Patent Publication Number: US-2012038724-A1

Title: Add-on type ultraviolet irradiation apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an add-on type accessory apparatus which is used in combination with an inkjet recording apparatus, and more particularly to an ultraviolet irradiation apparatus for curing ultraviolet-curable ink, and to an image forming apparatus using same. 
     2. Description of the Related Art 
     An inkjet recording apparatus is known which uses ink that is cured by irradiation of ultraviolet light (ultraviolet-curable ink) (Japanese Patent Application Publication No. 2005-319720 and Japanese Patent Application Publication No. 2008-168502). In general, in the case of a system which comprises a non-scanning type of ultraviolet irradiation apparatus in which an ultraviolet (UV) exposure unit is disposed at a prescribed distance in the paper conveyance direction from an inkjet head unit, control to convey paper is implemented until an image area on which an image is formed by the inkjet unit has passed the UV exposure unit, or on/off switching of UV exposure is controlled on the basis of the on/off switching of paper conveyance, in order for the whole of the image area to be reliably exposed with UV light. 
       FIG. 9  of Japanese Patent Application Publication No. 2005-319720 discloses a system which combines a full line type inkjet recording head and a non-scanning type ultraviolet irradiation apparatus, and discloses a composition in which conveyance is controlled in a sub-scanning direction until a recording end scanning line has passed through the range of ultraviolet irradiation (paragraphs 0040 to 0042 in Japanese Patent Application Publication No. 2005-319720). 
     On the other hand, Japanese Patent Application Publication No. 2008-168502 discloses a control method for starting UV irradiation when conveyance of paper is started and halting UV irradiation when the conveyance is halted. Furthermore, Japanese Patent Application Publication No. 2008-168502 proposes a device which prevents non-uniformity of the amount of irradiation, by adjusting the UV irradiation intensity in accordance with the conveyance speed or adjusting the conveyance speed in accordance with the irradiation intensity, or the like. 
     In such systems as described above, the apparatus design is based on the assumption that ultraviolet-curable ink is to be used, and the carriage control of the inkjet head unit, the paper conveyance control, the UV exposure control, and the like, are managed in the sequence of the overall apparatus. In other words, the apparatus is composed as a dedicated inkjet system which is specifically designed for use with UV-curable ink. 
     However, in the case of existing inkjet printers which do not comprise a UV curing system, for example, if it were possible to load ultraviolet-curable ink instead of normal ink and use the printer as a UV curing system, then users of existing printers would be able to obtain the merits of a UV curing system, which would be very desirable indeed. 
     From this viewpoint, the present inventor has undertook development of an add-on type UV curing auxiliary unit (an add-on type ultraviolet irradiation apparatus) which can be combined additionally with an existing inkjet printing system that is not provided with an UV curing system. In a system which uses an add-on type UV curing unit of this kind, it is desirable to minimize the interface signals which are exchanged between the UV curing unit and the inkjet recording apparatus. 
     On the other hand, in an add-on type system of this kind, similarly to a conventional dedicated system, it is desirable to achieve control for conveying paper continuously until the time at which the image area recorded by the inkjet head section passes the UV exposure unit, and to prevent the occurrence of fractures and discoloration caused by over-curing as a result of performing excessive exposure onto stationary paper at the end of recording or in the event of a jam or other abnormality, as well as implementing control to prevent the occurrence of abnormal heat generation, ignition, or the like, due to excessive exposure. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of the aforementioned circumstances, an object thereof being to provide an add-on type ultraviolet light irradiation apparatus which achieves a UV curing system simply by combination with an existing inkjet recording apparatus not comprising a UV curing system, and which enables suitable control of UV exposure in coordination with the conveyance of paper. A further object of the invention is to provide an image forming apparatus which uses this kind of add-on type ultraviolet light irradiation apparatus. 
     The following modes of the invention are proposed in order to achieve an aforementioned object. 
     In order to attain an object described above, one aspect of the present invention is directed to an add-on type ultraviolet irradiation apparatus for being coupled to an inkjet recording apparatus that does not include an ultraviolet irradiation device for curing ultraviolet curable ink, so as to make the inkjet recording apparatus usable as an ultraviolet curing type image forming apparatus, the add-on type ultraviolet irradiation apparatus comprising: a detection device which detects a conveyance operation of a recording medium performed by the inkjet recording apparatus; an ultraviolet light source for radiating ultraviolet light onto an ultraviolet curable ink deposited on the recording medium by the inkjet recording apparatus; and an exposure control device which controls exposure performed by the ultraviolet light source according to a detection signal obtained from the detection device. 
     According to this aspect of the invention, it is possible to achieve a UV curing system simply by coupling the ultraviolet irradiation apparatus according to this aspect of the present invention, to an existing inkjet recording apparatus which does not comprise a UV exposure function. 
     Desirably, the exposure control device is provided independently from a control device which controls an operation of the inkjet recording apparatus, and control of ultraviolet irradiation performed by the exposure control device and the operation of the inkjet recording apparatus performed by the control device are coordinated with each other via the detection signal from the detection device without transferring a control signal between the exposure control device and the control device, in such a manner that the inkjet recording apparatus functions as the ultraviolet curing type image forming apparatus. 
     The control device which controls the operation of the inkjet recording apparatus and the exposure control device which controls the operation of the add-on type ultraviolet irradiation apparatus are mutually independent, and while each of these control devices implements independent control, the operation of the inkjet recording apparatus and the operation of the ultraviolet irradiation apparatus are coordinated overall and a function of a UV curable image forming apparatus is obtained. Therefore, it is possible to apply an inkjet recording apparatus to a UV curing system, without requiring extensive modifications or improvements to the inkjet recording apparatus. 
     Desirably, when a halt of conveyance of the recording medium is detected by the detection device, the exposure control device implements control to halt the exposure performed by the ultraviolet light source. 
     According to this aspect of the invention, excessive UV exposure onto a stationary recording medium is avoided, and the occurrence of fractures or discoloration as a result of excessive exposure can be prevented, in addition to which the occurrence of abnormal heat generation, and the like, can also be prevented. 
     Desirably, when an operation of conveyance of the recording medium is detected by the detection device, the exposure control device implements control to start the exposure performed by the ultraviolet light source or to continue the exposure performed by the ultraviolet light source. 
     According to this aspect of the invention, it is possible to conduct automatic start-up of ultraviolet irradiation (start of exposure) and continued irradiation, in coordination with the image formation operation of the inkjet recording apparatus. 
     Desirably, the add-on type ultraviolet irradiation apparatus further comprises a platen which is disposed facing the ultraviolet light source and supports, from a rear surface side of the recording medium, the recording medium irradiated with the ultraviolet light from the ultraviolet light source. 
     A desirable composition is one where ultraviolet light is irradiated from an ultraviolet light source toward a recording medium which is supported on a platen. 
     Desirably, the add-on type ultraviolet irradiation apparatus further comprises a temperature sensor which detects a temperature of the platen, wherein the temperature sensor is used as the detection device. 
     The temperature of the platen is higher when ultraviolet light is irradiated while the conveyance of the recording medium is halted, compared to when ultraviolet is irradiated while conveyance of the recording medium is being performed. Therefore, it is possible to detect the state of conveyance of the recording medium indirectly, by detecting the temperature of the platen. In other words, it is possible to utilize a temperature sensor as a device which detects a conveyance operation of the recording medium. 
     Desirably, the recording medium is a continuous medium, the add-on type ultraviolet irradiation apparatus further comprises an arm position sensor for determining a position of a take-up tension arm which is included in the inkjet recording apparatus and adjusts a tension when the continuous medium is taken up onto a take-up roll of the inkjet recording apparatus, and the arm position sensor is used as the detection device. 
     According to this aspect of the invention, a detection device (arm position sensor) can be installed easily in the add-on type ultraviolet irradiation apparatus, and can also be achieved by a sensor having an inexpensive composition. 
     Desirably, the add-on type ultraviolet irradiation apparatus further comprises a timer which is reset in accordance with a signal obtained from the arm position sensor. 
     For example, a countdown timer is used, and UV exposure is switched off when the timer value reaches zero. Alternatively, a count-up timer is used and UV exposure is switched off when a specified timer value is reached. It is possible to prevent excessive UV exposure of the stationary recording medium by means of a simple device of this kind. 
     Desirably, the add-on type ultraviolet irradiation apparatus further comprises: a heater which heats the recording medium during ultraviolet irradiation performed by the ultraviolet light source; and a heater control device which controls the heater according to the detection signal obtained from the detection device. 
     A desirable mode is one where heating is performed by a heater simultaneously with the irradiation of ultraviolet light. In this case, the control of the heater, similarly to UV exposure, is controlled using a detection signal from the detection device, independently of the control system of the inkjet recording apparatus. 
     Desirably, when a halt of conveyance of the recording medium is detected by the detection device, the heater control device implements control to halt heating performed by the heater. 
     According to this mode, excessive heating of a stationary recording medium is avoided, and it is possible to prevent the occurrence of abnormal heat generation or deformation of the recording medium, or the like, caused by excessive heat. 
     Desirably, when an operation of conveyance of the recording medium is detected by the detection device, the heater control device implements control to start heating performed by the heater or to continue the heating performed by the heater. 
     According to this aspect of the invention, it is possible to conduct automatic start-up of the heater (start of heating) and continued heating, in coordination with the image formation operation of the inkjet recording apparatus. 
     In order to attain an object described above, another aspect of the present invention is directed to an image forming apparatus comprising: any of the add-on type ultraviolet irradiation apparatuses as defined in the above; and the inkjet recording apparatus that is coupled to the add-on type ultraviolet irradiation apparatus, wherein the inkjet recording apparatus includes: an inkjet head which ejects the ultraviolet curable ink; a conveyance device which conveys the recording medium on which the ultraviolet curable ink ejected from the inkjet head is deposited; and a control device which controls an operation of the inkjet recording apparatus, the operation including control of conveyance of the recording medium performed by the conveyance device and control of ejection of the ultraviolet curable ink from the inkjet head. 
     According to the present invention, it is possible to achieve a UV curing system simply by combination with an inkjet recording apparatus that is not provided with an ultraviolet irradiation device, and it is possible to achieve coordinated operation of inkjet recording and UV exposure in the overall system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of this invention as well as other objects and benefits thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is an external perspective diagram of an inkjet printer to which an add-on type ultraviolet irradiation apparatus relating to an embodiment of the present invention is applied; 
         FIG. 2  is a block diagram showing a composition of an inkjet printer relating to an embodiment of the present invention; 
         FIG. 3  is a block diagram showing an overview of the system composition according to an embodiment of the present invention; 
         FIG. 4  is a block diagram showing an overview of the system composition of a related-art UV curable printer relating to a comparative example; 
         FIG. 5  illustrates a timing chart of the operation of the system according to an embodiment of the present invention; and 
         FIG. 6  is a block diagram showing the composition of an add-on type ultraviolet irradiation apparatus relating to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an external perspective diagram of an image forming apparatus (inkjet printer) to which an add-on type ultraviolet irradiation apparatus relating to an embodiment of the present invention is applied. The image forming apparatus  10  shown in  FIG. 1  is a system formed by combining an inkjet recording apparatus  30  which includes a paper feed roll  12 , a platen  14 , a carriage  16 , an inkjet head  18 , a guide rail  20 , a take-up roll  22 , a take-up tension arm  24 , and leg sections  26  supporting these respective elements, with an ultraviolet irradiation apparatus  50  (corresponding to an “add-on type ultraviolet irradiation apparatus”) which includes a non-scanning type ultraviolet light source  42 , a platen  44 , an arm position sensor  46  and leg sections  48  supporting these respective elements. 
     Here, terms such as “paper feed” and “paper” are used, but the material of the recording medium  60  is not limited to “paper”. For example, it is possible to use various materials, such as polyvinyl chloride (PVC), which is employed widely in wide-format printers, or other resin sheets, cloth, fabric, or the like. 
     The paper feed roll  12  is supported by brackets  13  which are fixed to the leg sections  26 . A recording medium  60  (hereinafter, called “paper” for the sake of convenience) paid out from the paper feed roll  12  is wound up onto the take-up roll  22 , via the first platen  14  and the second platen  44 . The direction of conveyance of the recording medium  60  which is moved from the paper feed roll  12  to the take-up roll  22  is called the “sub-scanning direction”, the “paper conveyance direction” or the “y direction”. 
     The platen  14  is fixed to the upper end portion of the leg sections  26 . A pre-heater which pre-heats the recording medium  60 , a print heater which improves ink fixing and coloration, and the like, are provided with the platen  14 . 
     Guide rails  20  are arranged extending in the width direction of the recording medium  60  (the direction perpendicular to the sub-scanning direction: the “main scanning direction” or the “x direction”) above the platen  14 . The carriage  16  is supported movably in the main scanning direction along the guide rails  20 . The inkjet head  18  is mounted on the carriage  16 , which moves reciprocally in the width direction of the recording medium  60  (in the main scanning direction). 
     In the present embodiment, a head unit is employed in which four separate head modules corresponding to the four ink colors of cyan (C), magenta (M), yellow (Y) and black (K) are arranged on the carriage  16 , but the types of ink and the number of heads are not limited to this. For example, it is also possible to adopt a composition in which light cyan (LC) ink and light magenta (LM) ink are added, or a composition in which other colors, such as white ink, and the like, are added. 
     The head modules of the respective colors each have a plurality of nozzles (ink ejection ports). There are no particular restrictions on the nozzle arrangement, and the nozzles may be arranged in one nozzle row, in two staggered rows, or in a multiple-row matrix arrangement or other two-dimensional arrangement. For the ejection energy generation element, it is possible to use a piezoelectric element (piezo actuator), electrostatic actuator, heat generation element, or the like. 
     Although not shown in the drawings, a replaceable cartridge (ink tank) which stores ultraviolet-curable ink (UV ink) is provided in the inkjet recording apparatus  30  according to the present embodiment. The ink cartridges provided for the respective colors are connected to the inkjet head  18  by ink supply channels (not illustrated) which are formed independently. Ink droplets are ejected from the inkjet head  18  onto the recording medium  60  which has been conveyed on the platen  14 , thereby forming an image on the recording medium  60  by means of the ink droplets which are attached to the recording medium  60 . 
     The recording medium  60  on which the image has been formed by the inkjet head  18  is conveyed to the second platen  44 . The non-scanning type ultraviolet light source  42  has a paper-wide irradiation range in such a manner that the whole image formation range of the recording medium  60  in the width direction can be simultaneously irradiated with ultraviolet light. For the ultraviolet light source  42 , it is possible to use a UV lamp, or an LED element array in which a plurality of ultraviolet LED (light-emitting diode) elements are arranged. 
     When a recording medium  60  on which an image has been formed by the inkjet head  18  passes below the ultraviolet light source  42 , the ink which has been deposited onto the recording medium  60  is cured by ultraviolet light irradiated from the ultraviolet light source  42  onto the ink on the recording medium  60 . 
     The recording medium  60  on which an image has been fixed by curing the ink by UV light exposure is wound up onto the take-up roll  22 . The take-up tension arm  24  is a device for preventing excessive tension from being applied to the recording medium  60 , by applying a suitable flexure to the recording medium  60  before the take-up roll  22 . The take-up tension arm  24  is a swinging arm, the base end portion of which is supported swingably on the lower portions of the leg sections  26 . A roller  25  of a length corresponding to the paper width is installed on the take-up tension arms  24 . A roller  25  is pressed against the rear surface side of the recording medium  60  (the opposite side to the surface which is exposed with UV light) by the swinging action of the take-up tension arm  24 , thereby loosening the pulling of the recording medium  60  and adjusting the tension during take-up. 
     More specifically, the take-up tension arm  24  is swung at a suitable timing during the take-up operation of the paper onto the take-up roll  22  (during conveyance of the recording medium) (for example, the arm  24  is swung at a prescribed time interval, at a rate of once every several ten seconds), and the recording medium  60  which has passed under the ultraviolet light source  42  is pressed by the roller  25  from the rear surface side (the opposite side to the UV exposure surface). By this means, the length of the recording medium  60  (the amount paid out from the roll) between the second platen  44  and the take-up roll  22  becomes longer, and therefore the take-up tension of the recording medium  60  is loosened when the take-up tension arm  24  returns to its original position. In this way, the tension of the recording medium  60  during take-up onto the take-up roll  22  is adjusted. 
     An arm position sensor  46  is a non-contact type position determination device which determines the position of the take-up tension arm  24  (the angle of the arm which has swung). For the arm position sensor  46 , it is possible to use, for example, a light reflecting sensor which includes a light projecting unit that radiates light onto the side surface portion of the take-up tension arm  24 , and a light receiving unit that receives reflected light from the side surface portion. The light reflecting sensor is fixed at a prescribed position on a leg section  48 , radiates light onto the take-up tension arm  24 , and can determine the position of the take-up tension arm  24  from whether or not the reflected light is detected. 
     Instead of the reflective sensor described above, it is also possible to use a sensor of another mode, such as a light shielding sensor or a limit switch, or the like. 
     Description of Block Diagrams 
       FIG. 2  is a block diagram showing a composition of a control system of the image forming apparatus  10  relating to the present embodiment. As shown in  FIG. 2 , the inkjet recording apparatus  30  has a control apparatus  370  which forms a control device. For this control apparatus  370 , it is possible to use, for example, a computer equipped with a central processing unit (CPU), or the like. The control apparatus  370  functions as a control apparatus for controlling the whole of the inkjet recording apparatus  30  in accordance with a prescribed program, as well as functioning as a calculation apparatus for performing a variety of calculations. The control apparatus  370  includes a recording medium conveyance control unit  371 , a carriage drive control unit  372 , an image processing unit  374 , and an ejection control unit  376 . These respective units are achieved by a hardware circuit or software, or a combination of these. 
     The recording medium conveyance control unit  371  controls a conveyance drive unit  380  (including a drive motor which drives a conveyance roller, or the like, and a drive circuit for the motor), in order to perform conveyance of the recording medium  60 , and a take-up tension arm drive unit  381  (including a drive motor and a drive circuit for the motor) for operating the take-up tension arm  24  (see  FIG. 1 ). The recording medium  60  which is conveyed on the platen  14  (see  FIG. 1 ) is conveyed intermittently in swath width units in the sub-scanning direction, in accordance with a repeated scanning operation in the main scanning direction performed by the inkjet head  18  (a printing pass action). 
     The carriage drive control unit  372  controls a main scanning drive unit  382  (a drive motor and a drive circuit for the motor) for moving the carriage  16  in the main scanning direction. An input apparatus  390 , such as an operating panel, and a display apparatus  392 , are connected to the control apparatus  370 . The input apparatus  390  is a device which enables external operating signals to be input manually to the control apparatus  370 , and may employ various formats, such as a keyboard, a mouse, a touch panel, or operating buttons, or the like. The display apparatus  392  may employ various formats, such as a liquid crystal display, an organic EL display, a CRT, or the like. An operator is able to input print conditions, and input and edit additional conditions, and the like, by operating the input apparatus  390 , and is able to confirm the input details and various information such as search results, via the display on the display apparatus  392 . 
     Furthermore, an information storage unit  394  which stores various information and an image input interface  396  for acquiring image data for printing are provided in the inkjet recording apparatus  30 . 
     For the image input interface  396 , a serial interface, such as USB (Universal Serial Bus), IEEE 1394, an Ethernet (registered tradename), or a wireless network, or the like, or a parallel interface, such as a Centronics interface, or the like, can be used. It is also possible to install a buffer memory (not illustrated) in this portion for achieving high-speed communications. 
     The image data input via the image input interface  396  is converted into data for printing (dot data) by the image processing unit  374 . In general, the dot data is generated by subjecting the multiple-tone image data to color conversion processing and half-tone processing. The color conversion processing is processing for converting image data represented by an sRGB system, (for example, 8-bit image data for each color of RGB) into image data of the respective colors of ink used by the inkjet recording apparatus  30 . 
     A half-toning process is processing for converting the color data of the respective colors generated by the color conversion processing into dot data of respective colors by error diffusion, a threshold value matrix, or the like. The device carrying out the half-toning process may employ commonly known methods of various kinds, such as an error diffusion method, a dithering method, a threshold value matrix method, a density pattern method, and the like. The half-toning process generally converts a tonal image data having M values (M≧3) into tonal image data having N values (N&lt;M). In the simplest example, the image data is converted into dot image data having 2 values (dot on/dot off), but in a half-toning process, it is also possible to perform quantization in multiple values which correspond to different types of dot size (for example, three types of dot: a large dot, a medium dot and a small dot). 
     The binary or multiple-value image data (dot data) obtained in this way is used for driving (on) or not driving (off) the respective nozzles, or in the case of multiple-value data, is used as ink ejection control data (droplet ejection control data) for controlling the droplet volume (dot size). 
     The ejection control unit  376  outputs an ejection control signal to the head drive circuit  388  on the basis of dot data generated in the image processing unit  374 . Furthermore, the ejection control unit  376  comprises a drive waveform generation unit (not illustrated). The drive waveform generation unit is a device which generates a drive voltage signal waveform for driving the ejection energy generation elements (piezoelectric elements, and the like) which correspond to the respective nozzles of the inkjet head  18 . The waveform data of the drive voltage signal is stored previously in the information storage unit  394  and waveform data to be used is output as and when required. The signal (drive waveform) generated by the drive waveform generation unit is supplied to the head drive circuit  388 . The signal output from the drive waveform generation unit may be digital waveform data or an analog voltage signal. 
     The inkjet image recording apparatus  30  shown in the present embodiment employs a drive method in which a common drive power waveform signal is applied to the respective ejection energy generation elements of the inkjet head  18 , in units of one head module, and ink is ejected from the corresponding nozzles by turning switching elements (not illustrated) connected to the individual electrodes of the respective ejection energy generation elements on and off, in accordance with the ejection timing of the respective nozzles. 
     In this way, a drive signal for driving ejection of the respective nozzles is applied to the inkjet head  18  via the head drive circuit  388 , thereby controlling the ink ejection volume and ejection timing of the inkjet head  18 . By this means, a desired dot size and dot arrangement corresponding to the image data for printing are achieved. 
     Programs to be executed by the CPU of the control apparatus (system controller)  370  and various data required for control purposes are stored in the information storage unit  394 . 
     The ultraviolet irradiation apparatus  50  which is combined with the inkjet recording apparatus  30  comprises the arm position sensor  46  for detecting the position of the take-up tension arm  24 , and a control apparatus  510 , a light source drive circuit  520  and the ultraviolet light source  42 . The control apparatus  510  includes a timer  511  and a UV exposure control unit  512 . The UV exposure control unit  512  is a control device which controls emission of light by the ultraviolet light source  42  via the light source drive circuit  520 , while activating a timer  511 , on the basis of a detection signal obtained from the arm position sensor  46 . 
       FIG. 3  is a block diagram showing an overview of a system composition of the image forming apparatus relating to the present embodiment. The image forming apparatus  10  according to the present embodiment has two independent sequencers ( 630  and  650 ) which respectively and independently control the inkjet recording apparatus  30  and the add-on type ultraviolet irradiation apparatus  50  which constitute the inkjet printing system. The first sequencer  630  which controls the inkjet recording apparatus  30  corresponds to the control apparatus  370  shown in  FIG. 2 . The first sequencer  630  ( 370 ) controls the respective functional modules which are required for inkjet printing, such as scanning control of main scanning and sub-scanning, control of ink supply, control of head maintenance, control of printing by the head (ejection control), and the like. In  FIG. 3 , in order to simplify the drawing, only the carriage control module  632  and the paper conveyance control module  633  are depicted. The carriage control module  632  and the paper conveyance control module  633  respectively correspond to the carriage drive control unit  372  and the recording medium conveyance control unit  371  shown in  FIG. 2 . 
     In  FIG. 3 , the second sequencer  650  which controls the add-on type ultraviolet irradiation apparatus  50  corresponds to the control apparatus indicated by reference numeral  510  in  FIG. 2 . This second sequencer  650  is independent of the first sequencer  630 , and no control signals are exchanged between the two sequencers ( 630 ,  650 ). However, the second sequencer  650  detects a paper conveyance operation and halting of the paper conveyance operation by means of a sensor  652 , for the purpose of coordinated operation with the first sequencer  630 , and the results of this are used to control UV exposure within the range of management of the second sequencer  650 .  FIG. 3  shows a composition in which the second sequencer  650  controls the UV exposure control module  654  on the basis of the detection signal obtained from the sensor  652 . As one example of the sensor  652 , in  FIG. 1  and  FIG. 2 , a combination of the arm position sensor  46  and the timer  511  is employed. 
     Various modes can be used as a device (detection device) for obtaining a detection signal of paper conveyance operation/paper conveyance halt which is used to control UV exposure by the second sequencer  650 ; for example, the compositions indicated below can be adopted. Furthermore, it is also possible to use a plurality of detection devices in a suitable combination.
     (1) An encoder which detects the rotation of the paper feed roll (supply roller encoder)   (2) An encoder which detects the rotation of the take-up roll (take-up roller encoder)   (3) A combination of a limit switch on the take-up tension arm and a timer   (4) A paper speed determination sensor which determines the conveyance speed of the paper   (5) An encoder which detects movement of the carriage (carriage motor encoder)   (6) A temperature sensor which determines the temperature of the UV exposure platen (reference numeral  44  in  FIG. 1 )   

     The temperature sensor indicated in (6) does not detect the conveyance operation of the recording medium (paper) directly; rather, when UV exposure is continued while paper conveyance is halted, the temperature of the platen  44  is raised. The temperature of the platen  44  is higher when UV exposure is performed while conveyance is halted, compared to a case where UV exposure is performed during conveyance of the paper, and therefore halting of conveyance can be detected indirectly from the determination results of the temperature sensor. In other words, the temperature sensor may form an alternative device for determining a paper conveyance operation or conveyance halt. 
     The encoders given in the examples (1), (2), (4) and (5) above, and the like, are provided separately and independently as part of the composition of the add-on type ultraviolet irradiation apparatus  50 , separately from encoders already installed as part of the composition of the inkjet recording apparatus  30 . This is because, if an encoder, and the like, installed as part of the composition of the inkjet recording apparatus  30  is used, then extensive modifications are required, for instance, the connection of signal lines, and hence the merits of the add-on type apparatus are diluted. 
     As described above, the second sequencer  650  implements control such as automatic start of UV exposure (start of light emission), continuance of UV exposure, adjustment of the exposure amount, halting of UV exposure, and the like, in accordance with the detection results obtained from the sensor  652  which detects the conveyance operation based on the paper conveyance control performed under the management of the first sequencer  630 . 
     In this way, by using the first sequencer  630  and the second sequencer  650  simultaneously, the whole of the image forming apparatus  10  functions as a UV curing printer. 
     Comparative Example 
     For the purpose of comparison,  FIG. 4  shows a system composition diagram of a related-art UV curing printer. As shown in  FIG. 4 , in the related-art system  700 , the carriage control module  732 , the paper conveyance control module  733 , the UV exposure control module  734  and the other respective functional modules are controlled by a sequencer  730  which performs comprehensive control of the whole system, and the control of UV exposure is managed as part of the whole sequence. 
     This kind of composition is not compatible with an add-on type system, and in order to change from a “UV curing system” to a “non-UV curing system (for example, a system using an eco-solvent ink)”, or in order to change from a “non-UV curing system” to a “UV curing system”, preliminary preparations are required, for instance, in order to prepare sequences corresponding to two modes, in advance. Extensive modifications are required in order to achieve compatibility with commercially available printers, and this is difficult to achieve in actual practice. 
     Timing Chart of the Present Embodiment 
       FIG. 5  illustrates a timing chart of a control sequence according to the system of the present embodiment. (a) of  FIG. 5  illustrates a paper conveyance instruction signal which is managed by the control apparatus  370  of the inkjet recording apparatus  30  (first sequencer  630 ). At the timing indicated by arrow A, a print instruction is issued to the printer and the paper conveyance is started, while the paper conveyance is halted at the timing indicated by arrow B. 
     (b) of  FIG. 5  indicates the arm angle of the take-up tension arm  24 , and the dotted line represents the ON threshold value at which the arm position sensor  46  outputs an ON signal. 
     (c) of  FIG. 5  represents a reset timing of the timer  511  which is synchronized with the ON signal by the arm position sensor  46  (a timing at which the initial timer value of the countdown timer is loaded). When the take-up tension arm  24  swings to an angle exceeding the ON threshold value ((b) of  FIG. 5 ), the arm position sensor  46  outputs an ON signal ((c) of  FIG. 5 ). The timer  511  is reset at the rise-up timing of the sensor signal. 
     (d) of  FIG. 5  indicates change in the timer value. In the present embodiment, a countdown timer is employed and therefore the value of the timer gradually decreases from the value loaded at the reset timing. The initial timer value loaded at reset is set to a value longer than the swinging time interval of the take-up tension arm  24  (for example, 5 minutes). Paper conveyance is continued, and the timer  511  is reset at the timing that the take-up tension arm  24  has moved to the prescribed angle. When paper conveyance has ended and the take-up tension arm  24  has stopping moving, the countdown continues without resetting the timer  511 , and therefore the timer  511  ultimately runs out (the timer value reaches zero). 
     (e) of  FIG. 5  indicates the timing of on/off switching of the ultraviolet light source  42  and the heater  62  (see  FIG. 6 ) which is described below. The on/off switching of the ultraviolet light source  42  is managed by the timer  511 . The ultraviolet light source  42  switches on at the operation timing (initial reset timing) of the timer  511  and the ON state is continued during the countdown period. The ultraviolet light source  42  switches off when the timer is halted. 
     As described with respect to  FIG. 1 , since the ultraviolet light source  42  is disposed to the downstream side of the image formation unit of the inkjet head  18  in terms of the paper conveyance direction, there is no particular need to switch the ultraviolet light source  42  simultaneously with the start of printing (or immediately after the start of printing), and it is sufficient for the ultraviolet light source  42  to be switched on before the image area arrives at the ultraviolet light irradiation unit. 
     The light emission start timing of the ultraviolet light source  42  is not limited to a mode where the light source is started up automatically at the initial reset timing of the timer  511 , as described above. For example, it is also possible to start light emission from the ultraviolet light source  42  and to reset the timer  511  (load the initial value), by the operator manually operating a start switch, or the like. The portions marked by the broken lines in (d) and (e) of  FIG. 5  indicate cases where the ultraviolet light source is switched on manually by the operator at the timing indicated by the arrow C. 
     Control of Halting of UV Exposure Based on Halting or End of Paper Conveyance 
     According to the present embodiment, when paper conveyance is halted or ends, the take-up tension arm  24  also halts, whereupon the ultraviolet light source  42  switches off when the value of the timer  511  has reached zero. By this means, it is possible to prevent abnormal increase in the amount of UV irradiation as a result of the halting or the end of paper conveyance. 
     Countermeasures in the Event of an Abnormality, Such as a Jam 
     According to the present embodiment, when the control apparatus  370  (first sequencer  630 ) of the inkjet recording apparatus  30  halts paper conveyance due to the occurrence of a jam or other abnormality, the take-up tension arm  24  is also halted and ceases to move, and therefore the value of the timer  511  eventually reaches zero and the ultraviolet light source  42  switches off. Therefore, excessive UV exposure does not continue and it is possible to avoid abnormal heat generation, and the like. 
     Modifications for Reliable UV Exposure of the Image Area 
     By means of the user of the printer (the operator) additionally printing a white sheet image with a desired image which is a target of printing (in other words, by instructing output of a white sheet image), it is possible to perform UV exposure reliably on the image area on which an image has been formed before the white sheet image. Furthermore, the UV exposure can be composed so as to end (switching off of UV exposure) after a prescribed time period from the end of the image formation operation. 
     Example of Control for Suppressing UV Curing Non-Uniformities 
     With a shuttle scanning type of image formation based on movement of the carriage  16 , the conveyance of the paper in the sub-scanning direction is performed intermittently. In order to suppress UV curing non-uniformity caused by intermittent halting/conveyance of the paper, it is desirable to provide a device which reduces the amount of light exposure or switches exposure off when conveyance is halted. 
     The device for avoiding UV curing non-uniformities of this kind (non-uniformity avoidance device) may adopt a mode where the ultraviolet light source  42  is switched on and off, a mode where the amount of light emitted by the ultraviolet light source  42  is adjusted (light amount control is performed), a mode where a shutter provided with the ultraviolet light source  42  (a mechanical or a liquid crystal shutter) is controlled, or a suitable combination of these modes. A desirable mode is one where the amount of UV exposure light is controlled on the basis of a signal from a sensor which detects movement of the carriage  16  and movement of the recording medium  60 . 
     Further Action and Beneficial Effects of the Present Embodiment 
     According to the present embodiment, by connecting the add-on type ultraviolet irradiation apparatus  50  to the existing inkjet recording apparatus  30  which is not provided with a UV exposure function, a UV curing type image forming apparatus  10  can be obtained simply from the inkjet recording apparatus  30 , with hardly any modifications or improvements. There are no structural restrictions on the inkjet recording apparatus  30  and an ultraviolet curing system can be realized by connecting an add-on type ultraviolet irradiation apparatus  50  to inkjet recording apparatuses of various types. 
     Furthermore, by detaching the add-on type ultraviolet irradiation apparatus  50  from the inkjet recording apparatus  30 , and using general ink cartridges of eco-solvent (low-solvent) ink, aqueous pigment-based ink, aqueous dye-based ink, or the like, instead of the ultraviolet curable ink, then the inkjet recording apparatus  30  can be returned to a system using the original eco-solvent ink, or the like, and can be used as a conventional printer. 
     In this way, according to the present embodiment, it is extremely simple to change from a non-UV curing system (for example, a system using eco-solvent ink) to a UV curing system, and from a UV curing system to a non-UV curing system, and an existing inkjet recording apparatus can be utilized effectively. 
     Other Embodiments 
     Ultraviolet Irradiation Apparatus With Heater 
       FIG. 6  is a block diagram showing the composition of an add-on type ultraviolet irradiation apparatus relating to another embodiment of the present invention. In  FIG. 6 , elements which are the same as or similar to the composition shown in  FIG. 2  are labeled with the same reference numerals and further explanation thereof is omitted here. It is also possible to use the ultraviolet irradiation apparatus  50 ′ shown in  FIG. 6 , instead of the ultraviolet irradiation apparatus  50  described in relation to  FIG. 3 . 
     The ultraviolet irradiation apparatus  50 ′ shown in  FIG. 6  comprises a heater  62  which heats the recording medium  60 , as well as irradiating ultraviolet light from the ultraviolet light source  42 . The heater  62  is disposed close to the ultraviolet light source  42  (see  FIG. 1 ), and similarly to the ultraviolet light source  42 , has a paper width heating range in which the whole image formation range of the recording medium  60  in the width direction can be simultaneously heated. It is also possible to adopt a composition where a heater is provided inside the platen  44  and performs heating from the rear surface side of the recording medium  60 . 
     Furthermore, the ultraviolet irradiation apparatus  50 ′ comprises a heater drive circuit  530  for driving the heater  62  and a temperature sensor  66  which determines the temperature of the platen  44 . The control apparatus  510  includes a heater control unit  514 , and controls the heater  62  via the heater drive circuit  530 . The heater control unit  514  controls the on/off switching of the heater  62  by the same mechanism as the on/off control of the ultraviolet light source  42  which is described above in relation to (e) of  FIG. 5 . A timer for controlling the ultraviolet light source  42  and a timer for controlling the heater  62  are provided separately and can be controlled by different timer values. 
     Modification Example 1 
     In the embodiments described above, a printing system (roll-to-roll system) which conveys a continuous medium from a paper feed roll (recording medium supply side supply roll) to a take-up roll is described by way of example, but the present invention can also be applied to a cut sheet printing system. 
     Modification example 2 
     In the present embodiment, an inkjet recording apparatus which carries out image recording by a plurality of head scanning (moving) actions using a serial type (shuttle scanning type) of head is described as an example, but the range of application of the present invention is not limited to this and the present invention can also be applied to an inkjet recording apparatus using a page-wide full line type of head having a nozzle row of a length corresponding to the entire width of the recording medium (a single-pass type of image forming apparatus which completes an image of the prescribed resolution by means of one sub-scanning action). 
     Examples of Applicable Apparatuses 
     The present invention can be applied effectively to a wide-format printer which is suitable for recording a wide image formation range, such as a large poster or commercial wall poster, or the like. Here, a printer compatible with a recording medium size of super A3 (329 mm×483 mm) or greater is called a “wide format”. However, the range of application of the present invention is not limited to such a wide format machine. Furthermore, the present invention is not limited to a graphic printing application, and can also be applied to various inkjet systems (image forming apparatuses) which are capable of forming various types of image pattern, such as a wire pattern forming apparatus which forms an image of a wiring pattern on an electronic circuit substrate, a manufacturing apparatus for various devices, a resist printing apparatus using resin liquid as a functional liquid for ejection (which corresponds to an “ink”), a fine structure forming apparatus, and the like. 
     It should be understood that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.