Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2000-01-17
2001-10-23
Ryznic, John (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S452000
Reexamination Certificate
active
06305163
ABSTRACT:
The present invention relates to a method for adjusting supply pressure in a hydraulic system comprising a feeder pump for producing the supply pressure, at least two hydraulic actuators, measuring means for measuring the pressure level of the actuators, and a pressurized medium flow channel system. The invention relates also to a hydraulic system according to the method.
Particularly for forest machines, systems have been developed in which the progressive movement of the forest machine is implemented with mechanical legs instead of wheels. Such a forest machine causes less damage to the undervegetation in the forest. Moreover, on a difficult terrain it is easier to move with legs than with a forest machine equipped with wheels.
To secure sufficient reliability and locomotion, a walking machine implemented with presently known technology requires six legs, each leg requiring three degrees of freedom. Thus, the machine has a total of 18 so-called servo shafts. To use all these servo shafts with an optimum coefficient of occupation, each servo shaft would require a separate servo pump, i.e. a total of 18 servo pumps. In practical applications, a limit is set by e.g. the price, space requirements, reliability and maintainability to the number of pumps used and to the way of control used. In practice, the number of pumps is limited to one or two, and the way of control used in the actuators is valve gearing.
In view of the efficiency, it is important to adjust the hydraulic supply pressure in such a way that the supply pressure corresponds to the highest load pressure perceivable with the actuators. This method is known as such as a so-called load sensing (LS) hydraulic system. A typical load sensing hydraulic system is presented e.g. in German patent publication DE 35 46 336. In practice, load sensing hydraulic systems are implemented in a mechanical hydraulic manner by utilizing so-called load sensing valves manufactured for this purpose. When the actuator is moved, a particular load sensing channel is opened in these valves to a particular exchange counter-valve chain integrated in the valve system. By utilizing this chain, the highest load pressure P
Lmax
required for moving the actuators is obtained for the regulator of the pump. The pump regulator adjusts the pump supply pressure p
s
for a pre-set pressure difference &Dgr;
p
above the level corresponding to the maximum load pressure P
Lmax
securing that all movements can be made.
European patent application EP 104 613 presents an electrohydraulic system particularly for machines with several operating cylinders. One such example is a tractor shovel with separate cylinders for turning and lifting the shovel and a hydraulic motor for moving the tractor shovel. The system presented in the publication is aimed at achieving a volume flow which is optimal in the respective loading situation. The system applies two pumps with unequal maximum output flows. Thus, it is possible either to use any of the pumps alone or, with greater loads, to use both pumps simultaneously. The system is controlled on the basis of signals generated by sensors (speed/position) coupled to the cylinders as well as control commands given by the operator of the machine in such a way that the volume flows produced by the pumps correspond to the respective use and load situation. Nevertheless, this publication does not disclose the special feature of the present application that to determine the pressure need, only the pressure level of those actuators is used which effect positive work.
German patent application DE 35 35 771 presents a hydraulic system with a hydraulic pump to produce pressure in the hydraulic system, valves for controlling actuators, a pressure equalization valve, and a pressure sensing line. In the system, the pressure is measured in a pressure supply circuit. A shut-off valve is connected to this supply pressure line and to a pressure measurement line. As the pressure level is maintained substantially equal in these lines, the pressure equalization valve is closed irrespective of the absolute pressure level. In a situation in which the position of the control valve for an actuator is changed, the pressure of the supply pressure line is changed and the pressure of the pressure measuring line is reduced momentarily. Consequently, the pressure difference is thus increased and a return valve is opened, wherein the pressure of the supply pressure is limited by leading the flow of the hydraulic fluid along a return channel to a hydraulic fluid container. The system is a conventionally implemented load sensing system with pressure sensing lines for the control valves. The adjustment is made hydraulically and, moreover, the system does not apply sensors for detecting the directions of movement of the actuators.
German patent application DE 43 07 827 also presents an electro-hydraulic system. In the system, the settings are given to the actuator electrically, e.g. by means of potentiometers or the like, wherein a controller determines, on the basis of the setting value of each actuator, how high a supply pressure will be needed. This is determined primarily as a sum of the volume flow required for each actuator. After this, the controller adjusts the pump to achieve the desired volume flow. In the adjustment, conventional, hydraulic load sensing valves are applied. In addition, the system combines the load sensing lines of these valves to one pressure measuring line by means of exchange counter-flow valves, wherein it is possible to measure the highest pressure level of the actuators. The pressure measuring line is equipped with a pressure sensor to measure the pressure electrically. Also in the system presented in this German publication, a load sensing control valve is required, which complicates, among other things, the structure of the hydraulic system and need for its maintenance and adjustability.
German patent application DE 35 32 816 presents a control system for a hydraulic system. The hydraulic system comprises two or more actuators and at least one pump. In this publication, the basic idea is that in a situation in which the supply pressure produced by the pump is not sufficient, the supply pressure of each actuator is substantially reduced in the same proportion. The purpose of this is to secure that all the control valves will be adjustable and that the volume flow will not escape to any control valve. This auxiliary pump is controlled by the actuator which requires the highest pressure level. The system is a conventional pressure sensing hydraulic system which does not apply any electrical measuring and control means.
German patent application DE 33 47 000 discusses an electro-hydraulic control system. The electro-hydraulic system presented in the publication controls a bidirectional cylinder/hydraulic motor which comprises two actuator lines and the valves therein. These control systems of two different directions are independent of each other, except for the controller. The control system consists of two valves: a three-way valve and a 4/4 proportional valve. The control system is designed as a fail-safe type, wherein for example in case of damage to the controller, no control pressure is transferred to the hydraulic motor. Furthermore, the pressure level of the supply pressures of the hydraulic motor are measured on both sides of the piston, and these measurements can be used to adjust the control valves. The system is not aimed at adjusting the pump supply pressure but the pressures of the actuator lines. The hydraulic motor can be controlled differentially by providing the cylinder of the hydraulic motor with a fixed control on one side of the piston and, on the other side, with a control signal corresponding to the respective need for adjustment, to adjust the pressure and the volume flow. The system presented in the publication does not apply a control pump but the volume flow is produced with a constant volume pump. The presented system is relatively expensive, particularly in view of the valves.
Walking of a work machin
Plustech Oy
Quarles & Brady LLP
Ryznic John
LandOfFree
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