Abstract:
Disclosed is a linear actuator, including: a screw unit rotatably driven by a driving motor; a nut unit screwed to the screw unit; a slider attached to the nut unit, in which the slider and the nut unit move linearly by the rotation of the screw unit; a guide restricting both surfaces of the slider; a first antenna attached to a side wall of the guide opposite to the slider; a second antenna attached to a side wall of the guide opposite to the slider, and opposing the first antenna; and a third antenna attached to both side walls of the slider, being interposed between the first antenna and the second antenna, wherein position of the slider is detected based on a change in capacitance due to displacement of the third antenna with respect to the first antenna and the second antenna.

Description:
CLAIM OF PRIORITY 
   The present application claims priority from Japanese application JP 2005-196952 filed on Jul. 6, 2005, the content of which is hereby incorporated by reference into this application. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to a linear actuator. 
   2. Description of the Related Art 
   A linear actuator converts a rotational motion into a linear motion. For example, Japanese Patent Laid-Open. No. H11(1999)-264451 introduced a linear actuator, which converts the rotational motion of a motor mounted on the main body into the linear motion by a power transmission mechanism which comprises a feed screw unit driven in rotation via the motor and a feed nut unit screwed to the feed screw unit, thereby moving linearly the actuation member in the axial direction of the feed screw unit. In addition, Japanese Patent Laid-Open No. 2002-58271 disclosed a linear servo actuator capable of detecting the position of a slider with high accuracy. 
   To this end, the linear servo actuator comprises a magnetostriction potentiometer for detecting the position of a slider, and a position control unit in charge of feed back control of the position of the slider based on the difference between the position of the slider and a command position. 
   However, these conventional techniques have the following problems. 
   For instance, the linear actuator disclosed in Japanese Patent Laid-Open No. H11(1999)-264451 is not equipped with a position sensor for detecting the position of an actuation member. Thus, to execute the positioning of the actuation member, a sensor has to be installed additionally. 
   On the other hand, the linear servo actuator disclosed in Japanese Patent Laid-Open No. 2002-58271 is equipped with a magnetostriction potentiometer as a position sensor for detecting the position of the slider. The magnetostriction potentiometer comprises a magnetostriction scale and a magnetostriction head. Since the magnetostriction scale was mounted on a platen which is a fixed member and the magnetostriction head on the slider formed movably on the platen, the overall actuator size became too large. In addition, an expensive linear pulse motor implemented as a driving motor increased the price of the actuator. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a compact, high precision linear actuator. 
   To achieve the above objects and advantages, there is provided a linear actuator, including: a screw unit rotatably driven by a driving motor; a nut unit screwed to the screw unit; a slider attached to the nut unit, in which the slider and the nut unit move linearly by the rotation of the screw unit; a guide restricting both surfaces of the slider; a first antenna attached to a side wall of the guide opposite to the slider; a second antenna attached to a side wall of the guide opposite to the slider, and opposing the first antenna; and a third antenna attached to both side walls of the slider, being interposed between the first antenna and the second antenna, wherein position of the slider is detected based on a change in capacitance due to displacement of the third antenna with respect to the first antenna and the second antenna. 
   In an exemplary embodiment of the invention, the second antenna is grounded and the linear actuator of claim further comprises an oscillatory circuit applying a radio frequency AC voltage to the first antenna, a resistor connected between the oscillatory circuit and the first antenna, a signal processing circuit processing a signal from the first antenna, and a control unit calculating the position of the slider based on the signal from the signal processing circuit and controlling the drive of the driving motor. 
   In an exemplary embodiment of the invention, capacitance of a first condenser formed by the first antenna and the third antenna, capacitance of a second condenser formed by the second antenna and the third antenna, and capacitance of a third condenser formed by the first antenna and the second antenna change by a movement of the slider, and the position of the slider with respect to the guide is calculated by detecting from the oscillatory circuit a change in the AC voltage applied to the first antenna. 
   In an exemplary embodiment of the invention, the third antenna is formed of a conductor being bent to oppose the first antenna and the second antenna in continuation. 
   Another aspect of the invention provides a linear actuator, including: a rotating bar driven by a driving motor; a spiral spring screwed to the rotating bar; a slider attached to the spiral spring; and a guide accommodating the slider and the spiral spring, wherein the spiral spring and the slider move linearly inside the guide by the rotation of the rotating bar; the linear actuator further includes: a first antenna installed in a longitudinal direction of the guide; an second antenna, which is installed on the guide in a manner to be faced with the first antenna and which is grounded; a third antenna attached to the slider opposing the first antenna and the second antenna; an oscillatory circuit applying a radio frequency AC voltage to the first antenna; a resistor connected between the oscillatory circuit and the first antenna; a signal processing circuit processing a signal from the first antenna; and a control unit calculating the position of the slider based on the signal from the signal processing circuit and controlling the drive of the driving motor. 
   The linear actuator of the invention detects a displacement amount of the slider with respect to the guide in case the driving motor is OFF, and calculates an external force applied to the slider by utilizing the displacement amount and a spring constant of the spiral spring. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above aspects and features of the present invention will be more apparent by describing certain embodiments of the present invention with reference to the accompanying drawings, in which: 
       FIG. 1  is a front view illustrating a constitution of a linear actuator according to a first embodiment of the present invention; 
       FIG. 2  is a perspective view of the embodiment of  FIG. 1 ; 
       FIG. 3  is an enlarged view of a first stopper area in a linear actuator of the embodiment of  FIG. 1 ; 
       FIG. 4  illustrates the constitution of a displacement sensor in a linear actuator of the embodiment of  FIG. 1 ; 
       FIG. 5  shows another example of the constitution of a detecting unit; 
       FIG. 6  shows still another example of the constitution of a detecting unit; 
       FIG. 7  shows still another example of the constitution of a detecting unit; 
       FIG. 8  is a flow chart explaining an operation of a linear actuator of the embodiment of  FIG. 1 ; 
       FIG. 9  graphically illustrates a relation between distance from the initial position of a slider and output voltage of a displacement sensor; 
       FIG. 10  illustrates a constitution of a linear actuator for detecting the initial position of a slider using a separate method; 
       FIG. 11  is a front view illustrating a constitution of a linear actuator according to a second embodiment of the present invention; 
       FIG. 12  is a perspective view of the embodiment of  FIG. 11 ; 
       FIG. 13  is a front view of the embodiment of  FIG. 11  without a guide; 
       FIG. 14  is an explanatory diagram showing the installation of a rotating bar and a spiral spring; 
       FIG. 15  is an explanatory diagram showing the installation of a displacement sensor in a linear actuator of the embodiment of  FIG. 11 ; and 
       FIG. 16  illustrates a constitution of a linear actuator of the embodiment of  FIG. 11  for detecting an initial position of a slider with a different method. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings in  FIGS. 1 to 16 . 
   A linear actuator of the invention converts the rotational motion of a driving motor into the linear motion using a feed screw unit, thereby moving an article linearly. Particularly, the present invention realizes a compact (i.e., small outer diameter), cost-effective linear actuator. 
   Embodiment I 
     FIG. 1  is a front view illustrating a constitution of a linear actuator according to a first embodiment of the present invention, and  FIG. 2  is a perspective view of the embodiment of  FIG. 1 . 
   As shown in  FIGS. 1 and 2 , the linear actuator includes a driving motor  1  as a driving power source of a slider  5 . In the drawings, reference numeral  2  denotes a driving motor holder for fixing the driving motor  1 . Reference numeral  3  denotes a feed screw unit connected to the driving motor  1 , rotating together with the drive of the driving motor  1 . Reference numeral  4  denotes a joint connecting the driving motor  1  and the feed screw unit  3 . Reference numeral  6  denotes a feed nut unit, which is screwed to the slider  5  for linearly moving to move an article and the feed screw unit  3 , to convert a rotational motion into a linear motion. Reference numeral  7  denotes a guide guiding the slider  5  to move in a straight line by preventing the rotation of the slider  5 . Reference numeral  8  denotes a displacement sensor (not shown in  FIGS. 1 and 2  but will be described later with reference to  FIG. 4 ), which measures a displacement amount of the slider  5 . Reference numeral  9  denotes a base on which the driving motor holder  2  and the guide  7  are mounted. Reference numeral  10  denotes a control unit (not shown in  FIGS. 1 and 2  but will be described later with reference to  FIG. 4 ), which controls driving of the linear actuator. Reference numeral  25  denotes a second stopper. The second stopper  25  is deposited at the end portion on the side of driving motor  1  of the guide  7 . The second stopper  25  has a hole into which the feed screw unit  3  is inserted. Thus, an inner diameter of the hole should be slightly larger than an outer diameter of the feed screw unit  3  so as to prevent frictional resistance therebetween. The second stopper  25  restricts the movement of the slider  5  and the feed nut unit  6 , which move together with the driving motor  1 , towards the driving motor  1 . 
   An example of the driving motor  1 , the driving power source, is a DC brush motor. Although a stepping motor offering easy position control or a DC brushless motor may be used as the driving motor  1 , an inexpensive DC brush motor is preferred for the sake of saving cost. In addition, to increase thrust of the linear actuator, a reduction gear may be attached to the driving motor  1 . To realize a compact linear actuator, however, the outer diameter of the reduction gear must be smaller than the outer diameter of the driving motor  1 . The driving motor holder  2  is used for fixing the driving motor  1  to the base  9 . With help of the driving motor holder  2 , the driving motor  1  is installed in a manner that the output shaft of the driving motor  1  is parallel to the base  9 . As for the feed screw unit  3 , screw grooves are formed in almost the entire area of its outer peripheral surface. The feed screw unit  3  is connected to the output shaft of the driving motor  1  by a joint  4 . In detail, the center axis line of the output shaft of the driving motor  1  and the center axis line of the feed screw unit  3  are installed on the same line. The feed screw unit  3 , being connected to the driving motor  1  through the interposition of the joint  4  therebetween, rotates together with the output shaft of the driving motor  1 . Moreover, the length of the feed screw unit  3  is set to be greater than the stroke of the slider  5 . The feed screw unit  3  is made of hard metals, stainless steel for example, so that it may not be easily bent. Although thrust of the linear actuator increases as the pitch of the screw groove of the feed screw unit  3  decreases, the moving speed of the slider  5  slows down. 
   Meanwhile, it is necessary to set a proper pitch according to specifications required of a linear actuator. Even though a coupling for example is used for the joint  4 , other things can also be utilized as long as they generate the same effect. The slider  5  has a tubular shape, not a circular cylindrical shape, and at least one surface thereof is planar. In case of the slider  5  shown in  FIG. 2 , there are two planar surfaces in order to install a third antenna  17  of the displacement sensor  8  (to be described later). The length of the slider  5  is greater than the stroke of the linear actuator. The slider  5  is made of a metal having a smooth surface, or resin. However, to obtain a light-weight actuator, resin is preferably used. Inside the slider  5  is formed a through hole whose cross section in the longitudinal direction is a circular shape. This hole is for accommodating the feed screw unit  3  and therefore, is slightly larger than the outer diameter of the feed screw  3 . An article can be moved by connecting the article to this actuation unit  5 . In addition, the slider  5  may have plural holes to which articles to be fed can be attached. 
   The feed nut unit  6  has a tubular shape and forms together with the feed screw unit  3  a power transmission mechanism. Its role is to convert a rotational motion into a linear motion. Screw grooves having the same pitch with the feed screw unit  3  are formed on the inner peripheral surface of the through hole of the feed nut unit  6  so that the feed screw unit  3  can be screwed thereto. The feed nut unit  6  is fixed in a manner that the center axis of the through hole of the slider  5  and the center axis of the through hole of the feed nut unit  6  coincide with each other on the end portion of the driving motor  1  side of the slider  5 . Although the feed nut unit  6  can be made of a metal or resin, resin is preferably used to achieve a light-weight actuator. In addition, the slider  5  and the feed nut unit  6  can be molded together as one body. Also, a first stopper  11  (not shown in  FIGS. 1 and 2  but will be described later with reference to  FIG. 3 ), as a retaining means, is installed on the end portion opposite from the driving motor  1  of the feed screw unit  3 . 
     FIG. 3  is an enlarge view of the first stopper  11  area. 
   As shown in  FIG. 3 , the outer diameter of the first stopper  11  is larger than the outer diameter of the screw hole of the feed nut unit  6 , thereby having the slider  5  protrude to a great extent. This prevents the feed nut unit  6  from escaping from the feed screw unit  3 . The guide  7  is extendably formed in the longitudinal direction of the actuator, and fixed to the base  9 . The guide  7  has a recess whose cross-section has a U shape. By placing the slider  5  in this recess, the guide  7  hinders the rotation of the slider  5  along with the rotation of the driving motor  1  and at the same time guides the slider  5  to move in a straight line. Moreover, in order to reduce the frictional force between the guide  7  and the slider  5 , the recess of the guide  7  is slightly wider than the slider  5 . Desirably, the guide  7  is made of an insulating material, a resin for example, to be installed together with the displacement sensor  8 . 
   Although the above described the guide  7  as a member having a recess whose cross section has a U shape, any shape can be used as long as it can impede the rotation of the slider  5  along with the rotation of the driving motor  1  and make the slider  5  move in a straight line. For example, a tubular shape encompassing the outer peripheral of the slider  5  can also be used. The displacement sensor  8  is installed on the slider  5  and the guide  7 , and measures a displacement amount of the slider  5 . More details will be provided later. 
   The following now explains why the displacement sensor  8  is installed on the linear actuator of the present invention. The linear actuator of the invention is primarily used to execute position control of the slider  5  based on a displacement direction and a displacement amount of the slider  5  obtained by an external sensor or an external input means. In addition, as described before, a DC brush motor is used to realize a low-price driving motor  1 . In effect, the displacement sensor  8  for position control is not required if an expensive stepping motor or a DC brushless motor is used as the driving motor  1 , but the DC brush motor is not designed for position control. This is because the revolutions per minute of the motor vary by an external load of the linear actuator. Therefore, the linear actuator of the invention is provided with the displacement sensor  8  for executing position control of the slider  5 . Further description on the displacement sensor  8  detecting a displacement amount of the slider  5  will be provided with reference to  FIG. 4 . 
     FIG. 4  shows a constitution of the displacement sensor  8  installed on the linear actuator. As shown in  FIG. 4 , the displacement sensor  8  includes a detecting unit  12  mounted on the main body of the linear actuator, an input signal unit  13  applying a signal to the detecting unit  12 , and a signal processing unit  14  processing an output signal from the detecting unit  12 . The detecting unit  12  includes a first antenna  15 , a second antenna  16  arranged on an outer side of the guide  7  opposite to the first antenna  15  arranged on the other outer side of the guide  7 , a third antenna  17  arranged on the outer peripheral of the slider  5  opposing the first antenna  15  and the second antenna  16 , and a condenser  18  removing a DC component of a signal. The input signal unit  13  includes an oscillatory circuit  19  applying a radio frequency sine wave to the first antenna  15 , and a resistor  20  connected between the oscillatory circuit  19  and the first antenna  15 . The signal processing unit  14  includes a high-pass filter  21  passing only signals of a predetermined frequency or higher, a full wave rectifier  22  for detecting all waves that obtains absolute values of signals and rectifies signals, an integral circuit  23  executing gain adjustment and offset adjustment, and an A/D converter  24  converting an analog signal into a digital signal. The first antenna  15 , the second antenna  16 , and the third antenna  17  are installed in a way that the antenna surfaces are parallel with one another, and an insulator is inserted therebetween so that the antennas do not come in contact with each other. To this end, a condenser is formed by the first antenna  15  and the third antenna  17 , and by the second antenna  16  and the third antenna  17 . 
   Therefore, to form a condenser by the antennas, the guide is formed of an insulator. In addition, the first antenna  15 , the second antenna  16 , and the third antenna  17  have the same width, and they are installed so that their width directions (depth direction in  FIG. 4 ) are always coincident. The third antenna  17  is installed on three outer peripheral surfaces except for the surface where the feed nut unit  6  of the slider  5  is installed. In this way, the surface of the third antenna  17  facing the first antenna  15  and the other surface of the third antenna  17  facing the second antenna  16  have the same potential. In so doing, capacitance between the first antenna  15  and the second antenna  16  is increased and therefore, the detection sensitivity of the displacement sensor  8  is improved. In the first antenna  15 , the input signal unit  13  and the signal processing unit  14  are connected by the condenser  18 , and the second antenna  16  is grounded. 
   Since the third antenna  17  is installed on the slider  5 , it moves together with the slider  5  in the longitudinal direction of the linear actuator. In the case that a wire is connected to the third antenna  17 , the wire may be broken as the slider  5  moves. Thus, no wiring is done for the third antenna  17 , and the third antenna  17  is electrically floated. Although the first antenna  15 , the second antenna  16 , and the third antenna  17  are conductors, such as, copper foils, other things may also be used as long as they generate the same effect. Considering that a compact linear actuator is to be built, those antennas are desirably formed of thin sheet-type conductors. As the antenna conductors&#39; areas increase, capacitances thereof increase and the detection sensitivity of the displacement sensor  8  is improved. Thus, the greater the area of the respective antennas, the better. Each constituent of the displacement sensor  8 , i.e., the condenser  18 , the resistor  20 , the oscillatory circuit  19 , the high-pass filter  21 , the full wave rectifier  22 , the integral circuit  23 , and the A/D converter  24  are all mounted on the same circuit board. In the case that the first antenna  15  is installed away from the condenser  18 , a shield wire is used to connect the first antenna  15  and the condenser  18 , so as to get rid of any influence of noises from outside. 
   Therefore, since the linear actuator only needs to install the thin conductors which are the first antenna  15 , the second antenna  16 , and the third antenna  17 , the linear actuator can be made small. Although a well-known potentiometer may be used as the displacement sensor  8 , the potentiometer is not recommended because it only makes the linear actuator bulky. The oscillatory circuit  19  is connected to the first antenna  15  by the resistor  20  and the condenser  18  therebetween, and generates radio frequency sine waves. In effect, the condenser  18  is not an essential element for the actuator, so it can be removed if desired. The displacement sensor  8  measures potential between the condenser  18  and the resistor  20  with output voltage. This output voltage is processed at the signal processing unit  14 , and sent to the control unit  10 . An input side of the high-pass filter  21  for removing low frequency noises is connected to the condenser  18 , and an output side thereof is connected to the full wave rectifier  22 . Since it is good to send a signal within an oscillating frequency range of the oscillatory circuit  19  to the full wave rectifier  22 , the displacement sensor  8  ensures that all low frequency noises are removed by the high-pass filter  21 . 
   An input side of the full wave rectifier  22  is connected to the high-pass filter  21 , and an output side thereof is connected to the integral circuit  23 . A signal from the high-pass filter  21  is a sine wave oscillating between plus range and minus ranges. Before inputting a signal to the A/D converter  24 , the full wave rectifier  22  takes an absolute value of a signal within a minus range to a plus range. Moreover, the full wave rectifier  22  rectifies the signal. Meanwhile, an input side of the integral circuit  23  is connected to the full wave rectifier  22 , and an output side thereof is connected to the A/D converter  24 . The integral circuit  23  executes offset adjustment and gain adjustment on a signal from the full wave rectifier  22 . The offset adjustment and the gain adjustment are for adjusting the output sensitivity of the displacement sensor  8  that is set to increase the detection sensitivity. 
   An input side of the A/D converter  24  is connected to the integral circuit  23 , and an output side thereof is connected to the control unit  10  of the linear actuator. The A/D converter  24  converts an analog signal from the integral circuit  23  into a digital signal. The control unit  10  is connected to the A/D converter  24  and the driving motor  1 , and calculates a displacement amount based on a signal from the A/D converter  24 . Also, the control unit  10  acquires information about displacement direction and displacement amount of the linear actuator from a sensor or an input means of the outside. Based on the displacement direction and displacement amount, and the detection result from the displacement sensor  8 , the control unit  10  controls the rotational direction and ON/OFF of the driving motor  1 . 
   The following now describes the principle of measurement of the displacement sensor  8 . In the detecting unit  12  of the displacement sensor  8 , three condensers are formed by the first antenna  15 , the second antenna  16 , and the third antenna  17 . That is, a condenser C 1  is formed by the first antenna  15  and the third antenna  17 , a condenser C 2  is formed by the second antenna  16  and the third antenna  17 , and a condenser C 3  is formed by the first antenna  15  and the second antenna  16 , respectively. In general, capacitance C(F) is obtained by Equation 1 below.
 
 C=ε·S/d   [Equation 1]
 
where, ε (F/m) indicates a dielectric constant, S (m2) indicates an area of an opposite antenna, and d (m) indicates a distance between antennas opposite to each other. As seen in Equation 1, if the distance d between antennas opposite to each other is fixed, the capacitance C is proportional to the dielectric constant ε and the area S of an opposite antenna. Since the first antenna  15 , the second antenna  16 , and the third antenna  17  are arranged on planes parallel with one another and have the same width in the longitudinal direction, respectively, the capacitance in the condenser C 1  for example increases proportionally to an overlapped length between the first antenna  15  and the third antenna  17 . This equally happens in the second condenser C 2  and the third condenser C 3 . Since capacitances C 1 , C 2  and C 3  are changed by the linear motion of the slider  5 , the output voltage between the resistor  20  and the condenser  18  changes. This output voltage is processed by the signal processing unit  14  and the control unit  10  calculates a position of the guide  7  of the slider  5 . In addition, the length of an antenna becomes the measurement range of the displacement sensor  8 . In other words, the longer the antenna, the greater the measurement range. Therefore, antennas are arranged on the overall surface in the longitudinal direction of the guide  7  and the slider  5 . The detection sensitivity of the displacement sensor  8  is improved as a difference increases between the output voltage in the case the slider  5  is lead in closest to the driving motor  1  and the output voltage in the case that the slider  5  is protruded the most.
 
   Since capacitances of the condensers C 1 , C 2  and C 3  formed by the first antenna  15 , the second antenna  16 , and the third antenna  17  are small, frequency of the oscillatory circuit  19  should be large to make the impedances of these condensers C 1 , C 2  and C 3  small. However, if the frequency is too large, the output voltage is small. Thus, it is necessary to adjust the frequency. For instance, a proper frequency of the oscillatory circuit  19  is around 800 kHz. In this manner, the displacement sensor  8  is able to detect a displacement amount of the slider  5 . The detecting unit  12  may be installed on the linear actuator by other methods, the method of  FIGS. 5 to 7  for example, besides the one shown in  FIG. 4 . 
   The following now describes an installation method of the detecting unit  12  with reference to  FIGS. 5 to 7 . 
   Referring to  FIG. 5 , the first antenna  15  is arranged on the outside face of the guide  7  and the second antenna  16  is arranged on the slider  5 , on the side where the first antenna  15  is arranged. 
   In  FIG. 5 , the input signal unit  13  and the signal processing unit  14  are connected to the first antenna  15  with the condenser  18  therebetween, and the second antenna  16  is grounded. The second antenna  16  may be grounded using a wire or a grounding member (not shown) such as a conductive brush provided on the guide  7 . In case of connecting a wire to the second antenna  16 , it is proper for a linear actuator wherein the length of the wire falls within a range of stroke that does not influence the operation of the linear actuator. In case of grounding the second antenna  16  by a grounding member such as a brush, it is proper for a linear actuator whose driving motor&#39;s torque has a margin and the influence of a frictional load between the slider  5  and the guide  7  is small. As for the installation of the detecting unit  12  shown in  FIG. 5 , a condenser C 4  is formed by the first antenna  15  and the second antenna  16 . Since the overlapped area between the first antenna  15  and the second antenna  16  varies by the movement of the slider  5 , capacitance of the condenser C 4  changes. In this manner, the detecting unit  12  can detect a displacement amount of the slider  5 . In addition, the reason for installing the second antenna  16  on the side of the first antenna  15  of the slider  5  is to increase capacitance of the condenser C 4 . 
   Referring next to  FIG. 6 , the first antenna  15  is arranged on an outer surface of the guide  7  while the second antenna  16  is arranged on the outer surface on the other side of the guide  7 . Here, the slider  5  is an insulator. 
   In  FIG. 6 , the input signal unit  13  and the signal processing unit  14  are connected to the first antenna  15  by the condenser  18  therebetween, and the second antenna  16  is grounded. According to the installation method of the detecting unit  12  shown in  FIG. 6 , a condenser C 5  having the slider  5  inserted between the first antenna  15  and the second antenna  16  and a condenser C 6  without the slider  5  are formed. Since the electrode areas of the condenser C 5  and the condenser C 6  vary by the movement of the slider  5 , capacitances of the condensers C 5  and C 6  change. In this manner, the detecting unit  12  is able to detect a displacement amount of the slider  5 . Since capacitances of the condensers C 5  and C 6  are smaller than those shown in  FIG. 4  and  FIG. 5 , this is suitable for a linear actuator which has a small actuation unit  5  and a small distance between the first antenna  15  and the second antenna  16 . 
   Referring now to  FIG. 7 , the first antenna  15  is arranged on an outer surface of the guide  7  while the second antenna  16  is arranged on the outer surface on the other side of the guide  7 . Here, the slider  5  is a conductor and third antenna  17  at the same time. 
   In  FIG. 7 , the input signal unit  13  and the signal processing unit  14  are connected to the first antenna  15  via the condenser  18 , and the second antenna  16  is grounded. According to the installation method of the detecting unit  12  shown in  FIG. 7 , similar to the one shown in  FIG. 4 , a condenser C 7  is formed by the first antenna  15  and the third antenna  17 , a condenser C 8  is formed by the second antenna  16  and the third antenna  17 , and a condenser C 9  is formed by the first antenna  15  and the second antenna  16 . Since capacitances of the condensers C 7 , C 8  and C 9  change by the linear motion of the slider  5 , the detecting unit  12  is able to detect a displacement amount of the slider  5 . In this case, since the slider  5  is a conductor, its weight is increased. Therefore, this is suitable for a linear actuator in which the driving motor&#39;s torque has a margin and a light actuation unit  5  is not required. 
   So far, the installation method of the displacement sensor  8  at the linear actuator has been explained. However, such displacement sensor  8  may be installed on another instrument or different type of actuator and serve to detect a displacement amount of an actuating member. 
   As for the linear actuator of the present invention, a feed screw is used to convert the rotational motion of the driving motor  1  into the linear motion of the slider  5 . This is to realize a compact linear actuator having a small outer diameter, whereby plural linear actuators can be arranged nearby or at a small distance away from each other. To achieve a compact linear actuator, it is necessary to reduce the size of the driving motor  1 . However, if the size of the driving motor  1  is reduced, torque thereof is also reduced. Thus, to secure sufficient thrust for the linear actuator, the torque needs to be increased by a reduction gear. It turned out that the feed screw system not only reduced the size (outer diameter) of the linear actuator but also secured thrust therefor. 
   Also, to obtain a compact linear actuator, the base  9  and the driving motor holder  2  should not be larger than the outer diameter of the driving motor  1 . 
   With reference to the flow chart shown in  FIG. 8 , the following now describes the operation of the linear actuator. 
   As shown in  FIG. 8 , in step S 101 , the slider  5  returns to its initial position by executing the initial operation of the linear actuator. At the initial position, the slider  5  is accommodated at the closest place to the driving motor  1 . The driving motor  1  is driven to move the slider  5  towards the driving motor  1  and ultimately to the initial position. Whether the slider  5  reached the initial position or not is determined by checking whether the slider has touched the second stopper  25  and stopped. In addition, to detect the initial position of the slider  5 , an optical detecting sensor (not shown) may be installed around the second stopper  25  and detect whether an optical axis of the detecting sensor has been cut off by the slider  5 . Moreover, the most protruded location of the slider  5  may be set as the initial position. In step S 102 , displacement amount and displacement direction of the slider  5  are obtained. An operator may input these displacement direction and displacement amount from the outside, or the control unit  10  may obtain them from a sensor from the outside (not shown). In step S 103 , based on the displacement amount and the displacement direction obtained in step S 102 , a target voltage of the displacement sensor  8  is calculated. As depicted in  FIG. 9 , an output voltage of the displacement sensor  8  is in proportion to a distance from the initial position of the slider  5 , and a target voltage V 1  is calculated using the voltage V 0  at the present position X 0 , the displacement amount d and the displacement direction. In the case that the slider  5  is protruded, the target voltage V 1  can be obtained by Equation 2 below.
 
 V 1=− a×d+V 0  [Equation 2]
 
   Meanwhile, in the case that the slider  5  is lead in, the target voltage V 1  can be obtained by Equation 3 below.
 
 V 1=− a ×(− d )+ V 0  [Equation 3]
 
   The reason for using Equations 2 and 3 to calculate the target voltage is because the output voltage of the displacement sensor  8  is sometimes offset by its surrounding environment. Next, in step S 104 , the driving motor  1  is ON by rotating the driving motor  1  forward or backward according to the displacement direction obtained in step S 102 . In consequence, the output shaft of the driving motor  1  and the feed screw unit  3  rotate as one body. Moreover, the displacement direction of the slider  5  can be changed by the rotational direction of the driving motor  1 . Because the feed nut unit  6  screwed to the feed screw unit  3  is fixed to the slider  5  as described above and because the slider  5  does not rotate by the guide  7 , the feed nut unit  6  does not rotate. Instead, the feed nut unit  6  and the slider  5  start a linear motion. Later, in step S 105 , it is determined whether the slider  5  has reached its target position. This is accomplished by comparing a current output voltage of the displacement sensor  8  with the target voltage V 1  calculated in step S 103 . If the output voltage of the displacement sensor  8  and the distance from the initial position of the slider  5  satisfy the relation shown in  FIG. 9 , to move the slider  5  in the protrusion direction, it is determined that target position has been attained when the current output voltage is below the target voltage V 1 . In addition, to move the slider  5  in the lead-in direction, it is determined that target position has been attained when the current output voltage is above to the target voltage V 1 . 
   If, in step S 105 , it is determined that the output voltage of the displacement sensor  8  reached the target voltage V 1 , the driving motor  1  stops running (S 106 ). On the other hand, if, in step S 105 , it is determined that the output voltage of the displacement sensor  8  does not rech the target voltage V 1 , the driving motor  1  keeps running until the target voltage V 1  is obtained. As the gap between decision making steps is short, the detection precision of the displacement amount of the linear actuator is high. As described above, the linear actuator of the present invention controls ON/OFF of the driving motor  1  based on the information from the displacement sensor  8 , and therefore executes position control of the slider  5 . Moreover, by changing the length of the feed screw unit  3 , actuation unit  5  and guide  7 , the stroke of the linear actuator can be modified. The detection of the initial position of the slider  5  in step S 101  may also be accomplished by the following method. 
   As shown in  FIG. 10 , a third antenna  17   a  is arranged on the end portion of the slider  5  opposite to the driving motor  1 . This is an extended form of the third antenna  17  in the vertical direction. When the slider  5  returns to the initial position, the third antenna  17   a  is connected to the first antenna  15  and the second antenna  16 . Then the output voltage of the displacement sensor  8  becomes zero. In such case, it is determined that the slider  5  has reached its initial position. To detect the initial position, the method used in step S 103  for obtaining the target voltage has to be changed. After driving the driving motor  1  for a short amount of time, the target voltage V 1  is calculated by utilizing the Equations 2 or 3. This is possible because the output voltage of the displacement sensor  8  is zero when the slider  5  is at its initial position. Moreover, it is necessary to correct the displacement amount of the slider  5  when the driving motor  1  is driven for a short amount of time. Although the initial operation is to be executed in step S 101 , in the case that the current position of the slider  5  is known by driving the actuator repeatedly, step S 101  may be omitted. The following now describes a second embodiment of the linear actuator of the invention. 
   Embodiment 2 
     FIG. 11  is a front view illustrating a constitution of a linear actuator according to the second embodiment of the present invention. 
     FIG. 12  is a perspective view of the embodiment of  FIG. 11 . 
     FIG. 13  is a front view of the embodiment of  FIG. 11  without a guide  30  (to be described later). 
     FIG. 14  is an explanatory diagram showing the assembly of a rotating bar and a spiral spring. In  FIGS. 11 to 14 , reference numeral  26  denotes a driving motor, the driving power source of a slider  28 . Reference numeral  27  denotes a rotating bar connected to the driving motor  26 , and therefore rotates together with the driving motor  26 . Reference numeral  29  denotes a spiral spring, which is screwed to the rotating bar  27  moving straightly to move an article for example and converts a rotational motion into a linear motion. Reference numeral  30  denotes a guide guiding the slider  28  to move in a straight line. Explanations on the displacement sensor  8  and the control unit  10  are omitted because they are identical with those in the first embodiment. 
   Similar to the first embodiment, a DC brush motor is used as the driving motor  26 . Although a stepping motor offering easy position control or a DC brushless motor may be used as the driving motor  26 , an inexpensive DC brush motor is preferred for the sake of saving cost. In addition, to increase thrust of the linear actuator, a reduction gear may be attached to the driving motor  26 . To realize a compact linear actuator, however, the outer diameter of the reduction gear must be smaller than the outer diameter of the driving motor  26 . The rotating bar  27  is a member having propeller shaped wings on its end portion. The rotating bar  27  is connected to the output shaft of the driving motor  26 , and the center axis of the output shaft of the driving motor  26  and the center axis of the rotating bar  27  are arranged on the same line. The rotating bar  27  connected to the driving motor  26  rotates together with the output shaft of the driving motor  26  as one body. 
   As shown in  FIG. 14 , a blade  27   a  is installed to fit in a gap of the spiral spring  29  wound in a spiral shape. To make the interfacial surface between the spring  29  smooth, the a blade  27   a  is inclined by a predetermined angle with respect to the center axis. Moreover, the length of the rotating bar  27  in the longitudinal direction is greater than the stroke of the slider  28 . The rotating bar  27  is made of hard metals, stainless steel for example, so that it may not be easily bent. The slider  28  has a tubular shape, not a circular cylindrical shape, and at least one surface thereof is planar. 
   The slider  28  shown in  FIGS. 11 and 12  has two planar surfaces in order to install a third antenna  17  of the displacement sensor  8 . The length of the slider  28  is greater than the stroke of the linear actuator. The slider  28  is made of a metal having a smooth surface, or resin. However, to obtain a light-weight actuator, resin is preferably used. An article can be moved by connecting the article to this actuation unit  28 . In addition, the slider  28  may have plural holes to which articles to be fed can be attached. The spiral spring  29 , together with the rotating bar  27 , is a power transmission mechanism and is in charge of converting a rotational motion into a linear motion. A compressing spring for example is used as the spiral spring  29 . As aforementioned, the rotating bar  27  is combined with the spiral spring  29 . Even though the slider  28  is installed on the end portion of the spiral spring  29 , it is not fixed thereto. 
   In general, thrust of the linear actuator increases as the number of turns in the spiral spring  29  increases and the pitch decreases, while the speed of the slider  28  slows down. A proper pitch needs to be set according to specifications required of a linear actuator. The guide  30  is a cylindrical shaped member, and the driving motor  26 , the rotating bar  27 , the slider  28  and the spiral spring  29  is arranged in a straight line. Thus the slider  28  and the spiral spring  29  move in a straight line. The inner diameter of the guide  30  coincides with the outer diameter of the driving motor  26 , and the driving motor  26  is fixed to the end portion of the guide  30 . The inner diameter of the guide  30  and the outer diameter of the spiral spring  29  are adjusted to cause a frictional force sufficient for suppressing the rotation around the output shaft of the driving motor  26  can be applied to the spiral spring  29 . 
   In the case that the frictional force between the spiral spring  29  and the guide  30  is too small, the spiral spring  29  rotates along with the rotating bar  27 . In other words, the spiral spring  29  does not produce the linear motion in the longitudinal direction of the linear actuator. Similarly, if the frictional force is too great, the spiral spring  29  cannot produce the linear motion. In addition, in order to allow the rotating bar  27  to rotate without contacting the guide  30 , the outer diameter of a blade  27   a  of the rotating bar  27  is slightly smaller than the inner diameter of the guide  30 . As shown in  FIG. 15 , a rotation stopping unit  31  is formed at the other end portion of the guide  30  where the driving motor  26  is not installed. The rotation stopping unit  31  has a planar surface to come in contact with the planar surface of the slider  28 , and obstructs the rotation of the slider  28  along with the rotation of the driving motor  26 . To help the slider  28  move smoothly, there is a little space between the slider  28  and the rotation stopping unit  31 . For the installation of the displacement sensor  8 , the guide  30  is desirably formed of an insulator, a resin for example. The guide  30  and the rotation stopping unit  31  may be molded as one body. A first stopper  32  is installed on the end portion of the side of the driving motor  26  of the slider  28 . The first stopper  32  has a cylindrical shape, and its outer diameter is slightly smaller than the inner diameter of the guide  30 . 
   Now that the first stopper  32  comes in contact with the rotation stopping unit  31 , the slider  28  is not easily escaped from the guide  30 . In addition, the slider  28  and the first stopper  32  may be molded as one body. Meanwhile, a second stopper  33  is formed at the end portion of the side of the driving motor  26  of the rotating bar  27 . The second stopper  33  has a cylindrical shape and its outer diameter is slightly smaller than the inner diameter of the guide  30 . The spiral spring  29  moves vertically driven by the driving motor  26 . The second stopper  33  serves to limit the movement of the spiral spring  29  in the lower direction. In short, the linear actuator according to the second embodiment of the present invention utilized the rotating bar  27  and the spiral spring  29  instead of a typical feed screw. Also, the stroke of the linear actuator can be changed by changing sizes of the rotating bar  27 , actuation unit  28 , spiral spring  29  and guide  30 . 
   Referring to  FIG. 15 , the following now describes the installation method of the displacement sensor  8  at the linear actuator according to the second embodiment of the present invention. 
   As shown in  FIG. 15 , a first antenna  15  is arranged on the outside of the guide  30  and a second antenna  16  is arranged on the outside of the guide  30  opposing the first antenna  15 . Meanwhile, as shown in the drawing, a third antenna  17  is arranged on three parts of the outer peripheral surface of the slider  28 , respectively. For the installation of the first antenna  15  and the second antenna  16 , the outer peripheral of the guide  30  has planar surfaces. These surfaces are in parallel with the rotation stopping unit  31  as well as the third antenna  17  installed on the slider  28 . An input signal unit  13  and a signal processing unit  14  are connected to the first antenna  15  via a condenser  18 , and the second antenna  16  is grounded. Since the third antenna  17  is installed on the slider  28 , it moves along with the slider  28  in the longitudinal direction of the linear actuator. In the linear actuator according to the second embodiment of the present invention, the rotating bar  27  rotates driven by the driving motor  26 , and this rotation causes the spiral spring  29  to move in the longitudinal direction of the linear actuator, thereby moving the slider  28 . Because the control method is the same as the first embodiment, explanation thereof is omitted. 
   Next, the initial operation of the slider  28  is described. An initial position of the slider  28  is the closest place to the driving motor  26 . The driving motor  26  is driven to move the slider  28  towards the driving motor  26  and ultimately to the initial position. Whether the slider  28  reached the initial position or not is determined by checking whether the spiral spring  29  has touched the second stopper  33  and stopped. The detection of the initial position of the slider  28  may also be achieved by the following method. 
   As shown in  FIG. 16 , a third antenna  17   a  is arranged on the end portion of the slider  28  opposite to the driving motor  26 . This is an extended form of the third antenna  17  in the horizontal direction. When the slider  28  reaches the initial position on the side of the driving motor  26 , the third antenna  17  comes in contact with the first antenna  15  and the second antenna  16 . Therefore, when the slider  28  is led in the most towards the driving motor  26 , the first antenna  15  and the third antenna  17   a , and the second antenna  16  and the third antenna  17   a  contact with each other, thereby making the output voltage of the displacement sensor  8  zero. When the output voltage of the displacement sensor  8  is zero, it is determined that the slider  5  has reached the initial position. 
   Similar to the first embodiment, the method for calculating a target voltage has to be changed to detect the initial position. After driving the driving motor  26  for a short amount of time, the target voltage V 1  is calculated by utilizing the Equations 2 or 3. This is possible because the output voltage of the displacement sensor  8  is zero at the initial position. Moreover, it is necessary to correct the displacement amount of the slider  26  when the driving motor  26  is driven for a short amount of time. Although in this embodiment the initial position was set to a position where the slider  28  is the closest to the driving motor  26 , the most protruded position of the slider  28  can also be set as the initial position. In addition, the linear actuator according to the second embodiment of the present invention is able to measure an external force applied to the slider  28  by utilizing the spiral spring  29  and the displacement sensor  8 . 
   The following describes a method for measuring an external force. In the case that an external force is applied to the slider  28  in the direction of the driving motor  26 , provided that the driving motor  26  is off, the height of the spiral spring  29  shrinks. If the spring constant for the spiral spring  29  is known, the external force can be calculated based on a displacement amount of the length of the spring measured by the displacement sensor  8 . 
   In this way, the linear actuator of the second embodiment can be used as a sensor for detecting an external force. 
   In conclusion, the compact, high-precision linear actuator is achieved by installing the small-sized displacement sensor. 
   Although the preferred embodiment of the present invention has been described, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiment, but various changes and modifications can be made within the spirit and scope of the present invention as defined by the appended claims.