Scanner

A scanner includes a base frame ( 1 ); a table ( 2 ) provided on the base frame ( 1 ) to mount a sheet-like subject thereon; and a scanner head ( 3 ) movably supported on the base frame ( 1 ). The scanner head ( 3 ) can move straight along a subject mount surface ( 21 ) of the table ( 2 ).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A scanner shown in FIG. 1 is a large one that has a scan range of about 850 mm×1030 mm, for example. This scanner comprises a base frame 1 equipped with a console panel 11 on the side. A table 2 is provided on the base frame 1 to mount a sheet-like subject thereon. A scanner head 3 is supported movably on the base frame 1 so that the scanner head 3 can move straight along a subject mount surface 21 of the table 2 . FIG. 2A is a plan view of the table 2 and FIG. 2B a side view thereof. Preferably, the table 2 is composed of a metal plate. Specifically, the table 2 in this embodiment comprises an aluminum plate 2 a precisely machined and controlled to have a bend of 30 microns or below and an aluminum frame 2 b that supports the plate. An attractive mechanism 25 is formed in the table 2 to secure a subject on the subject mount surface 21 . The attractive mechanism 25 includes air ducts 22 formed inside the table 2 and narrow grooves 24 patterned in the subject mount surface 21 . Small bores 23 a, 23 b are formed to connect the ducts 22 , the subject mount surface 21 and the narrow grooves 24 together. On the periphery of the table 2 , positioning pins 26 are provided and employed as the reference to set a subject such as a printing plate of each size at the same location. FIG. 3 is a plan view of the scanner in a state, from which the table 2 and the scanner head 3 are removed. The base frame 1 has a rigid structure sufficient to prevent from a deforming by external force such as its own weight and an additional load. Provided at both edges in the shorter lateral direction on the table 2 are linear bearings 12 a, 13 a that extend in the longitudinal direction. The scanner head 3 includes a drive frame 31 of which both edges are supported on the linear bearings 12 a, 13 a at both sides of the base frame 1 via sliders 12 b, 13 b so that it can freely move in the longitudinal direction. A ball screw mechanism 14 is arranged at about the center on the base frame 1 and in parallel with the linear bearings 12 a, 13 a. The ball screw mechanism 14 includes a ball screw 14 a, which is suspended in the longitudinal direction over the base frame 1 to serve as a measurement axis. To support both ends of the ball screw 14 a, bearings 14 b, 14 c are employed. A motor 14 d is provided to rotationally drive the ball screw 14 a via a transmission that includes belts 14 e, 14 h and pulleys 14 f, 14 g, 14 i. Finally, a movable nut 14 j is secured on the drive frame 31 of the scanner head 3 . In the vicinity of the ball screw 14 a, a linear scale 15 is located in parallel with the ball screw 14 a. The linear scale 15 is of magnetic, optical or capacitive and includes a head unit 15 b that is secured on the drive frame 31 and relatively movable against a stationary scale 15 a. The linear scale 15 provides pulses indicative of a distance as the scanner head 3 moved. The pulses are introduced into an image capture circuit that contains a counter, not depicted, which counts the pulses for scan synchronization to capture an image from a CCD line image sensor 33 . When a head cover 32 of the scanner head 3 is opened, the CCD line image sensor 33 is seen secured on a sensor base not depicted. The CCD line image sensor 33 is employed to take an optical image of the subject through a slit 34 formed in a side of the scanner head 3 facing to the table 2 . FIG. 4 outlines optical elements in the scanner head 3 in a side view. As shown, the optical image of a subject 4 passed through the slit 34 is reflected at reflective mirrors 35 and 36 to change its optical path by 90°, and is focused through a lens 37 on the CCD line image sensor 33 for imaging. Such the configuration is effective to ensure a sufficient optical distance without the need for increasing a height, h, of the scanner head 3 too much. According to the scanner thus configured, the following effects can be achieved. (1) It employs direct scanning that scans, from above, the subject mounted on the upper surface of the table 2 . This is effective to eliminate an error due to the refractive distortion and bend of the table. (2) The scanner head 3 is supported at both sides and driven at the center. This is effective to precisely send the head without deviations in the direction of the drive shaft. (3) The linear scale 15 is located in the vicinity of the drive shaft. This is effective to reduce an error in movement, (4) Scan is synchronized with distance pulses from the linear scale 15 . This is effective to greatly improve a capturing precision against the distance. (5) The attractive mechanism 25 in the table 2 is operative to secure the sheet-like subject such as a printing in stable and the positioning pin 26 is possible to precisely secure it in place. In the above embodiment, the scanner head is designed to take an image reflected from the subject. To the contrary, if the subject is a transparent plate film, for example, the film is preferably illuminated from the rear surface to take an image transmitted through it. In this case, a flat light emissive plate is integrated into the table. For example, as shown in FIG. 5, a flat light emissive plate 5 is embedded in the upper surface of the table 2 . A light emissive surface of the plate 5 is employed as the subject mount surface 21 , on which a subject 4 is mounted. The flat light emissive plate 5 is composed of a transparent resinous plate 5 a, a reflective plate 5 b adhered thereto and a line light source 5 c arranged at their side. The resinous plate 5 a contains particulate dispersed therein for scattering light. FIGS. 6 A-B show another embodiment of a scanner which has a support mechanism for preventing the table 2 from bending, in a plan view and a cross-sectional view taken along the A-A′ line. The same reference numerals as those in the previous embodiment are employed to denote the corresponding components in this embodiment. The scanner head 3 is constructed to sandwich the table 2 between a substrate 38 and a drive frame 31 . This is the same as the above-described embodiment. In this embodiment, on the upper surface of the drive frame 31 , each two recessed guides 61 a 1 , 61 a 2 and 61 b 1 , 61 b 2 are formed at locations of certain distances apart from both edges in the short lateral direction of the table 2 . On the lower surface of the table 2 , two rails 62 a, 62 b are continuously formed in the longitudinal direction of the table 2 for sliding these guides 61 a 1 , 61 a 2 , 61 b 1 , 61 b 2 . Through the support mechanism including the guides 61 a 1 , 61 a 2 , 61 b 1 , 61 b 2 and rails 62 a, 62 b, the drive frame 31 of the scanner head 3 can support the table 2 at locations apart certain distances from both edges in the short lateral direction of the table 2 . In this case, even if the table 2 slightly bends due to its own weight and so forth, the bend can be corrected at least at the location of the scanner head 3 . As a result, it is possible to maintain a constant distance between the subject mount surface 21 of the table 2 and the scanner head 3 . This leads to possible high-precise image scanning. As obvious from the forgoing, according to the present invention, the scanner head equipped with the imager opposes to the subject mount surface, and the subject mounted on the subject mount surface is directly scanned through no glass plate and the like. Therefore, the problematic bend and refractive distortion of the glass plate is never caused. In addition, the location of the scanner head in the direction of the drive shaft is detected on the basis of the signal from the linear scale to control the scanning pitch for the image of the subject. Therefore, a precision of the scanning pitch to a running distance can be greatly improved. Accordingly, a high-precision scanner can be achieved. Having described the embodiments consistent with the invention, other embodiments and variations consistent with the invention will be apparent to those skilled in the art. Therefore, the invention should not be viewed as limited to the disclosed embodiments but rather should be viewed as limited only by the spirit and scope of the appended claims.