Abstract:
An electronic control unit monitors pressure conditions present at the input to the steering system and other vehicle data signals and then provides override control of a hydro-mechanical flow control device. An electro-hydraulic valve is connected to the supply side of the hydro-mechanical flow control device and reacts to the electronic monitoring of pressure and other vehicle data to alter the counter-balancing forces within the hydro-mechanical flow control device. This causes a by-pass diversion of steering fluid to the source by the hydro-mechanical flow control device and thereby relieves backpressure on the pump when relatively little steering assist is required to be provided by the steering system. The control of steering fluid diversion at such times results in a significant reduction in parasitic losses within the steering system and improves the operating and fuel efficiency of the vehicle.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to the field of power steering pumps as used in automotive vehicles and more specifically to the area of improved energy efficiencies in flow control devices that are used to control the delivery of hydraulic fluid to steering assist valves. 
     2. Description of the Prior Art 
     In conventional power assist steering systems, many techniques are used to position the location of a spool within a control valve to regulate the delivery of a constant amount of hydraulic fluid from a pump to a steering assist valve. Because the flow from the pump increases or decreases depending on the pump rotary speed, the flow regulation mechanism of the control valve is necessary to maintain a constant flow to the steering assist valve independent of pump rotary speed. In most cases, a by-pass orifice is established in the control valve that allows excess steering fluid to be diverted back to the pump input port or a fluid source. Corresponding control of the position of the spool regulates the flow into the bypass orifice to thereby maintain a constant steering fluid flow delivered to the steering assist valve while the pump is driven at rates that increase or decrease depending only on the speed of the engine or other power source. 
     In such conventional power assist steering systems the pump provides a relatively high outlet flow at high pressure even when no steering assist is required. One such instance occurs when the vehicle is in a stationary or parked position with the motor running and no steering effort is being applied by the vehicle operator. Similarly, the pump provides a high output flow when the vehicle is moving at a high speed and low power assist is desired to be applied to the actuator. High flow creates higher pressure and increases the torque required to drive the pump. Such drive torque demand on the engine directly affects vehicle horsepower output and fuel efficiency. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce fuel consumption and improve operating efficiency of a motorized vehicle by minimizing the pressure on the power steering pump, and reducing the consequent torque demanded by the pump when no or relatively low steering assist is required. 
     Another object of the present invention is to provide a system that reduces fluid flow to the steering system when little or no steering torque is required. The reduced flow reduces energy losses by reducing the flow circulating through orifice restrictions in the pump and steering system. 
     Another object of the present invention is to provide a system that reduces pressures on the supply side of a hydro-mechanical flow control device while maintaining regulated flow. 
     A further object of the present invention is to provide a hydro-mechanical flow control device with an electronic pressure control (“EPC”) system that senses fluid pressure output to the steering valve and utilizes input data from the vehicle to determine when and how much of the output flow from the engine driven power steering pump can be directed to the steering valve and/or to the by-pass orifice, and thereby relieve the pump pressure. 
     These objects are achieved by a control system for reducing parasitic losses in a power steering system of an automotive vehicle during periods of low demand for power assist and high pump output due to engine speed. A hydraulic pump is driven by the engine or other power source of the vehicle and provides an output of steering fluid to the steering system under pressure. A source of steering fluid is connected to the pump and supplies steering fluid to the pump and receives returned fluid from the power steering and control systems. A hydro-mechanical flow control device, containing a spring biased spool is connected to both the output of the pump and the input to the steering system. The flow control device reacts to predetermined flow and pressure conditions that exist between the output of the pump and the flow and pressure conditions present at the input to the steering system. The flow control device provides a diversion of steering fluid to the source when the pump output pressure is significantly greater than the pressure conditions present at the input to the steering system. The flow control device thus regulates and maintains constant the flow of steering fluid to the steering assist valve as the output of the pump increases or decreases depending on engine rotary speed. The improvement includes the use of an electronic control unit that monitors pressure conditions present at the input to the steering system and other vehicle data signals and then provides override control of the hydro-mechanical flow control device. An electro-hydraulic valve is connected to the supply side of the hydro-mechanical flow control device and reacts to the electronic monitoring of pressure and other vehicle data to alter the counter-balancing forces within the hydro-mechanical flow control device. This causes a by-pass diversion of steering fluid to the source by the hydro-mechanical flow control device and thereby relieves backpressure on the pump when relatively little steering assist is required to be provided by the steering system. The control of steering fluid diversion at such times results in a significant reduction in parasitic losses within the steering system and improves the operating and fuel efficiency of the vehicle. 
     In one particular aspect of the invention, an electrically regulated valve is strategically located in parallel with the input to the supply side of a hydro-mechanical flow control device to selectively relieve pressure at the supply side, when desired. When the electric valve is activated, the pressure at the supply side of the spool in the flow control device is reduced and the spool responsively moves towards that lower pressure to open or further open the by-pass orifice. Further opening of the by-pass orifice allows an increase of fluid from the pump to be diverted back to the pump inlet and thereby reduces pump output pressure and flow at the steering valve. Such pressure reduction relieves backpressure on the pumping mechanism of the pump thus reducing the torque on the pump shaft and consequently on torque losses from the engine. The electric valve is electronically controlled according to various inputs selected from the group of data that reflect vehicle speed, steering wheel angle, steering wheel turn rate, steering wheel torque, steering wheel straight ahead position, vehicle lateral acceleration, tie rod force, steering fluid flow rates, and pump output pressure. In this manner, the electrically regulated valve provides enhanced control to the hydro-mechanical flow control device at times when no or relatively low power steering assist is required. Such enhanced control results in a correspondingly regulated by-pass diversion of the pump output that minimizes back pressure on pump and pump driving shaft when no or low steering assist is required. This is achieved independent of the regulation provided by the flow control spool valve during normal vehicle operation when power steering assist is required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of a typical engine driven vehicle power steering system. 
         FIG. 2  is a schematic representation of a power steering pump system which includes a hydro-mechanical flow control device, with a bypass orifice, in a first normal mode incorporating an embodiment of the present invention. 
         FIG. 3  is a schematic representation of hydro-mechanical flow control device of  FIG. 2  in a second normal mode, and incorporates an embodiment of the present invention. 
         FIG. 4  is a schematic representation of hydro-mechanical flow control device of  FIGS. 2 and 3 , incorporating an embodiment of the present invention in an operational mode. 
         FIG. 5  is a cross-sectional view of an electro-hydraulic valve used in an embodiment of the present invention. 
         FIG. 6  is a block diagram of the EPC controls used in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a conventional power steering system  10 , as is typically employed in an automotive vehicle. Steering system  10  includes a steering gear  24  and a hydraulic assist piston  26  in a housing  30 . (Many possible steering mechanisms, such as, but not limited to, rack and pinion, re-circulating-ball, or worm-roller mechanisms may be used.) A steering wheel  50  is shown connected to a steering column  48  and, through a rotating universal joint  46 , to steering assist valve  45 . Tie rods  34  and  36  extend from steering gear  24  and provide the interconnection between the steering gear  24  and wheels  12  and  14 , which are mounted to the vehicle chassis (not shown). An engine driven pump assembly and reservoir  100  provides power steering hydraulic fluid under pressure through an outlet line  18  to torque driven steering assist valve  45 . A portion of the pumped fluid is supplied from steering assist valve  45  to the steering gear through output lines  21  and  22 . This pressurized fluid acts on hydraulic piston  26  to assist the steering effort exerted on the steering wheel by the vehicle operator. The fluid that is circulated in steering assist valve  45  and housing  30  is returned to a pump reservoir  20  through line  19 . 
     The power steering pump assembly  100  is schematically illustrated in  FIGS. 2-4  with emphasis on a flow regulation system. The schematic depicts a system that includes an engine driven pump assembly  16 , that receives its source of steering fluid for pumping from a an integrated reservoir  20 . (Alternatively, the pump can receive its source of steering fluid from a closed system that may also include a fluid cooling system.) The output of pump  16  is connected directly to an output line  180  which leads through a pump side inlet  404  and into a pump side chamber  402  of a flow control valve device  40 . The pump side chamber  402  also includes a pump side outlet  407  to which a line  181  is connected. A fixed orifice  430  is connected between line  181  and outlet line  18 . A feedback line  182  extends from line  18  at the outlet side of orifice  430  to the supply chamber side of the flow control valve device  40 . A feedback orifice  440  is located in line  182  in series with a damping orifice  442  and a supply side inlet  406  which leads to the supply side chamber  403 . A line  409  forms a bypass path that extends from a by-pass orifice  408  in flow control device  40  directly to the inlet of the pump  16 . 
     An electronic pump control (“EPC”)  500  of the present invention is depicted as being an integral part of the pump system and includes at least one pressure sensor  504 , shown in output line  18 , to sense the pressure of fluid being output from the pump assembly  100  and applied to the steering assist valve  45 . (It should be recognized that the location of the pressure sensor could be changed to other locations in the system where pump output pressure can be measured and that multiple sensors could be used, depending on the desired properties of the system and packaging limitations specified by the vehicle manufacturer. Also the pressure sensor could be eliminated if another type of sensor or a combination of sensors can be substituted to provide the same information.) An electronic control unit  502  functions to make appropriate calculations based on various data input signals received on data line  506  and to control the variable orifice of an electro-hydraulic valve  501 . Electro-hydraulic valve  501  is located in a line  191  that, when used, functions to relieve pressure and provide a flow path for a minor volume of fluid from junction  441  to bypass line  409 . 
     In the embodiment shown, flow control device  40  includes a valve bore valve bore  400  with a supply side inlet  406  and a by-pass return orifice  408 , in addition to the pump side inlet  404  and pump side outlet  407  mentioned above. A spool  410  is contained within the valve bore  400  for linear movement therein. A pump side chamber  402  is located within the valve bore  400  and to the left of spool  410 . The volume of pump chamber  402  is defined by the space existing within the valve bore  400  between the wall containing pump side inlet  404  and the valve face  411  of spool  410 . A supply side chamber  403  is located within valve bore  400  and to the right of spool  410 . The volume of supply side chamber  403  is defined by the space existing within the valve bore  400  between the wall containing supply side inlet  406  and the right side of spool  410 . A spool biasing spring  420  is located in supply side chamber  403  and extends from the wall containing supply side inlet  406  to the spring face  413  on the right end of spool  410 . Spool  410  contains a normally closed pressure relief check valve  415  that is in communication with supply side chamber  403  through a passage  412 . Check valve  415  is in communication with a relief passage  414  and by-pass orifice  408  that allows fluid to flow through check valve  415  when the steering system pressure exceeds a limiting value. 
     In  FIG. 2 , spool  410  is shown as being located in a position within the valve bore  400  that causes by-pass orifice  408  to be closed and prevents fluid in pump chamber  402  from flowing into by-pass line  409 . In this case, all steering fluid delivered from the pump  16  is delivered to the steering valve  45  through the fixed orifice  430 . This is due to the differential pressure present across the spool  410 . Differential pressure is derived from the fluid pressure in the supply side I chamber  403  combined with the biasing forces applied by spring  420  on spool  410  and the pump output pressure present in pump side chamber  402 . High pressure in supply side chamber  403  normally occurs when the steering system demand is greatest. One such instance is when the vehicle is stationary, the engine is at idle, and the vehicle operator is turning the steering wheel. Maximum torque is demanded during such times and therefore the fluid output from pump  16  is delivered to steering assist valve  45 , with no by-pass diversion occurring by flow control device  40 . In the case shown in  FIG. 2 , the differential pressure forces spool  410  to a position that causes by-pass return orifice  408  to be closed. 
     In  FIG. 3 , spool  410  is shown as being located in a position within the valve bore  400  that provides a partially open by-pass return orifice  408  and allows a portion of the fluid in pump chamber  402  to flow into by-pass orifice  408  and line  409 . The opening of by-pass orifice  408  means that a portion of the output from pump  16  flowing into the pump side chamber  402  is diverted and the output pressure in line  18  is reduced. The degree of opening of by-pass orifice  408  is due to the differential pressure existing across spool  410 . The position shown in  FIG. 3  reflects the case(s) where a lower torque demand is being made on the steering system and the pump pressure is higher than that produced when the engine is at idle. For instance, when the engine and pump  16  have their speeds increased, the flow output from pump  16  increases with a corresponding increase in pressure. As the torque demands of the steering system change, they are reflected as changes in pressure at steering assist valve  45 , and flow control device  40  responds to changes in differential pressure by providing a greater or lesser amount of fluid flow and pressure by controlling the opening of by-pass return orifice  408 . 
       FIG. 4  illustrates the effects of the present invention when it is activated under the influence of electronic control unit  502 . In this case, the sketch depicts the situation where it has been determined by the logic of electronic control unit  502  that the pressure measured by pressure sensor  504  is high, and other vehicle data indicates that the demands presented by the steering system are low and therefore do not require significant fluid flow. Other vehicle data is provided to electronic control unit  502  in the form of signals reflecting vehicle speed, steering wheel position and steering wheel turning rate. EPC electro-hydraulic valve  501  is connected to the feedback line  182  and tapping into it for the purpose of providing a low pressure path from the feedback line (supply side inlet  406 ) to the input of the pump  16 , when relatively little steering assist is required to be provided by said steering system. In this case, the pressure on supply side inlet  406  and supply side chamber  403  is reduced by the opening of EPC electro-hydraulic valve  501 . When that occurs, fluid in the feedback path flows from junction  441  through electro-hydraulic valve  501  and into return line  191  back to reservoir  20 . The reduction in pressure at supply side inlet  406  allows spool  410  to react to the higher pressure existing in pump side chamber  402  and move against the force provided by biasing spring  420 . This movement of spool  410  causes the by-pass orifice  407  to become fully opened. The opened by-pass orifice  407  serves to relieve the pressure on the output line  18  and also to relieve the amount of backpressure that is present on pump  16 . With such reduction in backpressure, the pump presents less of a load to the engine and has a positive impact on the overall efficiency of the vehicle. The results include better fuel consumption (mpg) characteristics. This condition is maintained until electronic control unit  502  senses a change in data and determines that flow control device  40  should be restored to its hydro-mechanical flow control operation to support steering assistance. Damping orifice  440  in the control line  182  is provided to prevent transient shudder during opening and closing of the EPC electro-hydraulic valve  501 . 
     An embodiment of EPC electro-hydraulic valve  501  is shown in  FIG. 5 . EPC electro-hydraulic valve  501  includes a housing  510 , an electrical coil  512 , and an armature  514 . A valve plunger  518  extends from armature  514  and intersects a junction  521  between an outlet passage  520  and an inlet passage  522 . A biasing spring  516  acts to maintain armature  514  and valve plunger  518  in a position where the junction  521  of outlet passage  520  and inlet passage  522  is normally closed. When sufficient current flows in coil  512 , armature  514  is caused to move against the bias forces provided by spring  516  and valve plunger  518 , traveling with armature  514 , opens the junction  521  between inlet passage  522  and outlet passage  520 . EPC electro-hydraulic valve  501  is controlled to open the junction  521  at an infinite number of positions between its fully closed position and its fully open position, depending on the amount of current flowing in coil  512 . 
     In calculating the amount of current that must be applied to EPC electro-hydraulic valve  501  that indirectly affects the amount of pressure relief provided to the output of pump assembly  100  through bypass orifice  408 , electronic control unit  502  analyzes various input data signals from the vehicle. The software in the electronic control unit  502  represented by the block diagram in  FIG. 6  includes the following sections: Input Signal Processing  530 ; Desired Flow Calculation  540 ; Desired Current Calculation  550  and Current Control  560 . 
     Input signals include a steering wheel rotation signal  3 ; system pressure signal  5  from pressure sensor  504 ; electro-hydraulic current at  6 ; and vehicle battery voltage  7 . These signals are filtered to a usable state and provided to the Desired Flow Calculation section  540 . Desired Flow Calculation section  540  also receives vehicle speed signals  1 ; steering wheel angle position  2 ; engine rpm  4 ; brake signal  8  and an ignition signal  9 . Additional signals such as steering system differential pressure  10 , steering wheel torque  11 , vehicle lateral acceleration  12 , tie rod loads  13 , and steering fluid flow rates  14  also may be employed. With these input signals, an algorithm is used to determine the appropriate amount of flow through steering assist valve  45  that is necessary for meeting desired steering feel requirements for the particular vehicle. That is, it determines the optimal flow that is necessary to maintain the desired steering feel to the operator. The output is a desired flow signal that is fed to the desired current section  550 . This section also receives the engine speed signal and filtered pressure signal. This section utilizes an algorithm to calculate the pressure that is required in the supply side chamber  403  of flow control device  40  to produce the desired fluid flow through steering assist valve  45 . 
     The desired electro-hydraulic current signal from the desired current section  550  is provided to the current control section  560  which activates electro-hydraulic valve  501  with a signal having a predetermined duty cycle to achieve the desired current through the coil of the electro-hydraulic valve  501 . By monitoring the actual electro-hydraulic current, and the battery voltage level, the current control section  560  adjusts the output duty cycle to hold the current flow at the desired level. 
     When the vehicle is being driven without turning or braking, the amount of fluid flow through steering assist valve  45  that is needed to maintain power steering assist is relatively small. The present invention allows only the small required flow to occur, while diverting the excess flow from pump  16  and relieving the backpressure that would otherwise be present on the pump rotary unit. This action is achieved because EPC  500  adjusts the supply side pressure of flow control device  40  to small magnitudes while providing the desired flow to the steering system through steering assist valve  45 . Consequently, pump  16  is not subject to the high pressures that are normally required to move spool  410  over by-pass orifice  408  in order to return excess flow. 
     It should be understood that the foregoing descriptions of the hydraulic flow valve, the power steering pump and the control circuit embodiments are merely illustrative of many possible implementations of the environments in which the present invention can be practiced. Those descriptions are not intended to be exhaustive of the many possible embodiments and implementations of the invention as set forth in the following claims.