Abstract:
An apparatus and method for steering a vehicle (not shown) provides an input member or steering wheel for receiving steering inputs from a user; a steering-torque sensor mechanically coupled to the input member for sensing the torque applied to the steering wheel; a control circuit electronically coupled to the steering-torque sensor for producing a signal corresponding to the measured driver steering torque; an output actuator electronically coupled to the control circuit and road wheel for receiving the electronic signal and translating it into mechanical motion; and an output member or road wheel mechanically coupled to the output actuator for steering the vehicle (not shown).

Description:
TECIINICAL FIELD  
         [0001]    The present disclosure relates to a vehicular steer-by-wire system, and more particularly, to a redundant sub-system for providing controllability in the event of a partial system failure.  
         BACKGROUND  
         [0002]    Modern vehicles are increasingly equipped with sophisticated electronic control systems for achieving finer control. Steer-by-wire is one variety of control system in which the conventional direct mechanical linkage between the input device (e.g., steering wheel or handwheel) and the output device (e.g., steered road wheel) is replaced with a system incorporating electronic input sensors, control circuitry, and output actuators.  
           [0003]    In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is typically mechanically connected 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.  
           [0004]    One desirable feature of conventional systems is the robust nature of their components. A pending mechanical failure generally exhibits detectable symptoms prior to significant failure. Thus, a feature desirable in a steer-by-wire system is a redundant sub-system to permit continuation of steering control in the case of a sensor failure.  
           [0005]    Under the prior art, steer-by-wire systems typically rely on a steering wheel position sensor, the output of which is used to compute a control signal to a road-wheel actuator. Redundancy is typically accomplished with duplicate components, such as duplicate steering wheel position sensors, which improve neither performance nor functionality during normal operation. Accordingly, it is desirable to provide redundancy in a steer-by-wire system without requiring significant duplication of components.  
         SUMMARY  
         [0006]    Redundant or fail-safe control of an output device, such as steered road wheels, is provided in the unlikely event of missing or erroneous steering-angle signals from an input device, such as a steering wheel.  
           [0007]    The steer-by-wire system includes an input member or steering wheel for receiving the steering inputs made by an operator, a steering-torque sensor connected to the steering wheel for sensing the level of torque being applied by the operator, a control circuit connected in electronic communication with the steering-torque sensor for controlling the actuation of at least one steered road wheel, an output actuator or motor electronically connected to the control circuit for effecting actuation of the steered road wheel, and an output member or tie rod connected between the motor and the road wheel for mechanically translating motor position into road wheel steering angle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is an elevational view of a vehicular steer-by-wire system.  
         [0009]    [0009]FIG. 2 is schematic circuit diagram of a control circuit.  
         [0010]    [0010]FIG. 3 is a flowchart for an error detection method of a vehicular steer-by-wire system. 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0011]    As shown in FIG. 1, a vehicular steer-by-wire system is indicated generally by the reference numeral  10 . The system  10  includes an input member or steering wheel  12 , a coupling or steering shaft  14  connected to the steering wheel  12 , a steering-angle sensor  16  connected to the shaft  14 , a steering-torque sensor  18  connected to the shaft  14 , an electronic controller  20  operably connected with the steering-angle sensor  16  and the steering-torque sensor  18 , an output or road-wheel actuator  22  coupled in signal communication with the controller  20 , and an output member or road-wheel  24  mechanically connected to the road-wheel actuator  22 .  
         [0012]    As may be recognized by those skilled in the pertinent art based on the teachings herein, various modifications may be made to this exemplary embodiment without departing from the scope or spirit of the present disclosure. For example, the steering wheel  12  may be replaced or supplemented with any of a number of input members for receiving the desired steering inputs of an operator, such as a control yoke or a joystick. In addition, although the exemplary steering-torque sensor  18  is mechanically coupled to the steering wheel  12  through the steering shaft  14 , various other torque-sensing schemes may be apparent to those skilled in the pertinent art based on the teachings herein, such as, for example, integral piezo-electric sensors and non-contact electromagnetic sensors. The steering-angle sensor is typically embodied by an optical encoder, but may alternatively be embodied by, for example, a potentiometer or other device for sensing angular displacement.  
         [0013]    The controller  20  is an electronic circuit comprising a digital micro-controller integrated circuit (“IC”) such as, for example, an HC68000 series micro-controller IC manufactured by Motorola Corporation. The controller  20  receives as input the electronic signal  27  produced by the steering-angle sensor  16  and the electronic signal  26  produced by the steering-torque sensor  18 , and produces as output a control signal  42  for the road-wheel actuator  22 . The control signal  42  has a power level that is capable of powering an actuator, and is input to the road-wheel actuator  22  that mechanically actuates the road wheel  24  according to the control signal  42 .  
         [0014]    As shown in FIG. 2, the controller  20  of FIG. 1 implements a control function indicated generally by the reference numeral  21 . The control function  21  receives as inputs a differential torque signal  26  from the steering-torque sensor  18 , a steering-angle signal  27  from the steering-angle sensor  16 , and a vehicle speed signal  29  indicative of the relative velocity of the vehicle (not shown) with respect to the travel medium (e.g., road or land surface, also not shown). A position ratio unit  39  corresponding to a desired steering-ratio function that varies according to the current value of the steering-angle signal  27  and the speed signal  29 , processes the steering-angle signal  27 .  
         [0015]    Under normal operation, the steering-angle sensor  16  detects the position and movement of the steering wheel  12  and sends a steering-angle signal  27  to the controller  20 . The controller  20  combines the steering-angle signal  27  with the vehicle speed signal  29  to produce the road wheel control signal  42  that is sent to the road-wheel actuator  22  for controlling the steering angle of the road wheel  24 . Thus, under normal operation, the output signal  26  produced by the torque sensor  18  is not required for determination of the command signal  42 . It shall be understood that the road wheel command signal  42  may also correspond to additional sensor signals and functions, as may be desirable for alternate applications.  
         [0016]    The control function  21  is used in the calculation of the road-wheel control signal  42 . The position ratio unit  39  receives the steering-angle signal  27 . The position ratio unit  39  also receives the vehicle speed signal  29 . The steering-angle signal  27  and the vehicle speed signal  29  are used as inputs to unit  39 , which comprises a multiplier, to generate a variable steering ratio signal at unit  39 . The resulting variable steering ratio signal is passed to a road wheel command switch  37 . It will be recognized that although the exemplary position ratio unit  39  comprises a multiplier, other means for serving the function of the multiplier may be substituted therefor, such as, for example, a non-linear algorithm or a three-dimensional look-up table.  
         [0017]    The integration sub-function  28  has an anti-windup feature and integrates the differential torque signal  26  over time to produce a signal  30  indicative of the torque applied to the steering wheel  12 . The system  10  can have the integration sub-function  28  in or out. When the integration sub-function  28  is out, a change in direction in the torque sensor  18  causes the corresponding control signal  42  to the road wheels  24  to be immediate. With the integration sub-function  28  in, the system  10  changes direction at a slower rate than the torque input signal  26 , as it unwinds the integration sub-function  28  before a direction change occurs. A variable gain function  32  scales the gain applied to the signal  30  based on the speed signal  29  to produce a speed-weighted steering-correction signal  34 . The speed-weighted signal  34  is then limited according to limiting function  36  in order to create a speed-limited steering correction signal  38 , and thus to avoid an excessive change in steering angle at higher vehicle speeds.  
         [0018]    The controller  20  generally receives signals from the sensors  16  and  18 , and determines whether each received signal is valid or erroneous, as described below. The switch  37  is used to selectively pass either the output of block  36  in a fail-safe or backup mode, corresponding to the torque signal  26 , or the output of unit  39  in a normal mode, corresponding to the position signal  27 , to a road-wheel position command generator  40 . In the backup mode where the controller  20  is receiving a valid signal  26  from the steering-torque sensor  18 , but not receiving a valid signal  27  from the steering-angle sensor  16 , the switch  37  causes the road-wheel position command generator  40  to produce a signal  42  corresponding to the speed-limited signal  38  for controlling the road-wheel actuator  22  in accordance with the differential torque signal  26 .  
         [0019]    In the normal mode of operation, the controller  20  receives a valid signal from the steering-angle sensor  16  and the switch  37  causes the road-wheel position command generator  40  to produce a signal  42  corresponding to the output of the steering-angle sensor  16  for controlling the road-wheel actuator  22 . Thus, the output of unit  39  is selected as an input of block  37  and is passed through to signal  42 . Block  40  controls the output selection of block  37  according to input signals  27 ,  29  and  26  that correspond respectively to hand wheel position, vehicle speed, and steering wheel torque. From these signals, block  40  determines how to route signal  38  and the signal from unit  39  through block  37 . When a position fault is detected, block  40  routes signal  38  as an output; when no position fault is detected, block  37  routes a signal from unit  39  as an output.  
         [0020]    In an alternate embodiment, the torque sensor is used to steer the system in the primary normal mode, and the position sensor is used in the secondary backup mode. Accordingly, when the alternate embodiment controller receives a valid signal from the steering-angle sensor but no valid signal from the steering-torque sensor, the switch causes the road-wheel position command generator to produce a signal corresponding to the output of the steering-angle sensor for controlling the road-wheel actuator.  
         [0021]    Turning to FIG. 3, the switch  37  of FIG. 2 operates in correspondence with a control algorithm, which is indicated generally by the reference numeral  44 . The control algorithm  44  embodies a method for determining whether the steering-angle sensor  16  may be providing an erroneous signal. Decision block  46  shows that a measured torque signal  26  received from the steering-torque sensor  18  that is in excess of a normal threshold value is considered to be potentially indicative of an erroneous signal from the steering-angle sensor  16 . If the measured torque value is not greater than the threshold value, the decision block fails and the function returns without setting a steering-angle sensor failure flag, thus indicating a valid signal. However, if the decision block detects a steering input torque above a normal threshold, the steering-angle sensor signal itself is differentiated to determine its current time-rate of change. As shown in decision block  48 , if the steering-angle rate of change is negligible, the steering-angle sensor failure flag is set to true as shown in function block  50 , thus indicating an erroneous steering-angle signal.  
         [0022]    As may be recognized by those of ordinary skill in the pertinent art, various other methods for determining the reliability of the respective signals from the steering-angle sensor  16  and the steering-torque sensor  18  may be employed without departing from the scope or spirit of the teachings herein. For example, even if the time-rate of change of the signal produced by the steering-angle sensor  16  is not negligible in the presence of an abnormally high steering-torque sensor signal  26 , the steering-angle sensor signal  27  may still be flagged as invalid if the signal  27  received from the steering-angle sensor  16  is highly discontinuous as might be indicative of other failure modes wherein the signal produced by the steering-angle sensor  16  is not truly indicative of the road wheel angle desired by the vehicle operator. Likewise, the validity of the signal  26  received from the torque sensor  18  may be determined in accordance with the steering-angle signal  27  and the speed signal  29 . For example, if the steering-angle signal  27  represents a large angular movement and the vehicle speed signal  29  indicates a slow vehicle speed, a very low torque signal  26  may be suspect depending on the level of power-assist and other possible input signals such as, for example, signals indicative of road surface conditions such as rain or ice.  
         [0023]    Any signal determined to be suspect may be assigned a confidence index as well as a set failure flag. Thus, if both the steering-angle sensor and the steering-torque sensor are suspected of failure, the control circuit  20  may still produce a control signal that is most likely to permit the operator to maintain control of the vehicle.  
         [0024]    Any suitable output actuator  22  may be substituted for the road-wheel actuator  22  for application to multiple vehicle types. For example, actuators suitable for marine use would be used to control one or more rudders on a boat, and actuators suitable for aviation use would be used to actuate one or more control surfaces on an aircraft.  
         [0025]    The natural instinct of an operator using the input device in the presence of restricted motion or seizure of the input device would be to turn it in the desired direction of travel, producing an increased torque. A signal from the steering-torque sensor may therefore be used to sense a torque level in a particular direction, even in the absence of measurable movement from the input device. This facilitates a method of utilizing a signal from the steering-torque sensor to control the output device or road wheel angle until the input torque is reduced. An output or road-wheel actuator is provided that converts the control output, which corresponds to one or both of the steering-angle sensor and steering-torque sensor signals, into motion of the output device or steered road-wheel.  
         [0026]    This disclosure contemplates the optional use of multiple torque sensors and multiple position sensors in order to provide additional hardware redundancy. One such embodiment comprises two torque sensors and two position sensors in place of the single torque sensor and single position sensor described in the primary exemplary embodiment.  
         [0027]    It shall be recognized that although it is currently preferable to incorporate a vehicle speed signal such as signal  29  of the exemplary embodiment, such signal is not required. Accordingly, an alternate embodiment controller does not receive nor require any signal indicative of vehicle speed.  
         [0028]    Vehicles incorporating the above described and like embodiments may be safely controlled in emergency situations such as those corresponding to partial failures of the steer-by-wire system. Steering control is also enhanced in non-failure modes of operation by using the signal representing the torque applied to the input device to enhance the rate of change of the output signals. Redundancy is enhanced while the number of additional components to implement this enhancement are minimized, thereby reducing the cost of providing the redundancy and reducing the packaging constraints within the vehicle.  
         [0029]    While exemplary embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the scope and spirit of the present disclosure. Accordingly, it will be understood that the present disclosure has been made by way of illustration only, and that such illustrations and embodiments as have been disclosed herein are to be construed in an exemplary sense, and not as limiting to the claims.