BACKGROUND OF THE INVENTION 
     The present invention relates to a serial printer. 
     A number of types must be provided on a printer head of a serial printer. Today, a printer is required to contain at a minimum more than one hundred types and, accordingly, the printer head of the printer becomes large, which results in various disadvantages. A printer head is required to perform a spacing action, a back spacing action, a carriage return action, a tabulating action, a type selecting action and a printing action. The time for all of these actions, especially for the type selecting and the printing actions, depends mainly upon the weight of the printer head. Therefore a large, heavy printer head causes an increase in the time required to perform these actions, which results in a decrease in the printing speed. In addition a large printer head, requires that the printer on which it is installed also be large and a large space is required for a large printer. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a serial printer in which the above disadvantages are obviated or mitigated. 
     One of the objects of the present invention is to provide a serial printer comprising a small and light weight printer head notwithstanding the fact that it includes many types. 
     Another object is to provide a serial printer comprising a means for selecting a desired type easily. 
     A further object is to provide a serial printer comprising a means for pushing the selected type outwardly toward a platen so that the selected type projects further outward than the adjacent types, thereby avoiding side-printing, i.e. needless printing with the types adjacent to the desired one. 
     A still further object is to provide a serial printer in which a shaft of the printer head and an output shaft of a DC servo motor of said type selecting means are connected coaxially, said DC servo motor rotating the printer head for selecting the desired type. The advantage of connecting the two shafts coaxially is that the rotation of the motor is transmitted directly to the shaft of the printer head so that the operation of selecting a desired type is effected accurately. 
     A still further object is to provide a serial printer in which a flexible coupling connects the above mentioned two shafts so that the printer head can swing or move pivotally between the printing position and the standing position while the DC servo motor is stationary. 
     A still further object is to provide a serial printer comprising a means for supporting a printer head which allows the printer head to swing or move pivotally about an axis located on a tangential line which contacts the printing point on a cylindrical platen section. Such an arrangement has the advantage that the type on the printer head always strikes the paper on the platen at a right angle and, thus, the letter is fully printed without light or shaded areas. 
     According to the present invention there is provided a serial printer comprising: 
     a platen; 
     a printer head which includes a shaft and a type wheel which is connected to said shaft and has a plurality of resiliently supported types thereon; 
     a means for selecting one desired type out of a plurality of types on said type wheel and for bringing said desired type to a printing position on the printer head; 
     a means for joining the type wheel shaft and an output shaft of the said type selecting means; 
     a means for pushing the selected type further outward, toward the platen, than the adjacent types; 
     a means for supporting the printer head and for allowing said printer head to pivotally move about an axis to strike the platen, and; 
     a striking means connected to the printer head for causing said printer head to strike the platen thereby enabling printing of the desired type. 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a serial printer according to the present invention; 
     FIG. 2 is a sectional view partly in section of the printer in FIG. 1; 
     FIG. 3 is an enlarged partial sectional view of a type wheel element according to the present invention; 
     FIG. 4 is a plan view of a means for pushing the type outwardly; 
     FIG. 5 is a side view of a stopper corresponding to a stopper which is designated by the reference numeral 27 in FIG. 4; 
     FIG. 6 is a side view of another stopper corresponding to a stopper which is designated by the reference numeral 24 in FIG. 4; 
     FIG. 7 is composed of graphs representing the relation between printing actions and axial shifting actions of the printer head; 
     FIG. 8 shows a stopper fixed to a carriage, the stopper corresponding to that depicted in FIG. 5; 
     FIG. 9 shows the mechanism of an axial shifting action of the printer head; 
     FIG. 10 is an overlapped illustration of the two stoppers which correspond to those illustrated in FIG. 4 as designated by the reference numerals 24 and 27, respectively, showing the difference in height between them; 
     FIG. 11 is a plan view of means for pushing the type outwardly, showing that a desired type is projected; 
     FIG. 12 shows a means for supporting a printer head according to the present invention; 
     FIG. 13 is a schematic block diagram illustrating the type selecting means, and; 
     FIG. 14 is a cross sectional view take on line X--X of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Throughout all of the above drawings like reference characters designate like or corresponding parts. 
     In FIG. 1, a sheet of paper 60 is shown set on a platen 1, a type wheel 3, ribbon cartridge 99, a support member 98, and a drive member 97. The type wheel 3 and the ribbon cartridge 99 are mounted on a carriage which is supported by the support member 98 and is driven to move along the platen 1 by the known mechanism through the drive member 97. 
     In FIG. 2 a platen 1 is shown which is cylindrical and rotatably fixed to unillustrated side plates of a serial printer according to the present invention. The platen 1 is not limited to a cylindrical construction and may be in the form of a flat plate or the like. Over the platen 1 is set a paper 60. A printer head comprises a shaft 2 and a type wheel 3. The type wheel 3 includes a pair of type wheel elements 3a and 3b, both of which have substantially the same construction. 
     In FIG. 3, an enlarged partial view of the type wheel element is depicted. Each type wheel element comprises a boss 4, a disc base 5 spreading radially from said boss 4, a number of arms 6 fixed to said disk base 5, and types 7, one of which is molded on the outside of the end of each arm. Accordingly, the types are arranged so as to face outwardly in a circle about the printer head shaft 2. As the arm 6 is resilient, the type 7 can surge or move while the joint between the disk base 5 and the arm 6 is stationary. 
     In this embodiment of the invention each type wheel element has 64 arms and, thus, the type wheel has 128 arms in all. This means that 128 types can be held on the type wheel because each arm has one type on its end. As the arm 6 bends near the disk base 5 and extends substantially parallel to the shaft 2 instead of spreading straight, the distance between individual types is minimized and, thus, the type wheel 3 is kept small in spite of having so many types. The boss 4, the disk base 5, the arms 6, and the types 7 of the type wheel element are normally molded integrally with thermal plastic material and the type surface is coated with known metallic materials such as, nickel, or chrome, undercoated with copper. The arm 6 and a part of disk base 5 may also be formed with resilient metallic material while only the boss 4 and the type 7 are molded with thermal plastic material. 
     Although essentially only one type wheel element is needed for a type wheel, in such a case more than 2 types must be arranged on one arm in order to increase the number of types in all. Such an arrangement induces sideprinting, so that it is preferable to arrange two type wheel elements in such a manner that one element covers the other which is upside down, as shown in FIG. 2, in order to avoid sideprinting. 
     As shown in FIG. 2, the shaft 2 and an output shaft 12 of a DC servo motor 11 are connected by a serration mechanism 8 and a joint 9 having an intermediate flexible resilient member and two joint ends on the opposite ends of said member. One of said joint end is connected to a serrated shaft 10 of said serration mechanism and the other joint end is connected to the output shaft 12. The shaft 2 is hollow at its lower part and is serrated inside in order to engage with the serrated shaft 10. The lower end of the serrated shaft 10 is connected to the flexible joint 9. The flexible joint 9 may be formed with a tightly wound coil spring, with one end of the spring being connected to the serrated shaft 10 and the other end to the outer shaft 12 of the DC servo motor 11. The serrated shaft 10 allows the shaft 2 to slide or move up and down along the shaft 10 as indicated by a double headed arrow C, but prevents relative rotation between the shaft 2 and the shaft 10 so that the rotation of the servo motor shaft is steadily transmitted to the shaft 2. The flexible joint 9 allows the movement or swinging of the shaft 2 as indicated by a double headed arrow D. 
     Referring again to FIG. 2, the servo motor 11 rotates the type wheel 3 until a desired type faces the paper 60 on the platen 1. The servo motor 11 is mounted under a carriage 96 which is supported by a support member 98 and a support plate 95 which is fixed to the unillustrated printer body. The carriage 96 has rollers 93 and 94, between which the support plate 95 is arranged. The support member 98 and the support plate 95 extend parallel to the platen 1. 
     In FIG. 2, a type pushing means is also seen comprising a base member 13, an upper lever plate 14, a lower lever plate 15, an upper pusher 16, a lower pusher 17, and springs 19 and 20. The type pushing means is set in the space between the two type wheel elements 3a and 3b as shown in FIG. 2. A plan view of this type pushing means is shown in FIG. 4. The upper lever plate 14 comprises a lever 22 and a cam 23. The hidden lower lever plate 15 comprises a lever 25 and a cam 26, respectively corresponding to the lever 22 and the cam 23 of the upper lever plate 14. The two levers 22 and 25 extend out of the type wheel through a slit between the two type wheel elements 3a, 3b, and are capable of contacting stoppers 24 and 27, respectively, as shown in FIG. 14. The stopper 27 is fixed to the carriage 96 as shown in FIG. 8, and the stopper 24 is also fixed to the carriage 96, in the same manner as the stopper 27 at the opposite side of the type wheel 3. The configurations of the stoppers 24, 27 are shown in FIG. 10 but they are not limited to the illustrations. 
     Referring again to FIG. 2, a guide lever 18 is fixed to the base member 13 and arranged to pass through an opening of a guide member 21 which is unmovably fixed to the carriage 96. In this way, the base member is prevented from rotating about the shaft 2 while the shaft 2 and the type wheel 3 are rotating and, thus, the upper pusher 16 and the lower pusher 17 always face at right angles to the platen 1. The two pushers 16 and 17 can slidably move toward and away from the platen 1 guided by the base member 13. Normally, the two springs 19 and 20 force the pushers 16 and 17 away from the type 7. 
     Referring to FIG. 4, a spring 61 connects the two lever plates 14 and 15 (not shown in FIG. 4) to each other and forces them to about against stoppers 28 and 29, respectively, with the result that the recesses of cams 23 and 26 (not shown in FIG. 4) normally contact the two pushers 16 and 17 (not shown in FIG. 4), respectively. The said stoppers 28 and 29 are fixed to the base member 13. 
     A roller may be arranged instead of the above mentioned cam mechanism with a pusher. In such case, the roller is always in pushing contact with a type on the type wheel element. 
     Other embodiments of the stoppers 24 and 27 are depicted in FIGS. 5 and 6. 
     In FIG. 5, a side view of the stopper 270, corresponding to the stopper 27, is illustrated. The stopper 270 is fixed to the carriage 97 (not shown in FIG. 5), in the same manner as the stopper 27, and has an abutment 271 at its end. The abutment 271 can rotate clockwise (in FIG. 5) about an axis 272, but normally a spring 273 forces it to abut against a stopper 274 as shown in FIG. 5. 
     In FIG. 6 a side view of the stopper 240, corresponding to the stopper 24, is depicted. The stopper 240 is fixed to the carriage 96 (not shown in FIG. 6), in the same manner as the stopper 24, and has an abutment 241 at its end. The abutment 241 can rotate counterclockwise (in FIG. 6) about an axis 242, but normally a spring 243 forces it to abut against a stopper 244 as shown in FIG. 6. 
     The operation of the stoppers 270 and 240 will now be described with reference to FIGS. 5 and 6. 
     In the case of using the stoppers 27 and 24, overlapping of the shifting action of the type wheel and the printing action (both will be described later) must be avoided in order to prevent the catching of the lever 22 or 25 on the top of the stopper 24 or 27. However, by using the stoppers 270 and 240, the above drawback can be obviated. In the shifting action, the levers 22 and 25 move vertically as indicated by double headed arrows 3 and F, and in the printing action they move transversely with respect to the platen (not shown) as indicated by double headed arrows G and H. 
     In the case where a character on the upper type wheel element is to be printed after the printing of a character on the lower type wheel element, the type wheel moves from the upper position (&#34;shift-out&#34; position) to the lower positon (&#34;shift-in&#34; position). In this case, the lever 22, depicted by solid lines in FIG. 6, at first moves to the right in FIG. 6, and during this operation the other lever 25, located at a position depicted by dotted lines in FIG. 5, is kept abutting against the abutment 271, and the type on the lower type wheel element is printed. Then, the lever 22 moves downward. The downward movement of the lever 22 can be started before it returns to its original position because even if the abutment 241 catches the lever 22, it doesn&#39;t prevent the downward movement of the lever 22 since the abutment 241 is rotatable about the axis 242. 
     FIG. 7a is a chart wherein the above described relation between the time and the movement of the type wheel is represented. In this chart the solid line ndej represents the downward movement of the type wheel and the solid line abc represents the transverse movement with respect to the platen toward and away from the platen of the type wheel. The downward movement starts at the point d and ends at the point e. The point d is located just after the starting point b of the returning movement of the type wheel. 
     FIG. 7b is a chart showing the same type of relation as the chart of FIG. 7a, but in this case the stoppers 24 and 27 are used instead of 240 and 270. As shown in FIG. 7b, the time of the starting point d&#39; of the downward movement is located near the ending point c&#39; of the returning movement of the type wheel. 
     In the case of upward movement of the type wheel, refering to FIG. 5, firstly the lever 25 moves rightwards in FIG. 5, then returns to the original position and, in the course of this returning movement, the lever 25 starts to move upwardly because the rotatable abutment 271 doesn&#39;t prevent the lever 25 from moving upwardly. This upward movement is represented by a broken line fg in FIG. 7a. 
     Due to the above described actions the vertical shifting action and the printing action can be overlapped with the result that the time between printing actions can be shortened. 
     Also illustrated in FIG. 2 is a support means which supports the printer head, comprising a link 30 and an upright 31. The link 30 is connected to the shaft 2 at one of its ends and is rotatably connected to the upright 31 at its other end. The shaft 2 can slide axially, though held by the link 30, as indicated by the double headed arrow C. Also, the link 30 can rotate about an axis 32 of the upright 31, and thereby the type wheel 3 swings or moves as indicated by the double headed arrow D. 
     In FIG. 2, a striking means is also depicted which comprises, a rotatable electromagnet 33 mounted on the carriage 96, an output lever 35 fixed to an output shaft 34 of the electromagnet 33, and a link 36 one end of which is connected to the lever 35 and the other end of which is connected to the aforementioned link 30. When the electro-magnet 33 is excited, the shaft 34 and the lever 35 rotate clockwise as shown by an arrow A in FIG. 2 and, accordingly, the link 36 which is rotatably connected to said lever 35 moves as shown by an arrow B. Thus, the link 36 forces the shaft 2 to rotate about the axis 32 with the result that the type wheel 3 moves toward the platen 1. When the electro-magnet 33 is turned off the shaft 2 is returned by the resiliency of the flexible joint 9 and by a spring 37. 
     In FIG. 9 the shifting mechanism is shown by which the type wheel 3 moves up and down in order to select one element out of the two type wheel elements of 3a and 3b. As shown in FIG. 9 the shaft 2 has an abutment 38 with which a pin 39 is always in contact. The pin 39 is fixed to a link 40 which is integrally formed with a shifting lever 42 which is rotatable about an axis 41. At one end of this shifting lever 42 is fixed a filament 43, the other end of said filament being fixed to the carriage 96. A pulley 44 cooperates with the filament 43. The pulley 44 is rotatably mounted on a lever 45 to which an armature 46 is fixed. When a solenoid 48 of an electro-magnet 47 is excited, the armature 46 moves toward the magnet and the lever 45 rotates about an axis 50 counterclockwise (in FIG. 9), making the shifting lever 42 rotate clockwise about the axis 41 due to the movement of the pulley 44 and the filament 43. Accordingly, the link 40 of the shifting lever 42 rotates clockwise about the axis 41 with the result that the pin 39 forces the shaft 2 to move upward. Thus, the two wheel elements 3a, 3b and the type pushing means which are fixed to the shaft 2 also move upward until the lever 42 abuts against a stopper 51. This condition wherein the type wheel is raised is called &#34;shift-out&#34;. 
     When the electro-magnet 47 is turned off, a spring 52 forces the shifting lever 42 and the link 40 to turn counterclockwise (in FIG. 9) on the axis 41 until the lever 42 abuts against a stopper 54. Thus, the pin 39 fixed to the end of the link 40 forces the shaft 2 to move downward. Accordingly, the two type wheel elements 3a, 3b and the type pushing means which are fixed to the shaft 2 also move downward. This condition wherein the type wheel 3 is in the lower position is called &#34;shift-in&#34;. 
     The relation between the shifting action and the operation of the type pushing means will now be described. 
     Referring to FIG. 4 and FIG. 10, when the type wheel is in the shift-out condition, the upper lever 22 and the lower lever 25 of the pushing means are located at the position depicted by the dotted line in FIG. 10. In this case the lower lever 25 abuts against the stopper 27 and, then, rotates about the shaft 2 as the shaft 2 is moved toward the platen 1 by the aforementioned operation of the rotatable magnet 33, while the upper lever 22 is over the stopper 24. On the other hand, when the type wheel is in the &#34;shift-in&#34; condition the levers 22 and 25 of the pushing means are located at the position depicted by the solid line in FIG. 10. In this case the upper lever 22 abuts against the stopper 24 and, then, rotates about the shaft 2 as the shaft 2 is moved toward the platen 1 by the operation of the rotatable electro-magnet 33, while the lower lever 25 is in the recess of the stopper 27. This condition is shown in FIG. 11. 
     The operation of the serial printer according to the present invention will now be described in detail. 
     The whole operation is completed by the type selecting action and printing action. The type selecting action is accomplished by the aforementioned shifting action and the action of type character selection in which one desired type is selected out of the many types on the selected type wheel element. The latter action is completed by rotation of the type wheel by the DC servo motor, which is connected to the shaft of the type wheel, until the desired type faces the paper on the platen. 
     Referring again to FIG. 2 and FIG. 9, when the upper type wheel element 3a is to be selected, i.e. the desired type is on the upper type wheel element 3a, the electro-magnet 47 is not excited and the shaft 2 is at the lower position or in the &#34;shift-in&#34; condition. At this time the types on the upper type wheel element 3a are arranged at the position corresponding to the printing line on the paper. In this case the upper lever 22 abuts against the stopper 24 so that the upper pusher 16 will be operated, as mentioned before referring to FIG. 4. 
     When the lower type wheel element 3b is to be selected, i.e. the desired type is on the lower type wheel element 3b, the electro-magnet 47 is excited and the shaft 2 is forced to move to the upper position or to the &#34;shift-out&#34; condition. In this case the lower lever 25 abuts against the stopper 27 so that the lower pusher 17 will be operated, as mentioned before. 
     FIG. 13 is a block diagram for showing an embodiment of a servo system employable for selecting a desired type from types on the type wheel of a serial printer of the present invention. It should be noted that in FIG. 13, the same elements as those shown in other drawings are designated by the same reference numerals. In the servo system of FIG. 13, a data converting circuit 101 is provided with eight input terminals each of which receive an input consisting of eight bits. One of the eight bits is a command information for determing whether the type wheel 3 should be positioned at either its shift-in position or shift-out position, and six other bits express a command as to which type character on the type wheel 3 should be selected and positioned at the printing position of the type wheel 3 adjacent to the platen 1. The remaining one bit is used as a parity check bit. Thus, when the above-mentioned input is applied to the data converting circuit 101, the circuit 101, issues a shift-in signal or a shift-out signal and also a type character selection signal from its output terminals. The shift-in or shift-out signal is sent, through a servo amplifier 102, to the electro-magnet 47, the operation of which was previously explained. However, it should be noted that the electro-magnet 47 is excited only when the shift-out signal is applied to the electro-magnet 47. That is to say, application of the shift-in signal to the electro-magnet 47 does not cause excitation of the electro-magnet 47. Thus, when the shift-out signal excits the electro-magnet 47, it operates so as to lift the type wheel 3, which is normally at its lower position where the upper type wheel 3a is positioned adjacent to the platen 1. As a result of the lifting of the type wheel 3, the lower type wheel 3b comes adjacent to the platen 1 and is maintained there until the printing operation is completed. When the shift-in signal is applied to the electro-magnet 47, the electro-magnet 47 is not excited and, therefore, lifting of the type wheel 3 does not occur and the upper type wheel element 3a is positioned adjacent to the platen 1. The type character selection signal of six bits which is issued from the data converting circuit 101, is sent to a digital to analogue converter 103 in which the signal is converted into an anologue voltage signal. It should be noted that, since the signal of six bits can be indicative of one of 64 different pieces of information, the analogue voltage signal can correspondingly be one of 64 different voltage signals. The analogue voltage signal issued from the digital to analogue converter 103 is supplied to a comparing circuit 104. In the circuit 104 the signal is compared with a voltage signal which is sent from a potentiometer 106 to the comparing circuit 104 and which is indicative of the actual position of the type character wheel 3. The term &#34;actual position&#34; of the type character wheel 3 is used here to mean the position where the type wheel 3 was positoned to perform the immediately previous printing operation. The comparing circuit 104 produces a voltage signal corresponding to or proportional to the difference between the signal from the digital to analogue converter 103 and the signal from the potentiometer 106. The voltage signal produced by the circuit 104 is subsequently amplified by a DC amplifier 105 and supplied to the DC servo motor 11 which rotates the shaft 2 of the type character wheel 3. It should be noted that the rotation of the shaft 2 is continued until the difference between the signal from D-A converter 103 and the signal from the potentiometer becomes nil. Thus, when the rotation of the shaft 2 is stopped, the type character wheel 3 reach a state where a desired type character commanded by the input to the data converting circuit 101 is positioned at the printing position of the type character wheel 3. 
     The printing action is accomplished by the selected type striking the platen on which a ribbon and a paper is set. As mentioned before, with reference to FIG. 2, when the rotatable electro-magnet 33 is excited, the shaft 2 is forced to move or swing about the axis 32. At that time, naturally, the flexible joint 9 is bent or expanded. 
     When the type wheel is in the &#34;shift-out&#34; condition and is forced to move toward the paper by the operation of the rotatable electro-magnet 33, the lever 25 of the lower lever plate 15 abuts against the stopper 27. As the shaft 2 moves further, the lever plate 15 rotates about it with the result that the cam 26 forces the pusher 17 to project outwardly so that the selected type is pushed further outward than the adjacent types. At this time, the lever 22 of the upper lever plate 14 is located over the stopper 24 and is not in contact with it, so that the upper lever plate 14 does not rotate about the shaft 2 and the cam 23 does not force the pusher 16 to project. 
     Then, the pushed out type strikes the platen 1 on which a ribbon and a paper is set. Thus, the desired letter or character is printed on the paper. 
     The rotatable electro-magnet 33 is turned off just before the type strikes the platen, so that the type wheel 3 strikes the platen 1 by the force of inertia. After the type wheel 3 strikes the platen 1, it is forced to move away from the platen 1 and return to the original standing position by the resiliency of flexible joint 9 and the force of the spring 37. 
     When the type wheel is in the &#34;shift-in&#34; condition and is forced to move toward the platen by the operation of the rotatable electro-magnet 33, the lever 22 of the upper lever plate 14 abuts against the stopper 24. As the type wheel moves further, the upper lever plate 14 rotates about the shaft 2 with the result that the cam 23 forces the pusher 16 to project so that the selected type is pushed further outward than the adjacent types as shown in FIG. 11. At that time the lever 25 of the lower lever plate 15 is in the recess of the stopper 27 and doesn&#39;t abut against it, so that the lower lever plate 15 does not rotate and the cam 26 does not force the pusher 17 to project. Then, the pushed out type strikes the platen and returns to the original standing position in the same way as mentioned before. 
     Referring to FIG. 12, the location of the axis 32 will be described. It is important that the center of the axis 32 be located directly under the printing point 62 on the platen 1, i.e. on the tangential line L at the printing point 62. In such an arrangement, the type wheel 3 always strikes the platen 1 perpendicular to the said line L and, thus, the letter is fully printed without light or shaded areas. This advantage will be substantially maintained even if the thickness of the paper changes, so that letters are always evenly printed. 
     As the distance between the printing point 62 and the center of the axis 32 is different than the distance between the printing point 62 and the center of the flexible joint 9, said joint 9 must be capable of expanding and contracting. In this regard, a bellows may be used instead of the above mentioned flexible joint 9 and the serration mechanism. 
     As many seemingly widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the above described specific embodiment thereof except as defined in the appended claims.
A serial printer is described which includes a platen, a printer head, a mechanism for selecting one desired type out of a plurality of types on the printer head, a mechanism for joining a printer head shaft and an output shaft of the type selector, a pusher for pushing the selected type further outward than the adjacent types, a support for supporting the printer head and a striking mechanism connected to the printer head for causing the printer head to strike the platen, thereby enabling printing of the desired type. The printer head includes a shaft and a type wheel which is connected to the shaft. The printer head also has a plurality of resiliently supported types thereon. The selector brings a desired type to a printing position on the printer head, and the support allows the printer head to pivotally move about an axis to strike the platen.