TECHNICALFIELD The present invention relates to a printer, and more particularly, to a roll printer having a paper detection device capable of detecting whether a roll of paper, which is wound in a stepped manner and supplied in predetermined lengths, is used up or not.

BACKGROUND OFTHEINVENTION A roll printer, also called as a label printer, a receipt printer, or a mini printer, prints an image on a section of a roll of paper and is generally used in commercial places to print transactional information, price calculations, tax invoices, or receipts, or to print an image in a desired label-type print region such as a sticker or trademark label. In addition, the roll printer may be adapted to a cash register, a label attaching device, a receipt generator, or a ticket dispenser. Since the printer uses a roll of paper wound on a cylindrical reel mounted on a frame to be continuously supplied in a stepped manner according to an image forming region, the printer is provided with a paper detection device for detecting in advance whether the roll of paper is used up and needs to be replaced. FIGS. 1 and 2 illustrate an embodiment of a paper termination detecting apparatus of a conventional printer, disclosed in US Patent No. 5,884,861. Referring to FIGS. 1 and 2, the paper termination detecting apparatus includes a frame for mounting a cylindrical reel 2 to supply a roll of paper 1 wound thereon in predetermined lengths in a stepped manner, and a lever 20 pivotally installed to be close to or spaced apart from one side of the roll of paper. The lever 20 has first and second detection projections 21 and 22 projecting toward a main frame 10, and a switch contact projection 23 projecting in the opposite direction, disposed along its longitudinal direction and spaced apart from each other. The first and second detection projections 21 and 22 may move in and out of an opening formed at the main frame 10. In this process, a limit switch 30 functions to output a predetermined signal to a main controller in response to movement of the switch contact projection 23 when the lever 20 is pivoted. Therefore, when one of the first and second detection projections 21 and 22 is in contact with the roll of paper 2 1, the limit switch 30 is spaced apart from the switch contact projection 23 to output a switch off signal. Accordingly, the main controller recognizes that the roll of paper remains in a normal state and therefore allows the printer to operate normally. Meanwhile, the main frame 10 has a tapered guide part formed at its interior so that the roll of paper 1 can be gradually lowered as a diameter of the roll of paper 1 decreases due to its continuous supply. Therefore, as the roll of paper 1 gradually becomes smaller and finally reaches a used-up state, as shown in FIG. 2, the roll of paper 1 is gradually lowered downward in the main frame 10 until the first detection projection 21 is released therefrom. Concurrently, the second detection projection 22 is inserted into a hollow part of the cylindrical reel 2, on which the roll of paper 1 is wound, to allow the lever 20 to pivot toward the main frame 10. As a result, the switch contact projection 23 of the lever 20 pushes the limit switch 30 to output an electrical signal so that the main controller recognizes that the roll of paper 1 is used up. A user is then alerted that the used-up roll of paper needs to be replaced with a new roll 3 of paper . However, in the paper termination detecting apparatus of the conventional printer, since the cylindrical reel 2 for winding the roll of paper 1 should have the hollow part, the apparatus cannot be adapted to a reel without the hollow part. In addition, when the cylindrical reel has a different diameter due to use of a different sized roll of paper, the first and second detection projections 21 and 22 should be adjusted and refastened in order to properly contact the roll of paper. Similarly, when different sized rolls of paper are used, since contact positions of the first and second detection projections are varied, it is necessary to adjust their space or replace the lever with another lever having an appropriate space. This troublesome and frequent adjustment may diminish detection precision. Accordingly, it is difficult to generally adapt the paper termination detecting apparatus of the conventional printer for using variously sized rolls of paper, and therefore the apparatus should be adapted for using only one size of paper roll. In addition, it is difficult to adapt the main frame where the roll of paper is mounted to enable free alternation between 4 a desk type and a wall-hanging type. In addition, the roll printer generates static electricity due to friction as the roll of paper mounted in the main frame rotates, which can cause printing errors. Further, if the roll printer is used in the wall- hanging type, when a paper roll receiving part is opened to change the roll of paper, the roll of paper mounted in the printer may drop out. Furthermore, even if the roll printer can use rolls of paper with a plurality of different sizes, when the roll of paper is smaller than the paper roll receiving part, the position of the roll of paper may shift, which can cause printing errors.

DISCLOSURE OF THE INVENTION In order to solve the foregoing and/or other problems, it is an aspect of the present invention to provide a roll printer having a paper detection device capable of using rolls of paper having various sizes, simplifying a size selection process by allowing a detection position of the paper detection device to be easily adjusted through a simple process, preventing build up of static electricity, and supporting a roll of paper when the printer is used in a wall-hanging type and a paper receiving part is opened. 5 It is another aspect of the present invention to provide a roll printer having a paper detection device capable of readily changing between a desk type and a wall- hanging type by simplifying a constitution of the paper detection device for adjusting an initial setting position. It is still another aspect of the present invention to provide a roll printer having a paper detection device capable of preventing a build up of static electricity on a roll of paper with installs a paper roll shaft at a lower surface of a frame for receiving the roll of paper. It is yet another aspect of the present invention to provide a roll printer having a paper detection device capable of preventing a roll of paper from dropping out when the roll printer is used in a wall-hanging type and a paper receiving part is opened. To this end, the paper detection device includes a rear paper tray for supporting the roll of paper when the paper receiving part is opened. Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of • the invention. The foregoing and/or other aspects of the present invention can be achieved by providing a roll printer having a non-contact paper detection device and printing an image on a roll of paper, the roll printer including: a paper roll receiving part for receiving a roll of paper to be supplied in a stepped manner; a paper detection device including a holder having a cam hole formed at its one end and a non-contact sensor engaged with the holder, the paper detection device being inserted into a guide hole passing through a side frame of the receiving part and formed from a bottom surface to a center part of the receiving part to detect whether the roll of paper is used up on the basis of variation of the diameter of the roll of paper; and a position adjustment unit having a cam pin inserted into the cam hole and a position adjustment lever projecting from its one side, the position adjustment unit being pivotally installed at the side frame so that the sensor is rotated in the guide hole within a predetermined angle as the holder is cam driven when the lever is operated to change a position of the cam hole, into which the cam pin is inserted, to adjust an initial setting position of the paper detection device depending on an initial diameter of the roll of paper. In addition, the roll printer having a paper detection device in accordance with the present invention may further include a rear paper tray for preventing the roll of paper from dropping out when the roll printer is used in a wall- hanging posture and the receiving part is opened. The rear paper tray may include a tray support rod passing through a fastening hole formed at both side frames of an inlet port of the receiving part, and a fastener formed at a rear surface of the rear paper tray, wherein the fastener is pivotally fastened to the tray support rod to install the rear paper tray at the inlet port of the receiving part. The roll printer having a paper detection device in accordance with the present invention may further include a metal paper roll shaft installed to pass through an insertion hole formed at a lower end of the side frame to cross the bottom frame, and a paper roll shaft cover grounded to a main body frame of the roll printer and fastened to the side frame for inserting the metal paper roll shaft, thereby preventing build up of static electricity on the roll of paper. According to another aspect of the present invention, a guide unit may be formed at an inner periphery of the bottom frame for guiding the roll of paper downward as the diameter of the roll of paper decreases due to consumption of paper.

BRIEFDESCRIPTION OFTHEDRAWINGS The above and other aspects and advantages of the present invention will become more apparent from the following detailed description of the invention and accompanying drawings in which: FIGS. 1 and 2 are cross-sectional views of a conventional roll printer; FIG. 3 is an exploded perspective view illustrating an embodiment of a roll printer in accordance with the present invention; FIGS. 4 and 5 are side views of a housing of the roll printer of FIG. 3; FIG. 6 is a perspective view of the housing of the roll printer of FIG. 3; and FIG. 7 is a perspective view of a roll of paper, in which a paper partition and a rear paper tray are installed.

BESTMODEFORCARRYING OUTTHEINVENTION A roll printer having a paper detection device in accordance with an exemplary embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Referring to FIGS. 3, 4, and 5, a roll printer having a paper detection device in accordance with the present invention includes a paper roll receiving part 100 for receiving a roll of paper, first and second paper detection devices 130A and 130B installed at a side frame 120A of the receiving part 100 to detect whether the roll of paper is used up on the basis of variation of the diameter of the roll of paper in a non-contact manner, and a position adjustment unit having a cam 133 for adjusting an initial setting position of the first and second paper detection devices 130A and 130B depending on a roll diameter upon initial setting of the roll of paper. As shown in FIG. 3, guide holes 121A and 121B are formed at the side frame 120A to accommodate the first and second paper detection devices 130A and 130B such that the detection devices can be moved according to manipulation of the cam 133. The guide holes 121A and 121B have a '~i' shape bent at a predetermined angle from an end to a center of a bottom frame 110. The guide holes 121A and 121B may have various shapes, in addition to the ' "~i ' shape, so that the detection devices 130A and 130B can be rotated in the guide holes 121A and 121B within a predetermined angle. The first and second paper detection devices 130A and 130B include detection device holders 132A and 132B, and sensors 131A and 131B engaged with the holders 132A and 132B, respectively in a projected manner, preferably using screws 139. Preferably, the sensors 131A and 131B employ an optical sensor or a photo sensor capable of detecting whether the roll of paper is used up in a non-contact manner or not. The sensors 131A and 131B installed at the holders 132A and 132B are inserted into the guide holes 121A and 121B formed at the side frame 120A. When a position adjustment lever 137 formed at the cam 133 is manipulated, the sensors 131A and 131B engaged with the holders 132A and 132B which follow the cam 133 are guided from a bottom surface to a center part along the guide holes 121A and 121B formed at the side frame 120A. As described above, in order to allow the holders 132A and 132B to be moved by the cam, the holders 132A and 132B are provided with cam holes 133A and 133B at one end thereof, respectively, and cam pins 135A and 135B are inserted into the cam holes 133A and 133B. When the cam 133 is rotated by adjustment of the position adjustment lever 137, the cam pins 135A and 135B formed at the cam 133 move in the cam holes 133A and 133B causing the holders 132A and 132B to rotate, which in turn causes the sensors 131A and 131B to move along the guide holes 121A and 121B. Meanwhile, the cam 133 is pivotally engaged with the cam mounting hole 123 formed at the side frame 120A by a screw 138. Here, the position adjustment lever 137 is located at a lever guide groove formed at an upper end of the side frame 120A, and the lever guide groove has graduations 124 at one side (see FIG. 6) for indicating displacement of the position adjustment lever 137. In accordance with the present invention, as shown in FIGS. 3 to 6, the paper roll receiving part 100 includes a paper roll supporting unit 150 installed at an inlet port of the receiving part 100 to support the roll of paper when the roll printer is used in a wall-hanging type. Accordingly, the roll printer can be used on a desk (a horizontal posture) and hanging on a wall (a vertical posture) . That is, when the roll printer is used in a wall-hanging type and the receiving part 100 is opened, the paper roll supporting unit 150 installed at the inlet port of the receiving part 100 prevents the roll of paper from dropping out due to gravity. The paper roll supporting unit 150 preferably has a semi-circular tray shape to allow the roll of paper to be seated thereon. In addition, the paper roll supporting unit 150 is installed at the paper roll receiving part 100 by a tray support rod 152 fastened to the inlet port of the receiving part 100. More specifically, fastening holes 122A and 122B are formed at both ends of the side frames to allow the tray support rod 142 to be inserted into the fastening holes 122A and 122B adjacent to the inlet port of the receiving part 100, and at least one fastening hole 151 is formed at a center of the periphery of the semi-circular supporting unit 150 to allow the support rod 142 to be inserted through the fastening hole 151 so that the paper roll supporting unit 150 can be pivotally installed at the inlet port of the paper roll receiving part 100. In accordance with the present invention, as shown in FIG. 3, at least one paper roll shaft 140 is installed at the bottom frame 110 to cross the bottom frame 110. A paper roll shaft cover 141 is fastened to an insertion hole of the side frame 120A, at which the paper roll shaft 140 is installed, to be grounded to a main body frame (not shown) and thereby prevent build up of static electricity. Preferably, the paper roll shaft cover 141 is formed of metal. The bottom frame 110 is formed to have a step to guide the roll of paper downward as the diameter of the roll decreases due to its consumption, regardless of whether the roll printer is used in a desk type or a wall- hanging type. Preferably, the paper roll shaft 140 is inserted through the insertion hole formed at a lower end of the side frame 120A to cross the stepped bottom frame 110. As shown in FIG. 5, the paper roll shaft 140 inserted through the insertion hole is fixed using the metal cover 141. Preferably, the paper roll shaft 140 is formed of metal to prevent build up of static electricity. In accordance with the present invention, a paper roll width supporting unit 160 for upholding the roll of paper depending on its width is installed in the paper roll receiving part 100. The paper roll width supporting unit 160 includes a plate formed to cross the receiving part 100 horizontal to the side frame 120A, and a support member 162 installed at the bottom frame 110 to support the plate 161. As shown in FIG. 7, the support member 162 is installed at the bottom frame 110 to allow one end of the support member to be exposed to the exterior of the side frame 120B of the receiving part 100. A user may manipulate the exposed support member 162 to adjust the position of the supporting unit 160. Preferably, a predetermined number of teeth are formed at the bottom surface of the bottom frame 110, at which the support member 162 is installed, to appropriately adjust the position of the paper roll width supporting unit 160 depending on the width of the roll of paper. The roll printer in accordance with the present invention can be manufactured by a simplified process, due to the simple structure of the paper detection device capable of readily setting an initial setting position when the roll printer is used in a desk type or a wall- hanging type. In addition, the roll printer having a paper detection device in accordance with the present invention is capable of preventing printing errors caused by static electricity build-up by installing the paper roll shaft for receiving the roll of paper at the bottom surface of the bottom frame to ground the roll of paper. Further, the roll printer having a paper detection device in accordance with the present invention is capable of preventing the roll of paper from dropping out when the roll printer is used in a wall-hanging type and the receiving part is opened, by including the rear paper tray for supporting the roll of paper. Moreover, the roll printer having a paper detection device in accordance with the present invention is capable of preventing printing errors due to movement of the roll of paper when a small-sized roll of paper is used, by including the paper roll width supporting unit installed in the receiving part to adjust the width of the roll of paper depending on the width of the roll of paper. While an exemplary embodiment λ of the present invention has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention defined by the appended claims and their equivalents.