Abstract:
An electric power steering control system for a vehicle that has a steering device coupled by a steering assembly to at least one tire steered by the steering device. The control system controls a current in a motor for generating torque applied to the steering assembly. The control system comprises a feed-forward component that generates a torque target signal representative of the torque applied by the motor to the steering assembly. The torque target signal is proportional to a multiplication of a steering torque from the steering device. There is also a sensor component that senses the motor current and provides a motor current signal. A motor control component receives the torque target signal and the sensed motor signal. The motor current signal is driven towards a value wherein the torque target signal is equal to a multiplication factor of the motor current signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional application 61/027,799 filed in the United States Patent and Trademark Office on Feb. 11, 2008, the complete disclosure of which is incorporated herein by reference and priority to which is claimed pursuant to 35 U.S. C. section 120. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to electric power steering control systems for vehicles and, in particular, to electric power steering control systems for vehicles with handle bars that control the current in a motor used to apply assist torque or damping torque to the handle bars depending upon the circumstance. 
         [0004]    2. Description of the Related Art 
         [0005]    It has been known to use a motor in an electric power steering system to provide steering assist functionality. The motor typically provides resistance during the return to center phase of steering to resist the normal return of the steering wheel to the neutral position after a turn. The motor has also been used to provide an assist torque during this phase to overcome the inherent resistance of the motor. Various schemes have been used to control the motor in an electric power steering system. 
         [0006]    U.S. Pat. No. 5,668,721 issued Sep. 16, 1998 to Ashok Chandy discloses an electric power steering control system that provides both a voltage mode and a current mode of motor control. The first mode of motor control is a voltage mode in which the motor is commanded with a voltage command based on the desired level of assist torque, used during high steering gain events. The second mode of motor control is a current mode in which the motor is commanded with a current command based on the desired level of assist torque, used for low gain torque assist steering events. However, this system of motor control in an electric power steering system may become unstable when crossing from one mode of control to another mode of control. 
         [0007]    There is accordingly a need for an improved system of motor control for an electric power steering system during both low and high gain assist events. 
       BRIEF SUMMARY OF INVENTION 
       [0008]    It is an object of the present invention to provide an improved system of motor control for an electric power steering system which gives a smooth, continuous control function which is substantially free from potential oscillations during different steering events. 
         [0009]    There is accordingly provided an electric power steering control system for a vehicle having a steering device coupled by a steering assembly to at least one tire steered by the steering device. The control system controls a current in a motor for generating torque applied to the steering assembly. The control system comprises a feed-forward component that generates a target torque signal representative of the torque applied by the motor to the steering assembly. The target torque signal is a multiplication of a steering torque from the steering device. There is also a sensor component that senses the motor current and provides a motor current signal. A motor control component receives the target torque signal and the sensed motor signal. The motor current signal is driven towards a value wherein the target torque signal is equal to a multiplication factor of the motor current signal. 
         [0010]    There is also provided an electric power steering control system for a vehicle having a steering device coupled by a steering assembly to at least one tire steered by the handle bar. The control system controls a motor for generating a torque applied to the steering assembly. The control system comprises a steering torque sensor for sensing steering torque generated by a driver of the steering device. The steering torque sensor provides a steering torque signal. A vehicle speed sensor provides a vehicle speed signal representative of the vehicle speed. A steering shaft angle sensor senses the angular displacement of the handle bars and provides a steering shaft angle signal. A feed forward component generates a target torque assist signal representative of the torque applied to the steering assembly. The feed forward component receives the steering torque signal, the car speed signal, and the steering shaft angle signal. A current sensor senses the current in the motor and provides a motor current signal. A control component controls the motor whereby the torque applied to the steering assembly is related to the target torque assist signal by a multiplication factor. The control component receives the target torque assist signal and the motor current signal, and provides a motor control signal. 
         [0011]    There is further provided a method of controlling an electric power steering system for a vehicle. The method comprises the steps of generating a target torque signal representative of the torque applied by the motor to the steering assembly, the target torque signal being a multiplication of a steering torque from the steering device; sensing a current of the motor and providing a motor current signal; and controlling the motor whereby the current is driven towards a value wherein the target torque signal is equal to a multiplication factor of the motor current signal. 
         [0012]    The present electric power steering control system uses a larger gear ratio gearbox which allows the motor to operate near its optimal rpm specification. The higher gear ratio also provides increased damping during road disturbance events, increased torque assist, and allows the vehicle to be assigned a more neutral castor for increased stability and reduced steering effort. The larger gear ratio gearbox also allows for a reduced current in the motor thereby reducing operating temperature and allowing for a smaller motor to be used. 
         [0013]    It is an advantage of the present invention to provide an electric power steering control system having steering assist, steering damping, and steering return functionality. It is another advantage of the present invention to provide a smooth transition between these functions. It is a still another advantage of the present invention to provide a pure torque multiplier for steering assist. It is yet still another advantage of the present invention to minimize electric power steering resistance effect for steering return, and to provide steering damping for disturbance rejection. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    The invention will be more readily understood from the following description of preferred embodiments thereof given, by way of example only, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a block diagram of an improved electric power steering system; 
           [0016]      FIG. 2  is an alternative block diagram of the electric power steering system of  FIG. 1 ; 
           [0017]      FIG. 3  is a flowchart diagram of a feed-forward controller algorithm of the electric power steering system of  FIG. 1 ; 
           [0018]      FIG. 4  is a flowchart diagram of an assist torque feedback component algorithm of the electric power steering system of  FIG. 1 ; 
           [0019]      FIG. 5  is a flowchart diagram of a feed back controller algorithm of the electric power steering system of  FIG. 1 ; 
           [0020]      FIG. 6  is a schematic view of an equivalent simplified circuit of a motor of the electric power steering control system of  FIG. 1 ; 
           [0021]      FIG. 7   a,    7   b  and  7   c  are voltage wave diagrams across elements of the equivalent simplified circuit of the motor of  FIG. 1 ; 
           [0022]      FIG. 8  is a current wave diagram of the circuit of  FIG. 6 ; and 
           [0023]      FIG. 9  is a view of a vehicle provided with the electric power steering system of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring to the drawings and first to  FIG. 1 , an electric power steering system indicated generally by reference numeral  10  is shown. The electric power steering system  10  includes a control system  12 , a motor  14 , a gearbox  16 , and a steering column  18 . The steering column  18  is connected with a steering assembly  20  for turning the steering tires of a vehicle. Such steering assemblies are well known in the art and accordingly steering assembly  20  is not described in detail herein. 
         [0025]    Referring now to  FIG. 9  a vehicle  11  provided with the power steering system is shown. The vehicle  11  includes handlebars  13  and wheels  15 ,  17 , and  19 . The steering assembly is shown at  20 . 
         [0026]    Referring back to  FIG. 1 , the electric power steering system  10  operates continuously over three phases of steering activity, namely, steering assist, steering return, and steering damping. Steering assist occurs when a driver of the vehicle applies torque to a steering device, e.g. handle bars  13  shown in  FIG. 9 . In order to steer the wheels  15  and  17  and turn the vehicle  11 . In this situation, the electric power steering system  10  provides an assist torque to the steering assembly  20  thereby decreasing the amount of torque the driver must apply in order to turn the vehicle. 
         [0027]    The steering return phase begins when the driver stops applying steering torque to the handle bars while the tires are still in an off center position. Road geometry generates a back driving alignment torque which tends to steer the wheels back to the neutral position, i.e. such that the vehicle will continue in a straight line. However, electric power steering systems have a built in resistance to the back driving alignment torque, which prevents the tires from returning immediately to the neutral position, causing the vehicle to continue to turn. In this situation the electric power steering system  10  generates an assist torque to counter balance the back driving torque thereby allowing the vehicle tires to return to the neutral position such that the vehicle continues in a straight line. 
         [0028]    Steer dampening occurs when the vehicle encounters an external disturbance in the road which generates a strong back driving torque that has the effect of turning the handle bars. Examples of external road disturbances include the vehicle hitting a rock or driving on a tractor track. In these situations the vehicles are typically travelling at a high speed and the steering angle rate is higher than is normally obtained from human input or back driving alignment torque geometry. In general, high steering stability is required during high vehicle speeds. The electric power steering system  10  generates a damping torque braking effect on the motor  14  to counteract and prevent the road disturbance torque from turning the handle bars. The damping torque is directly proportional to the induced speed of the motor. The active damping function allows the castor of the vehicle to be reduced thereby reducing low speed steering effort while not compromising high speed stability. 
         [0029]    Referring again to  FIG. 1 , the control system  12  comprises a steering torque sensor  22 , a vehicle speed sensor  24 , a motor current sensor  26 , a steering shaft angle sensor  28 , and an electric power steering controller  30 . The steering torque sensor  22  senses the torque applied to the steering device, e.g. the handle-bars (not shown), by a driver of the vehicle and provides a steering torque signal T IN . The vehicle speed sensor  24  provides a vehicle speed signal V S  representative of the vehicle speed. The motor current sensor  26  senses the current of the motor  14  and provides a motor current signal I M . The steering shaft angle sensor  28  senses the angular displacement of a shaft of the steering column  18  and provides a steering shaft angle signal θ S . The electric power steering controller  30  receives the signals T IN , V S , θ S  and I M  and provides a motor control signal V d . 
         [0030]    Referring now to  FIG. 2 , the electric power steering system  10  is described in more detail using an alternative view thereof in which the sensors  22 ,  24 ,  26  and  30  are omitted. It is understood by those skilled in the art that the electric power steering controller  30  can be implemented in either analog or digital form, or a combination of both. The description provided hereunder emphasizes a digital embodiment by way of example only. The electric power steering controller  30  comprises a feed-forward component  32 , an assist torque feedback component  34 , and a feedback component  36 . The feed-forward component  32  receives the steering torque signal T IN , the vehicle speed signal V S , and the steering shaft angle signal θ S  and provides an assist torque target signal T at . The assist torque feedback component  34  receives the motor current signal I M  and provides an estimated assist torque signal T aes . The assist torque target signal T at  and the estimated assist torque signal T aes  are added to each other to generate an assist torque error signal T aer . The feedback component  36  receives the assist torque assist error signal T aer  and provides the motor control signal V d . 
         [0031]    The steering torque signal T IN  multiplied by the transfer function of the feed-forward component  32  equals the assist torque target signal T at . The feed-forward component  32 , which can also be called a torque multiplier, simulates pure power assist steering and in the present embodiment is a tunable map which takes as inputs the vehicle speed signal V S  and the steering shaft angle signal θ S . The transfer function of the feed-forward component  32  is a continuous function which can be characterized in the three regions of steering operation described above as outlined in Table 1 below for different steering events. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Target Torque assist 
                   
                 Region of Operation 
               
               
                 (T at ) 
                 Conditions 
                 (Steering Event) 
               
               
                   
               
             
             
               
                 T at  &gt; 0 
                 T IN  &gt; 0 
                 Assist 
               
               
                   
                 V s  &lt; speed limit 
               
               
                 T at  = 0 
                 T IN  = 0 
                 Return 
               
               
                   
                 V s  &lt; speed limit 
               
               
                 T at  &lt; 0 
                 V s  &gt; speed limit or 
                 Damping 
               
               
                   
                 θ s  &gt; angle rate limit 
               
               
                   
               
             
          
         
       
     
         [0032]    Referring to  FIG. 3 , the algorithm of the transfer function of the feed forward component  32  is described. The steering torque signal T IN  is read in step S 102  and is compared to an input torque target value in step S 104 . The magnitude of the steering torque signal T IN  is limited to the absolute value of the input torque target value. The torque input error is calculated in step S 106 . The new assist torque target T at  is calculated in step S 108  by adding an old assist torque target signal to the product of the torque input error and a torque error gain factor, comparable to an integrator function. 
         [0033]    Referring to  FIGS. 2 and 4 , the algorithm of the transfer function of the assist torque feedback component  34  is now described. The motor current I M  multiplied by the transfer function of the assist torque feedback component  34  equals the estimated assist torque signal T aes . The motor current I M  is read in step S 202 . The estimated assist torque signal T aes  is calculated in step S 204  by multiplying the estimated torque constant K tes  by the estimated forward gear efficiency (ηgfes) and the gear ratio (n) and the motor current signal I M . 
         [0034]    As indicated above the assist torque feedback component  34  multiplies the motor current I M  by an estimated torque constant K tes , gear ratio and gear efficiency to calculate the estimated assist torque T aes . The motor current I M  is positive when forward driving, i.e. motor  14  driving gearbox  16 , and negative when backward driving, i.e. gearbox  16  driving the motor  14 . The estimated gear efficiency ηg is equal to the estimated forward gear efficiency (ηgfes) when the motor current I M  is greater than zero, and is equal to the estimated backward gear efficiency (ηgbes) when the motor current I m  is less than zero. The estimated gear efficiency ηg is speed dependent, and the torque constant K t  is temperature dependent. 
         [0035]    Referring now to  FIGS. 2 and 5 , the algorithm of the transfer function of the feedback component  36  is now described. The assist torque error signal T aer  multiplied by the transfer function of the feedback component  36  equals the motor control signal V d . The assist torque error signal T aer  is calculated in step S 302  by subtracting the assist torque target signal T at  from the estimated assist torque signal T aes . A duty cycle error signal V der  is calculated in step S 304  by multiplying the assist torque error signal T aer  by a duty error gain constant, and the result is limited in magnitude in step S 306 . The new duty cycle is calculated in step S 308 , where an old duty cycle value is added to the duty cycle error signal. This function comparable to an integrator function. The feedback component  36  is a series of tunable maps in the present embodiment. 
         [0036]    The operation of the electric power steering control system  10  is described for steering return. The steering torque input T IN  is zero during steering return. However, due to road geometry there is a back driving alignment torque that creates a pitman torque T pit . The pitman torque T pit  drives the inertia and damping of the mechanical system, i.e. the motor  14 , the gearbox  16 , the handle bar  13 , the steering column  18 , the linkages, tires  15  and  19 , and drives the steering shaft at an angular rate ω. The shaft angular rate ω multiplied by a motor velocity constant and the gear ratio equals the back e.m.f voltage V e  which feeds back to the motor creating the motor current I M , which further resists the return to center of the steering system. The assist torque feedback component  34  senses the motor current I M  which results in an assist torque error signal T aer  that is multiplied by the feedback component  36  thereby creating the motor control signal V d , which counteracts the back e.m.f voltage V e . The effect on the motor current I M  of this feedback is to drive the average motor current towards zero, when the steering torque signal T IN  is zero. The perception to the driver of the vehicle is that there is no effort required from them to return the handle bars to neutral, thereby the steering column  18  appears to free spin simulating non-power steering assist vehicles. 
         [0037]    Referring to  FIG. 6 , there is shown an equivalent simplified circuit of the motor  14  with the motor control signal V d  applied. The motor  14  includes an equivalent series resistance R and an equivalent series inductance L. The back e.m.f voltage V e , illustrated in  FIGS. 6 &amp; 7   a,  first generates the motor current I m  causing a torque which tends to oppose the action of the pitman torque T pit . However, due to the feedback described above, a pulse width modulated voltage signal is input to the motor as the motor control signal V d , illustrated in  FIG. 7   b,  which tends to counteract the effect of the back e.m.f voltage V e . The total voltage across the equivalent resistance R and equivalent inductance L of the motor  14  is shown in  FIG. 7   c.  Referring to  FIG. 8 , the motor current I m  across the motor equivalent circuit in  FIG. 6  is shown, which has an average value substantially close to zero, in the situation when the steering torque signal T IN  is zero. 
         [0038]    While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof. As is readily apparent the system and method of the present invention is advantageous in several aspects.