Patent Publication Number: US-2005132876-A1

Title: Position control device for a hydraulic actuator and interface plate for a servovalve implementing such a device

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The technical scope of the invention is that of devices enabling the positioning of a hydraulic actuator to be controlled.  
      These devices generally comprise a hydraulic circuit incorporating a pump (for example of variable size) supplying one or other of the two chambers of the actuator by means of a servovalve.  
      2. Description of the Related Art  
      The servovalve is a classical off-the-shelf component and comprises a slide made mobile by a torque motor. This slide may occupy at least three different positions: one central position in which the circuits are closed and two symmetrical positions allowing one or other of the chambers of the actuator to be connected to the hydraulic pump whereas the other chamber is connected to a hydraulic tank or reserve. The slide may also occupy, according to the control given by the torque motor, any intermediate position between the central position and one or other of the extreme positions, each intermediate position corresponding to a different fluid flow provided by the servovalve.  
      Such a servovalve is classically used in closed-loop actuator controls, that is to say controls with position servo control. Such a servovalve may also be used to control an open loop actuator, that is to say one with no position servo control.  
      In any event, so as to avoid the hydraulic pump working constantly, a controlled non-return valve is positioned between each chamber of the actuator and the servovalve. Thus, the pressure in each chamber of the actuator is maintained regardless of the supply level. This non-return valve, moreover, improves the positioning accuracy of the open loop actuator. However, this solution has disadvantages, both when operating in closed loop or open loop. Indeed, the sections of the actuator chambers are different (notably because of the presence of the cylinder rod in one of the chambers). This results in a small jolt at the end of the positioning process leading to an inaccuracy in the final positioning of the actuator.  
      Thus, when the actuator is a cylinder to ensure the elevation laying of a gun, the elevation laying angle obtained is erroneous with respect to the instruction given and the gun is incorrectly laid.  
      Moreover, under open loop operation positioning of the actuator can be seen to drift.  
      The aim of the present invention is to overcome such drawbacks by proposing a control device to position a hydraulic actuator able to be operated both in closed and open loop whilst ensuring positioning accuracy and laying speed.  
     SUMMARY OF THE INVENTION  
      The invention also enables this positioning to be ensured using standard off-the-shelf inexpensive components.  
      Thus, the invention relates to a position control device for a hydraulic actuator, such device comprising a hydraulic circuit comprising a pump supplying the two chambers of the actuator from a hydraulic tank or reserve by means of a servovalve, each outlet of the servovalve being connected to a chamber of the actuator by means of a non return valve, such device wherein at least one of the outlets of the servovalve connected a chamber of the actuator is also connected to the hydraulic tank by a calibrated nozzle upstream of the non return valve.  
      The device may comprise a calibrated nozzle connecting each outlet of the servovalve to the hydraulic tank.  
      The invention also relates to a servovalve interface plate enabling this device to be implemented at low cost using standard inexpensive components.  
      This plate incorporates at least four transversal drill holes intended to cooperate with the four hydraulic ports of the servovalve, that is to say a supply of pressurised fluid via a pump, an outlet to a reserve tank, a first control path for a first chamber of the actuator and a second control path for a second chamber of the actuator, such plate wherein it incorporates at least one leak circuit connecting one of the control paths to the tank outlet, such circuit in which is positioned a calibrated nozzle.  
      The servovalve interface plate may incorporate two leak circuits comprising a calibrated nozzle, each circuit connecting one of the control paths to the tank outlet.  
      The leak circuit(s) will advantageously be made in the form of holes perpendicular to one another and to the transversal drill holes corresponding to the different paths.  
      The outward opening of each hole may be stopped by a plug.  
      The nozzle(s) may be made in the form of screws having a calibrated axial orifice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other advantages of the invention will become more apparent from the following description of a particular embodiment of the invention, such description made with reference to the appended drawings, in which:  
       FIG. 1  is a simplified hydraulic schema of a control device according to the invention,  
       FIG. 2  is a simplified schema of the interface plate according to the invention,  
       FIGS. 3   a,    3   b  and  3   c  are three views of an embodiment of the interface plate,  FIGS. 3   a  and  3   b  being sections along planes AA and BB respectively in  FIG. 3   c.   
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows the schema of a device  1  to control the position of a hydraulic actuator  2 , for example a cylinder comprising a piston  2   c  integral with a rod  2   d  and delimiting two chambers  2   a  and  2   b . The cylinder ensures the elevation or traverse laying of a gun barrel (not shown). This device comprises a hydraulic circuit incorporating a pump  3  supplying the two chambers  2   a ,  2   b  of the actuator  2  by means of a servovalve  4 .  
      The latter is a classical off-the-shelf 4-way flow servovalve.  
      It incorporates four ports conventionally termed A, B, P and T. Port T is intended for the return to the tank  5 , inlet port P for the replenishment in fluid by the pump  3 , outlet ports A and B being intended to connect the servovalve  4  to the two chambers  2   a  and  2   b  of the cylinder  2 .  
      The pump  3  is thus connected by upstream ducting  6   a  to the hydraulic tank  5  (or fluid reserve) and discharges by downstream ducting  6   b  to the inlet port P of the servovalve  4 .  
      The servovalve  4  is conventionally shown in  FIG. 1  in its closed position (median block  4 c in the circuit) in which the four ports A, B, P and T are blocked.  
      The servovalve slide may occupy at least two different active extreme positions: 
          a first one (left block  4   a  in the circuit) in which the inlet port P is connected to the outlet port  1  whereas the other outlet B is connected to the tank  5  via port T.     a second one (right block  4   b  in the circuit) in which the inlet port P is connected to the outlet port B whereas the other outlet port A is connected to the tank  5  via port T.        

      The displacements of the servovalve slide are controlled classically by a torque motor  4 d integrated into the servovalve, such torque motor piloted by an electronic control unit  7 .  
      The extreme positions  4   a ,  4   b  correspond to the maximal fluid discharges supplied by the servo vale. The slide may classically occupy intermediate positions between the closed position and one or other of the two extreme positions. These intermediate positions correspond to different fluid discharges transmitted according to one or other of the control paths.  
      The outlet ports A and B of the servovalve  4  are connected by ducting  8   a  and  8   b  to chambers  2   a  and  2   b  of the cylinder  2  by means of two controlled non return valves  9   a ,  9   b.    
      These non-return valves are presented here in the form of a single hydraulic block  9  incorporating four outlets C, D, E and F. Each valve  9   a ,  9   b  comprises a ball applied to a seat by the pressure in the actuator  2  side. The ball may be moved away from the seat by a control pressure.  
      Valve  9   a  is positioned between outlets C and E and connects chamber  9   a  of the actuator and port A of the servovalve  4 . Valve  9   b  is positioned between outlets D and F and connected chamber  2   b  of the actuator and port B of the servovalve. Hydraulic links  9   c  and  9   d  (shown in dotted lines) ensure the control of valves  9   b  and  9   a  respectively.  
      Thus, an arrival of pressure at D naturally flows towards F through valve  9   b  and also lifts the ball of valve  9   a  thereby allowing fluid to pass from E to C. Inversely, an arrival of pressure at C flows towards E and allows fluid to pass from F to D.  
      Valves  9   a  and  9   b  allow the cylinder  2  to hold its position when controlled in open or closed loop.  
      According to the invention, at least one calibrated nozzle is positioned connecting one of outlet ports A or B of the servovalve  4  to the hydraulic tank  5 .  
      In the embodiment shown in  FIG. 1 , two nozzles  10   a  and  10   b  are provided.  
      A first nozzle  10   a  is connected by ducting  11   a  to ducting  8   a  and it drains part of the fluid circulating in it towards the tank  5 .  
      A second nozzle  10   b  is connected by ducting  11   b  to ducting  8   b  and it drains part of the fluid circulating in it towards the tank  5 .  
      The nozzles will be constituted by regulating nozzles of well defined calibers. The calibers will be selected so as to ensure a drop in pressure in ducting  8   a  or  8   b , which is inactive, without necessarily reducing the discharge in ducting  8   b  or  8   a , which controls the movement. Leak sections will be selected for the nozzles, for example, which enable a leak rate of between 4% and 7% of the fluid supplied by the pump  3 .  
      The nozzles  10   a  and  10   b  do not, therefore, perturb the normal operation of the servovalve  4  since they cause no significant loss in discharge rate in ducting supplied by the pump  3 .  
      Nevertheless, when the servovalve  4  is in the closed position shown in  FIG. 1 , the nozzles ensure a drop in the pressure of the fluid in ducting  8   a  and  8   b , upstream of the valve  9 .  
      This drop in pressure allows the non-return valves  9   a  and  9   b  to be stabilized. Indeed, maintaining the pressure in the actuator&#39;s chambers is carried out by the balls applied onto their seats by the pressure on the cylinder side. These balls are under the pressure of ducting  8   a  and  8   b  on the other side of their seat.  
      Thanks to nozzles  10   a  and  10   b,  the pressure in ducting  8   a  and  8   b  at inlets C and D is lower than that respectively at inlets E and F. The balls thus remain pressed onto their seats and any rebound, which could lead to a jolt in the positioning of the cylinder, is avoided.  
      Laying may thus be carried out accurately when the positioning of the cylinder is servo controlled in a closed loop and that position is held without drift when servo controlled in an open loop.  
      At least one nozzle with be provided connected to the ducting opposed to the movement required to be accelerated. Preferably, two nozzles will be provided, one connected to each ducting.  
      In practical terms, it is possible for the invention to be implemented by assembling ducting  11   a  and  11   b  such that is directly connected at any point in ducting  8   a  and  8   b .  
      According to a preferred embodiment of the invention, it will be implemented by providing a specific interface plate  13  integral with the servovalve  4 .  
       FIG. 2  shows a simplified hydraulic schema of interface plate  13  and  FIGS. 3   a ,  3   b  and  3   c  show a particular embodiment of this interface plate.  
      A servovalve  4  is made, in practical terms, in the form of a component incorporating a parallelepipedic base  12  fitted with inlet and outlet openings (or ports) A, B, T and P on a same upper face  12   a.    
      The interface plate  13  is thus also made in the form of a parallelepipedic block a lower face  13   a  of which is intended to be applied onto face  12   a  of the servovalve. This face has four openings Ta, Aa, Ba and Pa intended to lie respectively opposite openings T, A, B and P of the servovalve. In a classical manner not shown, each opening (Ta, Aa, Ba, Pa) of interface plate  13  incorporates a countersink able to house an O-ring. This O-ring will be applied during assembly against face  12   a  of the servo vale and will seal it.  
      An upper face  13   b  of the plate  13  has four openings Tb, Ab, Bb and Pb to which the different ducting of the hydraulic circuit will be connected. Ducting  8   a  and  8   b  leading to the cylinder are fastened to openings Ab and Bb, ducting  6   b  coming from the pump  3  is fastened to opening Pb and ducting  18 , which returns to the hydraulic tank  5 , is fastened to opening Tb. Fastening is made using appropriate hydraulic connectors.  
      Face  13   b  may also be applied against a hydraulic block (not shown) provided with holes corresponding to opening Tb, Ab, Bb and Pb. The latter openings will also be provided with countersinks (not shown) able to house O-rings thereby ensuring sealing with respect to the block. The block will be connected using appropriate ducting to the cylinder, the pump and the tank.  
      The different openings of the lower face  13   a  are connected to openings in the upper face  13   b  by holes drilled in the material of the plate  13 .  
      Thus, opening Ta is connected to opening Tb by drill hole  14 , opening Aa is connected to opening Ab by drill hole  15 , opening Ba is connected to opening Bb by drill hole  16  and opening Pa is connected to opening Pb by drill hole  17 .  
      Inside the plate, a first controlled hydraulic leak circuit  19   a  is formed by a channel onto which a nozzle  10   a  is positioned, which connects drill holes  15  and  14 . The plate, moreover, encloses a second leak circuit  19   a  formed by another channel onto which is nozzle  10   b  is positioned. This second leak circuit  19   b  connects drill holes  16  and  14 .  
      Thus, the plate  13  allows the servovalve  4  to be converted in an extremely simple manner to incorporate the nozzles proposed by the invention. The hydraulic circuit may thus easily be modified without it being necessary to provide additional ducting.  
      As may be seen more specifically in  FIGS. 3   a ,  3   b  and  3   c , the leak circuits  19   a  and  19   b  are made in the shape of orthogonal holes. Such an arrangement makes it easier to manufacture the interface plate  13 .  
      So as to avoid any interference, the holes are made in two parallel planes.  
      Thus, plane AA ( FIG. 3   a ) comprises two orthogonal holes  19   a   1  and  19   a   2  which form the first leak circuit  19   a . Hole  19   a   2  encloses the first nozzle  10   a,  made in the form of a screw having a calibrated axial orifice.  
      Plane BB ( FIG. 3   b ) comprises four holes  19   b   1 ,  19   b   2 ,  19   b   3  and  19   b   4  which are orthogonal two by two and thus form a rectangle. These four holes constitute the second leak circuit  19   b . Hole  19   b   1  encloses the second nozzle  10   b  also made is the form of a screw fitted with a calibrated axial orifice.  
      All the different holes are stopped and sealed by threaded plugs  20 .  
      As may be seen, it is easy for the leak characteristics of the device according to the invention to be modified without the need for complex dismounting. The nozzles  10   a  and  10   b  merely have to be unscrewed and replaced by nozzles having different leak diameters. The leak diameters of the nozzles will be selected according to the properties of the actuator, the load and the servo control circuit pressure in position.  
      Holes  21  passing right through the interface plate  13  are provided to allow the plate to be fastened onto the servovalve  4  by screws (not shown).  
      By way of a variant, it is naturally possible for only one nozzle to be provided connecting one of the supply paths from the cylinder to the tank. In the case, the other nozzle will be replaced by a non-perforated screw.  
      This variant allows one of the movements of the cylinder to be accelerated in a preferential manner.