Piezoelectric print head control device using adjacent dot data
This application is a continuation of application Ser. No. 08/000,036, filed Jan. 4, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a print head control device for controlling the operation of the print head of a wire dot printer, provided with printing wires driven by piezoelectric elements, i.e., magnetostrictive elements. 
     Further increase of the printing speed of the wire dot printer has been desired and efforts have been made to develop a wire dot printer provided with a print head provided with printing wires driven by piezoelectric elements. 
     2. Description of the Prior Art 
     A wire dot printer of such a type is proposed in Japanese Unexamined Patent Publication (Kokai) No. 2-241754. The wire dot printer selects a printing wire to be driven for printing from print data received from an external device, determines a charging period for charging piezoelectric elements for driving the selected printing wire and a discharging period for discharging the same piezoelectric elements, and charges and discharges the piezoelectric elements for the charging period and the discharge period, respectively. When charged and discharged, the piezoelectric elements drive the corresponding printing wire by its piezoelectric effect to print a dot on a sheet. 
     In some cases, the printing wire prints a ghost dot due to a rebounding action caused by kinetic energy. The printing wire is liable to print ghost dots when the wire is driven for a plurality of consecutive printing cycles. Ghost dots have a negative effect on print quality. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a print head control device for a wire dot printer, capable of preventing ghost dots to print characters and the like with a satisfactorily high print quality. 
     A print head control device in accordance with the present invention comprises a print head driving circuit and a print head control circuit. The print head driving circuit charges and discharges the piezoelectric elements to drive the printing wire for printing, and the print head control circuit controls the operation of the print head driving circuit according to print data received from an external device. 
     In a first aspect of the present invention, a print head control circuit comprises: a single dot detecting means for deciding from print data if a dot to be printed in the next print cycle is a single dot; a charging/discharging period setting means for setting a charging period long enough to cancel the rebounding force of the printing wire and a discharging period sufficient for retracting the printing wire when the single dot detecting means decides that a dot to be printed in the next print cycle is a single dot; and a control means for controlling the operation of the charging/discharging period setting means. 
     In a second aspect of the present invention, a print head control circuit comprises: a last dot detecting means for detecting from print data if a dot to be printed in the next printing cycle is the last dot among a row of consecutive dots; an intermediate dot detecting means for detecting from the print data if a dot to be printed in the next printing cycle is a dot among the intermediate dots of a row of consecutive dots; a first charging/discharging period setting means for setting a charging period and a discharge period so that a force for advancing and a force for retracting the printing wire are reduced when the intermediate dot detecting means detects an intermediate dot; a second charging/discharging period setting means for setting a charging period and a discharge period so that the charging period is longer than that for printing an intermediate dot and the discharge period is sufficient for retracting the printing wire when the last dot detecting means decides that a dot to be printed in the next printing cycle is the last dot; and control means for charging and discharging piezoelectric elements according to the charging period and the discharging period determined by the first or second charging/discharging period setting means. 
     In a third aspect of the present invention, a print head control circuit comprises: a single dot detecting means for deciding from print data if a dot to be printed in the next printing cycle is a single dot; a head dot detecting means for deciding from the print data if a dot to be printed in the next printing cycle is the head dot among a row of consecutive dots; a last dot detecting means for deciding from the print data if a dot to be printed in the next printing cycle is the last dot among a row of consecutive dots; an intermediate dot detecting means for deciding from the print data if a dot to be printed in the next printing cycle is a dot among the intermediate dots of a row of consecutive dots; a first charging/discharging period setting means for setting a charging period long enough to cancel the rebounding force of the printing wire and a discharging period sufficient for retracting the printing wire; a second charging/discharging period setting means for setting a charging period long enough to drive the printing wire for advancement and a discharging time reducing the printing wire retracting force when the intermediate dot detecting means decides that a dot to be printed in the next printing cycle is a dot among the intermediate dots of a row of consecutive dots; a third charging/discharging period setting means for setting a charging period longer than that for printing an intermediate dot and a discharge period sufficient for retracting the printing wire when the last dot detecting means decides that a dot to be printed in the next printing cycle is the last dot among a row of consecutive dots; and control means for charging and discharging piezoelectric elements according to the charging period and the discharging period set by the first, second or third charging/discharging period setting means. 
     When printing the last dot of a row of consecutive dots, the piezoelectric elements are charged for a charging period longer than that for printing other dots so that the printing wire is driven for advancement after printing a dot on the sheet to prevent the rebounding of the printing wire by cancelling the rebounding force of the printing wire. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a graph for explaining a mode of charging and discharging piezoelectric elements; 
     FIGS. 2(A), 2(B), 2(C) and 2(D) are graphs for explaining the operation of a conventional print head control device; 
     FIG. 3 is a block diagram for explaining the basic configuration of a print head control device according to the present invention; 
     FIG. 4 is a block diagram of a print head control device in a preferred embodiment according to the present invention; 
     FIG. 5 is a side view of a piezoelectric printing unit; 
     FIG. 6 is a circuit diagram of a print head driving circuit; 
     FIG. 7 is a table showing the contents of a ROM; 
     FIG. 8 is a flow chart of a program to be executed by the print head control device of FIG. 4; 
     FIGS. 9(A), 9(B), 9(C) and 9(D) are graphs for explaining the operation of a print head control device of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Examples of the related art will be described with reference to the accompanying drawings prior to the description of the preferred embodiments according to the present invention. 
     Referring to FIG. 1 showing a mode of charging and discharging piezoelectric elements, the piezoelectric elements are charged for a charging period T1 and discharged for a discharging period T2. When charged, the piezoelectric elements extend to advance a printing wire toward a sheet held on a platen for printing. When discharged, the piezoelectric elements contract to retract the printing wire. 
     FIG. 2(A) shows a dot pattern consisting of one single dot and four consecutive dots. When printing the single dot, the charging period T1 is A0 and the discharging period T2 is B0 as shown in FIG. 2(C), which are sufficient for operating the printing wire by a sufficiently high driving force as shown in FIG. (D). In some cases, the printing wire assembly rebounds by its kinetic energy after printing a dot, to print a ghost dot, which is liable to occur after printing the last dot of a row of consecutive dots. 
     If a ghost dot is printed after the single dot or the last one of a row of consecutive dots as shown in FIG. 2(B) due to the rebounding of the printing wire assembly, the print quality is deteriorated. Techniques relating with the foregoing problems are described in U.S. Pat. No. 5,078,520. 
     The present invention has been made to print characters and the like at a satisfactorily high print quality by preventing ghost dots, and a print head control device in accordance with the present invention has a construction as shown in FIG. 3. 
     Referring to FIG. 3, a print head driving circuit 14 charges and discharges piezoelectric elements 10 to advance and retract a printing wire 12, and a print head control circuit 16 controls the operation of the print head driving circuit 14 for charging and discharging the piezoelectric elements 10 according to print data received from an external device. 
     The print head control circuit 16 comprises a single dot detecting unit 18 for deciding from the print data if a dot to be printed in the next printing cycle is a single dot, a head dot detecting unit 20 for deciding if a dot to be printed in the next printing cycle is the head dot of a row of consecutive dots, a last dot detecting unit 22 for deciding if a dot to be printed in the next printing cycle is the last dot among a row of consecutive dots, an intermediate dot detecting unit 24 for deciding if a dot to be printed in the next printing cycle is a dot among the intermediate dots of a row of consecutive dots, a first charging/discharging period setting unit 26 for setting a charging period effective for cancelling the rebounding force of a printing wire 12 and a discharge period sufficient for retracting the printing wire 12 when the single dot detecting unit 18 decides that a dot to be printed in the next printing cycle is a single dot, a second charging/discharging period setting unit 28 for setting a charging period sufficient for driving the printing wire 12 and a discharge period suitable for applying a reduced retracting force to the printing wire 12 when the head dot detecting unit 20 decides that a dot to be printed in the next printing cycle is the head dot among a row of consecutive dots, a third charging/discharging period setting unit 32 for setting a charging period and a discharging period so that the printing wire 12 is advanced by a reduced advancing force and retracted by a reduced retracting force when the intermediate dot detecting unit 24 decides that a dot to be printed in the next printing cycle is a dot among the intermediate dots of a row of consecutive dots, a fourth charging/discharging period setting unit 30 for setting a charging period longer than that for printing an intermediate dot and a discharging period sufficient for retracting the printing wire 12 when the last dot detecting unit 22 decides that a dot to be printed in the next printing cycle is the last dot of a row of consecutive dots, and a control unit 34 for controlling charging/discharging operation for charging and discharging the piezoelectric elements 10 according to the charging period and the discharging period set by the first, second, third or fourth charging/discharging period setting unit. 
     A print head control device for a wire dot printer, in a preferred embodiment according to the present invention will be described hereinafter with reference to FIGS. 4 to 9. 
     Referring to FIG. 4, a piezoelectric print head 40 is driven by a print head driving circuit 14 consisting of a pair of transistors as shown in FIG. 6, which in turn is controlled by a print head control circuit 16. 
     The print head control circuit 16 comprises a line buffer 42, a dot pattern detector 43, a dot pattern analyzing unit 44, a ROM 46, a printing wire selecting unit 47 and a driving period setting unit 48. 
     Print data provided by an external device is applied to the line buffer 42. The dot pattern detector 43 detects the contents of the line buffer 42, and the dot pattern analyzing unit 44 decides that a dot to be printed in the next printing cycle is a single dot, the head dot among a row of consecutive dots, a dot among the intermediate dots of a row of consecutive dots or the last dot among a row of consecutive dots from the output of the dot pattern detector 43, and sends a signal representing the decision to the driving period setting unit 48. 
     The driving period setting unit 48 reads a charging period A0, A1, A2 or A3, and discharging period B0, B1, B2 or B3 corresponding to the output of the dot pattern analyzing unit 44, and provides control signals corresponding to the data read from the ROM 46 for controlling charging and discharging operation to print a dot according to print data. FIG. 7 shows an example of data stored in the ROM 46. 
     The control signal provided by the driving period setting unit 48 is applied to the print head driving circuit 14, and then, the print head driving circuit 14 drives the print head 40 according to the control signal to print dots on a sheet held on a platen in a dot pattern represented by the print data. FIG. 6 shows an example of a wire driving unit of the print head driving circuit 14. Charging operation is executed when a pulse T1 is applied to the base of one of the transistors of the wire driving unit, and discharging operation is executed when a pulse T2 is applied to the base of the other transistor. 
     The printing wire selecting unit 47 selects a printing wire according to the output of the dot pattern detector 43. The output of the printing wire selecting unit 47 is applied to the print head driving circuit 14. 
     Referring to FIG. 5, the print head 40 comprises a piezoelectric actuator consisting of a stack of a plurality of piezoelectric elements 10, parallel springs 50, a lever 52, a wire 54 and a printing wire 12. The piezoelectric elements 10 are charged and discharged by the print head driving circuit 14 in a mode as shown in FIG. 1 so that the stack of the piezoelectric elements 10 extends and contracts. The extension and the contraction of the stack of the piezoelectric elements 10 are transmitted through the parallel springs 50 to the lever 52, and then, the lever 52 multiplies the extension and the contraction of the stack of the piezoelectric elements 10. The lever 52 advances the printing wire 12 toward the sheet held on the platen and retracts the printing wire 12 through the wire 54. 
     The operation of the print head control device will be described hereinafter with reference to FIG. 8. 
     In step 400, a query is made to see if any dot has been printed in the preceding printing cycle. If the response in step S400 is negative, a query is made in step 402 to see if any dot is to be printed after a dot to be printed in the next printing cycle. 
     If no dot is to be printed in a printing cycle after the next, it is decided that a single dot is to be printed in the next printing cycle, the periods A0 and B0 are read from the ROM 46, and the charging period T1 and the discharge period T2 are set in step 404. 
     If no dot is printed in the preceding printing cycle and a dot is to be printed in a printing cycle after the next, it is decided that the head dot of a row of consecutive dots is to be printed, the periods A1 and B1 are read from the ROM 46 and the charging period T1 and the discharge period T2 are set in step 406. 
     If a dot is printed in the preceding printing cycle, a query is made in step 408 to see if any dot is to be printed in a printing cycle after the next. If a dot is printed in the preceding printing cycle and a dot is to be printed in a printing cycle after the next, it is decided that a dot among the intermediate dots of a row of consecutive dots is to be printed, the periods A2 and B2 are read from the ROM 46, and the charging period T1 and the discharge period T2 are set in step 410. 
     If a dot is printed in the preceding printing cycle and no dot is to be printed in a printing cycle after the next, it is decided that the last dot of a row of consecutive dots is to be printed in the next printing cycle, the periods A3 and B3 are read from the ROM 46, and the charging period T1 and the discharging period T2 are set in step 412. 
     In this embodiment, A2≦A1≦A0, A2≦A3, and B2≦B1&lt;B0 ≦B3. The period A0 is sufficiently long to cancel the rebounding force of the printing wire 12, the period B0 is sufficient for the printing wire 12 to be fully retracted, the period A1 is determined so that the printing wire 12 is advanced by a sufficient driving force, the period B1 is determined so that the printing wire 12 is retracted by a reduced driving force, the periods A2 and B2 are determined so that the printing wire 12 is advanced by a reduced driving force and retraced by a reduced driving force, and the period A3 is longer than the period A2. 
     When print data representing a dot pattern shown in FIG. 9(A) is applied to the line buffer 42, the periods A0, A1, A2, A3, B0, B1, B2 and B3 are read from the ROM 46, the periods T1 and T2 are set accordingly in step 404,406,410 and 412, and the piezoelectric elements 10 are charged and discharged in a mode shown in FIG. 9(C). 
     In this embodiment, since the charging period T1 is set for an increased period sufficient for cancelling the rebounding force of the printing wire 12 when printing a single dot and when printing the last dot of a row of consecutive dots, the movement of the printing wire 12 toward the platen is suppressed as shown in FIG. 9(D) and, consequently, the printing of ghost dots as shown in FIG. 9(B) can be prevented to print dots with a high print quality. 
     As is apparent from the foregoing description, the present invention extends the charging period in which the piezoelectric elements are charged to advance the printing wire to cancel the rebounding force of the printing wire when printing a single dot and when printing the last dot of a row of consecutive dots, no ghost dot is formed after the single dot and after the last dot of a row of consecutive dots, so that characters and the like are printed at a high print quality. 
     Although the invention has been described in its preferred form with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.