Shuttle printers

Abstract: A shuttle printer having a print shuttle unit (10) capable of reciprocating with a print head (11) mounted thereon, and a balance shuttle unit (20) capable of reciprocating to generate counterforce to momentum of the print shuttle unit. The shuttle printer includes a device for driving the print shuttle unit to reciprocate, and a device for driving the balance shuttle unit to reciprocate. The shuttle printer further includes a device (17) for detecting the position of the print shuttle unit, and a device for synchronously controlling the two driving devices in response to a result of detection by the print shuttle unit position detecting device.

Description: This invention relates to a shuttle printer having a reciprocable print shuttle unit for carrying a print head or a plurality of print heads. Recently, there has arisen demand for printers that are capable of printing graphic information, in addition to characters, and of effecting high-speed printing. Therefore, dot printers are widely used; the line printer versions of these are effective for high-speed printing. However, it is difficult from the viewpoint of mounting to provide a row of dot print elements for one line horizontally. Dissipation of heat also gives rise to a problem. Under these circumstances, shuttle printers have been developed which comprise a multiplicity of print heads (e.g. each having 24 pins) the print heads being mounted on a shuttle and the shuttle being reciprocable over a travel distance corresponding to the print area assigned to each print head. When such a print shuttle is reciprocated to effect printing, vibration is generated in the printer due to the momentum of the print shuttle, which depends on the mass and velocity thereof. Accordingly, it is desirable to take measures to prevent the generation of such vibration. In one approach, a counterweight (the balance shuttle unit) for cancelling the momentum of the print shuttle unit is provided, the balance shuttle unit being mechanically linked to the print shuttle unit so that, as the print shuttle unit is reciprocated by a motor, the balance shuttle unit moves, in linked relation to the reciprocating motion of the print shuttle unit, in a direction opposite to that of the print shuttle unit. The motion of the balance shuttle unit generates a counterforce to the force generated by the print shuttle unit so as to tend to cancel it. Thus, vibration of the printer is suppressed. However, the motor used to drive the print shuttle unit drives not only the print shuttle unit but also the balance shuttle unit, via a link mechanism. The load on the motor is then so heavy that it is difficult to drive the print shuttle unit at high speed. If however the motor is increased in size in order to effect high-speed driving, the overall size of the printer increases and production costs rise. A shuttle printer having a print shuttle unit and a balance shuttle unit is disclosed in US-A-4941405, the print and balance shuttle units being driven collectively by a single electric motor. In use, the electric motor oscillates the print shuttle unit back and forth by resonantly vibrating the print shuttle unit at a natural frequency thereof. The balance shuttle unit moves in a counter-reciprocating manner relative to the print shuttle unit by virtue of mechanical coupling between the print and balance shuttle units provided by springs held in tension. According to the present invention, there is provided a shuttle printer comprising a print shuttle unit for carrying a print head, a balance shuttle unit, means for detecting the position of the print shuttle unit and means for driving the print shuttle unit to reciprocate and the balance shuttle unit to counter-reciprocate so as to generate a force to counterbalance the momentum of the reciprocating print shuttle unit, characterised in that the driving means comprises an electric motor for driving the print shuttle unit, a further electric motor for driving the balance shuttle unit and drive circuit means for synchronously controlling said two electric motors in response to a signal from the print shuttle unit position detecting means, wherein the drive circuit means comprises first and second drive circuits for driving respective ones of the electric motors. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a fragmentary perspective view of a shuttle unit in a first embodiment of the present invention; Fig. 2 is a plan view of the shuttle unit in the first embodiment of the present invention; Fig. 3 is a sectional side view of the shuttle unit in the first embodiment of the present invention; Fig. 4 is a front view of a linear motor in the first embodiment of the present invention; Fig. 5 is a perspective view of a print head in the first embodiment of the present invention; Fig. 6 is a front view of a print head assembly in the first embodiment of the present invention; Fig. 7 is a circuit block diagram of the first embodiment of the present invention; Fig. 8 is a diagram showing a constant-speed drive circuit in the first embodiment of the present invention; Fig. 9 is a diagram showing a reversing drive circuit in the first embodiment of the present invention; Fig. 10 is a schematic view showing the operation of the first embodiment of the present invention; Fig. 11 is a timing chart of driving signals in the first embodiment of the present invention; Fig. 12 is a flowchart showing control processing in the first embodiment of the present invention; Fig. 13 is a circuit block diagram of a second embodiment of the present invention; Fig. 14 is a flowchart showing control processing in the second embodiment of the present invention; Fig. 15 is a flowchart showing control processing in the second embodiment of the present invention; and Fig. 16 is a flowchart showing control processing in a third embodiment of the present invention. Figs. 1 to 3 show in combination one embodiment in which the present invention is applied to a line printer. Fig. 1 is a perspective view of part of the line printer which includes a print shuttle unit and a balance shuttle unit. Figs. 2 and 3 are a plan view and a sectional side view of the same part of the line printer. A base frame 1 is secured to a casing 50. A pair of parallel stay shafts 2 and 3 extend horizontally and are each secured at both ends thereof to the base frame 1. It should be noted that in Fig. 1 illustration of the casing 50 and the base frame 1 is omitted, and in Fig. 2 illustration of the casing 50 is omitted. A print shuttle 12 is slidably fitted on the first stay shaft 2, which is disposed in the central portion of the base frame 1. The print shuttle 12 is equipped with a print head 11 comprising a row of a multiplicity of print pins. The print shuttle 12 is supported by the first stay shaft 2 and a roller 13 capable of traveling on the base frame 1. The print head 11 is of the electromagnetic release type, for example. As shown in Fig. 5, the print head 11 comprises a row of 12 (for example) print head assemblies 11a of 24-pin type arranged horizontally. Each print head assembly 11a is formed from 4 sets of 6 print elements which are respectively arranged in front upper, front lower, rear upper and rear lower stages in such a manner that the two sets of print elements in the front and rear upper stages are symmetric with respect to those in the front and rear lower stages. The print elements perform printing in units of dots by print pins. In each print head assembly 11a, wires 11b of the 24 pins are obliquely arranged in two groups of 12 pins, as shown in Fig. 6 When the print head 11 is driven, the distal ends of the print pins project in the direction of the arrow A, shown in Fig. 3, thereby striking printing paper, which is fed in the direction of the arrow B through a paper feed passage 4, through an ink ribbon (not shown). Thus, impact dot printing is carried out. This printer performs impact dot printing by reciprocating the print shuttle unit 10 through a distance corresponding to the width of the print head assembly 11a A yoke 14, which is a planar iron plate, is attached to the bottom of the print shuttle 12. A row of a plurality of rectangular plate-shaped permanent magnets 15 are disposed on the lower surface of the yoke 14 in a direction parallel to the axis of the first stay shaft 2. The permanent magnets 15 are each magnetized in the direction of the thickness thereof. That is, each permanent magnet 15 has two magnetic poles at the upper and lower end faces thereof. The permanent magnets 15 are formed by using rare-earth magnets, which have a strong magnetic property, for example, samarium-cobalt magnets. Accordingly, the permanent magnets 15 are thin and light in weight in comparison to ferrite magnets or others (e.g., the thickness and weight are each 1/5 of that in the case of the latter). Thus, the print shuttle 12, and the print head 11, the yoke 14 and the permanent magnets 15, which are attached to the print shuttle 12, form a print shuttle unit 10 which is movable along the first stay shaft 2. A row of electromagnetic coils 16 are secured to a coil base 18, which is formed from an iron plate secured to the base frame 1, so that the electromagnetic coils 16 face the permanent magnets 15 of the print shuttle unit 10 across a slight gap. Thus, the permanent magnets 15 and the electromagnetic coils 16 form a linear motor (first linear motor) for driving the print shuttle unit 10. Lead wires 19 are used to feed electric power to the electromagnetic coils 16. In this linear motor, as shown in Fig. 4, the permanent magnets 15 are divided into magnets 15a and 15d for contsant-speed control and magnets 15b and 15c for reversing, and the electromagnetic coils 16 are also divided into coils 16a (L1) and 16c (L2) for constant-speed control and a coil 16b (L3) for reversing. When the reversing coil 16b is driven, the coil 16b moves relative to the yoke 14 as far as the center of the pair of reversing magnets 15b and 15c to effect a reversing operation. When current is passed through the constant-speed control coils 16a and 16c in the forward direction, the print shuttle unit 10 moves rightward. When current is passed through the coils 16a and 16c backward, the print shuttle unit 10 moves leftward. In addition, a position detecting sensor 17 is provided, as shown in Fig. 2. The position detecting sensor 17 comprises slits formed in the yoke 14 of the print shuttle unit 10, and a transmissive photosensor that is attached to the base frame 1 so as to face the slits. In Figs. 1 and 3, illustration of the position detecting sensor 17 is omitted. The slits of the position detecting sensor 17 include a right-hand end slit, timing slits and a left-hand end slit, which are provided in the yoke 14. Thus, with regard to the print shuttle unit 10, a right-hand end detecting signal, a position signal, and a left-hand end detecting signal are output by the photosensor. A balance shuttle 22, which is formed in the same way as the print shuttle 12, is slidably fitted on the second stay shaft 3, which is disposed parallel to the first stay shaft 2. A counterweight 21 is mounted on the balance shuttle 22, and a yoke 24 is attached to the bottom of the balance shuttle 22. A row of permanent magnets 25, which are similar to the permanent magnets 15 of the print shuttle unit 10, are attached to the lower surface of the yoke 24. A roller 23 is rotatably attached to the balance shuttle 22 so that the balance shuttle 22 travels on the base frame 1. The balance shuttle 22 is supported by the roller 23 and the second stay shaft 3. Thus, a balance shuttle unit 20 is formed from the balance shuttle 22 and the counterweight 21, the yoke 24 and the permanent magnets 25, which are attached to the balance shuttle 22. The constituent elements of the balance shuttle unit 20 can move as one unit in parallel to the print shuttle unit 10. The balance shuttle unit 20 is formed so that the overall weight thereof is approximately equal to that of the print shuttle unit 10. A coil base 28 is secured to the base frame 1, and a row of electromagnetic coils 26, which are similar to the electromagnetic coils 16 shown in Fig. 4, are secured to the coil base 28 so as to face the row of permanent magnets 25 disposed on the balance shuttle 22 across a slight gap. Thus, the permanent magnets 25 and the electromagnetic coils 26 form a linear motor (second linear motor) for driving the balance shuttle unit 20. Lead wires 29 are used to supply electric power to the electromagnetic coils 26. By properly controlling the current passed through the electromagnetic coils 26, the balance shuttle unit 20 can be rectilinearly reciprocated at high speed along the second stay shaft 3. In addition, the balance shuttle unit 20 is also provided with a position detecting sensor 27, which is similar to the position detecting sensor 17 of the print shuttle unit 10, to output a position signal. Thus, when the print shuttle unit 10 is moved rightward, the balance shuttle unit 20 is moved leftward, whereas, when the print shuttle unit 10 is moved leftward, the balance shuttle unit 20 is moved rightward. In this way, the balance shuttle unit 20 generates counterforce to the momentum of the print shuttle unit 10 to cancel it, thereby preventing generation of vibration. Thus, since the print shuttle unit 10 and the balance shuttle unit 20 are independently driven by the respective driving devices, the print shuttle unit 10 can be reciprocated at high speed with a relatively small motor without using a large motor. In addition, the use of linear motors as driving devices enables a reduction in the overall size of the printer. Fig. 7 shows the arrangement of a controller 6 for controlling the operations of the first linear motor (15 and 16) and the second linear motor (25 and 26). The controller 6 is provided with a microprocessor (MPU) 60, a read-only memory (ROM) 61 stored with a program, a random access memory (RAM) 62 for work, a timer circuit 63, and a input/output (I/O) port 64 which receives an output signal from the position detecting sensor 17 and outputs a reversing control signal, a leftward constant-speed control signal and a rightward constant-speed control signal. A first linear motor driving circuit 7a for the print shuttle unit 10 drives the electromagnetic coils 16 of the first linear motor. A second linear motor driving circuit 7b for the balance shuttle unit 20 drives the electromagnetic coils 26 of the second linear motor. The second linear motor driving circuit 7b is connected to the electromagnetic coils 26 so that the constant-speed motor part of the second linear motor is opposite in polarity to that of the first liner motor. Fig. 8 shows a constant-speed motor driving circuit used in each of the linear motor driving circuits 7a and 7b. The constant-speed motor driving circuit comprises an H-shaped bridge circuit in which transistors Q1 to Q4 are connected in an H-shape, and flyback diodes d1 to d4 are connected to the transistors Q1 to Q4, respectively. More specifically, the constant-speed electromagnetic coils L1 and L2 are connected between the node of the series-connected transistors Q1 to Q3 and the node of the series-connected transistors Q2 to Q4. In response to a rightward driving signal, the transistors Q2 to Q3 turn on to pass current via the route: transistor Q2 → coil L2 → coil L1 → transistor Q3. In response to a leftward driving signal, the transistors Q1 to Q4 turn on to pass current via the route: transistor Q1 → coil L1 → coil L2 → transistor Q4. In this way, the print shuttle unit 10 and the balance shuttle unit 20 are each driven rightward and leftward. Fig. 9 shows a reversing motor driving circuit used in each of the linear motor driving circuits 7a and 7b. The reversing motor driving circuit comprises a transistor Q5, and a parallel circuit of the reversing electromagnetic coil L3 and a flyback diode D1, which is provided on the collector side of the transistor Q5. Accordingly, when a reversing driving signal is input to the base of the transistor Q5, current flows through the electromagnetic coil L3. Consequently, the coil L3 moves relative to the yoke 14 as far as the center of the permanent magnets 15b and 15c, as shown in Fig. 4. It should be noted that the constant-speed electromagnetic coils L1 and L2 of the balance shuttle unit 20 may be wound in a direction reverse to the winding direction of those of the print shuttle unit 10 so that the balance shuttle unit 20 moves leftward in response to the rightward driving signal, and it moves rightward in response to the leftward driving signal. This operation is carried out as shown in Figs. 10 and 11. That is, when the print shuttle unit 10 reaches the right-hand end during rightward constant-speed movement (leftward constant-speed movement of the balance shuttle unit 20), the reversing motor part 16b is driven to reverse the print shuttle unit 10. When the print shuttle unit 10 gets out of the reversing region, it is moved leftward at a constant speed (while the balance shuttle unit 20 is moved rightward at a constant speed). When the print shuttle unit 10 reaches the left-hand end, the reversing motor part 16b is driven to reverse the print shuttle unit 10. When the print shuttle unit 10 gets out of the reversing region, it is moved rightward at a constant speed again. Fig. 12 is a flowchart showing control processing in the first embodiment of the present invention. Reference symbol ST denotes processing steps. First, the microprocessor 60 outputs a rightward driving signal from the I/O port 64 at ST1 to drive the electromagnetic coils 16 and 26 of the linear motors through the driving circuits 7a and 7b, thereby moving the print shuttle unit 10 rightward and the balance shuttle unit 20 leftward. Next, the microprocessor 60 checks at ST2 whether or not a right-hand end detecting signal, which represents detection of the right-hand end slit, has been output from the position detecting sensor 17. If YES, the microprocessor 60 proceeds to ST4 to effect reversing control. If no right-hand end detecting signal is detected at ST2, the microprocessor 60 checks at ST3 whether or not a timing signal (position signal) from the position detecting sensor 17 has been detected. If NO, the process returns to ST2, whereas, if YES, the process shifts to constant-speed control. That is, the microprocessor 60 saves the measured value T of the timer circuit 63 at ST5 and restarts the timer circuit 63 at ST6 to commence measuring the interval of the position signal. At ST7, the microprocessor 60 compares the measured interval value T of the saved position signal with a constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore, a rightward driving signal is output at ST8 to accelerate the shuttle units 10 and 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a leftward driving signal is output at ST9 to decelerate the shuttle units 10 and 20. Then, the process returns to ST2. When the microprocessor 60 detects a right-hand end detecting signal at ST2, it turns on the reversing driving signal at ST4 to drive the reversing motor parts so as to reverse the shuttle units 10 and 20. As shown in Fig. 10, the microprocessor 60 checks again at ST10 whether or not a right-hand end detecting signal has been detected. If NO, the print shuttle unit 10 is judged to be within the reversing region. Accordingly, the process is repeated from ST4 to continue the reversing driving signal on. When a right-hand end detecting signal is detected, it is judged that the print shuttle unit 10 has got out of the reversing region. Therefore, the reversing driving signal is turned off at ST11, and the process proceeds to ST12 to effect leftward movement control. At ST12, the microprocessor 60 outputs a leftward driving signal from the I/O port 64 to drive the electromagnetic coils 16 and 26 of the linear motors through the driving circuits 7a and 7b, thereby moving the print shuttle unit 10 leftward and the balance shuttle unit 20 rightward. Subsequently, the microprocessor 60 checks at ST13 whether or not a left-hand end detecting signal, which represents detection of the left-hand end slit, has been output from the position detecting sensor 17. If YES, the process proceeds to ST20 to effect reversing control. If no left-hand end detecting signal is detected at ST13, the microprocessor 60 checks at ST14 whether or not a timing signal (position signal) from the position detecting sensor 17 has been detected. If NO, the process returns to ST13, whereas, if YES, the process shifts to constant-speed control. That is, the microprocessor 60 saves the measured value T of the timer circuit 63 at ST15 and restarts the timer circuit 63 at ST16 to commence measuring the interval of the position signal. At ST17, the microprocessor 60 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore, a leftward driving signal is output at ST18 to accelerate the shuttle units 10 and 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a rightward driving signal is output at ST19 to decelerate the shuttle units 10 and 20. Then, the process returns to ST13. When the microprocessor 60 detects a left-hand end detecting signal at ST13, it turns on the reversing driving signal at ST20 to drive the reversing motor parts so as to reverse the shuttle units 10 and 20. As shown in Fig. 10, the microprocessor 60 checks again at ST21 whether or not a left-hand end detecting signal has been detected. If NO, the print shuttle unit 10 is judged to be within the reversing region. Accordingly, the process is repeated from ST20 to continue the reversing driving signal on. When a left-hand end detecting signal is detected, it is judged that the print shuttle unit 10 has got out of the reversing region. Therefore, the reversing driving signal is turned off at ST22, and the process returns to ST1 for rightward movement control. In this way, the print shuttle unit 10 and the balance shuttle unit 20 are synchronously driven by a common signal in each of the reversing timing control and constant-speed control operations by a single controller 6 on the basis of the position signal of the print shuttle unit 10. Thus, the arrangement of the controller 6 is simplified. Further, since only the position detecting sensor 17 for the print shuttle unit 10 suffices for the control operation, it is possible to realize simplification of the arrangement. Fig. 13 shows the arrangement of a control part in a second embodiment of the present invention. A first controller 6a has a microprocessor 60, a ROM 61 for storing a program, a RAM 62 for work, a timer circuit 63, and an I/O port 64 which receives an output signal from the position detecting sensor 17 and outputs a reversing control signal, a leftward constant-speed control signal, a rightward constant-speed control signal and a state notice signal. A second controller 6b has a microprocessor 65, a ROM 66 for storing a program, a RAM 67 for work, a timer circuit 68, and an I/O port 69 which receives an output signal from the position detecting sensor 27 and the state notice signal and outputs a leftward constant-speed control signal and a rightward constant-speed control signal. A print shuttle unit linear motor driving circuit 7a drives the electromagnetic coils 16 of the linear motor for the print shuttle unit 10 in response to the output of the controller 6a. A balance shuttle unit linear motor driving circuit 7b drives the electromagnetic coils 26 of the linear motor for the balance shuttle unit 20 on the basis of the reversing control signal from the controller 6a and the leftward and rightward constant-speed control signals from the controller 6b. In this embodiment, reversing control for the print shuttle unit 10 and the balance shuttle unit 20 is effected by the first controller 6a on the basis of the output of the position detecting sensor 17, whereas constant-speed control for the two shuttle units 10 and 20 is effected by the respective controllers 6a and 6b on the basis of the outputs of the position detecting sensors 17 and 27, which are associated with the shuttle units 10 and 20, respectively. To obtain reversing synchronism between the first and second controllers 6a and 6b, the state notice signal is output from the controller 6a to the controller 6b. Fig. 14 is a flowchart of control processing in the second embodiment, showing the flow of control executed by the first controller 6a. Before starting rightward movement, the first microprocessor 60 outputs notice of rightward constant-speed control to the second microprocessor 65 at ST31, and outputs a rightward driving signal through the I/O port 64 at ST32 to drive the electromagnetic coils 16 of the first linear motor through the driving circuits 7a and 7b, thereby moving the print shuttle unit 10 rightward. Next, the first microprocessor 60 checks at ST33 whether or not a right-hand end detecting signal, which represents detection of the right-hand end slit, has been output from the position detecting sensor 17. If YES, the process proceeds to ST34 to effect reversing control. If no right-hand end detecting signal is detected at ST33, the first microprocessor 60 checks at ST35 whether or not a timing signal (position signal) from the position detecting sensor 17 has been detected. If NO, the process returns to ST33, whereas, if YES, the process shifts to constant-speed control. That is, the first microprocessor 60 saves the measured value T of the timer circuit 63 at ST36 and restarts the timer circuit 63 at ST37 to commence measuring the interval of the position signal. At ST38, the first microprocessor 60 compares the measured interval value T of the saved position signal with a constant-speed reference interval value Tref. If T≥Tref, the speed is judged to be lower than the reference speed. Therefore, a rightward driving signal is output at ST39 to accelerate the shuttle unit 10. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a leftward driving signal is output at ST40 to decelerate the shuttle unit 10. Then, the process returns to ST33. When the first microprocessor 60 detects a right-hand end detecting signal at ST33, reversing control is commenced. That is, the first microprocessor 60 first outputs notice of reversing to the second microprocessor 65 at ST34, and turns on the reversing driving signal at ST41 to drive the reversing motor part so as to reverse the shuttle unit 10. As shown in Fig. 10, the first microprocessor 60 checks again at ST42 whether or not a right-hand end detecting signal has been detected. If NO, the print shuttle unit 10 is judged to be within the reversing region. Accordingly, the process is repeated from ST41 to continue the reversing driving signal on. When a right-hand end detecting signal is detected, it is judged that the print shuttle unit 10 has got out of the reversing region. Therefore, the reversing driving signal is turned off at ST43, and the process proceeds to ST44 to effect leftward movement control. To perform leftward constant-speed control, the first microprocessor 60 outputs notice of leftward constant-speed control to the second microprocessor 65 at ST44, and outputs a leftward driving signal from the I/O port 64 at ST45 to drive the electromagnetic coils 16 of the first linear motor through the driving circuit 7a, thereby moving the print shuttle unit 10 leftward. Subsequently, the first microprocessor 60 checks at ST46 whether or not a left-hand end detecting signal, which represents detection of the left-hand end slit, has been output from the position detecting sensor 17. If YES, the process proceeds to ST47 to effect reversing control. If no left-hand end detecting signal is detected at ST46, the first microprocessor 60 checks at ST48 whether or not a timing signal (position signal) from the position detecting sensor 17 has been detected. If NO, the process returns to ST46, whereas, if YES, the process shifts to constant-speed control. That is, the first microprocessor 60 saves the measured value T of the timer circuit 63 at ST49 and restarts the timer circuit 63 at ST50 to commence measuring the interval of the position signal. At ST51, the first microprocessor 60 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≥Tref, the speed is judged to be lower than the reference speed. Therefore, a leftward driving signal is output at ST52 to accelerate the shuttle unit 10. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a rightward driving signal is output at ST53 to decelerate the shuttle unit 10. Then, the process returns to ST46. When the first microprocessor 60 detects a left-hand end detecting signal at ST46, it commences reversing control. That is, the first microprocessor 60 first outputs notice of reversing to the second microprocessor 65 at ST47 and then turns on the reversing driving signal at ST54 to drive the reversing motor part so as to reverse the shuttle unit 10. As shown in Fig. 10, the first microprocessor 60 checks again at ST55 whether or not a left-hand end detecting signal has been detected. If NO, the print shuttle unit 10 is judged to be within the reversing region. Accordingly, the process is repeated from ST54 to continue the reversing driving signal on. When a left-hand end detecting signal is detected, it is judged that the print shuttle unit 10 has got out of the reversing region. Therefore, the reversing driving signal is turned off at ST56, and the process returns to ST31 for rightward movement control. Fig. 15 shows the constant-speed control flow executed by the second controller 6b of the second embodiment. The second microprocessor 65 checks at ST61 whether or not notice of reversing has been output from the first microprocessor 60. If notice of reversing is received, the balance shuttle unit 20 is subjected to reversing control by the reversing driving signal from the first controller 6a. Therefore, the driving signal for the constant- speed control of the balance shuttle unit 20 is turned off at ST62, and the process returns to ST61. If it is judged that no notice of reversing has yet been output from the first microprocessor 60, the second microprocessor 65 checks at ST63 whether or not notice of rightward constant-speed control has been output from the first microprocessor 60. If NO, the process proceeds to ST64. If it is judged that notice of rightward constant-speed control has been output from the first microprocessor 60, the second microprocessor 65 commences drive in a direction reverse to the rightward direction, that is, leftward drive, at ST65. More specifically, the second microprocessor 65 checks at ST65 whether or not a timing signal (position signal) from the second position detecting sensor 27 has been detected. If NO, the process returns to ST61, whereas, if YES, the process shifts to leftward constant-speed control. That is, the second microprocessor 65 saves the measured value T of the second timer circuit 68 at ST66 and restarts the timer circuit 68 at ST67 to commence measuring the interval of the position signal. At ST68, the second microprocessor 65 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore, a leftward driving signal is output at ST69 to accelerate the shuttle unit 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a rightward driving signal is output at ST70 to decelerate the shuttle unit 20. Then, the process returns to ST61. If it is judged at ST63 that no notice of rightward constant-speed control has yet been output from the first microprocessor 60, the second microprocessor 65 checks at ST64 whether or not notice of leftward constant-speed control has been output from the first microprocessor 60. If NO, the process returns to ST61. If it is judged ST64 that notice of leftward constant-speed control has been output from the first microprocessor 60, the second microprocessor 65 commences drive in a direction reverse to the leftward direction, that is, rightward drive. More specifically, the second microprocessor 65 checks at ST71 whether or not a timing signal (position signal) from the second position detecting sensor 27 has been detected. If NO, the process returns to ST61, whereas, if YES, the process shifts to rightward constant-speed control. That is, the second microprocessor 65 saves the measured value T of the second timer circuit 68 at ST72 and restarts the timer circuit 68 at ST73 to commence measuring the interval of the position signal. At ST74, the second microprocessor 65 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore,a rightward driving signal is output at ST75 to accelerate the shuttle unit 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a leftward driving signal is output at ST76 to decelerate the shuttle unit 20. Then, the process returns to ST61. If constant-speed control is effected by a single driving signal as in the first embodiment, when the speed of the print shuttle unit 10 lowers due to the influence of disturbance, the print shuttle unit 10 is accelerated, and the speed of the balance shuttle unit 20, which is driven at a speed equal to that of the print shuttle unit 10, becomes higher than the reference speed because the balance shuttle unit 20 is not affected by the disturbance. Thus, there is a possibility that the balance shuttle unit 20 will overrun. In contrast, in this embodiment a common driving signal is used only for the reversing control of the first and second linear motors, and the constant-speed control is independently carried out. Therefore, even if the print shuttle unit 10 is accelerated to compensate for a lowering in the speed due to the influence of disturbance, the balance shuttle unit 20 can be maintained at a constant speed. Accordingly, it is possible to eliminate likelihood of overrun. In addition, since in this embodiment the control processing is distributively executed by two controllers (processors), it is possible to realize the desired control by using inexpensive processors, e.g., 8-bit processors. Fig. 16 is a flowchart of control processing in a third embodiment of the present invention, showing processing executed by the second microprocessor 65. The arrangement of this embodiment is the same as that of the second embodiment, which is shown in Fig. 13. The processing executed by the first microprocessor 60 is the same as that shown in Fig. 14. In this embodiment, position detection for the balance shuttle unit 20 is carried out to prevent overrun positively. The second microprocessor 65 checks at ST81 whether or not notice of reversing has been output from the first microprocessor 60. If YES, the balance shuttle unit 20 is subjected to reversing control by the reversing driving signal from the first controller 6a. Therefore, the second microprocessor 65 turns off the driving signal for constant-speed control at ST82, and resets the position counter C to "0" at ST83. Then, the process returns to ST81. If it is judged at ST81 that no notice of reversing has yet been output from the first microprocessor 60, the second microprocessor 65 checks at ST84 whether or not notice of rightward constant-speed control has been output from the first microprocessor 60. If NO, the process proceeds to ST85. If it is judged at ST84 that notice of rightward constant-speed control has been output from the first microprocessor 60, the second microprocessor 65 commences drive in a direction reverse to the rightward direction, i.e., leftward drive. More specifically, the second microprocessor 65 checks at ST86 whether or not a timing signal (position signal) from the second position detecting sensor 27 has been detected. If NO, the process returns to ST81, whereas, if YES, the second microprocessor 65 updates the position counter C to C+1 at ST87 and compares the value of the counter C with a set travel distance n. If the value of the position counter C is not less than n, it is judged that the balance shuttle unit 20 has moved the set travel distance or more. Accordingly, the second microprocessor 65 stops the movement of the balance shuttle unit 20 at ST89 and returns to ST81. Conversely, if the value of the position counter C is less than n, it is judged that the balance shuttle unit 20 has not yet moved the set travel distance. Accordingly, the process shifts to leftward constant-speed control. That is, the second microprocessor 65 saves the measured value T of the second timer circuit 68 at ST90 and restarts the second timer circuit 68 at ST91 to commence measuring the interval of the position signal. At ST92, the second microprocessor 65 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore, a leftward driving signal is output at ST93 to accelerate the shuttle unit 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a rightward driving signal is output at ST94 to decelerate the shuttle unit 20. Then, the process returns to ST81. If it is judged at ST84 that no notice of rightward constant-speed control has yet been output from the first microprocessor 60, the second microprocessor 65 checks at ST85 whether or not notice of leftward constant-speed control has been output from the first microprocessor 60. If NO, the process returns to ST81. If it is judged at ST85 that notice of leftward constant-speed control has been output from the first microprocessor 60, the second microprocessor 65 commences drive in a direction reverse to the leftward direction, that is, rightward drive. More specifically, the second microprocessor 65 checks at ST95 whether or not a timing signal (position signal) from the second position detecting sensor 27 has been detected. If NO, the process returns to ST81, whereas, if YES, the second microprocessor 65 updates the position counter C to C+1 at ST96 and compares the value of the counter C with the set travel distance n at ST97. If the value of the position counter C is not less than n, it is judged that the balance shuttle unit 20 has moved the set travel distance or more. Accordingly, the second microprocessor 65 stops the movement of the balance shuttle unit 20 at ST98 and returns to ST81. Conversely, if the value of the position counter C is less than n, it is judged that the balance shuttle unit 20 has not yet moved the set travel distance. Accordingly, the process shifts to rightward constant-speed control. That is, the second microprocessor 65 saves the measured value T of the second timer circuit 68 at ST99 and restarts the timer circuit 68 at ST100 to commence measuring the interval of the position signal. At ST101, the second microprocessor 65 compares the measured interval value T of the saved position signal with the constant-speed reference interval value Tref. If T≧Tref, the speed is judged to be lower than the reference speed. Therefore, a rightward driving signal is output at ST102 to accelerate the shuttle unit 20. If T<Tref, the speed is judged to be higher than the reference speed. Therefore, a leftward driving signal is output at ST103 to decelerate the shuttle unit 20. Then, the process returns to ST81. By virtue of the above-described arrangement, even if the balance shuttle unit 20 is moved at an excessively high speed because the movement of the print shuttle unit 10 is retarded by interference with the drive, which may be caused, for example, when the user's hand touches the print shuttle unit 10 during the movement, the balance shuttle unit 20 can be prevented from overrunning and colliding with the frame or other stationary member. In addition to the foregoing embodiments, the present invention includes modifications such as those described below: Although in the foregoing embodiments the print head is a wire dot-matrix print head, the present invention may also be applied to other dot print heads, e.g., an ink jet print head. Although in the second embodiment two controllers are provided, if a high-speed processor is used, constant-speed control of the two shuttles can be effected with a single controller by time sharing control. Although in the foregoing description the driving devices are linear motors, other actuators, e.g., DC motors, can also be used. According to the present invention, since a driving device is also provided for the balance shuttle, it is unnecessary to use a large-output driving device for the print shuttle. Therefore, it is possible to reduce the overall size of the printer and to lower the production cost thereof. Further, since the two driving devices are electrically connected to each other and synchronously controlled, it is possible to prevent the two shuttles from operating asynchronously.

Claims: A shuttle printer comprising a print shuttle unit (10) for carrying a print head (11), a balance shuttle unit (20), means (17) for detecting the position of the print shuttle unit and means (15, 16, 25, 26) for driving the print shuttle unit to reciprocate and the balance shuttle unit (20) to counter-reciprocate so as to generate a force to counterbalance the momentum of the reciprocating print shuttle unit (10), characterised in that the driving means comprises an electric motor (15, 16) for driving the print shuttle unit (10), a further electric motor (25, 26) for driving the balance shuttle unit (20) and drive circuit means (7) for synchronously controlling said two electric motors (15, 16, 25, 26) in response to a signal from the print shuttle unit position detecting means (17), wherein the drive circuit means (7) comprises first and second drive circuits (7a; 7b) for driving respective ones of the electric motors (15, 25; 16, 26). A printer according to claim 1, wherein the print shuttle unit (10) has a plurality of print heads provided thereon in a horizontal row (12). A printer according to claim 1 or 2, wherein the two electric motors (15, 16; 25, 26) are linear motors. A printer according to claim 3, wherein the linear motors comprise electromagnetic coils (16, 26). A printer according to claim 4, wherein said drive circuit means (7) controls current passed through the electromagnetic coils (16, 26) of the two linear motors. A printer according to claim 4 or 5, wherein in each motor the electromagnetic coils (16, 26) are divided into two groups functioning as a constant-speed motor part and a reversing motor part respectively, said constant-speed motor parts of the two linear motors being opposite in polarity to each other. A printer according to any one of the preceding claims, wherein the print shuttle unit position detecting means (17) comprises a slit formed in the print shuttle unit (10) and a photosensor provided on a stationary member so as to face said slit. A printer according to any one of the preceding claims, wherein the drive circuit means (7) has means for synchronously controlling the two electric motors (15, 16; 25, 26) in each of reversing timing control and constant-speed control. A printer according to any one of claims 1 to 7, comprising means (27) for detecting the position of the balance shuttle unit (20), wherein the drive circuit means (7) has means for controlling reversing timing of the two electric motors (15, 16; 25, 26) synchronously with each other and for effecting constant-speed control of the two electric motors (15, 16; 25, 26) independently of each other. A printer according to claim 9, wherein the drive circuit means (7) has means for stopping drive of the electric motor (15, 16) for the balance shuttle unit when overrun of the balance shuttle unit (20) is detected on the basis of an output from the balance shuttle unit position detecting means (27). A printer according to any one of the preceding claims, wherein the polarities of the respective connections to the two electric motors from the drive circuit means (7) are opposite. A shuttle printer according to any one of the preceding claims, wherein the first and second electric motors (15, 16; 25, 26) are each of the same configuration, each including electromagnetic coils (16, 26) and permanent magnets (15, 25), the first and second drive circuits (7a; 7b) being connected to the electromagnetic coils (16; 26) of their respective electric motors.

Inventors: NOJIRI KEIKO, YOSHINO SATOSHI, NOJIRI, KEIKO, YOSHINO, SATOSHI, Yoshino, Satoshi, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1993
Main classification: B41J25
IPC codes: B41J25, B41J2