BACKGROUND OF THE INVENTION 
       [0001]    The invention is directed to a hydraulic system. 
         [0002]    Hydraulic systems of that type include a hydrostatic piston machine, the volumetric flow of which can be steplessly adjusted using a valve control. The hydrostatic piston machine includes a plurality of pistons that can move in a reciprocating manner in cylinders, each piston limiting a working space having a volume that changes with the stroke of a piston and that can be connected via a low-pressure valve to a low-pressure connection, and via a high-pressure valve to a high-pressure connection. The low-pressure valves, at the least, are actuated using an actuator which, in turn, is activated by a control unit. It must be possible to switch the valves highly dynamically so that the working space can be blocked very rapidly or released for through-flow. The functionality, the essential properties such as volumetric flow, pulsation, torque output, and leakage of the piston machine that includes such a valve are highly dependent on the closing points of the valves. The functionality of the unit is influenced considerably by varying operating conditions. 
         [0003]    Since the operating conditions of the piston machine are not known, however, the exact switching point at which the valves must be actuated is difficult to find. Faulty states of the piston machine result, which are caused by inept actuation of the valves. For example, the cylinders may not be decompressed if the high-pressure valves are not deactivated at the right time. 
         [0004]    By determining the exact switching points, it is possible to avoid supplying current to the valves unnecessarily and for an excessive duration. An additional electrical loss caused by supplying current for an excessive duration can thereby be prevented. 
       SUMMARY OF THE INVENTION 
       [0005]    The problem addressed by the invention is that of further developing a hydraulic system in a manner such that the expected functionality of the hydrostatic piston machine is ensured under different operating conditions. 
         [0006]    In a hydraulic system according to the invention, the switching points at which the valves are actuated are dependent on operating conditions. Switching the valves in a manner that is controllable depending on the operating conditions ensures that the hydrostatic piston machine will function correctly. The valves achieve specific states at certain working points of the piston. Adapting the switching of the valves on the basis of the particular operating condition prevents faulty states of the piston machine from occurring and fundamentally improves efficiency. 
         [0007]    The switching points are preferably dependent on the rotational speed of the drive shaft. The sensing of the rotational speed can be realized particularly easily using a velocity sensor. The exact dependencies of the switching points of the valves on the rotational speed of the drive shaft are determined on the basis of physical models of the system. 
         [0008]    According to a particularly preferred embodiment of the present invention, the switching points for actuating the valves are dependent on temperature. The working conditions of a hydrostatic piston machine change with the temperature. At higher temperatures, the viscosity of the hydraulic fluid decreases, thereby making it more fluid and thinner. As a result, the valves can switch more rapidly, thereby enabling the switching points for actuating the valves to occur slightly later. Analogously, when viscosity is high, the switching point can occur slightly earlier. The temperature of the hydraulic fluid can be sensed in a simple, cost-effective manner using a measuring device that outputs signals to the control unit. The exact dependencies of the switching points of the valves on the temperature are determined on the basis of physical models of the system. 
         [0009]    It can be advantageous to make the switching points dependent on the pressure in the high-pressure line or the low-pressure line. Pressure can then be measured using a pressure-measuring device. The pressure-measuring device can be realized easily and cost-effectively, and outputs signals to the control unit which, in turn, activate the valves as a function of pressure. The dependence of the switching time of the valves on pressure is determined on the basis of physical models of the system. 
         [0010]    The switching points are preferably dependent on error functions. This means that targeted error detection and counteractive control can be realized by adaptively adjusting the switching points. 
         [0011]    According to a particularly preferred embodiment of the present invention, the switching points for actuating the valves are determined using a calculation using physical models of the system. In so doing, the entire system, including the relevant properties, are simulated in a simulation model under varying operating conditions. The various operating-condition scenarios are investigated in the simulation to determine the optimal switching points for actuating the valves (offline calculation). The models can also be programmed in the control unit, and the switching points for the actual operating point can be calculated directly by the control unit (online calculation). 
         [0012]    It can be advantageous to use an observer that is disposed in the control unit. It is then possible to determine the system state via observation on the basis of a few system variables detected using measurement technology in order to form a model. The model is used as the basis for calculating the exact switching times for actuating the valves. By comparing actual states with the expected states calculated by the observer, more exact statements regarding the switching point can be attained. 
         [0013]    The state observer includes a model of the system from which the expected switching points are obtained, and a regulator that changes the switching points depending on the measurable variables. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    An embodiment of a hydraulic system according to the present invention is shown in the drawings. The invention will now be explained in greater detail with reference to these drawings. 
           [0015]      FIG. 1  shows a highly schematicized depiction to explain the operating method of a valve-controlled hydraulic piston machine having variable volumetric flow; and 
           [0016]      FIG. 2  shows two graphs of piston stroke as a function of the angle of rotation of the rotational axis at operating conditions B A  and B B  with the switching points, which are based on static program maps, according to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    In the following, the operating method of a valve-controlled piston machine  1  having a digitally adjustable pump capacity/intake volume will be explained with reference to  FIG. 1 . These piston machines are also referred to Digital Displacement units (DDU). In the embodiment shown, the piston machine that is described is designed as an axial piston machine in swash plate design, wherein  FIG. 1  shows a highly simplified variant. In the description that follows, piston machine  1  is described as a hydraulic motor, although the descriptions of the hydraulic motor basically likewise apply to a pump having a variable pump capacity. 
         [0018]    According to the schematic depiction in  FIG. 1 , piston machine  1  includes a cylinder drum  2 , in which a large number of cylinder bores  4  is formed, each of which contains an axially displaceable piston  6 . Together with cylinder bore  4 , each piston  6  limits a working space  8 , the volume of which is independent of the stroke of piston  6 . Each piston  6  bears via a piston shoe  10  against a slanted swash plate that is connected to an output shaft  12 . Control curve  14  formed by the rotation of the awash plate is shown in the depiction in  FIG. 1 ; control curve  14  shows how the piston stroke and, therefore, the volume of the particular working space are dependent on the angle of rotation. As shown on the right in  FIG. 1 , each working space  8  is connected via an inlet valve  16  to a supply line  18  that is common to all working spaces  8 , and to which a system pressure or high pressure is applied. This high pressure can be created e.g. using a pump  20 . 
         [0019]    Moreover, each working space  8  is connected via a drain valve  22  to a drain line  24 , which is likewise common to all working spaces  8 , and which leads into a tank  26 . In the embodiment shown, drain valves  22  and inlet valves  16  are designed as electrically releasable and blockable non-return valves. Inlet valve  16  is preloaded in its home position shown into a closed position via a not-shown spring, and can be moved into an opened position by applying current to a solenoid actuator  28 , thereby allowing the pressure medium to flow out of inlet line  18  into particular working space  8 . In its home position shown, drain valve  22  is preloaded into the opened position using a spring. By supplying current to solenoid actuator  30 , drain valve  22  is moved into a blocking position in which the pressure medium cannot flow out of working space  8 . Solenoid actuator  28 ,  30  is activated by a control unit which is used to set the different modes (full mode, partial mode, idle mode), and so the intake volume of hydraulic motor  1  is steplessly adjustable, wherein the pulsation can also be reduced to a minimum by activating valves  16 ,  22  in a suitable manner. In the embodiment shown, valves  16 ,  22  are activated depending on the rotational speed of output shaft  12 , the rotational speed being detected using a rotational speed sensor  36  and reported via a signal line to control unit  34 . In principle, other characteristic data such as the torque acting on output shaft  12 , the intake volume of hydraulic motor  1 , or the angle of rotation of the swash plate can be taken into account, of course, in the activation of valves  16 ,  22 . 
         [0020]      FIG. 2  shows two graphs of piston stroke as a function of the angle of rotation of the rotational axis at operating conditions A and B with resultant switching points SP A  and SP B , which are determined based on static program maps. Piston stroke graph B corresponds to piston stroke graph A since it is based on the angle of rotation which remains the same at different operating conditions A and B. 
         [0021]    In piston stroke graph A, given operating conditions B A , switching point SP A  results from the setpoint assignment by the control unit in that activation of current supply I A  to valve A at time t, which corresponds to the time for magnetization plus the travel time of the valve, occurs earlier. Switching point SP A  for actuating valve A takes place at the desired instant. 
         [0022]    If operating conditions B B  occur, however, time t increases by Δt due to the changed operating conditions B B , and switching point SP B FOR  actuating valve B is delayed by Δt since current is supplied to valve I B  at the same time Analogously, current is supplied by Δt too soon if time t shortens. 
         [0023]    In the hydraulic system according to the invention, switching point SP E , at which valve B is actuated is dependent on operating condition B B , and therefore current supply I B′  to valve B starts at a point in time which results from a physical model having operating condition B B , and switching point SP B  starts at the desired point in time. 
         [0024]    It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above. 
         [0025]    While the invention has been illustrated and described as embodied in a device having a torque-limiting unit, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
         [0026]    Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention. 
         [0027]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  Piston machine 
           2  Cylinder drum 
           4  Cylinder bore 
           6  Piston 
           8  Working space 
           10  Piston shoe 
           12  Output shaft 
           14  Control curve 
           16  Inlet valve 
           18  Supply line 
           20  Pump 
           22  Drain valve 
           24  Drain line 
           26  Solenoid actuator 
           30  Solenoid actuator 
           34  Control unit 
           36  Rotational speed sensor 
         B A  Working condition A 
         B B  Working condition B 
         K A  Piston stroke graph A 
         K B  Piston stroke graph B 
         SP A  Switching point AA 
         SP B  Switching point B 
         I A  Current supplied to valve A 
         I B  Current supplied to valve B 
         I B′  Current supplied to valve B according to the invention 
         t Time for magnetization plus travel time of the valve 
         Δt Time for magnetization plus travel time of the valve
A hydraulic system includes a hydrostatic piston machine having a plurality of valves that can be actuated using an actuator depending on the motion of the pistons and a control unit for activating the actuators. The switching points at which the valves are actuated are dependent on operating conditions, so that the expected functionality and the essential properties of the hydrostatic piston machine are ensured under different operating conditions.