GB2266359A - Differential pressure control valve - Google Patents

Abstract

A differential pressure control valve, comprises a valve body having at least two service ports (4, 5), a spool (9) movably axially within the valve body and recessed to provide at least two cavities (24, 26) between the spool (9) and the inner wall of the valve body and each being in fluid connection with one of the ports (4, 5) respectively. The cavity (24) is constructed to generate an axial force on the spool related to the pressure within the cavity (24), while the cavity (26) is connected by an orifice (27) to a chamber (25) so as to generate a second, oppositely directed, axial force. A pressure differential between the service ports (4, 5) thus generates a resultant axial force on the spool (9). An actuator is mechanically coupled to the spool (9) for moving the spool (9) axially in opposite directions so as to generate a pressure differential between the service ports (4, 5). The valve is used in a vehicle CVT system or an anti-roll suspension system. Excessive movement of a ram controlled by the valve causes the ends of the spool to be exposed to the pressure in the ram, which in turn results in adjustment of the spool and the flow to and from the ram. <IMAGE>

Description

DlEFERlaNTIAI, PRliSSUltl! CON'I'ROL VALVE This invention relates to a device for the control of pressure in a hydraulic syste!n, in which thcre is a requirement for the control of a pressure differential on the two sides of a hydraulic ram, and that this pressure control shall be reversible. One such application of this type of pressure control is an automatic gearbox of the continuously variable transmission (CVT) type, particul arly for use in motor cars and commercial vehicles, e.g. those having an internal combustion engine delivering more than about 80 Kw.Another such application of this type of differential pressure control is for an anti-roll suspension, by the provision of additional pressure to the struts controlling the movement of the outer wheels of the vchicle during cornering.

CVT has been proposed and been a highly desirable objective of the automotive industry for a number of decades. A belt drive system for CVT has been reported '(Van Doorne) but it is believed that there is an undesirably low supper limit on the power that it can transmit.Recently, a heavy duty CVT has been reported (Greenwood, Driveline 84, London, I Mech E, March 1984, and Stubbs, Powder Transmission and Gear Conference, ASME, August 1980) wherein a so-called rolling traction drive is used with the variator ratio being, determined by the so-called precession angles adopted by .the rollers therein. Control of the aforesaid precession angle tends to be difficult and insensitive and the manufacture of the control unit based thereon often involves a substantial quantity of precision engineering.

We have now devised a differential pressure control valve suitable for a CVT, based on a discrete unir. THis valve is capable of providing a reversible differential pressure, being controlled by an electronic driver (such as a micro-processor) which provides a reversible current.

Furthermore, the valve incorporates an hydraulic pressure override, as is required- for certain CVT controls, when the hydraulically driven ram approaches the end of it's travel.

According to the first aspect of the present invention, there is provided a differential pressure control valve which comprises a body with at least two service ports, a spool mechanically coupled to a single actuator and movable axially within the body and processed to provide at least two cavities between the spool and the inner wall of the body and each being in fluid flow connection with one of the ports respectively, the cavities being constructed to generate an axial force related to the pressure within the cavity such that a pressure difference between the ports generates a resultant axial force on the spool and conversely an axial force on the spool generates å pressure difference between the two service ports.

A pressure difference across the differential pressure control valve (pressure port to work port) will be maintained by the use of the valve in conjunction with a pilot operated relief valve. The pilot pressure for normal operating pressures is determined by the higher of the two pressures in the service lines. The spring loaded main plunger of the relief valve is used to maintain this pressure difference. By limiting the maximum aperture of the pressure port(s), the maximum flow can be determined; which can be different in each of the two directions of movement of the cylinders. Because there is a constant pressure differential, the flow setting(s) will be pressure compensated, which will maintain a substantially constant flow over a wide range of loadings and temperature variations.By this means the maximum operational specds of- the cylinders will be controlled.

Typically the actuator will be a moving magnet type linear motor, and this is preferred, but we do not exclude the possibility of an alternative actuator such as a moving coil linear motor, an hydraulic actuator or a pneumatic actuator.

For control of a CVT, a hydraulic stop is required that will override the electronic signals from the micro-processor controller when the ram approaches the end of it's travel. There are two external signals, which are used to operate the main spool, overriding the force from the linear motor. When the pressure from one or other of these signals is reduced, the pressure from the opposite end of the spool moves the main spool thereby putting full hydraulic pressure onto the ram in order to prevent overtravel. This method of control is described in more detail herein after.

According to the second aspect of the present invention there is provided a differential pressure control valve according to the first aspect, a displacement detector and electronic controls.

The aformentioncd displacement detector would typically be a Hall Effect sensor, but we do not exclude the possibility of the use of a potentiometer, linear variable differential transformer (LVDT) or of an other type of displacement detector. The displacement detector would typically be fitted within the linear motor.

To provide damping of the movement of the ram(s) being controlled by the differential pressure control valve according to the present invention, the flow rate through the valve needs to be controlled at a reduced rate, as well as the differential pressure. For the provision of damping, the flow through the valve has to be restricted, and the spool has to be moved to specific positions as is more fully described hereinafter. To move the spool to known positions, the electronic control uses the positional feedback provided by mcans of the displacement detector. The valve spool will be held in the required positions to restrict the fluid flow from the pump to the ram(s), and from the ram(s) to the tank.Because a single spool is used to control both of these flows, the control of the flows can be inter-related.

According to - the third aspect of the present invention there is provided a differential pressure control valve according to the first aspect, but without the hydraulic stop required to override the electronic signals from the micro-processor controller when the ram approaches the end of it's travel. The third aspect of the present invention is for the antiroll suspension application preferred to hereinbefore.

The present invention will be further described by reference to the accompanying drawings which show, by way of example only, a preferred embodiment of a differential pressure control valve according to the present invention.

Figure 1 illustrates, in diagrammatic form, a differential pressure control valve and the interrelated items, including the pump, the hydraulic ram and relief valve.

Figure 2 illustrates the differenlial pressure control valve, partly in longitudinal section.

Figure 3 illustrates the fluid movements and pressures when the linear motor is pulling the main spool.

Figure 4 illustrates the fluid movemcnts and pressures when the linear motor is pushing the main spool.

Referring firstly to Figure 1, full flow connections are shown in full lines, and the pilot connections by broken lines, following the standard practice in the art. The supply of hydraulic pressure is from the pump (1),. which is a discrete device, and is fed to the two pressure ports of the differential pressure control valve (2) and (3). The service ports (4) and (5) of the differential pressure control valve are in fluid flow connection with the ram (6) or rams (not shown) of the CVT via connections (6a) and (6b). In normal operation, ports (6c) and (6d) in the ram remain covered. Under these circumstances, there will not be any flow within these pilot lines, and therefore the pressure will remain equalised and the pressure in the motor casing (7) and the spool end chamber (8) will be the same.Therefore, the main spool (9) is in equilibrium and will then be capable of being. driven by the linear motor (10). When the ram (6) moves too far to the left (as shown), port (6c) becomes uncovered and allows fluid to flow from the pilot line into the gearbox casing which will be at atmospheric pressure or near to that pressure. Fluid will then flow via connection (11) through restrict (12), and through restrictor (13), which make the pressure in motor casing (7) less than that in the spool end chamber (8). The main spool then moves to the left (as shown), and pressure port (2) is in fluid flow connection with service port (4), with service port (5) in fluid flow connection with the tank port (14).The full system pressure (up to the setting of the pilot operated relief valve (15)) is then applied to port (6a) of the ram, thereby providing resistance to any further movement to the left (as shown). When the ram (6) moves too far to the right (as shown), port (6d) becomes uncovered and allows fluid to flow from the pilot line into the gearbox casing which will be at atmospheric pressure or near to that pressure. Fluid will then flow via connection (16) through restrictor (17), and through restriclor (1 3), which make the pressure in spool end chamber (8) less than that in the motor casing (7).

The main spool then moves to the right (as shown), and pressure port (3) is in fluid flow connection with service port (5), with service port (4) in fluid flow connection with the tank port (14). The full system pressure is then applied to port (6b) of the ram, thereby providing resistance to any further movement to the right (as shown).

The main spool ((9) is driven by the linear motor (10) to generate the required pressure differentials, as is described hereinafter in more detail in relation to Figure 2.

Referring now to Figure 2, which illustrates the differential pressure control valve. The valve comprises a valve body (20), in which are disposed the main spool (9), a mounting plate (21), a fixed element (22) which is constrained by the body end cap t23), and the main spool is driven by 'the linear motor, as is described in more detail hereinafter.

The stator of the linear motor (24) is in rigid mechanical connection with the mounting plate (21) and the valve body (20). The armature of the linear motor (25) is mechanically linked to the main spool (9), and they move together, axially within the differential pressure control valve. The springs (26) and (27) support the armature and are used to assist in returning the armature and main spool to the central or neutral position.

Pilot port (11) is in fluid flow connection wi(h the casing of the linear motor (7). Pilot port (16) is in fluid flow connection with the spool end chamber (8). When the valve is being driven by the linear motor, the pressures in these two pilot lines are the same, and therefore the main spool is in balance so far as these pressures are concerned. As has been described hereinbefore in relation to Figure l, when the pressure at either pilot port (11) or pilot port (16) is reduced, the spool will be driven by the opposing hydraulic pressure which overrides the force that can become available from the linear motor.

The pilot pressure supply is from pressure port (2), via orifice (12) to chamber (7), and via orifice (17) to chamber (8). Both these chambers are pressure vessels.

The spool (9) is designed such that fluid flow from ports (2) or (3) will generate axial forces on the spool. These forces will reduce or eliminate the force generated by the springs (26) and (27), or will be such that the flow force is greater than the spring force. The spool is designed such that there will be underlay to provide sufficient pilot flow from the pilot operated relief valve to return to tank when the motor force equates to the differential pressure generated at the service ports (4) and (5). Described hereinafter in more detail in relation to Figure 3.

Referring now to Figure 3, which illustrates the main spool movement in the valve body when the linear motor is pulling the main spool. This spool movement will cause pressure port (2) to be in fluid flow connection with service port (4), and service port (5) to be in fluid flow connection with tank port (14). The flow of fluid is indicated by arrows (A) and (B) respectively. The force available from the linear motor will be according to the electrical signal. This force will move the spool to the left (as shown) until the force from the linear motor is balanced by the forces generated by the hydraulic pressure on the spool.The force working in the opposite direction to the linear motor force will be the pressure in chamber (24), (which equates to the pressure at service port (4)) on the differential area generated by the difference between the diameters (9b) and (9a). The force acting in the same direction as the linear motor is that generated by the pressure in chamber (25) on the cross sectional area of that chambers Chambers (25) and (26) are in fluid flow connection via hole or orifice (27), and the pressures in these chambers equate to the pressure at service port (5). When the pressure differential between service port (4) and service port (5) has increased to that required, the spool goes to a position of equilibrium, and there will not be. any further movement of fluid to service port (4) or fluid movement from service port (5) to tank.

In order to allow the pilot flow from the pilot operated relief valve (15) to return to tank, there will be spool underlap. The flow passes to tank, via the shuttle valve (18), service port (5) and tank port (14). There will be make up flow from the pump via the small opening of the pressure port (3).

Referring now to Figure 4, which illustrates the main spool movement in the valve body when the linear motor is pushing the main spool. This spool movement will cause pressure port (3) to be in fluid flow connection with service port (5), and service port (4) to be in fluid flow connection with tank port (14). The flow of fluid is indicated by arrows (D) and (C) respectively. The force available from the linear motor will be according to the electrical signal. This force will move the spool to the right (as shown) until the force from the linear motor is balanced by the forces generated by the hydraulic pressure on the spool. The force working in the opposite direction to the linear motor force is that, generated by the pressure in chamber (25) on the cross sectional area of that chamber.Chambers (25) and (26) are in fluid flow connection via hole or orifice (27), and the pressures in these chambers equate to the pressure at service port (5). The force acting in the same direction as the linear motor will be the pressure in chamber (24), (which equates to the pressure at service port (4)) on the differential area generated by- the difference between the diameters (9b) and (9a).

When the pressure differential between service port (5) and service port (4) has increased to that required, the spool goes to a position of equilibrium, and there will not be any further movement of fluid to service port . (5) or fluid movement from service port (4) to tank.

In order to restrict the fluid flow and provide damping, the spool movement, as shown in Figures 3 and 4, will be limited. Typically, the spool travel will be restricted to one third to one half of the maximum travel available. The displacement detector (28) will provide the signal to the electronics, and the signal will be used to limit the travel, or if there should be an external force applied to the. ram (6) which will cause the spool to be driven by the hydraulics, then the electronics can be used to reverse the force from the linear motor (10), thereby controlling the position of the spool (9). The restricted movement of the spool limits the maximum flow while damping is required, but allows a significantly greater flow when damping is not a requirement.

Claims (7)

1. A differential pressure control valve, comprising a valve body having at least two service ports, a spool movably axially within the valve body and recessed to provide at least two cavities between the spool and the inner wall of the valve body and each being in fluid connection with one of the ports respectively, the cavities being constructed to generate an axial force related to the pressure within the cavity such that a pressure differential between the service ports generates a resultant axial force on the spool and an actuator mechanically coupled to the spool for moving the spool axially in opposite directions so as to generate a pressure differential between the service ports.

2. A valve as claimed in claim 1, wherein fluid pressure overrides the actuatbr in the case when a ram being controlled by the valve approaches the end of its desired travel.

3. A valve as claimed in claim 1 or claim 2, wherein maximum flow is determined by limiting the aperture of pressure ports in communication with a source of fluid such that the maximum fluid flow may be the same or different in each direction of spool movement and a pilot operated relief valve is provided to maintain substantially constant flow over a wide range of loadings and temperature variations.

4. A valve as claimed in any one of the preceding claims, including a displacement detector and electronic controls, wherein the spool is driven to predetermined positions by the electronic controls to dampen fluid flow.

5. A valve as claimed in any one of the preceding claims, wherein fluid flow into and out of the service ports is controlled by a single spool thereby providing interrelated control of the two flows.

6. A valve as claimed in any one of the preceding claims wherein the actuator is a linear motor.

7. A differential pressure control valve substantially as herein described with reference to the drawings.