Abstract:
The invention relates to a drive ( 1 ) having an energy recovery function. The drive having an energy recovery function comprises a hydrostatic piston machine ( 9 ) and at least one storage element ( 13 ) which is connected to said hydrostatic piston machine. Said hydrostatic piston machine ( 9 ) and the at least one storage element ( 13 ) are connected together by means of a storage line. Said storage line is divided into a first storage line section ( 11 ) and into a second storage line section ( 12 ) by a throttle value unit ( 15 ). Said throttle valve unit ( 15 ) comprises a control pressure valve unit ( 16 ) and a built-in valve ( 17 ). The control pressure valve unit ( 16 ) produces a control pressure which acts upon the built-in valve ( 17 ).

Description:
BACKGROUND 
     The invention relates to a drive having an energy recovery function. 
     A drive having an energy recovery function is known from U.S. Pat. No. 6,712,166 B2. In the system proposed therein, pressure medium is conveyed on the one hand from a reservoir into a pressure store by a pump/motor arrangement as long as the pump/motor arrangement is driven by the mass inertia of the vehicle during a deceleration operation. In order to then also allow the vehicle to be braked by means of the hydrostatic drive, when the capacity limit of the pressure stores is reached, there is provision in the drive proposed therein to produce a flow resistance with the storage elements being bypassed and to convey pressure medium in a closed circuit by the pump/motor arrangement. 
     The system proposed therein has the disadvantage that a change-over is carried out from the store to the additional closed circuit when the storage elements have reached the capacity limit thereof and consequently can no longer be used for storing pressure energy whilst performing braking operations. The throttling is carried out in an additional circuit. However, it is not known from U.S. Pat. No. 6,712,166 B2 to provide a throttle valve unit which allows a throttle function in a store line between the pump/motor arrangement and the storage element. The provision of the two separate functions which must be linked to each other in a complex control system not only requires considerable adaptation work but also requires a significant level of structural complexity. This leads to high costs and the additional mechanical components which are required. 
     SUMMARY 
     The object of the invention is to provide a drive having an energy recovery function which, in addition to the braking power owing to the storage of energy, provides a controllable flow resistance for the hydrostatic piston engine which is operated as a pump. 
     According to one aspect of the present invention , the drive having an energy recovery function has a hydrostatic piston engine and at least one storage element which is connected thereto. The piston engine and the storage element are connected to each other by means of a store line. The store line is divided by a throttle valve unit into a first store line portion and a second store line portion. The throttle valve unit comprises a control pressure valve unit and an integrated valve, a control pressure which is adjusted by the control pressure valve unit acting on the integrated valve and thus allowing a variable throttling of the volume flow between the hydrostatic piston engine and the storage element. 
     According to claim  1 , the drive having an energy recovery function has a hydrostatic piston engine and at least one storage element which is connected thereto. The piston engine and the storage element are connected to each other by means of a store line. The store line is divided by a throttle valve unit into a first store line portion and a second store line portion. The throttle valve unit comprises a control pressure valve unit and an integrated valve, a control pressure which is adjusted by the control pressure valve unit acting on the integrated valve and thus allowing a variable throttling of the volume flow between the hydrostatic piston engine and the storage element. 
     The hydrostatic drive according to the invention has the advantage that the level of line complexity is low and in particular, even with the pressure store in a state of low charge, a high level of braking action is already enabled. In contrast to the drive known from the prior art, it is possible in this instance to increase the flow resistance, against which the hydrostatic piston engine conveys, using the throttle valve unit. This is necessary in particular when the pressure stores are still empty and therefore do not produce sufficient flow resistance. Owing to the provision of a control pressure using the control pressure valve unit, a simple integrated valve can be used to throttle the volume flow in the store line. The use of the control pressure as an adjustment value for the integrated valve further has the advantage that high forces can be produced at the valve and allow a rapid reaction. 
     In particular it is advantageous, on the one hand, to allow the control pressure to act on the integrated valve in one direction and, on the other hand, in the opposite direction, the supply pressure of the hydrostatic piston engine and the store pressure in the storage element. Depending on the control pressure produced by the control pressure valve unit, a constant counter-pressure is consequently established for the hydrostatic piston engine. Consequently, in particular as claimed according to a further preferred configuration, when an electrically activated pressure limitation valve is used to produce the control pressure, the counter-pressure for the hydrostatic piston engine can be adjusted in a simple manner. The control can then also advantageously be carried out by means of an electronic control unit which, in addition to the current travel status, can also take into account data entered by a user, such as, for example, brake pedal position or the like. 
     Furthermore, it is advantageous to supply the control pressure valve unit from a pressure medium source via a control pressure supply line and to depressurise this control pressure supply line into a tank volume via the pressure limitation valve. The adjustment is therefore carried out by discharging pressure from the control pressure supply line. In particular, the control pressure supply line can advantageously be connected to a supply-side connection of the hydrostatic piston engine by means of a throttle. According to another particularly preferred configuration, the connection of the control pressure supply line to the supply-side connection of the hydrostatic piston engine is carried out by means of a shuttle valve. Depending on the pressure relationships at the supply-side outlet of the hydrostatic piston engine and in the storage element, the control pressure supply line is connected either to the supply-side connection of the hydrostatic piston engine or to the storage element by the shuttle valve. 
     In order to prevent leakage through the electrically controllable pressure limitation valve, a switching valve is preferably arranged in the control pressure supply line upstream of the pressure limitation valve. The switching valve is also preferably electrically activated and is used to interrupt the control pressure supply line. A leakage through the pressure limitation valve which is arranged downstream is thus prevented. 
     In order to produce a control signal for the pressure limitation valve which is itself provided to produce the control pressure, an electronic control unit is preferably provided by means of which the pressure limitation valve can be electromagnetically controlled. The electronic control unit is connected, at the input side, to a pressure sensor which transmits a pressure value to the electronic control unit. The control signal is established by the electronic control unit in accordance with this measured pressure value and a desired pressure value which can be predetermined. The measured pressure value may be either, in the case of an adjustment of the integrated valve, the pressure produced at the supply-side connection of the hydrostatic piston engine or, if the integrated valve is controlled, the pressure in the storage element. 
     The integrated valve is preferably constructed in such a manner that it is acted on in the closing direction by the control pressure and a resilient force which acts in the same direction. In a manner already set out above, both the supply pressure of the hydrostatic piston engine and the store pressure of the storage element act in the opposite direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred configurations of the drive according to the invention are illustrated in the drawings and are explained in greater detail in the following description. 
         FIG. 1  is a first embodiment of a drive according to the invention; and 
         FIG. 2  is a second embodiment of a drive according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a hydraulic circuit diagram of a first embodiment of a drive  1  having an energy recovery function. In the embodiment illustrated, the drive  1  is provided for energy recovery in a travel drive  2 , for example, of a refuse collection vehicle or a fork lift truck. Such vehicles have intensive travel cycles, that is to say, braking and acceleration operations often closely follow each other. 
     The travel drive  2  comprises a differential  3  which acts on a first driven wheel  6  and a second driven wheel  7  by means of a first half-shaft  4  and a second half-shaft  5 . The differential  3  is connected to a hydrostatic piston engine  9  by means of an output shaft  8 . Further components which belong to the travel drive  2  are not illustrated in the drawings for reasons of clarity. The hydrostatic piston engine  9  may be either a separate piston engine which is used exclusively for energy recovery, or a hydraulic motor of a hydrostatic travel drive. The coupling to the differential  3  of the travel drive  2  by means of an output shaft  8  is also intended to be understood purely by way of example. 
     The hydrostatic piston engine  9  is connected to a pressure medium reservoir by means of an intake line  10 . In the most simple case, a pressure-free tank volume acts as a pressure medium reservoir. Alternatively, it is also possible to provide a hydraulic store as a pressure medium reservoir. In order to store kinetic energy during a braking operation, the hydrostatic piston engine  9  is operated as a pump owing to the mass inertia of the driven vehicle so that the hydrostatic piston engine  9  draws pressure medium from the pressure medium reservoir via the intake line  10 . The hydrostatic piston engine  9  conveys this pressure medium into a store line which is connected to the supply-side connection  14  thereof. The store line connects the hydrostatic piston engine  9  to a storage element  13 . The storage element  13  is, for example, a hydraulic membrane store. In particular, the storage element  13  is a high-pressure store. 
     The store line comprises a first store line portion  11  and a second store line portion  12 . The first store line portion  11  and the second store line portion  12  can be connected to each other by means of a throttle valve unit  15 . 
     The throttle valve unit  15  comprises a control pressure valve unit  16  in order to produce a control pressure. The control pressure provided by the control pressure valve unit  16  acts on an integrated valve  17 . The integrated valve  17  is constructed as a seat valve in the embodiment illustrated. A valve piston  18  is arranged so as to be able to be longitudinally displaced in the integrated valve  17 . The valve piston  18  has a control pressure face  19  on which the valve piston  18  is acted with a hydraulic force by the control pressure. In the same direction, the force of a valve spring  20  acts on the valve piston  18 . In the opposite direction, the sum of two hydraulic forces which is produced by the pressures in the first store line portion  11  and the second store line portion  12  acts on the valve piston  18 . To this end, the valve piston  18  has a first face  21  which can be acted on with pressure and a second face  22  which can be acted on with pressure. The first and the second faces  21 ,  22  which can be acted on with pressure are formed on the valve piston  18  by means of a stepped geometry of the valve piston  18 . The valve piston  18  which is preferably constructed in a rotationally symmetrical manner has, at the transition from the first face  21  which can be acted on with pressure to the second face  22  which can be acted on with pressure, a sealing edge  23  which co-operates in a sealing manner with a sealing seat  24  as long as the integrated valve  17  is in the closed position thereof. Owing to the valve spring  20 , the valve piston  18  is retained in a pressure-free state in the closed position thereof. 
     In order to produce a control pressure at the control pressure face  19 , a control pressure supply line  25  is connected to a shuttle valve  26 . The shuttle valve  26  connects the control pressure supply line  25  either to a supply pressure branch line  27  or to a store pressure branch line  28  as a pressure medium source. Depending on the pressures in the supply pressure branch line  27  and the store pressure branch line  28 , the control pressure supply line  25  is connected in each case to the line which has the greater pressure. A throttle  29  which is fixedly set or can be adjusted is preferably arranged within the control pressure valve unit  16  in the control pressure supply line  25 . 
     The control pressure supply line  25  is connected to the integrated valve  17  by means of a connection line  30  so that the pressure in the control pressure supply line  25  is supplied to the control pressure face  19  and acts on the valve piston  18  with a hydraulic force in the closing direction. In order to adjust the level of the pressure in the control pressure supply line  25 , a switching valve  31  and a controllable pressure limitation valve  32  are arranged in the control pressure supply line  25 . Using the switching valve  31 , with appropriate activation of an electromagnet  33 , the connection from the control pressure supply line  25  to the pressure limitation valve  32  can be completely disengaged in order to prevent a leakage flow through the pressure limitation valve  32 . In the idle position thereof, the switching valve  31  is acted on in the direction of this opened position by a pressure spring  34 . If the level of the control pressure in the control pressure supply line  25  is intended to be adjusted by the pressure limitation valve  32 , which is constructed so as to be adjustable, the electromagnet  33  is activated. When the electromagnet  33  is supplied with electrical power, the switching valve  31  is moved into the second switching position thereof in which a connection through which fluid can flow is produced from the control pressure supply line  25  to the pressure limitation valve  32 . 
     The pressure limitation valve  32  can be controlled by means of an electromagnet  37 . The electromagnet  37  is preferably a proportional magnet which is acted on with a control signal, whereby the opening pressure of the pressure limitation valve  32  is adjusted. Counter to the force of the electromagnet  37 , there acts on the pressure limitation valve  32  a hydraulic force which is produced on a measuring face by an input pressure of the pressure limitation valve  32  which is supplied by means of a measuring line  38 . The input pressure corresponds to the pressure of the control pressure supply line  25  when the switching valve  31  is open. If this hydraulic force which is produced by the pressure supplied from the input side of the pressure limitation valve  32  exceeds the force of the electromagnet  37  acting in the opposite direction, the pressure limitation valve  32  is displaced in the direction of the open position thereof In the open position of the pressure limitation valve  32 , the control pressure supply line  25  is connected to a depressurisation line  35 . The depressurisation line  35  opens in a tank volume  36 . 
     Depending on the resulting force on the pressure limitation valve  32 , a throttled connection is consequently produced between the control pressure supply line  25  and the depressurisation line  35 . Consequently, a pressure drop is brought about at the throttle  29  with the result that there is an adjustable control pressure on the integrated valve  17  at the control pressure face  19  thereof. 
     During a braking operation, pressure medium is conveyed from the intake line  10  into the first store line portion  11  by the hydrostatic piston engine  9  in a manner which has been described above. The pressure in the store line portion  11  acts on the face  21  which can be acted on with pressure. At the same time, the pressure in the storage element  13  acts on the second face  22  which can be acted on with pressure. If the sum of the hydraulic forces produced in this manner is greater than the sum of the force of the valve spring  20  and the hydraulic force at the control pressure face  19  acting in the opposite direction, the integrated valve  17  is moved in the direction of the open position thereof and the storage element  13  is filled with pressure medium. 
     As the pressure in the storage element  13  increases, the hydraulic force at the second face  22  which can be acted on with pressure also increases. Consequently, the integrated valve  17  is moved further in the direction of the open position thereof so that the throttling action at the integrated valve  17  is reduced. Consequently, owing to the increasing opening of the integrated valve  17  and the resulting lower throttle effect between the sealing edge  23  and the sealing seat  24 , the higher counter-pressure of the storage element  13  is taken into account. In a preferred configuration, the counter-pressure which is increasing owing to the pressure increase in the storage element  13  is compensated for in a precise manner by the decreasing throttling by the integrated valve  17 . For a constant control pressure at the control pressure face  19 , the vehicle is consequently subjected to a constant braking effect regardless of the respective charge state of the storage element  13 . 
     The level of the respective control pressure is determined by a control signal which is supplied to the electromagnet  37  of the pressure limitation valve  32  via a signal line  39 . The signal line  39  connects the electromagnet  37  to an electronic control unit  40 . The electronic control unit  40  establishes a value for the control signal which is supplied to the electromagnet  37  via the signal line  39 . As an input variable, the electronic control unit  40  uses at least the pressure signal of a pressure sensor  41 . The pressure sensor  41  is connected to the electronic control unit  40  via a sensor line  46 . In the first illustrated embodiment of  FIG. 1 , the pressure sensor  41  is arranged in the second store line portion  12 . The measured pressure value which is supplied to the electronic control unit  40  consequently corresponds to the pressure in the storage element  13 . Consequently, the system is controlled since it takes into account only the pressure in the storage element  13  but not the pressure produced at the supply side of the hydrostatic piston engine  9 . As an additional input variable, it is possible to determine, for example, a desired value or a brake pedal position via an additional signal line  42 . 
     In order to make the throttle valve unit  15  secure, a first safety valve  43  is provided in the control pressure valve unit  16  and connects the line  30  to the depressurisation line  35 , as long as the control pressure exceeds a critical value determined by the safety valve  43 . 
     A second safety valve  44  is connected to the second store line portion  12  and depressurises the store line portion into the tank volume  36 , as long as a critical pressure value is exceeded in this instance. A third safety valve  45  is connected to the first store line portion  10  and depressurises this and consequently the supply side of the hydrostatic piston engine  9  into the tank volume  36  if a critical pressure value is exceeded in the first store line portion  11 . As an alternative to the illustration with three safety valves  43 - 45 , it is also possible to dispense with the safety valves  43  and  44 . The securing is then carried out by means of the third safety valve  45 . In the event of an inadmissibly high increase in the control pressure, the integrated valve  17  is moved into the closed position thereof. Consequently, the supply-side pressure increases in the first store line portion  11  which leads to the third safety valve  45  opening. 
       FIG. 2  illustrates a second embodiment of the drive  1 ′ according to the invention. The elements which correspond to the elements of  FIG. 1  are given the same reference numerals. In order to prevent unnecessary repetition, this is not described again in its entirety. 
     In contrast to  FIG. 1 , a pressure sensor  41 ′ is arranged in the first store line portion  10 . Consequently, the pressure in the storage element  13  is not used as an input variable for the electronic control unit  40 , but instead the pressure produced by the hydrostatic piston engine  9  in the first store line portion  11 . In contrast to the control of the embodiment according to  FIG. 1 , the embodiment according to  FIG. 2  involves an adjustment since the actual counter-pressure produced for the hydrostatic piston engine  9  is taken into account when establishing the control signal for the electromagnet  37  of the pressure limitation valve  32 . Accordingly, owing to the new supply pressure of the hydrostatic piston engine  9 , a correction of the control signal is brought about directly and is transmitted to the control signal line  39  by the electronic control unit  40 . 
     The present invention is not limited to the embodiment illustrated. In particular, individual features of the embodiments illustrated can be combined with each other.