HYDRAULIC CONTROL UNIT

Abstract: A hydraulic control unit for automatically controlling the speed of a hydraulic motor subjected to a variable load in a hydraulic circuit wherein a power unit is connected to the hydraulic motor through a supply line and a return line. The hydraulic control unit comprises a metering orifice (12) which is spring biased in the closed direction and to which a control piston (14) is mechanically linked, with pressure from the hydraulic motor pressure port (A) acting on the control piston, and a balanced piston flow divider (11) through which excess flow is returned to tank (T) and which is connected at one end to the supply line at a point between the pump (P) and the metering orifice and at the other end to the return line. In operation, and with the pressure at the pressure port (A) decreasing, the metering orifice is progressively closed resulting in an increasing pressure drop occurring across the metering orifice. This pressure drop acts on the balanced piston flow divider to increaslingly direct flow back to tank. Conversely, with the pressure at the pressure port (A) increasing, a progressively reduced flow is directed back to tank. Two applications for the control unit are disclosed, one being the control of a pole raising device, the other being the control of separate hydraulic motors driving the wheels of an all-terrain vehicle.

Description: "Hydraulic Control unit" THIS INVENTION relates to a hydraulic control unit. With some hydraulic systems, it is desirable to automati¬ cally control the speed of a mechanism actuated by a hy¬ draulic motor, either linear or rotary, by controlling the rate of flow of hydraulic fluid to the motor. The object of the present invention is to provide a con¬ trol system for a hydraulic circuit which enables the speed of movement being effected to be controlled automa¬ tically. In one form the invention resides in a hydraulic control unit for a hydraulic circuit wherein a power unit is connected to a hydraulic motor through a supply line and a return line said unit comprising means for metering the flow through the supply line in response to the load pressure therein and means for diverting excess flow in the supply line to the return line. The invention will be better understood by reference to the following description of the specific embodiments shown in the accompanying drawings wherein:- Fig. 1 is a hydraulic circuit diagram of one embodi¬ ment; Fig. 2 is an idealized graph showing the flow charac¬ teristics of the embodiment; Fig. 3 shows a unit constructed in accordance with the invention fitted to a lighting pole; Fig. 4 is a hydraulic circuit diagram for the appli¬ cation shown in Fig. 3; and Fig. 5 is a hydraulic circuit diagram showing the application of the invention to a wheel driven vehicle. In the embodiment shown in Figs. 1 and 2 of the drawings the pump supply is connected to port P, the hydraulic motor's delivery line to port A, the hydraulic motor's return line to port B and the hydraulic fluid tank to port T. Flow is controlled by a balanced piston flow divider 11 divering excess flow supplied by the pump back to the tank. The remaining flow from the pump passes through a metering orifice 12 resulting in a drop in pressure from the P side of the orifice to the A side. The latter reduced pressure, together with a spring 13 act to balance the piston 11 against the pump line pressure so that a controlled flow passes through the metering orifice 12 to the hydraulic motor connected to port A. A control piston 14 is mechanically linked to the metering orifice and progressively opens the orifice with in¬ creasing pressure. This is illustrated with reference to Fig. 2. In the CONTROL ZONE (C), increasing load pressure at port A results in increased delivered flow at port A. Con¬ versely, reducing the load pressure at port A will give a reduced flow. As the orifice 12 is opened, the pressure drop across it is reduced for the same rate of flow through the orifice. The piston 11, previously balanced, will move to the left as shown in Fig. 1, thereby reducing the rate of flow back to the tank and hence increasing the flow rate through the orifice until the initial pressure drop across the orifice is re-established. One particular application of the invention is a hydraulic pole raising device where a hydraulic cylinder is attached to a hinged base pole and the cylinder is extended or retracted to raise or lower the main section of the pole. This application is illustrated in Figs. 3 and 4 of the drawings. As shown in Figs. 3 and 4 a mobile hydraulic unit 20 is provided with a lifting cylinder 21 one end of which is anchored to the base 22 of a pole and the upper end to the main section 23 of the pole which is arranged to rotate about a transverse hinge pin 24. The hydraulic unit comprises a tank 25, a pump 26 and a motor 27. Hydraulic fluid under pressure is fed through a control valve assem¬ bly 28 via a pressure relief valve 30 and one-way valve 31 and a control unit 29, substantially identical with that shown in Fig. 1, to the cylinder 21. The control unit's port A is connected to the head-end of the lifting cylinder. Due to the physical arrangement of the pole and cylinder, the load pressure on the head-end of the cylinder, and hence at the control unit's port A will depend on the angle of the main section of the pole above horizontal, the mass of the main section of the pole together with the weight of any attached fittings, and the location of the corresponding centre of mass. Examination of the geometry reveals that as the main section of the mast is raised from the horizontal position to the vertical, the load pressure on the lifting cylinder reduces as the main section approaches the vertical posi¬ tion. It is also noted that if the same rate of flow were to be maintained to the lifting cylinder, then the top of the main section of the pole would continue to accelerate in a circular path until the system reached the mechani¬ cally imposed limit of motion on the section. This effect is undesirable in that the structure and any attached fittings are subjected to impact loads which may damage the system and endanger the safety of surrounding personnel and structures. Inclusion of the hydraulic control unit 21 results in a deceleration of the mechanism as it approaches the verti¬ cal position between the decreasing pressure in the lift¬ ing cylinder would be sensed and the supplied rate of flow to the lifting cylinder correspondingly reduced. Con¬ sequently, the safety of the system is enhanced and the risk of injury or damage eliminated. A further application of the control unit is in an all terrain, wheel driven vehicle wherein a separate hydraulic motor is used for each wheel and a control unit of the present invention is used to control the rate of flow to each motor. Such a system is illustrated in Fig. 5. When wheel 33 is in contact with a surface which resists the turning of the wheel is therefore capable of supplying traction, the pressure in supply line 34 to hydraulic motor 35 increases up to the maximum supplied by the pump or the maximum sustainable by the supporting surface. As the pressure in the supply line rises, the control unit 36 which is identical with that shown in Figs. 1 and 2 also supplies a greater rate of flow to the hydraulic motor, thereby allowing the corresponding wheel to rotate more rapidly. The overall vehicle speed is set independently by varying the maximum rate of flow of hydraulic fluid to the drive circuit by adjustment of a variable displacement pump 37. The effect of the control circuit is to limit the rota¬ tional speed of the wheels to the maximum possible under the available traction conditions. The control unit may have .two further zones of operation shown in Fig. 2. The first is a Trickle zone (T), where a small flow (Qmin) will always be present at low pressure. This means that a rotary hydraulic motor will rotate very slowly or a cylin¬ der will extend or retract very slowly under no or low loads. The other zone is a Free Flow zone (F) where the pressure is sufficiently high to fully open the metering orifice and allow full pump flow to the outlet port A. Due to practical limitations, there will always be a small leak¬ age flow (Qlc) through the balanced piston flow divider. Transition between the zones depends on the control piston spring preload, stiffness and type. Also, the actual shape of the flow characteristic can be markedly affected by the changing of the viscosity of the hydraulic fluid. Whilst the invention has been described with particular reference to mechanical sensing and control units it is equally applicable to electronic sensing devices. For example the variable metering orifice and balanced piston may be replaced by electronic pressure sensing and an electrically controlled proportional spool valve.

Claims: THE CLAIMS defining the invention are as follows:- 1. A hydraulic control unit for a hydraulic circuit wherein a power unit is connected to a hydraulic motor through a supply line and a return line, said unit com¬ prising means for metering the flow through the supply line in response to the load pressure therein and means for diverting excess flow in the supply line to the return line. 2. A hydraulic control unit as claimed in claim 1 where¬ in the metering means comprises a variable flow orifice coupled to a control piston. 3. A hydraulic control unit as claimed in claim 1 or 2 wherein the means for diverting excess flow comprises a balanced piston flow divider. 4. A hydraulic control unit as claimed in claim 1 where¬ in the metering means comprises a spool valve. 5. A hydraulic control unit as claimed in claim 4 where¬ in the spool valve is controlled by electronic means for sensing the flow and pressure in the supply line. 6. A hydraulic control unit for a hydraulic circuit wherein a power unit is connected to a hydraulic motor through a supply line and a return line, said control unit comprising a balanced flow divider connected between the supply line and the return line, a metering orifice con¬ nected into the supply line between the connection of the divider to the supply line and the hydraulic motor and means responsive to the pressure in the supply line bet¬ ween the orifice and the hydraulic motor for varying the degree of opening of the orifice. 7. A hydraulic control unit for a hydraulic circuit substantially as herein described with reference to Figs. 1 and 2, 3 and 4 or 5 of the accompanying drawings.

Inventors: FELSCHE BERND HARTWIG, FELSCHE, BERND, HARTWIG
Applicants: CAVE HOLDINGS PTY LTD, CAVE HOLDINGS PTY. LTD.
Application year: 1986
Main classification: F15B11
IPC codes: F15B11, F15B13