SERVOMECHANISM

Abstract: A servomechanism made up of a relatively small number of components comprises a pintle (14) that is moved back and forth by a hydraulic actuator (53), an input member (54) disposed opposite to the pintle (14) and capable of reciprocating parallel to the pintle (14), a spool (57) capable of reciprocating parallel to both the input member (54) and the pintle (14), a pair of idle gears (58) pivoted to the spool (57), and a hydraulic circuit (59). The input member (54) has a moving rack (55) opposite to a rack (56) formed in the pintle (14). The spool (57) is mounted between these racks (55,56). The idle gears (58) are in mesh with the racks (55,56). When the input member (54) is moved to shift the spool (57) out of its neutral position, the actuator (53) acts to get the spool (57) back to its neutral position.

Description: The present invention relates to a servomechanism which increases an input displacement applied to the input member to produce a larger output displacement proportional to the input displacement. A servomechanism of this kind has its output member actuated by a hydraulic actuator or the like. A change-over valve is mounted in the circuit of driving the actuator so that an input displacement applied to the input member may control the valve. The output displacement from the output member is fed back to the valve in such a way that the position of the valve body of the change-over valve can be adjusted. In this way, the aforementioned augmented output proportional to the input can be produced. In this prior art mechanism, the output displacement from the output member is fed back to the change-over valve by means of a linkage or the like. Therefore, the mechanism comprises a large number of components and hence complex in structure. This makes the mechanism difficult to assemble. Moreover it is not easy to produce the mechanism with a small size and a light weight. In view of the foregoing, it is an object of the present invention to provide a servomechanism which is made up of a relatively small number of components and is not difficult to assemble, is small in size and lightweight, with a high reliability. It is another object of the invention to provide a servo­ mechanism which has the advantage that it can readily operate accurately, in addition to the features described in the above paragraph. These objects are achieved by a servomechanism comprising : an output member capable of reciprocating in certain directions ; an actuator for reciprocating the output member by utilizing hydraulic pressure ; an input member which is disposed opposite to the output member and which is reciprocated in a direction parallel to the output member by receiving an operation input; racks formed on the opposite portions of the input and output members, respectively ; a spool disposed between the racks of the input and output members and capable of reciprocating in a direction parallel to the input and output members ; idle gears pivoted to the spool and meshing with the racks ; and a hydraulic circuit which, when the spool is in the neutral position, locks the actuator and which, when the input member is moved to shift the spool out of its neutral position, acts the actuator so that the spool is returned to its neutral position. The object described secondly is achieved by further providing a spring resiliently pressing the spool in a certain direction. In this structure and under the condition that the spool is held in its neutral position and the actuator is locked, the operation of the input member, causes one of the idle gears to roll on the rack of the output member. Then, the spool moves a certain distance proportional to the input member. As a result, the hydraulic circuit is changed to its other configuration to operate the actuator so that the output member is moved in such a direction as to get the spool back to its neutral position. In certain conditions, the spool remains substantially stationary. In such a case, the output member moves a distance proportional to the moving distance of the input member invariably in the reverse direction to the input member. Fig. 1 is a cross-sectional view of an embodiment of the invention ; Fig. 2 is a cross-sectional view taken along line II-II of Fig. 1; Fig. 3 is a cross-sectional view taken along line III.III of Fig. 1; Fig. 4 is a cross-sectional view of another embodiment of the invention; and Fig. 5is a cross-sectional view taken along line V-V of Fig. 4. Referring to Figs. 1-3, there is shown a rotary fluid energy converter. A servomechanism according to the invention is used to adjust the position of the pintle of this converter as described later. The converter comprises a cylindrical housing 1 with a bottom portion and a torque ring 2 is rotatably mounted on the inner surface of the housing by means of first static pressure bearings 3. The housing 1 is provided at one end with an opening 1a. The inner surface of the housing presents a surface 4 tapering toward the opening 1a, and ring 2 being in contact with said tapering surface 4. The ring 2 is shaped like a cup and has an outer peripheral wall 2a presenting the same apical angle as the tapering surface 4. A rotating shaft 6 is formed integrally with the ring 2 and protrudes away from one of its axial ends. The front end portion of the shaft 6 extends outwardly from the housing 1 through the opening 1a. The first bearings 3 are constituted with shoes 5 rigidly secured at the outer surface of the ring 2 at required positions, each shoe 5 being pressed on the tapering surface 4 of the housing 1. Each shoe 5 is provided with three pressure pockets 7a, 7b, 7c being axially adjacent to each other. Hydraulic pressure is introduced into the pockets 7a, 7b, 7c. There is provided an odd number of bearings 3 which are regularly circumfe­ rentially spaced apart from one another. The inner surface of the torque ring 2 has flat surfaces 2c at positions corresponding to the bearings 3. Pistons 8 are disposed at positions corresponding to the inner flat surfaces 2c. The front ends 8a of the pistons 8 are pressed against their corresponding surfaces 2c by means of second static pressure bearings 9. The bearings 9 are made plane so that the front ends 8a of the pistons 8 may come into close contact with their corresponding surface 2c. Each of the front end 8a has a pressure pocket 11 into which hydraulic pressure is introduced. The base end of each piston 8 is held by a piston retainer 12. A space 13 is formed between the retainer 12 and the piston 8 to be fed with fluid. The piston retainer 12 consists of a pintle 14 having a sliding portion 14a, and cooperating with an annular cylinder barrel 15. The sliding portion 14a bears on the housing 1. The pintle 14 can rotate about an axis n being parallel to the symmetry axis m of the housing 1 and the torque ring 2 about which said torque ring rotates. The barrel 15 is rotatably fitted over the outer periphery of the pintle 14. The barrel 15 is provided with a plurality of cylinders 16 being regularly circumferentially spaced apart from each other and extending radially. The axis of each cylinder 16 is substantially perpendicular to the outer surface of the pintle 14. The pistons 8 are fitted in the cylinders 16 so as to be slidable. The base surface 8b of each piston 8 and the inner surface of each cylinder 16 form a chamber 13. The barrel 15 is connected to the torque ring 2 by means of an Oldham coupling 20 or similar, so that the barrel can rotate at the same angular velocity as the ring 2. The pintle 14 takes the form of a truncated cone whose outer surface presents an apical angle substantially equal to the apical angle formed by the peripheral wall 2a of the ring 2. The pistons 8 are so held that they can move perpendicularly to the peripheral wall 2a of the ring 2. The sliding portion 14a of the pintle 14 is shaped in the form of a longitudinally elongated block and is trapezoidal in groove 19 formed in the housing 1. More precisely, the pintle 14 is held in such a way that it can slide perpendicularly to the axis m. This makes it possible to set the distance D between the axis n of the pintle 14 and the axis m to any desired value, including zero. As shown in Fig. 2, the inside of the housing 1 is divided into a first region A and a second region B by an imaginary line P that is drawn along the direction in which the pintle 14 slides. Those chambers 13 which are moving across the first region A are placed in communication with a first fluid communication line 21 and the chambers 13 which are moving across the second region B communicate with a second fluid communication line 22. The first fluid communication line 21 comprises fluid passages 23, a port 24 extending through the pintle 14, and a fluid inlet/outlet port 25 formed in the housing 1, corresponding to one end of port 24. The chambers 13 are in communication with the inside of the barrel 15 through the passages 23. One of the ends of the port 24 extends to the outer periphery of the pintle 14 on the side of the first region A, while the other end extends to the inclined surface 14b of the sliding portion 14a of the pintle 14, which is on the side of the second region B. A pressure pocket 27 is formed between the outer periphery of the pintle 14 and the inner surface of the cylinder barrel 15, at one end of the port 24, in order to form a third static pressure bearing 26. Another pressure pocket 29 is formed between the inclined surface 14b of the pintle 14 and the inner surface of the housing 1, at the other end of the port 24, to form a fourth static pressure bearing 28. The pocket 27 extends circumferentially, and acts to place all the chambers 13 present in the first region A in communication with the port 24 extending through the pintle. The pocket 29 is elongated in the sliding direction of pintle 14. When the pintle 14 is caused to slide, the pocket 29 prevents the port 24 from being disconnected from the fluid inlet/outlet port 25. The second fluid communication line 22 comprises the fluid passages 23 already mentioned, a port 34 extending through the pintle, and a fluid inlet/outlet port 35 formed in the housing 1 at a position corresponding to one end of the port 34. The other end of the port 34 extends to the outer surface of the pintle 14 on the side of the second region B, while the other end extends to the inclined surface 14c of the sliding portion 14a of the pintle on the side of the first region A. At the other end of the port 34, a pressure pocket 37 is formed between the pintle 14 and the cylinder barrel 15 to form another third static pressure bearing 36. At said one end of the port 34, a further pressure pocket 39 is formed between the inclined surface 14c of the pintle and the inner surface of the housing 1 to form a fourth static pressure bearing 38. Pockets 37 and 39 are similar in structure to pockets 27 and 29. A pressure inlet passage 41 is formed along the axis of each piston 8. The fluid pressure within each chamber 13 corresponding to each piston 8 is introduced into the pressure pocket 11 in the corresponding second static pressure bearing 9 via the pressure inlet passage 41. The hydraulic pressure within the pocket 11 is directed into the pressure pockets 7a, 7b, 7c in the corresponding first static pressure bearing 3 via fluid passages 42a, 42b, 42c formed in the ring 2. These passages 42a, 42b, 42c and the pressure pockets 11 constitute sliding valve elements 50, which act to selectively cut off the flow into the pockets 7a, 7b, 7c, making use of the axial movement of the piston 8 relative to the torque ring 2. When the axial distance between the center of each first static pressure bearing 3 and the center of the piston 8 is within a certain limit, the pressure pockets 11 are in communication with all the fluid passages 42a, 42b, 42c. However, when the distance exceeds the limit, the pocket 11 interrupt communication with this fluid passages 42c or 42a which is farther from piston 8. Restrictors 40a, 40b, 40c are mounted in the passages 42a, 42b, 42c, respectively. The directions and area of the static pressure bearings 3 and 9 are set to such a value that the force acting on the torque ring 2 due to the static pressure of the fluid introduced into the first bearings 3 is identical in magnitude but opposite in direction to the force acting on the torque ring 2 due to the static pressure introduced into the second bearings 9. The area of the second bearings 9 is set to such a value that the force acting on the piston 8 due to the static pressure applied to the bearing 9 is cancelled by the force working on the piston 8 due to the static pressure of the fluid within the chambers 13. Further, the area of the third static pressure bearings 26 and 36 is set to such a value that the force acting on the barrel 15 due to the static pressure introduced into the bearings 26 and 36 is cancelled by the force acting on the barrel 15 due to the static pressure of the fluid within the spaces 13 that exist in the corresponding regions A and B. The angle at which the surfaces 14b and 14c are inclined is set to such a value that the force acting on the pintle 14 due to the static pressure of the fluid introduced to the bearings 28 and 38 is cancelled by the force acting on the pintle 14 due to the static pressure of the fluid introduced to the third bearings 26 and 36 existing in the regions A and B in opposite relation to the inclined surfaces 14b and 14c on which the bearings 28 and 38 are respectively mounted. Indicated by numeral 43 is a seal member and 44 is an assistant bearing for supporting the rotating shaft 6. The fluid energy converter of the variable displacement type constructed as described above further includes a stepping motor 51 for converting an electrical digital signal into mechanical displacement and the servomechanism 52 for reciprocating the pintle 14 in proportion to the output displacement delivered by the motor 51. The servomechanism 52 comprises the pintle 14 capable of reciprocating in certain directions and acting as the output member, an actuator 53 for reciprocating the pintle 14 by utilizing hydraulic pressure, an input member 54 that is disposed opposite to the pintle 14 and is reciprocated in a direction parallel to the pintle 14 by receiving the operation input, racks 55 and 56 formed on the opposite portions of the input member 54 and pintle 14, respectively, a spool 57 disposed between the racks 55 and 56 and capable of reciprocating in a direction parallel to the input member 54, idle gears 58 pivoted to the spool 57 and meshing with the racks 55 and 56, and a hydraulic circuit 59, which, when the spool 57 is in its neutral position, locks the actuator 53 and which, when the spool 57 is moved out of its neutral position by movement of the input member 57, acts the actuator 53 so that the spool 57 is brought back to its neutral position. More specifically, the actuator 53 consists mainly of a pair of hydraulic cylinders 61, 62 disposed at the longitudinal ends of the sliding portion 14a of the pintle 14. The cylinders 61 and 62 comprise cylindrical pistons 61b and 62b respectively, slidably fitted in holes 61a and 62a formed in end surfaces 14d and 14e, respectively, of the sliding portion 14a of the pintle 14. Springs 61c and 62c are mounted in holes 61a and 62a, respectively, to urge outwards pistons 61b and 62b, respectively. The outer ends of the pistons 61b and 62b are continuously pressed against the inner surfaces 1a and 1b of the housing 1 with the seal members 61d and 62d, respectively. Inlet/outlet ports 61e and 62e communicating with the holes 61a and 62a are formed in the inner surfaces 1a and 1b, respectively, of the housing 1. The input member 54 is square in cross-section and slidably received in a cover 63 which is of U-shaped cross-section. The member 54 has a tapped hole 54a extending along its axis, so that the member 54 is screwed to a threaded portion 64a formed on the output shaft 64 of the motor 51. The spool 57 has lands 65 and 66 near its ends. Both ends of the spool 57 are slidably fitted in a port block 67 disposed between the housing 1 and the cover 63. The block 67 and the spool 57 form high-pressure passages 68 and 69 on the inner side of the lands 65 and 66 and low-pressure passages 73 and 74 on the outside of the lands 65 and 66, which communicate with a case drain via ports 71 and 72. High pressure ports 75 and 76 which are continuously in communcation with the high-pressure passages 68 and 69 are opened in the inner surface of the block 67. Ports 77 and 78 communicate with the ports 61e and 62e in the cylinders 51 and 62, respectively. The servomechanism is set so that when the spool 57 is held in its neutral position, the lands 65 and 66 close the ports 77 and 78, respectively. A flat portion is formed at the center of the spool 57. Two idle gears 58 are rotatably mounted at opposite sides of the flat portion by means of pin shafts 81. A spring 82 is mounted between the lower end of the spool 57 and the inner surface of the port block 67 to continuously resiliently urge the spool 57 upwardly. The hydraulic circuit 59 is constituted of the pressure ports 75, 76, the high-pressure passages 68, 69, the entrance/exit ports 77, 78, the low-pressure passages 73, 74, and return ports 71,72. The pressure ports 75 and 76 are in communication with the fluid communication line on the high-pressure side, namely first fluid communication line 21 in the discussed embodiment. The operation of the illustrated mechanism shall now be described. The body of the mechanism essentially operates in the manner as described in Japanese Laid-Open Patent specification N o 77179/1983. More specifically, when high-pressure fluid is supplied into the chambers 13 present in the first region A through the first fluid communication line 21, a couple of forces rotating the torque ring 2 in the direction indicated by arrow S is produced. Thus, the system functions as a motor. When the ring 2 is rotated in the direction indicated by arrow R by an external force, high-pressure fluid is discharged from the first fluid communication line 21. Thus, the system functions as a pump. The pintle 14 is reciprocated along the trapezoidal groove 19 to vary the eccentricity, i.e., the distance between the axis n of the pintle and the axis m of the housing 1. Thereby, the displacement can be controlled. The mechanism which controls the variable displacement operates in the manner described below. When the stepping motor 51 is cut out and the spool 57 is maintained at its neutral position as shown in Fig. 1, the lands 65 and 66 on the spool 57 close the ports 77 and 78. Therefore, the hydraulic cylinder 61 and 62 of the actuator 53 are locked, retaining the pintle 14 at a certain position. Under this condition, the stepping motor 51 is cut on by an instruction from a computer (not shown). When the output shaft 64 rotates only a given angle, the input member 54 of the servomechanism 52 screwn to threaded portion 64a of the shaft 64 moves in a direction parallel to the direction along which the pintle 14 operates. When the input member 54 moves upward from the location shown in Fig. 1, the idle gear 58 meshing with the rack 55 of member 54 rolls upwardly on the stationary rack 56 of pintle 14.Then, the spool 57 that is connected to the center of this gear 58 by means of the pin shaft 81 upwardly moves a distance half of the moving distance of the input member 54. The pressure port 75 is in communication with the inlet/outlet port 77 through the high-pressure passage 68. As a result, a portion of the high-pressure fluid in the first fluid communication line 21 is supplied into the inlet/outlet port 61e of one cylinder 61 via the ports 75 and 77. The pressure fluid is then introduced into the cylinder hole 61a. At this time, the other port 78 communicates with the return port 72 via the other low-pressure passage 74. The pressure of the pressure fluid supplied into cylinder 61 moves the pintle 14 downward. Then, the idle gear 58 downwardly rolls on the rack 55 of the input member 54, shifting the spool 57 downward until it returns to its neutral position. In this state, the inlet/outlet ports 77 and 78 are closed again. Accordingly, the pintle 14 moves the same distance as the moving distance of the input member in the reverse direction, before coming then to a halt. When the input member 54 moves downward, a similar situation applies but in an inverted way. In other words, the pintle 14 upwardly moves the same distance as the moving distance of the input member 54. In this way, as the motor 51 is rotated forward or rearward, the pintle 14 moves the corresponding distance in the corresponding direction. The displacement can be appropriately changed in response to the digital signal supplied to the stepping motor 51. By the rolling movement of the gears 58 which are mounted between the input member 54 and the pintle 14 that acts as the output member, the input displacement is transmitted from the input member 54 to the spool 67, and the displacement fed back is transmitted from the pintle 14 to the spool 57. Hence, the servomechanism is made up of a much less number of components and simpler in structure than the mechanism using linkage or the like. Therefore, the novel servomechanism can be made in much smaller size and lighter in weight than the conventional servomechanism. Also, since the spool 57 is continuously resiliently pressed in a certain direction by the springs 82, the idle gears 58 are always resiliently brought into mesh with the racks 55 and 56. Thus, the gears 58 are prevented from rattling on the racks 55 and 56. In other words, the mechanism does not backlash. This allows fine adjustement and accurate control for access to a desired position. In the illustrated embodiment, the distance travelled by the output member is equal to the amount of displacement of the input member, but various modifications and changes may be made thereto. For example, as shown in Figs. 4 and 5, idle gears having different radii may be used in a pair, in which case the output member can be moved a distance proportional to the input displacement. The distance may be increased or decreased, depending on the combination of racks and gears. In the embodiment shown in Figs. 4 and 5, an idle gear 58b is in mesh with the rack 56, and a smaller idle gear 58a is in mesh with the rack 55. According to another feature of the invention, the pintle 14 can be moved a distance forward or rearward, depending on the direction of the rotation of the motor, the distance travelled by the pintle corresponding to the angle through which the stepping motor 51 rotates. This makes it possible to appropriately vary the displacement in response to the digital signal supplied to the motor 51. Therefore, when the stepping motor is operated according to the signal from a digital control apparatus, such as a computer, constant-pressure operation, constant-power operation, or two-pressure control can be easily and accurately performed. Moreover, the system can readily accommodate itself to changing of control mode. Furthermore, since the stepping motor operates directly on the digital signal from the computer or the like, no digital-to-analog converter circuit is needed. In addition, as the system does not undergo a drift due to temperature variations or other phenomenon, it does not need compensators of any kind or similar circuits. Consequently, the novel mechanism is characterized in that it is simple in structure, yet can perform accurate variable-displacement operation, as defined in the appended claims. According to a further feature of the invention, the displacement can be appropriately varied in response to the digital signal fed to the stepping motor 51. The variation of displacement is converted into digital signal by an encoder and can be presented on a display unit for visual check. Therefore, for detecting the output, since the threaded rod 84 is screwn to the nut 83 disposed in the cylinder 61 of the actuator 53 and is connected with the encoder E, the mechanism for transmitting the signal indicative of the position of the operating pintle 14 to the encoder is not bulky, contributing to simplification of the structure. Additionally, since the nut 83 is continuously pressed against pintle 14 by the spring 61c in the actuator 53, it is possible to cause the nut 83 to accurately follow the movement of the pintle 14 without the need to use a special fixing element for rigidly fixing the nut to the pintle 14. Consequently, the position of the operating pintle 14 can be detected with high accuracy without encountering difficulties such as those related to the difficulties for assembling or manufacturing the mechanism, or the complexity of the structure. Hydraulic pressure may be employed to urge the nut toward the pintle. In this case, it is necessary to seal the lower side of the nut 83 and to place its inside part in communication with the drain. The present invention is also characterized in this respect as defined in the appended claims. It is to be noted that the invention is not limited to the control over the position of the pintle, but that it may well be used in various other applications. Since the novel servomechanism is constructed as described thus far, it does not suffer from the disadvantage that it is made up of a large number of components, making the structure complex. Therefore, it is easy to make it in small size and lightweight. Further, the servomechanism operates reliably. The servomechanism has the advantage that accurate control can be performed, in addition to the aforementioned advantages.

Claims: 1. A servomechanism characterized in that it comprises : an output member (14) capable of reciprocating in certain directions ; an actuator (53) for reciprocating the output member (14) by utilizing hydraulic pressure ; an input member (54) which is disposed opposite to the output member (14) and which is reciprocated in a direction parallel to the output member (14) by receiving an operation input ; racks (55, 56) formed on portions facing each other of the input (54) and output (14) members, respectively ; a spool (57) disposed between the racks (55,56) of the input (54) and output (14) members and capable of reciprocating in a direction parallel to the input (54) and output (14) members ; idle gears (58) pivoted to the spool (57) and meshing with the racks (55,56) ; and a hydraulic circuit (59) which, when the spool (57) is in its neutral position, locks the actuator (53) and which, when the input member (54) is moved to shift the spool (57) out of its neutral position, acts the actuator (53) so as to bring the spool back to its neutral position. 2. A servomechanism as set forth in claim 1 characterized in that it further includes a stepping motor (51) for converting an electrical digital signal into mechanical displacement and a means moving in proportion to the output displacement of the motor (51), whereby the operation signal is applied to the input member (54). 3. A servomechanism as set forth in claim 1, characterized in that it further includes nut (83) continuously pressed against the output member (14) reciprocated by the hydraulic actuator (53), a threaded rod (84) screwn to the nut and acting to convert the movement of the nut (83) following the output member (14) into rotary reciprocating movement, and a detecting means (E) connected to said threaded rod (84) and acting to convert the rotary displacement of said rod (84) into an output representative of the displacement of the output member (14).

Inventors: KITA, YASUO
Applicants: SHIMADZU CORPORATION
Application year: 1986
Main classification: F15B9
IPC codes: F04B1, F15B9, F04B49