Patent Publication Number: US-6658747-B2

Title: Position sensor for a hydraulic actuator and hydraulic system using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a position sensor for a hydraulic actuator, and more particularly, to a position sensor capable of contactlessly detecting one or more operating positions of a hydraulic actuator utilizing the flow of working fluid and a hydraulic system using such a position sensor. 
     2. Related Art 
     A hydraulic system such as a machine tool is generally provided with hydraulic actuators that are operated under the control of a controller, so as to actuate various operating sections of the hydraulic system to thereby carry out desired operations. To this end, operating positions of the hydraulic actuators are detected by position sensors and supplied to the controller. 
     To detect the operating position of a hydraulic actuator, a limit switch serving as a position sensor is widely used. On the other hand, as a hydraulic actuator, a cylinder actuator is employed that has a cylinder body, a piston disposed to be movable therein, and a rod formed integrally with the piston and that is configured to move the rod back and forth by supplying and discharging working fluid to and from a cylinder chamber defined by the cylinder body and the piston. 
     The limit switch is provided with an operative element, i.e., a switch, disposed in the vicinity of a predetermined moving position of the rod. The operative element is actuated when a dog formed in the rod is brought in contact with the operative element during the rod movement, thereby detecting the operating position of the actuator. 
     In the case of position detection using limit switches, a hydraulic system requires a large number of limit switches each arranged to detect a corresponding one of objective operating positions of an associated cylinder actuator. This may cause difficulties in arranging some of the limit switches at their desired locations in narrow spaces around cylinder actuators associated therewith and in performing maintenance of these limit switches. Furthermore, a number of wires are required to connect the limit switches and the controller. Depending on circumstances in which limit switches are disposed, operative elements are sometimes exposed to the outside. This permits dusts and fluid drops to adhere to the operative elements, resulting in occurrences of operating failures and erroneous operations. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a position sensor capable of contactlessly detecting one or more operating positions of a hydraulic actuator utilizing a flow of working fluid, without using a contact-type position sensor such as a limit switch. 
     Another object of the present invention is to provide a hydraulic system provided with the aforesaid non-contact type position sensor. 
     A position sensor according to the present invention comprises: at least one gear disposed in a fluid passage of a hydraulic actuator so as to be rotatable by a flow of working fluid in the fluid passage; a first sensing element, disposed to face the gear, for generating a first sensor output each time the gear rotates for a predetermined angle; a second sensing element, disposed to face the gear, for generating a second sensor output which is out of phase with the first sensor output each time the gear rotates for the predetermined angle; and a detecting section for determining an operating position of the hydraulic actuator based on the first and second sensor outputs supplied from the first and second sensing elements. 
     In the present invention, the gear of the position sensor rotates when working fluid flows in a fluid passage of a hydraulic actuator, and first and second sensor outputs that are out of phase with each other are supplied from the first and second sensing elements to the detecting section each time the gear rotates for a predetermined angle. From a phase relation between the first and second sensor outputs, the detecting section determines the direction of rotation of the gear indicative of the direction of the working fluid flow and, by extension, the direction of operation of the hydraulic actuator. Further, the detecting section can determine an amount of rotation of the gear indicative of an amount of operation of the hydraulic actuator based on the number of times for which the first or second sensor output is generated. Thus, an operating position of the hydraulic actuator can be determined based on the first and second sensor outputs. 
     As mentioned above, the position sensor of the present invention, having a sensing section comprised of a gear disposed in a fluid passage and two sensing elements disposed to face the gear, is capable of contactlessly detecting an operating position of a hydraulic actuator without using a contact-type sensing element such as a limit switch. In addition, the sensing section of the position sensor is not required to be disposed near the hydraulic actuator. Thus, it is easy to prevent dusts and fluid drops from adhering to the sensing section, thereby eliminating operating failures and erroneous operations of the sensing section. Also, a wire may be shortened in length that connects the sensing section with a detecting section of the position sensor. Furthermore, unlike a conventional position sensor having sensing sections such as limit switches that are provided for individual operating positions being detected, the position sensor of this invention can detect one or more operating positions of a hydraulic actuator by means of a single sensing section comprised of a gear and sensing elements. In other words, the sensing section of the position sensor of this invention serves as one or more limit switches. For this reason, it is enough to provide each position sensor with a single sensing section, even if two or more operating positions should be detected for each actuator. Accordingly, installation and maintenance of position sensors in a hydraulic system can be carried out with ease. The required number of wires connecting the sensing section of a position sensor with a detecting section thereof can be also reduced, and a frequency of occurrences of wire disconnection may be reduced. 
     In the present invention, preferably, each of the first and second sensing elements is comprised of a magnetic proximity sensing element disposed to face a peripheral portion of the gear, and the at least one gear is made of a metal material capable of influencing a magnetic field. 
     With this preferred arrangement, each sensing element supplies the sensor output to the detecting section each time a tooth portion of the gear passes in front of the sensing element, whereby the operating position of a hydraulic actuator can be contactlessly detected with reliability. 
     Preferably, the position sensor comprises first and second gears that are disposed in the fluid passage so as to be in mesh with each other and to be rotatable by a flow of the working fluid. More preferably, the first and second gears are disposed in the fluid passage so as to receive the flow of the working fluid at their portions where they are in mesh with each other. 
     According to this preferred arrangement, the flow of the working fluid acting on the first and second gears is converted into gear rotation with efficiency and accuracy, so that the first and second gears rotate in a manner appropriately following the flow of the working fluid, thereby improving the accuracy of detecting the operating position of the hydraulic actuator. 
     Preferably, the position sensor further comprises a sensor block that includes a sensor block body formed with first and second fluid passage portions constituting part of the fluid passage and having a first outer face to which respective one ends of the first and second fluid passage portions open, and a first plate attached to the first outer face of the sensor block body. The first plate is formed with a gear-accommodating space for receiving the at least one gear so as to be rotatable, the gear-accommodating space being communicated with the respective one ends of the first and second fluid passage portions. 
     With this preferred arrangement, the gear can be easily disposed in the fluid passage so as to be rotatable by a flow of working fluid by simply attaching the first plate that accommodates the gear to the sensor block body. 
     More preferably, the gear-accommodating space is formed so as to receive first and second gears to be in mesh with each other and to be rotatable by the flow of the working fluid. More preferably, the gear-accommodating space is formed so as to be communicated with respective one ends of the first and second fluid passage portions in vicinity of portions of the first and second gears where they are in mesh with each other. 
     According to these preferred arrangements, the flow of the working fluid is permitted to properly act on the gears. 
     Preferably, the position sensor further comprises a second plate attached to an outer face of the first plate on a side remote from the sensor block. The second plate is formed with an element-attaching section to which the first and second sensing elements are attached so as to face the gear. 
     With this preferred arrangement, the first and second sensing elements can be accurately disposed to face the gear by simply attaching the second plate, mounted with the sensing elements, to the first plate, whereby the position sensor can be simplified in construction and the detecting accuracy of the sensing elements can be improved. 
     A hydraulic system according to the present invention comprises: a working fluid source for supplying working fluid; one or more hydraulic actuators that are operable in response to supply of the working fluid; one or more fluid passages extending between the working fluid source and the one or more hydraulic actuators; one or more valves disposed in the one or more fluid passages for allowing or prohibiting the supply of the working fluid from the working fluid source to the one or more hydraulic actuators through the one or more hydraulic passages; a controller for drivingly controlling the one or more valves; and one or more position sensors disposed in the one or more fluid passages, each of the one or more position sensors being configured as mentioned in the above. 
     In the hydraulic system of the present invention, each of the one or more valves is drivingly controlled by the controller, to supply working fluid from the working fluid source through the associated fluid passage to a corresponding hydraulic actuator, thereby operating the same. At this time, the operating position of the hydraulic actuator is detected by the position sensor and is provided for control of the hydraulic actuator by means of the controller. The position sensor is configured as mentioned above, so that the aforementioned advantages can be attained such that the operating position of the hydraulic actuator can be contactlessly detected, to permit the controller to properly control the drive of one or more hydraulic actuators. 
     In the hydraulic system of the present invention, preferably, each of the one or more position sensors is disposed in the fluid passage between the valve and the hydraulic actuator, which individually correspond to the position sensor. 
     According to this preferred arrangement, the position sensor is disposed in a fluid passage region in which a flow of the working fluid is produced which adequately corresponds to the flow of the working fluid actually affecting on the operation of the hydraulic actuator, thus permitting the position sensor to accurately detect the operating position of the hydraulic actuator. 
     Preferably, each of the one or more valves is comprised of an electromagnetic changeover valve. 
     For instance, each electromagnetic changeover valve has first and second input ports and first and second output ports. The first and second input ports are connected to the working fluid source and a reservoir section for storing the working fluid therein. The first and second output ports are connected to first and second ports of a hydraulic actuator corresponding to the electromagnetic changeover valve, respectively. Under the control of the controller, the electromagnetic changeover valve assumes a first changeover position where the first and second input ports are individually communicated with the first and second output ports and a second changeover position where the first input port is communicated with the second output port and the second input port is communicated with the first output port. Alternatively, the electromagnetic changeover valve assumes the first or second changeover position or a neutral position where communication between the first and second input ports and the first and second output ports is prohibited. 
     With the aforesaid preferred arrangement using an electromagnetic changeover valve, the valve can be drivingly controlled by the controller with ease, with accuracy and with improved response. 
     More preferably, the hydraulic system has one or more position sensors each of which is configured as mentioned above. That is, each position sensor comprises a sensor block including a sensor block body formed with first and second fluid passage portions and having first outer face thereof to which respective one ends of the first and second fluid passage portions open, and each sensor block body has a second outer face thereof to which another end of the second fluid passage portion opens. The hydraulic system further comprises one or more valve blocks each attached to the second outer face of the sensor block body of a corresponding one of the one or more position sensors. Each valve block has a valve-attaching portion thereof to which a corresponding one valve is attached. The valve block is formed with a first fluid passage portion in alignment with another end of the second fluid passage portion formed in the sensor block body associated therewith. 
     More preferably, each sensor block body has a third outer face to which another end of the first fluid passage portion formed therein opens. Each sensor block body is further formed with third, fourth and fifth fluid passage portions having their opposite ends that open to the second and third outer faces of the sensor block body, respectively. Each valve block is formed with second, third and fourth fluid passage portions in alignment with respective one ends of the third, fourth and fifth fluid passage portions formed in the sensor block body associated therewith. 
     With the preferred arrangements, corresponding ones of the fluid passage portions formed in the valve block and the sensor block body can be communicated with one another by simply attaching the valve body, mounted with the valve, to the sensor block body. This facilitates the assembly of the hydraulic system and permits a simplified construction thereof. 
     More preferably, the hydraulic system further comprises one or more manifold blocks each attached to the third outer face of the sensor block body of a corresponding one of the one or more position sensors. Each manifold block is formed with first through fourth fluid passage portions in alignment with respective other ends of the first, third, fourth and fifth fluid passage portions formed in the sensor block body associated therewith. 
     This preferred arrangement makes it possible to simplify the fabrication and construction of the hydraulic system 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a hydraulic system according to one embodiment of the present invention; 
     FIG. 2 is an exploded perspective view of the position sensor shown in FIG. 1; 
     FIG. 3 is a section view showing part of a fluid passage formed in a sensor block of the position sensor shown in FIGS. 1 and 2; 
     FIG. 4 is a view for explaining an action of rotation of gears of the position sensor; 
     FIG. 5 is a view for explaining an action of opposite rotation of the gears; and 
     FIG. 6 is a perspective view showing four manifold blocks that are formed integrally with one another. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIGS. 1-5, a hydraulic system according to an embodiment of the present invention will be explained. 
     As shown in FIG. 1, the hydraulic system of this embodiment is configured to control the supply and discharge of working fluid to and from a plurality of hydraulic actuators such as hydraulic cylinder actuators (one of which is shown by reference numeral  2  in FIG. 1) by means of electromagnetic changeover valves (hereinafter referred to as solenoid valves)  8  that operate under the control of a controller  6 , thereby drivingly controlling the actuators  2  to carry out the desired operation such as moving a workpiece (not shown) to a predetermined position. 
     Each solenoid valve  8 , which is a four-port, two-position type, has first and second input ports and first and second output ports. The first and second input ports are connected through an oil passage to the discharge side of an oil pump  3  and an oil tank  4 , respectively. The first and second output ports are connected through the oil passage to first and second ports  2 A,  2 B of the cylinder actuator, respectively. 
     When a first solenoid coil  8 Sa of each solenoid valve  8  is supplied with electric power from the controller  6 , the solenoid valve  8  assumes a first changeover position  8 SA in which the first and second input ports are in communication with the first and second output ports, respectively. In this case, the working oil is supplied from the pump  3  through the oil passage to the first port  2 A of the cylinder actuator  2  and enters into a first cylinder chamber of the cylinder actuator  2 , thereby causing a rod  2   a  of the actuator  2  to advance, while permitting a piston  2   b  of the actuator  2  to discharge the working oil from a second cylinder chamber through the second port  2 B. On the other hand, when a second solenoid coil  8 Sb of the solenoid valve  8  is energized, the solenoid valve  8  assumes a second changeover position  8 SB where the first input port is communicated with the second output port and the second input port is communicated with the first output port, so that the working oil is supplied to the second cylinder chamber of the cylinder actuator  2  to cause the rod  2   a  to move backward, while discharging the working oil from the first cylinder chamber. 
     The hydraulic system of this embodiment comprises a plurality of position sensors (the arrangement associated with one position sensor is shown in FIG. 1) each of which is configured to contactlessly detect, based on the flow of working oil in the oil passage, information indicative of an actuator operating position and used for actuator control by means of the controller  6 . Each position sensor is disposed in the oil passage between the solenoid valve  8  and the cylinder actuator  2 . As for the position sensor, a detailed explanation will be given later. 
     In order to simplify the construction of the hydraulic system and ease the assembly and disassembly thereof, the hydraulic system of this embodiment is configured to combine a manifold block  7 , a sensor block  9  and a solenoid valve block  5  associated with each cylinder actuator  2  into one piece with use of, e.g., four bolts  18 , and corresponding ones of oil passage portions, which are formed in these three blocks  7 ,  9  and  5  and which constitute part of the oil passage, can be communicated with one another by simply combining these blocks into one piece, with the detecting section  13 - 16  of the position sensor attached in advance to the sensor block  7  and with the solenoid valve  8  attached to the solenoid valve block  5 . 
     In FIG. 2, reference numerals  5   d  and  10   d  denote bolt-insertion holes formed in the solenoid valve block  5  and a main body  10  of the sensor block  9  so as to permit the bolts  18  to be inserted therethrough, and reference numeral  7   d  denotes threaded holes formed in the manifold block  7  so as to permit the bolts  18  to be threadedly engaged therewith. 
     As shown in FIGS. 1 and 2, the sensor block  9  is comprised of the sensor block body  10 , a first plate  11  attached to one end face (first outer face)  10   a  of the sensor block body  10 , and a second plate  12  attached to an outer end face of the first plate  11 . The first and second plates  11 ,  12  are connected to the sensor block body  10  integrally therewith by means of, e.g., six bolts  17 . In FIG. 2, reference numerals  11   e  and  12   e  denote bolt-insertion holes formed in the first and second plates  11 ,  12  so as to permit the bolts  17  to be inserted therethrough, and reference numeral  10   e  denotes threaded holes formed in the sensor block body  10  so as to permit the bolts  17  to be threadedly engaged therewith. 
     The sensor block body  10  is formed with first through fifth oil passage portions  101 - 105 . Respective one ends of the first and second oil passage portions  101  and  102  open to an end face  10   a  of the sensor block body. Another end  102   b  of the second oil passage portion and respective one ends  103   a - 105   a  of the third, fourth and fifth oil passage portions open to an outer face (second outer face)  10   b  of the sensor block body on the side close to the solenoid valve block. Another end  101   b  of the first oil passage portion and respective other ends  103   b ,  104   b  and  105   b  of the third, fourth and fifth oil passage portions open to an outer face (third outer face)  10   c  of the sensor block body on the side close to the manifold block, respectively. 
     Each solenoid block  5  is provided with a solenoid-valve mounting section  5   a  to which a solenoid valve  8  is attached. Each solenoid block  5  is formed with first through fourth oil passage portions  81 - 84 . Respective one ends of the first through fourth oil passage portions  81 - 84  are aligned with another end  102   b  of the second oil passage portion and respective one ends  103   a - 105   a  of the third through fifth oil passage portions that are formed in the sensor block body  10 . Respective other ends of the first through fourth oil passage portions  81 - 84  are connected to the second output port, the first output port, the first input port and the second input port of the solenoid valve  8 , respectively. 
     Each manifold block  7  is formed with first through fourth oil passage portions  71 - 74 . Respective one ends  71   a - 74   a  of the first through fourth oil passage portions are aligned with respective other ends  101   b ,  103   b ,  104   b  and  105   b  of the first, third, fourth and fifth oil passage portions formed in the sensor block body  10 , respectively. Further, the first and second oil passage portions formed in the manifold block  7  have their other ends  71   b  and  72   b  opening to one end face of the manifold block  7  and are connected to the second and first ports  2 B,  2 A of the cylinder actuator  2  through hoses  23 ,  22  constituting part of the oil passage. Respective other ends  73   b ,  74   b  of the third and fourth oil passage portions formed in the manifold block  7  open to an upper face of the manifold block  7  and are connected to the oil tank  4  and the discharge side of the oil pump  3  through hoses  21 ,  20  constituting part of the oil passage, respectively. 
     In the following, the position sensors provided in a hydraulic system according to the present embodiment will be explained with reference to FIGS. 2,  4  and  5 . 
     Each of the position sensors is provided with first and second gears  13 ,  14  made of a metal material capable of influencing a magnetic field. In relation to the gears  13 ,  14 , the end face  10   a  of the sensor block body  10  is formed with two axial holes  10   f  that permit shafts, not shown, for supporting the gears  13 ,  14  to be inserted thereinto. Meanwhile, such shafts may be omitted to simply the construction. The first plate  11  is formed with a gear-accommodating space  11   a  for receiving the gears  13 ,  14 . The gear-accommodating space  11   a , which has first and second gear receiving sections each having a circular shape as viewed from end side, is formed as a whole into a cocoon shape. The gear-accommodating space  11   a  is formed at its vertically central portions with first and second passages  11   b  and  11   c  each having a semi-circular shape as viewed  5  from side end. The first and second passages  11   b ,  11   c  have their inner ends that are communicated with one ends  101   a ,  102   a  of the first and second oil passage portions formed in the sensor block body  10 , respectively, and cooperate with the gear-accommodating space  11   a  to constitute an oil passage portion (shown by reference numeral  106  in FIG. 1) that is interposed between the first and second oil passage portions  101 ,  102  of the sensor block body  10 . 
     The first and second gears  13 ,  14  of the position sensor are received in the gear-accommodating space  11   a  of the first plate  11  in a manner meshing with each other and being rotatable. When working oil flows in the oil passage as the working oil is supplied to or discharged from the cylinder actuator  2 , the flow of the working fluid in the aforementioned oil passage portion  106  acts on that portion of the gears  13 ,  14  where they are in mesh with each other, thereby causing the gears to rotate. 
     Each position sensor comprises first and second sensing elements  15 ,  16  each constituted by a magnetic proximity sensing element. The sensing elements  15 ,  16  are attached to mounting holes  12   a ,  12   b  formed in the second plate  12 , respectively, so as to face a peripheral portion of the first or second gear  13  or  14  on both sides of the meshing portion of the gears  13 ,  14 . The first and second sensing elements  15 ,  16  are configured to generate first and second sensor outputs which are 90 degree out of phase with each other, each time the first and second gear  13 ,  14  rotate for a predetermined angle so that a tooth portion  13   a  or  14   a  of the gear  13  or  14  passes in front of the sensing element. Each position sensor further comprises a detecting section  6   a  accommodated in the controller  6 . The detecting section  6   a  is configured to determine an operating position of the hydraulic actuator  2  based on the first and second sensor outputs supplied from the first and second sensing elements  15 ,  16 . 
     In the following, the operation of the hydraulic system of this embodiment will be explained. 
     In association with a given cylinder actuator  2 , it is assumed that the solenoid valve  8  is supplied at its second solenoid coil  8 Sb with electric power and assumes the second changeover position  8 SB, so that the rod  2   a  of the cylinder actuator  2  is in its most-backward position. 
     When the first solenoid coil  8 Sa of the solenoid valve  8  is supplied with electric power from the controller  6  so that the solenoid valve  8  is changed over from the second position  8 SB to the first position  8 SA, working oil is supplied from the oil pump  3  to the second input port of the solenoid valve  8  through the hose  20  and the oil passage portions  74 ,  105  and  84 . Then, the working oil is supplied from the second input port to the first cylinder chamber of the cylinder actuator  2  through the second output port, the oil passage portions  82 ,  103  and  72 , the hose  22  and the first port  2 A of the cylinder actuator  2 , whereby the rod  2   a  is caused to advance. 
     With this advancing movement of the rod  2   a , the working oil in the second cylinder chamber of the cylinder actuator  2  is discharged to the oil tank  4  through the hose  23 , the oil passage portions  71 ,  101 ,  106 ,  102  and  81 , the solenoid valve  8 , the oil passage portions  83 ,  104  and  73  and the hose  21 . As for the position sensor, the working oil enters from one end  101   a  of the oil passage portion  101  into the oil passage portion  106 , an then enters into a gap  13   b  between adjacent teeth portions of the first gear  13  and into a gap  14   b  between adjacent teeth portions  14   a  of the second gear  14 , thereby causing the first gear  13  to rotate anti-clockwise as shown by arrow A′ in FIG.  5  and causing the second gear  14  to rotate clockwise as shown by arrow B′. Then, the working oil flows from the oil passage portion  106  to one end  102   a  of the oil passage portion  102 . 
     As the first and second gears  13 ,  14  rotate in this manner, the first and second sensing elements  15 ,  16  of the position sensor generate the first and second sensor outputs that are out of phase with each other, each time a tooth portion  13   a  or  14   a  of the gear  13  or  14  passes in front of the sensing element associated therewith. The detecting section  6   a  of the controller  6  determines the direction of rotation of the gear  13  or  14  based on a phase relation between the first and second sensor outputs, and, based on the direction of gear rotation, determines that the rod  2   a  of the cylinder actuator  2  is moving forwardly. The detecting section  6   a  counts up the number of times of generating the first or second sensor output, and, based on counted number of times, determines an amount of advancement of the rod  2   a  from its most-backward position and by extension a moving position of the rod  2   a  (more generally, the operating position of a hydraulic actuator). 
     When the rod  2   a  of the cylinder actuator  2  advances up to its most-forward position, the piston  2   b , for instance, is in abutment with a stopper, not shown, in a condition that the working oil is kept supplied to the first cylinder chamber of the cylinder actuator  2 . Further, the total number of times of generating the sensor output of the first or second sensing element  15  or  16 , which is counted up from the start of advancement of the rod  2   a  from its most-backward position, reaches a predetermined value indicative of arrival to the most-forward position, so that the detecting section  6   a  detects that the rod  2   a  reaches its most-forward position. 
     Subsequently, when the solenoid valve  8  is changed over from the second position  8 SB to the first position  8 SA, the working oil is supplied from the oil pump  3  through the oil passage to the second cylinder chamber of the cylinder actuator  2 , so that the rod  2   a  retreats from its most-forward position to its most-backward position while discharging the working oil from the first cylinder chamber to the oil tank  4  through the oil passage. At this time, in relation to the position sensor, the working oil enters from one end  102   a  of the oil passage portion  102  into the oil passage portion  106 , thereby causing the first and second gears  13 ,  14  to rotate clockwise and anti-clockwise, respectively, as shown by arrows A and B in FIG.  4 . Then, the working oil flows from the oil passage portion  106  to one end  101   a  of the oil passage portion  101 . During the rotation of the gears, the first and second sensor outputs are supplied from the first and second sensing elements  15 ,  16  to the detecting section  6   a . Since the phase relation between the first and second sensor outputs is opposite to the case where the rod moves forwardly, the detecting section  6   a  determines that the rod  2   a  retreats, and detects an amount of the backward movement of the rod  2   a  from its most-forward position and by extension the current moving position of the rod (more generally, the current operating position of a hydraulic actuator) based on the number of times for which the first or second sensor output is generated. 
     When the rod  2   a  retreats up to the most-backward position, the piston  2   b  is brought in abutment with a stopper, not shown, and is maintained there, in a condition that the working oil is kept supplied to the second cylinder chamber of the cylinder actuator  2 , and the detecting section  6   a  detects that the rod  2   a  reaches its most-backward position. 
     In this hydraulic system, under the control of the controller  6 , working fluid is also supplied to and discharged from other cylinder actuators, so as to cause the rod of each cylinder actuator to move forward or backward. 
     As explained above, the hydraulic system of this embodiment is provided with a plurality of cylinder actuators  2  each having the manifold block  7 , the sensor block  9  and the solenoid valve block  5  that are combined into one piece, and thus includes plural sets of blocks  7 ,  9  and  5  respectively corresponding to the plurality of cylinder actuators  2 . The plural sets of blocks may be formed separately from or integrally with one another. 
     In a case where plural sets of blocks  7 ,  9  and  5  are formed integrally with one another, the hydraulic system may include a plurality of, e.g., four manifold blocks  7  that are formed integrally with one another so as to constitute an elongated block, as shown by way of example in FIG.  6 . Each manifold block  7  is formed with first and second oil passage portions  71 ,  72  as in the case of the manifold block  7  shown in FIGS. 1 and 2. In addition, it is formed with an upstream side of a third oil passage portion  73  and a downstream side of a fourth oil passage portion  74 . In FIG. 6, reference numerals  71   a  and  72   a  denote respective one ends of the first and second oil passage portions, and  71   b  and  72   b  denote respective other ends of these oil passage portions which are connected to the second and first ports  2 B,  2 A of the cylinder actuator  2  through hoses  23 ,  22 , respectively. Further, the elongated manifold block comprised of the four blocks is formed with those two holes so as to vertically extend therethrough, which individually constitute a downstream side of the third oil passage portion and an upstream side of the fourth oil passage portion and which are common to the four manifold blocks  7 . These two holes are in communication with the third and fourth oil passage portions of each manifold block, respectively, and have their upper ends  73   b ,  74   b  individually opening to an upper face of the elongated block and have their lower ends that are closed. The upper ends  73   b ,  74   b  are connected to the oil tank  4  and the oil pump  3  through hoses  21 ,  20 , respectively. Each block  7  constituting the elongated manifold block is mounted with a sensor block  9  and a solenoid valve block  5 . With this arrangement, sensing sections of four position sensors can be formed into one piece, so that wires connecting the sensing elements  15 ,  16  of the position sensors and the controller  6  can be collectively provided and can be shortened in length, even if four cylinder actuators  2  are disposed at different locations that are apart from one another. As a consequence, the construction of the hydraulic system can be simplified, the appearance thereof can be improved, and disconnection failures can be reduced. 
     The present invention is not limited to the foregoing embodiment, and may be modified variously. 
     For example, in the embodiment, a case has been explained where a position sensor of this invention is applied to a cylinder actuator configured to cause a rod to linearly move between its most-backward position and its most-forward position. However, a position sensor of this invention is applicable to a hydraulic actuator such as a hydraulic rotary actuator, i.e., a hydraulic motor, other than the cylinder actuator, so as to detect the operating position of such an actuator. Further, a hydraulic system to which the present invention is applied is not required to have a plurality of hydraulic actuators. The present invention is applicable to a hydraulic system provided with a single hydraulic actuator. 
     In the embodiment, a case has been explained in which working fluid is supplied and discharged through a two-position solenoid valve. However, the present invention is applicable to a hydraulic system using a three-position solenoid valve having a neutral position in addition to first and second changeover positions. With such an arrangement, a cylinder actuator rod (more generally, a movable member of an actuator) can be stopped at its arbitrary moving position by changing the solenoid valve over to the neutral position, and accordingly, the controller  6  is permitted to detect the direction of rod movement and the moving distance of the rod from its most-backward position or its most-forward position based on sensor outputs from sensing elements of the position sensor and to cause the rod to stop at a predetermined operating position by changing the solenoid valve over to the neutral position when such an operating position is reached. 
     In the hydraulic system according to the embodiment, the construction of the manifold block  7 , the sensor block  7  and the solenoid valve block  5  may be modified variously. For example, a pilot check valve, a throttle valve and the like may be interposed between oil passage portions formed in, e.g., the sensor block  9  and the manifold block  7 . 
     In other respects, the present invention may be modified within the scope of this invention.