Patent Application
20060150623
Not Applicable
Not Applicable
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
Therefore, it is desirable to provide a mechanism to determine when various factors cause variation in motor speed and compensate for that variation.
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.
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.
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.
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.
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.
With initial reference to
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.
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.
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.
With continuing reference to
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.
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.
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.
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.
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.
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
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
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
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.
