compensation routine as provided by low-pass filter  124  in FIG. 5, it is contemplated that one could advantageously be provided prior to low-pass filters  154 ,  156 , and  157 . 
     An example of the effect of multiple low-pass filters and maximum function can be seen by way of graph  200  in FIG. 7, which shows two voltage-time fault lines represented by curves  210  and  220 . Curve  220  corresponds to a filter-multiplier combination having a faster time constant and smaller multiplication constant than curve  210 . It can be seen that by taking the maximum of the two curves, an engineer can more closely tailor the fault discrimination curve to the requirements curve. 
     Another strategy for approximating requirements curve is depicted by way of process diagram  250  in FIG.  8 . In this case, a non-linear low-pass filter is applied to the torque error signal. Torque signals T 1  and T 2  are input at  252  and  254 . Signal conditioner  251  includes offset and trim summers  256  and  258 , and the two signals are summed at summer  260 . The output of summer  260  is torque error  261 . Torque error  261  is split for the purposes of filtering out signal bias errors in the incoming signals in signal bias correction procedure  268 . Signal bias correction procedure  268  compensates for signal bias errors in the torque error  261  in a manner similar to that shown in FIG.  5 . Because the torque signals T 1  and T 2  may exhibit non-linearities, particularly at the extremes of its range, signal bias error correction procedure  268  employs 3 low-pass filters  280 ,  282 , and  284 . Although three low-pass filters are shown, any number of low-pass filters necessary to compensate for non-linearities in the control input sensors may be used. One long-time low-pass filter  282  is used in the center portion of the range of output of T 1  and T 2 . Long-time low-pass filters  280  and  284  are used at the extreme negative and positive ends of the range of output of T 1  and T 2 , as will now be described in further detail. 
     Diff Torque Trimmed signal (DTT)  262 , which is essentially T 2 −T 1 , is applied to logic boxes  264  and  266  to control the input of control input error  261  in three low-pass filters, such that if DTT  262  is less than−4 Newton-meters, torque error  261  is applied to a first long time low-pass filter  280 ; if DTT  262  is between −4 and 4 Newton-meters then torque error  261  is applied to a second long time low-pass filter  282 . If DTT  262  is greater than 4 Newton-meters, then torque error  261  is passed to a third long time low-pass filter  284 . 
     Each low-pass filter  280 ,  282 , and  284  output to a limit function  286 ,  288 , and  290 , respectively, so that a fault is determined when the bias exceeds a predetermined value. 
     The learned value for each long time low-pass filter  280 ,  282 , and  284  is saved in EEPROM, or other non-volatile memory, at each loop of the procedure described in process diagram  250 . The output of bias error correction procedure  268  is subtracted from torque error  261  in summer  270  and results in bias-compensated torque error  271 . This value is applied to non-linear low-pass filter  274  which employs a look-up table  272  to determine the adjustment to non-linear low-pass filter output  275  in the known manner. The non-linear low-pass filter output  275  is passed to absolute function  276 . The output of absolute function  276  (A) is then compared with a threshold (B). If absolute function output  276  is greater than threshold B, then a fault condition is determined. 
     FIG. 9 shows a graph  300  depicting an exemplary application of the error discrimination technique described above with reference to process diagram  250  in FIG.  8 . Curves  112 ,  114 ,  116 , and  118  represent outputs from non-linear low-pass filter such as described above is applied to a step function of 620 mV, 1.2 V, 1.8V, and 4 V is applied, respectively. As described above with reference to FIG. 3, points  124 ,  126 ,  122 ,  128 ,  130 ,  132 , and  134  are plotted as the time required to generate a fault at the respective voltages applied to the non-linear filter. It can be seen that the non-linear filter technique generates a fault discrimination much closer to requirements curve  60  than the linear filter technique shown by FIG.  3 . 
     It will be understood that the methods described above may be implemented using analog control circuitry or digitally using a digital control input sensor or an analog sensor with analog-to-digital converter and microprocessor, or a combination of analog and digital processes. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.