WO1997048908A1

WO1997048908A1 - Pressure and temperature independent flow control valve system

Info

Publication number: WO1997048908A1
Application number: PCT/US1997/010541
Authority: WO
Inventors: Nianzhu Luo
Original assignee: Sauer Sundstrand - Control Concepts, Inc.
Priority date: 1996-06-20
Filing date: 1997-06-18
Publication date: 1997-12-24

Abstract

A housing has an inlet chamber (104) for receiving pressurized fluid. A check valve (140) is coupled to the inlet chamber (104) and movable, by a pressure force exerted by the fluid in the inlet chamber (104), between normally closed and open positions for transmitting the fluid from the inlet chamber (104) to the load. A pilot spool (124) includes a fixed orifice (124b) coupled to the inlet chamber (104) and a variable orifice (124c) coupled to tank. The variable orifice (124c) size is controlled by the pilot spool (124) position. A solenoid (112) is responsive to a control signal for moving the pilot spool (124) to reduce the size of the variable orifice (124c).

Description

PRESSURE AND TEMPERATURE INDEPENDENT FLOW CONTROL VALVE SYSTEM

FIELD OF THE INVENTION

The invention relates to hydraulic control valves generally, and in particular, to valves suitable for controlling the raising and lowering of an unbalanced load.

BACKGROUND OF THE INVENTION

Systems for the control of raising and lowering unbalanced loads are known in the art. For example, the cut crop receiving header and feeder house mechanism of an agricultural harvesting machine, such as a combine, is an unbalanced load. It is generally known that controlling the flow to raise and lower the header using on/off electrohydraulic valves causes a fluid shock wave, resulting in "bang" or flow jerk, leading to an abrupt displacement of the header. This flow jerk produces stress and may damage the combine components. U.S. Patent 4,202,250 to Zeuner et al. describes one approach to controlling the header, using a programmed poppet valve to minimize the flow jerk.

With the advent of more sophisticated harvester control functions such as automatic header height or floatation control, various harvester manufacturers are now using proportional valves for header control. Proportional valves allow various computer control strategies to be implemented for these functions. Combine harvesters are typically capable of accommodating crop headers of various weight and size. The control systems must be able to sense or adapt to the various weight headers that may be attached to the harvester. The characteristics of the raise and lower valves play a key role in the system performance. To make the system cost effective, proportional flow control valves are widely used. Most commercially available low cost proportional flow control valves are load dependent; that is, they are sensitive to source and load pressure variations and the temperature of the hydraulic fluid. In general, for fixed source pressure and load conditions, the transfer relationship for these valves is constant. That is, the flow versus control parameter (solenoid current) is fixed. However, if the load or source conditions change, the transfer relationship exhibits shift and/or slope changes. This change in flow results in variations in system performance. Various methods are known to address this variation in system performance. Electronic solutions generally require a closed loop control system with several sensors. These controllers use control algorithms such as calibration or adaptive control to compensate for variations in valve performance. Hydraulic solutions generally require the addition of pressure compensation. The current state of the art requires the use of a separate pressure compensator spool to achieve flow characteristics which are independent of load. Compensator spools adds complexity and in some cases can cause instability. Further, some compensated valves always require flow and thus cannot be completely shut off. SUMMARY OF THE INVENTION

The present invention is a valve system. The system includes a normally closed main valve for receiving pressurized fluid and transmitting the fluid to a load. A pilot valve mechanism includes a first orifice in fluid communication with the main valve and a second orifice coupled to tank. The second orifice has a size controlled by a position of the pilot valve mechamsm. An actuating mechanism is responsive to a control signal for moving the pilot valve mechanism, to determine the size of the second orifice. The position of the pilot valve mechanism changes so that flow through the main valve is determined by the control signal and is substantially independent of the pressure and temperature of the fluid. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a control system according to the invention.

FIG. 2 is a schematic diagram of the raise assembly shown in FIG. 1.

FIG. 3 is a schematic diagram of the lower assembly shown in FIG. 1.

FIG. 4 is a cross sectional view of a first exemplary embodiment of a control valve system according to the invention.

FIG. 5 is an exploded isometric view of the main components of the raise pilot valve shown in FIG. 4.

FIG. 6 is a cross sectional view of the raise pilot valve outer sleeve, taken across section line 6-6 of FIG. 5. FIG. 7 is a cross sectional view of the raise pilot spool, taken across section line

7-7 of FIG. 5. FIG. 8 is an exploded isometric view of the main components of the lower assembly shown in FIG. 4.

FIG. 9 is a cross sectional view of the lower valve cartridge outer sleeve, taken along section line 9-9 of FIG. 8. FIG. 10 is a cross sectional view of the lower valve main poppet, taken along section line 10-10 of FIG. 8.

FIG. 11 is a block diagram showing a mathematical model of the raise assembly shown in FIG. 4.

FIG. 12 is a block diagram showing a mathematical model of the lower assembly shown in FIG. 4.

FIG. 13 is a cross sectional view of a second exemplary embodiment of a control valve system according to the invention.

OVERVIEW

FIG. 1 is a schematic diagram of an exemplary system according to the invention, suitable for use in an open center system 100. The invention is a valve system 100 in which the raise assembly 110 includes a single set of components for providing proportional flow control and performing pressure and temperature compensation. Lower assembly 150 also includes a set of flow control components that control flow substantially independent of temperature and pressure. Valve system 100 is suitable for use in a system having a fixed displacement pump 91 , a relief valve 92, and an actuating cylinder 98 for raising and lowering the header 99 of an agricultural combine harvester

(not shown).

Raise assembly 110 has a normally closed main check valve 140 for receiving pressurized fluid from the pump 91 via port P, and transmitting the fluid to cylinder 98. A normally open proportional pilot valve 120 allows some or all of the pressurized fluid to be unloaded to tank. A main spring 126 biases pilot 120 to the open position. A first actuating mechanism, such as proportional solenoid 112, is responsive to a first control signal for moving raise pilot 120 away from the open position. When pilot 120 is at least partially closed by solenoid 112, the pressure at the inlet 104 to check valve 140 is sufficient to open check valve 140 and transmit fluid to cylinder 98. FIG. 2 is a more detailed schematic of raise assembly 110. Raise pilot valve 120 has a spool 124. Spool 124 has a first orifice 124b in fluid communication with the raise main valve 140 (via chamber 104), and a second orifice 124c coupled to tank. Second orifice 124c is a variable orifice having a size set by the position of raise pilot valve spool 124. Thus the size of orifice 124c is set by the control signal that control solenoid 112. A compensation spring 128 applies a force on pilot spool 124 tending to move spool 124 away from its open position. The position of pilot spool 124 changes to vary the size of orifice 124c, so that flow through check valve 140 is determined by the first control signal and is substantially independent of the pressure and temperature of the fluid.

FIG. 1 also shows lower assembly 150, in which a normally closed main poppet valve 164 receives fluid from an inlet chamber 106 coupled to cylinder 98. When open, main lower valve 164 transmits the fluid via passage 162a to a chamber 108. Chamber 108 is fluidly coupled to tank via a port T₂. A chamber 164c receives fluid from inlet chamber 106 via orifice 164f. Fluid in chamber 164c applies a control force tending to close main poppet 164. Fluid from inlet chamber 106 is also used to applied a force via passage 162h in a direction tending to open main poppet 164. A normally closed pilot valve 166 is coupled to main valve 164. The balance of the fluid forces on main valve 164 is such that the flow (or absence of flow) through pilot valve 166 controls the opening of main valve 164.

Pilot poppet valve 166 is coupled to inlet chamber 106 via a control orifice 164f. Pilot poppet valve 166 controls pilot flow through variable pilot orifice 164b and passage 164a to chamber 108, in order to control the opening of main poppet 164. A spring 165 biases pilot poppet 166 to its normally closed position. A second actuating means, which may be a proportional solenoid 152, opens pilot poppet 166 to release fluid from chamber 164c to tank via pilot orifice 164b, passage 164a, chamber 108 and port T₂. When the pressure in chamber 164c is reduced, the force balance on main poppet 164 changes, causing main poppet 164 to move to a new equilibrium position.

Main valve 164 provides a variable orifice 164e. Variable orifice 164e has a size controlled by a position of main valve 164. The position of main valve 164 changes, so that a flow rate through main valve 164 is determined by the second control signal and is substantially independent of the pressure and temperature of the fluid.

FIG. 3 is a detailed schematic diagram of lower assembly 150. Lower assembly 150 includes two poppet valves 164 and 166. Pilot poppet 166 is positioned inside the bore 164c of main poppet 164. Fluid is provided to chamber 106 behind main poppet 164. Orifice 164f connects chamber 106 to inner bore 164c of main poppet 164, so that in equilibrium, with pilot poppet 166 closed, pressure in bore 164c equals the load pressure.

System 100 raises and lowers an unbalanced load 99, providing load independent performance. Flow to cylinder 98 is dependent on the average current to solenoid 112 and is independent of variations in supply, load and temperature. System 100 uses a single set of components (raise pilot 120) that controls flow and compensates for variations in supply pressure, load pressure and temperature, without the use of separate compensation components or assemblies. Similarly, flow from cylinder 98 is dependent on the average current to solenoid 152 and is independent of variations in supply, load and temperature. FIG. 4 is a cross sectional view of a first exemplary hydraulic control valve system 100 according to the invention. Items in FIG. 4 that correspond to items shown schematically in FIGS. 1-3 have the same reference numerals, for ease of understanding. The first exemplary embodiment is an open center system. Valve system 100 transmits pressurized fluid via a work port W to the load. The load may be actuating cylinder 98. System 100 provides load independent performance without the additional expense and complexity of separate pressure compensators.

System 100 includes a housing 102. The raise assembly 110 and lower assembly 150 are integrated into housing 102, which may be a stackable body. Within raise assembly 110, housing 102 has an inlet chamber 104 for receiving pressurized fluid from a pressure port P, via passages P_A and P_B.

The main raise valve is a check valve 140 coupled to inlet chamber 104. Check valve 140 is normally in the closed position shown in FIG. 4 to prevent flow to cylinder 98 when raise section 110 is unenergized. Check valve 140 is responsive to a pressure force exerted by the fluid in inlet chamber 104, to move to an open position. In the open position, check valve 140 transmits the fluid from inlet chamber 104 to the load via passage Wj and work port W.

A pilot valve assembly 120 has a normally open pilot spool 124. Spool 124 includes a fixed orifice 124b coupled to inlet chamber 104, and a variable orifice 124c coupled to tank via passages T_1A, T_1B and tank port T, . Variable orifice 124c has a size controlled by the position of pilot spool 124. In the unenergized condition, normally open spool 124 bypasses pump flow to tank at very low pressure. This minimizes energy loss of system 100 and improves response time of valve system 100.

A biasing spring 126 biases pilot spool 124 towards an open position at which the size of variable orifice 124c has a maximum value. An actuating mechamsm, such as proportional solenoid 112, is responsive to a control signal for moving pilot spool 124 towards check valve 140, thereby reducing the size (metering area) of variable orifice 124c. The decrease in metering area results in an increase in the pressure drop through pilot valve 120. Pressure in chamber 104 increases until it reaches a value greater than the load pressure in work port W, causing check valve 140 to open. Once check valve 140 opens, flow to cylinder 98 is a function of the current supplied to solenoid 110, and the force generated thereby.

For any given condition, the position of spool 124 is determined by balancing the sum of the forces acting on it. The pressure drop across front orifice 124b and flow forces through spool 124 provide feedback to compensate for variations in supply pressure, load pressure and temperature. Flow compensation spring 128 adds an additional feedback loop to improve stability and metering accuracy.

Because of the combination of springs 126 and 128 and fixed orifice 124b, the position of pilot spool 124 changes without further movement by solenoid 112, to compensate for changes in pressure and temperature of the fluid. That is, given a flow rate that is initially determined by the control signal to solenoid 112, pilot spool 124 moves (without any change in the position of solenoid rod 112a) to compensate for changes in pressure and temperature. Thus, the flow rate through check valve 140 is determined by the control signal transmitted to the solenoid 112, and is substantially independent of the pressure and temperature of the fluid. With reference to lower assembly 150 in FIG. 4, housing 102 has a cavity 106 for receiving fluid from the load via work port W and passages W_2A and W_2B.

Lower valve assembly 160 is coupled to cavity 106 and to tank. Assembly 160 has a normally closed main poppet 164, to prevent flow out of cylinder 98 when lower section 150 is unenergized. Main poppet 164 is movable to an open position by a pressure force exerted by fluid in cavity 106, for transmitting the fluid from cylinder 98 to tank via passage T_2A and port T . Main valve 164 has a variable orifice 164e for releasing fluid from cylinder 98 to tank. Variable orifice 164e has a size controlled by a position of main poppet 164. A lower pilot valve 166 is coupled to cavity 106 and to tank. Pilot 166 is normally in a closed position to maintain a fluid pressure force which urges main poppet 164 towards its closed position, and prevents fluid from flowing out of cylinder 98 when lower section 150 is unenergized.

An actuating mechamsm, which may be a proportional solenoid 152, is responsive to a control signal for moving the pilot valve 166 to an open position. In the open position, pilot valve 166 releases fluid from cylinder 98 so that that main poppet 164 opens to release fluid from cylinder 98 to tank.

In the unenergized state the poppets 164, 166, which are held closed by the load pressure and force of spring 165, act as a load check valve. Energizing solenoid 152 generates a force which causes pilot poppet 166 to open, creating a flow path to tank.

The forces on pilot poppet 166 are balanced, allowing its position to be dependent only on current to solenoid 152 and the force of spring 165. Pilot flow through variable pilot orifice 164b generates a pressure drop across main poppet 164, causing main poppet 164 to follow the position of pilot poppet 166, creating the main flow path to tank. Displacement of main poppet 164, though limited by pilot poppet 166, is controlled by balancing the opening and closing pressure forces on main poppet 164. Variations in load are compensated for through this balancing of forces. Proportional flow metering characteristics are controlled by notches 164e in main poppet 164.

That is, given a flow rate that is initially determined by the control signal to solenoid 152, poppet 164 moves (without any change in the position of solenoid rod 152a) to compensate for changes in pressure and temperature. Thus, the flow rate through the main valve 160 is determined by the control signal, independently of the pressure and temperature of the fluid.

As explained in detail below, further exemplary embodiments of the inventions include a closed center system using a similar construction, so that a single set of flow control components provides proportional flow control and pressure and temperature compensation.

These and other aspects of the present invention are described in detail with reference to the exemplary embodiments.

DETAILED DESCRIPTION

FIGS. 4-10 show the first exemplary embodiment of the system 100. FIG. 11 is a block diagram of a mathematical model of raise assembly 110 of FIG. 4.

Referring to FIGS. 4-7, housing 102 includes a raise bore 102a for housing raise assembly 110, and a lower bore 102b for housing lower valve 160. Assembly 110 is designed as a pair of cartridges 120, 140 to improve manufacturabibty and serviceability of the complete valve system 100. Raise assembly 110 is located on the left side of housing 102. At the top of bore 102a, raise pilot cartridge 120 is held in place by solenoid 112. Raise check valve cartridge 140 screws into the bottom of bore 102a. Lower assembly 150 is located on the right side of housing 102. Lower valve assembly 160 screws into the bottom of bore 102b. At the top of bore 102b, a plug 154 is held in place by solenoid 152 for sealing the lower cartridge.

RAISE ASSEMBLY FIG. 5 is an exploded isometric view of the main components of the raise pilot valve 120 shown in FIG. 4. FIG. 6 is a cross sectional view of the raise pilot valve outer sleeve 122, taken across section line 6-6 of FIG. 5. FIG. 7 is a cross sectional view of raise pilot poppet 124, taken across section line 7-7 of FIG. 5. Conventional seals, washers and retaining rings are shown in the appropriate locations in FIG. 4, but are omitted from FIG. 5 for ease of understanding.

Outer sleeve 122 screws onto housing 102. A seal 113, which may be an O-ring, is provided between die top of pilot cartridge 120 and solenoid 112, creating a chamber 122c between spool 124 and solenoid 112. Pilot cartridge 120 comprises an outer sleeve 122, a pilot spool 124, a biasing spring 126, a retaining ring 130, washers 132, 134, a compensating spring 128, and seals 136 and 142.

Sleeve 122 has a central bore 122e, which is sized to slidably receive the outer circumference 1241 of spool 124. Sleeve 122 has a plurality of metering holes 122a. An inner circumferential groove 122b is provided around the inner circumference of sleeve 122. Sleeve 122 has two outer circumferential grooves 122d for receiving seals 136 and 142, which may be O-ring seals. Seals 136 and 142 are located on opposite sides of metering holes 122a. Pilot spool 124 has a central chamber 124a. The entrance 124b to chamber 124a forms a first flow metering orifice. The area of orifice 124b may be determined by the equation:

where: A is the area of orifice 124b; Q is the maximum pump flow; C_d is the flow coefficient; rho is the mass density; and delp is a constant chosen by the designer, representing a pressure drop across orifice 124b. For normal pressure compensation, delp may be about 50 to 100 psi. The size of delp may be adjusted to accommodate different sizes for solenoid 112. For example, a larger value of delp may be used for a larger solenoid (i.e., a solenoid that provides a larger force for a given input current). Spool 124 has a plurality of holes 124c around its outer circumference 1241.

Holes 124c provide fluid communication between chamber 124a and inner circumferential groove 122b of sleeve 122. Groove 122b distributes fluid pressure uniformly around metering holes 124c of spool 124, so that it is not necessary to align holes 124c of spool 124 with holes 122a of sleeve 122 (i.e., alignment in the tangential direction is not needed). Spool 124 has a first bearing surface 124i which is engaged by pin 112a of solenoid 112 to actuate spool 124. A second bearing surface 124g is engaged by flow compensation spring 128. A third bearing surface 124k is engaged by washer 134. Spool 124 is mounted within outer sleeve 122 using spring 126, retaining ring

130, and washers 132 and 134. Retaining ring 130 is attached to circumferential notch

124j on spool 124.

When solenoid 112 is unenergized (as shown in FIG. 4), pin 112a does not directly engage spool 124. Spring 128 is preloaded when solenoid 112 is unenergized.

Spring 128 biases spool 124 to limit retraction beyond the position shown in FIG. 4.

Spool 124 is held (due to the force of spring 126) in an open position in which ledge 122f and surface 124k are coplanar. Thus, in the unenergized state, washer 134 ordinarily rests on ledge 122f of outer sleeve 122 and ledge 124k of spool 124. A variable orifice is formed between chamber 124a and passage T_1B. The variable orifice comprises partially blocked metering holes 124c of spool 124. In the unenergized (open) position shown in FIG. 4, holes 124c are open or partially blocked by the inner surface 122e of sleeve 122.

Spool 124 has a longitudinal central passage 124f, with a radial passage 124e passing therethrough. Passages 124e and 124f provide fluid communication so that pressures in chambers 122c and 124a equalize. A damping orifice 129 may be mounted in receptacle 124d within passage 124f, to reduce overshoot and oscillation in the transient response of valve system 100.

Retaining ring 130 engages washer 132, compressing spring 126 between washers 132 and 134. Spring 126 applies a force on poppet 124 in the upward direction towards solenoid 112. Because spring 126 is constrained between retaining ring 130 and ledge 122f, the maximum length (and minimum spring force) of spring 126 occurs in the position shown in FIG. 4. The length of spring 126 decreases linearly (and the spring force increases linearly) if spool 124 moves downwards towards check valve 140. The length (and spring force) of spring 126 remains constant if spool 124 moves further upwards closer to solenoid 112, because the spring length is then determined by the distance between retaining ring 130 and ledge 124k.

Compensation spring 128 is positioned between solenoid 112 and ledge 124g of pilot spool 124. Compensation spring 128 applies a force against spool 124 in the downward direction, opposite the force of spring 126. Compensation spring 128 minimizes the influence of the flow force. In the unenergized state, the force from spring 128 is sufficient to maintain spool 124 in the position shown in FIG. 4, but is not sufficient to compress spring 126 further and close variable orifice 124c. Thus in the unenergized position shown in FIG. 4, both springs 126 and 128 are preloaded. Compensation spring 128 has a preload force that is about equal to the preload force of spring 126. For example, the preload force of spring 128 may be from 1.0 to 1.2 times the preload force of spring 126. Oil from pump 91 flows from the pressure port P into chamber 104 and then through fixed orifice 124b, variable orifice 124c and back to tank.

In the unenergized state, the sum of the forces on poppet 124 keep it in the position shown in FIG. 4. This results in variable orifice 124c being held open, so that all pressurized fluid in chamber 104 is bypassed to tank, and pump 91 is unloaded at low pressure. Check valve 140 prevents any flow of oil between raise assembly 110 and cylinder 98 as long as the pressure in inlet chamber 104 is less than the load pressure P_L in passage W,. Check valve 140 has a poppet 140j. Passages 140d and 14e through poppet 140j connect bore 140f of check valve 140 with passage W₂. Thus, the load pressure P_L is applied to the bottom surface 140h of poppet 140j, as well as the portion of the poppet 140j below seat 140g. The spring 140i of check valve 140 applies sufficient force to maintain poppet 140j closed unless the pressure in valve inlet 140a exceeds the load pressure P_L.

The force of spring 126, the pressure on surface 124m (in chamber 104), and the reduced pressure in chamber 124a push spool 124 upwards, tending to keep variable orifice 124c open. At the same time, the force of solenoid 112 and the force of compensation spring 128 push spool 124 downwards, tending to close variable orifice 124c. Under static conditions, the forces acting on spool 124 are balanced. A change to any one of the forces causes spool 124 to shift and seek a new equilibrium position. Energizing solenoid 112 generates a force proportional to the average current through the coil of solenoid 112 is applied to spool 124, which is added to the pressure and spring forces acting on spool 124. Spool 124 moves downward towards check valve 140, increasing the blocked portion of holes 124c, thereby reducing the size of the variable orifice. The size of fixed first orifice 124b is determined by the (fixed) area of the front of spool 124, and the size of variable orifice 124c is controlled by the position of spool 124. Orifices 124b and 124c form a pressure divider which feeds chamber 124a. Thus, when orifice 124c becomes smaller, the pressure in chamber 124a increases. As a result, the pressure in chamber 104 and check valve inlet 140a increases. Check valve 140 opens and allows flow to cylinder 98. Check valve 140 remains open until the pressure in inlet 140a is reduced to the load pressure P_L.

As described above, the position of spool 124 is determined by balancing the forces acting on it. As flow is diverted to cylinder 98, the flow through 124b and 124c decreases. This reduces the pressure drop across 124b and the resulting flow force acting on chamber 124a. Spool 124 seeks a new equilibrium position that results in a flow to cylinder 98 controlled only by the average current input to solenoid 112.

An increase in the average current generates more downward force on spool 124, causing spool 124 to shift toward check valve 140. This shift makes orifice 124c smaller, further reducing the flow to tank and increasing the flow to cylinder 98. The reduction in flow to tank reduces the pressure drop across fixed orifice 124b and the resulting flow force acting on chamber 124a. This change in the force balance causes spool 124 to shift back upwards toward solenoid 112. As spool 124 shifts upward, the force generated by compensator spring 128 increases and limits the amount of spool movement. This additional compensation spring force, which is proportional to the force generated by flow across fixed orifice 124b, improves the metering and stability characteristics of spool 124. Damping orifice 129 in spool 124 is used to limit the overshoot associated with the transient response while also adding to the overall stability Temperature, supply and load pressure compensation is performed by using orifice

124b, variable orifice 124c and spool travel. As the temperature of hydraulic fluid increases, its viscosity decreases. This results in more fluid passing through orifices 124b and 124c, and less to cylinder 98. More flow through the pressure divider increases the pressure in chamber 124a which causes pressure drop across orifice 124b, causing spool 124 to shift downward toward check valve 140. As spool 124 shifts downward, orifice 124c closes down, thereby restricting the flow to tank and increasing the flow to cylinder 98. Similarly, variations in supply and load pressure are compensated for through the same process. In each case, spool 124 seeks a new equilibrium point so as to maintain the flow to cylinder 98 at the original setpoint defined by the average current in solenoid 112. LOWER ASSEMBLY

FIGS. 4 and 8-10 show an exemplary lower valve assembly 160 according to the invention. FIG. 8 is an exploded isometric view of the main components of the lower assembly 150 shown in FIG. 4. FIG. 9 is a cross sectional view of the lower valve cartridge outer sleeve 162, taken along section line 9-9 of FIG. 8. FIG. 10 is a cross sectional view of lower valve main poppet 164, taken along section line 10-10 of FIG. 8.

Conventional seals, wear rings and spring are shown in FIG. 4, but are omitted from

FIGS. 8-10 for ease of understanding.

Lower assembly 150 is a proportional load check servo assembly comprising a solenoid 152, which screws into the top of bore 102b, and a valve cartridge 160, which screws into the bottom of bore 102b. A plug 154 is provided to seal the top of the bore 102b. Plug 154 may be sealed by O-rings 156 and 157. Plug 154 has a central bore 154a, in which a wear ring 158 is mounted.

Outer sleeve 162 of lower valve 160 is sealingly mounted in bore 102b. Sleeve 162 has a central bore 162h for receiving a middle section 164j of main poppet 164, and a smaller diameter nose section 164a for receiving the nose section 164d of poppet 164. A seat 162k is formed between nose section 162a and section bore 162h. A plurality of metering holes 162c are provided around the outer circumference of sleeve 162. Holes 162c connect bore 162h with a cavity 106 in housing 102. Cavity 106 is connected to work port W by passages W_2A and W_2B, and is always coupled to the load pressure in cylinder 98. Section 162a of sleeve 162 faces cavity 108. Cavity 108 is connected to tank by a passage T_2A and tank port T₂. Thus section 162a is constantly coupled to tank. Sleeve 162 has a third bore section 162i, and a passage 162e connecting the outer circumference of sleeve 162 to bore section 1621. Passage 162e is constantly in fluid communication with tank passage T_2B, which is coupled to tank via passage T_2A, and thus, bore section 162i is constantly coupled to tank. The bottom section 162g of sleeve 162 has a threaded section 162j for receiving a plug member 169. Sleeve 162 also has a plurality of circumferential grooves 162b, 162d and 162f on its exterior surface, for receiving seals, which may, for example, be O-rings as shown in FIG. 4. The seals in grooves 162b and 162d isolate the work pressure cavity 106. Main lower poppet 164 has a tapered section 164h for seating in seat 162k. The nose 164d of poppet 164 fits inside nose section 162a of sleeve 162. A plurality of longitudinal grooves 164e are formed in nose 164d of poppet 164. Poppet 164 has a tapered section 164h for seating in seat 162k. Nose 164d has a bore 164a for receiving a pin 163 slidably mounted in plug 154. Bore 164a is constantly exposed to tank pressure in cavity 108. The rear section 164g of poppet 164 has a bore 164c in which pilot poppet 166, washer 167 and biasing spring 165 are located. Pilot valve orifice 164b connects bore 164a to bore 164c. In the position shown in FIG. 4, pilot poppet 166 is seated in orifice 164b, and completely blocks the orifice. Poppet 164 has an orifice 164f connecting bore 164c with the exterior surface of poppet 164, so that bore 164c is constantly coupled to the load pressure P_L in cavity 106.

Pilot poppet 166 is housed inside of main poppet 164. Pilot poppet 166 has a tapered nose 166b for providing a variable flow orifice as a function of the position of poppet 166. A pin 166a provides a flat bearing surface on poppet 166, which pin 163 engages when solenoid 152 is actuated. Pilot poppet 166 is engaged by spring loaded washer 167, to bias poppet 166 to the closed position, seated in main poppet 164. Washer 167 is smaller in diameter than counterbore 164, and does not create a seal; in equilibrium, all of bore 164c is maintained at the load pressure P_L via cavity 106, metering holes 162c, bore 162h, and orifice 164f. The front end of nose 166b and the rear end 166d of pilot poppet 166 are both constantly exposed to tank pressure, so that the force balance on poppet 166 is essentially controlled by the opening force from solenoid 152 and the closing force from spring 165.

Pilot poppet 166 is designed so that the sum of the pressure forces acting on it are always zero. This is accomplished by exposing the sides 166c of poppet 166 (including a portion of poppet nose 166b below seat 164i) to the pressure in bore 164c, and exposing both ends 166b and 166d of poppet 166 to tank pressure. The elimination of longitudinal pressure forces allows control of pilot poppet 166 to be strictly a function of the forces applied by solenoid 152 and spring 165.

A plug 168 is provided behind poppet 164. Plug 168 has an external circumferential groove 168d for holding a seal 171. Plug 168 has a central bore 168a, for receiving a wear ring 168b. Wear ring 168b (which may be formed of plastic) slidably receives the rod 166c of poppet 166. An internal seal 172 is provided beneath wear ring 168b. Plug 168 and seals 171 and 172 isolate the load pressure P_L in bore 164c of main poppet 164 from tank pressure in passage 169b. A chamber 109 is formed between poppet 164 and plug 168. Bore 164c opens into chamber 109, so both are coupled to work port W via passages W_2B and W_2A, cavity 106, holes 162c, bore 162h, and passage 164f.

Beneath plug 164, a washer 174 and an adjusting knob 169 are provided. Knob 169 has threads on its outer circumferential surface for engaging the threads 162j of sleeve 162. The biasing force of spring 165 varies linearly with the position of knob 169. When the biasing force is set, a lock nut 170 is tightened into place to maintain the selected biasing force.

In operation, load pressure P_L enters chamber 106, generating a force equal to the pressure P_L times the longitudinal projection of neck area 164h of main poppet 164. This force is applied to poppet 164 in the longitudinal direction away from solenoid 152. Chamber 106 is connected to chamber 164c through orifice 164f of poppet 164. With pilot poppet 166 closed, there is no flow through orifice 164f, and the pressure in chamber 164c is equal to that of 106. The pressure in chamber 164c generates a force equal to the top area of main poppet 164 multiplied by the pressure in chamber 164c. This force is applied to poppet 164 in the longitudinal direction towards solenoid 152, tending to close poppet 164 or maintain it closed. Under static conditions, the position of main poppet 164 is determined by summing the opening pressure force against surface 164h, the opening force applied by solenoid 152, the closing pressure force in chambers 164c and 109, and the closing force of spring 165.

In the unenergized state, pilot poppet 166 is held closed by the force of spring 165. With no pilot flow, the pressure drop across orifice 164f is zero, and the pressure in chambers 106 and 164c is equal. Main poppet 164 is held closed as long as the sum of the pressure force in chamber 164c plus the force of pilot spring 165 exceeds the pressure force in chamber 106. The area gains of main poppet 164 are such that the closing force is always greater than the opening force when solenoid 152 is in the unenergized state, thus holding main poppet 164 closed. Energizing solenoid 152 generates a force proportional to the average current through its coil. This force acts on pushrod 163, causing pilot poppet 166 to open. The distance traveled by poppet 166 is determined by the sum of the forces of solenoid 152 and spring 165. Opening pilot 166 creates a flow path through orifice 164f, chamber 164c, pilot opening 164b and bore 164a back to tank. This flow creates a pressure drop across orifice 164f, reducing the pressure in chamber 164c and the closing pressure force. The resulting force imbalance causes main poppet 164 to open, creating a flow path through main poppet 164 from cylinder 98 to tank.

As main poppet 164 opens, pilot opening 164b closes down causing the pilot flow to stop. This mechanism, by which main poppet 164 follows pilot poppet 166, is called a follower or servo device. Poppet 164 has a variable orifice size. When slots 164e begin to clear seat 162k, the portion of slots 164e below seat 162k (and exposed to load pressure P_L in chamber 162h) forms the orifice. The greater the portion of slots 164e below the seat 162k, the larger the orifice. Thus, the effective orifice created by main poppet 164 is determined by its position, which is dependent on the balance of the forces acting on poppet 164. Slots 164e may be formed as milled notches. Slots 164e provide timing and fine metering characteristics and minimize the flow forces by maximizing the flow jet angle.

As a result of this force balance, flow from cylinder 98 to tank is defined only by the average current in solenoid 152. An increase in the average current to solenoid 152 generates more force causing pilot poppet 166 to open. Main poppet 164 follows, seeking a new balance point, which increases the effective orifice and flow from cylinder 98. Conversely, a decrease in the average coil current reduces the force acting on pilot poppet 166 and increases the upward force pushing main poppet 164 toward solenoid 152. This causes main poppet 164 to shift toward solenoid 152, closing down the flow path to tank. Temperature and load pressure compensation is performed by orifice 164f and the pressure forces in chambers 106 and 164c. As the temperature of hydraulic fluid increases, its viscosity decreases. This results in more fluid passing through orifice 164f, chamber 164c and pilot orifice 164b when pilot poppet 166 opens. More flow through this path reduces the pressure drop across orifice 164f. This reduces the effective orifice created by slots 164e of main poppet 164, which in turn reduces the flow from cylinder 98. Similarly, variations in load pressure P_L are compensated for through the same process. In each case, main poppet 164 seeks a new balance point so as to maintain the flow from cylinder 98 at the original setpoint defined by the average current in solenoid. MATHEMATICAL ANALYSIS OF THE VALVE SYSTEM FIG. 11 is a block diagram of the mathematical model for the raise section 110.

The output of the model, Q(s), is the flow through orifice 124c to tank. Because the inlet flow volume is fixed (for a fixed displacement pump 91), control of the flow to tank also controls the flow to cylinder 98.

There are two inputs to the model: Fs(s) which is the force generated by solenoid 112; and Pl(s) which is the resulting pressure in chamber 104.

Solenoid force Fs(s) is summed (in node 1102) with the preload force Kl x,_o of spring 126 (where Kl is the spring constant of spring 126 and x,_o is the initial displacement of the spring) and the net pressure force. The pressure force is given by Pl(s)-P2(s) (determined at node 1106) times the cross-sectional area (gain node 1104) of spool 124. P2 is the pressure in chamber 124a.

The resulting force 1118 on spool 124 is then summed (in node 1120) with the preload force K2x_2o of compensation spring 128 and the feedback force 1122 to control the position X(s) of spool 124. The preload force K2 x_2o is determined by the product of

K2 (the spring constant of spring 128) and x_2g (the initial displacement of the spring). The feedback force 1122 is generated by the position X(s) of spool 124 times the spring constant K2 of compensation spring 128. The total force acting on the spool is indicated by reference number 1123.

Block 1124 is the spool position transfer function. Spool position X(s) is a function of the sum 1123 of the forces acting on spool 124, the mass M of spool 124, the viscous spool damping coefficient B, and the spring constant Kl of spring 126.

The pressure drop 1105 [the difference Pl(s)-P2(s)] across orifice 124b generates a flow 1115 that is summed with the flow Q(s) through orifice 124c and the flow 1117 generated by the change spool position X(s) times its cross-sectional area As. The resulting flow 1111 is then multiplied by the bulk modulus of the fluid divided by the volume of fluid in chamber 104 (gain 1110), and then integrated in block 1108 to generate pressure P2(s).

The flow 1127 through orifice 124c is determined by the position X(s) and the flow gain 1126 of orifice 124c. A positive displacement for X(s) moves spool 124 toward check valve 140, closing down orifice 124c.

Pressure P2(s) times the pressure coefficient of orifice 124c (gain block 1130) generates a flow 1129 that is summed (in node 1128) with the flow 1127 generated by the spool position X(s) times the flow gain 1126 of orifice 124c. The resulting flow, Q(s), is the controlled flow through orifice 124b to tank. With a fixed displacement pump, the flow to cylinder 98 is also controlled.

Mathematically, compensation spring 128 adds a feedback loop 1119, 1121, 1122 around the spool position transfer function 1124. The feedback gain K2 is equal to spring rate of compensation spring 128.

FIG. 12 is a block diagram of the mathematical model for the lower assembly 150. The inputs the lower assembly 150 include the load pressure P^, , the solenoid force F(s) in block 1202 and the preload force 1204 (KpKlO) of lower pilot spring 165. The output of the lower assembly is the valve flow 1218.

In summing block 1206, the spring preload force 1204 is subtracted from the force F(s) of solenoid 152. The resulting force 1207 on pilot poppet 166 is input to the transfer function 1208. In transfer function 1208, the three main factors affecting the position of pilot poppet 166 are: (1) the mass Mx of pilot poppet 166, (2) the damping coefficient Bx of pilot poppet 166, and (3) the spring constant Kp of spring 165. ^αs" is the Laplace operator. The resulting value X(s) from transfer function 1208 is the displacement of pilot poppet 166. At summing node 1209, the displacement Y(s) of main poppet 164 is subtracted from the displacement X(s) of pilot poppet 166. The position difference 1211 is input to block 1210, where gain Kqx is applied. At block 1210, the flow gain defines the pilot flow 1215 as a function of X(s). At block 1212, the pilot flow 1215 is reduced by the pressure feedback Kcx from block 1214, to form the pilot flow Qx(s). Then at block 1216, the pilot flow Qx(s) is summed with the main flow Qy(s) from block 1246, to form the total lower flow 1218. Pilot flow Qx(s) provides an input 1213 to summing node 1224 of a feedback loop. Summing node 1224 determines the rate of fluid flow into bore 164c. Summing node 1224 has three inputs: (1) the flow Q2(s) through orifice 164f into bore 164c, plus

(2) the volumetric displacement 1239 caused by the opening velocity Ydot(s) of main poppet 164 (downward velocity in HG. 4) multiplied by the area Ap of the largest diameter section 164g of main poppet 164, minus (3) the pilot flow Qx(s) out of bore

164c. The output from summing node 1224 is input to gain 1222, which multiplies the flow rate by the bulk modulus Be of the fluid divided by the volume V2 of bore 164c.

The result 1221 is integrated in integrator 1220 to define the pressure P2(s) in bore 164c. Pressure P2(s) is multiplied by the pressure gain Kcx in node 1214 to form the feedback flow that is subtracted in node 1212 to form the pilot flow Qx(s).

The flow Q2(s) through orifice 164f is determined as follows: the pressure P2(s) in bore 164c is subtracted from load pressure P_L in subtracting node 1228, to form a pressure drop 1227 across orifice 164f. In node 1226, the gain K2 of orifice 164f defines the flow rate Q2(s) as a function of the pressure drop 1227.

Volumetric displacement 1239 is determined as follows. In node 1236, three main forces are combined: (1) a force 1233 tending to open poppet 164, determined by multiplying load pressure P_L in bore 162h times the area difference 1232 between the area Ap of poppet section 164g and the area Aseat of seat 162k; minus (2) a force 1235 tending to close poppet 164, determined by multiplying the pressure P2(s) in bore 1 4c by area Ap of poppet section 164g; minus (3) a flow force 1249, described below. Preferably, the ratio of the area (Ap) of the poppet section 164g to the area (Aseat) of seat 162k is within the range from about 1.5 to about 3.0. This ratio influences the response time of lower assembly 150; if the ratio is too large, the response would be too slow. Transfer function 1240 determines the velocity Ydot(s) from the sum of the fluid forces on poppet 164. In transfer function 1240, velocity Ydot(s) of main poppet 164 is determined as a function [l/(My.s + By)] of the mass My of poppet 164 and the damping coefficient By of main poppet 164. The velocity Ydot(s) times are Ap of poppet section 164g determines the volumetric displacement rate 1239 of fluid forced into bore 164c by movement of spool 164. The position Y(s) of spool 164 is determined in block 1242 as the integral of Ydot(s). Position Y(s) of poppet 164 is multiplied by flow gain Kqy in node 1244 to determine flow component 1245 as a function of the position of poppet 164. Additionally, the product 1263 of load pressure P_L multiplied by a pressure gain Key in node 1262 is subtracted from flow component 1245 in subtracting block 1246 to form the main flow Qy(s) through seat 162k.

The structure of lower assembly 160 includes fluid feedback, which tends to urge poppet 164 towards the closed position. Displacement Y(s) of poppet 164 and flow Qy(s) through the variable orifice of seat 162k both contribute to the feedback. The product of displacement Y(s) and the area gain Kw of main poppet 164 (when open) determines the opening area 1261 of poppet 164. Because slots 164e are not exposed until poppet 164 moves by a distance greater than zero, there is a deadband in which movement of poppet 164 does not increase flow area. Essentially, block 1260 includes a negative area offset, so area 1261 may have a negative value. In block 1256, a flag 1255 is set to a value of one if the area is greater than zero, or flag 1255 is set to a value of zero, of the area is less than or equal to zero. The area 1261 is provided to switch 1254, which ensures the deadband correction. Switch 1254 is a logical switch used for mathematical simulation. The resulting flow area 1251 is multiplied by (rho/u)cos(theta) in block 1250, where rho is the mass density of the fluid, u is the area 1261 input to switch 1254, and theta is the flow jet angle. The result 1247 of block 1250 is multiplied by the square of the volumetric flow rate Qy(s) and by flag 1255 in multiplier 1248. The resulting flow force 1249 tends to urge poppet 164 towards the closed position (if area 1261 is positive and flag 1255 is equal to one).

CLOSED CENTER EMBODIMENT FIG. 13 is a cross sectional view of a second exemplary embodiment of the invention. Valve system 200 is suitable for use in a closed center system having a fixed displacement pump (not shown). For ease of understanding, the reference numerals of elements in system 200 that correspond to elements of system 100 (FIGS. 4-10) have the same last two digits as the reference numerals in FIGS. 4-10. The elements of raise section 210 that are different from those of raise section 110 are described in detail below. Components that are configured similarly and perform the same function as the components shown in FIGS. 4-10 are not described in detail again. For example, the lower section 250 of system 200 is identical to the lower section of system 100, and is not described again herein.

The raise section 210 of system 200 uses components that are substantially similar to those of raise section 110, except that the housing 202 includes fluid communication paths different from those in housing 102. Also, the positions of metering holes 222a and 224c are different from those of metering holes 122a and 124c.

Raise pilot spool 224 has the same general shape and outside dimensions as raise pilot spool 124 (FIGS. 4, 5 and 7), but chamber 224a extends further into spool 224, and orifice holes 224c are positioned closer to solenoid 212 (and further from fixed orifice 224b). Also, outer sleeve 222 has the same general shape and outside dimension as sleeve 122, but holes 222a and groove 222b are positioned further from solenoid 212 than the corresponding holes 122a and groove 122b in sleeve 122. As a result, in the unenergized position shown in FIG. 13, holes 224c are completely blocked, and variable orifice 224c is completely closed; thus, there is no bypass flow through pilot spool 124. The bottom of inlet chamber 204 has a plug 245, and is not directly coupled to the main check valve 240.

Applying current to actuate solenoid 212 moves spool 224 downward, thereby increasing the size of variable orifice 224c, to decrease the pressure at the inlet 240a to main check valve 240. Main valve 240 is similar to valve 140 in that check valve 240 provides fluid to the load when the pressure in inlet 240a is greater than the load pressure P_L in valve outlet 240b. Thus, when solenoid 212 is energized, fluid flows from port P through passages Pj, P₂, and P₃, cavity 204, chamber 224a, orifices 224c, groove 222b, holes 222a, passages Mj and M₂, check valve 240, passages W_1B and W_2B to work port W. There is no seal between the external surface of spool 224 and the inner surface of sleeve 222. Thus, even when pilot spool 224 is in the completely closed position (as shown in FIG. 13), a small amount of fluid leaks from chamber 204 around pilot spool 224 and out of sleeve 222 into passage Ml. To prevent this fluid from building up pressure and eventually opening check valve 240, a bleed orifice 241 is provided in passage Mi to bleed the leaked fluid off to tank. In all odier respects, the structure and operation of the components in raise assembly 210 is similar to the operation of the corresponding components in raise assembly 110, described above.

Valve system 200 is intended for use in a closed center system, in which the pressurized fluid entering pressure port P is not bypassed to tank within system 200.

Valve system 200 may be used in conjunction with other closed center valve systems (not shown) in a stacked assembly (not shown); an unload valve (not shown) may be used to allow the pressurized fluid to bypass all of the valves in the stack if no fluid flow is being applied to any load in the system. In a further variation, a load sense passage (not shown) may be connected to passage M,. A suitable load sense check valve (not shown) would be placed in load sense passage to prevent backwards flow into passage M, from the load check valve. Adding a load sense passage enables the use of valve system 200 with a conventional variable displacement pump, instead of the fixed displacement pump described above. The variable displacement pump receives the load sense signal from the load sense passage, and the pump displacement is controlled using the load sense signal.

Although the invention has been described with reference to exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the true spirit and scope of the present invention.

Claims

What is Claimed: 1. A valve system, comprising: normally closed main valve means for receiving pressurized fluid and transmitting the fluid to a load means; pilot valve means including a first orifice in fluid communication with the main valve means and a second orifice coupled to tank, the second orifice having a size controlled by a position of the pilot valve means; and actuating means responsive to a control signal for moving the pilot valve means to determine the size of the second orifice, wherein the position of the pilot valve means changes so that flow through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid.

2. A valve system according to claim 1 , wherein the pilot valve means include a bore having an inner surface and a spool slideably mounted in the bore.

3. A valve system according to claim 2, wherein the spool has a central chamber with the first orifice at one end thereof, and the second orifice connects the central chamber to an outer surface of the spool, said second orifice being at least partially blocked by the inner surface of the bore when the spool moves away from an open position of the pilot valve means.

4. A valve system according to claim 3, wherein the bore has a circumferential groove in fluid communication with tank, and said second orifice is in fluid communication with said groove when the pilot valve means is in the open position.

5. A valve system according to claim 2 further comprising a housing having an inlet chamber for receiving the pressurized fluid, wherein the spool has a central chamber with the first orifice at one end thereof, said first orifice connecting the central chamber to the inlet chamber, said main valve means connects the inlet chamber to the load means and opens when pressure in the inlet chamber is greater than pressure in the load means, and the position of the spool is determined by a pressure force due to a difference in pressure between the inlet chamber and the central chamber for a given value of the control signal.

6. A valve system according to claim 5, wherein the main valve means is a check valve.

7. A valve system according to claim 1 , wherein the pilot valve means has an open position, at which the variable orifice has a maximum size, when a magnitude of the control signal is below a threshold value, and the size of the variable orifice is reduced when the control signal increases above the threshold value.

8. An open center valve system for transmitting pressurized fluid to a load means, comprising: a housing having an inlet chamber for receiving the pressurized fluid; a check valve coupled to the inlet chamber and movable, by a pressure force exerted by the fluid in the inlet chamber, between a normally closed position and an open position for transmitting the fluid from the inlet chamber to the load means; pilot valve means including a fixed orifice coupled to the inlet chamber and a variable orifice coupled to tank, the variable orifice having a size controlled by a position of the pilot valve means; biasing means for biasing the pilot valve means towards an open position at which the size of the variable orifice has a maximum value; and actuating means responsive to a control signal for moving the pilot valve means to reduce the size of the variable orifice, wherein the position of said pilot valve means changes to compensate for changes in pressure and temperature of the fluid, without further moving by the actuating means, so that a flow rate through the check valve is determined by the control signal, and is substantially independent of the pressure and temperature of the fluid.

9. A valve system according to claim 8, wherein (a) the biasing means is a first spring having a maximum length when the pilot valve means is in the open position; (b) the pilot valve means include: a sleeve having a bearing surface, a spool slidably mounted within the sleeve, the spool having a first surface that is engaged by the biasing means, the first surface being coplanar with the bearing surface when the pilot valve means is in the open position, and a retaining ring for preventing extension of the first spring beyond the maximum length; and (c) the system further comprises a second spring urging die spool away from a position of the spool corresponding to the open position of the pilot valve means, the second spring being mounted between the actuating means and a second surface of the spool, the second spring applying a force on the spool to change a position of the spool to compensate for changes in pressure and temperature of die fluid, without further moving by the actuating means.

10. A valve system according to claim 9, wherein tiie first and second springs apply respective first and second preload forces on the spool, the first and second preload forces being about equal.

11. A valve system according to claim 9, wherein the first and second springs apply respective first and second preload forces on the spool, the second preload force being between 1.0 and 1.2 times the first preload force.

12. A valve system, comprising: normally closed main valve means having an inlet and an outlet for transmitting the fluid to a load means; pilot valve means including a first orifice in fluid communication with pressurized fluid and a second orifice coupled to the inlet, the second orifice having a size controlled by a position of the pilot valve means; and actuating means responsive to a control signal for moving the pilot valve means to determine the size of the second orifice, wherein the position of the pilot valve means changes so that flow through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid.

13. A valve system according to claim 11 , wherein the second orifice is blocked when a magmtude of the control signal is less than a threshold value and the size of the second orifice increases when the magnitude increases beyond the threshold value.

14. A closed center valve system for transmitting pressurized fluid to a load means, comprising: a housing having a chamber for receiving the pressurized fluid; a check valve having an inlet and an outlet coupled to the load means, said check valve being movable, by a pressure force exerted by the fluid in the inlet, between a normally closed position and an open position for transmitting the fluid from the inlet to the load means; pilot valve means including a fixed orifice coupled to the chamber and a variable orifice coupled to the inlet of the check valve, the variable orifice having a size controlled by a position of the pilot valve means; biasing means for biasing the pilot valve means towards a closed position at which the variable orifice is blocked; and actuating means responsive to a control signal for moving the pilot valve means to increase the size of the variable orifice, wherein the position of said pilot valve means changes to compensate for changes in pressure and temperature of the fluid without further moving by the actuating means, so that a flow rate through the check valve is determined by the control signal, and is substantially independent of the pressure and temperature of the fluid.

15. A valve system, comprising: normally closed main valve means for receiving fluid from a load means and transmitting the fluid to tank via a variable orifice, the variable orifice having a size controlled by a position of die main valve means; pilot valve means having a normally closed position to maintain a fluid pressure force to urge the main valve means towards a closed position; actuating means responsive to a control signal for moving the pilot valve means to release fluid from a surface of the main valve means, so that that said main valve means opens to release fluid to tank, wherein the position of said main valve means changes, so that a flow rate through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid.

16. A valve system according to claim 15, wherein the main valve means has a poppet and a seat the variable orifice includes a slot for coupling the load means to tank, and a portion of the slot is blocked by the seat, the portion varying with a position of the poppet, the slot being completely blocked when the main valve means is in the closed position.

17. A valve system according to claim 15, wherein the main valve means has a main poppet and a seat, the main poppet has a central bore, and the pilot valve means has a pilot poppet slideably mounted within the central bore of the main poppet.

18. A valve system according to claim 17, wherein the pilot poppet has two ends exposed to tank, each of the two ends having an area projection in a longitudinal direction, the area projections being equal.

19. A valve system according to claim 15, wherein the main valve means has a main poppet and a seat, said main poppet having a pressure surface fluidly coupled to the load means, said pressure surface having a pressure surface area, said seat has a seat area, and the ratio of the pressure surface area to the seat area is within a range from about 1.5 to about 3.0.

20. A valve system, comprising: a housing having a cavity for receiving fluid from a load means; main valve means coupled to the cavity and to tank and movable, by a pressure force exerted by the fluid in the cavity, between a normally closed position and an open position for transmitting the fluid from the load means to tank, the main valve means having a variable orifice for releasing fluid from the load means to tank, the variable orifice having a size controlled by a position of the main valve means; pilot valve means coupled to the cavity and to tank and having a normally closed position to maintain a fluid pressure force to urge the main valve means towards its closed position; actuating means responsive to a control signal for moving the pilot valve means to release fluid from the load means so that that said main valve means opens to release fluid from the load means to tank, wherein the position of said main valve means changes to compensate for changes in pressure and temperature of die fluid without further moving by the actuating means, so that a flow rate through the main valve means is determined by the control signal, independently of the pressure and temperature of the fluid. 21. A valve system, comprising: normally closed raise main valve means for receiving pressurized fluid and transmitting the fluid to a load means; raise pilot valve means including a first orifice in fluid communication widi the raise main valve means and a second orifice coupled to tank, the second orifice having a size controlled by a position of the raise pilot valve means; first actuating means responsive to a first control signal for moving die raise pilot valve means to determine the size of the second orifice, wherein the position of the raise pilot valve means changes so that flow through die raise main valve means is determined by die first control signal and is substantially independent of the pressure and temperature of the fluid; normally closed lower main valve means for receiving fluid from the load means and transmitting die fluid to tank via a variable orifice, the variable orifice having a size controlled by a position of die lower main valve means; lower pilot valve means having a normally closed position to maintain a fluid pressure force to urge the lower main valve means towards a closed position; second actuating means responsive to a second control signal for moving the lower pilot valve means to release fluid from a surface of the lower main valve means, so tiiat that the lower main valve means opens to release fluid to tank, wherein the position of the lower main valve means changes, so that a flow rate through the lower main valve means is determined by the second control signal and is substantially independent of the pressure and temperature of the fluid.

AMENDED CLAIMS

[received by the International Bureau on 28 October 1997 (28. 10.97 ) ; original claims 2 and 3 cancel led ; original c l aims 1 ,4 , 5 , 10-14 and

21 amended ; new c laim 22 added ; rema in ing c l a ims unchanged (8 pages ) ] 1. (Amended) A valve system, comprising: normally closed main valve means for receiving pressurized fluid and transmitting the fluid to a load means; pilot valve means including a bore having an inner surface and a spool slideably mounted in the bore, the spool having an outer surface without an undercut, wherein the spool has: (a) a central chamber with a first orifice at one end thereof, the first orifice being in fluid communication with the main valve means, (b) a second orifice coupled to tank, the second orifice having a size controlled by a position of the pilot valve means, the second orifice transmitting fluid from the central chamber to the outer surface of the spool, the second orifice being at least partially blocked by the inner surface of the bore when the spool moves away from an open position of the pilot valve means; and actuating means responsive to a control signal for moving the pilot valve means to determine the size of the second orifice, wherein the position of the pilot valve means changes so that flow through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid. 2. (Canceled) 3. (Canceled) 4. (Amended) A valve system according to claim 1 , wherein the bore has a circumferential groove in fluid communication with tank, and said second orifice is in fluid communication with said groove when the pilot valve means is in the open position. 5. (Amended) A valve system according to claim 1 further comprising a housing having an inlet chamber for receiving the pressurized fluid, wherein said first orifice connects the central chamber to the inlet chamber, said main valve means connects the inlet chamber to the load means and opens when pressure in the inlet chamber is greater than pressure in the load means, and the position of the spool is determined by a pressure force due to a difference in pressure between the inlet chamber and the central chamber for a given value of the control signal. 6. A valve system according to claim 5, wherein the main valve means is a check valve. 7. A valve system according to claim 1, wherein the pilot valve means has an open position, at which the variable orifice has a maximum size, when a magnitude of the control signal is below a threshold value, and the size of the variable orifice is reduced when the control signal increases above the threshold value. 8. An open center valve system for transmitting pressurized fluid to a load means, comprising: a housing having an inlet chamber for receiving the pressurized fluid; a check valve coupled to the inlet chamber and movable, by a pressure force exerted by the fluid in the inlet chamber, between a normally closed position and an open position for transmitting the fluid from the inlet chamber to the load means; pilot valve means including a fixed orifice coupled to the inlet chamber and a variable orifice coupled to tank, the variable orifice having a size controlled by a position of the pilot valve means; biasing means for biasing the pilot valve means towards an open position at which the size of the variable orifice has a maximum value; and actuating means responsive to a control signal for moving the pilot valve means to reduce the size of the variable orifice, wherein the position of said pilot valve means changes to compensate for changes in pressure and temperature of the fluid, without further moving by the actuating means, so that a flow rate through the check valve is determined by the control signal, and is substantially independent of the pressure and temperature of the fluid. 9. A valve system according to claim 8, wherein (a) the biasing means is a first spring having a maximum length when the pilot valve means is in the open position; (b) the pilot valve means include: a sleeve having a bearing surface, a spool slidably mounted within the sleeve, the spool having a first surface that is engaged by the biasing means, the first surface being coplanar with the bearing surface when the pilot valve means is in the open position, and a retaining ring for preventing extension of the first spring beyond the maximum length; and (c) the system further comprises a second spring urging the spool away from a position of the spool corresponding to the open position of the pilot valve means, the second spring being mounted between the actuating means and a second surface of the spool, the second spring applying a force on the spool to change a position of the spool to compensate for changes in pressure and temperature of the fluid, without further moving by the actuating means. 10. (Amended) A valve system according to claim 22, wherein the first and second springs apply respective first and second preload forces on the spool, the first and second preload forces being about equal. 11. (Amended) A valve system according to claim 22, wherein the first and second springs apply respective first and second preload forces on the spool, the second preload force being between 1.0 and 1.2 times the first preload force. 12. (Amended) A valve system, comprising: normally closed main valve means having an inlet and an outlet for transmitting the fluid to a load means; pilot valve means including a bore having an inner surface and a spool slideably mounted in the bore, the spool having an outer surface without an undercut, wherein the spool has: (a) a central chamber with the first orifice at one end thereof, the first orifice being in fluid communication with pressurized fluid, (b) a second orifice coupled to the inlet, the second orifice having a size controlled by a position of the pilot valve means, the second orifice transmitting fluid from the central chamber to the outer surface of the spool, said second orifice being at least partially blocked by the inner surface of the bore when the spool moves away from an open position of the pilot valve means; and actuating means responsive to a control signal for moving the pilot valve means to determine the size of the second orifice, wherein the position of the pilot valve means changes so that flow through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid. 13. (Amended) A valve system according to claim 11 , wherein the variable orifice is blocked when a magnitude of the control signal is less than a threshold value and the size of the second orifice increases when the magnitude increases beyond the threshold value. 14. (Amended) A closed center valve system for transmitting pressurized fluid to a load means, comprising: a housing having a chamber for receiving the pressurized fluid; a check valve having an inlet and an outlet coupled to the load means, said check valve being movable, by a pressure force exerted by the fluid in the inlet, between a normally closed position and an open position for transmitting the fluid from the inlet to the load means; pilot valve means including a bore having an inner surface and a spool slideably mounted in the bore, the spool having an outer surface without an undercut, the spool having: (a) a central chamber with the fixed orifice at one end thereof, the fixed orifice coupled to the chamber, and (b) a variable orifice coupled to the inlet of the check valve, the variable orifice having a size controlled by a position of the pilot valve means, the variable orifice transmitting fluid from the central chamber to the outer surface of the spool, said variable orifice being at least partially blocked by the inner surface of the bore when the spool moves away from an open position of the pilot valve means; and biasing means for biasing the pilot valve means towards a closed position at which the variable orifice is blocked; and actuating means responsive to a control signal for moving the pilot valve means to increase the size of the variable orifice, wherein the position of said pilot valve means changes to compensate for changes in pressure and temperature of the fluid without further moving by the actuating means, so that a flow rate through the check valve is determined by the control signal, and is substantially independent of the pressure and temperature of the fluid. 15. A valve system, comprising: normally closed main valve means for receiving fluid from a load means and transmitting the fluid to tank via a variable orifice, the variable orifice having a size controlled by a position of the main valve means; pilot valve means having a normally closed position to maintain a fluid pressure force to urge the main valve means towards a closed position; actuating means responsive to a control signal for moving the pilot valve means to release fluid from a surface of the main valve means, so that that said main valve means opens to release fluid to tank, wherein the position of said main valve means changes, so that a flow rate through the main valve means is determined by the control signal and is substantially independent of the pressure and temperature of the fluid. 16. A valve system according to claim 15, wherein the main valve means has a poppet and a seat, the variable orifice includes a slot for coupling the load means to tank, and a portion of the slot is blocked by the seat, the portion varying with a position of the poppet, the slot being completely blocked when the main valve means is in the closed position. 17. A valve system according to claim 15, wherein the main valve means has a main poppet and a seat, the main poppet has a central bore, and the pilot valve means has a pilot poppet slideably mounted within the central bore of the main poppet. 18. A valve system according to claim 17, wherein the pilot poppet has two ends exposed to tank, each of the two ends having an area projection in a longitudinal direction, the area projections being equal. 19. A valve system according to claim 15, wherein the main valve means has a main poppet and a seat, said main poppet having a pressure surface fluidly coupled to the load means, said pressure surface having a pressure surface area, said seat has a seat area, and the ratio of the pressure surface area to the seat area is within a range from about 1.5 to about 3.0. 20. A valve system, comprising: a housing having a cavity for receiving fluid from a load means; main valve means coupled to the cavity and to tank and movable, by a pressure force exerted by the fluid in the cavity, between a normally closed position and an open position for transmitting the fluid from the load means to tank, the main valve means having a variable orifice for releasing fluid from the load means to tank, the variable orifice having a size controlled by a position of the main valve means; pilot valve means coupled to the cavity and to tank and having a normally closed position to maintain a fluid pressure force to urge the main valve means towards its closed position; actuating means responsive to a control signal for moving the pilot valve means to release fluid from the load means so that that said main valve means opens to release fluid from the load means to tank, wherein the position of said main valve means changes to compensate for changes in pressure and temperature of the fluid without further moving by the actuating means, so that a flow rate through the main valve means is determined by the control signal, independently of the pressure and temperature of the fluid. 21. (Amended) A valve system, comprising: normally closed raise main valve means for receiving pressurized fluid and transmitting the fluid to a load means; raise pilot valve means including a bore having an inner surface and a spool slideably mounted in the bore, the spool having an outer surface without an undercut, wherein the spool has: (a) a central chamber with the first orifice at one end thereof, the first orifice being in fluid communication with the raise main valve means, and (b) a second orifice coupled to tank, the second orifice having a size controlled by a position of the raise pilot valve means, the second orifice transmitting fluid from the central chamber to the outer surface of the spool, the second orifice being at least partially blocked by the inner surface of the bore when the spool moves away from an open position of the pilot valve means; first actuating means responsive to a first control signal for moving the raise pilot valve means to determine the size of the second orifice, wherein the position of the raise pilot valve means changes so that flow through the raise main valve means is determined by the first control signal and is substantially independent of the pressure and temperature of the fluid; normally closed lower main valve means for receiving fluid from the load means and transmitting the fluid to tank via a variable orifice, the variable orifice having a size controlled by a position of the lower main valve means; lower pilot valve means having a normally closed position to maintain a fluid pressure force to urge the lower main valve means towards a closed position; second actuating means responsive to a second control signal for moving the lower pilot valve means to release fluid from a surface of the lower main valve means, so that that the lower main valve means opens to release fluid to tank, wherein the position of the lower main valve means changes, so that a flow rate through the lower main valve means is determined by the second control signal and is substantially independent of the pressure and temperature of the fluid.

22. (Newly Added) A valve system according to claim 14, wherein (a) the biasing means is a first spring having a maximum length when the pilot valve means is in the closed position; (b) the pilot valve means include: a sleeve having a bearing surface, the spool slidably mounted within the sleeve, the spool having a first surface that is engaged by the biasing means, the first surface being coplanar with the bearing surface when the pilot valve means is in the closed position, and a retaining ring for preventing extension of the first spring beyond the maximum length; and (c) the system further comprises a second spring urging the spool away from a position of the spool corresponding to the closed position of the pilot valve means, the second spring being mounted between the actuating means and a second surface of the spool, the second spring applying a force on the spool to change a position of the spool to compensate for changes in pressure and temperature of the fluid, without further moving by the actuating means.