Abstract:
An actuator includes: a rotor assembly including an output shaft having a male screw portion and having a pin attached thereto, and a sleeve having a female screw portion and threadedly engaged with the male screw portion of the output shaft so as to convert a rotary motion of the sleeve into a linear motion of the output shaft; a stator assembly disposed around the rotor assembly; a housing attached to the stator assembly; a coil spring; and a spring holder having slits to guide the pin into an axial direction. In the actuator, the coil spring and the spring holder constitute a fail-safe mechanism and are contained inside the housing, and the coil spring is arranged at such a location that the output shaft is caused to move in a direction to achieve a fail-safe operation. Thus, the actuator incorporates a fail-safe mechanism as an integral unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an actuator for a flow control valve to control flow in a fluid channel, and more particularly to an actuator integrally provided with a fail-safe function to mandatorily actuate a valve so as to provide a safe condition for a fluid channel when electric power supply is accidentally cut off for some reason.  
         [0003]     2. Description of the Related Art  
         [0004]     A flow control valve is conventionally used for gas flow control performed with a shutoff valve provided in a fluid channel of a gas supply equipment, and also for switching control of a refrigerant channel for air-conditioning. Such a flow control valve is driven by an actuator in which that the rotary motion of a motor shaft is converted into the linear motion of a valve by means of a pinion gear or a screw structure.  
         [0005]     Recently, a permanent magnet (PM) type stepping motor, which achieves a high positioning accuracy by means of step input, is increasingly combined with a mechanism to convert a rotary motion into a linear motion so that it can be used as an actuator.  
         [0006]     A flow control valve is used in, for example, a gas meter, which is equipped with a safety mechanism to prevent troubles. Such a safety mechanism has what is called a fail-safe function incorporated in a gas shutoff valve system, and operates such that a flow control valve is automatically shut so as to close a gas channel when power supply is stopped due to electric power failure or due to trouble of a control circuit.  
         [0007]      FIG. 7  is a schematic structural view of a conventional gas shutoff valve system provided with such a fail-safe function (refer to, for example, Japanese Patent Application Laid-Open No. 2003-042324).  
         [0008]     The gas shutoff valve system shown in  FIG. 7  includes a valve seat  101 , a valve plug  102  adapted to block the valve seat  101 , a valve shaft  103  connected to the valve plug  102 , a guide  104  to cause the valve shaft  103  to move linearly, a spring  105  to press the valve plug  102  toward the valve seat  101 , a stepping motor  106  as a rotary device having coils  111  and a rotor  112  with an output shaft  107 , a feed screw  108  as a feeding element formed on the output shaft  107 , a moving block  109  engaging threadedly with the feed screw  108  so as to thrust the valve plug  102 , a rotation stopper  110  to prohibit the moving block  109  from rotating, and a lid  113 . When something abnormal occurs in gas supply, pulsing currents with respective phases different from each other are applied to the coils  111  so as to rotate the rotor  112 , whereby the moving block  109  moves toward the lid  113 , and the valve plug  102 , urged by the spring  105 , is caused to move together so as to block the valve seat  101  thus stopping gas from flowing through.  
         [0009]      FIG. 8  is a schematic structural view of another conventional gas shutoff valve system (refer to, for example, Japanese Patent Application Laid-Open No. 2003-222259).  
         [0010]     In the gas shutoff valve system shown in  FIG. 8 , when there is something abnormal detected in gas supply, pulsing currents with respective phases different from each other are applied to respective wires  122  of coils  121  thereby rotating a rotor  123 , and then a moving block  124 , which engages threadedly with a feed screw  127  fixedly connected to the rotor  123 , and which has a groove engaging with a rib  126  for prohibition of rotation, is caused to linearly move in the axial direction due to the feed screw  127  rotating together with the rotor  123 . A valve plug  128  is engagingly attached to the moving block  124 , and when the moving block  124  moves toward a valve seat  130 , a sealing pad  129  of the valve plug  128  is brought into contact with the valve seat  130 . And, when the moving block  124  further moves toward the valve seat  130 , a coil spring  131  is further contracted so that a spring retaining portion  131  of the moving block  124  is brought into contact with a cylinder portion  133  of a sealing pad holding member  132 , and the sealing pad  129  sags and is contracted, which causes the repulsion of the moving block  124  to exceed the thrust of the feed screw  127  consequently stopping the rotation of the rotor  123 . At this time, the valve plug  128  is firmly pressed against the valve seat  130  by force of the coil spring  131 , and gas flow is shut off. If electric power is turned off in this state, the rotor  123  keeps its position due to holding torque, and the valve plug  128  is kept pressed against the valve seat  130  so as to keep the valve closed.  
         [0011]     In the gas shutoff valve system shown in  FIG. 7  as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-042324, the spring  105  to press the valve plug  102  against the valve seat  101  is provided in a gas channel  114 , which makes the system complicated, and the spring  105  is difficult to replace, which results in deteriorated workability in maintenance.  
         [0012]     In the gas shutoff valve system shown in  FIG. 8  as disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-222259, since the coil spring  131  is attached at the valve plug  128 , the valve plug  128  is prohibited from downsizing. Also, the coil spring  131  must be prepared so as to fit to the configuration of each valve plug  128 , which leads to an increased number of components.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention has been made in light of the above problems, and it is an object of the present invention to provide an actuator which leverages a motor to actuate a flow control valve in a fluid channel, and which has built-in capability of performing a fail-safe operation, wherein maintenance work can be performed easily.  
         [0014]     In order to achieve the object, according to an aspect of the present invention, an actuator comprises: (a) a rotor assembly including *an output shaft having a screw portion constituted by a male screw formed on an outer circumference of the output shaft, having a plain rod portion toward the other end thereof, and having a pin attached to the plain rod portion so as to protrude therefrom toward an outside of the pin in a radial direction, *a sleeve having a screw portion constituted by a female screw formed on an inner circumference of the sleeve, the sleeve rotatably supported by bearings disposed respectively at both axial ends of the sleeve, wherein the screw portion of the sleeve is threadedly engaged with the screw portion of the output shaft so as to convert a rotary motion of the sleeve into a linear motion of the output shaft, and *a magnet fixedly disposed at an outer circumference of the sleeve; (b) a stator assembly functioning to rotate the magnet, and including **a coil, **a bobbin having the coil wound therearound, and **stator yokes coupled to each other so as to sandwich the bobbin; (c) a housing having a hollow-cylindrical configuration, and attached to the stator assembly; and (d) a fail-safe mechanism in the housing, the fail-safe mechanism comprising a coil spring disposed around the output shaft, and a spring holder disposed around the output shaft, and having slits to guide the pin attached to the output shaft into an axial direction movement. In the actuator described above, the coil spring and the spring holder constitute a fail-safe mechanism and are contained inside the housing. Consequently, a fail-safe mechanism is incorporated in the actuator as an integral unit, and therefore the mounting work and the maintenance work can be implemented easily.  
         [0015]     In the aspect of the present invention, the coil spring may be arranged at such a location that the output shaft is caused to move in a direction to achieve a fail-safe operation. Consequently, a versatile fail-safe operation is available according to the arrangement of the coil spring.  
         [0016]     In the aspect of the present invention, the coil spring may be an extension spring and located between the pin attached to the output shaft and the spring holder so that when electric power supply to the coil of the stator assembly is cut off, the output shaft is caused to move toward the stator assembly by force of the coil spring. This arrangement is suitable for a fail-safe operation achieved by opening a valve.  
         [0017]     In the aspect of the present invention, the actuator may further comprise a stopper plate disposed on one of the bearings located closer to the housing, and the coil spring may be an extension spring and located between the pin attached to the output shaft and the stopper plate so that when electric power supply to the coil of the stator assembly is cut off, the output shaft is caused to move away from the stator assembly by force of the coil spring. This structure is suitable for a fail-safe operation achieved by shutting valve.  
         [0018]     In the aspect of the present invention, the coil spring may be a compression spring and located between the pin attached to the output shaft and the spring holder so that when electric power supply to the coil of the stator assembly is cut off, the output shaft is caused to move away from the stator assembly by force of the coil spring. This arrangement is suitable for a fail-safe operation achieved by shutting a valve.  
         [0019]     In the aspect of the present invention, the actuator may further comprise a stopper plate disposed on one of the bearings located closer to the housing, and the coil spring may be a compression spring and located between the pin attached to the output shaft and the stopper plate so that when electric power supply to the coil of the stator assembly is cut off, the output shaft is caused to move toward the stator assembly by force of the coil spring. This arrangement is suitable for a fail-safe operation achieved by opening a valve.  
         [0020]     In the aspect of the present invention, the housing may include a flange portion at one end portion thereof, and may be fixedly attached to the stator assembly by means of a front plate pressing on the flange portion. Thus, the housing is securely attached to the stator assembly.  
         [0021]     In the aspect of the present invention, the actuator may further comprise a rotor case having a cup-like configuration, the rotor case being disposed between the rotor assembly and the stator assembly so as to constitute a hermetic partition therebetween. Consequently, a gap between the rotor assembly and the stator assembly is completely sealed up to ensure a safe condition.  
         [0022]     In the aspect of the present invention, the actuator may further comprise a sealing member disposed around the housing. This structure seals up a gap between the housing and the equipment applied, thus a safe condition is ensured. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a side view (partly cross sectioned) of an actuator according to a first embodiment of the present invention;  
         [0024]      FIG. 2  is a cross sectional view of a fluid channel in which the actuator of  FIG. 1  is mounted;  
         [0025]      FIG. 3  is an explanatory view of the fluid channel of  FIG. 2  for explanation of a fail-safe operation, wherein a flow control valve is closed;  
         [0026]      FIG. 4  is a side view (partly cross sectioned) of an actuator according to a second embodiment of the present invention;  
         [0027]      FIG. 5  is a cross sectional view of a fluid channel in which the actuator of  FIG. 4  is mounted;  
         [0028]      FIG. 6  is an explanatory view of the fluid channel of  FIG. 5  for explanation of a fail-safe operation, wherein a flow control valve is open;  
         [0029]      FIG. 7  is a schematic structural view of a conventional gas shutoff valve system; and  
         [0030]      FIG. 8  is a schematic structural view of another conventional gas shutoff valve system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0032]     Referring to  FIG. 1 , an actuator  1  according to a first embodiment of the present invention generally comprises a rotor assembly  5 , a stator assembly  13 , and a housing  17 .  
         [0033]     The rotor assembly  5  includes an output shaft  2 , a sleeve  3 , and a magnet  4  with multi-polar magnetization. The output shaft  2  has a screw portion  2   a  and a plain rod portion  2   b , the sleeve  3  is formed of synthetic resin and has a screw portion  3   a  located at an elevated portion on its inner circumferential surface, and the magnet  4  is shaped hollow-cylindrical and fixedly disposed at the outer circumference of the sleeve  3 . The screw portion  2   a  is located at a portion of the outer circumferential surface of the output shaft  2  and engages threadedly with the screw portion  3   a  of the sleeve  3 . A head piece  25  is attached to an open end of the plain rod portion  2   b  of the output shaft  2 , and a valve plug (not shown in  FIG. 1 ) is attached to the head piece  25  so as to oppose a valve seat (not shown in  FIG. 1 ) disposed in a fluid channel (not shown in  FIG. 1 ) thereby functioning to open and close the fluid channel.  
         [0034]     The stator assembly  13  is disposed outside the magnet  4  of the rotor assembly  5  and includes first and second stator units  13   a  and  13   b , each of which has stator yokes  7  and  8  combined so as to house a bobbin  9  having a coil  6  wound therearound. The stator assembly  13  further includes a wiring board unit  14  and a front plate  23  which is placed on the first stator unit  13   a  coaxially stacked on the second stator unit  13   b . The stator yokes  7  and  8  each include a plurality of pole teeth facing the outer circumference of the magnet  4  of the rotor assembly  5 . The bobbin  9  has a substantially squared C-shaped cross section and is provided integrally with a terminal block  10 , and terminal pins  11  are implanted in the terminal block  10 . A cover  12  is attached so as to enclose the coil  6  thereby preventing dusts from coming in to sit on the coil  6 . The front plate  23  is for attachment of the actuator  1  to an outside apparatus. The constituent components described above are resin injection molded for solid structure. The wiring board unit  14  includes a printed board  14   a  and a pin retainer  14   b . The pin retainer  14   b  is disposed outside the terminal blocks  10  and holds the printed board  14   a . Wire cables  16  connected to the printed board  14   a  lead out through the pin retainer  14   b.    
         [0035]     The housing  17  is shaped in a hollow-cylinder, disposed around the plain rod portion  2   b  of the output shaft  2 , and has a flange portion  17   a  toward one end thereof. A spring holder  26  shaped in a cylinder with solid and hollow portions, and having slits  26   a  extending in the axial direction, a coil spring  27  working as an extension spring adapted to return to an initial condition after contraction, a stopper plate  28 , a pin  29 , and two retainer pins  30  are fitted in the housing  17 , and O-rings  31  and  32  are disposed in respective grooves formed on the outer circumferential surface of the housing  17 . The pin  29  is inserted orthogonally through the plain rod portion  2   b  of the output shaft  2  and has its both ends sticking out. The coil spring  27  is disposed between the pin  29  and the stopper plate  28  and in contact with both thereof so as to exert its action on both thereof.  
         [0036]     The actuator  1  further comprises a rotor case  20  shaped like a cup and disposed between the rotor assembly and the stator assembly, a bearing  18  and a sleeve bearing  33  to support respective ends of the sleeve  3 , and collar members  19  and  34 .  
         [0037]     &lt;Manufacturing Process&gt; 
         [0038]     The stator assembly  13  is manufactured as follows.  
         [0039]     The stator yokes  7  and  8 , which are made of a soft-magnetic material, such as a galvanized steel plate (SECC), an electromagnetic soft steel (SUY), and a silicon steel plate, and which each have a plurality of pole teeth arrayed in a comb-like arrangement, are coupled to each other so as to sandwich the bobbin  9  having the coil  6  wound therearound, such that respective pole tooth arrays intermesh with each other with a gap formed therebetween at a phase difference of 180 degrees in terms of electrical angle. Thus, the first stator unit  3   a  is completed. The second stator unit  3   b  is fabricated in the same way.  
         [0040]     The first stator unit  3   a  is stacked on the second stator unit  3   b  in a mold (not shown) such that the first stator unit  3   a  is shifted from the second stator unit  3   b  by an electrical angle of 90 degrees. Synthetic resin, for example, polybutylene terephthalate (PBT), is injected in the mold. The synthetic resin injected fills up the gap between the respective pole tooth arrays of the stator yokes  7  and  8  on each of the first and second stator units  3   a  and  3   b , and at the same time forms lower bosses  21  and upper bosses  22 . Thus, the first and second stator units  3   a  and  3   b  are rigidly coupled to each other by resin.  
         [0041]     The front plate  23  having an opening at its center is put on and welded to the first stator unit  3   a  such that holes formed in the front plate  23  are engaged with the aforementioned upper bosses  22 . Then, the wiring board unit  14 , which includes the printed board  14   a  and the pin retainer  14   b  formed of synthetic resin, is attached to the outer circumference of the first and second stator units  3   a  and  3   b  coupled together. Thus, the stator assembly  13  is completed.  
         [0042]     The rotor assembly  5  is manufactured as follows.  
         [0043]     The magnet  4  is fixed onto the outer circumference of the sleeve  3  which is provided with the screw portion  3   a  constituted by a female screw, the bearings  18  and  33  are attached respectively to both axial ends of the sleeve  3 , and the sleeve  3  thus furnished is put in the rotor case  20 . The rotor case  20  is shaped into a cup-like configuration and has an inner diameter slightly larger than an outer diameter of the magnet  4  so as to allow the magnet  4  to rotate.  
         [0044]     The stopper plate  28  is placed on the bearing  18 , and the one end of the housing  17  having the flange portion  17   a  is inserted between the bearing  18  and the rotor case  20  so that the housing  17  presses the stopper plate  28  for fixed attachment. As described above, the housing  17  is hollow-cylindrical and houses the spring holder  26  and the coil spring  27 .  
         [0045]     The output shaft  2  having the pin  29  as a rotation stopper inserted orthogonally therethough is put through the coil spring  27 , and the screw portion  2   a  of the output shaft  2  is brought into engagement with the screw portion  3   a  of the sleeve  3 . The output shaft  2  has the screw portion  2   a  and the plain rod portion  2   b  which are continuous with each other. At a later process, the head piece  25  as a stopper member is to be attached to the open end of the output shaft positioned toward the plain rod portion  2   b . A screw hole  25   a  is formed at the center of the head piece  25 , and a valve plug for controlling flow of fluid such as gas is fitted into the screw hole  25   a . The pin  29  serves also as a stopper for the coil sprint  27 .  
         [0046]     The spring holder  26  is attached around the plain rod portion  2   b  such that the both ends of the pin  29  sticking out from the plain rod portion  2   b  are put through the slits  26   a  of the spring holder  26 , and then the two retainer pins  30  are put through respective holes formed at the other end of the housing  17  opposite to the one end having the flange portion  17   a  and through respective grooves formed at one end of the spring holder  26  so that spring holder  26  is not allowed to move with respect to the housing  27 . Thus, the rotor assembly  5  is completed.  
         [0047]     Respective two portions of the housing  17  and also the spring holder  26 , through which the two retainer pins  30  are put, are positioned to oppose each other in the embodiment, but do not have to be so positioned, as long as the spring holder  26  is prevented from rotating with respect to the housing  17 .  
         [0048]     The rotor case  20  is inserted between the magnet  4  and the stator yokes  7  and  8 , and therefore is preferably made of a non-magnetic material, for example, stainless steel, and preferably has a minimum possible thickness with a sufficient strength. Also, the rotor case  20  is preferably fabricated by method of drawing in view of cost. The O-ring  31  is made of an elastic material, such as rubber, is fitted in the groove formed on the outer circumference of the housing  17  so as to hermetically make contact with the rotor case  20 , and serves to prevent the housing  17  from coming off the rotor case  20  as well as to seal up the gap between the housing  17  and the rotor case  20 .  
         [0049]     The rotor case  20  having the rotor assembly  5  housed therein is press-fitted in the center opening of the stator assembly  13 , and an additional front plate  24  having a center opening is placed on the front plate  23  so as to press the flange portion  17   a  of the housing  17 . The additional front plate  24  may be fixedly connected to the front plate  23  by mechanical means or by welding. If the front plate  23  is so structured as to press the flange portion  17   a  of the housing  17 , then the additional front plate  24  can be eliminated. And, as mentioned earlier, the head piece  25  is attached to the open end of the output shaft  2 , thus completing the actuator  1 .  
         [0050]     &lt;Operation&gt; 
         [0051]     The actuator  1  shown in  FIG. 1  has a fail-safe function such that when something abnormal occurs in the actuator  1 , for example, power supply is stopped, or a wire for a coil is broken thus interrupting current, a flow control valve is either automatically driven for a safe condition, or automatically closed to shut off gas flow.  
         [0052]     Referring to  FIG. 2 , the housing  17  of the actuator  1  is put in a hole connecting to a fluid channel  38  such that the step portion of the additional front plate  24  is fitted loosely in a circular recess having a larger diameter than the aforementioned hole. The O-ring  32  provided at the end of the housing  17  seals up the gap between the housing  17  and the wall of the hole. A valve plug  35  is attached to the head piece  25  by press-fitting or by screwing, a valve seat  36  is provided in the fluid channel  38 , and the valve plug  35  and the valve seat  16  in combination constitute a flow control valve  37 .  
         [0053]     When current is applied to the coil  6  of each of the first and second stator units  3   a  and  3   b , the stator yokes  7  and  8  are magnetized with N and S poles (or S and N poles), respectively, whereby the magnet  4  is rotated. Then, the screw portion  3   a  of the sleeve  3  fixed to the magnet  4  is rotated together, and the output shaft  2  which has the screw portion  2   a  engaging threadedly with the screw portion  3   a  is urged to rotate but is prevented from rotating due to the pin  29  set in the axially extending slits  26   a  of the spring holder  26  and therefore is caused to linearly move in the axial direction. When the output shaft  2  moves toward the rotor sleeve  3  causing the pin  29  to compress the spring coil  27 , the valve plug  35  disengages from the valve seat  36 , thus the flow control valve  37  is opened allowing gas to flow through the fluid channel  38  as shown in  FIG. 2  (the arrows indicate the direction of gas flow). The opening degree of the flow control valve  37  is controlled by the number of steps of the actuator  1 .  
         [0054]     &lt;Fail-Safe Operation&gt; 
         [0055]      FIG. 3  shows a safe condition, where the valve plug  35  makes tight contact with the valve seat  36  to keep the flow control valve  37  securely closed in order to stop gas flow when power supply is interrupted due to a power failure, or when current flow is interrupted by a wire broken due to a trouble in a control circuit (not shown).  
         [0056]     When current flow to the coil  6  is accidentally interrupted, the pin  29  is pressed toward the valve plug  37  by a restoring force of the coil spring  27  with respect to the stopper plate  28  as a reference position, and the output shaft  2  having the pin  29  fixed thereto moves toward the valve seat  36  forcing the sleeve  3  and the magnet  4  to rotate due to the screw portion  2   a  threadedly engaging with the screw portion  3   a  until the pin  29  hits the solid portion of the spring holder  26 . Thus, the valve plug  35  is brought into hermetic contact with the valve seat  36  by means of the coil spring  27  so as to make the flow control valve  37  closed thereby shutting off gas flow as shown in  FIG. 3 .  
         [0057]     A second embodiment of the present invention will hereinafter be described with reference to  FIG. 4 . An actuator  50  according to the second embodiment of the present invention is structured to operate such that when a load exceeding a predetermined value is accidentally applied to a valve plug, a flow control valve is opened so as to prevent damages to the valve plug and the actuator. The actuator  50  shown in  FIG. 4  is structured basically same as the actuator  1  according to the first embodiment shown in  FIG. 1 , and therefore in explaining the example of  FIG. 4 , any component parts corresponding to those in  FIG. 1  are denoted by the same reference numerals, and description will be focused on the difference from the actuator  1  shown in  FIG. 1 .  
         [0058]     In the actuator  1  according to the first embodiment shown in  FIG. 1 , the coil spring  27  as an extension spring is provided between the stopper plate  28  and the pin  29  so as to force the valve plug  35  to move to such a position as to make the flow control valve  37  closed, thus performing a fail-safe operation. On the other hand, in the actuator  50  according to the second embodiment shown in  FIG. 4 , a fail-safe operation is performed such that a coil spring  27  as an extension spring is provided between the solid portion of a spring holder  26  and a pin  29  so as to force a valve plug  35  to move to such a position as to make a flow control valve  37  open.  
         [0059]     The difference from the actuator  1  is also found in the configuration of the spring holder  26 , and in the position of the coil spring  27 . Specifically, the spring holder  26  has shorter slits  26   a  in the actuator  50  than in the actuator  1 , and the pin  29  is located closer to a sleeve  3  in the actuator  50  than in the actuator  1  thereby allowing the coil spring  27  to be provided between the solid portion of the spring holder  26  and the pin  29 .  
         [0060]     &lt;Manufacturing Process&gt; 
         [0061]     A stator assembly  13  of the actuator  50  shown in  FIG. 4  is fabricated in the same way as the stator assembly  13  of the actuator  1  shown in  FIG. 1 , and therefore an explanation thereof will be omitted.  
         [0062]     A rotor assembly  5  is fabricated as follows.  
         [0063]     A magnet  4  is fixedly attached onto the outer circumference of a sleeve  3 , which is formed of, for example, PBT resin, and which is provided with a screw portion  3   a  constituted by a female screw.  
         [0064]     An output shaft  2  includes a screw portion  2   a  and a plain rod portion  2   b . The screw portion  2   a  is constituted by a male screw, and a head piece  25  as a stopper is attached to an open end of the plain rod portion  2   b . The head piece  25  has a screw hole  25   a  formed at its center, and the aforementioned valve plug  35  for flow control of fluid such as gas is fitted into the screw hole  25   a.    
         [0065]     Bearings  18  and  33  are attached respectively to both axial ends of the sleeve  3 , the sleeve  3  having also the magnet  4  attached thereto is put in a rotor case  20 , a stopper plate  28  is placed on the bearing  18 , and one end of a housing  17  having a flange portion  17   a  is inserted inside the rotor case  20  so that the housing  17  presses the stopper plate  28  for fixed attachment. The housing  17  is hollow-cylindrical and houses the spring holder  26  and the coil spring  27 .  
         [0066]     The coil spring  27  is put in the spring holder  26 , and the output shaft  2  with the pin  29  as a rotation stopper is inserted through the spring holder  26  and the coil spring  27 . The pin  29  is put orthogonally through the plain rod portion  2   b  of the output shaft  2  so as to have its both ends sticking out from the outer circumference of the plain rod portion  2   b.    
         [0067]     The screw portion  2   a  of the output shaft  2  is engaged threadedly with the screw portion  3   a  of the sleeve  3  such that the both ends of the pin  29  as a rotation stopper are set in respective slits  26   a  of the spring holder  26 . Then, two retainer pins  30  are put through respective holes formed at the housing  17  and through respective grooves formed at the solid portion of the spring holder  26  so that spring holder  26  is not allowed to move with respect to the housing  27 . Thus, the rotor assembly  5  is completed.  
         [0068]     Respective two portions of the housing  17  and also the spring holder  26 , through which the two retainer pins  30  are put, are positioned to oppose each other in the embodiment, but do not have to be so positioned, as long as the spring holder  26  is prevented from rotating with respect to the housing  17 .  
         [0069]     The rotor case  20  is inserted between the magnet  4  and stator yokes  7  and  8 , and therefore is preferably made of a non-magnetic material, for example, stainless steel, and preferably has a minimum possible thickness with a sufficient strength. Also, the rotor case  20  is preferably fabricated by method of drawing in view of cost. An O-ring  31  is made of an elastic material, such as rubber, is fitted in the groove formed on the outer circumference of the housing  17  so as to hermetically make contact with the rotor case  20 , and serves to prevent the housing  17  from coming off the rotor case  20  as well as to seal up the gap between the housing  17  and the rotor case  20 .  
         [0070]     The rotor case  20  with the rotor assembly  5  housed therein is press-fitted in the center opening of the stator assembly  13 , and an additional front plate  24  having a center opening is placed on a front plate  23  so as to press the flange portion  17   a  of the housing  17 . The additional front plate  24  is fixedly connected to the front plate  23  by mechanical means or by welding. If the front plate  23  is so structured as to press the flange portion  17   a  of the housing  17 , then the additional front plate  24  can be eliminated. And, as mentioned earlier, the head piece  25  is attached to the open end of the plain rod portion  2   b  of the output shaft  2 . In this connection, depending on the length of the output shaft  2 , current is applied to coils  6  so as to move the output shaft  2  in the direction to compress the coil spring  27  thereby making the open end of the plain rod portion  2   b  stick out from the end face of the housing  17 . Thus, the actuator  50  is completed.  
         [0071]     &lt;Operation&gt; 
         [0072]     The above-described actuator  50  shown in  FIG. 4  is adapted to open the flow control valve  37  when a pressure exceeding a predetermined value is applied to the valve plug  35  in order to protect the valve plug  35  as well as the actuator  50  from being damaged.  
         [0073]     Referring to  FIG. 5 , the housing  17  of the actuator  50  is put in a hole connecting to a fluid channel  38  such that an O-ring  32  provided at the end of the housing  17  seals up the gap between the housing  17  and the wall of the hole. The valve plug  35  is attached to the head piece  25  by press-fitting or by screwing, a valve seat  36  is provided in the fluid channel  38 , and the valve plug  35  and the valve seat  16  in combination constitute the flow control valve  37 .  
         [0074]     When current is applied to the coil  6 , the stator yokes  7  and  8  are magnetized with N and S poles (or S and N poles), respectively, whereby the magnet  4  is rotated. Accordingly, the screw portion  3   a  of the sleeve  3  fixed to the magnet  4  is rotated together, and the output shaft  2  which has the screw portion  2   a  engaging threadedly with the screw portion  3   a  is urged to rotate but is prevented from rotating due to the pin  29  set in the axially extending slits  26   a  of the spring holder  26  and therefore is caused to linearly move in the axial direction. The output shaft  2  moves toward the valve seat  36  causing the pin  29  to compress the spring coil  27 , and stops at a predetermined position for a predetermined opening degree of the flow control valve  37  (the arrows indicate the direction of gas flow). The opening degree of the flow control valve  37  is controlled by the number of steps of the actuator  1 .  
         [0075]     &lt;Fail-Safe Operation&gt; 
         [0076]      FIG. 6  shows a damage-preventive state, in which the flow control valve  37  is opened (or opened wider) so as to relax gas pressure on the valve plug  35  when the pressure of gas flowing through the fluid channel  38  exceeds a prescribed value, whereby the valve plug  35  and the actuator  50  are prevented from being damaged.  
         [0077]     When the gas pressure in the fluid channel  38  exceeds a prescribed value, the gas pressure is detected by a sensor (not shown), and an output signal is fed to a control circuit (not shown). Then, the control circuit outputs a control signal to interrupt power supply to the coils  6 . When exciting current is interrupted to the coils  6 , the coil spring  27  has its configuration restored causing the output shaft  2  to move toward the sleeve  3  forcing the sleeve  3  and the magnet  4  to rotate due to the screw portion  2   a  threadedly engaging with the screw portion  3   a  until the pin  29  hits the stopper plate  28 . Consequently, the valve plug  35  is pulled away from the valve seat  36  so as to make the flow control valve  37  open wider, whereby the valve plug  35  and the actuator  50  are prevented from being damaged due to gas overpressure.  
         [0078]     In the second embodiment described above, the stopper plate  28  is not absolutely required if the pin  29  is otherwise stopped so as to be prohibited from coming into contact with the end of the sleeve  3  including the bearing  18 .  
         [0079]     In the embodiments described above, an extension coil spring is used for a fail-safe operation in case of emergency, but the present invention is not limited to use of an extension coil spring but may alternatively use a compression coil spring which is extended by application of an external force and is restored for contraction by release of the external force.  
         [0080]     When a compression coil spring is used, components must be attached to the compression coil spring in such a manner as to allow its spring force to duly act on the components. Specifically, for example in case of the first embodiment shown in  FIG. 1 , both ends of the compression coil spring are engagingly attached to the stopper plate  28  and the pin  29 , respectively, and in case of the second embodiment shown in  FIG. 4 , both ends of the compression coil spring are engagingly attached to the solid portion of the spring holder  26  and the pin  29 , respectively. Also, the compressed height of the coil spring must be taken into consideration when setting the position of the pin  29 .  
         [0081]     Alternatively, either an extension coil spring or a compression coil spring may be fastened with an accumulator latch, wherein when reduction in voltage or current of power supply is detected, the latch is electromagnetically released so as to force the output shaft into a safe condition.  
         [0082]     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.  
         [0083]     This application is based on Japanese Patent application JP 2004-208122, filed Jul. 15, 2004, the entire content of which is hereby incorporated by reference. This claim for priority benefit is being filed concurrently with the filing of this application.