Control system for variable displacement hydraulic pumps in automotive type closed circuit hydrostatic drives

Abstract: A control system for variable displacement hydraulic pumps in automotive type, closed circuit hydrostatic drives, comprises a flow control valve (1), a pressure reducing valve with fixed adjustment (2), a pressure reducing valve with lever actuated variable adjustment (3), a load sensing spool type valve (4), a double check valve (5), an auxiliary pump (10), an hydraulic proportional servo actuator (73) with mechanical feedback positioning system, two electrically actuated three-ways two-positions directional control valves (74) for drive direction control. At deceleration, the load induced pressure acting on valve (4) swivels the variable pump towards minimum displacement for effective braking.

Description: This invention concerns a control system for variable displacement pumps in automotive type, closed circuit hydrostatic power drives. As it is known, an automotive type hydrostatic transmission allows to obtain a continuously and infinitely variable-ratio drive between primary engine (usually endothermic) and the output power shaft of the driving motor, namely an hydrostatic motor with fixed or variable displacement, in such a way that the whole system behaves as an hydrodynamic drive with a continuously and infinitely variable ratio. Furthermore, in an automotive type hydrostatic transmission the variable displacement pump swept volume is directly related to the pump rotation speed, therefore to the engine rotation speed, in such way that, at low rotation speed, the pump displacement is close to zero, hence is kept low the pump absorbed torque. In this way the engine can quickly increase its rotation speed, in order to reach higher power performances. Last, in an automotive type hydrostatic transmission the load induced pressure causes a reduction in pump displacement which is inversely proportional to this pressure, so that the engine is kept free from overloading that would come at high working pressure and it can regularly function also with quite high working pressure values. The traditional automotive type hydrostatic drives are characterised by the lack of braking action during the vehicle deceleration, asking for a frequent use of the mechanic brakes or for the presence of a proper hydraulic flow control valve, piloted by the braking circuit hydraulic pressure, this causes overheating and wear of the braking system components. Again, the known automotive type hydrostatic transmissions need high engine rotation speeds, hence not allowing a smooth control of the transmitted power and keep the engine at high specific fuel consumption rotation speed. Finally, the mentioned transmissions usually do not allow a smooth feeling of the lever actuated throttle function, this resulting in sharp operations of the control, the control over the transmission absorbed torque is also scarcely reproducible, due to the control mechanical hysteresis, which amount is mainly random. DE-39 03 188 A1 disclosed a control system for variable displacement hydraulic pumps in automotive type, closed circuit hydrostatic drives, as defined in the preamble of claim 1. This invention's technical aim is to offer a pump control system which avoids the traditional control systems drawbacks. This purpose is pursued using an automotive type pump control as defined in claim 1. More details can be found in the following description, illustrating an example of system layout: Fig. 1 shows an hydraulic layout of the control system according to this invention. Fig. 2 shows a cutaway drawing of the flow control valve. Fig. 3 shows a cutaway drawing of the pressure reducing valve with fixed adjustment. Fig. 4 shows a cutaway drawing of the pressure reducing valve with lever actuated adjustment. Fig. 5 shows a cutaway drawing of the load sensing spool type valve. Fig. 6 shows a cutaway drawing of the double check valve. In fig. 1 there are: the flow control valve (1), the pressure reducing valve with fixed adjustment (2), the pressure reducing valve with lever actuated adjustment (3), the load sensing spool type valve (4), the double check valve (5), the auxiliary pump (10), the hydraulic proportional servo actuator mechanically feed-back (73), the electrically operated three-ways two-positions directional control valves (74). In fig. 2 there are: the flow control valve (1) which is formed by a shutter (6) movable into a bushing (7) that is closed on one of its sides by a spacer (8). On the spacer (8) is designed a seat for a differential area relief valve that controls the auxiliary pump (indicated with (10) in fig. 1) delivery. The shutter (6) is pushed on the seat (9) by the spring (11), which is fit on the spring retainer (12). The spring retainer (12) position is adjustable by acting on the adjustment screw (13) which is fit on the cover (14). The flow control valve is operated via the auxiliary pump generated flow that crosses a calibrated orifice (15) in such a way that it generates a pressure drop which is proportional to the pump flow, therefore to the pump rotation speed. In fig. 3 there are: the pressure reducing valve with fixed adjustment (2) which is formed by a spool (16) with a bushing (17) on one of its ends which is axially hold into position between the spool's (16) shoulder (18) and the spring ring (19). The spool (16) can move inside the sleeve (20). Inside the bushing are designed two coaxial cylindrical seats (21 & 22) separated by the shoulder (23)..The sleeve (20) feature three radial cavities. (24, 25 & 26). The spool (16), the bushing (17) and the sleeve (20) define cavities which are approximately annular (27, 28, 29, 30 & 31). These cavities are respectively: at the pressure existing after the calibrated orifice; at the return line pressure; at the pressure proportional to the rotation speed; at the pressure existing after the calibrated orifice; at the pressure existing before the calibrated orifice. The cavity (27) is closed on one side by the cover (32), while the cavity (31) is closed on one side by the plug (33). The spool (16) is pushed by the spring (34) which is inside the cavity (31), fit on the spring retainer (35). The spring retainer adjustment is acted via the adjustment screw (36) fit on the plug (33). All the different pressures on cavities (27, 28, 29, 30 & 31) act on the spool (16) and bushing (17); this, combined with the force generated by the spring (34), create a pressure inside cavity (29) which is proportional to the pump rotation speed, so that the pressure reducing valve with fixed adjustment (2) is piloted by the pressure drop generated by the flow control valve (1), this pressure drop being proportional to the rotation speed. In fig. 4 there are: the pressure reducing valve with lever actuated adjustment (3), which is formed by a spool (37) movable into a bushing (38). The spool (37) is pushed by the spring (39), fit on the spring retainer (40) and is also pushed by the spring (41) fit on the spring retainer (42). The spring (41) is also fit on the spring retainer (43) which position can be adjusted acting on the adjustment screw (44) fit on the piston (45). The piston (45) is moved via the eccentric pin (46) of the shaft (47), which is actuated by an external lever. The piston (45) and the external lever are placed into rest position by the force of the backup spring (48). The spool (37) and the bushing (38) define cavities approximately annular (49, 50 & 51) which are, respectively: at the modulated pressure, which is generated by the pressure reducing valve with lever actuated adjustment (3); at a pressure, which is proportional to the pump rotation speed, that is generated by the pressure reducing valve with fixed adjustment (2); at the return line pressure. The pressure reducing valve with lever actuated adjustment (3) works as a pressure reducing valve with lever controlled adjustment, having a backup spring whose force, that is proportional to the valve adjustment pressure, is adjustable acting on the screw (44). In fig. 5 there are: the spool type load sensing valve (4) which is formed by a spool (52) that can move into the valve casing (53). The spool is pushed by the piloting piston (54) and the adjustment spring (55), which is fit on the spring retainer (56) and is adjustable acting on the screw (57) fit on cover (58). The piloting piston (54) is movable into a guide bushing (59) which is closed on one side by the cover (60). The spool (52), the valve casing (53), the cover (58), the piloting piston (54) and the guide bushing (59) define cavities which are approximately annular (61, 62, 63, 64 & 65) and are, respectively: at the modulated pressure that is generated by the pressure reducing valve with lever actuated adjustment (3); at the return line pressure; at the modulated pressure that is generated by the spool type load sensing valve (4); at the pressure after the calibrated orifice (15); at the modulated pressure that is generated by the spool type load sensing valve (4). At the free end of the piloting piston (54), into the cavity (66) of the guide bushing (59), acts the load induced pressure, that is the highest between the two pressures that are present into the connection lines from variable pump to hydrostatic motor. The above described spool type load sensing valve (4) allows to generate a modulated pressure which is inversely proportional to the load induced pressure but directly proportional to the pump rotation speed; moreover, the modulated pressure value depends on the adjustment controlled by the screw (57). The modulated pressure acts on the hydraulic proportional servo actuator mechanically feed-back into position (73) of the variable pump and is controlled by the electrically operated three-ways two-positions directional control valves (74), that choose the direction of movement or, on option, by a four-ways three-positions electrically operated directional control valve. The hydraulic proportional servo actuator mechanically feedback into position (73) varies the pump displacement in such a way that the pump displacement is directly proportional to the modulated pressure that is generated by the spool type load sensing valve (4). The variable pump displacement is so directly proportional to the pump rotation speed and inversely proportional to the load induced pressure. In fig. 6 there are: the double check valve (5), that is formed by a ball (67) which is pushed by the load induced pressure towards the seat (68) or the seat (69) depending on which of these two pressures that are present into the connection lines between variable pump and hydrostatic motor is the highest. In the cavities (70 & 71) there are, respectively, the two different pressures that are into the connection lines: in such a way, the hole (72) is at the highest of the two mentioned pressures, that is the load induced pressure. As it can be noted, the invention perfectly matches the aim. In particular, since the load induced pressure acts also during the vehicle deceleration, being the mentioned pressure generated by the hydrostatic motor when it is dragged, the variable pump swivels towards the minimum displacement, dragging the engine at high rotation speed: this causes an effective braking action, with no need for the presence of piloted flow control valves on the braking circuit, avoiding overheating and wear of the mechanical brake elements. The known automotive type hydrostatic drives behave exactly opposite way, that is in such a transmission the variable pump swivels towards the maximum displacement during the vehicle deceleration, dragging the engine at low rotation speed, thus causing a scarcely effective braking action. This invention also allows to reach the engine highest output torque at the intermediate rotation speed range, that correspond to the maximum output torque and to the minimum specific fuel consumption rotation speed range. This is permitted by the particular pressure reducing valve adjustment, which allows to reach high modulated pressure values, even with low rotation speeds. The pressure reducing valve with lever actuated adjustment (3) acts to keep the modulated pressure at a maximum value that is in correspondence with the primary engine maximum output torque, avoiding overloading due to the highest rotation speeds. In this way the engine is utilised at the maximum output torque even at the intermediate rotation speeds, in such a way to reduce the fuel consumption and to permit a better control smoothness. Further, the above described system allows a perfect control of the modulated pressure throttle function: this is advisable during the slow speed movements of the vehicle, or whenever should be necessary to use part of the engine output power to operate other devices, or to quickly arrest the vehicle. Such a result is obtained due to the use of the pressure reducing valve with lever actuated adjustment (3), which is basically a pressure reducing valve: in this way the modulated pressure value is simply determined by the control lever angular position, or else the lever position allows a direct, reproducible and smooth control on modulated pressure values. In known automotive type hydrostatic drives the throttle function is obtained using, on choice, adjustment variation devices on the pressure reducing valve (2) or variable diaphragm valves, in which the modulated pressure values are controlled by the means of the variation of the cross section of a port: in this case the valve adjustment is influenced by the fluid viscosity, thus by the fluid temperature, this resulting in a scarcely smooth and/or reproducible operation of the throttle function. This invention also permits a variable pump control function, by the means of an external pressure reducing valve, allowing a remote proportional hydraulic control of the hydrostatic power drive: for example via an hydraulic proportional joystick or with a pressure reducing valve placed on the vehicle control panel. In the same way it is possible to control the hydrostatic power drive using an electrically operated pressure reducing valve, mounted on the variable pump or, on choice, mounted in-line on the hydraulic circuit connections: in this case the control is remotely operated via electric wires, featuring a potentiometer or an electronic joystick control or else a radio control. Moreover, the described control system works practically uninfluenced by the mechanical hysteresis present in the variable pump displacement control device: this because its particular design features an hydraulic proportional servo actuator which is mechanically feed-back into position, allowing to highly reduce the mechanical friction effects on the pump displacement variations. Last, this device can be implemented of a pressure cut off valve that can cut down the pump displacement whenever the pump working pressure would exceed a pre-set value: this in order to avoid that the pump delivery should be discharged through the anti-shock valves, thus reducing fuel consumption and the fluid working temperature.

Claims: A control system for variable displacement hydraulic pumps in automotive type closed circuit hydrostatic drives, comprising: a variable displacement pump adapted to be connected to a hydrostatic motor by means of connection lines; an auxiliary pump (10) connected to a flow control valve (1) and a pressure reducing valve with fixed adjustment (2); a pressure reducing valve with lever actuated variable adjustment (3), connected to a load sensing spool valve (4); an hydraulic proportional servo-actuator (73) with mechanic feedback positioning system, connected to said variable displacement pump and adapted to drive directional control valves (74); a double check valve (5) connected to said variable displacement pump; said load sensing spool valve (4) being designed so as to generate a modulated pressure which is inversely proportional to a load induced pressure and directly proportional to the rotation speed of the variable displacement pump, the variable displacement of said variable displacement pump being directly proportional to said modulated pressure and acting upon the servo-actuator (73) which in turn changes the pump displacement, said load sensing spool valve (4) comprising: - a valve casing (53), and - a spool (52) movable in said valve casing (53); characterized in that - a first cavity (66) is defined in said valve casing (53) and connected to a load induced pressure; - a piloting piston (54) is movable in said valve casing (53) between said first cavity (66) and a first end of said spool (52) so as to induce the load pressure to said spool (52); - a second cavity (61) is defined between a second end of said spool (52) and said valve casing (53) and connected to said pressure reducing valve with lever actuated variable adjustment (3); - a spring (55) is arranged in said second cavity (61) and acting on the second end of said spool (52) and in opposed direction to said piloting piston (54); - a third cavity (62) is connected to a fluid return line; - a fourth cavity (64) at a pressure after a calibrated orifice (15) of said flow control valve (1); and - a fifth cavity (65) is defined between the first end of said spool (52) and said valve casing (53), said fifth cavity (65) being, at the modulated pressure generated by said load sensing valve (4). The control system of claim 1, characterized in that said modulated pressure is applied to said hydraulic proportional servo-actuator (73) with mechanic feedback positioning system. The control system of claim 1, characterized in that said modulated pressure is controlled by said directional control valves (74). The control system of claim 3, characterized in that said modulated pressure is controlled by a electrically operated three-way two-position directional control valves (74). The control system of claim 3, characterized in that said modulated pressure is controlled by a electrically operated four-way three-position directional control valves. The control system of claim 1, characterized in that said flow control valve (1) comprises a shutter (6) movable into a bushing (7), said flow control valve being supplied by said auxiliary pump (10) through the calibrated orifice (15), a pressure drop proportional to the rotation speed of the variable pump being derived. The control system of claim 1, characterized in that said pressure reducing valve with fixed adjustment (2) comprises a further first spool (16) movable within a sleeve (20) provided at one of its end with a bushing (17), said sleeve (20) being provided with three radial cavities (24, 25, 26), a pressure proportional to the rotation speed of the variable displacement pump being generated. The control system of claim 1, characterized in that said pressure reducing valve with lever actuated adjustment (3) comprises a further second spool (37) movable into a bushing (38) and pushed by a spring (41) fitted on a spring retainer (42); the position of said spring retainer (42) being adjustable by means of an adjustment screw (44) arranged on a spring pushing piston (45) which is axially moved by means of an eccentric pin (46) in turn operated by said lever. The control system of claim 1, characterized in that said double check valve (5) comprises a ball (67) pushed by said load induced pressure towards two distinct seats (68, 69), depending on which of the connection lines between said variable displacement pump and said motor is at the highest pressure, said highest pressure being supplied, through said connection line, through an orifice. The control system of claim 1, characterized in that it further comprises an external pressure reducing valve adapted to remotely proportionally control said variable displacement pump. The control system of claim 1, characterized in that an electrically operated pressure reducing valve is mounted on said variable displacement pump.

Inventors: BREVINI CORRADO, BREVINI MAURIZIO, BREVINI, CORRADO, BREVINI, MAURIZIO
Applicants: SAMHYDRAULIK S P A, SAMHYDRAULIK S.P.A.
Application year: 1995
Main classification: F16H61
IPC codes: F16H61