JP2005161788A - Image forming method and apparatus using thermal head - Google Patents

Abstract

Density unevenness due to speed fluctuation is prevented in a thermal printer.
A transport amount detection unit and a line timer unit obtain a relative moving speed between a thermal head and a recording medium. The intra-tone energization amount control unit 60 controls the amount of heat generated by the heating element 19 in a direction to cancel out density fluctuations caused by the actual speed deviation from the reference speed based on the relative movement speed. The control includes a method of modulating the thermal head energization pulse P64 by a pulse width and a method of modulating a drive voltage and a drive current. Alternatively, a method of modulating the number of drive pulses corresponding to the number of gradations for driving the heat generating element 19 can be used.
[Selection] Figure 3

Description

  The present invention relates to an image forming method and apparatus for forming an image on a recording medium moving relative to the thermal head using a thermal head such as a thermal printer. More specifically, the present invention relates to a technology for controlling energization to a heat generating element constituting a thermal head.

  For example, a thermal printer that uses a thermal head to form an image on a recording medium that is moving relative to the thermal head, such as a thermal transfer printer such as a sublimation printer, is known (for example, Patent Documents). 1-3).

Japanese Patent Laid-Open No. 08-197818 Japanese Patent Laid-Open No. 08-197762 Japanese Patent No. 3419932

  The thermal printer uses a thermal head in which heating elements are arranged, selectively energizes the heating elements based on print data corresponding to image information, and is a recording medium (for example, printing paper) that is relatively moved on a platen roller. On the other hand, an image is formed on a recording medium by forming a dot pattern.

  In such a thermal printer, when the resistance value of the heating element, the environmental temperature, the supply voltage value to the heating element, the relative movement speed, and the like change, the print quality also changes. In order to obtain good print quality, the resistance value, environmental temperature, supply voltage value to the heating element, relative movement speed, etc. are detected, and the energization width for obtaining an appropriate dot pattern is determined according to the detection result. Output as timing signal.

  For example, in a configuration in which the thermal head is fixed and the recording medium is moved, the image quality is affected if the recording medium conveyance speed during printing fluctuates. For this reason, the structure which uses a pulse motor for a drive system is considered.

  FIG. 15 is a diagram illustrating a configuration example in the case of using a pulse motor. Based on the reference clock P90, the line drive and the head energization timing are forcibly synchronized. That is, a line pulse P91 is generated using a line pulse generator (Gen; Generator) 910 based on the reference clock P90, and a speed pulse for driving the pulse motor 902 using this line pulse P91 as a trigger for each line. The speed pulse generator 912 for generating P92 and the gradation signal generator 930 for driving the thermal head 908 are used.

  A line pulse P91 is generated based on the reference clock P90, and this is used as a trigger for each line. In the speed control system, the speed pulse P92 of the pulse motor 902 is generated, and the pulse motor 902 is driven via the pulse motor drive circuit 914. To do. In the printing system, the gradation signal P93 of the thermal head 908 is generated by a gradation signal generator (head control Gen) 930 and supplied to the heating element 909 of the thermal head 908 via the head driver 932.

  While the recording medium 9 is nipped and conveyed by a capstan roller 903 and a pinch roller 904 driven by a pulse motor 902, printing is performed on a printing portion sandwiched between a thermal head 908 and a platen roller 906. Since the rotation amount of the pulse motor 902 is controlled by the speed pulse P92, energization control of the heating element 909 can be easily performed at the aimed image position.

  However, the pulse motor has a loud driving sound due to torque fluctuation, and the motor itself is expensive. For this reason, there is a technique for reducing cost and reducing noise by driving with a DC (direct current) motor in consideration of cost and driving sound. For example, in terms of cost, it can be suppressed to about 200 yen by DC motor drive, compared to 350 yen for pulse drive. In addition, the drive sound eliminates vibration due to torque fluctuations seen in pulse drive, and is suitable for noise reduction.

  However, since the rotational speed of the DC motor varies depending on the load, for example, as shown in FIG. 16, when the image is actually printed, the image expands or contracts in accordance with the speed. As with the pulse motor, in order to synchronize with the energization timing of the thermal head of each line, it is necessary to use a DC motor drive equipped with a high-accuracy servo circuit to force the speed constant. For example, the printer has a severe load fluctuation, and it is difficult and difficult to design a servo that can accommodate the servo response within the line cycle (= several mSec order), and the cost increases. Therefore, if an attempt is made to improve the print quality, an expensive drive system is required.

  In addition, no matter how much the motor itself rotates at a constant speed, the conveyance speed depends on the gear distortion until the driving force is transmitted to the recording medium and the fluctuation of the friction coefficient between the head and the recording medium depending on the content of the image. Fluctuations occur and eventually the image expands or contracts.

  As a method for solving the image expansion / contraction and density unevenness, a technique for controlling the energization start timing and energization time of the thermal head in synchronization with the printing cycle has been proposed (for example, see Patent Document 4). This technology can be thought of as switching the line pulse trigger from time management to line time management.

Japanese Patent No. 3365132

  However, even with this technique, depending on the thermal head driving method, as shown in FIG. 17, the expansion and contraction of the image due to the conveyance speed can be eliminated, but it is dark when the conveyance speed is high and thin when it is slow. Further, it has been found that density unevenness occurs due to the speed fluctuation of the recording medium (change in moving speed relative to the thermal head).

  The inventor of the present application has examined this cause. When the energization start timing and energization time of the thermal head are controlled in synchronization with the printing cycle, the drive stop period of the thermal head changes. I found out that there was a difference in quantity.

  FIG. 18 is a diagram for explaining this point. As shown in the timing chart of FIG. 18A, since the energization amount Ton of the thermal head is made constant, the non-energization time Toff (OFF time) of the thermal head is expanded and contracted. Since the non-energization time of the thermal head corresponds to the cooling time of the thermal head, the expansion / contraction of the non-energization time affects the thermal head heat generation start temperature Tstart of the next line.

  For example, the heat generation chart of the thermal head in FIG. 18B shows the heat generation fluctuation of the thermal head due to the change in the conveyance speed. That is, when the conveyance speed is slow, the non-energization time of the thermal head becomes long, and the head cools down by a long amount. For example, Tloss shown in the figure is the heat change. Then, in the next line, the head temperature is decreased by the amount of cooling Tloss, and the current heat generation amount (the same heat generation amount as in the previous printing) rises, so the target heat generation line was cooled last time. The head temperature rises only to a temperature that is lower by the amount ΔTloss. Since printing is performed based on the head temperature, it can be seen that the result of printing on the recording medium becomes thin as a result. In other words, even if the energization amount (heat generation amount) of the thermal head is the same, the difference in energization start temperature becomes the temperature difference of the thermal head, and the print result is produced as a shading.

  The present invention has been made in view of the above circumstances, and can prevent density unevenness even when the energization start timing and energization time of the thermal head are controlled in synchronization with the printing cycle. The purpose is to provide.

  In the image forming method and apparatus according to the present invention, the relative movement speed between the thermal head and the recording medium is obtained, and the density fluctuation caused by the actual speed deviation from the reference speed is obtained based on the obtained relative movement speed. The amount of heat generated by the heat generating element of the thermal head was controlled in the direction to cancel out the light.

  In order to control the amount of heat generation, there are a method of modulating the drive pulse for driving the heat generating element, and a method of modulating the drive voltage and drive current of the drive pulse. Further, a method of modulating the number of drive pulses corresponding to the number of gradations for driving the heating element can be used.

  According to the present invention, based on the relative movement speed between the thermal head and the recording medium, the amount of heat generated by the heating element is controlled in a direction to cancel the density fluctuation caused by the actual speed deviation from the reference speed. It is possible to compensate for the temperature decrease caused by the non-energization period that changes according to the speed fluctuation. Thereby, it is possible to prevent density unevenness due to the non-energization period that changes according to the speed fluctuation.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Outline of printing mechanism>
FIG. 1 is a view showing an outline of a printing mechanism around a thermal head in a thermal printer which is an example of an image forming apparatus according to the present invention.

  Here, as the thermal printer 1, a sublimation type thermal transfer printing method is used. In the sublimation type thermal transfer printing method, an ink ribbon 9a that sublimates by heating and a photographic paper 9b to which the sublimated dye ink adheres are used as the recording medium 9 (see also FIG. 3 described later).

  As shown in FIG. 1A, the printing mechanism unit 10 has a configuration in which the thermal head 18 and the recording medium 9 are moved relative to each other. The thermal head 18 is fixed, and is rotated by a drive circuit (not shown). The recording medium 9 is inserted between the stun roller 12 and the pinch roller 14 disposed so as to be able to sandwich the recording medium 9 so as to be opposed to the capstan roller 12, so that the recording medium 9 is transported. Yes.

  Further, the recording medium 9 is transported and inserted between the platen roller 16 disposed so as to be opposed to the thermal head 18 so that the recording medium 9 can be sandwiched, so that the recording medium 9 and the thermal head 18 are relatively moved. An image is formed by pressing and heating the recording medium 9 against the heating element 19 constituting the thermal head 18 to transfer the dye ink adhering to the ink ribbon 9a of the recording medium 9 onto the photographic paper 9b.

  As shown in FIG. 1B, the thermal head 18 has a plurality (a large number) of heating elements 19 arranged in a line in accordance with the pixel pitch. In the thermal head 18, each heating element 19 is provided for each pixel position in the line direction according to the width of a thermal head energization pulse P 64 output from a heating value control unit (not shown) (see FIG. 3 described later). One line is heated at the same time, and one line can be printed simultaneously. If the width of the thermal head energization pulse P64 is narrow, the amount of heat generated by the heat generating element 19 decreases accordingly, and if the pulse width is wide, the amount of heat generated by the heat generating element 19 increases correspondingly. That is, since the heating elements 19 have a structure in which the amount of energization can be individually controlled, it is possible to express shading (density modulation) for each heating element 19 in the line.

<Relationship between element drive and density modulation>
FIG. 2 is a diagram for explaining the relationship between heat generation driving of the heat generating element 19 and density modulation in the recording medium 9. The energization amount for each heating element 19 is obtained from a density relationship with predetermined image data. For example, when the image data of a certain heating element 19 is “127”, as shown in FIG. 2A, the energization amount of the heating element 19 reaches 300 gradations by the image data → gradation curve L10. Energized. Then, a printing result having a density of “1.0” is obtained by the emission characteristic curve L12 of the recording medium.

  Here, in the present embodiment, as shown in FIG. 2B, the number of gradation pulses P62 having a predetermined duty ratio (= Ton / (Ton + Toff)) corresponding to the number of gradations in one line. A pulse number driving method to be supplied to the heating element 19 is adopted. Then, in synchronization with the gradation pulse P62, the duty ratio of the thermal head energization pulse P64 that is set to a predetermined de-duty ratio at the reference speed VO is controlled according to the speed fluctuation, that is, pulse width modulation (Pulse Width Modulation). ) And supplied to the heating element 19 to prevent density fluctuations due to speed fluctuations. At this time, the control is simplified by setting all the thermal head energization pulses P64 of a certain gradation to a common duty ratio. This will be specifically described below. The gradation pulse P62 is generated based on the image data by a gradation pulse generation unit (a circuit configuration example is not shown).

<Overview of thermal printer>
FIG. 3 is a block diagram showing an outline of a control system in the thermal printer 1 which is an example of the image forming apparatus according to the present invention. FIG. 4 is a timing chart showing timings of various signals in the control system shown in FIG.

  The control system rotationally drives the motor of the printing mechanism unit 10 or drives the heat generating element 19 of the thermal head 18 according to the image pattern to generate heat according to the image pattern, and the mechanical drive control unit 30 that relatively moves the recording medium 9 and the thermal head 18. It is comprised from the calorific value control part 50 which performs.

  The mechanical drive control unit 30 drives the printing mechanism unit 10 according to a control signal supplied from a system control unit (not shown) that controls the overall operation of the thermal printer 1, and includes a line pulse generation unit 32, a DC motor servo circuit 34, And a DC motor drive circuit 36.

  The drive output signal P36 of the DC motor drive circuit 36 is supplied to the DC motor 40 that constitutes the printing mechanism unit 10. The DC motor 40 that is driven by the DC motor drive circuit 36 transmits the rotational force to the capstan roller 12 via the gear group 42. As a result, the recording medium 9 sandwiched between the pinch roller 14 moves in a predetermined direction.

  The heat generation amount control unit 50 includes a line timer unit 52, an intra-gradation energization amount setting unit 54, a latch circuit 56, an intra-gradation energization amount control unit 60, and a head driver 70. The line pulse generation unit 32 of the mechanical drive control unit 30 is also used as a component of the heat generation amount control unit 50. The details of the operation of each functional unit constituting the heat generation amount control unit 50 will be described later.

  Here, in the printing mechanism unit 10 of the present embodiment, the platen roller 16 is provided with a conveyance amount detection unit 20 that detects the conveyance amount of the recording medium 9 by monitoring the rotation thereof. The carry amount corresponds to the relative movement speed between the thermal head 18 and the recording medium 9, and the carry amount detection unit 20 functions as a relative movement speed detection unit.

  FIG. 5 is a diagram illustrating a configuration example of the transport amount detection unit 20 that functions as a speed detection unit according to the present invention. FIG. 6 is a diagram showing the relationship between movement of the recording medium 9 and lines.

  In the present embodiment, an FG method is used in which the platen roller 16 and the FG sensor detect the movement amount of one line of the recording medium 9. Specifically, as shown in FIG. 5, the disk-shaped FG plate 22 is fixed to the platen roller 16 that rotates one-on-one with the conveyance of the recording medium 9. The FG plate 22 is a rotating plate that is painted on a black (or white or other color) line (hereinafter referred to as a printing line) for each rotation angle corresponding to the movement of one line of the recording medium 9 on a transparent plate. is there. By forming a printing line for each rotation angle corresponding to the movement of one line, the relative movement speed detection accuracy corresponds to the conveyance amount of one line, and the accuracy is high.

  Note that the print lines may be formed at a rotation angle corresponding to several lines of movement instead of a rotation angle corresponding to one line of movement. The ratio can be changed simply by changing the printing. In any case, a line of a predetermined color is applied to the rotating plate, and an FG pulse P20 having a substantially constant ratio can be obtained by a ratio between a portion where the printed line is formed and a portion where the printed line is not printed. Just keep it.

  For example, since the density irregularity period (interval) that can be confirmed with the naked eye is about 0.2 mm (subjective), detection for every two lines may be performed if the period (interval; line interval) of one line is 0.1 mm. Further, detection may be performed every four lines as long as the cycle (interval; line interval) of one line is 0.05 mm. In this way, the cost and size of the sensor can be reduced.

  By detecting this printed line with a transmissive optical sensor 24 such as a photo interrupter, as shown in FIG. 6, the rising and falling are alternately performed for each movement of one (or several) lines of the recording medium 9. Thus, an FG pulse P20 is obtained as a synchronization signal repeated repeatedly. The FG pulse P20 is used as a trigger for one line, and has no meaning in the Thigh period and the Tlow period.

  In this embodiment, the print line is formed on the transparent disk and the print line is detected by the transmission type optical sensor 24. However, the print line is provided on the opaque disk, and the reflection type optical sensor 24 is used. The type sensor 24 may be configured to detect the printed line. The reflective type has a relatively small installation area for the optical sensor 24 than the transmissive type, so that space efficiency is better.

  Also, instead of forming a printed line on a disk, a slit window is formed on an opaque rotating plate, and the slit window is detected by a transmissive optical sensor 24 that detects a change in the amount of transmitted light. It is good. The FG pulse P20 having a substantially constant ratio can be obtained by the ratio of the slit window portion that transmits light formed on the rotating plate and the opaque portion as a non-window portion that blocks light.

  In addition, although the mechanism which provides such a mechanism in the DC motor 40, the capstan roller 12, or the pinch roller 14 to obtain the FG pulse P20 as a synchronization signal is also conceivable, in this case, due to the influence of gear unevenness or conveyance slip, A result different from the relative movement speed in the portion of the thermal head 18 where the printing process is actually performed is detected.

  On the other hand, in the present embodiment, the rotation of the platen roller 16 is detected to monitor the relative movement speed in the portion of the thermal head 18 where the printing process is actually performed. There is an advantage that the relative movement speed can be detected with high accuracy without being affected by the generated speed fluctuation on the upstream side.

  A detection error caused by the sliding of the platen roller 16 cannot be corrected. When it is necessary to detect the relative moving speed with higher accuracy, a mechanism for detecting the moving state of the recording medium 9 itself using a Doppler speedometer or the like may be used in the vicinity of the thermal head 18. Similarly to the case of the printed line, the rising edge of the pulse or the falling edge may be used as a trigger for one line.

  Returning to FIG. 3, the description will be continued. The FG pulse P20 obtained by the carry amount detection unit 20 is input to the line pulse generation unit 32. The line pulse generator 32 generates a line pulse P32 based on the FG pulse P20 (see FIG. 6).

  The line pulse P32 obtained by the line pulse generation unit 32 is input to the DC motor servo circuit 34 and also to the line timer unit 52 of the heat generation amount control unit 50.

  The mechanical drive control unit 30 uses the line pulse P32 obtained by the line pulse generation unit 32 as a trigger for driving the DC motor 40 of the printing mechanism unit 10, and uses the rotational speed of the DC motor 40 (the recording medium 9). Corresponding to the transport speed) and drive timing. The drive control of the DC motor 40 in the mechanical drive control unit 30 is the same as a well-known technique in a general thermal printer, and a detailed description thereof is omitted.

  In the present embodiment, the control accuracy of the rotational speed (the conveyance speed of the recording medium 9) by the mechanical drive control unit 30 is within the correction range of the speed fluctuation correction function by the heat generation amount control unit 50 described later. What is necessary is an inexpensive design. For example, when the correction capability by the heat generation amount control unit 50 is large, a drive control system for the DC motor 40 by the mechanical drive control unit 30 may be removed and the DC motor 40 may be driven at a constant current. The motor is not limited to a DC motor, and a stepping motor or other motors can be used.

<Head heat generation control according to transport speed>
Next, details of the operation of the heat generation amount control unit 50 will be described. The present embodiment is characterized in that energization data corresponding to the line cycle is latched, and the conveyance speed fluctuation is reflected in the energization amount of all thermal head energization pulses P64 in the line.

  First, the heat generation amount control unit 50 uses the line pulse P32 obtained by the line pulse generation unit 32 as a reference for the energization timing of the heat generating element 19 constituting the thermal head 18.

  FIG. 7 is a diagram showing the relationship between the line pulse P32 and the energization timing. In the present embodiment, the energization start timing and energization time of the thermal head are controlled in synchronization with the printing cycle. Specifically, as shown in FIG. 7, a variable period for expressing gradation density in one line is defined as gradation control period Tcnt1. A period excluding the gradation control period Tcnt1 in one line is defined as a thermal head drive stop period Tstp1.

  By providing the drive stop period Tstp1, when the relative movement speed between the thermal head 18 and the recording medium 9 is a reference value, the drive stop period Tstp1 is secured as a cooling time.

  However, the gradation control period Tcnt1 is a part representing gradation, and it is necessary to ensure a predetermined period regardless of variations in the relative moving speed between the thermal head 18 and the recording medium 9. For this reason, when the relative movement speed deviates from the reference value (that is, the transport speed varies), the drive stop period Tstp1 also changes. In particular, the line cycle is shortened at a high speed, and gradation control is performed by this drive stop period Tstp1. Prevents chipping of period Tcnt1.

  For this reason, in the present embodiment, the gradation corresponding to the image pattern is controlled by controlling the energization time within each gradation described below to the heating element 19 of the thermal head energization pulse P64 within the gradation control period Tcnt1. Control to reveal the concentration. That is, the cycle (time width) of one thermal head energization pulse P64 corresponding to one gradation is fixed, and the drive stop period Tstp2 is also provided in each thermal head energization pulse P64. In this way, the cooling time can be secured without being affected by the relative movement speed between the thermal head 18 and the recording medium 9.

  Returning to FIG. 3, the description will be continued. The line timer unit 52 obtains the line time of one line as a count value by counting the line pulse with the reference clock P50 in the set. The count value is output as the line time signal P52. Since the line time signal P52 represents a line cycle, the change in the count value of the line time signal P52 becomes the change in the relative moving speed between the thermal head 18 and the recording medium 9. Therefore, by monitoring the line time signal P52, it is possible to monitor the fluctuation of the relative moving speed, that is, the conveyance speed of the recording medium 9.

  The line time signal P52 obtained by the line timer unit 52 is input to the in-tone energization amount setting unit 54. The intra-gradation energization amount setting unit 54 has a lookup table (LUT) 55 corresponding to the gradation energization curve L14 of FIG. This look-up table 55 associates the relative movement speed with the energization amount in the gradation of the next line.

  The gradation energization curve L14 of FIG. 8 that has previously obtained the relationship between the energization amount of each gradation with respect to the relative movement speed (line speed) is written in the lookup table 55 of the intra-gradation energization amount setting unit 54. The energization amount of each gradation with respect to the line time signal P52, that is, the duty ratio of the gradation pulse P62 can be obtained. By associating a duty ratio that changes the amount of heat generated according to the speed fluctuation, the head temperature (specifically, the heating element in detail) is caused by the speed fluctuation (ΔV1 on the high speed side and ΔV2 on the low speed side centered on the reference speed V0). 19) is corrected (details will be described later). By using the look-up table 55, it is not necessary to perform an operation or the like, and correction information can be easily obtained.

  The in-tone energization amount setting unit 54 abuts the line time signal P52 indicating the relative movement speed (line cycle) detected using the conveyance amount detection unit 20 against the lookup table 55 and indicates the corresponding energization amount. The amount signal P54 is obtained, and the obtained energization amount signal P54 is input to the latch circuit 56.

  The latch circuit 56 causes the energization amount signal P54 obtained using the lookup table 55 to be reflected on one line to be printed by latching with the line pulse P32 so as to be reflected in the drive of the heating element 19 of the next line. . The latch circuit 56 inputs the delayed energization amount signal P56 to the in-tone energization amount control unit 60.

  As shown in FIG. 3B, the intra-gradation energization amount control unit 60 has a down counter 62 inside. The down counter 62 is configured such that the reference clock P60 is input to the count terminal and the number of gradation pulses P62 corresponding to the gradation is input to the trigger terminal.

  The in-tone energization amount control unit 60 is based on the relative movement speed detected using the conveyance amount detection unit 20 and cancels out the density fluctuation caused by the actual speed deviation from the reference speed. In order to control the energization amount, the individual thermal head energization pulses P64 are subjected to pulse width modulation. Specifically, first, with the gradation pulse P62 as a trigger, the energization amount signal P56 output from the latch circuit 56 is loaded into the down counter 62, the thermal head energization pulse P64 is turned on, and the count down is performed with the reference clock P60. However, the heating element 19 of the thermal head 18 is energized until “0”, and the energized state after “0”.

  By causing the down counter 62 to perform the same operation again by the trigger of the next gradation pulse P62, the energization amount in the gradation is made the same for all the gradation pulses P62 for representing a certain gradation. By controlling, overall control is simplified.

  The thermal head energization pulse P64 obtained in this way is supplied to the heat generating element 19 through the head driver 70 as a drive pulse of the heat generating element 19 which is pulse width modulated in accordance with the speed fluctuation. The pulse width of the modulated thermal head energization pulse P64 is proportional to the amount of applied energy supplied to the heating element 19 by the power source 72. That is, when the width of the modulated thermal head energization pulse P64 is narrow, the supply time (energization time) of energy applied from the power source 72 to the heat generating element 19 is shortened accordingly, and when the pulse width is wide, the corresponding amount. The energy supply time becomes longer. Since the pulse width of the thermal head energization pulse P64 is controlled (pulse width modulation) according to the speed fluctuation, as a result, the density unevenness caused by the fluctuation of the conveyance speed can be corrected.

  For example, as shown in FIG. 3B, if the energization amount signal P56 output from the latch circuit 56 is “150”, it is provided inside the intra-gradation energization amount control unit 60 using the gradation pulse P62 as a trigger. “150” is loaded into the down counter 62. By the reference clock P60, a thermal head energization pulse P64 is generated in which the energization period is from the countdown until the counter value becomes “0”. This is repeated for each gradation pulse P62.

  FIG. 9 is a diagram for explaining the basic effect of the pulse width modulation of the thermal head energization pulse P64 in accordance with the speed fluctuation. When controlling the energization start timing and energization time of the thermal head in synchronization with the printing cycle, first, as shown in FIG. 9A, the thermal head energization pulse P64 to the heating element 19 in one line is firstly shown. Control is performed so that the gradation density according to the image pattern is expressed according to the number of applications. Here, a variable period for expressing the gradation density is defined as a gradation control period. The time width of one thermal head energization pulse P64 corresponding to one gradation is fixed.

  In this case, when the conveyance speed fluctuates, the line cycle changes and the gradation control period is constant. As a result, as indicated by an arrow Y in FIG. The thermal head drive stop period Tstp1 excluding the gradation control period fluctuates, which causes a difference in the cooling amount of the thermal head, which causes density unevenness.

  On the other hand, in the present embodiment, the fluctuation amount of the cooling amount of the thermal head corresponding to the speed fluctuation is supplemented by the heat generation amount using pulse width modulation. That is, density unevenness is prevented by compensating for the amount of heat generated by increasing or decreasing the amount of heat generated by the thermal head that has been overcooled due to fluctuations in the conveyance speed.

  That is, as shown in FIG. 9B, all the thermal head energization pulses P64 in the line have a non-energization period, and the non-energization period is controlled (increased / decreased) by pulse width modulation, thereby generating a heat value. Can be realized. For example, in accordance with the increase / decrease of the non-energization period as shown in FIGS. 9C1 to 9C3, the calorific value can be changed as in the calorific value change characteristic shown in FIG. 9D. I understand. Thereby, the cooling amount of the thermal head due to the drive stop period of the thermal head 18 (specifically, the heating element 19) can be corrected.

  In addition, a drive stop period Tstp2 is always provided within one cycle of the thermal head energization pulse P64, and a controllable non-energization period Tstp3 is provided within a range excluding this drive stop period Tstp2, thereby ensuring the necessary cooling time. Therefore, the thermal head 18 (specifically, the heat generating element 19) can be prevented from being destroyed.

  That is, by changing the non-energization period corresponding to the change in the relative movement speed and controlling the energization period Ton corresponding to the actual printing cycle (line cycle), it is possible to prevent a decrease in print density. .

  FIG. 10 is a diagram for explaining the overall image of the effect of pulse width modulation of the thermal head energization pulse P64 in accordance with the speed fluctuation. For example, looking at the M-th line in FIG. 10A, as shown in FIG. 10B, the energization amount of each gradation of the thermal head is shown in FIG. The heat generation point can reach the target heat generation line by the energization pulse of type A shown in FIG. In the next M + 1th line, the thermal head is cooled by an excess of ΔTloss as the conveyance speed of the Mth line becomes slower.

  Therefore, by using the energization pulse of type B shown in FIG. 10C2 in which the non-energization time between each gradation is short, the amount of heat generated by the heating element 19 as shown in the heat generation curve in the period B in FIG. Therefore, the amount of heat generated by the thermal head (specifically, the heating element 19) can be increased by ΔTloss, and the target amount of heat generated can be obtained by covering ΔTloss.

  In FIG. 10A, looking at the N-th line, as shown in FIG. 10B, the energization amount of each gradation of the thermal head is shown in FIG. The heat generation point can reach the target heat generation line by the energization pulse of type A shown in FIG. In the next N + 1 line, the thermal head is heated by an excess of ΔTup because the conveyance speed of the Nth line is increased.

  Therefore, by using a type C energization pulse shown in FIG. 10 (C3) with a long non-energization time between each gradation, the amount of heat generated by the heat generating element 19 is increased as shown in the heat generation curve of the C period in FIG. Therefore, the amount of heat generated by the thermal head (specifically, the heat generating element 19) can be reduced by ΔTup, and the target amount of heat generated can be obtained by covering ΔTup.

  In this way, by using the control method of the present embodiment in which the pulse width of the thermal head energization pulse P64 that expresses the gradation is controlled in accordance with the speed fluctuation, the heat generation amount of the thermal head is changed, thereby the transport speed. It was possible to compensate for the fluctuation of the cooling time of the thermal head according to the condition.

  In other words, even if there is unevenness in the conveyance speed, it is possible to prevent the influence from appearing as density unevenness. This leads to the design of an inexpensive mechanical drive control unit 30. For example, even if an inexpensive DC motor is used, it is not necessary to make the conveyance speed constant, so that it is possible to design an inexpensive and rough servo for speed control due to load fluctuation.

  Further, even if there is gear unevenness or conveyance slip, the influence can be prevented from appearing as density unevenness. Gear eccentricity, gear tooth unevenness and conveyance slip are factors of fluctuations in the conveyance speed. Conventionally, gear unevenness or the like caused density unevenness as it is, but the control method of this embodiment should be adopted. Can take measures. In addition, the accuracy of the gear that affects the speed fluctuation can be relaxed, and an inexpensive gear configuration is also possible.

  Further, even if there is a frictional fluctuation of the recording medium 9, it is possible to prevent the influence from appearing as density unevenness. For example, the heat generation amount (energization amount) of the thermal head is changed according to the image content, but the friction coefficient between the head and the recording medium also changes depending on the heat generation amount. Since this is also a variation factor of the conveyance speed, density unevenness has occurred in the past, but it is possible to take measures by adopting the control method of this embodiment.

<Modification>
FIG. 11 is a block diagram illustrating a first modification of the in-tone energization amount setting unit 54. In the above embodiment, the in-tone energization amount setting unit 54 directly abuts the line time signal P52 indicating the relative movement speed (line cycle) detected using the conveyance amount detection unit 20 against the lookup table 55, The energization amount signal P54 indicating the corresponding energization amount was obtained. Also, all processing is performed digitally.

  On the other hand, in the first modification, the relationship between the energization amount of each gradation with respect to the shift amount of the speed with respect to the reference speed V0 is obtained and written in the lookup table 85. Then, a deviation amount of the speed with respect to the reference speed V0 is obtained, and the deviation amount is abutted against the lookup table 85, thereby obtaining an energization amount signal P54 indicating the corresponding energization amount.

  Therefore, the in-tone energization amount setting unit 54 includes a reference cycle value setting unit 80 that outputs a count value P80 corresponding to the reference speed V0, a subtraction processing unit 82, and a lookup table 85.

  The subtraction processing unit 82 takes the difference between the line time signal P52 input from the line timer unit 52 and the count value P80 corresponding to the reference speed V0 input from the reference cycle value setting unit 80, and looks up the subtraction result By abutting against the table 85, an energization amount signal P54 corresponding to the difference P82 between the count value P80 corresponding to the reference speed V0 and the actual relative movement speed is obtained, and the obtained energization amount signal P54 is input to the latch circuit 56. .

  Similar to the above embodiment, by using the lookup table 85, correction information can be easily obtained. In addition, the lookup table 85 can be made small by adopting a mechanism for deriving the difference with respect to the reference time instead of the line period itself. As a result, the in-tone energization amount setting unit 54 can be reduced in size and the efficiency of the memory effective area can be improved.

  FIG. 12 is a block diagram illustrating a second modification of the in-tone energization amount setting unit 54. This second modification is characterized in that a sawtooth wave that is reset in an analog manner by the line pulse P32 is generated, and the voltage immediately before the reset is read as digital data.

  For this reason, the in-tone energization amount setting unit 54 of the second modification has a sawtooth wave generation unit 90, an A / D conversion unit 92, and a lookup table 95. The look-up table 95 may have the same characteristics as the look-up table 55 described in the above embodiment.

  The sawtooth wave generation unit 90 generates a sawtooth wave (sawtooth wave) signal P90 by analog resetting with the line pulse P32 and inputs it to the A / D conversion unit 92. The A / D conversion unit 92 digitally converts the input sawtooth wave and passes the A / D conversion output to the lookup table 95 in synchronization with the line pulse P32. By doing so, the in-tone energization amount setting unit 54 can read the digital value P92 corresponding to the sawtooth voltage immediately before the sawtooth wave generation unit 90 is reset by the line pulse P32.

  The intra-tone energization amount setting unit 54 can obtain the energization amount signal P54 corresponding to the relative movement speed by abutting the digital value P92 against the lookup table 95, and inputs this to the latch circuit 56.

  FIG. 13 is a block diagram illustrating a first modification of the in-tone energization amount control unit 60. In the above embodiment, the energization amount signal P54 corresponding to the line period is obtained by the in-gradation energization amount setting unit 54, latched by the latch circuit 56, and the in-gradation energization amount control unit 60 performs the line operation. It was reflected in the energization amount of all gradations. On the other hand, in the first modification, the voltage or current applied to the heating element 19 is modulated according to the energization amount signal P54.

  That is, the heat generation amount of the heat generating element 19 is determined by the applied energy. The applied energy is determined by electric power × application time. In the above embodiment, the on-time of the thermal head energization pulse P64 for one gradation is controlled while keeping the power constant, that is, the application time is controlled by modulating the pulse width of the thermal head energization pulse P64, and the heat generation amount. Control was realized.

  On the other hand, as shown in FIG. 13A, the in-tone energization amount control unit 60 of the first modification is in a direction to cancel the density fluctuation caused by the actual speed deviation from the reference speed. In order to control the energization amount of the heat generating element 19, the power source 72 supplied to the head driver 70 is replaced with the variable power source 64, and the power source voltage Vdrv supplied to the head driver 70 is controlled by the energization amount signal P56. The amount of generated heat is controlled by modulating the drive voltage of the thermal head energization pulse P64.

  Alternatively, as shown in FIG. 13B, the in-tone energization amount control unit 60 according to the first modified example arranges an output current source 68 at the subsequent stage of the head driver 70, so that the head driver 70 supplies the heating element 19. The amount of heat generated is controlled by modulating the magnitude of the drive current supplied to the current by the energization amount signal P56.

  As in the first modification, even when the drive voltage or drive current of the thermal head energization pulse P64 supplied to the heating element 19 is controlled, the correction amount can be reflected in the energization amount in the gradation.

  FIG. 14 is a block diagram illustrating a modification of the heat generation amount control unit. This modification can also be realized by modulating the image data (number of gradations) with the energization amount signal P56 by the gradation pulse generation unit 69, instead of reflecting the energization amount in the gradation. Further, it can be realized only by an analog circuit by combining with the other modified examples described above. As a result, the signal processing circuit can be simplified.

It is the figure which showed the outline | summary of the printing mechanism part around a thermal head. It is a figure explaining the relationship between the heat_generation | fever drive of a heat generating element, and the density modulation in a recording medium. It is the block diagram which showed the outline | summary of the control system in a thermal printer. 4 is a timing chart showing timings of various signals in the control system shown in FIG. 3. It is the figure which showed one structural example of the conveyance amount detection part which functions as a speed detection part which concerns on this invention. FIG. 6 is a diagram illustrating a relationship between movement of a recording medium and a line. It is the figure which showed the relationship between a line pulse and energization timing. It is a figure explaining an example of the gradation electricity supply curve for correct | amending the emitted-heat amount. It is a figure explaining the basics of the effect of carrying out pulse width modulation of a thermal head energization pulse according to speed change. It is a figure explaining the whole image of the effect of carrying out pulse width modulation of a thermal head energization pulse according to speed change. It is a block diagram explaining the 1st modification of the energization amount setting part in a gradation. It is a block diagram explaining the 2nd modification of the energization amount setting part in a gradation. It is a block diagram explaining the 1st modification of the energization amount control part in a gradation. It is a block diagram explaining the modification of a emitted-heat amount control part. It is a figure explaining the example of 1 structure in the case of using a pulse motor. It is a figure explaining the problem of a prior art (the 1). It is a figure explaining the problem of a prior art (the 2). It is a figure explaining the problem examination of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Thermal printer, 9 ... Recording medium, 10 ... Printing mechanism part, 18 ... Thermal head, 19 ... Heating element, 20 ... Conveyance amount detection part, 22 ... FG board, 24 ... Optical sensor, 30 ... Mechanical drive control part 32 ... Line pulse generator, 34 ... DC motor servo circuit, 36 ... DC motor drive circuit, 40 ... DC motor, 42 ... Gear group, 50 ... Heat generation control unit, 52 ... Line timer unit, 54 ... In gradation Energization amount setting unit, 55 ... Look-up table, 56 ... Latch circuit, 60 ... In-gradation energization amount control unit, 62 ... Down counter, 69 ... Gradation pulse generation unit, 70 ... Head driver, 80 ... Reference cycle value setting , 82 ... subtraction processing unit, 90 ... sawtooth wave generation unit

Claims (11)

An image forming method for forming an image on a recording medium moving relative to the thermal head using a thermal head in which heating elements are arranged,
The relative movement speed between the thermal head and the recording medium is obtained, and based on the obtained relative movement speed, the thermal head is arranged in a direction to cancel the density fluctuation caused by the actual speed deviation from the reference speed. An image forming method comprising controlling the amount of heat generated by the heat generating element. The image forming method according to claim 1, wherein the heat generation amount is controlled by performing pulse width modulation on a driving pulse for driving the heat generating element. The image forming method according to claim 1, wherein the heat generation amount is controlled by modulating a driving voltage of a driving pulse for driving the heat generating element. The image forming method according to claim 1, wherein the heat generation amount is controlled by modulating a drive current of a drive pulse for driving the heat generating element. The image forming method according to claim 1, wherein the heat generation amount is controlled by modulating the number of drive pulses corresponding to the number of gradations for driving the heat generating element. An image forming apparatus that forms an image on a recording medium that is moving relative to the thermal head using a thermal head in which heating elements are arranged.
A speed detector for obtaining a relative moving speed between the thermal head and the recording medium;
Based on the relative movement speed obtained by the speed detector, the amount of heat generated by the heating element of the thermal head is controlled in a direction to cancel out density fluctuation caused by the actual speed deviation from the reference speed. An image forming apparatus, comprising: The image forming apparatus according to claim 6, wherein the heat generation amount control unit controls the heat generation amount by performing pulse width modulation on a drive pulse for driving the heat generating element. The image forming apparatus according to claim 6, wherein the heat generation amount control unit controls the heat generation amount by modulating a drive voltage of a drive pulse for driving the heat generating element. The image forming apparatus according to claim 6, wherein the heat generation amount control unit controls the heat generation amount by modulating a drive current of a drive pulse for driving the heat generating element. The image forming apparatus according to claim 6, wherein the heat generation amount control unit controls the heat generation amount by modulating the number of drive pulses corresponding to the number of gradations for driving the heat generating element. The image forming apparatus according to claim 6, wherein the heat generation amount control unit drives the heat generating element with a number of drive pulses corresponding to a gradation of an image.