Abstract:
An electric syringe according to the invention injects a dental anesthetic by pressing a rubber plug of a cartridge filled with an anesthetic. The electric syringe includes: a push member configured to press and move the rubber plug of the cartridge; a drive motor configured to generate a drive force; a transmission mechanism part configured to transmit the drive force to the push member; and a control unit configured to control a moving of the push member by controlling the drive motor, wherein the control unit controls the drive motor to move the push member to gradually increase an injection speed of the anesthetic in the beginning of the injection and to move the push member to inject the anesthetic in a constant injection speed after a predetermined time has elapsed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric syringe for a dental anesthetic in which used when injecting an anesthetic in a dental treatment. 
     2. Description of the Related Art 
     As a generally used tool for injecting an anesthetic in a dental treatment, there is known a hand-operated syringe. When using the hand-operated syringe, since there is used an extremely fine needle for an anesthetic, there is required a large pushing force when injecting the anesthetic into the gum of a patient and thus there is required a great deal of labor to maintain a constant injection speed. In view of the above, there has been spread an electric syringe for a dental anesthetic (in which hereinafter referred to simply as an electric syringe) that aims at relieving a dental treatment load. 
     As a related art of such conventional electric syringe, there are known, for example, [An electric syringe for a dental treatment] disclosed in JP-A-7-213610, [An electric injection apparatus for a dental treatment] disclosed in JP-A-2001-70444, and [Mounting apparatus for a receiving cylinder in a cartridge-type injection apparatus for a dental treatment] disclosed in JP-A-2002-191694. 
     In the electric syringes disclosed in the documents, no consideration is given to the injection speed of the anesthetic; that is, a given quantity of anesthetic is output without controlling the injection speed. In a case where the injection of the anesthetic is started into the gum from such electric syringe, just after needling, a large quantity of anesthetic is injected, to thereby apply an excessively large pressure onto the tissue of the gum around a needlepoint of the needle, which makes a patient feel a pain. 
     Also, to start the injection of the anesthetic from the electric syringe, it is necessary to press down an operation button by an operator; however, when the operation button is operated in this manner, the vibrations of the operation transmits to the electric syringe and thus the needle is also moved, which also makes the patient feel a pain. 
     In the conventional electric syringes, the operation button is further requested so as to able to restrict the malfunction thereof. 
     Further, for the anesthetic, there presently exist two kinds of cartridges: that is, a cartridge for 1.0 ml and a cartridge for 1.8 ml. In the conventional electric syringes, the two kinds of cartridges are used by replacing cartridge holders that respectively hold their associated cartridges. 
     However, it is not desirable to use the cartridges while replacing a plurality of kinds of cartridge holders, because such usage results in the complicated management of the cartridge holders. Therefore, there have been demands for common use of the cartridge holders. 
     Also, there has been required a mechanism which is capable of mounting such cartridge holders simply. 
     SUMMARY OF THE INVENTION 
     It is therefore a first object of the invention is to provide an electric syringe that enhances the operation efficiency thereof without making a patient feel a pain and ease the pain of the patient. 
     Also, since the operation of the electric syringe is a medicine injection operation which must be executed carefully, there have been needs for avoiding positively such an operation which does not match the will of an operator (e.g. a dentist). Thus, it is therefore a second object of the invention to provide an electric syringe that prevents the malfunction thereof. 
     Further, it is a third object of the invention to provide an electric syringe which can use the 1.0 ml cartridge and 1.8 ml cartridge in common without replacing the cartridge holders and also in which, when the electric syringe is held at a set position, a rack for a push member is in contact with the rubber plug of the anesthetic at a constant pressure. 
     Still further, conventionally, there has been the need for a connecting mechanism which can use the 1.0 ml and 1.8 ml cartridges in common. Thus, it is a fourth object of the invention to provide an electric syringe which includes a cartridge holder connecting portion serving as a simple mechanism capable of strong connection of both of the 1.0 ml and 1.8 ml cartridges using a single cartridge holder. 
     To sum up, it is a main object of the invention to provide an electric syringe for a dental anesthetic which is enhanced in the whole operation efficiency thereof and can provide high reliability to both of a dentist and a patient. 
     In order to achieve the object, according to an aspect of the invention, there is provided an electronic syringe for injecting a dental anesthetic by pressing a rubber plug of a cartridge filled with an anesthetic, thereby injecting the anesthetic from a needlepoint of a dental needle connected to the cartridge, the electric syringe including: a push member configured to press and move the rubber plug of the cartridge; a drive motor configured to generate a drive force; a transmission mechanism part configured to transmit the drive force to the push member; and a control unit configured to control a moving of the push member by controlling the drive motor, wherein the control unit controls the drive motor to move the push member to gradually increase an injection speed of the anesthetic in the beginning of the injection and to move the push member to inject the anesthetic in a constant injection speed after a predetermined time has elapsed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more apparent by describing in detail a preferred exemplary embodiment thereof with reference to the accompanying drawings, wherein: 
         FIG. 1  is an external view of an electric syringe for a dental anesthetic according to the invention, explaining the outer appearance thereof; 
         FIG. 2  is an external view of the electric syringe for a dental anesthetic, explaining the outer appearance thereof; 
         FIG. 3  is an internal mechanism view of the electric syringe for a dental anesthetic, explaining the internal structure thereof; 
         FIG. 4  is an internal mechanism view of the electric syringe for a dental anesthetic, explaining a clutch mechanism portion thereof; 
         FIG. 5  is an internal mechanism view of the electric syringe for a dental anesthetic, explaining a cartridge holder connecting part thereof; 
         FIGS. 6A through 6D  are explanatory views of the operation of the cartridge holder connecting part; 
         FIGS. 7A through 7D  are explanatory views of the operation of the cartridge holder connecting part; and 
         FIG. 8  is an explanatory view of the injection speed control to be made in the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, a preferred embodiment of an electric syringe for a dental anesthetic (in which hereinafter simply referred to as an electric syringe) according to the invention will be described with reference to the accompanying drawings.  FIGS. 1 and 2  show external views of the electric syringe according to the embodiment, showing the outer appearance of the electric syringe;  FIG. 3  shows an internal mechanism of the electric syringe, showing the interior structure thereof;  FIG. 4  shows an internal mechanism of a clutch mechanism portion of the electric syringe;  FIG. 5  shows an internal mechanism of a cartridge holder connecting part of the electric syringe;  FIGS. 6 and 7  show explanatory views of the operation of the cartridge holder connecting part; and  FIG. 8  shows an explanatory view of the injection speed control to be made in the electric syringe. Referring to the relation between  FIGS. 1 and 2 , when it is assumed that  FIG. 2  is a front view of the electric syringe,  FIG. 1  is a right side view of the electric syringe. 
     The electric syringe includes a main body part  100  (see  FIGS. 1 and 2 ), a cartridge holder connecting part  200  (see  FIGS. 1 and 3 ), a cartridge holder  300  (see  FIGS. 1 and 3 ), a dental needle  400  (see  FIGS. 1 and 3 ), and a cartridge  500  (see  FIG. 3 ). 
     The main body part  100  is formed in such a manner as shown in  FIG. 2 : that is, two cover cases  1  and  2  are assembled together from right and left and a top plate  3  is then disposed on the assembled cover cases  1  and  2 . In the main body part  100 , as shown in  FIG. 1 , there are disposed a lock removing button  4  and an operation confirming lamp  5 ; and, as shown in  FIG. 2 , there are further disposed a safety sensor  6  as a first operation switch, and a start/stop sensor  7  as a second operation switch. 
     As shown in  FIG. 3 , on the top side of a control substrate  8 , there is disposed an operation display part  9  with a display panel on which display and operation symbols are printed, and bonded to the front side thereof; and on the display panel of the operation display part  9 , there are disposed a battery remaining amount display portion, a speed set display portion, a power supply switch, and a speed set switch. 
     The operation confirming lamp  5  receives supply power through the control substrate  8  and visually indicates an operation state of the electric syringe. 
     As shown in  FIG. 2 , two batteries  10  are set respectively on the two sides of the cover cases  1  and  2 . Further, there is disposed a charging terminal (not shown) which belongs to a charging substrate  11  shown on the lower side of  FIG. 3 ; and, in case where the main body part  100  is placed on a charger (not shown), the charging battery  10  is charged through the charging terminal. The charging battery  10  supplies supply power to various parts of the electric syringe as described later. 
     Next, description will be given below of the internal structure of the electric syringe. 
     Firstly, in the interior of the electric syringe, as shown in  FIG. 3 , there is disposed a drive motor  12 . 
     The drive motor  12  is connected to a control drive circuit formed on the control substrate  8  and is configured in such a manner that the drive force thereof is controlled. Description will be given later of how the drive motor  12  is driven and controlled. 
     On the control substrate  8 , there is also arranged a CPU (not shown) in which controls the whole operational status of the electric syringe. The CPU controls the drive circuit and a sound output unit, which will be described later. In the embodiment, the control substrate  8  (more specifically, the CPU and the control drive circuit arranged on the control substrate  8 ) corresponds to a control unit of the invention. 
     The drive motor  12  applies a drive force to a transmission mechanism part. On the main shaft of the drive motor  12 , there is rotatably supported a spur gear  13 . 
     The transmission mechanism part is a generic name of a gear train which is used to transmit the drive force given from the drive motor  12  through the spur gear  13 ; and includes an idler gear  14 , a spur gear  15 , a bevel gear  16 , a bevel gear  17 , a spur gear  18 , a double gear  19 , and another double gear  20 . 
     The double gear  20  is configured to be engaged with a rack  21   a  of a push member  21  and thus the drive force is transmitted to the push member  21  through the transmission mechanism part. 
     The drive motor  12  and the transmission mechanism part are both stored in a gear case  22 . The gear case  22  is held by and fixed to positioning bosses (each of which is formed so as to have a projecting shape) respectively disposed in the interior portions of the cases  1  and  2 . Since the gear train is positioned with the gear case  22  as the reference thereof, the gear train is engaged with high accuracy to thereby be able to reduce generation of noises. 
     Next, description will be given below of the transmission operation of the transmission mechanism part. 
     The idler gear  14  is rotatably supported on a shaft  23  fixed to the gear case  22 , and the idler gear  14  arranged so as to are engaged with the spur gear  13 . The drive force is to be transmitted to the idler gear  14  from the spur gear  13 . 
     The spur gear  15 , as shown in  FIGS. 3 and 4 , is rotatably supported on and fixed to a rotary shaft  24  by a setscrew  24   a . The rotary shaft  24  is supported on and fixed to the inner races of two bearings  26  whose outer races are held by a guide  25 , and the rotary shaft  24  allowed to be rotated. 
     The spur gear  15  is engaged with the idler gear  14  and thus, to the spur gear  15 , a drive force is transmitted from the idler gear  14 . The drive force of the spur gear  15  is transmitted through the rotary shaft  24  to the bevel gear  16 . 
     The bevel gear  16  is engaged with the bevel gear  17  and thus, to the bevel gear  17 , a drive force is transmitted from the bevel gear  16 . The transmitting direction of the drive force changes substantially by 90 degrees by the bevel gears  16  and  17 . 
     A clutch shaft  27  is inserted through the bevel gear  17 . The section of the clutch shaft  27  has such a structure as that of a spline shaft or a serrated shaft (in which hereinafter referred to as a sliding groove); and, the clutch shaft  27  is fitted into a sliding hole formed in the bevel gear  17  so as to be coincident with such sliding groove, and thus the clutch shaft  27  is slidably mounted. 
     The clutch shaft  27  is also inserted through the spur gear  18 . In the spur gear  18  as well, there is formed a sliding hole which is coincident with the sliding groove of the clutch shaft  27 . The clutch shaft  27  is fitted with the spur gear  18  coincident with such sliding groove and is thus slidably mounted. 
     The bevel gear  17  and spur gear  18  are formed as a double gear in appearance, while the bevel gear  17  corresponds to the large gear portion of the double gear and the spur gear  18  corresponds to the small gear portion thereof. 
     The drive force, which has been transmitted to the bevel gear  17 , is then transmitted through the clutch shaft  27  to the spur gear  18 . 
     The double gear  19  is rotatably supported on a shaft portion  28  as is shown in  FIG. 4 . The large gear portion of the double gear  19  is engaged with the spur gear  18 , while the drive force of the spur gear  18  is transmitted to the double gear  19 . 
     The double gear  20  is rotatably supported on a shaft portion  29  shown in  FIG. 4 . The large gear portion of the double gear  20  is engaged with the small gear portion of the double gear  19  and thus the drive force of the double gear  19  is transmitted to the double gear  20 . The shaft portions  28  and  29  are respectively fixed by the gear case  22  and gear case cover  30 . 
     The rack  21   a  formed in the push member  21  is further engaged with the small gear portion of the double gear  20 , while the drive force is transmitted to the push member  21  through the rack  21   a.    
     The moving direction of the push member  21  is restricted by bearing bushes  31  and  32  shown in  FIG. 3  in such a manner that the push member  21  horizontally moves only in the right and left direction in  FIG. 3 . Due to the drive force transmitted to the rack  21   a  of the push member  21 , the push member  21  is moved in the left direction so as to press against the rubber plug  501  (see  FIG. 5 ) of a cartridge  500 . 
     In the embodiment, a great deal of speed reduction is provided by the gear train of the transmission mechanism part and thus the push member  21  is moved more sufficiently slowly than the rotation of the drive motor  12 , thereby being able to reduce the minimum unit of the anesthetic injection quantity. 
     The transmission mechanism part, as shown in  FIG. 4 , includes a clutch mechanism portion for releasing the transmission of the drive motor  12 . 
     The clutch mechanism portion includes the bevel gear  17 , spur gear  18 , clutch shaft  27  and spring  37 . 
     Since the sliding groove of the clutch shaft  27  is simply inserted through the sliding hole of the previously described bevel gear  17 , the clutch shaft  27  is allowed to slide in the axial direction thereof. The bevel gear  17  is fixed to the inner race of a bearing  33  whose outer race is held by the gear case  22  in such a manner that the bevel gear  17  is allowed only to rotate. 
     The clutch shaft  27  is also slidably fixed to the inner race of a bearing  35  whose outer race is held by the gear case cover  34 . Due to existence of the bearings  33  and  35 , the clutch shaft  27  allowed to be freely rotated; and due to the mutual fitting engagement between the sliding groove and sliding hole, the bevel gear  17 , spur gear  18  and clutch shaft  27  are allowed to rotate stably as an integral body. 
     Since the sliding groove of the clutch shaft  27  is fitted with the sliding hole of the bevel gear  17 , the clutch shaft  27  allowed to slide upwardly and downwardly; and in particular, the clutch shaft  27  is allowed to slide along the sliding holes of the bevel gear  17  and spur gear  18 . 
     On the clutch shaft  27 , there is supported the lock removing button  4  through the bearing  36 , while such consideration is given to the clutch shaft  27  that, even when the clutch shaft  27  is rotated, the lock removing button  4  is allowed surely to be prevented against rotation. 
     In the lower portion of the clutch shaft  27 , a hole  27   a  is formed. In the bevel gear  17 , a cylindrical-shaped tube portion  17   a  is formed. A spring  37  is disposed so as to extend over the hole  27   a  and tube portion  17   a . Normally, the clutch shaft  27  is biased in direction represented by an arrow A in  FIG. 4  and can be stabilized in a state where it is positioned on the upper side shown in  FIG. 4 . 
     In the case where the lock removing button  4  is situated on the upper side as in the embodiment, the sliding holes of the bevel gear  17  and spur gear  18  are both engaged with the sliding groove of the clutch shaft  27 ; and in a case where the drive force is transmitted to the bevel gear  17 , the drive force is transmitted to the spur gear  18  through the clutch shaft  27 . Normally, the sliding groove and sliding holes of the bevel gear  17 , clutch shaft  27  and spur gear  18  are engaged with each other, so that the drive force generated due to rotation transmitted from the drive motor. 
     On the other hand, in a case where the lock removing button  4  is pressed down and is thereby positioned on the lower side (the state not shown), since the sliding groove of the clutch shaft  27  is engaged only with the sliding hole of the bevel gear  17  but is not engaged with the sliding hole of the spur gear  18 , even when the drive force is transmitted to the bevel gear  17 , the drive force is transmitted to the clutch shaft  27  but not to the spur gear  18 , so that the drive force in the transmission mechanism part is released. 
     As described above, according to the transmission mechanism part of the embodiment, in a case where the lock removing button  4  is pressed down and thus the transmission of the transmission mechanism part is cut off by the clutch mechanism portion, the push member  21  can be pressed so as to be movable; and thus by releasing the lock removing button  4 , the transmission of the transmission mechanism part is connected by the clutch mechanism portion. 
     Description will be given below of the cartridge holder connecting part  200 . 
     The cartridge holder connecting part  200 , as shown in  FIG. 5 , includes a mounting and removing ring  38 , a first ball  39   a , a second ball  39   b , a mounting and removing ring biasing spring  40 , a push member biasing spring  41 , a spring storage portion  42 , a connecting ring  43 , a stopper  44 , a ball pusher  45  and a ball pusher biasing spring  46 . 
     Next, description will be given below in more detail of the above mechanisms as well as the above mounting and removing operations with reference to  FIGS. 6A through 6D  and  FIGS. 7A through 7D . Firstly,  FIG. 6A  shows the state of the cartridge holder connecting part  200  in which the cartridge holder  300  is not connected yet. As shown in  FIG. 6A , the connecting ring  43  is configured so as to store the first ball  39   a  and second ball  39   b  therein. The connecting ring  43  is configured to prevent the second ball  39   b  from moving into the tube portion of the connecting ring  43  (for example, by forming the positioning hole of the second ball  39   b  in a truncated cone shape). 
     The ball pusher  45  is biased in direction represented by an arrow C in  FIG. 6A  due to the biasing force of the ball pusher biasing spring  46 . The ball pusher  45  is formed as a cylindrical body having a multi-stage-shaped outer peripheral surface and, in more detail, as shown in  FIG. 6A , the ball pusher  45  includes two kinds of stage portions, that is, a lower stage portion having a small diameter and an upper stage portion having a large diameter. 
     Similarly, the mounting and removing ring  38  is biased in direction represented by an arrow C in  FIG. 6A  due to the biasing force of the mounting and removing ring biasing spring  40  which is to be stored in the spring storage portion  42  of the connecting ring  43 . The mounting and removing ring  38  is formed as a cylindrical body having an inner peripheral surface with a groove and, in more detail, the ring  38  includes two groove portions  38   a  and  38   b.    
     In the state shown in  FIG. 6A , the first ball  39   a  in contact with the upper stage portion of the multi-stage-shaped outer peripheral surface of the ball pusher  45  moves into the groove portion  38   a  of the mounting and removing ring  38 , and the mounting and removing ring  38  is pressed in the arrow mark c direction by the mounting and removing ring biasing spring  40 ; and the ball pusher  45  is also pressed in direction of the arrow C by the ball pusher biasing spring  46 , so that the mounting and removing ring  38  and first ball  39   a  are both positively restricted in the movements thereof. 
     Now, it is assumed that, as shown in  FIG. 6B , the cartridge holder  300  with the cartridge  500  for 1.0 ml or 1.8 ml of the anesthetic previously incorporated therein is inserted into the above cartridge holder connecting part  200  in direction represented by an arrow D in  FIG. 6B  (in direction of arrow B in  FIG. 5 ) and reaches a position where it can be contacted with the ball pusher  45  ( FIG. 6B  shows the above state). 
     The cartridge holder  300  includes an outer peripheral surface with a groove in which a circular-ring-shaped groove portion  300   a  is formed. In the groove portion  300   a , there is stored a ring-shaped step portion  300   b.    
     As shown in  FIG. 6C , in a case where the cartridge  300  is further inserted in direction represented by an arrow E in  FIG. 6C , the ball pusher  45  is also pressed and moved in direction of the arrow E against the biasing force of the ball pusher biasing spring  46 . Due to the above, between the first ball  39   a  and the multi-state-shaped outer peripheral surface of the ball pusher  45 , there is generated such clearance that allows the first ball  39   a  to move; the groove portion  300   a  is situated just below the second ball  39   b ; and the mounting and removing ring  38  biased in direction represented by an arrow F in  FIG. 6C  by the mounting and removing ring biasing spring  40  is pressed against the first ball  39   a  in direction represented by an arrow G 1  in  FIG. 6C . Therefore, the first ball  39   a  is moved in direction represented by an arrow H 1  in  FIG. 6C . And, since the mounting and removing ring  38  is pressed against the second ball  39   b  in direction represented by an arrow G 2 , the second ball  39   b  is moved in direction represented by an arrow H 2 . 
     As shown in  FIG. 6D , the mounting and removing ring  38  moves further in direction represented by an arrow I, while the mounting and removing ring  38  is contacted with the stopper  44  and thus the movement of the ring  38  is restricted, so that the mounting and removing ring  38  is stabilized in such a state as shown in  FIG. 6D . In the stabilization, because mechanical sounds are generated, the operator confirms that the cartridge holder  300  has been connected. 
     After completion of the connection of the cartridge holder  300 , the first ball  39   a  is contacted with the mounting and removing ring  38  and he lower stage portion of the multi-stage-shaped outer peripheral surface of the ball pusher  45  and also the second ball  39   b  is contacted with the mounting and removing ring  38  and groove portion  300   a , whereby the cartridge holder  300  can be firmly fixed to the cartridge holder connecting part  200 . 
     In the case where the cartridge  500  and cartridge holder  300  are inserted into the cartridge connecting part  200  as described above, the cartridge  500  and the cartridge holder  300  are automatically fixed to the electric syringe. Therefore, it is quite easy to mount the cartridge  500  and the cartridge holder  300 . 
     When pulling out and remove the cartridge  500  and cartridge holder  300  from the cartridge holder connecting part  200 , as shown in  FIG. 7A , the operator may move the mounting and removing ring  38  in direction represented by an arrow J against the biasing force of the mounting and removing ring biasing spring  40 . 
     In a case where the mounting and removing ring  38  is further pushed and is thereby finally moved up to a state shown in  FIG. 7B , the groove portion  38   a  can be situated just above the first ball  39   a  and the groove portion  38   b  can be situated just above the second ball  39   b . Then, the ball pusher  45  and cartridge holder  300  are both biased in direction represented by an arrow K by the ball pusher biasing spring  46 . In the above state, since the ball pusher  45  pushes the first ball  39   a  in direction represented by an arrow L 1 , the first ball  39   a  is moved in direction represented by an arrow M 1 ; and also, because the step portion  300   b  of the cartridge holder  300  pushes the second ball  39   b  in direction represented by an arrow L 2 , the second ball  39   b  is moved in direction represented by an arrow M 2 . 
     As shown in  FIG. 7C , in a case where the ball pusher  45  is moved still further in direction represented by an arrow N to thereby push up the first ball  39   a  in direction represented by an arrow O, the movement of the mounting and removing ring  38  is also restricted by the first ball  39   a ; and further, in a state where the ball pusher  45  is pressed against the first ball  39   a , in case where the first ball  39   a  is contacted with the upper stage of the multi-stage-shaped outer peripheral surface of the ball pusher  45 , the movement of the mounting and removing ring  38  is stopped. In the state above, the cartridge  500  and cartridge holder  300  can be freely pulled out from the cartridge holder connecting part  200 . 
     Finally, the cartridge holder  300  is removed, which provides such a state as shown in  FIG. 7D . The state shown in  FIG. 7D  is the same as the state shown in  FIG. 7A . 
     That is, the mounting and removing operations of the cartridge  500  and cartridge holder  300  with respect to the cartridge holder connecting part  200  are carried out in the above described manner. 
     Incidentally, the cartridge  500  includes two kinds of cartridges differing in length: that is, one cartridge for 1.0 ml of the anesthetic, the other for 1.8 ml of the anesthetic. In either of the two kinds of cartridges, when mounting the cartridge  500 , in a case where the cartridge  500  is mounted in a state where the lock removing button  4  is pressed down to thereby allow the push member  21  to move, the push member  21  can be moved on by the push member biasing spring  41  while maintaining its contact with the rubber plug  501 , and finally the mounting of the cartridge  500  is completed in a state where the push member  21  is contacted with the rubber plug  501  at a constant pressure, thereby being able to use either of the two kinds of cartridge  500  regardless of the lengths thereof. 
     Thus, it is possible to use the two kinds of cartridges for 1.0 ml and 1.8 ml of the anesthetic that are different in length from each other. 
     Next, description will be given below of the operation of the thus-structured electric syringe. 
     Firstly, the operator takes up the electric syringe that is placed on a charger (not shown) and presses down the power supply switch of the operation display part  9  to thereby turn on the power. Then, on the battery remaining amount display of the operation display part  9 , an indication is shown that the charge is completed or not. 
     The electric syringe is configured to select one of an injection speed from among a plurality of the injection speed as shown in  FIG. 8  ( FIG. 8  shows three injection speeds that are configured to be selectable). 
     The operator may press down the speed set switch of the operation display part  9  and, while confirming the speed set display, may select the desired one of the injection speeds. 
     Next, of the two kinds of cartridges for 1.0 ml of anesthetic and for 1.8 ml of anesthetic that are different in the length, either one cartridge  500  is mounted onto the cartridge holder  300 . 
     When setting the cartridge holder  300  onto the electric syringe, the lock removing button  4  is pressed down and the transmission of the transmission mechanism part is cut off by the clutch mechanism part; and in this state, the push member  21  is pushed and moved into the cartridge holder connecting part  200 . 
     Further, in a state where the lock removing button  4  is pressed down and the transmission of the transmission mechanism part is cut off, the cartridge holder  300  is mounted. In the mounting, as shown in  FIG. 5 , when the push member  21  in contact with the rubber plug  501  of the cartridge  500  is pushed and moved toward the main body part  100  side, while the contact state of the push member  21  at a constant pressure is maintained due to the spring force of the push member biasing spring  41 , the push member  21  is finally moved to its start position. 
     Then, the aforementioned operation is carried out to fix the cartridge holder  300  to the cartridge holder connecting part  200 , thereby providing such a state as shown in  FIG. 5 . Then, the dental needle  400  is mounted, which can complete the initial operation for the operation of the electric syringe. 
     Next, in a case where the electric syringe is gripped by hand, the window hole portion of the safety sensor  6 , which is a sensor of a light reflection type, is closed by the gripping hand and an input for operation of the sensor  6  is thereby enforced. And, in the above state, the window hole portion of the start/stop sensor  7 , which is also a sensor of a light reflection type, is closed with a forefinger to thereby enforce the operation input thereof. In this manner, only when the safety sensor  6  and start/stop sensor  7  are both closed and operated, the control substrate  8  is allowed to control the drive motor  12  so as to start the anesthetic injection operation; and therefore, even in a case where only one of the two sensors is closed in error, the electric syringe can be surely prevented against the malfunction thereof. 
     Since the safety sensor  6  and start/stop sensor  7  are both a sensor of a light reflection type and are structured such that the window hole portions thereof are only closed, the electric syringe can be prevented against movement, which can in turn reduce the undesirable slight movement of a needlepoint of the needle which causes a pain in the patient. 
     The electric syringe is operated in this manner and, to release the air existing in the needle, while allowing the anesthetic to reach the needlepoint of the dental needle  400 , the operation of the electric syringe is confirmed. 
     Next, the dental needle  400  is stuck into the gum of the patient and the start/stop sensor  7  is closed to thereby start the injection. During the injection, the sound output unit arranged on the control substrate  8  outputs a buzzing sound or a melodic sound. Such buzzing sound or a melodic sound not only can inform that the anesthetic is under injection but also can relieve the patient from anxiety. 
     Also, the control substrate  8 , as shown in  FIG. 8 , drives the drive motor  12  to thereby control the moving amount of the push member  21  through the transmission mechanism part so that the injection can be started at a low injection speed so as to be able to provide a very small anesthetic injection quantity in the beginning of the injection, the injection speed can be increased substantially at a given variation rate and, after the passage of a given period, the injection speed can provide a constant speed. 
     In a case where the injection speed is set low in the beginning of the injection, a pain, which is felt by the patient in the beginning of the injection, can be reduced and thus the patient can be relieved from anxiety. 
     After injection of a given quantity of anesthetic, the forefinger, which is closing the start/stop sensor  7 , is moved, thereby completing the injection operation. 
     That is, the electric syringe is operated in the above-mentioned manner. 
     According to the invention, the injection speed is controlled in such a manner that the injection quantity in the beginning of the injection can be set small; there is given such consideration that the patient can be relieved from anxiety due to generation of a buzzing sound or a melodic sound; and, the slight movement of the needlepoint of the needle in the injection operation can be prevented. Due to the above, there can be provided an electric syringe that is enhanced in the operation performance thereof so as to be able to prevent the patient from feeling a pain in the injection operation. 
     Also, since the injection operation can be executed only when the safety sensor and start/stop sensor are both operated, as in a sensor of a light reflection type, it is possible to use a switch which can be operated easily only by closing (shielding) the portion in which the sensor is disposed. According to the above configuration, there can realize not only the prevention of the malfunction of the electric syringe but also the enhancement in the operation performance thereof. 
     Further, according to the invention, there can be provided an electric syringe structured in the following manner: that is, provision of a clutch mechanism portion enables the clutch operation of a transmission mechanism part, which makes it possible to move a push member; by biasing the push member using a push member biasing spring, a cartridge for 1.0 ml and a cartridge for 1.8 ml can be used in common without replacing a cartridge holder; and, at a set position, a rack for the push member can be contacted with a rubber plug for an anesthetic at a constant pressure. 
     In addition, in the case of the connection with respect to the cartridge holder connecting part, the connection can be carried out automatically simply by inserting the cartridge holder with the cartridge assembled thereto into the cartridge holder connecting part, the present electric syringe is easy to handle. 
     To sum up, according to the invention, generally, there can be provided an electric syringe for a dental anesthetic that is enhanced in the operation performance as a whole and is thereby improved in the reliability for both of a dentist and a patient. 
     Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.