Abstract:
A steering system comprises a steering wheel adapted to receive a driver&#39;s rotational input. Left and right wheel units are rotatable along a steering angle for adjusting a direction of the vehicle during the driving movement. A steering mechanism converts the rotational input to a variation of the steering angle of the wheel units. The steering mechanism comprises a steering shaft connected to the steering wheel, and gear steering units for each said wheel unit. The gear steering units are connected to the steering shaft for mechanically converting the rotational input to steering outputs for both said wheel units to concurrently vary the steering angle of said wheel units. Independent steering units for each wheel unit each adjusting the steering angle of a respective one of the wheel units independently from the rotational input and from the other wheel unit.

Description:
FIELD OF THE APPLICATION 
     The present application relates to steering systems and, more particularly, to the independent control of the left and right wheel steer angle. 
     BACKGROUND OF THE ART 
     Conventional vehicle steering systems are designed based on a fixed relation between left and right (inner, outer) steer angles to give the best performance at low speeds (close to Ackerman geometry) or can be designed for improved high-speed performance with anti-Ackerman geometry. Due to the fixed relation, the performance cannot be ideal at all speeds. Active steering control has one of the most significant potentials to generate forces to improve handling performance of vehicles at all speeds, where steer angle is actively modified based on driver steering input and vehicle forward speed. Such modification in steer angle with fixed relation between left and right wheels cannot, however, maximize the tire&#39;s ability to generate its maximum lateral forces, as it does not consider the tire normal loads which change significantly during maneuvers. In fact, such modification may saturate one tire force while the other tire may still develop more forces. 
     Analytical studies through computer simulation show that the best handling performance in terms of ideal path, ideal yaw rate, as well as equalization of left and right tires&#39; ability to generate lateral force for a given steering angle at any speed, requires different steering angles for left and right wheels depending on vehicle speed, as well as understeer characteristics of the vehicle. A control algorithm has been developed to show its effectiveness under all conditions. Conventional steering mechanisms use a single degree of freedom (DOF) rack-pinion connected to a 4-bar linkage mechanism for each wheel, following a pro-Ackerman steering ratio between inner and outer wheels. Conventional Active Front Steering (AFS) controls add an active command to this mechanism controlling both front wheels simultaneously. 
     Ackerman geometry requires the inner wheel to be steered more than the outer wheel, as shown by the following relation: 
                       cot   ⁢           ⁢     δ   o       -     cot   ⁢           ⁢     δ   i         =       2   ⁢           ⁢     T   f       L             (   1   )               
where δ o  and δ i  are the outer and inner wheel steering angles, T f  is the half front track width, and L is the wheel base. This geometric ratio is necessary at low speeds to avoid tire scrub. However, at high speeds, higher steering angle results in higher side slip at the inner tire compared to the outer tire. The lateral force generated by a tire primarily depends on two factors: side slip angle and normal load on the tire. During turning, a load shift from the inner wheels to the outer wheels takes place, which can be significant at high speeds. Low normal load on the inner tire not only reduces its ability to generate lateral force, but also decreases the slip angle at which maximum lateral force can be generated. At high speeds, the inner tire&#39;s frictional force is prone to reaching saturation due to low normal load and high-slip angle, while the outer tire may still have the ability to generate more lateral force. Hence, the inner tire becomes a deciding factor for the amount of active steering control.
 
     To overcome this situation, anti-Ackerman geometry is employed in race cars, where, the primary concern is higher lateral forces at high-speed turning. The anti-Ackerman geometry requires outer tires to be steered more, hence generating high lateral force from the more capable tire. However, at the same time, the anti-Ackerman geometry causes tire scrub at low speeds, hence introducing noise and tire wear. 
     Although such steering concepts can be realized by steer-by-wire and wheel actuator systems, their reliability and safety in vehicle applications is always a major concern. 
     SUMMARY OF THE APPLICATION 
     It is therefore an aim of the present disclosure to provide a steering system and active independent steering method that address issues associated with the prior art. 
     Therefore, in accordance with the present application, there is provided a steering system comprising: a steering wheel adapted to receive a driver&#39;s rotational input; left and right wheel units adapted to rollingly interface a vehicle to a ground for driving movement of the vehicle, each wheel unit being rotatable along a steering angle for adjusting a direction of the vehicle during the driving movement; a steering mechanism for converting the rotational input to a variation of the steering angle of the wheel units, the steering mechanism comprising a steering shaft connected to the steering wheel, and gear steering units for each said wheel unit, the gear steering units connected to the steering shaft for mechanically converting the rotational input to steering outputs for both said wheel units to concurrently vary the steering angle of said wheel units, and independent steering units for each said wheel unit, the independent steering units each adjusting the steering angle of a respective one of the wheel units independently from the rotational input and from the other wheel unit. 
     Further in accordance with the present application, there is provided a method for adjusting a steer angle of wheels of a vehicle comprising: measuring a steering input from the driver of the vehicle and a vehicle velocity; calculating a reference yaw rate for the vehicle from the steering input and from the vehicle velocity; measuring an actual yaw rate for the vehicle; determining a corrective value for a first wheel of the vehicle as a function of the reference yaw rate and the actual yaw rate, and of the steering input; adjusting the steer angle of said first wheel by said corrective value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top perspective view of a steering system in accordance with a preferred embodiment of the present application; 
         FIG. 2  is a side perspective view of the steering system of  FIG. 1 ; 
         FIG. 3  is an enlarged rear perspective view of a steering unit of the steering system of  FIG. 1 ; 
         FIG. 4  is an enlarged front perspective view of the steering unit of  FIG. 3 ; 
         FIG. 5  is a block diagram showing a method used by a controller unit for active independent steering of wheels of a vehicle; and 
         FIG. 6  is a flow chart illustrating a method for adjusting a steering angle according to another preferred embodiment of the present application. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, and more particularly to  FIG. 1 , a steering system for independent steering of the wheels is generally shown at  10 . The steering system has a steering wheel  11 , a steering mechanism  12 , and wheels  13 , a left wheel  13 A and a right wheel  13 B. Letters A and B will be affixed to the reference numerals in the drawings and the following description to illustrate the same component, as positioned on the left side (A) or the right side (B) of the vehicle. 
     The steering wheel  11  is manipulated by the driver to orient the wheels  13  and hence steer a vehicle. 
     The steering mechanism  12  converts the driver input on the steering wheel  11  to orientation movements of the wheels  13 . 
     The wheels  13  are the interface between the road and the vehicle. The orientation of the wheels  13  along a steer angle is controlled by the steering mechanism  12 . In  FIGS. 1 to 4 , the steering system  10  is illustrated as being a front wheel steering system. However, it is considered to use the steering system of the present application for rear-wheel steering, or for 4-wheel steering as well. 
     The steering mechanism  12  has a steering shaft including a steering column  20  connecting the steering wheel  11  to a remainder of the steering shaft  21  via a universal joint  22 . This is one configuration amongst numerous others used to transfer the driver&#39;s rotational input to the wheels  13 . 
     Gear steering units  30 A and  30 B are both connected to the steering shaft  21 . The gear steering units  30 A and  30 B are respectively associated with the left wheel  13 A and right wheel  13 B. As the gear steering units  30 A and  30 B are similar in configuration, a single gear steering unit  30  will be described hereinafter without affixed letters A or B. 
     Referring concurrently to  FIGS. 2 to 4 , gear steering unit  30  has a planetary gear system  31 . The planetary gear system  31  has a sun gear  32  fixed to the steering shaft  21  so as to rotate therewith. Three planet gears  33  are in orbit of the sun gear  32  and are meshed therewith, although more or less planet gears  33  could be used as well. However, there must be space between the planet gears  33  for additional gears, as will be described hereinafter. Moreover, the range of movement of the planet gears  33  must take into account the presence of such additional gears. 
     The planet gears  33  are meshed to an interior of ring  34 . The ring  34  is connected to a structure of the vehicle, but rotatable about its own axis, which axis is coincident with that of the sun gear  32  and of the steering shaft  21 . 
     Referring to  FIGS. 2 and 4 , a planet carrier  35  interconnects the planet gears  33  to pinion  36 . The pinion  36  is hollow, so as to accommodate the steering shaft  21  without being secured to it. Accordingly, the steering shaft  21  and the pinion  36  rotate independently from one another, about coincident axes. 
     The pinion  36  is meshed with rack  37 . Accordingly, a rotation of the pinion  36  results in the translation of the rack  37 . Linkage  38  interrelates the rack  37  to the wheel  13 , whereby a translation of the rack  37  is converted to a rotation of the wheel  13  in its steering angle. The rack  37  and linkage  38  are illustrated schematically in  FIGS. 1 to 4  for simplicity purposes. However, it is understood that the rack  37  is provided with an appropriate support for translational movement, and for responding to a rotation of the pinion  36 . The linkage  38  is used in combination with various other mechanisms to operatively connect the wheel to the vehicle (e.g., suspension, brake, drive, etc.), and is for instance a four-bar mechanism. The linkage  38  forms part of a wheel unit with wheel  13 . 
     As best seen in  FIG. 3 , the steering system  10  has independent steering units each having an independent-steer gear  40  meshed with an interior of the ring  34 . A rotation of the independent-steer gear  40  is controlled by motor  41 . Motor  41  is typically an electric motor connected to a controller unit so as to be actuated in rotating the independent-steer gear  40 . An actuation of the independent-steer gear  40  causes rotation of the ring  34 . 
     Gear steering units  30 A and  30 B are similar to one another and are both connected to the steering shaft  21 , whereby a rotation of the steering shaft  21  results in a translation of the same magnitude of both the racks  37 A and  37 B. In the illustrated embodiment, the steering units  30 A and  30 B are mirror images of one another. 
     As the steering wheel  11  is mechanically coupled to the racks  37 A and  37 B, the wheels  13  of the vehicle may be steered despite failure of the electric system of the vehicle. The driver&#39;s input on the steering wheel  11  results in a modification of the steer angle of the wheels  13 , with both wheels  13  pivoting to the same steer angle. It is considered to add a power-assist system to provide additional force to a driver&#39;s input. However, the steering system  10  as embodied in  FIGS. 1 to 4  features a direct mechanical link between the steering wheel  11  and the wheels  13 , whether assisted or not. 
     However, the motors  41 A and  41 B of the independent steering units are actuated independently from one another by the controller unit of the steering system  10 , so as to individually adjust the steering angle of the wheels  13 A and  13 B. 
     As shown in  FIGS. 1 and 2 , the driver input rotates the sun gear  32  of each planetary gear system  31 , which rotates the planet gears  33  and the planet carrier  35 . As the planet gears  33  rotate about the sun gear  32  as a response to the rotational actuation from the sun gear  32 , the planet carrier  35  also rotates about the sun gear  32 . As the planet carrier  35  is connected to the pinion  36 , a rotation of the planet carrier  35  moves the rack  37 , giving the desired steer angle to the wheels  13 . The number of teeth for the gears and rack can be selected to obtain a desirable ratio between steering angle input at the steering wheel  11  and the steer angle output at the wheels  13 . 
     When a modification of the steer angle at a wheel  13  is required as determined by the controller unit, the controller motor  41  as shown in  FIGS. 2 and 3  will rotate the ring  34  in the appropriate direction via the independent-steer gear  40  to increase or decrease the wheel steer angle. The rotation of the gear  40  will cause a rotation of the ring  34 . The rotation of the ring  34  will result in a rotation of the planet gears  33  and of the planet carrier  35  about the sun gear  32 , without causing a rotation of the sun gear  32  itself. This is achieved without altering the motion of the sun gear  32  or the steering shaft  21  attached to it. Controller motors  41 A and  41 B can be activated independently to modify the steer angle of each wheel  13  to the level determined by the controller unit. 
     It is pointed out that numerous other configurations are considered for the steering system  10 . For instance, the independent steering units may act directly on the wheels  13 , or on the racks  37 . Moreover, other mechanisms may be used as alternatives to the planetary gear systems  31  to convert the rotational movement from the steering shaft  21  to a steering output for the wheels  13 . 
     For a desired maneuver, the driver will give a standard steering command by turning the steering wheel. The controller motors  41  being stationary, the system will work as conventional systems. The vehicle response to the steering input will be sensed to calculate the error in the yaw rate and/or other responses compared to the ideal for the forward speed and steering command. The control algorithm proposed based on speed, steering input, as well as understeer characteristics of the vehicle, will decide the steer angle for which wheel should be increased or decreased. In this decision, the wheel load shift due to the maneuver and the resulting vertical load at wheel is also taken into account since it affects the tire&#39;s ability to generate force. The controller unit therefore decides which tire&#39;s steer angle should be increased or decreased to realize the ideal response, while one tire&#39;s lateral force must not saturate before the other. The controller unit will, therefore, send the appropriate signal to the motor  41 A and/or motor  41 B for appropriate modification of the wheel steer angle. The modification can be done by adding or subtracting steer angle independently at each wheel from the one given by driver command. This modification is done by rotating the rings  34  and thus will not affect the steering column  22  or the driver&#39;s input. The modification will be done without the knowledge of the driver. 
     In order to achieve improved steering and handling performance at low and high speeds, it is considered to use steering system  10  with independent control for both front wheels. Accordingly, a method is illustrated at  50  in  FIG. 6  for the controller unit to adjust the steer angle of the wheels. The controller unit would make it possible to follow an Ackerman ratio at low speeds, while using an anti-Ackerman ratio at high speeds. Conventional AFS techniques risk saturation of the inner tire forces, while Active Independent Front Steering (AIFS) control utilizes the tire frictional forces, in an attempt to equalize the tire workload between the two front tires. Tire workload is a measure of utilization of the tire&#39;s frictional forces, and is defined as the ratio of total forces being generated from the tire and the maximum ability to generate forces at instantaneous normal load. For simplicity purposes, tire longitudinal forces are neglected and both tires are assumed to have similar lateral stiffness characteristics and road conditions. Hence, tire workload is simplified as the ratio of the lateral force to the normal load: 
     
       
         
           
             
               
                 
                   
                     Tire 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     workload 
                   
                   = 
                   
                     
                       F 
                       y 
                     
                     
                       F 
                       z 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In order to achieve an ideal state of a response, active feedback control is employed in the vehicle. The controller unit changes one of the input parameters to achieve one of the reference or set-point state parameters. A conventional AFS controller unit may be based on a simple feedback proportional-integral (PI) control strategy. A conventional controller unit controls steering angle of both front wheels  13  simultaneously following a near Ackerman ratio between inner and outer wheels. According to step  52 , the controller unit measures the steering input from the driver of the vehicle, for instance by a magnitude sensor on the steering wheel  11 , as well as the vehicle velocity. Then, according to step  54 , the controller unit calculates the reference yaw rate from forward velocity and the driver steering input command as shown in equation (3) below. According to steps  56  and  58 , the response can be used to define an error or corrective value in the yaw rate as: 
     
       
         
           
             
               
                 
                   
                     R 
                     ref 
                   
                   = 
                   
                     L 
                     
                       tan 
                       ⁡ 
                       
                         ( 
                         
                           δ 
                           st 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     Ω 
                     ref 
                   
                   = 
                   
                     
                       
                         V 
                         x 
                       
                       
                         R 
                         ref 
                       
                     
                     = 
                     
                       
                         
                           V 
                           x 
                         
                         · 
                         
                           tan 
                           ⁡ 
                           
                             ( 
                             
                               δ 
                               st 
                             
                             ) 
                           
                         
                       
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Ω 
                   
                   = 
                   
                     
                       Ω 
                       ref 
                     
                     - 
                     
                       Ω 
                       act 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In equations (3) to (5), R ref  is the ideal radius of curvature for wheel steer angle δ st  for a vehicle with a wheel base L. Ω ref  is thus the ideal yaw rate for a forward velocity V x . In equation (5), the error in yaw rate is the difference between Ω ref  and the actual yaw rate Ω act  of the vehicle measured with a yaw rate sensor in step  56 . 
     To minimize this error, an actively controlled additional steering command has to be added to the driver input. The input of active steering command is proportional to the error in the yaw rate. The steer gain factor K st  can be defined as: 
                     K   st     =         δ   st       Ω   ref       =           δ   st     ·   L         V   x     ⁢     tan   ⁡     (     δ   st     )           ≈     L     V   x                   (   6   )               
The corrective steering angle can be calculated in step  58  from the gain factor and the yaw rate error as:
 
Δδ st   =K   st ·ΔΩ  (7)
 
and the actively controlled steering command with PI controller unit can finally be calculated as:
 
δ c   =k   1 Δδ st   +k   2 ∫Δδ st   (8)
 
     In equation (8), δ c  is the corrective steering command required to realize the reference yaw rate and Δδ st  is established in equation (7). The first and second terms on the right-hand side of equation (8) represent the proportional and integral part of the feedback controller, and k 1  and k 2  are the weight assigned to proportional and integral parts. These weights can be adjusted to realize the best performance of the controller. According to step  60 , the steer angle is corrected, for instance, using the steering system  10  described in  FIGS. 1 to 4 . 
     For commonly designed understeer vehicles and for any steering command by the driver, the AIFS will first identify the instantaneous outer wheel and the corrective steering angle required at that forward speed according to equations (4) to (8) for the outer wheel. The corrective angle will then be added to the outer wheel only, thus introducing anti-Ackerman geometry. In doing so, a larger force is developed at the more capable outer wheel to realize the reference yaw rate while the tire work load of the inner and outer wheels are brought closer. The control strategy is shown in  FIG. 5 . The conventional AFS on the other hand increases the tire work load of both tires risking the saturation of the inner tire. 
     If the vehicle characteristic is oversteer, as shown in  FIG. 5 , the inner wheel is identified and the corrective steer angle is subtracted from the inner wheel angle to realize the reference yaw rate. For this oversteer case, however, the tire work load is not equalized. In order to equalize tire work load of an oversteer vehicle both wheels need to be controlled. For an AIFS to operate on both front wheels simultaneously, it is suggested to distribute the active command on both wheels, by controlling the previously uncontrolled wheel in proportion to the controlled wheel. For an example, a control scheme represented in equation (9) can be used:
 
δ i =δ st +δ c  
 
δ o =δ st +0.4δ c   (9)
 
Due to the oversteer nature, the inner wheel is considered as the controlled wheel and only 40% of the controller command is added to the outer wheel. The distributed AIFS is able to equalize the tire workload better than the conventional AFS control. Although conventional AFS reduces the tire workload for both front tires compared to non-control vehicle, it is incapable of equalizing them. The understeer characteristic of the vehicle can also be detected from the sign of either the yaw rate error or the corrective angle (δ c ), as shown in  FIG. 5 .
 
     The system will enhance the handling performance and stability of the vehicle. In addition, the system can be used to introduce a steer angle at a selected wheel without any driver input for enhancement of stability, with or without other stability enhancement techniques. In case of failure, the controller motors will be locked in place and the system will work as a conventional system, and is thus failsafe. Any power-steering system applicable to conventional system can be used with modification to realize power steering with the proposed design. It is pointed out that the intervention of the controller motors is within saturation limits, in order to avoid outputting steering angle beyond acceptable values. 
     This application presents via an embodiment a mechanical system consisting of tandem planetary gear with separate racks for each wheel that can be designed to give the best ratio between inner and outer wheel steer angle at low speed, while each wheel steer angle can be modified independently by motors depending on the controller command. Any modification to wheel steer angle will not be felt by the driver through the steering wheel under the driver&#39;s control. In other words, the driver can give same steering angle for a maneuver at any speed and, if the predefined path for that steering angle is not followed at any other speed, the controller unit through the developed steering mechanism will modify the left only, right only or both wheel steer angles without any knowledge of the driver. In doing so, the vehicle will follow the ideal path and ideal yaw rate while ensuring that the wheel that can generate more force is only modified. The mechanism will allow for such control while remaining failsafe. In the event of failure in the control system and/or motors, the system will behave like an uncontrolled conventional steering system. 
     The proposed control strategy design of the mechanism will provide the best possible handling performance of the vehicle, reducing the skill requirement of the driver. It also has potential for further development, mainly through controller unit for automatic parking, maximizing performance on split-mu surface, to provide variable steering ratio, and can be combined with other control systems such as control through tire traction and braking force generation. The independently controllable steering system can revolutionize the stability enhancement techniques for a vehicle through introduction of lateral force at a selected wheel, as well as integrated control strategies. The above is possible since a steer angle at a selected wheel can be introduced without intervention of the driver. 
     The adaptive control strategy and the mechanism for independent control of steered wheel presented is also applicable to rear wheel and/or 4-wheel steering system.