Patent Publication Number: US-7219591-B2

Title: Hydraulic control arrangement

Description:
The invention relates to a hydraulic control arrangement for controlling a consumer in accordance with the preamble of claim  1 . 
   From DE 100 45 404 C2 a control arrangement is known, wherein a hydraulic consumer, for instance a double-acting cylinder for moving a load, may be supplied with pressure medium via a continuously adjustable directional control valve. In the pressure medium supply to the cylinder and in the drain from the cylinder, respective releasable cut-off valves are provided, wherein the supply-side cut-off valve is taken into an opened position by the pressure downstream from the directional control valve. By actuation of a topping piston, the drain-side cut-off valve may be taken into an opened position that allows the pressure medium to drain from the consumer towards the directional control valve. In this solution, a drain regulation is effected via the drain-side cut-off block by feeding back the drain pressure to the topping piston of the cut-off block in front of the spool control edge of the continually adjustable directional control valve determining the drain. 
   In DE 199 31 142 C2 a control arrangement is disclosed wherein a supply regulation takes place via an individual pressure compensator arranged upstream of the regulating valve. This individual pressure compensator is subjected to the force of a spring in the opening direction and to the pressure in the supply in the direction towards the consumer. 
   DE 36 39 174 C2 discloses a control arrangement for a single-acting hydraulic consumer, wherein a continually adjustable directional control valve is preceded by a LS (load-sensing) pressure compensator which is subjected to the force of a spring and to the individual load pressure in the closing direction, and to the pressure between the pressure compensator and the directional control valve in the opening direction. The known control arrangement further comprises a load-compensated, releasable cut-off valve whereby a drain regulation may be carried out. 
   In DE 102 16 958 a hydraulic control arrangement is shown wherein upstream from a supply measuring orifice a LS pressure compensator is provided with is subjected to the force of a spring and to the highest pressure downstream from the supply measuring orifice and upstream from a drain measuring orifice in the opening direction. In the closing direction, the pressure upstream from the supply measuring orifice acts on the LS pressure compensator. 
   All of the above described practical examples share the drawback that the control edges (supply control edge, drain control edge) determining the supply cross-section and the drain cross-section of the measuring orifices must be adapted to each other with extreme accuracy. 
   It is furthermore a drawback in these solutions that either only the pressure medium volume flow to the consumer or the volume flow draining from the consumer may be controlled in a load-independent manner. Moreover particularly in the control of double-acting cylinders in the event of so-called pulling loads—namely, of loads where the pressure in the drain is higher than in the supply—there is a risk of an insufficient supply of the supply-side cylinder chamber. Such an insufficiency may lead to cavitations causing damage to the consumer or to the hydraulic switching elements associated with the latter. Such an operating condition may occur, e.g., during downhill travel or whenever a load is initially raised, then overcomes a dead center, and subsequently exerts a pull on the hydraulic consumer. 
   In order to avoid such insufficient supply of the consumer, it is, e.g., possible to use anti-cavitation valves. Owing to the comparatively low differential pressure between the suction side and the tank pressure during replenishing, however, these valves need to have a very large cross-section. 
   One alternative possibility is to provide biasing valves in the pressure medium drain. Such biasing valves are, however, accompanied by a high energy loss, for the supply pressure, particularly with small loads, must be raised strongly. 
   Another option is to use brake valves. These do, however, equally require a comparatively high pressure on the supply side in order to control the volume flow on the drain side and thus avoid an insufficiency of the supply side. 
   In other words, these known options for avoiding an insufficient supply (anti-cavitation valve, biasing valve, countertorque lowering valve) require considerable complexity in terms of circuit technology and moreover incur energy losses. 
   In contrast, the invention is based on the object of furnishing a hydraulic control arrangement for controlling a consumer, whereby an insufficient supply may be avoided with low expense, and wherein the pressure medium volume flow to the consumer and the pressure medium volume flow draining from the consumer may be controlled in a load-independent manner. 
   This object is achieved through a hydraulic control arrangement having the features of claim  1 . 
   The hydraulic control arrangement in accordance with the invention is executed with an adjustable supply measuring orifice and an adjustable drain measuring orifice, wherein a pressure compensator arranged in the pressure medium delivery is subjected in the opening direction to a constant force and in the closing direction to the lower one of the pressures downstream from the supply measuring orifice and upstream from the drain measuring orifice. In the pressure medium drain a releasable cut-off block is provided which acts as a check valve in the direction of supply and regulates the drain pressure medium volume flow in the pressure medium drain. 
   Owing to the interaction of the pressure compensator and of the load-compensated cut-off block, both the pressure medium volume flow towards the consumer and the pressure medium volume flow draining from the consumer may be maintained constant independently of the load pressure. Furthermore the consumer may be biased with the aid of the pressure compensator and the cut-off block in any operating conditions, so that the insufficiency in the pressure medium supply mentioned at the outset is safely avoided. 
   The bias of the consumer may be adapted to the system in a simple manner by adjusting the constant force acting on the pressure compensator in the opening direction. 
   The solution in accordance with the invention may be realized at low expense, with the above described additional valve arrangements, such as anti-cavitation valves, biasing valves, countertorque lowering valves etc., not being required. Due to the bias of the consumer, air discharges on the suction side may reliably be avoided. 
   It was furthermore found that in the hydraulic control arrangement in accordance with the invention, a reduced increase of the supply pressure is necessary, so that energy saving in comparison with conventional solutions is made possible. 
   In a particularly preferred embodiment of the invention, a flow rate of a pump of the hydraulic control arrangement is controlled depending on the higher one of the pressures downstream from the supply measuring orifice and upstream from the drain measuring orifice. This flow rate is regulated such that a pump pressure exceeding the load pressure by a particular pressure difference Δp is present in the pump line (LS system). 
   The supply and drain measuring orifices are preferably formed by a continuously adjustable directional control valve, the work ports of which are connected to a supply line and a drain line, the pressure port of which is connected with a delivery line, and the tank port of which is connected with a return line. 
   The solution in accordance with the invention allows to make the supply cross-section controlled open by a supply control edge larger than the drain cross-section controlled open by a drain control edge, or to control the drain cross-section open at a later time, so that due to the reduced demands to the supply control edge, the harmonization of the two control edges is simplified. The velocity of the consumer will then always be determined by the drain cross-section. 
   The constant force acting on the pressure compensator is in one variant of the invention applied through a control pressure acting on a control surface of a pressure compensator piston. As an alternative, the constant force may also be applied through a spring or the like to the pressure compensator piston. 
   The adaptation of the bias of the consumer may then be altered in a simple manner by adjusting the constant pressure. The latter may through suitable switching also be lowered to Zero, so that the pressure medium may flow off from the consumer without the pressure medium being supplied to the supply side. 
   In the control of a double-acting consumer, e.g., a differential cylinder, releasable cut-off blocks each having a topping piston adapted to be actuated by a control pressure are provided preferably both in the supply and in the drain. The structure of the control arrangement is particularly simple if this control pressure corresponds to the constant control pressure acting on the pressure compensator in the opening direction. 
   The lower pressure downstream from the supply measuring orifice and upstream from the drain measuring orifice may be tapped by means of an inverse shuttle valve. 
   Further advantageous developments of the invention are subject matter of further subclaims. 

   
     In the following a preferred practical example of the invention shall be explained by referring to a single FIGURE showing a circuit diagram of a hydraulic control arrangement for controlling a double-acting consumer. 
   

   The like control arrangements are used particularly for controlling the consumer of a mobile working tool, e.g., in a stacker or a tractor. In the represented practical example, the consumer has the form of a differential cylinder  2  adapted to be connected with a pressure medium supply  6  via a valve arrangement  4 . 
   The pressure medium supply  6  comprises in the represented practical example a fixed displacement pump  8  whereby the pressure medium is sucked from a tank T and conveyed in a pump line  10 . From the pump line  10  a bypass line  12  branches off having arranged therein an inlet pressure compensator  14  which is subjected to the pressure in the pump line  10  in the opening direction and to the highest load pressure of all the consumers as well as the force of a spring  16  in the closing direction. This highest load pressure is in a known manner tapped via a shuttle valve cascade from all the consumers of the system and is present at the inlet pressure compensator  14  via a load reporting line  18 . The pump  8  may have the form of a fixed displacement pump with a speed-regulated drive mechanism, or a variable displacement pump. 
   The pressure medium flowing back from the cylinder  2  returns to the tank T—depending on the direction of movement of the cylinder  2 —via a return line  20  or a return line  22 . 
   The valve arrangement having, e.g., the form of a valve disk of a mobile control block, includes a pressure port P, a LS port LS, two return ports R, as well as two work ports A, B, wherein the latter are connected via work lines  24 ,  26  with a bottom-side cylinder chamber  28  or a piston rod-side annular chamber  30 , respectively. 
   As may furthermore be seen from the FIGURE, the two return lines  20 ,  22  are connected with the two return ports R, the pump line  10  with the pump port P, and the load reporting line  18  with the LS port LS. The valve arrangement essentially consists of a pressure compensator, hereinafter referred to as the individual pressure compensator  32 , a continually adjustable directional control valve  34  indicated by dash-dotted lines, as well as two releasable cut-off blocks  36 ,  38 . The continuously adjustable directional control valve  34  customarily has a directional control element determining the direction of pressure medium flow and a velocity element respectively formed by a supply measuring orifice and a drain measuring orifice. In the represented circuit diagram, four measuring orifices are represented schematically, with only two measuring orifices being effective, however, depending on the adjustment of the directional control element. I.e., in the case of a pressure medium flow to the work port A, the measuring orifice designated by reference number  40  acts as a meter-in orifice, whereas the drain measuring orifice is designated by reference number  42  and is active in the case of a pressure medium flow from the work port B to the tank T. Upon reversing the direction of flow, the measuring orifices represented in addition then become active as a supply measuring orifice  44  and as a drain measuring orifice  46 . The two drain measuring orifices  42 ,  46  are arranged in a return passage  48  or  50  connected with a respective one of the return ports R. 
   The directional control valve includes two work ports A′ and B′ that are connected via a supply line  52  and a drain line  54  with the work ports A, B. The construction of the cut-off blocks  36 ,  38  is known per se, so that detailed explanations are not necessary. With the aid of a topping piston  56  or  58 , respectively, these cut-off blocks  36 ,  38  may be taken from a spring-biased basic position, in which they each act as a check valve, into a through position allowing a return flow of the pressure medium from the cylinder  2 . Actuation of the topping piston  56 ,  58  is effected through a constant control pressure p x  tapped from a suitable control pressure supply. In the opposite direction, the pressure (viewed in the direction of drain) downstream from the respective cut-off block  36 ,  38 , which is tapped via a passage  60  or  62  from the supply passage  52  or from the drain passage  54 , respectively, acts on an annular surface of the topping piston  58 . By thus feeding back the load pressure into the drain, the cut-off block  36 ,  38  operates in a load-compensated manner and regulates the pressure in the respective associated line, which then acts as a drain passage, to a constant value in a load-independent manner. The respective cut-off block located in the supply then operates as a check valve. 
   Inside a delivery passage  64  connected to the pressure port P of the valve arrangement  4 , the individual pressure compensator  32  is arranged which is subjected to the constant control pressure p x  in the opening direction and to the force of a pressure compensator spring  66  and to a pressure in a reporting passage  68  in the closing direction. This pressure is the smaller one of the pressures in the supply passage  52  and in the drain passage  54 , i.e., the smaller one of the pressures downstream from the supply measuring orifice  40  and upstream from the drain measuring orifice  42  (for a pressure medium flow towards the work port A and from the work port B to the tank T). This pressure is tapped by the supply passage  52  and by the drain passage  54  via a respective tapping passage  70 ,  72  that each lead to an inlet of an inverse shuttle valve  74 . The output thereof is connected to the reporting passage  68 . 
   From the tapping passages  70 ,  72  respective LS branch passages  76 ,  78  branch off which are each connected to an inlet of a shuttle valve  80 , the outlet of which is connected with the inlet of another LS shuttle valve  82  of the above mentioned LS shuttle valve cascade, through which the highest load pressure of all the consumers supplied by the pump  8  is tapped. This highest load pressure is then present in the load reporting line  18  and acts on the spool of the inlet pressure compensator  14  in the closing direction. 
   For an improved understanding of the invention, the operation of the control arrangement in accordance with the invention shall now be explained by way of an extension of the cylinder  2 , i.e., the pressure medium is conveyed by the pump  8  via the work port A into the cylinder chamber  28  and again displaced from the annular chamber  30  via the work port B towards the tank T. Here the measuring orifice  40  acts as a supply measuring orifice, and the measuring orifice  42  as a drain measuring orifice; the measuring orifices  44 ,  46  are closed. 
   Basic Operation 
   In order to extend the cylinder  2 , the directional control valve  34  is adjusted such that a supply control edge of the directional control valve  34  opens the cross-section of the supply measuring orifice  40  and accordingly a drain control edge of the cross-section of the drain measuring orifice  42  opens. The directional control valve  34  is designed such that the opening cross-section of the supply measuring orifice  40  is larger than that of the drain measuring orifice. The pressure medium then flows via the pressure compensator  32 , the operation of which shall be explained in more detail in the following, the opened supply measuring orifice  40 , the supply passage  52 , and the cut-off block  36  acting as a check valve, via the work port A and the work line  24  into the cylinder chamber  28 . Accordingly the pressure medium is displaced from the annular chamber  30  and flows via the work line  26 , the work port B, and the cut-off block  38  opened by the constant control pressure p x , the drain passage  54 , the opened drain measuring orifice  42 , the return passage  50 , via the return port R and the return line  22  back to the tank. The pressures in the supply passage  52  and in the drain passage  54  are compared through the intermediary of the inverse shuttle valve  74  as well as the shuttle valve  80 . The higher one of the two pressures is conveyed via the shuttle valve  80  as a LS signal to the LS shuttle valve  82  and from there (if no other higher load pressure is present) to a control surface of the inlet pressure compensator  14  that acts in the closing direction. The lower one of the two pressures is reported via the inverse shuttle valve  74  and the reporting passage  68  to the individual pressure compensator  32  and there compared with the constant control pressure p x . By means of this arrangement, the pressure medium volume flow through the supply passage  52  is adjusted by the supply measuring orifice  40  in such a way that a constant pressure is maintained upstream from the drain control edge in the drain passage  54  opening the drain measuring orifice  42 . If the lower pressure tapped via the inverse shuttle valve becomes higher than the constant pressure p x  (minus the force of the pressure compensator spring  66 ), the pressure compensator spool of the individual pressure compensator  32  throttles the pressure medium flow more strongly. If the lower pressure tapped via the inverse shuttle valve  74  is too low, the flow opening of the individual pressure compensator  32  is enlarged, and correspondingly more pressure medium is conducted to the supply measuring orifice  40 . 
   Driven Load 
   In the case of a driven load, the pressure in the supply passage  52  is higher than the pressure in the drain passage  54 . Apart from the pressure loss across the cut-off block  36  in the supply acting as a check valve, this higher pressure is passed on as a LS signal, and the inlet pressure compensator  14  is adjusted such that the pressure in the pump line  10  is above this highest load pressure by a predetermined pressure difference Δp. The lower pressure on the drain side is reported to the individual pressure compensator  32  and is about 0 bar while the directional control valve  34  is not actuated. The individual pressure compensator  32  is then opened completely by the constant control pressure p x . When the directional control valve  34  is actuated, pressure medium is conveyed by the pump  8  into the cylinder chamber  28  of the cylinder  2  until the pressure on the drain side, i.e., in the drain passage  54 , reaches the value of the constant pressure p x  (minus the force of the pressure compensator spring  66 ). The pressure compensator spool of the individual pressure compensator  32  reduces the opening cross-section of the individual pressure compensator so that the pressure upstream from the drain measuring orifice  42  is maintained constant. 
   Upon opening the drain measuring orifice  42 , the pressure in the drain passage  54  initially drops so that the individual pressure compensator  32  opens to some degree and the pressure medium volume flow across the supply measuring orifice  40  increases until the pressure in the drain passage  54  again reaches the regulating pressure of the individual pressure compensator. The pressure medium volume flow on the supply side is accordingly regulated such that the pressure upstream from the drain measuring orifice  42  remains constant at the regulating pressure of the individual pressure compensator  32 . Thus the opening cross-section of the drain measuring orifice  42  jointly with the regulating pressure of the individual pressure compensator  32  determines the pressure medium volume flow. The cylinder  2  is then clamped, with this clamping force being adaptable by suitably selecting the control pressure p x . 
   Pulling Load 
   In the case of a pulling load, the pressure in the drain passage  54  is higher than the pressure in the supply passage  52 . This pressure in the drain passage  54  is maintained constant by the load-compensated cut-off block  38 , with this value being comparatively low. This regulating pressure of the cut-off block  38  is output as a LS signal via the shuttle valves  80 ,  82  to the pressure medium supply  6 : the pressure in the pump line  10  is adjusted to a comparatively low stand-by pressure. 
   On the supply side, i.e., in the supply passage  52 , only a very low pressure or no pressure at all is present in the case of a pulling load. This lower pressure is reported via the inverse shuttle valve  74  to the individual pressure compensator  32 . The latter is fully opened while the directional control valve  34  is not actuated. When the directional control valve  34  is actuated and the supply measuring orifice  40  is opened, the pump pressure is sufficient for raising the pressure in the supply passage  52  to such an extent that the individual pressure compensator  32  is moved into a regulating position. When the pressure in the supply reaches the regulating pressure of the individual pressure compensator  32 , the latter is closed. In this way the supply-side pressure may be maintained at a constant value that corresponds to the regulating pressure of the individual pressure compensator  32 . 
   Upon opening the drain control edge, i.e., upon opening the drain measuring orifice  42 , the pressure in the drain passage  54  is throttled by the load-compensated cut-off block  38  from the load pressure in the annular chamber  30  or in the work line  26  to a constant level in the drain passage  54  and maintained constant. Thus the moving velocity of the cylinder is even in the case of a pulling load determined by the opening cross-section of the drain measuring orifice  42  jointly with the regulating pressure of the load-compensated cut-off block  38 . 
   If the cylinder  2  is displaced more rapidly, the pressure in the supply passage  52  initially drops in the case of a pulling load, and correspondingly the pressure acting on the individual pressure compensator  32  in the closing direction is reduced so that the latter increases its opening cross-section until the pressure in the supply (supply passage  52 ) again reaches the regulating pressure of the individual pressure compensator  32 . The cylinder  2  thus remains clamped with this regulating pressure even in the case of a pulling load. 
   By turning off the constant control pressure p x  acting on the two cut-off blocks  36 ,  38  and the individual pressure compensator  32  in the opening direction it is possible to realize a safety function, for the two cut-off blocks  36 ,  38  act as check valves, and the individual pressure compensator  32  is closed. This safety function may be realized, e.g., through a separate switching valve whereby the constant control pressure p x  may be turned off. This safety function may also be enabled by suitably designing the control edge of the directional control valve  34 . 
   If the constant control pressure is merely turned off for the individual pressure compensator  32 , it is possible to move the cylinder  2  without pressure medium being conveyed into the supply chamber. 
   The constant control pressure p x  may be varied in order to adapt the pressure drop across the control edges of the directional control valve  34 , and thus the pressure medium volume flow at a given measuring orifice opening, to different operating conditions. 
   Instead of the constant control pressure p x  and the pressure compensator spring  66  it is also possible to mount on the left side (view in accordance with the single FIGURE) a spring whereby a substantially constant force is applied to the pressure compensator piston of the individual pressure compensator  32 . It is also possible to employ other means for maintaining this force constant. 
   What is disclosed is a hydraulic control arrangement for controlling a hydraulic consumer, comprising an adjustable supply measuring orifice and an adjustable drain measuring orifice as well as a pressure compensator arranged in a pressure medium delivery. The control arrangement moreover comprises a releasable cut-off block that acts as a check valve in the direction of supply and may be released in the direction of drain by means of a control pressure. In accordance with the invention, the pressure compensator is subjected to a constant force in the opening direction and to the lower one of the pressures downstream from the supply measuring orifice and upstream from the drain measuring orifice in the closing direction. 
   List of Reference Numbers 
   
       
         2  cylinder 
         4  valve arrangement 
         6  pressure medium supply 
         8  fixed displacement pump 
         10  pump line 
         12  bypass line 
         14  inlet pressure compensator 
         16  spring 
         18  load reporting line 
         20  return line 
         22  return line 
         24  work line 
         26  work line 
         28  cylinder chamber 
         30  annular chamber 
         32  individual pressure compensator 
         34  directional control valve 
         36  cut-off block 
         38  cut-off block 
         40  supply measuring orifice 
         42  drain measuring orifice 
         44  supply measuring orifice 
         46  drain measuring orifice 
         48  return passage 
         50  return passage 
         52  supply passage 
         54  drain passage 
         56  topping piston 
         58  topping piston 
         60  passage 
         62  passage 
         64  delivery passage 
         66  pressure compensator spring 
         68  reporting passage 
         70  tapping passage 
         72  tapping passage 
         74  inverse shuttle valve 
         76  branch passage 
         78  branch passage 
         80  shuttle valve 
         82  LS shuttle valve