Abstract:
A method for operating a hydraulic system has at least one supply device, in particular a hydraulic pump ( 3 ) supplying different hydraulic consumers. A synchronizing device ( 33, 35 ) ensures that, if at least one hydraulic consumer is not supplied sufficiently, the deficit in volumetric flow for that consumer is compensated such that all the consumers compensate for the deficit equally.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for operating a hydraulic system having at least one supply device, in particular a hydraulic pump supplying different hydraulic consumers. Moreover, the invention relates to the corresponding hydraulic system. 
     BACKGROUND OF THE INVENTION 
     In motor-driven systems and devices with hydraulic systems, for example, in loader-like and excavator-like construction machinery, for reasons of costs, typically the output of the diesel engine is used to drive the hydraulic pump without reserves. Likewise, for reasons of costs, in many cases the hydraulic pump is not designed such that with a simultaneously maximum volumetric flow demand of all consumers, sufficient supply of all consumers would be ensured. 
     In working operation, this limitation leads to the diesel engine being operated at its output limit and the pump flow rate in parallel operation of hydraulic consumers not being sufficient for the desired maximum working speeds. For safety functions, priority valves must be used to supply the preferred consumers first before delivery to other consumers is released. All other consumers must share the remaining flow. 
     For such construction machinery, the prior art t supplies the consumers by directional control valves with compensators connected upstream. The valve spools of the directional control valves determine the size of the opening of the metering orifices for supply of the consumers. Viewed from the pump, a series of variously high resistances is presented by the operating principle of the upstream compensators copying the pressure of the external loads to upstream from the metering orifices and still increasing it by the amount of force of their control springs. When the flow rate of the pump is insufficient, the pump pressure collapses, and the working medium flows over the path of least resistance. The consumer with the highest load can thus be shut down. Its “saved” volumetric flow is thus available to all other consumers. 
     For the machine operator, this system behavior is not acceptable since, for typical machine control with a joystick, several functions are run at the same time. If one consumer inadvertently stops, the operator will experience difficulties with the controls. 
     The attempt to solve this problem by using valves with compensators connected downstream from the metering orifice does not lead to the desired success, even though downstream compensators do not copy the load pressure to upstream from the metering orifice. The highest load pressure in the system is copied to downstream from the metering orifice, as a result of which, when the pump pressure collapses, all resistances remain the same viewed from the pump. Disadvantageously in these systems, the amount upstream from the metering orifice must be separated. This separation is not easily possible. Another particular disadvantage is that the load signaling system of directional control valves with downstream compensators dictates a continuous discharge flow from the controlled volumetric flow of the consumer with the highest load. This operation constitutes an energy loss. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method for operating a hydraulic system with relatively improved operating behavior when the supply device is overtaxed. 
     According to the invention, this object is basically achieved by a method providing that when the consumer is undersupplied, all consumers in the hydraulic system are used to compensate for the volumetric flow deficit of the undersupplied consumer. In the prior art in systems with an upstream compensator in the case of undersupply, the consumer with the highest load can shut down and its saved volumetric flow then benefits the other consumers. The invention conversely calls for a correspondingly reduced volumetric flow to be made available to all consumers in the case of undersupply. Therefore, no danger exists that the machine operator controlling several consumers at the same time, in order to simultaneously run several functions, will be confronted with a situation in which one consumer is shut down while the other consumers continue to operate (remain in motion). 
     In embodiments in which a volumetric flow of pressurized fluid dependent on the size of the opening of an assigned adjustable metering orifice is supplied via the metering orifice from a pump delivery flow to each consumer, on the orifices a pressure difference is produced referenced to the size of the orifice opening and the pump flow rate. Preferably, when the pressure difference drops on at least one metering orifice to below a setpoint, a correction signal is produced. Depending on its signal value the size of the opening of all metering orifices is synchronously reduced. The correction signal is maintained until the setpoint of the pressure difference is reached again. The correction signal therefore is generated when the hydraulic pump is overtaxed. The pump flow rate is then no longer sufficient to produce the necessary dynamic pressure on the metering orifice of the consumer with the highest load, by which the pressure difference on this orifice drops below a specified setpoint. 
     Preferably, a synchronous pressure is produced in a synchronous channel as the correction signal. Since the correction signal is thus present in the form of a pressure signal, it is preferably caused to take effect directly in the valve system. 
     In especially preferred embodiments, the synchronous pressure is produced by a synchronous compensator supplied on the one hand with the pump pressure and on the other with the highest load pressure of the system plus the force of its control spring. When its control pressure difference is not reached, a pressure source is connected to the synchronous channel. 
     In embodiments in which the supply of the consumers is controlled by proportional directional control valves, whose valve spools can be triggered hydraulically for changing the metering orifices by sensor pressure, the synchronous pressure is supplied to the face side of the valve spools triggered with the sensor pressure. The synchronous pressure produced in operating states of undersupply therefore results in valve spools of all directional control valves being reset by an amount depending on the synchronous pressure against the respective sensor pressure. All consumers are then supplied with a correspondingly reduced volumetric flow for compensation of the undersupply. 
     In especially advantageous embodiments, the pressure difference on the metering orifices of the directional control valves are controlled by an assigned individual compensator, and the system pressure is controlled by a system compensator. 
     Accordingly, the differential pressure of the synchronous compensator is preferably set to a somewhat lower value than the differential pressure of the system compensator. This setting ensures that in normal operation of the system, the differential pressure in the system is definitively determined by the system compensator. 
     The pressure difference of the synchronous compensator is preferably set to the pressure difference of the individual compensators or higher. 
     In especially preferred embodiments, the system of valves and pumps is dimensioned such that at the maximum possible volumetric flow demand the maximum synchronous pressure does not exceed the pretensioning force of the centering springs of the valve spools. This arrangement ensures that in the case of undersupply of the system, the valve spools cannot be reset to their neutral position by the synchronous pressure. 
     In systems with consumers which, for example for reasons of safety, are especially preferred, the supply of the correction system to the respectively preferred consumer can be stopped by a priority circuit. 
     The subject matter of the invention is also a hydraulic system which can be operated according to this method. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a schematic hydraulic circuit diagram of a prior art hydraulic system for supply of two hydraulic consumers; 
         FIGS. 2 and 3  are schematic hydraulic circuit diagrams of two different system pressure regulators for use in hydraulic systems of  FIG. 1 ; 
         FIG. 4  is a flow chart illustrating the operating principle of the invention; 
         FIG. 5  is a schematic hydraulic circuit diagram of the hydraulic system according to an exemplary embodiment of the invention designed for implementing the method according to an exemplary embodiment of the invention; 
         FIG. 6  is a schematic operating diagram of a directional control valve for the synchronous control method according to an exemplary embodiment of the invention and with a logic circuit for triggering with synchronous pressure; and 
         FIG. 7  is a schematically simplified side elevational view in section of a synchronous compensator according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a hydraulic system corresponding to the prior art for supply of two consumers (not shown). A system pressure regulator is connected upstream from the pump line  1 .  FIGS. 2 and 3  show two embodiments of system pressure regulators that can be used for hydraulic systems of the type shown in  FIG. 1  to keep the pressure difference of the pump pressure and the maximum load pressure LSmax constant.  FIG. 2  illustrates a hydraulic pump in the form of a constant delivery pump  3 . The pump pressure side is connected to a three-way compensator  5  supplied with the pump pressure and with LSmax, plus the force of one control spring  7 . Compensator  5  works as a pilot-controlled pressure limitation valve keeping constant the pressure difference between the pump line  1  and LSmax.  FIG. 3  conversely shows the use of a variable delivery pump  9  whose controller is formed by a directional control valve  11  that adjusts the required flow rate within the control circuit “pump adjustment mechanism.” 
     The supply of the consumers (not shown in  FIG. 1 ) by the supply lines A 1 , B 1  and A 2 , B 2  takes place by way of proportional directional control valves  13 . The valve spools  15  of control valves  13  with their metering edges define the sizes of the openings of metering orifices  17 . One individual compensator  19  is connected upstream from the respective directional control valve  13  supplied conventionally for upstream compensators with the dynamic pressure p 1 ′ and p 2 ′ prevailing on the respective metering orifice  17  of the directional control valve  13  and with the loading pressure of the pertinent consumer LS 1  and LS 2  plus the force of its control spring  21 . A selector valve  23  to which the load pressures LS 1  and LS 2  are supplied decides which load pressure is supplied as LSmax to the system pressure regulator (not shown in  FIG. 1 ). For controlling the volumetric flows supplied to the consumers by the supply lines A 1 , B 1  and A 2  and B 2 , the control valves  13  can be triggered hydraulically by a sensor pressures X a1  and X a2  supplied to the face side of the respective valve spool  15  or a sensor pressure X b1  and X b2  being supplied to the opposite face side thereof. 
     If the pump pressure collapses when the pump output is overtaxed during operation of the system shown in  FIG. 1 , on the individual compensator  19  with the highest load pressure, only a reduced pressure difference as a pressure excess for controlling the pressure difference is available on the pertinent metering orifice  17 . If this dynamic pressure on the most highly loaded directional control valve  13  drops to the load pressure or below, this consumer stops while the consumers under a low load continue to move. 
       FIG. 4  illustrates the different state arising by the method according to the invention. If the directional control valves  13  during system operation are opened to the extent that the pump flow rate is no longer sufficient to throttle the necessary dynamic pressure upstream from the metering orifices  17 , the dynamic pressure then drops according to a quadratic function, see box  25  (first box from the bottom). In the next box  27  to the top, the control law of a synchronous compensator ( 33  in  FIG. 5 ) reduces again the volumetric flow demanded by the consumers down to the possible pump flow rate by the correction signal in the form of a synchronous pressure X syn  constituting compensation of the control pressures prevailing on the valve spools  15 . The compensating synchronous pressure X syn  opposes the control pressures X, see box  29 , and thus reduces the opening cross sections of all metering orifices  17 . This operation takes place until the differential pressure setpoint which is set on the synchronous compensator  33  is reached again, see box  31 . 
       FIG. 5  illustrates the method according to an exemplary embodiment of the invention using a hydraulic system with three-way directional control valves  13  for supplying three consumers. The supply lines are omitted, and the directional control valves  13  are shown simplified for the sake of clarity. One individual compensator  19  in the same arrangement as shown in  FIG. 1  is connected upstream from two of directional control valves  13 . The directional control valve  13  for the consumer N is integrated into the system without an individual compensator. The system pressure is regulated according to the example of  FIG. 2  by a three-way compensator  5  connected to the pump line  1  at the output of the constant delivery pump  3 . 
     The synchronous compensator  33  used to produce a synchronous pressure X syn  in a synchronous channel  35  is supplied with the pump pressure and with the maximum control block load pressure L STB  plus the force of a control spring  37 . The choice of which load pressure is supplied as the maximum load pressure L STB  both to the synchronous compensator  33  and to the system compensator  5  takes place as in the system of  FIG. 1  by selector valves  23 . 
     The synchronous compensator  33  works as the pump regulator in a control circuit in which the valve spools  15  of all directional control valves  13  participate. The basic principle is a sensor circuit monitoring the level of the current pressure difference on the control block (directional control valve  13 ). If this pressure difference is in the specified region, the synchronous compensator  33  remains passive, i.e., it is pressed by the desired pressure difference against its control spring  37  and relieves the synchronous channel  35  after the tank  39 . In the other case, the synchronous compensator  33  assumes an open position and supplies from the supply line  41  the volumetric flow into the synchronous channel  35  to produce a synchronous pressure X. The synchronous channel  35  can be connected in parallel to each face side of all valve spools  15 , the decision—supply of control pressure/sensor pressure—being made by the respective selector valve  43  to which on the one hand the sensor pressure X . . . on the one hand and the synchronous pressure X syn  on the other are supplied. 
     If the synchronous pressure Xsyn rises and pushes through to the face side of the valve spool  15 , it can be assumed that it is that side of the valve spool  15  opposite the side triggered with the sensor or pilot pressure. If, for example, a directional control valve  13  is triggered with 7 bar and delivers 50 l/min and at this point, the synchronous pressure rises from 0 to 2 bar, the spool  15  deflected with 7 bar is reset to the spool position corresponding to 5 bar control pressure by 2 bar counterpressure. As a result, the volumetric flow supplied to the consumers is reduced. The corresponding applies to the valve spools  15  of the directional control valves  13  of the other consumers. The synchronous pressure is built up, i.e., the synchronous compensator  33  remains in the open position until the desired pressure difference on the control block has again reached the setpoint. 
     The differential pressure of the synchronous compensator  33  is set somewhat lower than the differential pressure of the system pressure regulator. In normal saturated operation, the differential pressure in the system is then definitively determined by the system pressure regulator. The differential pressure of the individual compensators  19  is ideally set to the value of the pressure difference of the synchronous compensator  33 . Then, the synchronous compensator  33  recognizes incipient undersaturation of the system compensator  5 , while the individual compensators  19  are still saturated. For incipient undersupply, this method does not cause any errors in synchronous control since, before the individual compensators  19  would completely open due to incipient undersaturation and then could no longer regulate, the synchronous compensator  33  already begins to produce a compensating synchronous pressure X syn  to reset all deflected valve spools  15 . 
     As alternatives to using the selector valves  43 , according to  FIG. 6 , a logic circuit on the valve spool  15  of the directional control valves  13  can choose to what face side the sensor pressure or synchronous pressure is supplied. 
       FIG. 7  shows a cross section of the synchronous compensator  33  whose spool  45  is shifted so far to the left by the load pressure LS and the force of the control spring  37  in the figures that the metering edge  47  begins to connect the supply line  41  to the synchronous channel  35 , while the connection to the tank  39  is cut off. When the pressure P pu  rises until the desired differential pressure is reached and the spool  45  is reset to the right, the synchronous channel  35  is relieved again to the tank  39 . 
     If in this text orifices such as metering orifices are addressed, the pertinent details also apply to throttles such as metering throttles. These details also apply to the nozzles used. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.