Abstract:
A solenoid operated valve with a single inlet for connection to a pump and a control signal outlet for connection to a power assist steering actuator and a bleed port for connection to a pressure control valve. The valve has a poppet rod extending through an annular valve seat, with a pressure equalization passage extending through the poppet and rod to balance the inlet pressure forces acting on the poppet and rod. The valve provides a low residual flow when the solenoid is de-energized and proportionally increases flow as current in the solenoid is increased and gives substantial linearity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to vehicle power assist steering systems and particularly to systems of the type providing a variable assist to the vehicle operator in response to changing conditions, as for example road speed. Such systems for providing variable steering effort assist in a vehicle have employed a solenoid operated flow control valve to control flow of hydraulic fluid to a power steering actuator, typically a hydraulic piston and cylinder type actuator. Presently, it is known to provide a solenoid operated flow control valve having a single inlet and two outlets with one outlet connected to the steering actuator and the other outlet connected to a pressure control valve or bleed valve for maintaining a controlled supply pressure to the power steering actuator wherein the flow through the pressure control valve is inversely proportioned to the solenoid energization current. 
     This arrangement has been necessitated in variable assist power steering systems because of the range of pressures experienced from the supply pump which has its speed varying with the engine shaft speed. In such a variable assist power steering system arrangement, the pressure control valve serves to bleed hydraulic fluid from the flow control valve to the inlet of the pump or sump when the pressure in the flow control valve valving chamber exceeds a pre-selected value thereby enabling the solenoid operated valve to control flow to the steering actuator at a generally controlled pressure. 
     Heretofore, solenoid operated valves for variable assist power steering assist systems have provided a flow response which is decreased with increasing current flow to the solenoid from a vehicle on-board electronic controller which received an input from a vehicle speed sensor. The electronic controller provided an electrical current to the solenoid for controlling flow to the steering actuator. However, at high current levels and low flow the position of the valve pintle close to the valve seat created a Bernoulli effect which permitted the pressure acting on the pintle head to drive the valve suddenly to a closed position maximizing the power assist to the steering and creating a sudden change in the steering response. In such an arrangement, where a failure occurs in the power supply to the solenoid during operation, a sudden decrease in steering effort required by the vehicle operator could result in the operator&#39;s over steering the vehicle and create a hazardous condition 
     It is known to provide a solenoid pressure balanced spool valve for constant level assist power steering systems as described in U.S. Pat. 5,607,137. However, spool valves have a limited range of flow response and may not be suitable for variable assist power steering applications. 
     Accordingly, it has been desired to provide a solenoid operated proportional flow control valve for a hydraulic power steering system which gives the desired flow control, particularly at low current levels to the solenoid and which eliminates the effect of the pressure forces which tend to drive the valve to a suddenly closed position. 
     BRIEF SUMMARY OF THE INVENTION 
     It therefore has been desired to provide a solenoid operated flow control valve for a variable assist power steering system with flow to the steering actuator directly proportional to the solenoid current, yet prevent sudden valve closing and to provide a residual or minimum flow with the solenoid de-energized. 
     It is an object of the present invention to provide a flow control valve which provides flow from an inlet to a first outlet which is proportional to the energization current of the solenoid actuator of the valve and bleed flow to control pressure is provided at a second outlet. 
     It is another object of the invention to provide a solenoid operated flow control valve having the flow directly proportional to the solenoid energization current and to have the valve configured as a pintle-type valve to provide a relatively low manufacturing cost to render the valve suitable for high volume mass production vehicle variable assist power steering systems. 
     It is a further object of the present invention to provide a solenoid operated proportional flow control valve which is able to maintain a low or residual flow in the de-energized state to the first outlet without the pressure forces closing the valve. 
     The solenoid operated flow control valve of the present invention provides a poppet passing through the valve seat to the inlet side and having a reverse taper to provide directly proportional flow and a through passage in the poppet which communicates the inlet pressure with the opposite end of the poppet to balance the pressure forces acting on the poppet in the axial direction, thus eliminating the tendency of the pressure forces to abruptly close the valve at low flow positions of the poppet. The present invention provides calibration of the valve in the de-energized state to provide a residual low flow to the control signal port. The valve of the present invention is particularly suitable for variable assist power steering applications in motor vehicles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a variable assist power steering system employing the flow control valve of the present invention; 
     FIG. 2 is a cross-section of the valve of the present invention in the de-energized condition without the solenoid; 
     FIG. 3 is a cross-section of the valve of the present invention, with the solenoid fully energized; and, 
     FIG. 4 is a graph of flow as a function of solenoid energization current for the valve of FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a variable assist power steering system is indicated generally at  10  and includes a module  12  having mounted therein a pressure balance flow control valve in accordance with the invention indicated generally at  14 , a pump  16  and a pressure control valve  18 . The module  12  may be mounted in a convenient location on the vehicle adjacent the hydraulic steering actuator  20 . A sump or pump return is denoted by reference numeral  22  which is illustrated as separate from module  12 ; however, it will be understood that alternatively the sump may be formed integrally with pump  16 . 
     The pressure control valve  14  is operated by a solenoid indicated generally at  24  which receives energization current from an electronic controller  26  connected to the onboard vehicle power supply  28 ; and, controller  26  also receives an input from a vehicle road speed sensor  30 . It will be understood that the hydraulic steering actuator  20  may comprise a piston and cylinder acting on the steering linkage or may comprise a pressure piston powered rack and pinion arrangement. 
     The system of FIG. 1 operates by flow control valve  14  receiving at its inlet pressurized fluid from the pump along conduit  32  and providing a controlled flow at its outlet along conduit  34  to the steering actuator  20 . The pressure of the fluid in the valving chamber of valve  14  is controlled by bleeding fluid from the second outlet of the valve along conduit  36  through a pressure control valve which returns fluid along line  38  to the sump  22 . It will be understood that the pressure control valve may be a simple pressure relief valve or an electrically controlled valve if desired. 
     Referring to FIGS. 2 and 3, the valve  14  is shown with the solenoid  24  fully energized with the valve in the fully open condition in FIG. 3; and, the valve  14  is shown in the low flow condition with the solenoid de-energized in FIG.  2 . 
     The valve  14  has a valve body  40  which has an inlet passage  42  which has formed therein an annular valve seat  44  which may be formed on an insert  46  which has a plurality of passages  52  formed around the valve seat  44 . It will be understood that the inlet passage  42  is adapted for connection to the inlet conduit  32  of the system of FIG.  1 . 
     A primary outlet is formed in the passage  42  downstream of valving surface  44  and the outlet is denoted by a plurality of ports  48  which are adapted for connection to the conduit  34  supplying the steering actuator  20 . A secondary or bleed outlet  50  is formed in the valve body  40  and spaced from the primary outlet  48 ; and, the secondary outlet  50  is adapted for connection to the conduit  36  supplying the inlet pressure control valve  18 . The inlet passage  42  communicates with the primary control signal ports  48  via passages  52  provided in the form of grooves about the periphery of the insert  46  as denoted by reference numeral  52  in FIGS. 2 and 3. 
     A valve operating member  54  is slidably disposed in a bore  56  formed in valve body  40  and extends in one direction outwardly thereof in a direction opposite the inlet; and, valve operating member  54  extends in the opposite direction through valve seat  44  and has a tapered conical valving surface  58  formed on the undersurface of a poppet  60  formed on the end of the valve extending through the valve seat  44 . Valve member  54  has an annular ferromagnetic armature  62  attached to the end of the valve member extending outwardly of the body  40 ; and, the armature and valve member  54  are biased in a leftward direction or in a direction tending to move the valving surface  58  toward the valve seat  44  to minimize flow by a spring  64  having one end registered against the armature and the opposite reaction end registering against the body  40 . 
     An armature guide or cover denoted by reference numeral  66  having an enlarged diameter or bell-mouthed open end is received over the armature and sealed thereover by suitable seal ring  68 . The armature guide  66  is secured to the body by threaded engagement as denoted by reference numeral  70  with a reduced diameter portion of the body. The interior of the armature guide chamber denoted by reference numeral  72  is communicated with the pressure in the inlet passage by means of a pressure equalization passage  74  which is formed through the valve member and is open at one end to chamber  72  and extends therethrough to poppet  60  and opens to the inlet passage  42 . 
     Referring to FIG. 3, an annular flux collector  76  is received over the armature guide and has a flange  78  formed thereon which retains an encapsulated coil  80  over the armature guide. The flux collector and coil are secured thereon by a suitable snap ring  82  which engages a groove  84  formed in the armature guide. Encapsulated coil  80  has formed therewith an electrical receptacle  86  which has electrical terminal ends  88  formed therein which are connected to the coil  80  in an unshown manner well known in the art. 
     The construction of the valve shown in FIGS. 2 and 3 thus enables the coil at low current to overcome some of the bias of spring  64  and maintain the valve seat  44  open sufficiently to provide a minimal amount of flow through control port  48 ; and, in a power steering application, such as that of FIG. 1, to conduit  34  to the hydraulic steering actuator  20 . With the coil de-energized the valve member  54  is moved to the position shown in FIG. 3 by spring  64  which does not completely close the valving surface  58  against seat  44  thereby permitting some flow to control signal ports  48 . The valve is shown in the fully energized fully open position in FIG. 3 wherein maximum flow occurs through port  48  conduit  34  and to the steering actuator  20 . Thus, in the event of loss of power to the coil  80 , the valve member  54  is moved to the position shown in FIG.  2  and some minimal flow which in the application of FIG. 1 is to the steering actuator thereby maintaining a minimal power assist to the vehicle operator for steering. The present invention thus prevents a sudden change in steering assist in the event of power failure to the solenoid or failure of the solenoid to respond to energization. 
     Referring to FIG. 4, a curve of the flow through a valve made in accordance with the present invention as a function of coil current is indicated showing that the response is substantially linear from low energization to full energization. 
     Although the invention has hereinabove been described with respect to the illustrated embodiments, it will be understood that the invention is capable of modification and variation and is limited only by the following claims.