Patent Publication Number: US-2021188345-A1

Title: Steering System for an Automated Vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 62/951,414 filed Dec. 20, 2019, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of autonomous vehicles. More specifically, the invention relates to a steer-by-wire steering system for an autonomous vehicle having a high level of redundancy and integrity. 
     BACKGROUND OF THE INVENTION 
     Steer-by-wire steering systems are increasingly used in the automotive field, especially with the rise of autonomous vehicles. Such steer-by-wire typically use sensors detecting a rotation of a steering wheel and send a signal representing this detected rotation to an actuator acting on a steering rack connecting steered wheels. 
     In other types of autonomous vehicles, another version of the steer-by-wire system is used where a separate steering actuator is assigned to each steerable wheel of the autonomous vehicle. This variation allows an independent control of a steering angle of each steerable wheel. Advantageously, such a steer-by-wire system may orient the steerable wheels either according to the Ackermann principle or to allow crabbing movement of the vehicle when all wheels are steered parallelly. 
     As is often the case with product design, one of the objectives is often to keep the cost of the vehicle as low as possible such that the number of sensors and actuators in both of these designs is kept to an acceptable minimum. However, in some other types of autonomous vehicles, such as those used in the mass transit market, higher safety levels are required where the automotive safety levels are not sufficient and where the above described steer-by-wire systems may not be capable of meeting safety requirements which often require higher availability and integrity. There is therefore a need for autonomous mass-transit vehicles meeting more stringent safety requirements. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a steering system for an autonomous vehicle that overcomes or mitigates one or more disadvantages of known steering systems for autonomous vehicles, or at least provides a useful alternative. 
     The present invention provides the advantage of meeting safety requirements for at least some categories of autonomous mass-transit vehicles. 
     In accordance with an embodiment of the present invention, there is provided an autonomous vehicle comprising a body having a passenger compartment, four steering systems, at least two controllers, a first power source and a second power source. Each steering system is equipped with a wheel and a first actuator connecting the wheel to the body for steering the wheel. Each one of the four wheels supports a different end and a different side of the body. The at least two controllers are operatively connected to the first actuators for controlling their actuation. The first power source powers a first subset of the first actuators while the second power source powers a second subset of the first actuators. 
     Optionally, the four steering systems of the autonomous vehicle may be a front right steering system, a front left steering system, a rear right steering system and a rear left steering system. The autonomous vehicle may further comprise coupling links respectively between the front right steering system and the front left steering system and between the rear right steering system and the rear left steering system. The coupling links are operative to respectively transfer a steering movement from a first one of the coupled steering systems to a second one of the coupled steering systems. 
     Optionally, each one of the coupling links may comprise a spring element and a damping element where each spring element allows a difference in steering angle between the wheels of the respective coupled steering system. The coupling link may be one of a mechanical or a hydraulic coupling link. 
     Alternatively, the four steering systems of the autonomous vehicle may be a front right steering system, a front left steering system, a rear right steering system and a rear left steering system. The autonomous vehicle may further comprise coupling links respectively between the front right steering system and the rear right steering system and between the front left steering system and the rear left steering system. 
     Optionally, each one of the coupling links may comprise a spring element and a damping element where each spring element allows a difference in steering angle between the wheels of the respective coupled steering system. The coupling link may be one of a mechanical or a hydraulic coupling link. 
     Alternatively, each one of the four steering systems may further comprise a second actuator and one controller. The second actuator also connects the wheel to the body for steering the wheel. Each controller is operatively connected to the first actuator of its respective steering system and to one of the second actuators of another steering system. The first power source powers a first subset of the second actuators while the second power source powers a second subset of the second actuators. 
     Optionally, each second actuator may act on its respective wheel in opposition to the first actuator of the respective steering system. 
     Alternatively, each second actuator may act on its respective wheel in parallel to the first actuator of the respective steering system. 
     The first and the second actuators may be hydraulic actuators, in which case the first and the second power sources would be hydraulic pumps. Alternatively, the first and second actuators may be electro-mechanical actuators, in which case the first and the second power sources would be electrical power sources. 
     Optionally, the controllers may be organized in two pairs. Each pair of controllers control two of the four steering systems located at a different end of the body. 
     Optionally, the controller of each steering system may be connected in active mode to the first actuator of the respective steering system, while being connected in passive mode to one of the second actuators of a different one of the four steering systems. 
     Optionally, the first actuators and the second actuators of each steering system may operate in active-active mode. In this case, each controller of each steering system further comprises a force-fight compensation system adapted to adjust one of a stroke and a force of at least one of the first actuator and the second actuator of their respective steering systems. 
     Optionally, the controllers may be high-integrity controllers. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a schematic diagram of a first concept of a steering system in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of a second concept of a steering system in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic diagram of a third concept of a steering system in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic diagram of a fourth concept of a steering system in accordance with an embodiment of the present invention; 
         FIG. 5  is a schematic diagram of a fifth concept of a steering system in accordance with an embodiment of the present invention; 
         FIG. 6  is a schematic diagram of position and force fight reduction function of an actuator controller in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically depicts a first embodiment of the present invention. An autonomous vehicle  10  typically comprises a body  12  having a passenger compartment for transporting passengers. The vehicle  10  comprises at least four wheels  14 , each supporting a different end and a different side of the body as well as a steering system  16  for steering steerable wheels  18 . 
     A command to steer the autonomous vehicle  10  may either come from a main vehicle controller which either detects that steering is required, or which uses a memorized path and sends a command to steer accordingly. Alternatively, the autonomous vehicle  10  could be remote controlled and receive the steering command remotely. 
     The steering system  16  of the instant embodiment steers two steerable wheels  18  through two actuators  20 . Each actuator  20  is connected at one end to the body  12  and at its moveable end to a different one of the steerable wheels  18 . In the present embodiment, the steerable wheels  18  are solely located at a front  21  of the vehicle  10 . The wheels  14  located at a back  22  of the vehicle  10  are non-steerable. It should be noted that the front  21  and the back  22  of the vehicle  10  could be inverted without departing from the present invention. 
     Steerable wheels  18  are typically organized in pairs on a common “axle”. Although there is clearly no axle used in the present embodiment, that term will nevertheless be used to denote two corresponding wheels located at a common longitudinal distance in the vehicle  10 , but on two different sides of the vehicle  10 . 
     Each actuator  20  is independently controlled by a controller  24  and independently powered by its own power source  26 . The actuators  20  may either be hydraulic, in which case the power source  26  is a hydraulic pump, or electro-mechanic, in which case the power source  26  is electric. The actuators  20  may be linear or rotational. In case of failure, each actuator  20  may be back driven. 
     Upon receiving the steering command, for example, by the main vehicle controller, each controller  24  independently connected to one actuator  20  is operative to send a signal to its respective actuator  20  that is indicative of a stroke to reach. In turn, this stroke corresponds to a desired steering angle of that steerable wheel  18 . The controller  24  therefore sends a signal to the actuator  20  to which it is connected that is proportional to the desired steering angle. Each actuator  20  may be equipped with its own stroke sensor  27  operative to send back a signal (a stroke reading) to its controller  24 , indicative of the stroke it has reached. This creates a feedback loop allowing the controller  24  to readjust its command in case the actuator  20  has not reached the desired stroke. Moreover, angle sensors  28  may be installed in proximity to a steering pivot  30  of each steerable wheel  18  to monitor the actual steering angle of its respective steerable wheel  18 . Typically, each angle sensor  28  also feeds its actual steering angle reading to its respective controller for monitoring. If the controller  24  detects a significant discrepancy between the actual angle reading and a theoretical angle yielded by the stroke read by the stroke sensor  27 , then the controller  24  may decide on a mitigating action, such as switching the actuator  20  in passive mode, or stopping the vehicle by communicating a request to the main vehicle controller. In the present document, the expression “controlling in Passive mode” shall be interpreted as switching one actuator in Passive mode, that is letting this actuator being back-driven. 
     In the present description, and in all embodiments of the invention, the one or many controllers  24  may be of the high-integrity type. High integrity types of controllers  24  further add to the integrity of the whole system. 
     In the present embodiment, redundancy is provided by a steering link  32  connecting both steerable wheels  18 . It is used as a redundant steering means in case one of the actuators  20  fails. Because each steerable wheel  18  is independently controlled, their steering angle may be slightly different. However, in normal operation, this steering angle will follow the Ackermann principle so an Ackermann geometry may be built in the steering link  32 . According to the Ackermann geometry, in a turn, the inner wheel steers more than the outer wheel. In this case, the steering link  32  comprises a steering rod and two steering arms, one connected to each steerable wheel  18 . In other cases, however, and as will be further discussed below, it may be desirable for the steered wheels  18  to acquire a parallel toe angle. To accommodate this, the steering link  32  is designed with free play so that the steered wheels  18  may steer either parallel to each other or with an Ackermann angle in between them. This free play insures a certain independence of steering of the corresponding steered wheels  18  on a common axle. If one of the actuators  20  fails, the steered wheel  18  on the failed side may want to steer in a totally unpredictable manner. To prevent this, a predetermined threshold of angle difference between both steerable wheels  18  of a common axle is designed in the steering link  32 . This predetermined threshold may correspond to the end of the free play in the steering linkage. Passed this threshold, the steering link  32  acts as a solid linkage and couples both steerable wheels  18 . To achieve this, the steering link  32  may be equipped with one or both of a resilient spring element and a damping element. The steering link  32  may either be a mechanical link or a hydraulic connection between the steerable wheels  18 . For light vehicles, a pneumatic connection could even be considered. 
     To provide even further redundancy, the steering link  32  could be dualized, that is two steering links  32  may be used in combination to both steer the steerable wheels  18  to which they are attached. For example, the steering links  32  may be attached one atop the other or in some appropriate way. They may be concentric rods or placed side-by-side. Under some circumstances, it may even be appropriate to use such dualized steering links  32  and to remove one of the actuators  20  driving one of the steerable wheels  18  to which the steering links  32  are attached. In case of failure of one steering link  32 , the other steering link  32  is still sufficient to transfer steering movement between both steerable wheels  18  to which it is attached. This would however be to the detriment of the integrity of the system since there would be no way to steer the steerable wheels  18  in case of failure of the only remaining actuator  20 . 
     Optionally, a second steering system  16 , similar to the one used at the front  21  of the vehicle  10 , may be used at the back  22  of the vehicle. This embodiment is depicted in  FIG. 2 , now concurrently referred to. In this embodiment, all four wheels  14  of the vehicle  10  are steerable. Similarly to the steerable wheels  18  of the front axle, the steerable wheels  18  of the rear axle are steered independently, each being steered through their own actuator  20  controlled by their own controller  24 . As can be seen in  FIG. 2 , the exact same steering system architecture is used at the rear of the vehicle  10  as at the front of the vehicle and works exactly the same way. The rear steering system  16  will therefore not be further described here. 
     This four-wheel steering system however provides an advantage over that of the two-wheel steering system: the crabbing capacity (also known as Dog Tracking). Crabbing happens when the vehicle  10  moves sideways at an angle. This maneuver may be induced by steering all wheels  14  at the same angle. This capacity is useful when, for example, the vehicle  10  is required to dock along a platform, for example to embark or disembark passengers. 
     A variant of this design still provides some level of redundancy while using less components (and is therefore arguably cheaper to manufacture). This embodiment is depicted in  FIG. 3 , now concurrently referred to. This embodiment is similar to the previous embodiment depicted in  FIG. 2 , except that only two controllers  24  and two power sources  26  are used instead of four of each. Each controller  24  is linked to two steered wheels  18  located on the same side of the body  12 . Hence, one controller  24   a  controls the steering of the right steerable wheels  18   a  by being connected to the right actuators  20   a  while the other controller  24   b  controls the steering of the left steerable wheels  18   b  by being connected to the left actuators  20   b . Both right stroke sensors  27   a  are connected to the controller  24   a  for providing feedback on the stroke of the front right and rear right actuators  20   a . Both right angle sensors  28   a  are also connected to the right controller  24   a  for providing feedback on the steering angle of the front right and rear right steerable wheels  18   a . Similarly, both left stroke sensors  27   b  are connected to the left controller  24   b  for providing feedback on the stroke of the front left and rear left actuators  20   b . Both left angle sensors  28   b  are also connected to the left controller  24   b  for providing feedback on the steering angle of the front left and rear left steerable wheels  18   b . The right and left controllers  24   a ,  24   b  are referred to as such not based on their actual location, but rather because they belong respectively to either the right or the left steering system  16   a ,  16   b.    
     The right power source  26   a  powers both of the right actuators  20   a  while the left power source  26   b  powers both of the left actuators  20   b . The right and left power sources  26   a ,  26   b  are referred to as such not based on their actual location, but rather because they belong respectively to either the right or the left steering system  16   a ,  16   b . Again, the right and left power sources  26   a ,  26   b  may either be hydraulic, in which case the actuators  20  are hydraulic actuators or the right and left power sources  26   a ,  26   b  may be electric in which case the actuators  20  are electro-mechanical actuators. 
     Interestingly, in the present embodiment, it cannot be said that the steering links  32  belong to either the right or left steering systems  16   a ,  16   b  since they still connect the right and left steerable wheels  18  on a common axle. In other words, the front steering link  32   a  connects the front right and front left steerable wheels  18  while the rear steering link  32   b  connects the rear right and rear left steerable wheels  18 . 
       FIG. 4  is now concurrently referred to. Alternatively, each steering link  32  could belong to a different one of the steering systems  16   a ,  16   b  by having a right steering link  32   a  connecting the right front steerable wheel  18   a  to the right rear steerable wheel  18   a  and a left steering link  32   b  connecting the left front steerable wheel  18   b  to the left rear steerable wheel  18   b . Note that for clarity reasons, the controllers  24  and the power sources  26  as well as their connections have been omitted to clearly see the steering links  32   a ,  32   b , but that it should be understood that implementations of such controllers  24 , power sources  26  and their respective connections may be achieved according to either  FIG. 2  or  FIG. 3 . 
     In another embodiment of the present invention, the steering links  32  are completely avoided. This embodiment is represented in  FIG. 5 , now concurrently referred to. The autonomous vehicle  10  of this embodiment comprises four steerable wheels  18  supporting a different end and a different side of the body  12 , the steering system  16 , four controllers  24  and at least two power sources  26 . This steering system  16  is made of four sets of actuators  20 , each having a first actuator  20   c  and a second actuator  20   d  connected in opposition. Alternatively, the first actuator  20   c  and the second actuator  20   d  could also be connected in parallel, one beside the other (not shown). Each set of actuators  20  is connected to a different one of the four steerable wheels  18 . 
     The first power source  26   c  powers all first actuators  20   c , while the second power source  26   d  powers all second actuators  20   d . Again, these power sources  26   c ,  26   d  may be a hydraulic pump, when the actuators  20  used are hydraulic actuators or the power sources  26   c ,  26   d  may be an electric power source when the actuators  20  used are electro-mechanical actuators. In the present example, the actuators  20  are controlled to work in Active-Passive mode. Active means that the controller is controlled and provides the force to steer the wheel. Passive means that the actuator is not controlled or is not responsive to the command and is simply back driven by the moveable element to which it is attached. Hence, in Active-Passive mode, one actuator provides a force to steer the wheel while the other is being back driven and provides no force. Active-Active mode means that both controllers provide a force to steer the wheel. 
     Upon receiving the command from the main vehicle controller, each one of the four controllers  24  controls a steering of a different one of the four steerable wheels  18 . Each controller  24  is connected in Active mode to the first actuator  20   c  of the respectively steerable wheel  18  it controls. Each controller  24  may additionally be connected in Passive mode to the second actuator  20   d  of at least one of the other steerable wheels  18 . None of the controllers  24  is connected to both the first actuator  20   c  and the second actuator  20   d  of one given steerable wheel  18 . This intertwined connection ensures that each controller  24  is in control of a different steerable wheel  18  through its Active connection to the first actuator  20   c  of that steerable wheel  18  while acting as a back-up controller for at least one other steerable wheel  18  by controlling in Passive mode its second actuator  20   d , should the controller  24  or the first actuator  20   c  of that other steerable wheel  18  fail. Stated differently, each controller  24  is assigned to the control of a different steerable wheel  18  and controls two actuators  20 : one first actuator  20   c  steering a first steerable wheel  18  is controlled in Active mode and one second actuator  20   d  connected to a second steerable wheel  18  is controlled in Passive mode. If, for example, a first controller  24  detects a failure of its Actively controlled first actuator  20   c  (e.g. as discussed earlier, if there exist a significant discrepancy between the steering angle reading received from the steering angle sensor  28  and the theoretical steering angle yielded by the stroke reading received by the stroke sensor  27 ), then it can switch its connected first actuator  20   c  in Passive mode immediately. Simultaneously and in a similar manner, a second controller  24  will be informed of the mode transition of the first controller in Passive mode and will switch this second actuator  20   d  in Active mode. Transition from one controller to the other must be within an acceptable time threshold to prevent losing control of the vehicle  10 . 
     In the present embodiment, and as an example, controller  24   x  controls in Active mode the steering of the front right steerable wheel  18   x  through the first actuator  20   cx . Controller  24   x  is also connected in Passive mode to the second actuator  20   dy  of the front left steerable wheel  18   y . Controller  24   x  receives the feedback signals from the stroke sensor  27   cx  and angle sensor  28   x  of the steerable wheel  18   x  as well as from the stroke sensor  27   dy  and angle sensor  28   y  of the steerable wheel  18   y.    
     Controller  24   y  controls in Active mode the steering of the front left steerable wheel  18   y  through the first actuator  20   cy . Controller  24   y  is also connected in Passive mode to the second actuator  20   dx  of the front right steerable wheel  18   x . Controller  24   y  receives the feedback signals from the stroke sensor  27   cy  and angle sensor  28   y  of the steerable wheel  18   y  as well as from the stroke sensor  27   dx  and angle sensor  28   x  of the steerable wheel  18   x . The control of both front steerable wheels  18   x ,  18   y  is therefore interconnected so that one controller  24   x ,  24   y  acts as a redundant controller for the other one by controlling the second actuator  20   dx ,  20   dy  of the other controller  24   y ,  24   x.    
     Similarly, the rear steerable wheels  18   z ,  18   w  are also interconnected. Controller  24   z  controls in Active mode the steering of the rear right steerable wheel  18   z  through the first actuator  20   cz . Controller  24   z  is also connected in Passive mode to the second actuator  20   dw  of the rear left steerable wheel  18   w . Controller  24   z  receives the feedback signals from the stroke sensor  27   cz  and angle sensor  28   z  of the steerable wheel  18   z  as well as from the stroke sensor  27   dw  and angle sensor  28   w  of the steerable wheel  18   w.    
     Controller  24   w  controls in Active mode the steering of the rear left steerable wheel  18   w  through the first actuator  20   cw . Controller  24   w  is also connected in Passive mode to the second actuator  20   dz  of the rear right steerable wheel  18   z . Controller  24   w  receives the feedback signals from the stroke sensor  27   cw  and angle sensor  28   w  of the steerable wheel  18   w  as well as from the stroke sensor  27   dz  and angle sensor  28   z  of the steerable wheel  18   z . The control of both rear steerable wheels  18   z ,  18   w  is therefore interconnected so that one controller  24   z ,  24   w  acts as a redundant controller for the other one by controlling the second actuator  20   dz ,  20   dw  of the other controller  24   w ,  24   z.    
     These interconnections are only provided as an example and the same level of redundancy may be provided by interconnecting a controller  24  in Passive mode with any one of the wheels to which it is not connected to the first actuator  20  in Active mode. For example, the controller  24   x  could be connected to any one of the second actuators  20   dy ,  20   dz  or  20   dw . The other controllers  24   y ,  24   w ,  24   z  must also therefore be connected in Passive mode to any one of the other second actuators  20  so that each steerable wheel  18  is controlled in Active mode by one controller  24  and in Passive mode by another controller  24 . 
     In another variant of the present embodiment, the actuators may be controlled in Active-Active mode so that both the first actuator  20   c  and the second actuator  20   d  attached to the same steerable wheel  18  may use a force to induce a steering movement to the steerable wheel  18 . In this case, the controllers  24  and actuators  20  may be interconnected as previously described, but a force-fight control is added to the controllers  24  so as to prevent having the first and second actuators  20   c ,  20   d  of a set of actuators force against each other or forcing differently and creating mechanical fatigue. Using the feedback provided by force-fight feedback reduction controller (FFRC) sensors  29  connected to each one of the first and second actuators  20   c ,  20   d , the controllers  24 , using a force-fight reduction algorithm, are capable of reducing the induced additional force created by a possible misalignment of a set of actuators  20 , by either adjusting the stroke of or the force applied by one or both actuators  20  so that the steerable wheel  18  to which the set of actuators  20  is connected is steered at the desired angle without additional force being incurred in the actuators  20 . This is especially important in the case where the vehicle  10  is a self-propelled electric vehicle which needs to manage properly its battery, which also feeds the power sources  26 , in order to maximize its range. Consequently, the following force-fight solution is applied. 
     When adding the force-fight function to the controller  24 , the FFRC sensors  29  are required in the actuators  20 . The FFRC sensors  29  may detect one or more of a load, force, pressure or current, depending on the type of actuator  20  used. For each steerable wheel  18 , the FFRC sensors  29  of both connected first and second actuators  20   c ,  20   d  are operative to send the detected signal to both their respective controller  24  as well as to the controller  24  of the other connected actuator. In other words, whereas each steerable wheel  18  is steered by both connected first actuator  20   c  and second actuator  20   d , the FFRC sensors  29  of both the first and second actuator  20   c ,  20   d  send their respective signals to both the controller  24  controlling the first actuator  20   c  and to the controller  24  controlling the second actuator  20   d.    
       FIG. 6  is now concurrently referred to. As already mentioned, during operation, the loads supplied by both actuators  20  of a set of actuators acting on a steerable wheel  18  may not be equal or acting in the same direction. A force-fight reduction controller  34 , which may be implemented in each individual controller  24 , implements a so-called force-fight limiting compensation function whose goal is to equalize the load share between all active actuators  20  acting on a given steerable wheel  18  to less than a percentage of the actuator maximum force. Typical numbers range between 10 to 30%. 
     This is achieved by continually biasing a control current command output  36  (such as for commanding an electro-hydraulic servo valve (EHSV) or motor torque for example) of every actuator position controller  38  of actuators  20  which are part of one set of actuators so as to balance the measured loads applied by each active actuator  20  on a given steerable wheel  18 . The compensation can be considered as being equivalent to a position command bias for aligning the motion and position of any given actuator  20  with that of the other actuator  20  in a set of actuators. As such, the force-fight reduction controller  34  provides an adaptive improvement to the sets of actuators  20 . In other words, the force-fight reduction controller  34  acts as a continuous mutual recalibration with one another of both actuators  20  of one set of actuators. 
     One force-fight reduction controller  34  is associated with every actuator position controller  38  controlling the steering angle of one steerable wheel  18  steered by one set of at least two actuators nominally acting in an All Active configuration. Both the actuator position controller  38  and the force-fight reduction controller  34 , for a given actuator  20  or a given set of actuators  20 , may be implemented as a pair in their corresponding controller  24 . The actuator position controller  38  and the force-fight reduction controller  34  may operate at a sampling frequency which typically ranges between 250 and 500 Hz (i.e. 4 ms and 2 ms sampling period, respectively). 
     At each computation cycle, the force-fight reduction controller  34  computes and adds a limited-authority compensation to the control current command  36  computed by the actuator position controller  38 . The force-fight reduction controller  34  authority is typically limited to approximately +/−20% of control current command  36 , while the combined authority of the actuator position controller  38  and force-fight reduction controller  34  is limited to the nominal value of the control current command. 
     The force-fight reduction controller  34  comprises a force-fight enable logic function  40  and a force-fight compensation function  42 . The force-fight enable logic function  40  determines if the force-fight reduction controller  34  supplies a compensation based on availability of the required actuator sensor data and the status of the force-fight reduction controller  34 , while the force-fight compensation function computes the value of this compensation. If failure occurs in either one of the first actuator  20   c  or second actuator  20   d  connected to one common steerable wheel  18 , then the force-fight reduction control function is disactivated since the failed actuator  20  will have been placed in Passive mode by its controller  24 . 
     The present invention has been described with regard to preferred embodiments. The description as much as the drawings were intended to help the understanding of the invention, rather than to limit its scope. It will be apparent to one skilled in the art that various modifications may be made to the invention without departing from the scope of the invention as described herein, and such modifications are intended to be covered by the present description. The invention is defined by the claims that follow.