Abstract:
An apparatus controls fluid flow to an actuator. A feedback mechanism responds to a pressure differential that occurs across the actuator by altering the fluid flow. That pressure differential indicates acceleration of the actuator that can occur when the load acting thereon varies. Thus as the actuator accelerates, the apparatus reduces the flow of fluid which counteracts the acceleration and maintains the actuator speed relatively constant. A unique directional control valve incorporates the feedback mechanism that functions regardless of the direction in which the actuator moves.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to the control of hydraulic actuators, and more particularly to controlling the speed of a hydraulic motor under varying load conditions.  
         [0005]     2. Description of the Related Art  
         [0006]     Some hydraulic motors have inherently low damping which makes accurate speed control difficult under varying load conditions. As a result, the device being moved by the motor may overshoot or undershoot a desired position or operate at too great a velocity if velocity is the controlled attribute.  
         [0007]     For example, hydraulic motors are used to open and close the weapons bay doors on military aircraft. If the door does not open fully because of an unexpectedly large load acting of the motor, such as a high speed wind, the weapons may not fully deploy. Conversely if the load is unexpectedly small and the motor operates longer than necessary, the door will be forced against a mechanical stop which can damage the door or the motor. In addition to changing load conditions, other factors, such as variation of hydraulic fluid flow to the motor, also affect the speed of the motor. Thus, if the motor is operated based on an assumed speed and the actual speed is different, the member being moved may not be properly positioned.  
         [0008]     Therefore, it is desirable to provide a mechanism to determine when various factors cause variation in motor speed and compensate for that variation.  
       SUMMARY OF THE INVENTION  
       [0009]     An apparatus is provided to control a fluid powered actuator that has a first port and a second port. A directional control valve includes a sleeve with a longitudinal bore into which an inlet and a first workport open. A valve spool is slidably received within the bore and has one position in which the inlet is in fluid communication with the first workport, and another position in which communication between the inlet and the first workport is blocked. A feedback mechanism that applies pressure from the first and second port of the fluid power actuator to the valve spool, wherein when pressure in the first port exceeds pressure in the second port a force is produced which tends to move the valve spool from the first position to the second position.  
         [0010]     A significant pressure differential occurs between the first and second ports as the fluid power actuator accelerates, which happens as the load acting on that actuator varies. The feedback mechanism responds to that pressure differential by causing the directional control valve to reduce the flow of fluid to the fluid power actuator, thereby counteracting the acceleration and maintaining the actuator speed relatively constant.  
         [0011]     In a preferred embodiment of the present apparatus, the directional control valve has first and second workports to which the ports of the fluid power actuator connect. The valve spool has a first annular groove and a second annular groove. In a first position of the valve spool, the first annular groove connects the first workport to the inlet and the second annular groove provides connection between the second workport and the outlet. In a second position, the first annular groove couples the first workport to the outlet and the second annular groove connects the second workport to the inlet. The valve spool has a third position in which the inlet and the outlet are isolated from both the first and second workports.  
         [0012]     The load pressure feedback mechanism of that preferred embodiment comprises a first feedback piston slideably received in a first aperture at one end of the valve spool and forming a first spool cavity there between. A second feedback piston is slideably received in a second aperture at an opposite end of the valve spool, thereby creating a second spool cavity there between. A first passage is provided in the valve spool to convey pressure in the first annular groove to the first spool cavity, and a second passage conveys pressure in the second annular groove to the second spool cavity.  
         [0013]     The directional control valve preferably is pilot operated and has a first chamber in the bore at one end of the valve spool and a second chamber in the bore at an opposite end of the valve spool. In this case, the apparatus further may comprise a variable orifice coupling a supply line to the inlet of the directional control valve and a pilot valve which alternately couples a node to the supply line or a return line in response to a pressure differential across the variable orifice. A valve assembly couples the node to either the first chamber and to the second chamber to move the valve spool into the first or second position, respectively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic diagram of a hydraulic motor control circuit that includes a load pressure feedback mechanism;  
         [0015]      FIG. 2  illustrates a state of the directional control valve in  FIG. 1  for operating the motor in one direction; and  
         [0016]      FIG. 3  illustrates the state of the directional control valve for operating the motor in the opposite direction. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     With initial reference to  FIG. 1 , a hydraulic circuit  10  controls the flow of pressurized fluid from a supply line  12  to a fixed displacement hydraulic motor  14 . The fluid exiting the hydraulic motor  14  is directed into a return line  16  that leads to a reservoir (not shown). Although the inventive concepts are being described in the context of controlling a fixed displacement hydraulic motor, they can be utilized with a variable displacement motor.  
         [0018]     The hydraulic motor  14  can be driven in either of two directions depending upon the position of a pilot-operated, directional control valve  18 . The directional control valve  18  has a sleeve  20  that rests within an aperture in a body  21  which combined form a valve housing. The sleeve has a longitudinal bore  22  and a transverse inlet port  24  to receive pressurized fluid from the supply line  12  and convey that fluid into the bore. First and second outlet ports  26  and  28  provide passages on opposite sides of the inlet port for fluid to flow from the bore  22  through an common outlet port  29  into the return line  16 . A first workport  32  extends from the longitudinal bore  22  transversely through the valve sleeve  20  at a position between the inlet port  24  and the first outlet port  26 . A second workport  34  provides another opening from the longitudinal bore  22  at a position between the inlet port  24  and the second outlet port  28 . The hydraulic motor  14  is connected to the two workports  32  and  34 .  
         [0019]     A valve spool  36  is slidably received within the longitudinal bore  22  of the sleeve  20 . The valve spool  36  has first and second annular grooves  38  and  40  around the exterior that provide paths between the various ports in different positions of the valve spool, as will be described. A first feedback piston  44  is slidably positioned within an aperture  42  at one end of the valve spool  36  that is within a spring chamber  56  of the longitudinal bore  22 . A first spool cavity  58  is formed within the spool aperture  42  adjacent the interior end of the first feedback piston  44  and is connected by a first passage  60  to the second annular groove  40 . A first pintle  46  is received within a hole in the first feedback piston  44  and has an exposed end that engages a wall  53  of the spring chamber  56 . A ring clip  48  is secured within an exterior annular notch near this end of the valve spool  36  and engages a first spring retainer  50  through which the valve sleeve extends. A second spring retainer  52  abuts a shoulder  55  on the spool  36  farther away from that one end. A compression spring  54  is located between the two spring retainers  50  and  52 . When pilot pressure is not being applied to the directional control valve  18 , the compression spring  54  forces the spring retainers  50  and  52  against opposing walls  53  and  57  of the spring chamber  56 , which centers the valve spool  36  within the longitudinal bore  22 . In that centered position, the annular spool grooves  38  and  40  do not provide paths between the ports  24 ,  26 ,  28 ,  32  and  34  and the directional control valve  18  is in a closed state.  
         [0020]     A second aperture  62  is formed at the opposite end of the valve spool  36  from the first aperture  42 . A second feedback piston  64  is slidably received within this second aperture  62  and abuts a second pintle  66  that engages a wall  65  of the body  21  which forms another end of the longitudinal bore  22 . A nose chamber  68  is located between the body  21  and the end of the sleeve  20  adjacent the second feedback piston  64 . A second spool cavity  70  is created between the second feedback piston  64  and the bottom of the second spool aperture  62 . A second passage  72  couples the second spool cavity  70  to the first annular groove  38  around the spool  36 .  
         [0021]     With continuing reference to  FIG. 1 , the return line  16  connects directly to the first and second outlet ports  26  and  28 . The supply line  12  is coupled by a variable orifice  80  to a secondary supply line  13  that leads to the inlet port  24 . The variable orifice  80  controls the motor speed and can be dynamically varied by an electrical actuator, such as a solenoid. The directional control valve  18  is a pilot operated device in which the spool  36  moves within the longitudinal bore  22  in response to the application of pressure to the spring chamber  56  or the nose chamber  68 . Application of that pressure is controlled by a three-way, proportional pilot valve  82  and two solenoid valves  86  and  88 . The pilot valve  82  selectively couples a node  84  to either the supply line  12  or the return line  16  in response to a pressure differential across the variable orifice  80 . The force of a spring in the pilot valve  82  defines the differential pressure setting at which that valve opens. A first solenoid valve  86  selectively couples the spring chamber  56  of the directional control valve  18  to either the node  84  or the return line  16 . A second solenoid valve  88  selectively couples the nose chamber  68  to either the node  84  or the return line  16 . Which one of the first and second solenoid valves  86  and  88  connects the node to the directional control valve  18  determined the rotational direction on the directional control valve  18  motor  14 .  
         [0022]     The speed at which the hydraulic motor  14  rotates is proportional to the flow from the supply line  12  which is controlled by the variable orifice  80 . The differential pressure across the variable orifice  80  corresponds to the supply line flow and is sensed by the three-way pilot valve  82  which is driven into a position that is proportional to the magnitude of that pressure. The pilot valve position produces a control pressure at node  84  that corresponds to the flow from the supply line into the directional control valve  18 . That control pressure is applied by one of the two solenoid valves  86  or  88  to either the spring chamber  56  or the nose chamber  68  to select the direction of the hydraulic motor  14 . The magnitude of the control pressure at node  84  determines the amount that the directional control valve  18  opens and thus the speed of the motor  14 .  
         [0023]     Assume a fixed pressure setting of the pilot valve  82 . The sensed differential pressure across the variable orifice  80  will be less than that pressure setting under relatively low flow conditions. In that case, the pilot valve  82  conveys the supply line pressure to node  84  and that pressure travels through the active solenoid valve  86  or  88  to increase the opening of the directional control valve  18 . Opening the directional control valve  18  farther drives the motor  14  to a higher speed until the sensed pressure across the variable orifice  80  matches the pressure setting of the pilot valve  82 . At that time, the pilot valve  82  assumes a position the maintains that motor speed.  
         [0024]     Similarly during a relatively high flow condition, the sensed differential pressure exceeding the pressure setting causes the pilot valve  82  to close off node  84  from the supply line  12  and couple that node to the return line  16 . In this state, both the spring chamber  56  and the nose chamber  84  of the directional control valve  18  are connected to the return line  16 , either by a deactivated solenoid valve  86  or  88  or through the activated solenoid valve and the pilot valve  82 . With both of these directional control valve chambers  56  and  68  at the return line pressure, the spring  54  forces the valve spool  36  toward the center, or closed, position to slow the hydraulic motor  14 . Slowing of the hydraulic motor  14  eventually results in a low flow condition occurring through the variable orifice  80 . At that time, a differential pressure is produced which again causes the pilot valve  82  to open a path between the supply line  12  and the node  84  to increase the flow of pressurized fluid to the motor  14 .  
         [0025]     A key feature of the hydraulic circuit  10  is a motor acceleration feedback mechanism provided by the two feedback pistons  44  and  64  incorporated in the directional control valve  18 . The two feedback pistons  44  and  64  bear against the valve housing through two pintles  46  and  66 . The pintles apply an axial load with a minimal lateral load that would adversely affect valve performance. The pressures at the two workports  32  and  34 , conveyed by the respective spool passages  60  and  72  to the first and second spool cavities  58  and  70 , act on the interior ends of the two feedback pistons  44  and  64 . Because the motor torque is proportional to the differential workport pressure, that pressure differential provides a reasonable approximation of motor acceleration, which is the first derivative of motor speed. Feedback of the differential workport pressure (i.e. motor acceleration) is employed as a dampening coefficient in a servo-loop created in the directional control valve  18 .  
         [0026]     As noted previously, the size of the variable orifice  80  controls the motor speed and can be dynamically varied by an electrical actuator. The feedback mechanism provided by the feedback pistons  44  and  64  control the acceleration of the hydraulic motor  14  to maintain a relative constant speed under varying load conditions.  
         [0027]      FIG. 2  illustrates the state of the directional control valve  18  that results from applying pressure from node  84  to the spring chamber  56  by activating the first solenoid valve  86  in  FIG. 1 . That action drives the valve spool  36  downward in the orientation of the valve in the drawings, so that the first annular groove  38  is positioned to create a path between the inlet port  24  and the first workport  32 . This applies fluid from the supply lines  12  and  13  to the motor  14 . After passing through the motor, the fluid reenters the directional control valve  18  via the second workport  34  from which it flows through a path provided by the second annular groove  40  to the second outlet port  28  and into the return line  16 .  
         [0028]     In this state, the first passage  60  through the spool  36  applies the fluid pressure returning from the motor at the second workport  34  to the first spool cavity  58  at the inner end of the first feedback piston  44 . The second spool passage  72  conveys the motor driving pressure in the first workport  32  to the second spool cavity  70  at the inner end of the second feedback piston  64 . An increase in the pressure differential across the motor causes a correspondingly acceleration of the motor and is denoted by a greater pressure in the second spool cavity  70  than occurs in the first cavity  58 . That pressure differential between the spool cavities  58  and  70  creates a net force which moves the spool  36  upward in  FIG. 2 , reducing and maybe even closing communication between the supply input port  24  and the first workport  32 . The change in the directional control valve  18  reduces the flow of fluid to the motor  14 , whereby dampening the acceleration.  
         [0029]     As the pressure differential across the motor deceases so does the motor acceleration which is denoted by a reduction in the difference in pressure between the first and second spool cavities  58  and  70 . This pressure reduction causes the spool  36  to return to the position illustrated in  FIG. 2  at which the first annular groove  38  again provides a larger path between the inlet port  24  and the first workport  32 .  
         [0030]      FIG. 3  illustrates the directional control valve  18  in a position which operates the motor  14  in the opposite direction from that illustrated in  FIG. 2 . Here, the spool  36  is positioned so that the second annular groove  40  provides a path between the inlet port  24  and the second workport  34 . The fluid exhausting from the motor  14  enters the first workport  32  and then flows via the first annular groove  38  to the first outlet port  26 . In this state, the pressure of the fluid fed to the motor  14  via the second workport is communicated through the first spool passage  60  to the first spool cavity  58  at the inner end of the first feedback piston  44 . The pressure of the fluid leaving the motor  14 , which flows through the first annular groove  38 , is applied via the second spool passage  72  to the second spool cavity  70  adjacent the second feedback piston  64 .  
         [0031]     As the motor  14  accelerates, the resultant pressure differential is reflected in the first and second spool cavities  58  and  70  with the pressure in the first spool cavity  58  being greater. This creates a force that tends to move the spool  36  downward in  FIG. 3 , closing off communication between the inlet port  24  and the second workport  34 , thereby reducing the motor acceleration. As the acceleration decreases, the pressure differential across the motor similarly decreases which results in the pressures in the first and second spool cavities  58  and  70  tending to equalize. As that occurs, the spool  36  moves to enlarge the path between the inlet port  24  and the second workport  34 . In this manner, the operation of the feedback piston limits the acceleration of the hydraulic motor  14 .  
         [0032]     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.