Abstract:
A hydraulic system includes an engine driven adjustable hydraulic pump which supplies fluid to a hydraulic consumer, and an electronic control unit. The adjustable pump includes flow controller for controlling a pressure difference between the consumer and the adjustable pump to a pre-determined control pressure difference. The flow controller includes an actuator controlled by the control unit, through which the pressure difference can be controlled. The electronic control unit receives an engine speed signal and controls the actuator to control the pressure difference, in order to make power delivery of the adjustable pump conform to the operating conditions of a vehicle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a hydraulic system for an engine driven vehicle. 
     BACKGROUND OF THE INVENTION 
     Agricultural machines such as tractors, construction machines loaders or other types of operating machines, typically have a hydraulic system with which one or more hydraulic consumers such as hydraulic cylinders, hydraulic motors or other hydraulically driven components. Such hydraulic systems include hydraulic pumps that can be connected directly or over a connecting gearbox with fast or slow fixed gear ratios to the drive shaft of a drive engine. Thereby the maximum volume flow that can be conveyed by the hydraulic pump varies with the rotational speed of the drive engine. The faster the rotational speed of the drive engine the larger is the volume flow that can be conveyed by the hydraulic pump. In adjustable load-sensed controlled hydraulic pumps, so called adjustable pumps, as they are being applied today in the state of the art, the maximum volume flow conveyed can be made to conform to the demand of the hydraulic consumers. This is performed usually over a so-called conveyed volume-flow-controller that controls or maintains a predetermined control pressure difference between the pressure of the outlet of the adjustable pump and the load sensing signal (in the following called LS-signal). The conveyed volume flow controller of a LS-controlled adjustable pump now operates in such a way that it adjusts the conveyed volume flow of the adjustable pump in such a way that the predetermined control pressure difference, that can be adjusted on the conveyed volume flow controller can be provided as an input by means of an adjusting spring and is maintained constant at all times. The exact method of operation of such a pressure-based volume flow controller can be reviewed in the relevant literature and as such is the state of the art. 
     The conveyed volume flow that can be delivered by a hydraulic valve to a hydraulic consumer depends directly upon this control pressure difference. A certain control pressure is adjusted by means of the adjusting spring and an adjusting piston of the conveyed volume flow controller in that it forces the adjustable pump to maintain a control pressure difference corresponding to this adjusted pressure between the outlet of the adjustable pump and the consumer (L-S Signal). In order to attain this control pressure difference, it pivots the conveyed volume flow control adjusting unit upward in order to begin to convey a corresponding conveyed volume flow that can be controlled or adjusted as a function of the adjusting piston. Here the adjusting piston is connected hydraulically with the conveyed volume flow controller and changes its position as a function of the control pressure difference existing or provided as input at the conveyed volume flow controller. The conveyed volume flow control unit may for example include a pivoting disk that is connected with a control or lifting piston where the rotating movement of the pivoting disk is converted into a linear movement of the lifting piston. The conveyed volume flow conveyed by the adjustable pump flows through the lines and the valves of the hydraulic system and thereby generates certain pressure losses in the lines and each of the valves leading to the consumer. The pressure that then develops behind the valves or at the consumer is reported back as load pressure (LS-signal) to the adjusting pump (over a load sensing line (L-S-line) that is connected to the conveyed flow controller) and impels the adjustable pump to convey such an amount of flow that the pressure at the outlet of the adjustable pump is higher by the control pressure difference than the pressure at the consumer delivered by the L-S-Signal. 
     The further a valve now is from the adjustable pump, the higher are the pressure losses occasioned by the longer lines, which leads to the effect, that valves that are located further away from the adjustable pump than other valves, permit less volume flow to reach the consumer, although those valves are configured identically. In order to compensate for this effect, a known practice is to apply valves that transmit an increased load signal to the pump, as is disclosed for example in EP 176 0 325 A2. 
     Accordingly a simplified conclusion is that a certain pressure is required to force a certain volume flow through a line and/or a valve. Since the pressure losses increase during the flow through the lines or valves, it would therefore be advantageous to maintain the cross sections and the conduct of the lines and the bores as large as possible, as well as keeping the losses over the valves as low as possible by the configuration if a certain volume of hydraulic fluid is to be provided to a consumer. If the losses become too great and thereby the volume flow is reduced, this can be compensated for by increasing the cross section of the valve openings, in other words, by changing the cross section at the valve openings, volume flows can be changed, that is, they can be increased or decreased. 
     Other possibilities of changing the volume flow lead to a change in the adjusting force provided at the conveyed volume flow controller of the adjustable pump. In that way, EP 0 439 621 B1 discloses that for a precision operation of the hydraulic system, the control pressure difference at the adjustable pump can be reduced by manual operation of an adjustable force at the conveyed volume flow controller, which results in a lower maximum volume flow in the hydraulic system or in the valves. 
     Now the problem is that is that it may be advantageous for environmental or economical reasons to operate a hydraulic system of an operating machine in the lower range of rotational speeds. This has the result that with today&#39;s adjustable pump sizes too little volume flow is available for the applications, which leads to the application of larger adjustable pumps, so that large volume flows can be conveyed at low engine rotational speeds. This results in turn in very large volume (not utilized) flows being conveyed at high engine rotational speeds that lead to very large power losses in the overall power balance. These problems could be overcome at least partially, by raising the control pressure difference of the pump, that finally would lead to a higher fuel consumption of the machine, since a certain power output is required, or a certain conveyed volume flow is required, in order to attain the necessary control pressure difference. Moreover the possibility exists of designing all lines and valves for the maximum pump power output that would in turn lead to very high cost for the individual components and to space problems on the operating machine. EP 349 092 B1 discloses a further possibility, to permit possible high volume flows at low engine rotational speeds, but to limit the volume flow at high engine or rotational speeds. Here the maximum conveyed volume flow of the adjustable pump is limited, where the conveyed volume flow amount of the adjustable pump is measured or monitored (for example, by a measurement of the position of the conveyed volume flow control mechanism, for example the adjustment angle of an adjusting disk or a pivot disk). These adjustable pumps and the corresponding electronic control systems however are costly and expensive. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to provide a hydraulic system that makes available a large conveyed volume flow at low drive rotational speeds, which however is limited to a certain maximum value at higher rotational speeds. 
     This and other objects are achieved by the present invention, wherein a hydraulic system is configured so that the conveyed volume flow controller includes an actuator that can be controlled by a control unit, by means of which the control pressure difference can be varied by the electronic control unit at the conveyed volume flow controller, where the electronic control unit detects a signal dependent upon the drive rotational speed, as a function of which a control signal can be generated by the control unit for the controllable actuator, which varies the control pressure difference at the conveyed volume flow controller. In order to detect the drive rotational speed and generate a corresponding control signal for the actuator that can be remotely controlled a predetermined control pressure difference at the conveyed volume flow controller can be purposefully varied as a function of the drive rotational speed. 
     That is, that at a low drive rotational speed the control pressure difference for example amounts to P=P1 bar. This adjustment is useful, for example, to convey a volume flow up to a magnitude of V1 1/min through the valves. The adjustable pump, however, on the basis of its maximum conveyed volume flow, has the possibility of conveying conveyed volume flows of Vmax 1/min that would result in high losses in the lines and the valves. At idle rotational speed this pump now conveys, for example, a maximum volume flow of V idle 1/min (V idle&lt;V1&lt;Vmax) and with increasing rotational speed (U+) the conveyed volume flow (V) increases. Now it is conceivable that upon reaching the V1 1/min limit the control pressure difference P bar is reduced further (P=P1−P(U); where P(U) is to be understood as a function of the drive rotational speed and increases with increasing drive rotational speed) and thereby also reduces the conveyed volume flow of the adjustable pump, so that at maximum drive rotational speed of, for example, Vmax U/min, V (1) 1/min is conveyed by the adjustable pump at all times. If the drive rotational speed drops again, the control pressure difference is again increased and therewith a conveyed volume flow again increases slowly (P=P1−P (U); where P (U) is reduced with a reducing drive rotational speed) until the original value (P=P1) has again been reached. The corresponding functions and calculation algorithms are preferably stored in memory in the electronic control unit. A corresponding control signal is generated by the control unit and is conducted to the actuator at the conveyed volume flow controller for purposes of control of the same. The control pressure difference is changed at the conveyed volume flow controller by controlling the actuator. 
     When a predetermined drive rotational speed value is exceeded the control pressure difference of the adjustable pump can be reduced. Here the predetermined drive rotational speed that triggers the readjustment of the control pressure difference can be provided as input depending on the application preferably over an input module on an operator&#39;s display of an operating implement or over another appropriate input interface of the control unit. However, depending on the magnitude of the power of the adjustable pump, a fixed drive rotational speed can be provided and stored in memory in the control unit. 
     When a predetermined drive rotational speed is exceeded the control pressure difference can be changed proportionally to the drive rotational speed, where the control arrangement reduces the control pressure difference with increasing drive rotational speed and increases it with reduced drive rotational speed. With respect to this the control signal generated by the control unit preferably conforms uniformly to a change in the drive rotational speed, so that the operator of the system does not directly sense the change in the conveyed volume flow. 
     For special applications provision is made for utilizing the maximum possible conveyed volume flow of the adjustable pump. For this purpose adjusting devices are provided as a function of which a control signal can be modified and the control pressure difference can be changed by means of the adjusting devices, in such a way that the control pressure difference can be reduced or increased independently of the drive rotational speed. In that way an operator can quasi “override” the control of the adjusting devices taken over from the control arrangement at the conveyed volume flow controller and deactivate the control function of the control unit that is dependent upon the drive rotational speed by corresponding input at the adjusting devices, for example, at an input module or an input button with an adjusting wheel or a potentiometer, and modify the control signal by a direct input of an input signal that can be provided so that the signal provided by the adjusting devices is prioritized despite a control signal originally generated that was proportional to the drive rotational speed. Thereby it is possible to circumvent a control dependent upon the drive rotational speed and to operate the hydraulic system with a high control pressure difference, or to adjust any desired control pressure difference at any desired rotational speed. There are particular applications in which the operator would like to necessarily utilize the maximum conveyed volume flow of the adjustable pump regardless of the power losses. Such an application would, for example, be a front loader in which the operator would like to attain the shortest possible cycle times and with that more (turnover) power. In such a case an adjustable pump with a large conveyed volume flow would not help as long as all the lines and valves are not also increased. Here it may be useful to purposely increase the control pressure difference in order to assure that the adjustable pump can fully conform in order to convey a greater volume flow. 
     Here the hydraulic system can also include a temperature sensor that detects the temperature in the hydraulic system and delivers a corresponding signal to the control unit. In particular since the viscosity of the hydraulic fluid is a function of the temperature, so that it may be advantageous at low temperatures or at a high viscosity of the hydraulic fluid to adjust the control pressure difference in the conveyed flow controller as a function of the viscosity or the temperature, for example, to raise it. Moreover it may be advantageous at extremely low temperatures to counteract cavitations problems caused by the viscosity, so that the control pressure difference at the conveyed volume flow controller is limited or reduced and an increase occurs only at a certain temperature and particularly independently of or dependent upon the drive rotational speed. Similarly it may be advisable to make the control pressure difference at the conveyed volume flow controller conform to the lower flow losses at higher temperatures, that is, for example, to reduce it. In that way conditions may occur for which an adjustment of the control pressure difference at the conveyed volume flow controller is advantageous as a function of the drive rotational speed alone as well as in combination with the temperature in the hydraulic system. Such control functions or control algorithms may be implemented in the electronic control unit and stored in memory as corresponding condition diagrams. On the basis of these control functions or control algorithms corresponding control signals can be generated at the conveyed flow controller as a function of the drive rotational speed as well as the temperature for the control of the adjusting devices or for the adjustment of the control pressure difference. 
     The actuator at the conveyed volume flow controller include, for example, an electric motor, that can be controlled by the control unit and can adjust the adjusting spring in the conveyed volume flow controller. Moreover, it is also conceivable that an electro magnet be applied that can adjust the adjusting spring in the conveyed volume flow controller. Preferably the adjustment of the conveyed volume flow controller or of the adjusting spring can be performed directly at the pre-load of the adjusting spring of the conveyed volume flow controller. As already described above, this adjustment can be performed electrically or electromagnetically, but also hydraulically, pneumatically or purely mechanically, where an electric or electromagnetic adjustment is preferred, since this can be handled more readily compared to other means of adjustment. This adjustment can now increase or decrease the spring pre-load which automatically adjusts the control pressure difference. This adjustment may be performed, for example, by means of proportional magnets that are effective in both directions. Obviously it is also conceivable to permit an adjustment in only one direction. Since it is always possible that the electronic system at the operating vehicle fails, it is useful to provide measures that prevent a failure of the entire hydraulic system in the case of a failure of the electronics. For this reason the application of a stepper motor for the adjustment of the control pressure difference is particularly appropriate. The stepper motor has the advantage that it is provided with a certain self-locking arrangement and can be operated very precisely into a certain position (adjusting angle), that it does not leave unless it is provided with a new control signal or a very strong force is applied. Such a stepper motor can be connected simply to the adjusting screw for the adjustment of the control pressure difference of the conveyed volume flow controller and can adjust the adjusting screw very precisely and very rapidly depending on the control signal, so that the control pressure difference can be adjusted very sensitively. In the case of a failure of the electronics the stepper motor would simply remain in its last position and thereby assure that a certain minimum operation of the hydraulic system is assured. 
     The hydraulic system, according to the invention, is applied in operating vehicles that are used in agriculture, that is, in agricultural vehicles, such as tractors, with or without front loaders as well as telescopic front loaders. Moreover, such a hydraulic system is also appropriate for application in construction machines, for example dredges or wheel loaders. 
     The hydraulic system, according to the invention, permits optimum operation of a hydraulic system in all operating conditions for the drive of the vehicle and is used in particular to reduce power losses and to make available large volume flows at low drive rotational speed. If necessary, very large conveyed volume flows are possible despite small line cross sections and valves. It is thereby possible to retain the existing valves and lines without the need for a larger adjustable pump. Moreover, in the case of failure of the electrical system assurance can be provided despite electronic control of the conveyed volume flow electronic controller that the existing hydraulic system continues to be available. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a hydraulic circuit diagram of a hydraulic system according to the invention with an electric motor as an adjusting device; 
         FIG. 2  is a hydraulic circuit diagram of an alternative hydraulic system with a proportional valve as adjusting device; and 
         FIG. 3  is a side view of a vehicle with a hydraulic system according to  FIG. 1  or  2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a hydraulic system  10  operates a hydraulic consumer  12 , for example, a hydraulic cylinder for raising and lowering a front loader  14 . The hydraulic system  10  includes a hydraulic reservoir  16 , an adjustable hydraulic pump  18  with a conveyed volume flow controller  20  for controlling a pressure difference between adjustable pump  18  and the consumer  12 , a pressure limiter  22  to limit the operating pressure for the adjustable pump  18 , as well as an adjustable piston  23  for the adjustment and limitation of the conveyed volume flow of the adjustable pump  18 , that can be adjusted by a conveyed volume flow adjusting arrangement  24 . Moreover, a stop  23  or an adjustable spindle, is provided for the adjustable piston  23 , that can be brought into engagement with the adjustable piston  23  and with which a maximal conveyed volume flow of the adjustable pump  18  can be adjusted. The adjustable pump  18  is driven by an internal combustion engine  25 . A hydraulic control valve  26  is provided between the consumer  12  and the adjustable pump  18  by means of which the hydraulic consumer  12  can be controlled. A load pressure line  28  is connected between the consumer  12  and the control valve  26  that is connected with the conveyed volume flow controller  20 , where the load pressure line  28  is provided with a pressure relief orifice  29  connected to the reservoir  16  and a check valve  30  closing in the direction of the consumer  12 , where the check valve  30  is arranged between pressure relief orifice  29  and the consumer  12 . Moreover, the hydraulic system  10  includes an electronic control unit  32  that is connected with rotational speed or engine speed sensor  34  and an operator control device or adjuster  36 . The conveyed volume flow controller  20  is provided with actuator  38  that are configured as an electric motor, preferably as a stepper motor and can be controlled by the electronic control unit  32 . 
     The engine  25  is directly connected with the adjustable pump  18 , where these are here represented only as examples. Obviously, here step-up gears or reduction gears can also be inserted. The drive shaft of the engine  25 , which also could be an electric motor, is directly provided with the rotational speed sensor  34  that conducts a rotational speed signal to the electronic control unit  32 . Moreover, the electronic control unit  32  can receive input signals from the operator control  36 , which it then considers in the generation of control signals for the actuator  38 . In the generation of a control signal, a rotational speed signal delivered by the rotational speed sensor  34  is considered primarily as a function of which the electronic control unit  32  generates the control signal for the actuator  38 . If, however, an additional input is provided over the input device  36 , then the control signal that is based on the rotational speed signal is modified. This has the background that no rotational speed dependant control of the actuator  38  is to occur in the conveyed volume flow controller  20  that can be signaled by an operator over the input device  36  but rather input control magnitudes can be provided as input by the operator for the control of the actuator  38 . For example, the operator can provide as input over the actuator  38  the maximum conveyed volume flow value or another adjustable conveyed volume flow value for the adjustable pump  18  that is to be adjusted independently of the rotational speed of the drive engine  25  by the electronic control unit  32 . 
     The conveyed volume flow controller  20  that is initially adjusted over a pre-load spring  40  with a fixed control pressure difference value can now be adjusted by readjusting the pre-load of the pre-load spring  40  over the actuator  38  so that the control pressure difference can be raised as well as lowered. Depending on the existing pressure relations of the system pressure existing between the control valve  26  and consumer  12  and the system pressure existing at the outlet of the adjustable pump  18  the result is a pressure difference that is provided as input to the conveyed volume flow controller  20  over the load sensing pressure line  28  and over a control pressure line  42 . According to the initially adjusted control pressure difference, the adjusting piston  23  was brought into a corresponding control position; it is connected with the control pressure regulator  20  over the pressure limiter  22 . Corresponding to the control position of the adjusting piston  23 , in turn, the conveyed volume flow adjusting arrangement  24  of the adjustable pump  18  is adjusted. Hence the conveyed volume of the adjustable pump  18  is controlled or regulated, that is over the control pressure difference value adjusted at the conveyed volume flow controller  20 . The control pressure difference at the control pressure regulator  20  can be adjusted by means of the pre-load spring  40  over the actuator  38 , the control pressure difference is adjustable and therewith can be controlled or adjusted or regulated. Thereby an adjustment of the actuator  38  at the conveyed volume flow controller  20  and thereby the conveyed volume of the adjustable pump  18  can be performed as a function of the signal delivered by the rotational speed sensor  34 . 
     Preferably for that purpose threshold values are implemented or deposited in memory in the electronic control unit  32 , on the basis of which a corresponding control program can be started, so that, for example, upon reaching a predetermined rotational speed of the engine  25  the pressure difference can be reduced further, in order to reduce the conveyed volume correspondingly and to limit the conveyed volume flow. 
     Now an operator can “level off” or “over steer” the predetermined threshold values over the input device  36 , so that a free determined control of the actuator  38  can be performed regardless of the rotational speed by means of the input device  36 . For example, the control pressure differential can be set upon a constant value by means of the input device  36 , where the control arrangement  32  performs the control of the actuator  38  regardless of the rotational speed of the drive engine  25 . The input device  36  then may include several designated switches or an input display or an adjustable potentiometer, with which adjustment magnitudes can be provided as input. Moreover, an activation or deactivation of the control of the conveyed volume flow controller  20  can be performed by means of the input device  36 . 
     As already mentioned the temperature sensor  37  detects the temperature of the hydraulic fluid and delivers a corresponding temperature signal to the control unit  32 . The control unit  32  can now change or control or regulate the control pressure difference at the conveyed volume flow controller by adjusting the actuator  38  as a function of the rotational speed alone or in combination with the temperature. In that way, the control pressure difference at the conveyed volume flow controller can be reduced or increased in addition as a function of the temperature of the hydraulic fluid of the hydraulic system  10 . Correspondingly control signals can be generated by control functions or control algorithms implemented in the control unit  32  as a function of the drive rotational speed and/or the temperature. 
     In an alternative embodiment of the invention shown in  FIG. 2 , in place of the electric motor  38  shown in  FIG. 1 , an electromagnetic proportional magnet  38 ′ is shown in  FIG. 2 . The proportional magnet  38 ′ is preferably effective in both directions where generally an adjustment of the conveyed volume flow controller  20  is entirely conceivable in only one direction, so that, for example, only one reduction of the control pressure difference is possible. 
       FIG. 3  shows an agricultural vehicle  44  in the form of a tractor that is equipped with a front loader  14  that is operated by a hydraulic system shown in  FIG. 1  or  2 . Here other applications are conceivable for the hydraulic system, according to the invention, for example for application in construction machines or telescopic loaders. The hydraulic system, according to the invention, can also be used for the supply of other hydraulic consumers not explicitly cited here, for example, for the supply of three point attachment arrangements for agricultural tractors. 
     While the present invention has been described in conjunction with a specific embodiment, it is understood that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, this invention is intended to embrace all such alternatives, modifications and variations which fall within the spirit and scope of the appended claims.