Abstract:
In series between pump and tank are two throttle devices, one of which includes a controllable throttle valve having a piston, of which one side supports a valve head, which forms the boundary of an annular gap between an inflow duct and an outflow duct. A hydraulic control path extends from the inflow duct through the throttle opening to the tank, running through a precontrol bore in the piston. To prevent an unwanted leakage fluid stream, one end of the precontrol bore at the annular gap in the region of the inflow duct is positioned so that it remains opened for a closed annular gap, and the other end of the precontrol bore on the other side of the piston is arranged where the piston, in response to the controllable throttle valve being completely opened, pushes against a stop face and, in so doing, automatically closes this other end.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a control device for the continuous motion of a hydraulic control motor. 
   BACKGROUND INFORMATION 
   Proportional valve arrangements, as are used, for example, as servo valve arrangements for operating a hydraulic control motor in the form of, e.g. a working cylinder  27  ( FIG. 1 ), may be made up of individual modules D 1  through D 4 . In the form of closing valves that are precontrolled and controlled in a pressure-proportional manner, these modules then form four controllable throttle devices D 1 , D 2 , D 3 , D 4  of a hydraulic bridge circuit represented in  FIG. 1 . 
     FIG. 1  shows the basic circuit diagram of the so-called open-center variant (variant according to the open-principal) of such a servo valve arrangement, in the neutral position, in which all four throttle devices are open, so that a fluid conveyed by a pump P from a tank T may flow back nearly unhindered through the throttle devices to tank T. Lines LA and LB run from points A and B, respectively, to working chambers a and b, respectively, of working cylinder  27 , which are separated from each other by a working piston AK. In the neutral position shown, the pressure in the two working chambers a, b is the same, so that working piston AK remains at rest. 
   Not all modules D 1  through D 4  must be controllable. In order to be able to control the movement of the working piston, it is basically sufficient to have a series connection of two throttle devices D 1  and D 4 , of which one must be controllable. In this context, the working piston may be provided a spring, which pushes it in a direction, or working chamber b may be kept at another controlled or constant pressure, whose magnitude is between the pump pressure and the pressure of the tank (mostly atmospheric pressure). 

   DETAILED DESCRIPTION 
   The present invention relates to a controllable module (e.g., D 1 ) or a pair of modules (e.g., D 1  with D 3 ). According to  FIG. 2 , such a module D 1  is basically made up of a housing bore  1 , which is introduced into a valve block (housing G), and in which a piston  3  may be moved axially back and forth. A sealing element  4  on the circumference of the piston separates pressure chambers  2  and  5  from each other and simultaneously functions as a low-friction guide of piston  3  in housing bore  1 . If the fluid flow indicated by arrows, from an inflow duct  14 , through an annular gap  17 , to a discharge duct  13  is reduced or completely interrupted, a throttle needle  9  is moved into a throttle opening  8  by an actuating force. This throttle opening  8  is arranged in a fixed disk  6 , which may also be integrated into a housing G or a cover D and forms, together with piston  3  and housing bore  1 , pressure chamber  5 . The axial actuating force on throttle needle  9  may be applied mechanically, electromotively, electromagnetically, hydraulically, pneumatically, etc. The insertion of throttle needle  9  into throttle opening  8  reduces the flow cross-section for the fluid stream flowing through a choke bore  12  into a tank T, via a duct  10 . This throttling of the fluid stream causes a high pressure to build up in pressure chamber  5 . Piston  3  is moved in the direction of annular gap  17  as a result of the imbalance between, on one hand, the forces acting on the left side of piston  3  and, on the other hand, the forces acting on the right side of piston  3 . This reduces the width of annular gap  17 , which produces the connection of inflow duct  14  to outflow duct  13 . Consequently, the fluid pressure in inflow duct  14  increases. The fluid pressure in chamber  5  is proportional to the actuating force on throttle needle  9 . This fluid pressure is proportional to the fluid pressure in inflow duct  14 , which means that, on the whole, a proportionately is established between the actuating force acting on throttle needle  9  and the fluid pressure produced in inflow duct  14 . 
   When two such modules D 1 , D 3  of a total of four modules carry out the same closing function, cf.  FIG. 3 , for the functioning of a hydraulic bridge circuit, as may be conventional in the case of servo valves, then these modules may be designed to have a nearly identical construction and to be jointly controlled by the throttle unit of one module D 1 , made up of throttle needle  9  and throttle opening  8 . For this joint control, a hydraulic connection is provided between, on one hand, pressure chamber  5  of a module D 1  provided with a throttle unit, and, on the other hand, the corresponding pressure chamber ( 53  in  FIG. 4 ) of one or more other modules, which are then constructed without a throttle unit and a choke bore  12 , and are also controlled. The hydraulic connection may be constructed in the form of an internal housing duct ( 1 _ 3  in  FIG. 4 ). 
     FIG. 4  shows the diagrammatic representation of a servo-valve arrangement constructed in this manner.  FIG. 4  is based on the state described in  FIG. 3 , where D 1  and D 3  are open, while D 2  and D 4  are closed, so that the pump pressure (pressure at the outlet of pump P) acts via opened throttle device D 1  on working piston AK, in the direction of the arrow in working chamber a of working cylinder  27 . 
   When used in a closed-center system (system having a closed center), the two throttle needles  9  and  92  keep corresponding throttle openings  8 ,  82  closed in the neutral state. If working piston AK is moved, for example, to the right, then throttle needle  9  is moved to the left, in order to open the throttle opening. In the case of an open-center system, only one of throttle needles  9 ,  92  (the active one) is pushed into its throttle opening  8 ,  82  by an actuating force, while no force acts on the other (passive) throttle needle, so that this throttle needle remains outside of its throttle opening. The throttle needles exchange their active and passive roles as a function of the direction in which working piston AK should be displaced. 
   In  FIG. 4 , throttle device D 1  (and D 3 ) are passive, the pressure conditions (taking into consideration the different sizes of the pressurized surfaces on the two sides of piston  3 ) ensuring that annular gap  17  is wide open. Since D 4  (and D 2 ) are nearly closed (unlike the neutral position of  FIG. 1 ), a pressure greater than the pressure in Tank T prevails at point A, and, in working chamber a, pressure is exerted, via line LA, on working piston AK, in the direction of the arrow. 
   How is it that D 4  is now newly closed? This is based on the fact that D 2  is also nearly closed and D 4  is controlled by D 2 , for pressure chamber  52  of D 2  is connected to pressure chamber  54  of D 4  by a duct  2 _ 4 , so that the same pressure prevails in the two pressure chambers. 
   D 2  is nearly closed in the following manner: 
   First of all, throttle opening  82  of D 2  is opened even further, so that fluid may flow relatively unhindered from pump P, via inflow duct  142 , through a choke bore  122  in piston  32 , and via throttle opening  82  and duct  2 _T, to tank T. Then, throttle opening  82  is further closed by moving throttle needle  92  to the left. This causes a higher pressure to build up in pressure chamber  52 , which results in piston  32  moving to the left. In this manner, the path from inflow duct  142  to outflow duct  132  (and therefore to point B, to throttle device D 3 , and to tank T) is nearly closed to the fluid coming from the pump, and the pressure at point B is nearly reduced to the pressure in tank T. 
   As aspect of the modules manufactured according to an example embodiment of the present invention may be simple and cost-reducing construction. Since sealing is provided here on the end faces, unlike conventional longitudinal slide-valve-sleeve units and rotary slide-valve-sleeve units sealed on the circumference, the fit between the slide and the bore, which fit may be encumbered with tight tolerances and may therefore be expensive to produce, may be omitted. This also may allow special materials and their expensive processing with regard to surface treatment and heat treatment to be omitted. In the case of the described module, a piston and housing made of a light-metal alloy may be sufficient, even for high fluid pressures. 
   In the neutral position, spring elements  11 , which are represented in  FIG. 2  and may also be replaced by a large (e.g., recessed) spring element  11   a  (shown in D 4  in  FIG. 4 ), have the task of pressing piston  3  in the direction of annular gap  17  in such a manner, that the throttling of the fluid stream in annular gap  17  produces a predefinable pressure difference, which allows a pressure force to be exerted on end face  16  ( FIG. 2 ) of piston  3  in response to the occurrence of the throttling effect at the throttle unit, which is made up of throttle needle  9  and throttle opening  8 . The pressure force overcomes the friction of sealing element  4 . In addition, this friction is also overcome by the spring force of spring element  11 . 
   Throttle opening  8  is arranged in a disk  6 , which forms the rear seal of pressure chamber  5 . The fluid flowing through throttle opening  8  into chamber  7  is fed back through line  10  in  FIG. 2  (or  1 _T in  FIG. 4 ) into tank T of the system. 
   Represented in  FIG. 4  is an operating position, in which module D 2  and module D 4  are activated and therefore brought into the “closed” position, which means that the fluid stream is directed to working chamber a of working cylinder  27 . Modules D 1  and D 3  are not driven or activated and are therefore in the “open” position. When the system represented in  FIG. 4  (or also just the module, which is represented in  FIG. 2  and is connected in series to a throttle valve) is used in open-center engineering practice, the high fluid pressure in inflow chamber  23  may produce a very high leakage fluid stream through choke bore  12  in the direction of the tank, which may correspond to a considerable reduction in the moving capacity of working piston AK of working cylinder  27 . As aspect of an example embodiment of the present invention is to prevent this. 
   The disadvantage mentioned above may be prevented according to an example embodiment of the present invention, in which piston  3  may be designed so that precontrol bore  12  opens out at end face  16  of the piston ( FIG. 2 ) and the bore opening there, of the precontrol bore, is closed by the abutting of end face  16  against stop face  60  of disk  6  (or against another appropriate stop face for piston  3 ), when piston  3  is pressed against stop face  60  of disk  6  (or against another appropriate stop face for piston  3 ) due to the application of pressure. This situation is represented in module D 1  in  FIG. 4 . 
   Precontrol bores  12 ,  122  do not have to be situated in controllable throttle devices D 1 , D 2 , but may instead be situated in additionally controlled throttle devices D 3 , D 4  (cf. reference numerals  123  and  124 ). When throttle device D 2  should be controlled so as to pass over from the opened state into the closed state in a  FIG. 4  configuration modified in this manner, a precontrol fluid stream will initially flow from pump P through inflow duct  14 , outflow duct  13 , precontrol bore  124  in the piston of throttle device D 4 , connecting duct  2 _ 4 , pressure chamber  52 , throttle opening  82 , and connecting duct  2 _T, to tank T. When throttle needle  92  is pushed into the throttle opening  82 , the pressure in pressure chamber  52  will increase and move piston  32  in the closing direction. The pressure in pressure chamber  54  of D 4  simultaneously increases, so that the piston of D 4  also moves in the closing direction. 
   The control device of an example embodiment of the present invention may be suited for hydraulic power-steering systems having an open-center design, because, in this case, the control device may solve the problem of unwanted leakage in the case of annular gap  17  being completely opened. However, the control device of an example embodiment of the present invention may also be suitable for use in a closed-center system.