Abstract:
An apparatus and method for a steering system which compensates a drive current for certain variables in a steer-by-wire and electrical power assist steering system. The present invention utilizes a steering wheel angle sensor, a road wheel angle sensor, and a controller for determining a precise time and amount of compensation for the drive current.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a vehicle steering control system, and more particularly to a motor driver circuit for an electric steering control system. 
     BACKGROUND OF THE INVENTION 
     In the case of a so called “steer-by-wire” system for a motor vehicle, an electric motor actuator is provided for generating torque and applying the generated torque to a rack or linear steering member for steering of a vehicle. In the steer-by-wire system, there is no mechanical connection between the operators steering wheel and the motor actuator. The electric motor actuator generates the torque required to turn a vehicle&#39;s road wheels. However, in an electrical power assist steering system (referred to as EPAS), there is a mechanical connection between the steering wheel and the motor actuator wherein the motor actuator assists a driver&#39;s applied torque. A precise level of generated electrical current is essential to enable the motor actuator to produce an appropriate magnitude of torque for either a steer-by-wire or an electric power assist steering system. The magnitude and direction of torque generated by the motor actuator is a function of a number of variables including a steering wheel angle input, the vehicle&#39;s road wheel angle, and an electric motor actuator current signal which is generated by a motor driver circuit. Although generally not compensated for, the appropriate magnitude of applied torque is also affected by factors such as temperature fluctuations, component-to-component variations, wear of components, and other factors. In an effort to more precisely match motor current to the desired torque in view of the aforementioned variables, designers of steer-by-wire systems have introduced devices which measure the amount of electric current generated by the motor driver circuit and which compensate the measured current by a predetermined value or “offset”. 
     However, in the above-mentioned prior art steering systems, no compensation is provided for environmental factors such as temperature and other sources of errors in the applied current which can change over time. The accuracy of the current compensation is thus severely limited in these systems. The error in compensation causes an inappropriate amount of current to be applied to the motor actuator resulting in either too much or too little steering torque being applied. There is therefore a need for a steering system that will take into account environmental factors, and other variables including time dependent variables when compensating the current applied to a motor actuator for steer-by-wire and EPAS systems. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved steering control system in which the above-mentioned problems are addressed. A more specific object of the present invention is to provide a steering. control system which continuously monitors a control signal and an electric current applied to the motor actuator so as to improve the quality of current compensation in a steer-by-wire or EPAS system. 
     In order to achieve the above mentioned objects, there is provided according to the present invention a steering control system for a vehicle having an electric motor actuator, a motor driver circuit for generating and applying an electric current from the motor driver circuit and generating a feedback current signal. The steering control system includes a steering wheel angle sensor for sensing a steering wheel angle and generating a steering wheel angle signal; a vehicle road wheel angle sensor for sensing a vehicle&#39;s road wheel angle and generating a vehicle road wheel angle signal; a controller for setting an offset current signal and for processing the feedback current signal. The steering wheel angle signal, and the road wheel angle signal are used to generate a control signal for the motor driver circuit. The controller compares the control signal to a predetermined threshold value and subtracts the offset current signal from the measured electric current which is applied to the motor actuator when the control signal is greater than the predetermined threshold value. The offset current signal is set equal to an initial offset value when the control signal is less than the predetermined threshold value. 
     According to the present invention, the controller receives the generated steering wheel angle signal, road wheel angle signal, and feedback current signal to generate the control signal. The control signal is applied to the motor driver circuit and the motor driver circuit generates an electric current which is applied to the motor actuator. The current sensor measures and generates the feedback current to the controller. The controller continuously monitors the control signal and electric current to determine an adequate amount of compensation for the steering control system. Thus, the compensation of the electric current is consistently updated to reflect changes in the steering system during operation. 
     These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a steering control system for a vehicle according to the present invention; 
     FIG. 2 is a flowchart of a compensation method performed by the controller used in the system according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a steering control system  10  for a vehicle according to the present invention includes a steering wheel  12  connected to an input shaft  14 . A pinion gear  17  has gear teeth which are meshingly engaged with gear teeth (not shown) on a linear steering member or rack  16 . The rack  16  is coupled to a vehicle&#39;s road wheels  18  through a steering linkage in a known manner. The pinion gear  17  together with the rack  16  forms a rack and pinion gear set. The rotation of the pinion gear  17  is translated into lateral movement of the rack  16  causing steering angle changes for road wheels  18 . 
     When the steering wheel  12  is turned, a steering wheel angle sensor  13  senses a steering wheel angle of the steering wheel  12  and generates a steering wheel angle signal  15  for a controller  26 . In the preferred embodiment, the steering wheel angle sensor  13  is an absolute steering wheel angle sensor, wherein the steering wheel angle signal  15  is an analog signal, meaning a signal is produced as the steering wheel  12  is turned which identifies the steering position of the steering wheel  12 . A second embodiment of the steering wheel angle sensor  13  is a relative steering wheel angle sensor wherein the steering angle signal  15  is a digital signal generated by an algorithm that estimates the change in steering wheel position in relation to a specified reference point. A road wheel angle sensor  22  is mechanically coupled to a motor actuator  20  and generates an output related to the turning angle of the road wheels  18 . In the preferred embodiment, the road wheel angle sensor  22  is either a digital or analog encoder. In the case of the digital encoder, the motor actuator  20  generates electrical pulses which are applied to the digital encoder. In the case of the analog encoder, the motor actuator  20  generates an analog signal which is applied to the analog encoder. Concurrently with the turning of the steering wheel  12 , the motor actuator  20  is energized and an output gear (not shown) of the motor actuator  20  begins to rotate. The angle and number of rotations of the motor actuator  20  corresponds directly with a turning angle of the road wheels  18 . The road wheel angle sensor  22  senses the angle and number of rotations of the output gear (not shown) of the motor actuator  20  and generates a road wheel angle signal  24  for the controller  26 . The controller  26  processes the steering wheel angle signal  15 , the road wheel angle signal  24  and a feedback current signal  34  to generate a control signal  36 . A current sensor  28  generates the feedback current signal  34 . The control signal  36  is applied to a motor driver circuit  30 . The motor driver circuit  30  generates an electric current  32  for a motor actuator  20 . As the electric current  32  is applied to the motor driver circuit  30 , the current sensor  28  continuously measures the electric current  32 . The current sensor  28  generates the feedback current signal  34  which is applied to the controller  26 . Simultaneously, the rack and pinion gear set converts the rotary motion of the steering wheel  12  into linear motion of the rack  16 . When the rack  16  moves linearly, the road wheels  18  pivot about their associated steering axes and the vehicle is steered. The motor actuator  20  is connected with the rack  16  through a known manner. The motor actuator  20 , when energized, provides torque to enable the vehicle operator to steer the vehicle. 
     Referring to FIG. 2, a flowchart of a compensation method performed by the controller  26  in the present invention. The controller  26  in a preferred embodiment has permanent and temporary memory storage capabilities. A step  38  is the entry point for the method  37 . At a step  40  the controller  26  is permanently programmed with a predetermined threshold value which is determined by vehicle testing. At a step  42  the controller  26  is permanently programmed with an initial offset value which functions as a default amount of offset within the steering control system  10 . The initial offset value is automatically subtracted from the electric current  32  during system operation when the steering control system  10  has not received an adequate amount of information to determine a precise amount of required compensation. For example, when the vehicle is initially powered up, the steering wheel angle signal  15 , the road wheel angle signal  24 , and the feedback current signal  34  have not been generated which causes a zero output for the control signal  36  thereby rendering the precise amount of required compensation in the system undeterminable. 
     At a step  44  the controller  26  takes samples of the generated control signal  36  by processing the generated steering wheel angle signal  15 , road wheel angle signal  24 , and feedback current signal  34 . Step  44  occurs when a vehicle operator turns the steering wheel  12 . At a step  46  the controller  26  compares the control signal  36  to the predetermined threshold value. When the control signal  36  is less than the predetermined threshold value, the sampled feedback current signal  34  is temporarily stored and set as an offset current signal at step  48 . At a step  50  the offset current signal is subtracted from the electric current  32  which is generated by the motor driver circuit  30  for the motor actuator  20 . In the preferred embodiment the electric current  32  is pulse-width-modulated. When the control signal  36  is greater than the predetermined threshold value at step  46 , the controller  26  will set the offset current signal equal to the initial offset value and subtract the initial offset value from the electric current  32  at step  50 , thereby improving the quality of current compensation. The method then loops back to step  44 . 
     Although the present invention has been described with regard to a steer-by-wire system, the invention is not limited to such a system. The present invention may be used with equal utility in other embodiments and is not limited to those embodiments disclosed, and variations and modifications may be made without departing from the scope of the present invention.