Patent Publication Number: US-6217239-B1

Title: Temperature control apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a temperature control apparatus for controlling a heating temperature of a thermal head utilized in a recording apparatus, such as a high-resolution printer. 
     2. Description of the Related Art 
     An ink is known that includes fine capsules, such as micro-capsules, filled with heat-sensitive color developing dye or ink for high-resolution printing in a high resolution color printer. A recording sheet consists of a base sheet with a layer of the micro-capsules covering the base sheet. The layer of micro-capsules includes a plurality of types of micro-capsules, each type corresponding to a specific color, which seeps from the micro-capsule onto the recording sheet when the corresponding micro-capsule is heated to a predetermined temperature. The predetermined temperature varies dependent on the type of micro-capsule. Each seeped color is developed and fixed by light of a predetermined wavelength, which also varies dependent on the type of micro-capsule. Therefore, each type of micro-capsule seeps a predetermined color when heated to the predetermined temperature, and the seeped color is developed and fixed on the base sheet of the recording sheet by irradiation with the light of the specific wavelength. Thus, ink or dye of a full-color image, to be recorded on a recording sheet, can be controlled through selective breakage of the micro-capsules as seepage of the dye or ink, which occurs through control of a localized heating and irradiation with a specific wavelength of light. 
     The recording process utilizing the recording sheet with the layer of the micro-capsules is complicated and time-consuming as the localized heating and light irradiation must be repeatedly executed in order to develop and fix a plurality of colors. 
     In a printer for producing pixels via a thermal head having one or more heating elements, it is necessary to control a heating temperature of the heating elements through a time controlled application of the electric current. Usually, the heating temperature is measured by a thermistor or another type of temperature sensor. However, due to a small-size of the printer the direct measurement is difficult as the heating elements are extremely small. In this case, the temperature of the heating element cannot be directly measured and is estimated from a resistance of a thermistor disposed adjacent to the heating element within the thermal head. The temperature measured is an ambient temperature of a peripheral area around the heating element. 
     Due to the temperature not being directly measured, the heating temperature is inaccurate, and the printing quality of the printer, using the thermal head, is thus limited. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a temperature control apparatus for controlling the heating temperature of the thermal head when utilized in a recording apparatus. 
     A temperature control apparatus according to the present invention controls a heating element according to a current flowing through the heating element. 
     Preferably, the current flowing through the heating element is switched by a switching device, an analog signal indicative of the current is compared to a threshold value by a comparator, and the switching device is controlled according to a result of the comparison. 
     When a plurality of heating elements is provided, each heating element is independently and accurately controllable by the temperature control apparatus according to the present invention through a direct temperature measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the description of the preferred embodiments of the invention set forth below together with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectioned elevational view showing a high-resolution printer for pressure-sensitive and temperature-sensitive recording using an embodiment of a temperature control apparatus; 
     FIG. 2 is a plan view showing a thermal head viewed from a platen roller in FIG. 1; 
     FIG. 3 is a cross-sectioned elevational view of a recording sheet used in the printer; 
     FIG. 4 is a cross-sectional view showing different types of micro-capsules utilized in the embodiment; 
     FIG. 5 is a graph showing a characteristic relationship between temperature and elasticity coefficient of a shape memory resin of the micro-capsules; 
     FIG. 6 is a graph showing a characteristic relationship between glass-transition temperature and breaking pressure of a capsule wall of the different types of micro-capsules; 
     FIG. 7 is a block diagram showing a temperature control apparatus of the embodiment according to the present invention; 
     FIG. 8 is a graph showing a characteristic relationship between temperature and resistance of the heating resistor; and 
     FIG. 9 is a timing chart of an operation of the temperature control apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiment of the present invention is described with reference to the attached drawings. 
     FIG. 1 is a cross-sectioned elevational view showing a high-resolution color printer  10  for pressure-sensitive and temperature-sensitive recording using an embodiment of a temperature control apparatus. The color printer  10  comprises a thermal head  30 , platen rollers  41 ,  42  and  43 , and spring units  51 ,  52  and  53 . The color printer  10  is a line printer for recording a full-color image line by line on a recording sheet  20  that includes cyan, magenta and yellow micro-capsules. 
     The color printer  10  comprises a housing  11 , which is rectangular parallelepiped in a longitudinal direction (“line direction”, hereinafter) being perpendicular to a longitudinal direction of the recording sheet  20 . An inlet slit  12  is provided on an upper surface of the housing  11  for inserting the recording sheet  20 , and an outlet slit  13  is provided in a right side surface of the housing  11 . The recording sheet  20  passes along a conveyer path (single-chained line coinciding with the recording sheet  20 ) from the insert slit  12  to the outlet slit  13 . The thermal head  30  extends along the conveyer path under the platen rollers  41 ,  42  and  43 . A series of heating elements  31 , a series of heating elements  32  and a series of heating elements  33  are provided on an upper surface of the thermal head  30  corresponding to the platen rollers  41 ,  42  and  43 , respectively. 
     FIG. 2 is a plan view of the thermal head  30 , representatively showing the series of heating elements  33 , viewed from the platen roller  43 . The series of heating elements  33  are aligned along a line direction. Similarly, the series of heating elements  31  and the series of heating elements  32  are also aligned along the line direction. 
     The heating elements  33  are heated by a driver unit  90 , which includes a plurality of temperature control apparatuses  90 ′ corresponding to the heating elements  33 , respectively. The temperature control apparatuses  90 ′ are controlled by a control circuit C 00  mounted on a printed circuit board (PCB)  62  (FIG.  1 ). The heating elements ( 31 ,  32 ,  33 ) of each series are selectively heated by the temperature control apparatuses  90 ′, and each series of heating elements  31 ,  32  and  33  is heated to a different temperature. 
     The platen rollers  41 ,  42  and  43  are rubber rollers extending in the line direction for pressing the total width of the recording sheet  20  at the positions corresponding to the heating elements  31 ,  32  and  33 , respectively. The platen rollers  41 ,  42  and  43  are resiliently biased toward the thermal head  30  and exert different predetermined pressures on the thermal head  30 , by means of the spring units  51 ,  52  and  53 , respectively. The platen rollers  41 ,  42  and  43  press with the different pressures at the positions of the heating elements  31 ,  32  and  33  uniformly along the total width of the recording sheet  20 . The platen rollers  41 ,  42  and  43  are rotationally driven by motors (not shown), at respective predetermined speeds in a counterclockwise direction in FIG.  1 . The recording sheet  20  is thus conveyed downstream toward the outlet opening  13  by the rotating platen rollers  41 ,  42  and  43  along the conveyer path. The motor is driven by a driver circuit (not shown) formed on the PCB  62 . 
     The heating elements  31 ,  32  and  33 , and the platen rollers  41 ,  42  and  43  correspond to three primary colors cyan, magenta and yellow. When the heating elements  31  operate in conjunction with the platen roller  41 , the color cyan is developed; when the heating elements  32  operate in conjunction with the platen roller  42 , the color magenta is developed; when the heating elements  33  operate in conjunction with the platen roller  43 , the color yellow is developed. A number of series of heating elements and a number of platen rollers are changed in accordance with a number of types of micro-capsule. 
     When the recording sheet  20  is inserted from the insert slit  12  into the housing  11  on the conveyer path, the recording sheet  20  is conveyed by the platen rollers  41 ,  42  and  43  at a predetermined speed toward the outlet slit  13 . During the movement, the recording sheet  20  is selectively heated by the heating elements  31 ,  32  and  33 , as well as being pressed by the platen rollers  41 ,  42  and  43  against the thermal head  30  at the positions of the heating elements  31 ,  32  and  33 . Image pixels are formed on the recording sheet  20  where the selective heat is directed. Then, the recording sheet  20  is forwarded through the outlet slit  13 , being ejected from the housing  11 . 
     The temperatures of the heating elements  31 ,  32  and  33  are set to increase in order. The temperature of the heating elements  32  is higher than the temperature of the heating elements  31 , and the temperature of the heating elements  33  is higher than the temperature of the heating elements  32 . Since the above serial color printer  10  performs the recording operation as the recording sheet  20  moves downstream, by using the above arrangement, the temperatures of the heating elements  32  and  33  are readily obtainable by additional heating of the heating elements  32  and  33 , respectively, thus simplifying a thermal control of the heating elements  31 ,  32  and  33 . Conversely, the pressures exerted by the platen rollers  41 ,  42  and  43  are set to decrease in order, that is, the pressure exerted by the platen roller  41  is lower than the pressure exerted by the platen roller  42 , and the pressure exerted by the platen roller  43  is lower than the pressure exerted by the platen roller  42 . 
     A battery  63 , acting as a voltage source for the control circuit and so forth, is held in a compartment at a side opposite to the surface of the outlet opening  13 . 
     A structure of the recording sheet  20  is described with reference to FIG. 3., which is a cross-sectioned elevational view of the recording sheet  20 . 
     The recording sheet  20  comprises a base member  21  made of white paper, a layer of micro-capsules  22 , and a sheet of protective transparent film  23  covering the layer of micro-capsules  22 . 
     The layer of micro-capsules  22  is formed from three types of micro-capsules: a first type of micro-capsules  24  each of which includes a shell wall  24   a  filled with a cyan core material  24   b , a second type of micro-capsules  25  each of which includes a shell wall  25   a  filled with a magenta core material  25   b , a third type of micro-capsules  26  each of which includes a shell wall  26   a  filled with a yellow core material  26   b . The core materials  24   b ,  25   b  and  26   b  are liquid dyes or inks for developing the colors of cyan, magenta and yellow, respectively. The micro-capsules  24 ,  25  and  26  are uniformly distributed in the layer of micro-capsules  22  and adhered by a wax-based binder (fixing material). Shell walls  24   a ,  25   a  and  26   a  of the micro-capsules  24 ,  25  and  26  are of diameters of several micro-meters and are formed of a synthetic resin material. The transparent film  23  prevents the image formed on the recording sheet  20  from discoloration and fading due to ultra-violet radiation, oxidation. In FIG. 3, for the convenience of illustration, although the capsule layer  22  is shown as having a thickness corresponding to the diameter of the micro-capsules  24 ,  25  and  26 , in reality, the three types of micro-capsules  24 ,  25  and  26  may overlay each other, and thus the capsule layer  22  may have a larger thickness than the diameter of a single micro-capsule  24 ,  25  or  26 . 
     In FIG. 4, the three types of micro-capsules  24 ,  25  and  26  consist of shell walls  24   a ,  25   a  and  25   a , respectively, and respective core materials  24   b ,  25   b  and  26   b , respectively. The synthetic resin material of the walls  24   a ,  25   a  and  26   a  is a white shape memory resin, for example, polynorbornene, trans-1, 4-polyisoprene, polyurethane and so forth. In general, as shown in a graph of FIG. 5, the shape memory resin exhibits a coefficient of longitudinal elasticity, which abruptly changes at a glass-transition temperature boundary Tg. In the shape memory resin, micro-Brownian motion is frozen in a low temperature area “a”, which is lower than the glass-transition temperature Tg, and thus the shape memory resin exhibits a glass-like phase. On the other hand, micro-Brownian motion of the molecular chain becomes increasingly energetic in a high-temperature area “b”, which is higher than the glass-transition temperature Tg, and thus the shape memory resin exhibits a rubber elasticity. 
     As shown in a graph of FIG. 6, the micro-capsule wall  24   a  is prepared so as to exhibit a characteristic breaking pressure having a glass-transition temperature T 1 ; the micro-capsule wall  25   a  is prepared so as to exhibit a characteristic breaking pressure having a glass-transition temperature T 2 ; and the micro-capsule wall  26   a  is prepared so as to exhibit a characteristic breaking pressure having a glass-transition temperature T 3 . For example, the glass-transition temperature T 1  may be set to a temperature selected from a range between 65° C. and 70° C., and the temperatures T 2  and T 3  are set so as to increase in turn by 40° C. from the temperature set for T 1 . In this embodiment, the glass-transition temperatures T 1 , T 2  and T 3  are 65° C., 105° and 145° C., respectively. 
     Note, by suitably varying compositions of the shape memory resin and/or by selecting a suitable one from among various types of shape memory resin, it is possible to obtain the respective shape memory resins, with the glass-transition temperatures T 1 , T 2  and T 3 . 
     In FIG. 4, the wall thickness d 4  of cyan micro-capsules  24  is larger than the wall thickness d 5  of magenta micro-capsules  25 , and the wall thickness d 5  of magenta micro-capsules  25  is larger than the wall thickness d 6  of yellow micro-capsules  26 . Consequently, the breaking pressure increases as the wall thickness (d 4 , d 5 , d 6 ) increases. 
     As shown in FIG. 6, the wall thickness d 4  of the cyan micro-capsule wall  24   a  is selected such that it is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 1  and an upper limit pressure P 0 , when each micro-capsule  24  is heated to a temperature between the glass-transition temperatures T 1  and T 2 , as shown by a hatched area “c”; the wall thickness d 5  of the magenta micro-capsule wall  25   a  is selected such that it is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 2  and the critical breaking pressure P 1 , when each micro-capsule  25  is heated to a temperature between the glass-transition temperatures T 2  and T 3 , as shown by a hatched area “d”; the wall thickness d 6  of the yellow micro-capsule wall  26   a  is selected such that each yellow micro-capsule  26  is broken and compacted under a breaking pressure that lies between a critical breaking pressure P 3  and the critical breaking pressure P 2 , when each micro-capsule  26  is heated to a temperature between the glass-transition temperature T 3  and an upper limit temperature T 0  as shown by a hatched area “e”. 
     Note, when the glass-transition temperatures T 1 , T 2 , T 3  are set as mentioned above, the upper limit temperature T 0  may be set to a temperature selected from a range between 185° C. and 190° C. Also, the critical breaking pressures P 3  may be, for example, 0.02 MPa; the critical breaking pressure P 2  may be, for example, 0.2 MPa; the critical breaking pressure P 1  may be, for example, 2.0 MPa; and the upper limit pressure P 0  may be, for example, 20 MPa. 
     For example, if the selected heating temperature and breaking pressure fall within a hatched cyan area “c”, as shown in FIG. 6, only the cyan micro-capsules  24  are broken and squashed. Also, if the selected heating temperature and breaking pressure fall within the hatched magenta area “d”, only the magenta micro-capsules  25  are broken and squashed. Further, if the selected heating temperature and breaking pressure fall within the hatched yellow area “e”, only the yellow micro-capsules  26  are broken and squashed. Then, the recording sheet  20  is colored by the corresponding dye or ink for forming the color image. 
     The temperature control apparatus  90 ′ in the driver unit  90  (FIG. 2) is now described in detail with reference to FIGS. 7 to  9 . 
     FIG. 7 is a block diagram showing the temperature control apparatus  90 ′. The color printer  10  forms the image line by line and a number of each of heating elements  31 ,  32  and  33  corresponds to a number of pixels of one line. The heating elements  31 ,  32  and  33  are heating resistors, and, herein, an m th  heating element  33  is designated by a reference “Rm”. A characteristic relationship between temperature T (corresponding to T 3 ) and resistance r of the heating resistor Rm is shown in FIG.  8 . The temperature coefficient in FIG. 8 is negative, that is, the resistance lowers as the temperature rises. Although the following description refers to the temperature control apparatus  90 ′ of a heating element  33  (FIG.  2 ), obviously the description is applicable to the heating elements  31  and  32 . 
     The heating resistor Rm has opposite terminals, one of which is connected to a power supply of a constant direct voltage Vh, and the other of which is connected to the temperature control apparatus  90 ′. Signals L 2 , L 3 , L 4  and L 5  and a reference voltage Vref are input to the temperature control circuit  90 ′. The signal L 3  is a data of pixels in one line of an image to be recorded. The signal L 2  is a latch signal for receiving the data signal L 3  at a proper timing. The signal L 4  is a data-extracting signal operating synchronously with the latch signal L 2  for extracting the data signal L 3  for each heating resitor Rm. 
     The temperature control apparatus  90 ′ includes a current sensor  100 , which is connected to the heating resistor Rm through a sensing resistor Rs and a switching device Tr. The current sensor  100  is a differential amplifier, for example. The switching device Tr is an nMOS, for example, having a drain D and a source S connected to the resistors Rm and Rs, respectively. When the switching device Tr is closed, a current Is through the heating resistor Rm is introduced to the sensing resistor Rs, causing a voltage drop between opposite terminals of the sensing resistor Rs. The current sensor  100  amplifies the voltage drop to a proper level and outputs an analog signal Va corresponding to the voltage drop. A sensing circuit C 12  incorporates the sensing resistor Rs and the current sensor  100 . Since the resistance of the heating resistor Rm decreases as the temperature rises, the analog signal Va increases as the temperature increases. When the signal Va exceeds a threshold value the switching device Tr is opened, as mentioned below. 
     The analog signal Va is input to a temperature control circuit C 11 , which includes a comparator COMP and a holding circuit. The comparator COMP includes an operational amplifier  102 , an inverter  101 , first resistor R 1 , second resistor R 2  and a pull-up resistor R 4  which is connected to a supplied voltage VDD. The analog signal Va is input to the comparator COMP, and an output Vc of the comparator COMP is input to a clear input of a JK-flip-flop  103  being the holding circuit. The JK-flip-flop  103  holds and outputs a status signal (switching signal) L 8  to an input G of the switching device Tr for opening and closing the switching device Tr. The reference voltage Vref is connected through the first resistor R 1  to a non-inverted input of the operational amplifier  102 , and the second resistor R 2  is connected between the non-inverted input and an output of the operational amplifier  102 . The analog signal Va is input to an inverted input of the operational amplifier  102 . When the analog signal Va exceeds a threshold value Vb, which is based on the reference voltage Vref and is defined by the following formula (1), the inverter  101  outputs the signal Vc, being low level, to CLR so as to clear the data in the JK-flip-flop  103 . Thus, the status signal L 8  becomes low level and the switching device Tr opens.              Vb   =           V   DD     ·   R1       R1   +   R2       +       Vref   ·   R2       R1   +   R2                 (   1   )                         
     Therefore, the threshold value Vb is adjustable by the resistors R 1  and R 2 . 
     The temperature control circuit C 11  is controlled by a clock generating circuit C 10 . The signals L 3  and L 4  are input to a data input D and a clock input of a D-flip-flop  94 , which extracts data dm from a total data series for the total resistors (Rm) corresponding to the total pixels of one line, indicating that heating is to be performed by the resistor Rm. The extracted data dm is held by a D-flip-flop  95  connected at a data input D to a data output Q of the D-flip-flop  94 . The output from the data output Q of the D-flip-flop  94  is also transferred to the next temperature control circuit  90 ′ of the resistor Rm+1. The signal L 5  is inverted by an inverter  93  and input to an AND-gate  96 . An output L 14  of D-flip-flop  95  is also input to the AND-gate  96 . An output L 6  of the AND-gate  96  is input to a clock input of the JK-flip-flop  103  of the temperature control circuit C 11 . 
     When the data dm is held by the D-flip-flop  95  for heating the heating resistor Rm, and the signal L 5  is low level, the clock output L 6  from the AND-gate  96  becomes high level. Therefore, when the data dm and the strobe signal L 5  indicate that the resistor Rm is to be heated and, simultaneously, the strobe signal L 5  is low (L 6  is high), the JK-flip-flop  103  receives a high level clock signal L 6 . At this time, if the output of the comparator COMP is high, the switching device Tr is closed so that the heating resistor Rm is heated. Otherwise, the switching device Tr is opened so that the heating is stopped. 
     An operation of the temperature control apparatus  90 ′ is now described with reference to a timing chart in FIG.  9 . 
     At time “t 1 ”, the digital image-pixel signal L 3  of one pixel line and the data extracting signal L 4  are input to the D-flip-flop  94  of the temperature control apparatus  90 ′. The D-flip-flop  94  extracts the data dm indicating whether the heating resistor Rm is to be heated. The extracted data dm is held by the D-latch  95 . When the latch signal L 2  becomes low for a short time, as shown by reference S 8 , at time “t 2 ”, the output L 14  (not shown in FIG. 9 ) of the D-flip-flop  95  is kept high. 
     At time “t 3 ”, the strobe signal L 5  (S 9 ) becomes low (S 10 ), and the inversion L 6  (S 11 ) becomes high(S 12 ). Then the signal L 8  (S 18 ) becomes high (S 19 ) so that the switching device Tr (OFF) is closed (ON), and the heating of the heating resistor Rm is started. As the temperature rises, the signal Va gradually increases, as shown by a reference S 13 . When the signal Va exceeds the threshold value Vb based on the reference voltage Vref at time “t 4 ”, the signal Vc (S 16 ) becomes high (S 17 ), and L 8  becomes low, as shown by S 20 , by opening the switching device Tr. The current Is (FIG. 7) is stopped and the signals Va and Vc become low level as shown by S 21  and S 22 , respectively, at time “t 5 ”. 
     The temperature control apparatus  90 ′ of the present invention is easily adaptable to environmental ambient changes and heat hysterisis, for example, if the heating resister Rm is rather hot before the heating, the signal Va may be higher, as shown by S 14 , than “Va” shown by S 13 , and the signal Va will reach the threshold value Vb at time “tu” being prior to “t 4 ”. If the heating resister Rm is cold before the heating, the signal Va may be lower, as shown by S 15 , than “Va” shown by S 13 , and the signal Va will reach the threshold value Vb later at time “td” after “t 4 ”. Therefore, no significant effect occurs, and the temperature control apparatus  90 ′ operates without difficulty. 
     In the above embodiment, the temperature of the heating resistor Rm can be measured from the current Is, flowing through the heating resistor Rm. Thus, a more accurate control can be realized than that in the prior art. 
     The resistors R 1 , R 2  and R 4  are adjusted, so that the relationship between the temperature and the current is optimized. 
     The heating temperature (T 1 , T 2  and T 3  ) of the heating resistor Rm of the heating elements  31 ,  32  and  33  may be controlled by changing the reference voltage Vref. It is also possible that the reference voltages Vref for the heating elements  31 ,  32  and  33  are equal and the heating temperature (T 1 , T 2  and T 3 ) of the heating resistor Rm of the heating elements  31 ,  32  and  33  is controlled by the threshold value Vb which is adjusted by the resistors R 1  and R 2 , respectively. 
     In the above embodiment, the temperature coefficient of the heating resistor Rm is negative, however, it is also possible to use a heating resistor of a positive temperature coefficient. In this case, the differential amplifier of the current sensor  100  is substituted by an inverting amplifier. 
     Finally, it will be understood by those skilled in the art that the foregoing description is of a preferred embodiment of the temperature control apparatus, and that various changes and modifications may be made to the present invention without departing from the spirit and scope thereof. 
     The present disclosure relates to subject matters contained in Japanese Patent Application No.10-096590 (filed on Mar. 25, 1998) which is expressly incorporated herein, by reference, in its entirety.