Abstract:
The invention relates to a hydraulic system, having a hydrostatic, valve-controlled piston engine with a plurality of valves actuatable by an actuator as a function of the motion of the pistons, and with a control unit for triggering the actuators, which is arranged for generating an electrical attraction current in a first time segment and a maintenance current in a second time segment. The object of the invention is for the valves to switch with quick reactions and function safely, so that the expected functionality and safe operation of the hydrostatic piston engine under various operating conditions is ensured. This is attained in that the control unit includes a current regulating device, which triggers the actuators in current-regulated fashion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on German Patent Application 10 2009 056 776.3 filed on Dec. 3, 2009. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention is based on a hydraulic system. 
         [0004]    2. Description of the Prior Art 
         [0005]    Such hydraulic systems have a hydrostatic piston engine, whose volume flow is continuously variably adjustable via a valve controller. The hydrostatic piston engine is a plurality of pistons, movable back and forth periodically in cylinders, each piston defining one work chamber whose volume varies with the stroke of a piston and which can be made to communicate with a low-pressure connection via a low-pressure valve and with a high-pressure connection via a high-pressure valve. At least the low-pressure valves are actuated by means of an actuator, which in turn is triggered by a control unit. The valves must be capable of being switched highly dynamically, so that the work chamber can be very quickly blocked off or opened for a flow through it. To enable activating these valves as fast as possible, it is necessary upon actuation to have the largest possible attraction current flow through the actuator. The high actuation current produces a magnetic force in the actuator that is proportional to the current and that can mechanically actuate the valve. The high attraction current, however, leads to a corresponding power loss in the ohmic resistor of the actuator, which can heat the valve severely and impermissibly if triggering lasts a relatively long time. For maintaining the switching position of the valve, all that is needed in addition is a comparatively low maintenance current. Typically, the triggering of the valves is effected by means of a change in the voltage that is applied at the valve. A voltage change profile is specified in the control unit by software-based pilot control, and the pilot control in turn is based on experimentally ascertained data. Thus the voltage change profile includes an activation voltage profile for the attraction phase of the actuators as well as a reduced maintenance voltage profile for the maintenance phase. 
         [0006]    The attraction current resulting from the activation voltage, and the maintenance current that varies with the temperature of the valve, can assume high values in an uncontrolled way. High current levels, or overswings in the current course, among other effects cause damage to electronic components or cable connections, for instance from overloading or thermal overheating. Moreover, imprecisions arise in the switching times of the valves, which depend strongly on the operating conditions at the time. That in turn is critical for the safety and sturdiness of the system 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0007]    It is the object of the invention to further develop a hydraulic system such that the valves switch with fast reactions and functionally safely, so that the expected functionality and safe operation of the hydrostatic piston engine is ensured under various operating conditions. 
         [0008]    In the hydraulic system of the invention, the control unit includes a current regulating device, which triggers the actuators in current-regulated fashion. 
         [0009]    By means of the apparatus according to the invention, the advantage is obtained that the actuator current is regulated, and thus from the detection of the actuator current course, the exact switching time is ascertained, and thus fast reaction times of the valve can be attained. 
         [0010]    For perfect function of a valve-controlled hydrostatic piston engine, it is essential that errors such as short circuits, line interruptions, and overloading or overheating, be detectable reliably and quickly. Sources of error are quickly detected by read-out of the actuator current for a higher-order plausibility check with either the model value or the expected value, and permit fast counter-control for safe, sturdy operation. 
         [0011]    By limiting the actuator current to a maximum value, it is unnecessary to design a control unit for higher currents, which leads to a cost saving. A regulated current also lead to correspondingly less lost heat, so that the components and possibly still other component groups combined in a housing are subject to less temperature stress. 
         [0012]    Advantageously, the current-regulated triggering of the actuator is effected in the second time segment during the maintenance phase. In the actuated state of the valve, the maintenance current for maintaining the switching position is relatively slight. A relatively small regulated current also leads to correspondingly less thermal power loss. Reducing the power loss leads to structural compactness of the control unit. Regulating the actuator current can also be done during the first time segment or in both time segments. By precise monitoring of the current, both excessively high current levels and overswings in the current course are avoided. As a result of the regulation of the current course, the operation of the valve-controlled hydrostatic piston engine functions exactly as a result of the precise ascertainment of the switching points as a function of the current course, as well as safely, since overly high current levels and overswings in the current course are avoided. 
         [0013]    In an especially preferred feature of the present invention, the current-regulated triggering of the actuator is effected by means of a clocked trigger voltage output by the control unit. This means that the attraction and/or maintenance voltage is varied in its effective voltage value by pulse width modulation. This has the advantage that the effective voltage value, based on a basic voltage such as the battery voltage, can be adjusted solely by pulse width modulation. Instead of the direct output of the clocked trigger voltage, the control unit can send an ON or OFF signal via a communications interface, and downstream electronic components convert these control commands into digital or PWM signals. 
         [0014]    In an advantageous embodiment of the apparatus of the invention, for simple measurement of the actuator current, the current regulating device has a measuring resistor or shunt downstream of the actuator. The shunt is a low-impedance resistor, whose detected voltage drop furnishes the actual current value of the actuator. 
         [0015]    Moreover, the current regulating device has a differential amplifier, which detects the voltage drop applied via the measuring resistor and furnishes a differential amplifier voltage which corresponds to the actual current value of the actuator. 
         [0016]    Preferably, the current regulating device includes a current regulator, connected downstream of the differential amplifier, that compares the output signal of the differential amplifier, as an actual current value, with a set-point current value and controls the voltage supply to the actuator as a function of the differential current. An alternative to this purely electronic hardware version is a microprocessor with software stored in it for read-in of the actual current value, comparing it with the set-point value, and controlling or regulating the actuator current by evaluation of the differential current. 
         [0017]    In an especially preferred feature of the present invention, the current regulating device has first switching elements and a pulse width modulator for triggering the switching elements. Current regulation can be attained economically, without software regulation, by means of minimum and maximum current regulation thresholds via a current regulator output signal, in that first switching elements are triggered via pulse counter modulation, as a function of the different current thresholds. 
         [0018]    The PWM output signal of the current regulator can be carried to these switching elements, which for instance are field effect transistors. 
         [0019]    If the current regulating device is implemented as an integrated circuit, further improvements are obtained with regard to diagnostic capabilities. In addition, the space required for the electronic components is reduced, and the thermal management is improved. 
         [0020]    It may be advantageous to accomplish the communication within the overall control unit via a bus system. As a result, additional economies of space and error-free data transmission can be achieved. 
         [0021]    Preferably, the control unit includes a voltage-increasing device, which generates a higher voltage from an operating voltage of the control unit, and that is supplied to the control unit. As a result, the attraction phase of the actuator can be shortened considerably. 
         [0022]    In an advantageous feature of the apparatus of the invention, the voltage-increasing device has a boosting circuit, which particularly in response to a corresponding trigger signal charges a buffer memory to a higher voltage. The boosting circuit proves advantageous because in that case only one voltage source, the operating voltage, is required. Since the increased voltage is available only for at least the duration of the attraction phase, the energy consumption of the boosting circuit is quite low. Moreover, the buffer memory version is simple and favorable to implement. 
         [0023]    Preferably, the buffer memory is switched to the actuator by second switching elements during the attraction phase. As a result, the increased voltage is switched to the actuator only for the duration of the attraction phase. 
         [0024]    If the current regulating device, for regulating the current of the actuator, triggers the second switching elements during the attraction phase, then the actuator current can be regulated during the attraction phase as well. 
         [0025]    Because the current regulating device, for regulating the current of the actuator, can trigger the first switching elements during the maintenance phase, the actuator current can be regulated during the maintenance phase as well. 
         [0026]    The invention is not limited to a purely electronic hardware version. In general, the implementation or representation of the current regulation is dependent on the particular application. If inexpensive versions are of primary importance, if the demands from the outset are virtually static, and if hardware components without relatively expensive microprocessors and software are already available, then the purely hardware version is certainly to be preferred. However, if the demands are dynamic and have to be expandable, then implementation becomes more expensive, and possibly other or additional components (such as microprocessors and software) must be employed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0027]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawings, in which: 
           [0028]      FIG. 1  is a highly schematic illustration to explain the functional principle of a valve-controlled hydrostatic piston engine with a variable volume flow; and 
           [0029]      FIG. 2  is a schematic circuit diagram of one embodiment of the current regulating device of the invention in a control unit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    First, the functional principle of a valve-controlled piston engine  1 , whose displacement/absorption volume is digitally adjustable (DDU), will be explained in conjunction with  FIG. 1 . The piston engine described, in the exemplary embodiment shown, is embodied as an axial piston engine  1  of the swash plate type, and  FIG. 1  shows a very highly simplified developed view of it. In the ensuing description, the piston engine  1  is described as a hydromotor; however, in principle the descriptions of the hydromotor pertain accordingly to a pump with an adjustable displacement volume. 
         [0031]    In the schematic view in  FIG. 1 , the piston engine  1  has a cylinder drum  2 , in which a plurality of cylinder bores  4  are embodied, in each of which one piston  6  is guided axially displaceably. Each of the pistons  6 , with the cylinder bore  4 , defines a work chamber  8  whose volume is independent of the stroke of the pistons  6 . These pistons are each braced via a respective piston shoe  10  on an obliquely positioned swash plate that is connected to a power takeoff shaft  12 . In the view in  FIG. 1 , the control curve  14  formed because of the rotation of the swash plate is shown, which reflects the dependency of the piston stroke and thus of the volume of the particular work chamber on the angle of rotation. As shown on the right in  FIG. 1 , each work chamber  8  communicates via an inlet valve  16  with an inlet line  18  common to all the work chambers  8 , and that line is subjected to system pressure or high pressure. This high pressure can be generated for instance via a pump  20 . 
         [0032]    Each work chamber  8  furthermore communicates via an outlet valve  22  with an outlet line  24 , likewise common to all the work chambers  8 , which discharges into a tank  26 . 
         [0033]    In the exemplary embodiment shown, the outlet valves  22  and the inlet valves  16  are each embodied as electrically openable and closable check valves. The inlet valve  16 , in its basic position shown, is prestressed into a closing position via a spring, not shown, and can be put into an open position by the supply of current to a magnet actuator  28 , so that the pressure medium can flow out of the inlet line  18  into the respective work chamber  8 . The outlet valve  22 , in its basic position shown, is prestressed in the open direction via a spring. By the supply of current to a magnet actuator  30 , this outlet valve  22  can be put into a closed position, in which the pressure fluid cannot flow out of the work chamber  8 . The triggering of the magnet actuators  28 ,  30  is effected via a control unit  34 , by way of which various modes (full mode, partial mode, idle mode) can be set, so that the absorption volume of the piston engine  1  is continuously variably adjustable, and by suitable triggering of the valves  16 ,  22 , the pulsation can be reduced to a minimum as well. In the exemplary embodiment shown, the triggering of the valves  16 ,  22  is effected as a function of the rpm of the power takeoff shaft  12 , which is detected via an rpm pickup  36  and reported to the overall control unit  34  via a signal line. In principle, it is understood that still other characteristic data, such as the torque acting on the power takeoff shaft  12 , the absorption volume of the piston engine  1 , or the angle of rotation of the swash plate, can be taken into account in the triggering of the valves  16 ,  22 . 
         [0034]      FIG. 2  shows the basic construction of a control unit  40 . This control unit  40  includes, among other elements, a current regulating device  42  for regulating the current of a coil  44  of a solenoid valve, not shown in further detail. 
         [0035]    The coil  44  is connected to ground  54 , on its end toward ground, via a measuring resistor  46  and a field effect transistor  48 , which transistor is switched on and off by means of an external trigger signal  50 , via a gate trigger circuit  52 . A differential amplifier  56  detects the voltage drop applied via the measuring resistor  46  and delivers the corresponding measured value, which is additionally conducted to the outside as a signal  58 , to a regulating amplifier  60 . This amplifier, taking into account an external set-point value signal  62 , triggers two field effect transistors  64  and  66  in such a way that the voltage, supplied via a diode  68  to the supply-voltage-side end of the coil  44 , corresponds to the desired set-point value. 
         [0036]    The field effect transistors  64  and  66  are triggered in pulse width modulated fashion by the regulating amplifier  60 , so that the coil  44  either is connected via the field effect transistor  64  to the supply voltage or, via the field effect transistor  66 , contacts the ground terminal  54 . 
         [0037]      FIG. 2  also shows a voltage-increasing device  80 . The voltage-increasing device  80  has a boosting circuit  82 , which on the input side receives an external set-point value signal  84 , and during that time generates a higher voltage U H =60 V, compared to the operating voltage U B =24 V. This higher voltage serves to charge a downstream capacitor  86 , which is connected to a ground terminal  88 . The voltage U H  furnished by the capacitor  86  is dimensioned such that during the attraction phase, if a field effect transistor  90 , which is connected between the capacitor  86  and the coil and which is followed by a diode  92  connected in series with it, is made conducting as a function of a gate trigger circuit  94 . The gate trigger circuit  94  is triggered by an external set-point value signal  96 . The coil  44  is connected on the ground side to the voltage-carrying end of the capacitor  86  via a diode  98 . A freewheel diode  100  protects the field effect transistor  48  from voltage peaks upon being shut off. 
         [0038]    In operation, the boosting circuit  82 , before the attraction phase of the valve, is supplied with an external set-point value signal  84 , so that the boosting circuit  82 , over a period of time predetermined by the duration of the signal  84  on the input side, charges the capacitor  86  with the higher voltage U H =60 V. During the attraction phase of the valve, the external set-point value signal  96  is applied to the gate trigger circuit  94 , which makes the field effect transistor  90  conducting, so that the coil  44  is subjected to the voltage U H  applied to the capacitor  86 . During this attraction phase, the capacitor  86  discharges, with a time constant which is predetermined by its capacitance. The maintenance phase of the valve begins after the termination of the triggering of the gate trigger circuit  94  and the end of the resultant blocking of the field effect transistor  90  with the external set-point value signal  62  to the regulating amplifier  60 , so that during this maintenance phase, the coil is supplied with a regulated voltage via the field effect transistors  64  and  66 . The regulated voltage is a clocked voltage, which alternates between operating voltage and ground potential. It results from the pulse width modulated triggering of the field effect transistors  64 ,  66  by the regulating amplifier  60 , whose trigger signals depend on the difference between the set-point current value and the actual current value of the coil  44 . 
         [0039]    When the attraction current in the coil  44  is regulated, an output signal  63  of the regulating amplifier  60  leads to the gate trigger circuit  94 , so that the field effect transistor  90  is triggered, as a function of the actual current of the coil  44  and of the set-point value from set-point value signal  62 . As a result, excessively high currents are avoided during the attraction phase. 
         [0040]    For the invention, a clocked trigger voltage is not compulsory for the current-regulated triggering of the actuators. To accomplish the fastest possible buildup of the magnetic field, the trigger voltage can be switched on or off in unclocked fashion accordingly in the attraction phase or at the end of the maintenance phase. 
         [0041]    The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.