Abstract:
A hybrid electrical power assist system (EPAS) for an automotive vehicle that utilizes the advantages of a conventional EPAS system to provide power assist based upon several distinct sensor inputs, in a heavy duty vehicle that requires significantly more assistance forces than a conventional EPAS can provide. The hybrid system utilizes a hydraulic amplifier that is connected to receive the torque output from an EPAS motor actuator and responsively provide a fluid under differential pressure through a pair of high pressure lines to a steering piston.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to the field of electrical power assist steering systems for automotive vehicles and more specifically to the area of increasing the output of such systems to enable them to be used in heavy duty vehicles. 
         [0003]    2. Description of the Prior Art 
         [0004]    Generally, power assisted steering systems for automotive vehicles are classified as either hydraulic power assisted steering (“HPAS”) or electrical power assisted steering (“EPAS”). A pure HPAS system is the type in which a fixed displacement hydraulic pump is directly driven from the vehicle engine to supply pressurized fluid through a hydro-mechanical control valve to a steering gear where the pressure is differentially applied to a piston or other steering actuator mechanism on the steering gear. A pure EPAS system is the type in which an electrically powered motor is controlled by sensor reactive circuitry to apply assisting torque directly to the steering gear or other steering actuator mechanism. All control to the steering gear in a pure EPAS system is electromechanical in nature and no hydraulic systems are involved. EPAS systems generally provide for a greater use of sensors throughout the vehicle and allow steering assist to be adjusted in accordance with driver input through the steering wheel and other factors, such as speed of the vehicle, rate of steering wheel torque and many other variables that were not available in pure HPAS systems. 
         [0005]      FIG. 1  illustrates a block diagram of a typical prior art EPAS system as employed in an automotive vehicle. In such a system, a control module  100  contains an electrical controller  110  which feeds an output control signal to a power electronics module  120 , which, in turn, supplies electrical power on line  140  to an actuating torque motor  150 . Torque motor  150  contains a gear  160  on its output shaft which is engaged directly or through a gear mechanism with a steering gear  212  within a rack housing  210 . In this depiction, the driver input is represented as a steering wheel  250 . A torque transducer  240  is located on the steering wheel shaft to provide informational data to the controller  110 . Such informational data includes the torque being applied by the driver to the steering wheel. Also, it may sense the position of the steering wheel as well as its distance from center. Steered vehicle wheels are represented as output mass  260  connected to steering gear  212 . Controller  110  is connected to receive input data from the vehicle such as vehicle speed, steering wheel position and steering wheel input torque. Based upon such data, controller  110  utilizes an algorithm to determine how much assistance torque to apply to the steering gear  212  through torque motor  150  and its gear  160 . 
         [0006]    It is highly desired to employ EPAS systems in heavy duty vehicles. However, when using EPAS systems with a conventional 12 volt DC electrical power system, there are practical limitations that must be overcome or accommodated. Due to the larger steering loads encountered by placing such systems in heavier vehicles, there would be a requirement for higher capacity power electronics and larger motor components. The increased cost of these components makes EPAS systems uncompetitive with HPAS systems in high load applications. Even when cost is ignored, the maximum current available from the vehicle electrical system is a real-life barrier to implementation. As a practical alternative, several variations of hybrid systems have been developed in which a hydraulic-mechanical link of an HPAS system is maintained, to some degree, while one or more control functions applied to a hydraulic assist are electrically controlled to provide greater response to various vehicle data. 
         [0007]    There continues to be a need for an improved EPAS system or some version of a hybrid EPAS system in which steering assist could be applied to heavy duty vehicles, while minimizing the detrimental effects of the system on the operating efficiencies of the vehicle. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a hybrid EPAS system that applies a hydraulically amplified steering assist fluid under differential pressure to the steering gear of a vehicle. The hydraulically amplified steering assist is controlled in response to the torque output produced by a conventional EPAS actuator motor and a controller which responds to electrical sensors providing vehicle data useful to provide enhanced steering control. 
         [0009]    In the described embodiment, an over-center variable displacement pump is utilized as the hydraulic amplifier. The over-center variable displacement pump has a pair of pump chamber output ports that are connected to either side of a steering gear piston chamber to provide fluid under differential pressure to the piston and assist in moving the steering gear. The over-center variable displacement pump also has an adjustable ring cam which, with a fixed center vane rotor, defines a variable pump chamber to determine the differential pressure output via the pump chamber output ports. Depending on the adjustment of the ring cam, the fluid at one port will have a higher pressure than the fluid at the other port. If no adjustment is made to the ring cam, it is biased to assume a neutral position and no differential pressure is present at the pump chamber output ports. When adjusted between its extreme locations, the ring cam controls the pump to vary the output pressure differential that is applied to the steering gear. The amount of torque required to adjust the ring cam by the EPAS motor is significantly less than the amount of differential pressure output from the over-center variable displacement pump, thereby resulting in the desired force amplification. 
         [0010]    A fixed displacement hydraulic pump is used to supply fluid to the variable displacement pump under relatively low pressure. The fixed displacement pump provides make-up fluid to replace any losses that occur between the internal chambers, seals and passages of the variable displacement pump and the fluid reservoir. By providing such make-up fluid, cavitation is prevented from occurring in the variable displacement pump. For efficiency of construction, space, and future service, the rotors of both pumps are preferable mounted on a common shaft connected to a common accessory drive on the engine of the vehicle. Alternatively, separated drives could be applied to the pumps, if so desired. 
         [0011]    It is an object to the present invention to provide a power steering assist system which includes a variable displacement hydraulic pump having a pair of pump chamber output ports connected to provide differentially pressurized fluid to a steering gear apparatus; a control circuit which reacts to vehicle driver input and other vehicle data to provide an electrical signal to actuate a torque motor; the torque motor reacting to the electrical signal to adjust the displacement characteristics of the variable displacement hydraulic pump; and the first variable displacement pump reacting to the adjusted displacement characteristics to provide differentially pressurized fluid to the steering gear apparatus. 
         [0012]    It is another object of the present invention to provide a second hydraulic pump of lesser output capacity than the first pump to act as the source of fluid to the variable displacement pump. 
         [0013]    It is a further object of the present invention to provide a method of controlling the differential pressure of hydraulic fluid applied to the steering gear of a steering assist system in an automotive vehicle by utilizing the steps of providing hydraulic fluid output from an over-center variable displacement hydraulic pump, having a movable cam, through a pair of outlet lines to opposing sides of the steering gear; providing hydraulic fluid under differential pressure to the steering gear; controlling the differential pressure output from the over-center variable displacement hydraulic pump with a an electrically actuated torque motor which adjusts the movable cam; deriving data from the vehicle; and actuating the torque motor in response to the data to provide the desired differential pressure to the steering gear. 
         [0014]    It is still a further object of the present invention to provide a power assist steering system which utilizes a fixed displacement pump in tandem and on a common shaft with an over-center variable displacement pump. The two pumps are commonly driven by the same engine of an automotive vehicle. The fixed displacement pump provides relatively low pressure to said variable displacement pump. The variable displacement pump contains a ring cam that is infinitely adjustable between two extreme positions about a relatively fixed center vane rotor to change the pump chamber configuration and the differential pressure characteristics of the pump output. An EPAS controller and electrically driven torque motor provide adjustment control to the ring cam to position the ring cam with respect to the pump vane rotor and, therefore, the differential pressure output by the variable displacement pump to the steering gear. 
         [0015]    Advantages include:
       a) the utilization of a gearing mechanism to convert electrical motor torque to a force that is amplified by a hydraulic pump;   b) the utilization of a an over-center variable displacement pump to amplify and provide a differential pressure having a magnitude and flow to the steering gear piston that is a function of torque applied by the electric motor;   c) a feedback mechanism in the over-center variable displacement pump to resist cam displacement caused by torque from the electric motor, and to balance the cam in a neutral position the event of electrical system failure;   d) allowing for a single pump assembly which contains two pumps which share a common shaft; and   e) utilizing a smaller second pump to provide fluid supply to the larger over-center variable displacement pump to compensate for internal leakage and to pressurize the larger pump to prevent cavitation.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic of a typical prior art EPAS system. 
           [0022]      FIG. 2  is a schematic of an embodiment of the present invention. 
           [0023]      FIG. 3A  is a representation of an over-center variable displacement pump in which no torque is applied to the ring cam. 
           [0024]      FIG. 3B  is a representation of an over-center variable displacement pump in which torque is applied to position the ring cam to the right. 
           [0025]      FIG. 3C  is a representation of an over-center variable displacement pump in which torque is applied to position the ring cam to the left. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    In  FIG. 2 , a preferred embodiment of the invention is shown in which a conventional EPAS type controller module  100  is connected to a torque motor  350  connected to a hydraulic amplifier  300 . The amplifier  300  provides a fluid under differential pressure on output lines  319  and  329  to a steering apparatus  202 . Steering apparatus  202  contains a steering rack  278  within a housing  270 . Linear movement of the steering rack  278  along its length provides the forces necessary to affect directional steering of the vehicle wheels represented as output mass  260 . Steering gear  278  has a piston  280  mounted thereon and defines separate steering actuation chambers  272  and  274 . Differential pressure of fluid output from amplifier  300  applied to actuation chambers  272  and  274  on either side of the piston acts to assist the driving forces applied to steering rack  278  by the vehicle operator (“driver”) input in the form of a steering wheel  250 , or the like. Steering wheel  250  is connected to rotate a pinion gear  220 ; and a torque sensor  240  monitors the amount of torque applied to pinion gear  220  by steering wheel  250 . 
         [0027]    Controller module  100  contains an EPAS control circuit  110  which receives vehicle input data  130 , including vehicle speed, steering wheel input torque, steering wheel position and turning velocity. Control circuit  110  is programmed with an algorithm to provide a predetermined output signal to a power electronics driver  120  where it is stepped up to drive torque motor  350  with a current sufficient to provide a predetermined measure of rotational torque force output. The output of the motor  350  is directly connected to a reduction gear mechanism  360 . 
         [0028]    The heart of hydraulic amplifier  300 , in this embodiment, is the over-center variable displacement hydraulic pump  310 . While the rotor  312  is relatively fixed about its rotation center  321  (see  FIGS. 3A-3C ), its pump chamber  323  is subject to change in configuration. A ring cam  316  is mounted within a cam control chamber  331  which is shown as separate chambers  331   a  and  331   b . Rotor  312  has a plurality of vanes  314  slideably mounted in corresponding slots  315  distributed evenly about its outer periphery. When rotor  312  is rotationally driven, vanes  314  slide outward due to centrifugal forces to engage the inner circular wall surface of ring cam  316 . Ring cam  316  is mounted to be moved within the limits of the cam chamber  331  about a pivot pin  321 . Seals  327  and  328  provide separation of pressure to the left and right of cam control chambers  331   a  and  331   b  while allowing ring cam  316  to be moved about pivot pin  321 . Ring cam  316  has a central void which, along with the outer periphery of pump rotor  312 , forms the variable pump chamber  323 . In this embodiment, the central void of ring cam  316  is a circular space and is shown in  FIG. 2  as being concentric with pump rotor  312 . That is, the center  302  of ring cam  31   6  lies on center  301  of pump rotor  312 , when the pressures applied to outer wall surface of ring cam  316  are in balance. Balancing springs  322  and  324  are located on opposite sides of ring cam  316 , and function to bias ring cam  316  towards the concentric configuration noted above. 
         [0029]    Pump  310  has a pair of outlet ports  318  and  326  on opposite sides of variable pump chamber  323 . High pressure output lines  319  and  329  lead directly from outlet ports  318  and  326  to actuation chambers  272  and  274 , respectively. Rack gear  278  is responsive to applied driver input pressure from pinion gear  220 , and is assisted by the differential pressure output from pump  310  applied to actuation chambers  272  and  274  acting on piston  280 . Feedback orifices  332 ,  334  and  336  are provided between output lines  319  and  329  to present reduced but corresponding pressures to cam control chambers  331   a  and  331   b  via feedback lines  335  and  337 , respectively. 
         [0030]    A second pump  370  is of a fixed displacement variety and has a relatively low pressure capability, as compared to the variable displacement pump  310 . Pump  370  serves to provide make-up pressure to the variable displacement pump  310  and thereby prevent cavitation that may otherwise occur due to leakage within and among the various pump cavities. For convenience in packaging and service, the two pumps are mounted to have a common shaft  380  that interconnects and rotationally drives their respective rotors. Shaft  380  is driven as an accessory of the associated vehicle engine in a conventional manner, such as by a continuous belt or a gear system (not shown). Alternatively, if employed in a hybrid or an electric motor driven vehicle, the pumps could be driven by an auxiliary power source. 
         [0031]    Pump  370  is connected to draw fluid from a reservoir on input line  377  and provide fluid under a relatively low pressure that varies with the speed of the engine on output line  371 . The output pressure is equally provided to check valves  374  and  376 . From the check valves  374  and  376 , the fluid is supplied as make-up fluid to both sides of pump chamber  323  via lines  373  and  375 . Control orifice  372  is provided between the inlet and outlet of pump  370  to to provide backpressure on the inlet of the variable displacement pump. 
         [0032]    In operation, as control module  100  reacts to the need to provide steering assist, motor  350  is energized with a predetermined amount of current to cause a predetermined torque to be generated to the left or right, as appropriate. This torque is applied to gear  360 . Gear  360  responsively rotates against ring cam  316  to force it over center with respect to the relatively stationary rotor center  301 . Movement of ring cam  316  is limited by various factors, including the amount of pressure existing in the cam control chambers  331   a  and  331   b  due to pressure feedback from lines  335  and  337  as well as springs  324  and  322  all acting to counterbalance the torque pressure presented via gear  360 . 
         [0033]    As can seen in the simplified and exaggerated depictions in  FIGS. 3A ,  3 B and  3 C, the operation of the over-center variable displacement pump  310  functions to apply a differential pressure at its output ports  318  and  326  according to the position of ring cam  316 . 
         [0034]    In  FIG. 3A , ring cam  316  is shown in its center position, as it is also shown in  FIG. 2 . In this center position, the center  302  of the ring cam  316  and center  301  of pump rotor  312  are concentric and the pump chamber  323  is symmetric. In this condition, the output pressure at output ports  318  and  326  is equal no matter how fast rotor  312  is driven. Effectively, there is no differential pressure that will be applied to the steering gear piston when no torque is applied by gear  360 . This condition also illustrates the “limp home” mode where no assist is applied to the steering system in the event the EPAS controller or electrical system should fail. Ring cam  316  is balanced by both springs  322  and  324  as well as the balanced feedback in lines  325  and  327  to remain on center and not produce any differential in output fluid pressure. 
         [0035]    In  FIG. 3B , ring cam  316  is shown as being moved slightly right by clockwise directed torque applied by gear  360 . Ring cam  316  pivots slightly counter-clockwise about pivot pin  321  and its center  302  moves to the right of pump rotor center  301 . Assuming a clockwise rotation of pump rotor  312 , the movement of ring cam  316  changes the configuration of pump chamber  323  so that the pressure of fluid at output port  326  is greater than the pressure of fluid of output port  31   8 , as indicated by the “IN” and “OUT” arrows. When this occurs, right steering pressure “RP” is dominantly applied to actuation chamber  274 , while a lesser pressure is applied to actuation chamber  272  which affects the assist applied to piston  280  (see  FIG. 2 ). The differential pressure causes right steering to be assisted. 
         [0036]    In  FIG. 3C , ring cam  316  is shown as being moved slightly left by counter-clockwise directed torque applied by gear  360 . Ring cam  316  pivots slightly clockwise about pivot pin  321  and its center  302  moves to the left of pump rotor center  301 . Assuming a clockwise rotation of pump rotor  312 , the movement of ring cam  316  changes the configuration of pump chamber  323  so that the pressure of fluid at output port  318  is greater than the pressure of fluid of output port  326 , as indicated by the “IN” and “OUT” arrows. When this occurs, left steering pressure “LP” is dominantly applied to actuation chamber  272 , while a lesser pressure is applied to actuation chamber  274  which affects the assist applied to piston  280  (see  FIG. 2 ). The differential pressure causes left steering to be assisted. 
         [0037]    As can be seen by the drawings and accompanying explanation, the present invention allows the advantages of a conventional EPAS system to be utilized in power assist steering systems that require significantly more power than is available in a strictly electrical system within conventional vehicles. While the embodiment shown here is the preferred embodiment, it shall not be considered to be a restriction on the scope of the claims set forth below.