AN APPARATUS AND A METHOD FOR AUTOMATICALLY ERASING A BLACKBOARD

Abstract: The invention discloses an apparatus and a method for automatically erasing a blackboard, and more particularly, to an improved apparatus and method for automatically erasing the blackboard wherein the automatic erasing apparatus horizontally reciprocates relative to a blackboard. In the invention, two wiper members are positioned at outer ends of the erasing means rotatably installed at both ends with respect to the carriage. The eccentric axes are vertically mounted on the variable plates. By the variable plates, the left erasing means and one wiper member are moved in the right direction or in the left direction to contact or not to contact the right erasing means and the other wiper member.

Description: AN APPARATUS AND A METHOD FOR AUTOMATICALLY ERASING A BLACKBOARD Technical Field The present invention relates to an apparatus and a method for automatically erasing a blackboard, and more particularly, to an improved apparatus and method for automatically erasing handwritings written by chalk on blackboard. Background Art In general, a stuffed eraser made of fabrics is usually used to erase characters written on a conventional blackboard. However, erasing the blackboard with a stuffed eraser causes the chalk dust to pollute and dirty the surroundings of the chalkboard. As the chalk dust blows, there are disadvantages that the user's respiratory system and/or nasal mucous membranes may be threatened by the chalk dust in the air. There is another disadvantage in that the user must physically shake the chalk dust from the eraser after each erasing operation. There is a further disadvantage in that, since the surface of the blackboard has a prescribed roughness to prevent sliding of the chalk, when the user writes characters on the blackboard with the chalk, chalk particles become embedded in minute concave portions of the surface of the blackboard, so that the fabric eraser cannot completely and clearly erase the characters on the surface of the blackboard, and blurry traces from the erased characters remain on the board. In order to solve these problems of the prior art, numerous types of automatic erasing apparatuses have previously been proposed. Certain of these known automatic erasing apparatuses are disclosed in Japanese patent laid-open publications 8-113000 and 9-193599. Each of these proposed automatic erasing apparatuses comprises a carriage laterally movable- on the blackboard, on which the brushes are installed in the longitudinal direction, the portion in contact with the blackboard. The carriage includes a washing liquid supplier for supplying the washing liquid such as water on a surface of the board, and a wiper made of rubber longitudinally extended in contact with the blackboard plate. On the carriage, a drier for drying the board plate with hot air from the left direction of the wiper. These automatic erasing apparatuses are operated as follows: spraying washing liquid on a surface of a blackboard through a first nozzle to dissolve a film layer of chalk attached on the blackboard; rotating an erasing device such as a brush or a sponge in engagement with the surface of the blackboard to isolate the dissolved chalk from the surface of the blackboard; again spraying the washing liquid on the surface of the blackboard through a second nozzle to wash down the dissolved chalk; temporarily storing the used liquid containing the dissolved chalk in a tank to settle down the fine particles of the chalk; filtering and pumping up the washing liquid to return it to the tank in order to reuse it; and scrubbing the liquid remaining on the surface of the blackboard with a rubber scrubber in engagement with the blackboard and drying the blackboard with hot air. The erasing apparatuses illustrated in JP 8-113000 and JP 9-193599 both reciprocate laterally of the elongated blackboard. An eraser device of the erasing apparatus traverses in engagement with the surface of the blackboard during the erasing operation, and traverses in disengagement with the surface of the blackboard when returning to its original position. Japanese patent laid- open publication 9-193599 shows the brushes separated from the board surface during the returning operation. These apparatuses have solved the problems caused by the chalk dust. However, both publications show the wipers being in contact with the board surface at all times. Moreover, the previously proposed automatic erasing apparatuses also have several disadvantages that are generally recognized in use. First, rotation of the erasing device makes the sprayed washing liquid be scattered, resulting in low erasing efficiency. In order to perform an erasing operation, the washing liquid is supplied between the board and the erasing device. However, the rotation of the erasing device, i.e., centrifugal force makes the sprayed washing liquid be scattered. As a result, the substantive amount of the washing liquid to be used is little and more amounts should be supplied on board. Secondly, the relationship between the falling speed of the washing liquid and the moving speed of the erasing device should be considered. Along the moving direction of the erasing device the washing liquid is supplied around the erasing device. Considering that the sprayed washing liquid falls along the board surface, lateral-moving speed is adjusted. Either way to make slow the moving speed of the erasing device or to widen the distance between the erasing device and the washing liquid spraying nozzle should be considered. This occasionally causes the carriage size with the erasing device and the washing liquid spraying nozzle to become bigger. Thirdly, the configuration shows the wipers m being always contact with the board surface. In case of a long term suspending period, the rubber portion of the wipers may stick to the board surface. Further, movement of the wipers with the carriage, thereby damaging the board surface may exfoliate the fabric layer on the board surface. During returning operation of the wipers to the original position, the washing liquid remaining on the wiper may be re-stained on the board surface. Fourth, for erasing the carriage moves m one direction, e.g. from left to right and does not perform the erasing operation m the reverse direction. Supposing that the carriage is put on left and moves to right for erasing, a user must wait until the erasing apparatus finishes onr complete operation, i.e., returns to its original position after erasing the blackboard. Moreover, the erasing apparatus m the prior art automatic devices has a decreased lifetime because the erasing apparatus repeats the returning operation on one erasing operation. Fifth, during the returning operation, the wiper moves on a dry surface, thereby causing the unnecessary frictions with the board plate. Sixth, the erasing apparatus is fixed to perform the reciprocation relative to the entire blackboard, which lengthens the waiting time and causes deterioration. Seventh, the prior art erasing apparatus cannot erase only a portion of the blackboard (for example, half of the blackboard) . If it is desirable to move the carriage from the original position to the desired position, the contents on the board surface may be undesirably erased due to the direct contact with the board surface and the wipers. Disclosure of the Invention It is an object of this invention to provide an improved apparatus and method for automatically erasing a blackboard with an erasing device which overcomes the above disadvantages, which prevents the sprayed washing liquid between the erasing device and the blackboard from being scattered due to rotation of the erasing device and improves the erasing efficiency by spiral rotation. Another object of this invention is to provide an apparatus and a method for improving the operating speed of the erasing device by efficiently using the washing liquid and achieving the compact size. It is a still another object of this invention to provide an apparatus and a method for improving the erasing operation with a dual erasing device wherein one erasing device being m direct contact with the board surface is to perform the erasing operation by friction and the other erasing device not being m contact (or indirect contact) with the board surface is rotated to spray the washing liquid. It is a further another object of the invention to provide an apparatus and a method for improving the erasing operation irrespective of moving direction of the carriage on which the erasing device and the wiper are mounted. It is a still further object of the invention to provide an apparatus and a method for improving the erasing operation wherein the wiper following the erasing device along the operating direction thereof functions to remove the washing liquid. The wipers are constructed to be m selective contact with the board surface—it may stick to the surface or return to the original position after completion one erasing operatιon--to prevent wear of wipers or board surface. It is a still further another object of the invention to provide an apparatus and a method for performing a partial erasing operation by making the wipers and the erasing device separate from the board in case of the purpose of non-erasing operation. To accomplish the above objects, the present invention provides an apparatus and a method for automatically erasing a blackboard, the apparatus comprising a carriage mounted for lateral movement on the blackboard; an erasing device mounted to the carriage for reciprocation (moving back and forth) relative to a surface of the blackboard plate; a washing liquid supplier for supplying washing liquid around contact area with the erasing device and the board plate; a washing liquid guide means for guiding the washing liquid to be spirally rotated around the erasing device during the erasing operation; and wipers mounted to the carriage for wiping the liquid remaining on the surface. In the apparatus of the invention, m case of movement m a right direction of the carriage to perform the erasing operation, a left wiper moves m the right direction following the erasing device on the blackooard plate on which the washing liquid exists. In case of movement m the left direction of the carriage to perform the erasing operation, a right wiper moves m the left direction following the erasing device on the blackboard plate on which the washing liquid exists. The erasing operation is properly performed regardless of the moving direction of the carriage. The washing liquid may be sprayed by a nozzle mounted on the upper portion of the blackboard or a plurality of nozzles disposed on longitudinal direction on the board. One nozzle simplifies a mounting structure and a plurality of nozzles permit the simultaneous spray throughout the area being m contact with the erasing device at the time of initiating the erasing operation. Sprayed washing liquid is scattered by rotation of the erasing device. The scattered washing liquid confronts the washing liquid guide means disposed around the erasing device and moves toward the erasing device. The washing liquid m the direction of the erasing device is re-scattered by rotation and confronts the washing liquid guide means. The washing liquid rotates around the erasing device. Due to the gravity downward, the washing liquid proves the spiral motion around the erasing device and moves downward the blackboard. Washing liquid, which moves m spiral, lowers the chalk cohesion from the surface to perform the erasing operation with friction. Because the washing liquid around the erasing device moves m spiral, the increased volume of washing liquid lowers the cohesion of the chalk powder than the case without the washing liquid guide mans, for improving erasing efficiency. In case of absence of the washing liquid guide means, most washing liquid is scattered by rotation of the erasing device and the amount to reach the board surface is small m comparison of the sprayed amount. In case of existence of the washing liquid guide means, the amount to reach the board surface is maintained constant. Upon operation by the erasing device and the washing liquid, the washing liquid remaining on the board surface is wiped by the wipers which laterally move, to complete the erasing operation. Alternatively, the erasing device may be m constructed m dual m order to improve the efficiency. In case of the dual erasing devices, it is preferable to make one erasing device contact and the other in indirect contact with the board surface, m which the term 'indirect contact" refers to the non- contact as well as relatively less contact. In this case, the indirect contact erasing device should be placed ahead of the contact erasing device m the erasing direction. The contact erasing device follows the indirect contact erasing device. Tne washing liquid guide means makes tne washing liquid to be moved m spiral. The indirect contact erasing device (or the erasing device having less contact area) functions to mainly lower the cohesion between the chalk powder and the board. The erasing operation is made m the consequence of the steps of lowering cohesion by the indirect contact erasing device; lowering cohesion by the direct erasing device; and erasing operation by friction, to obtain the excellent erasing result. It is further preferable to employ the wipers oppositely disposed with respect to the erasing device on the carriage. Both wipers, a right wiper and a left wiper, are selectively contacted or separated from the board surface. One wiper, which follows the erasing device m any direction, is m contact with the board surface and the other wiper, which proceeds the erasing device, is separated from the board surface. It is preferable to employ an actuating control means for automatically controlling movement of the wipers m accordance with the erasing direction. In case that the erasing operation of the carriage is performed m right direction, the left wiper which follows the erasing device m right direction is slidably moved to wipe the washing liquid. In case that the carriage is performed m tne left direction, the right wiper following the erasing device is slidably moved to wipe the washing liquid. Accordingly, the erasing operation may be performed m any direction of the carriage. Because the either wiper, whicn proceeds the erasing device, is separated from the board surface, the wiper does not move on the blackboard plate m a dried condition, so that wear of the wiper or the blac board plate may be prevented. In the erasing apparatus of the invention, any direction of the carriage guarantees the smooth erasing operation. The user may not need to return the carriage to the original position because the erasing operation is performed m any direction of the carriage, thereby minimizing the actuating time of the erasing device and prolonging the lifetime of the apparatus. It is desirable to simultaneously control operations of the wiper and the erasing device. The contact or separated state of the wiper and the contact or indirect contact of the erasing device may be separately controlled, however, it is preferred to link them to control m order to accomplish the compact size. The erasing apparatus is constructed m which one of the left and right wipers may be m contact with the board surface only during the erasing operation and both wipers are m non-contact with the board surface during the suspension period. By doing so, the wipers may not stick to the board plate even though the erasing operation is not performed for a long period. If it is desirable to move the erasing apparatus of the invention without erasing operation, the wiper and the erasing device do not contact with the board plate. Further, a means for moving the carriage to a desired position may be employed. This allows initiating erasing operation at a desired position m lateral and avoids unnecessary erasing portions, so that it is possible to perform partial erasing operation on board plate, thereby contributing the apparatus to prolong the lifetime. It is possible to initiate the erasing operation at either side, left or right, on plate, which guarantees the minimum distance to move the carriage and improvement of erasing operation. In moving, the wipers are separated from the board plate, which prevents wear. On the carriage, the erasing device is mounted, both of which are m contact or indirect contact (m case of dual erasing devices, either device may be partially contacted) . The actuating control means may further employ a means for making the erasing device separate from the board plate during the period of suspension. By doing so, when the carriage recovers the original position or moves to tne desired position, the erasing device and the wipers are separated from the board plate, which prevents the erasing devices from abrasion or degradation . In moving the carriage to a desired position, because the wipers and the erasing device are separated from the board plate, the contents on the board plate on which the wipers and the erasing device pass are maintained. Accordingly, it is possible to erase the partial portion on the board plate. The erasing apparatus of the invention may further employ a forced drying means such as a ventilator for immediately drying the board plate after erasing operation. The blowing direction may be altered depending on the advancing operation of the erasing device with one ventilator, to accommodate tne compact size of the apparatus . The forced drier having a rotating fan m the body case with a drying outlet is axially mounted on the carriage. By rotating the body housing location/direction of the drying outlet is altered to selectively ventilate on the left or right side of the board plate. The actuating control means controls ventilation by rotating the body housing of the forced drier in the direction of the board plate from the wiper following the erasing device during the erasing operation. Regardless of the moving direction of the carriage, ventilation from the ventilator is applied on the board plate, which is wiped by the left or right wiper, thereby ventilating on the remaining washing liquid and drying the board plate. In this case, the ventilating temperature applied on the plate may be a normal temperature. The temperature may be higher than the room temperature. To make the right/left wipers m contact or indirect contact with the board plate is achieved by the embodiments as described herembelow. Variable contact means having right and left eccentric axles rotatably mounted between the upper support plate and the lower support plate. The upper rectangular variable plate has a circular hole through which the right eccentric axle protrudes. The lower rectangular variable plate has an elliptical hole through which the left eccentric axle protrudes . Outwardly inserted on the opposite eccentric axles, any end of the variable plates is close to the board plate depending on the clockwise rotation or counterclockwise rotation at the neutral state of the first and second eccentric axles. The right wiper is mounted on a right portion of the variable plate to contact with the board plate only when the right portion of the variable plate is moved close to the board plate. The left wiper is mounted on a left portion of the variable plate to contact with the board plate only when the left portion of the variable plate is moved close to the board plate. The actuating control means function to control rotation of the eccentric axles, thereby achieving the contact or separation operation of tne left and the right wipers . The device for contact or separation operation of the wipers utilizes the first and the second eccentric axles and the variable plates outwardly inserted to the axles to move depending on rotation of the axles. Rotating the first and the second eccentric axles makes the contact or separation operation of the wipers. An actuating motor or system may be further used for simple construction. To rotate the first and second eccentric axles to neutral rotation state makes the variable plates non- rocking state and enables the right and left wipers to be separated from the board plate. At the upper and lower portions of the eccentric axles, a couple of variable plates are integrally mounted. The variable plates and the upper and the lower ends of the erasing device are axially supported at the intermediate position of the variable plates. The erasing device is separated from the board plate at the non-rocking state of both variable plates at neutral state of the first and the second eccentric axle. The erasing device is m contact with the board plate when the opposite sides of both variable plates become close to the board plate. A couple of the erasing devices are m parallel mounted between the right wiper and the left wiper. A couple of the variable plates are integrally mounted at the upper and the lower positions of the eccentric axles. The erasing devices are separated from the board plate at the non-rocking state of the variable plates in the neutral state of the first and the second eccentric axles. When the left variable plate becomes close to the board plates the left erasing device contacts with and the right erasing is separated from the board plate. When the right variable plate becomes close to the board plate, the right erasing device contacts with and the left erasing device is separated from the board plate. Upper and lower portions of the erasing devices are axially mounted at the intermediate position between the left and the right variable plates. By axially mounting the erasing devices on the variable plate, one of the right and the left wipers is made to associate with contact operation to the board plate, so that the erasing device contacts the board plate. In case that the first and second eccentric axles are moved to the neutral state, the right and the left wipers and the erasing device are made separate from the board plate. The contact and separation operations of the right and left wipers and the erasing device are accomplished by rotation of the first and the second eccentric axles, thereby accommodating the actuating system to make the construction m compact. Another preferred embodiment of the contact or separation operation of the wipers with respect to the board plate is explained herembelow. In the embodiment, the variable plates are axially mounted on the carriage to make the opposite portions move forward and backward on board m accordance with the clockwise rotation or counter-clockwise rotation of the neutral state. The right wiper is fixed at the right portion of the variable plates, to contact the board plate m case of rotation of the variable plates at the neutral state to make the right portion of the variable plate toward the board plate. The left wiper is fixed at the left portion of the variable plates, to contact the board plate m case of rotation of the variable plates at the neutral state to make the left portion of the variable plate toward the board plate. By controlling rotation of the variable plate, the actuating control means enables the right and the left wipers to perform the contact/separate operation. Rotatable variable plates may achieve the contact or separation operation of the wipers. An actuating motor or system may be further used for simple construction. By rotating the variable plates to the neutral state, the right and left wipers are separated from the board plate . Construction of the erasing device will be explained m detail . Between the right wiper and the left wiper a couple of the erasing devices are mounted m parallel. The variable plates are rotatably and integrally mounted m separate m the longitudinal direction. The erasing devices are separated from the board plate m the neutral state of the variable plates. When the left variable plate becomes close to the board plate, the left erasing device contacts with and the right erasing is separated from the board plate. When the right variable plate becomes close to the board plate, the right erasing device contacts with and the left erasing device is separated from the board plate. Upper and lower portions of the erasing devices are axially mounted at the intermediate position between the right and the left variable plates. In axially mounting the erasing device with the variable plates, one of the right and the left wipers is made to associate with contact operation to the board plate, so that the erasing device contacts the board plate . In case that the variable plates are moved to the neutral state, the right and the left wipers and the erasing device are made separate from the board plate. The contact and separate operations of the right and left wipers and the erasing device are accomplished by rotation of the variable plates, thereby accommodating the actuating system to make the construction m compact. In case that a couple of the erasing devices are mounted between the right wiper and the left wiper, the erasing devices may be m association with each other or independently rotated. It is preferable of the right and the left wipers of which exterior materials are made of rubber suitable for wiping the washing liquid. The erasing operation of the invention is accomplished by the steps of: supplying washing liquid to the contact area of the board plate and the erasing device; rotating the erasing device being m contact with the board plate around the axle center in the longitudinal direction; guiding the washing liquid to the erasing device and relieving the chalk powder by moving in spiral the washing liquid; detaching the chalk powder by friction between the rotated erasing device and the board plate; moving the carrier with rotated erasing device and the washing liquid supplier m horizontal direction on board; and removing the washing liquid on the board by moving the wipers disposed at opposite sides of the erasing device. According to the erasing apparatus of the invention, the washing liquid rotates m spiral around the erasing device, to improve washing efficiency. Each wipers at opposite ends of the erasing device are constructed m contact with or separate from the board plate. It is preferable to insert the process that the left wiper contacts with the board plate and the right wiper is separated from the plate m case that the erasing device is moved m the right direction, and the right wiper contacts with the board plate and the left wiper is separated from the plate in case that the erasing device is moved m the left direction. Any direction of movement of the erasing device guarantees the wipers to move on the plate to remove the washing liquid, which ensures the erasing operation. In the erasing operation, because the wiper ahead of the erasing device to be moved is separated from the board plate, the wiper does not to move m contact on a dried plate and it is prevented from abrasion between wipers and the plate. Upon completion of the erasing operation, the method according to the present invention includes a step of maintaining the right and left wipers to be separated from the board plate, so that the wipers do not stick to the board plate even though erasing operation is not performed for a long period. The invention further includes a step of initiating the erasing operation at a desired position by laterally moving the right and left wipers and the erasing device, thereby avoiding erasing operation on the unnecessary portions. It is possible to partially erase the necessary portions . In moving to the desired position, the left and the right wipers with the erasing device are maintained m separate from the board plate, thereby avoiding wear of the wipers and plate. Because the erasing device is moved m a separate state as with the right and left wipers, it has an effect to protect the partial portions on plate by non-contact with the board plate. The remaining portions on the plate are readily removed as normal. According to the method of the invention, the step of ventilating on the board plate is added as such ventilation is made m left of the left wiper m case of movement m right for erasing. Similarly, ventilation is made m right of the right wiper m case of movement in left for erasing. This step ensures dried condition of the board plate by evaporating the washing liquid. Brief Description of the Drawings Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the present invention will be described with additional specificity and detail through use of the accompanying drawings in which : Fig. 1 is a front view showing the outline of an erasing device mounted on a blackboard according to a preferred embodiment of the present invention. Fig. 2 is a vertical sectional view taken from the section II-II of Fig. 1; Fig. 3 is a transverse sectional view taken from the section III-III of Fig. 2; Fig. 4 is a vertical sectional view taken from the front direction of the construction according to the preferred embodiment of the present invention; Fig. 5 is a vertical sectional view taken from the front direction of the construction according to the preferred embodiment of the present invention; Figs. 6 to 8 are plan views showing the operating principle of the variable contact means according to the preferred embodiment of the present invention; Fig. 9 is a plan view showing a controller and a P.C.B. circuit board for controlling the operation of the erasing apparatus according to the present invention; Figs. 10A to 10C are plan views showing a modified embodiment according to the present invention; Fig. 11 is a vertical sectional view taken from the right direction of the erasing apparatus according to another preferred embodiment of the present invention; Fig. 12A and 12B are plan views snowing the operating state according to another preferred embodiment of the present invention; Fig. 13 is a vertical sectional view taken from the front direction of an erasing apparatus according to a further preferred embodiment of the present invention; Fig. 14 is a vertical sectional view taken from right direction of an erasing apparatus according to a further preferred embodiment of the present invention; Figs. 15A and 15B are plan views showing the operating state of an erasing apparatus according to a further preferred embodiment of the present invention; Fig. 16 is a vertical sectional view taken from the front direction of an erasing apparatus according to still another preferred embodiment of the present invention; Fig. 17 is a vertical sectional view taken from the right direction of an erasing apparatus according to still another preferred embodiment of the present invention; Figs. 18A and 18B are plan views showing the operating state according to still another preferred embodiment of the present invention; and Fig. 19 is a perspective view showing another embodiment of the moving guide means with a single motor for transmitting power to the upper and the lower paths. Best Mode for Carrying Out the Invention It will be readily understood that the components and steps of the present invention, as generally described and illustrated m the figures herein and accompanying text, could be arranged and designed m a wide variety of different configurations while still utilizing the inventive concept. Thus, the more detailed description of the preferred embodiments of the apparatus and metnod of the present invention is not intended to limit the scope of the invention, as claimed, but it is merely representative of the presently preferred embodiments of the invention. The presently preferred embodiments of the invention will be best understood by reference to the drawings, wherein like parts or steps are designated by like numerals throughout. Figs. 1 to 9 show the first embodiment of the invention in which Fig. 1 is a front view showing the outline of an erasing device mounted on a blackboard according to a preferred embodiment of the present invention; Fig. 2 is a vertical sectional view taken from the section II-II of Fig. 1; Fig. 3 is a transverse sectional view taken from the section III-III of Fig. 2; Fig. 4 is a vertical sectional view taken from the front direction of the construction according to the preferred embodiment of the present invention; Fig. 5 is a vertical sectional view taken from the front direction of the construction according to the preferred embodiment of the present invention; Figs. 6 to 8 are plan views showing the operating principle of the variable contact means according to the preferred embodiment of the present invention; and Fig. 9 is a plan view showing a controller and a P.C.B. circuit board for controlling the operation of the erasing apparatus according to the present invention. Reference numeral 100 indicates a blackboard. Reference numeral 200 indicates an erasing apparatus of the board. The blackboard 100 essentially comprises a blackboard plate 110, and a board frame 120 enclosing the contour of the blackboard plate 110. The blackboard plate 110 is made of steel or synthetic resin and has a board surface 111 on which the various characters and symbols are written. The board frame 120 is mounted to the blackboard plate to enclose the contour of the blackboard plate 110, and supports the blackboard plate 110 for reciprocation relative to a surface of the blackboard plate 110. The erasing apparatus 200 includes a moving guide means 130 mounted on the blackboard frame 120 and a carrier 201 mounted on the blackboard 100 m order to guide the lateral reciprocation of the erasing apparatus 2 00 . The carriage 201 further includes a housing 210 having an opening portion toward the board plate 110, and an upper support plate 211a and a lower support plate 211b which are fixed to walls of the housing 210. A slide 212 including a bearing 132b protrudes toward the rear side of the blackboard 100 to integrally form with the support frame 211a. Moving guide means 130 for movably supporting the carriage 201 on the blackboard 100 includes a rack gear 131a, a guide rod 132a, a pinion gear 131b and the bearing 132b. The rack gear 131a is horizontally positioned along the upper frame of the blackboard frame 120, and the pinion gear 131b is rotatably supported on the upper support frame 211a to engage to the rack gear 131a. The guide rod 132a is movably mounted on the slide 212 of the upper support frame 211a to engage to the bearing 132b. The carriage 201 including the bearing 132b of the slide 212 of the upper support frame 211a and the guide rod 132a is movable m a horizontal direction on the blackboard 100, and controls rotation of the pinion gear 131b in engagement with the rack gear 131a, so that reciprocation on the blackboard can be made. Referring to Fig. 19, the rack gear 131a and the guide rod 132a may be mounted at the upper and lower ends of the board frame 120, and the pinion gear 131b may be mounted at the upper and lower ends of the carriage, so as to form a power transmitting device m which power from the power source is divided to two paths to enable a couple of pinions 131b to rotate in an equal speed. For the power transmitting device, actuating means of the erasing device can be used. By this construction, the carriage is moved by equal power applied to the upper and the lower ends and suspended by the equal brake power. In the embodiment, movement and suspension of the carriage 201 are controlled by a controller 270 which includes a position detection means 140 for detecting the moving state of the carriage 201 at the right side and left side on the board 100 (Fig. 1) . The position detection means 140 includes a contactor 141a and a limit switch 141b. The contactor 141a is mounted at upper both sides of the board frame 120. The limit switch 141b is mounted at the upper both sides of the housing 210 of the carriage 201 to output the electrical signal at the time of making contact the contactor 141a. The detection signal from the limit switch 141b is transmitted to the controller 270 in order to control the running/stopping operation of the carriage 201. For the position detection means 140 an optical sensor, micro-switch or contact sensor may be utilized as well as the limit switch 141b. In case of disorder of the position detection means 140, a stopper may be mounted to restrict movement of the carriage at the opposite ends of the blackboard. The erasing apparatus 200 including the carriage 201 further includes an erasing device 230, wipers 240, variable contact means 220, washing liquid supplier 250, drier 260 and a controller 270 on the carriage 201. Referring to Fig. 3, two erasing devices 230L and 230R to contact with the Jooard plate 111 and wipers 240L and 240R at right and left of the erasing devices 230L and 230R are installed on the carriage 201, as shown m Fig. 3. The variable contact means 220 installs the erasing devices 230R and 230L and the wipers 240R and 240L on the carriage 201 to be in contact or separate from the plate 111, and selectively contacts the left member L consisting of the left erasing device 230L and left wiper 240L or the right member R consisting of the right erasing device 230R and right wiper 240R to the board plate 111 during the erasing operation. Referring to Figs. 2 to 5, the variable contact means 220 includes a right eccentric axle 221R and a left eccentric axle 221L between the support frames 221a and 221b. The right eccentric axle 221R is axially supported at the right portion of the upper and lower support frames 211a and 211b. The left eccentric axle 221L is axially supported at the left portion of the upper and lower support frames 211a and 211b. Between the support frames 211a and 211o of the eccentric axles 221R and 221L, the center axle is eccentrically disposed from the rotation axle center. The variable contact means 220 includes an upper variable plate 222a and a lower variable plate 222b outwardly inserted into the upper and lower side of the eccentric axles 221R and 221L. Referring to Figs. 3 and 4, the variable plates 222a and 222b include a circular through hole 223R and ellipse trough hole 223L, in wnich the eccentric ax ^ es 221R and 221L are disposed. The variable plates 222a and 222b are suspended m the axial direction of the eccentric axles 221R and 221L. The ellipse shape of the through hole 223L is derived to enable the variable plates 222a and 222b to be moved by rotation of the eccentric axles 221R and 221L. The variable contact means, referring to Figs. 2, 3 and 6, between the variable plates 222a and 222b includes the eccentric axle cover 225 enclosing the eccentric axles 221R and 221L, and a washing liquid guide means 226 enclosing the right and left erasing device 230R and 230L of the board plate 111. The eccentric axle cover 225 and the washing liquid guide means 226 are integrally formed m a fixed state with the variable plates 222a and 222b. Referring to Figs. 3 and 6, the washing liquid guide means 226 is disposed near the edge of the board plate 111 of the variable plates 222a and 222b and has an omega (W) shape. The erasing devices 230R and 230L are installed m two concave portions 226b. The shape of the washing liquid guide means 226 may be modified to provide spiral movement and to prevent the washing liquid to be scattered. In case of the dual erasing device, it is preferable to form the washing liquid guide means having an omega (W) shape. The dual erasing device has an advantage that a nozzle may be mounted on the center portion of the W shape and the washing liquid guide means 226 acts itself as a nozzle having a plurality of holes to spray the washing liquid. At the front end 226a of the washing liquid guide means 226, protrusions 227 are laterally formed from the front end 226a of the washing liquid guide means 226. On a front side 227a facing the board plate 111 of the protrusions 227, a plurality of the inserts 227b (two inserts m the preferred embodiment) for inserting the wipers 240R and 240L are formed m a longitudinal direction of the washing liquid guide means 226. The space between the rear side 226c of the washing liquid guide means 226 and the front side 225a of the eccentric axle cover 225 is used as a ventilation path 228 to guide ventilation on the board plate 111 from the drier 260 toward the right wiper 240R. For guiding ventilation, an elongated plate 225b is shown at right of the right wiper 240R from the eccentric axle cover 225 . The erasing devices 230R and 230L are rotated m contact with the board plate 111, tnereby erasing writings on the plate 111 by fπctional force. Referring to Figs. 3 and 5, tne erasing devices 230R and 230L are m a roll shape with a brush 232 made of synthetic resin on the support shafts 231, and mounted m the concave portion 226b of the washing liquid guide means 226 m the shape of W. The upper and lower support shafts 231 of the erasing devices 230R and 230L are rotatably mounted between the variable plates 222a and 222b. An actuating device for rotation of the erasing devices 230L and 230R is assembled to the upper variable plate 222a. Referring to Fig. 5, the driving device comprises a motor Ml fixed at the upper variable plate 222a and a gear group Gl on the upper variable plate 222a to enable the driving force of the driving motor Ml to be delivered to the erasing devices 230R and 230L. In the gear group Gl, a gear 230a mounted at the driving shaft of the driving motor Ml, a gear 230b axially supported on the upper variable plate 222a m engagement with the gear 230a, a gear 230c mounted at the support shafts 231 of the left erasing device 230L are disposed m sequence to deliver the driving power of the driving motor Ml to the left erasing device 230L. A gear 230d m engagement with the gear 230c and axially supported on the upper variable plate 222a, and a gear 230e m engagement with the gear 230d and mounted on the support shaft 231 of the right erasing device 230R are disposed m sequence to deliver the driving power of the driving motor Ml to the right erasing device 230R. Any rotational direction of the driving motor Ml ensures the erasing devices 230R and 230L to be rotated m the same direction. The driving motor Ml functions to rotate the erasing devices 230R and 230L and laterally move the carriage 201 on the blackboard 100. Referring to Fig. 5, gear 230f functions to deliver the driving force of the driving motor Ml to the pinion 131b of the moving guide means 130. The gear 230f is integrally rotated with the gear 230d, and engaged with the gear 230g supported under the upper support frame 211a (Fig. 2) . On the upper support frame 211a, a gear 230h is rotatably installed m engagement with the gear 230g. The gear 230h is integrally engaged with a gear 2301 mounted at the rotation shaft of the pinion 131b of the moving guide means 130. The driving force of the driving motor Ml is delivered to the erasing devices 230R and 230L, and through the gears 230a to 230d and the gears 230f to 230ι to the pinion 131b, thereby achieving movement of the carri age 2 01 . The wipers 240R and 240L is moved on the board plate 111 to wipe out the remaining washing liquid. The wipers 240R and 240L made of the rubber wedge are inserted to the inserts 227b m the protrusions 227 of the washing liquid guide means 226, and assembled at the upper and lower variable plates 222a and 222b. Because the wipers 240R and 240L may be wet by the washing liquid and moved m the attached condition on the board plate 111, so that water resistance and abrasion resistance of the wipers are required. The surface of the wipers 240R and 240L is coated by polymerization such as polyvmyl chloride or polyurethane upon halogenization to the contact area of tne wiper. The variable contact means 220 assembled with the erasing devices 230R and 230L and the wipers 240R and 240L is explained. The eccentric axles 221R and 221L are rotated at a predetermined neutral rotation position to move the variable plates 222a and 222b, thereby contacting and making separation with/on the board plate 111 of the erasing devices 230R and 230L and the wipers 240R and 240L. In order to perform variable contact operation, the actuating device for rotational driving the eccentric axles 221R and 221L is mounted on the upper support frame 211a of the carriage 210. Referring to Fig. 4, the actuating device comprises a driving motor M2 fixed on the upper support frame 211a and the gear group G2 for delivering the driving power of the driving motor M2 to the eccentric axles 221R and 221L, The gear group G2 actuates a gear 221a mounted on the driving axle of the motor M2 and a gear 221b m engagement with the gear 221a and mounted on the axle support of the right eccentric axle 221R, to deliver the driving power of M2 to the right eccentric axle 221R. The gear 221c m engagement with the gear 221b and rotatably supported on the upper support frame 211a and the gear 221d supported on the axial support of the left eccentric axle 221L are in sequence actuated to deliver the driving power of the driving motor M2 to the left eccentric axle 221L. Regardless of the direction of rotation of the driving motor M2, the eccentric axles 221R and 221L are rotated m the same direction as the motor M2. Referring to Figs. 6 to 8, operation of the variable contact means 220 by rotation of the eccentric axles 221R and 221L is explained. In the drawings, LO indicates a rotational axle center of the left eccentric axle 221L, LA for a rotational reference of the left eccentric axle 221L to indicate the rotation position of tne left eccentric axle 221L, RO for a rotational axle center of the right eccentric axle 221R, and RA for a rotational reference of the right eccentric axle 221R to indicate the rotation position of the right eccentric axle 221R. X indicates a horizontal line m parallel with the board plate 111 through the rotational axle center LO and RO of the eccentric axles 221L and 221R, LY for a horizontal line m vertical through the rotational axle center LO of the left eccentric axle 221L, and RY for a horizontal line m vertical with the horizontal line X through the rotational axle center RO of the right eccentric axle 221R. Fig. 6 shows a neutral state m which the erasing devices 230R and 230L and the wipers 240R and 240L are both separated from the board plate 111 and the variable plates 222a and 222b are not actuated. In the neutral state, the rotational reference LA of the left eccentric axle 221L is located at a prescribed angle θ (approximately 16° m the preferred embodiment) m clockwise with respect to the horizontal line LY . The rotational reference RA of the right eccentric axle 221R is located at a prescribed angle θ (approximately 16° m the preferred embodiment) m counterclockwise with respect to the horizontal line RY. The rotational position of the eccentric axles 221R and 221L is a neutral rotation position. The outer circumference of the erasing devices 230R and 230L is separated from the board plate 111 for the interval Lt, Rt(Lt=Rt). The wipers 240R and 240L are separated from the board plate 111 for the interval LT, RT(LT=RT). It satisfied the following relationship: LT=2*Lt, RT=2«Lt. Fig. 7 shows operational state of the variable contact means 220 when the carriage 201 moves m the right direction m which the left member L comprising the left erasing device 230L and the left wiper 240L in contact with the plate 111. The driving motor M2 is rotated to actuate the gears 221a and 221b, so that the right eccentric axle 221R is rotated m clockwise. The gears 221c and 221o are simultaneously actuated to rotate the left eccentric axle 221L m clockwise. In rotation of the driving motor M2, the eccentric axles 221R and 221L are rotated m clockwise for a predetermined angle (approximately 90° m the preferred embodiment) to move the rotational reference points LA and RA of the eccentric axles 221R and 221L from the position m Fig. 6 to the position m Fig. 7. The variable plate devices 222a and 222b are actuated from the neutral state m Fig.6 to be close to the board plate 111. The left variable contact member L moves forward the board plate 111 for an eccentric distance Ldmax of the left eccentric axle 221L (Fig. 7) . The eccentric distance Ldmax and the relationship of the intervals LT and Lt are set to 2.Lt<Ldmax or LT<Ldmax, so that the left variable contact member L contacts the board plate 111. To avoid excessive contact pressure of tne left member L on the board plate 111, it is preferable to set the interval LT, Lt or the eccentric distance Ldmax to Ldmax<3.Lt. The right variable contact member R moves forward the board plate 111 for the eccentric distance Rdmin (<Ldmax) of the right eccentric axle 221R (Fig. 7) . The eccentric distance Rdmm and the intervals RT and Rt (Fig. 6) are set to Rdmιn<Rt<RT. The right variable contact member R keeps separated from the board plate 111, without contacting with the board plate 111. In case of the dual erasing device, it is possible to keep the partial contact state because cohesion of the chalk powder should be lowered. In case that the eccentric distance Rdmm is determined to be equal with the interval Rt, i.e., Rdmιn=Rt, it is possible to contact the erasing device 230R on the board plate 111 without contacting the wiper 240R of the right variable contact group R on the board plate 111. However, the preferred embodiment shows the non-contact state of the right variable contact group R with the board plate 111 during the contact state of the left variable contact group L with the board plate 111. This applies to the vice versa state that the right variable contact group R contacts the board plate 111 while the left variable contact group L does not contact the board plate 111. At this time the wipers do not contact at all from the board plate. The erasing devices may partially contact the board plate. Fig. 8 shows the operating principle of the variable contact means 200, i.e., the right variable contact group R comprising the right erasing device 230R and the right wiper 240R m contact with the board plate 111 during the erasing operation to move the carriage 201 m the left direction on board. If the driving motor M2 is counter-rotated, the gears 221a and 221b are actuated to rotate the right eccentric axle 221R m counterclockwise direction. Simultaneously with the gears 221c and 221d, the left eccentric axle 221L is rotated m counterclockwise direction. By counter-rotation of the driving motor M2, the rotation references RA and LA of the eccentric axles 221R and 221L are moved from Fig. 6 position to Fig. 8 position to rotate the eccentric axles 221R and 221L in counterclockwise for a predetermined angle (approximately 90° in this embodiment) , so that the variable plates 222a and 222b are actuated to be close to the board surface 111 from the neutral state (non-actuation state) of Fig. 6. The right member R moves forward the board plate 111 for the eccentric distance Rdamx of the left eccentric axle 221L (Fig. 8) . The eccentric distance Rdmax and the interval RT and Rt(Fιg. 6) are set to 2.Rt<Rdmax or RT<Rdmax . The entire body of the right member R is m contact with the board plate 111. To avoid excessive contact pressure of the right member R on the board plate 111, it is preferable to set Rdmax<3.Rt. The left member L moves forward the board plate 111 for the eccentric distance Ldmin (<Rdmax) of the left eccentric axle 221L (Fig. 8) . The relationship is shown as Ldmin < Lt < LT . The left member L does not contact the board plate 111 and keeps separated from the board plate 111. In case of the dual erasing devices, it is inevitable to partially contact with the board plate. The variable contact means 220 enables the left member L or the right member R to be selectively contact with the board plate 111. In order to control operations of the variable contact means 220 by the controller 270, referring to Figs. 3 and 4, m the preferred embodiment, three sensors 221a are mounted on the upper support frame 211a of the carriage 201 to determine whether or not the right and left members R and L are the neutral state separated from the board plate 111, whether or not only left member L contacts the board plate 111, and whether or not only right member R contacts the board plate 111. Each sensors 221f respond to the classified actuating states and are disposed around the gear 221b m opposite from a moving body 221e mounted m rotation on the gear 221b the right eccentric axle 221R. When the sensors 221f and the carriage 221e each other, a detecting signal is output to the controller 270, which indicates the actuating state of the variable contact means 220. The washing liquid supplier 250 of Fig. 1 supplies the washing liquid to the contact area with the erasing devices 230R and 230L and the board plate 111. The washing liquid is sprayed on the board following the erasing device and lowers cohesion of the chalk. The washing liquid supplier 250 includes a pump 251, a tank 253 for storing the washing liquid, such as water or a surfactant, and a pipe 252 communicating with an outlet of the pump 251, upwardly transferring the washing liquid from the pump 251 and spraying the washing liquid between the erasing devices 230R and 230L, and the blackboard plate 110 engaging with the erasing devices 230R and 230L. As for the nozzle, a nozzle mounted on upper portion of the board may be used or the nozzle having a plurality of holes may be used to simultaneously spray throughout the area being contact with the erasing device and the board plate m longitudinal. The nozzle having a plurality of holes may be mounted mside the washing liquid guide means or the washing liquid guide means acts itself as the nozzle having the holes. The tank 253 is detachably mounted under the lower support frame 211b of the carriage 201, to wnich the pump 251 is attached. Withm the tank 253, a filter 254 is detachably mounted for filtering the used washing liquid. The pump 251 delivers the filtered washing liquid to the pipe 252. The forced drier as shown m Figs . 3 and 4 functions to dry the board plate 111 by means of ventilation after erasing operation by the wipers 240R and 240L. The forced drier 260 includes a housing 262 longitudinally supported on the support axle 261 with the support frames 211a and 211b of the carriage 201 at the left end of the variable plates 222a and 222b of the variable contact means 220, a rotation fan 263 rotatably held m the housing 262, and a ventilation outlet 262a formed along the side of the housing 262. The rotation fan 263 is actuated by the fan motor (not shown the drawings) . With the housing 262, a heater (not shown) may be included to satisfy needs for warm air. In the invention a halogen lamp may be adopted as a heater. To alter direction of the ventilation outlet 262a association with the right and left members R and L by the variable contact means 220, the housing 262 is interconnected with the left eccentric axle 221L by using a power transmission means 264. The power transmission means 264 comprises a gear 261a drivingly interconnected with the lower portion of the left eccentric axle 221L under the lower variable plate 226b, a pinion gear 261b is in engagement with the gear 261a and axially supported on the lower support frame 211b, a first sprocket 261c integrally formed on the pinion gear 261b, and a second sprocket 261d mounted on the lower support axle 261 of the housing 262 and interconnected with a roller chain 261e to the sprocket 261c. By this constitution, rotations of the left eccentric axle 221L are delivered to the housing 262 via gears 261a and 261b, the first sprocket 261c, the roller chain 261e and the second sprocket 261d, so that the housing 262 is rotated to alter direction of the ventilation outlet 262a. In the neutral state in which the right and left members R and L are both separated from the board plate 111, the ventilation outlet 262a is shown toward the left wiper 240L (Fig.3). When the eccentric axles 221L and 221R are rotated in clockwise in a predetermined angle to contact the left member L with the board plate 111, the housing 262 is rotated in counterclockwise to direct the ventilation outlet 262a toward the board plate from left of the left wiper 240L to enable ventilation on the plate 111. When the eccentric axles 221R and 221L are rotated in counterclockwise for a predetermined angle to contact the right member R with the board plate 111, the housing 262 is rotated m clockwise to direct the ventilation outlet 262a toward the ventilation path 228 between the washing liquid guide means 226 and the eccentric axle cover 225, and through the ventilation path 228 ventilation is made on the board plate 111 at right of the right wiper 240R. The controller 270 m the carriage 201, referring to Fig. 9, comprises an erasing device/running controller 270a for controlling rotation of the erasing devices 230R and 230L and movement of the carriage 201 by using the motor Ml, a variable contact controller 270b for controlling operation of the variable contact means 220 by using the motor M2, a washing liquid supplying controller 270c for controlling operation of the washing liquid supplier 250 by using the pump 251, and a drying controller 270d for controlling operation of the forced drier 260 (rotation fan 263 and heater) . To the controller 270 a panel 271 for directing the operational command of the erasing apparatus 200 to the operator's controller 270 is connected. The panel 271 includes a on-off switch SI, a left direction erase button S2 for moving the carriage 201 the left direction to erase the board plate, a right direction erase button S3 for moving the carriage 201 in the right direction to erase the board plate, a stop button S4 to stop the carriage 201 or erasing operation, a movement button S5 for moving the carriage 201 to a desired position without erasing operation, a hot air button S7 for ventilating hot air to the forced drier 260, a high speed button S6 and a low speed button S8 for switching the operation speed of the carriage 201. The controller 270 corresponds to the operation controlling means. Operations of the erasing apparatus 200 are explained herembelow. In the erasing apparatus 200, the right member R and the left member L are both completely separated from the board plate to be a neutral state, i.e., when erasing operation of the erasing apparatus 200 is completed or suspended or the stop button S4 of the panel 271 is pressed. The following is described case of ON state of the switch SI when the carriage 201 is positioned at the left side of the blackboard 100. When the right direction erase button S3 of the panel 271 is pressed to erase writings on the board plate 111, the motor Ml is rotated by the signal from the erasing device/running controller 270a to the motor Ml. The motor Ml then actuates the gears 230a to 230e (Fig. 5) to rotate the erasing devices 230R and 230L in counterclockwise direction, and actuates the gears 230f to 2301 (Fig. 2) to rotate the pinion 131b of the moving guide means 130 m clockwise direction. The erasing devices 230R and 230L are then rotated to move the erasing device 201 from left to right on the board . The motor M2 is rotated m clocKwise by the signal from the variable contact controller 270b of the controller 270 to the motor M2. The motor M2 actuates the gears 221a to 221d (Fig. ) to rotate the eccentric axles 221R and 221L m clockwise direction. By this operation, only the left member L including the left erasing device 230L and left wiper 240L contacts tne board plate 111. The motor M2 is stopped when the variable contact means 220 contacting the left member L with the board plate 111 is detected by the sensor 221f, and the eccentric axles 221R and 221L are rotated for a predetermined angle m clockwise . In engagement with the eccentric axles 221R and 221L being rotated for a predetermined angle, the housing 262 of the forced drier 260 is rotated m counterclockwise direction for a predetermined angle, and the ventilation outlet 262a of the housing 262 moves toward the right end on the board plate 111. By the signal applied to the washing liquid pump 251 of the washing liquid supplier 250 from the washing liquid supplying controller 270c of the controller 270, the pump 251 is actuated to spray the washing liquid engagement with the pipe 252 from the upper side of the erasing devices 230R and 230L toward the board plate 111. The rotation fan 263 is rotated by the signal applied to the forced drier 260 from the drying controller 270d of the controller 270 m order to actuate the fan motor (not shown) of the forced drier 260, thereby initiating drying operation from the ventilation outlet 262a. The erasing apparatus 200 is actuated m such that the carriage 201 is moved m the right direction to perform the erasing operation (referred to right direction erasing operation) . In the right direction erasing operation, the left erasing device 230L contact with the board plate 111 is moved with the carriage 201 m the right direction, and the washing liquid is supplied to the contact area of the left erasing device 230L and the board plate 111. Thereafter, the writings on the board is detached and wiped down by the washing liquid. In the right erasing operation, the washing liquid is rotated m spiral around the erasing device by the washing liquid guide means by laying emphasis on reducing adhesive power of the chalk powder. The right erasing device may perform erasing operation by reducing cohesion of the chalk powder m complete separation or partial contact . The left wiper 240L m contact with the board plate 111 follows the left erasing device 230L along the right direction of the carriage 201 on the board plate 111. The washing liquid remaining on the board plate 111 is wiped by the left wiper 240L downward to be collected the tank 253. Ventilation from the ventilation outlet 262a of the forced drier 260 is made on the board plate 111 at rear side of the left wiper 240L, thereby drying the board plate 111 after being wiped by the wiper 240L. In dry weather, the natural drying effects are high. In humid weather, however, it is desirable that the housing with the forced drier traverses slowly while the forced drier is actuated and controlled by pressing the buttons S2, S3, S7 and/or S8 to completely dry the blackboard plate 110. Therefore, by controlling rotational speed of the motors Ml and M2 and operation of the forced drier, depending on the weather, the blackboard plate 110 is maintained m an optimum condition for use. As described above, right erasing operation, the carriage of the erasing apparatus 200 moves to the right end of the blackboard 100. The limit switch 141b mounted on upper end of the housing 210 faces the contact member 141a mounted at the upper right end of the board frame 120 and output the detected signal to the controller 270. The controller 270 stops operations of motor Ml, pump 251, and the forced drier 260, and consequently stops the carriage 201, the erasing devices 230L and 230R, the washing liquid supply, and ventilation to the board plate 111, thereby completing right erasing operation by the erasing apparatus 200. The controller rotates m counterclockwise the motor M2 by the signal applied to the motor M2 from the variable contact controller 270b, and rotates the eccentric axles 221R and 221L to the neutral state. When the neutral state of the variable contact means 220 is detected by the sensor 221f, operation of the motor M2 is stopped, thereby returning the neutral state (Fig. 6) of the variable contact means 220. Accordingly, in the right erasing operation, the left variable contact member L including the wiper 240L and the erasing device 230L is completely separated from the board plate 111 and the right and left members R and L are maintained in a complete separated state from the board plate 111. In case that the carriage 201 of the erasing apparatus 200 is located at the right end of the blackboard 100, a button 2 of the panel 271 is pressed when the carriage 210 is moved in the left direction to erase the board plate 111 (referred "left erasing operation" ) . The motor Ml is counter-rotated by the signal applied to the motor Ml from the erasing device/running controller 270a of the controller 270. By the counter-rotation of the motor Ml, the gears 230a to 230e are actuated to rotate in clockwise direction the right and left erasing devices 230R and 230L and the gears 230f to 230i are actuated to rotate in counterclockwise direction the pinion 131b of the moving guide means 130. Rotation of the erasing devices 230R and 230L enables the carriage 201 of the erasing apparatus 200 to move from right to left on blackboard 100. The motor M2 is driven in counterclockwise by the signal applied to the motor M2 from the variable contact controller 270b of the controller 270. The right and left eccentric axles 221R and 221L of the variable contact means 220 the neutral state is rotated m counterclockwise by the gears 221a to 221d. Among the right and left members R and L, referring to Fig. 8, only the right member R including the right erasing device 230R and the right wiper 240R contacts the board plate 111. Operation of the motor M2 is stopped when the variable contact means 220 contacting the board plate 111 with the right member R is detected by the sensor 221f shown m Fig. 3, and the eccentric axles 221L and 221R are rotated for a predetermined angle m counterclockwise . By the counterclockwise rotation of the eccentric axles 221R and 221L for a predetermined angle, the housing 262 of the forced drier 260 is rotated in clockwise for a predetermined angle, and the ventilation outlet 262b of the housing 262 directs toward the ventilation path 228. The forced drier 260 outputs ventilation through the ventilation path 228 at the right direction of the right wiper 240R, i.e., at the rear side of the wiper 240R with the carriage 201 moving m left on the board plate 111. With the above, the washing liquid supplier 250 and the forced drier 260 operate m the same manner with the right erasing operation. The carriage 201 of the erasing apparatus moves from right to left on the blackboard 100 to erase the board plate 111 and removes the washing liquid from the plate 111. The left erasing operation to dry the board plate 111 is made as to the right erasing operation. The right erasing device 230R contacts the board plate 111 and rotates to erase the writings on the board. The washing liquid on the board plate 111 is removed by the wiper 240R that moves following the right erasing device 230R on the board plate 111. The board plate 111 is dried such a manner that the air blow from the ventilation outlet 262a of the forced drier 260 is delivered through the ventilation path 228 to the right wiper 240R. In the left erasing operation, the carriage 201 of the erasing apparatus 200 moves toward the left end of the blackboard. When the limit switch 141b mounted at upper left end of the housing 210 contacts the contacting member 141a at left end of the upper frame of the board frame 120, the switch 141b outputs the detected signal to the controller 270. As the right erasing operation, the controller 270 stops the operations of motor Ml, pump 251, and the forced drier 260, and consequently stops the carriage 201, the erasing devices 230R and 230L, the washing liquid supplying operation, and ventilation on the board plate 111, thereby completing the left erasing operation by the erasing apparatus 200. The controller 270 rotates the motor M2 by the signal applied to the motor M2 from the variable contact controller 270b and rotates the eccentric axles 221L and 221R toward the neutral rotation position. This is performed until the sensor 221f detects the neutral state of the variable contact means 220. The variable contact means 220 returns the state of Fig. 6 and the right member R (wiper 240R and the erasing device 240R) m contact with the board plate 111 is separated from the board plate 111, thereby maintaining the right and left members R and L to be separated from the board plate 111. In the right or left erasing operation, the number of rotations of the erasing devices 230R and 230L are set 200-300 r.p.m. Large number of rotations of the erasing devices 230R and 230L is to achieve the optimum erasing effect on the board plate and to achieve dehydration effect from the polluted washing liquid from the erasing devices 230R and 230L. In order to initiate partial operation of the board plate 111 at the desired position lateral on blackboard 100, an user must press a movement button S5 of the panel 271. If it is desirable to move the carriage 201 m the right direction, a left side of the movement button S5 is pressed. If it is desirable to move the carriage 201 m the left direction, a right side of the movement button S5 is pressed. Depending to the button manipulations, the controller 270 rotates the motor Ml m clockwise or counterclockwise according to the moving direction of the carriage 201 by using the movement button S5, to move the carriage 201 m the desired direction. In this case, the controller 270 does not actuate the motor M2. The right and left members R and L are maintained m separate from the board plate 111. The variable contact means 220 is maintained the neutral state . When the carriage 201 reaches the desired position, the user presses the stop button S4 and the controller 270 stops the motor Ml. The carriage 201 stops at the desired position. Depending on the direction of erasing operation, the user presses one of the buttons S3 and S2 of the panel to perform the right or left erasing operation. The right erasing button S3 is pressed to erase the right portion of the board plate 111 from the stopped carriage position, and the left erasing button S2 is pressed to erase the left portion of the board plate 111 from the stopped carriage position. In the erasing apparatus 200, the carriage 201 may be moved m any direction to erase a desired portion on the board . Therefore, it is desirable to initiate erasing operation at the desirable position on the board plate 111. Before re-mitiatmg the erasing operation, the right and left members R and L are completely separated from the board plate 111 m case of the states such as suspension of the carriage 201 or moving the carriage 201 to the desired position. Even though the erasing apparatus 200 is not used for a long period, the wipers 240R and 240L do not stick to the board plate 111. Because the wipers 240R and 240L or the erasing devices 230R and 230L do not move m contact with the dried board plate 111, wear of the wipers 240R and 240L or the erasing devices 230R and 230L may be prevented. Figs. 10A, 10B and 10C show the alternative type of the first embodiment. The first embodiment shows two couples of wipers mounted in opposite protrusions 227 of the washing liquid guide means 226, respectively. However, a couple of wipers 240R and 240L are mounted at the opposite protrusions 227 as shown m Fig. 10A. Further, the first embodiment shows two erasing devices 230R and 230L axially supported between the upper and lower variable plates 222a and 222b of the variable contact means 220. However, one erasing device 230 may be axially supported between the variable plates 222a and 222b as shown in Fig. 10B or IOC. Each wipers 240R and 240L mounted at the protrusions 227 of the washing liquid guide means 226 of the erasing device 230 are shown each two mounted in opposite (Fig. 10B) or each one mounted in opposite (Fig. IOC) . The washing liquid guide means 226 is in a shape of partial circle to surround the erasing device and the other portions not in contact with the board plate, to guide the washing liquid flow even with one erasing device . With one erasing device 230, the wipers 240R and 240L are selectively contacted with the board plate by movement of the variable plates 222a and 222b in accordance with the rotation or counter-rotation from the neutral state of the eccentric axles 221R and 221L, and the erasing device 230 is in contact with the board plate 111 in case of contact with the wipers 240R and 240L. In order that the wipers 240R and 240L at the neutral state of the variable plates 222a and 222b and the wipers 240R and 240L are separated from the board plate 111, and the erasing device 230 and the wipers 240R and 240L at the neutral state of the erasing device 230 should be separated from the board plate 111, the erasing device 230 is axially supported at the center or front end of the variable plates 222a and 222b. With respect to the eccentric distances Ld and Rd of the eccentric axles 221R and 221L, it is desirable to appropriately set the distance between the erasing device 230 and the board plate 111, and the distance between the board plate 111 and the wipers 240L and 240R. Referring to Fig. 11 and Figs. 12A and 12B, the second preferred embodiment is explained. Fig. 11 is a vertical sectional view taken from the right direction of the erasing apparatus according to another preferred embodiment of the present invention; and Fig. 12A and 12B are plan views showing the operating state according to another preferred embodiment of the present invention. For the essential structure except for the variable contact means for the erasing devices and wipers, the same reference numerals are used throughout the second, third and fourth embodiments. Reference numeral 201 indicates a carriage movably supported m lateral on the blackboard 100 and including a housing 210 and the upper and lower support frames 211a and 211b therein. The numeral 320 indicates a variable contact means mounted on the carriage 201. The numerals 230L and 230R indicate a couple of the erasing devices which are axially supported on the variable contact means 320, respectively. The numerals 240R and 240L indicate the wipers assembled to the variable contact means 320, which are disposed at both erasing devices 230R and 230L. The variable contact means 320 includes a rotary frame 322 integrally formed with an upper plate 322a and a lower plate 322b spaced from the support frames 211a and 211b, and a rear plate 322c longitudinally extended between the plates 322a and 322b. The upper plate 322a and the lower plate 322b of the rotary frame 322 are axially supported by hmge axles 321 and 321b on the upper support frame 211a and the lower support frame 211b. The rotary frame 322 is rotated with respect to the h ge shafts 321a and 321b. Between the upper plate 322a and the lower plate 322b of the rotary frame 322, the erasing devices 230R and 230L are parallel disposed at the front opening toward the board surface 111. A support axle 231 of the erasing devices 230R and 230L is axially supported on the upper plate 322a and the lower plate 322b of the support shaft 231. In the variable contact means 320, the washing liquid guide means 226 at rear side of the erasing devices 230R and 230L is/are integrally formed between the upper plate 322a and the lower plate 322b of the rotary frame 322. The wipers 240R and 240L are mounted at the protrusions 227 at opposite ends of the washing liquid guide means 226. On the upper plate 322a of the rotary frame 322 of the variable contact means 320, the motor Ml for rotating the erasing devices 230R and 230L. The motor Ml actuates a gear g3 and a gear g4, in engagement with the gear g3, on the support shaft 231 of the erasing devices 230R and 230L, thereby rotating the erasing devices 230R and 230L. On the lower support frame 211b of the carriage 201, a motor M2 for rotating the variable contact means 320 around the hinges 321a and 321b is mounted downward from the lower plate 322b. The gear gl mounted on the driving shaft of the motor M2 is engaged with the gear g2 mounted on the lower hmge 321b. The gear gl is rotated m counterclockwise by the motor M2 e.g. a stepping motor or a vibration motor from the neutral state of the rotary frame 322 shown as an imaginary line m Fig. 12A. The rotary frame 322 is rotated m clockwise with respect to the hinges 321a and 321b by actuating the gear g2 m engagement with the gear gl . The left portion of the rotary frame 322 moves forward the board plate 111. The left member L consisting of the left erasing device 230L and the left wiper 240L close to the left portion of the rotary frame 322 contacts with the board plate 111. At the neutral state of the rotary frame 322, the left member L and the right member R consisting of the right erasing device 230R and the right wiper 240R are separated from the board plate 111 for a predetermined distance. As shown m Fig. 12B, when the gear gl is rotated in clockwise for a predetermined angle by the motor M2 from the neutral position of the rotary frame 322 (an imaginary line m Fig. 12b) , the rotary frame 322 is rotated in counterclockwise with respect to the axial center of hinges 321a and 321b, thereby making the right portion of the rotary frame 322 move forward the board plate 111. The right member R consisting of the right erasing device 230R and the right wiper 240R close to the right portion of the rotary frame 322 contacts with the board plate 111. By rotating the gear gl by the motor M2, either the right member R or the left member L is selectively contacted with the board plates 111. Both members R and L are separated from the board plate 111 at the neutral rotating position of the rotary frame 322. Except for the above essential structure, the erasing apparatus 200 including the variable contact means 320 shows the same configuration as the first embodiment. The washing liquid supplier 250, forced drier 260 or controller 270 may be found the same as in the first embodiment . The housing of the forced drier 260 is supported between the upper and lower support frames 211a and 211b of the carriage 201 at the left portion of the rotary frame 322. The housing is connected to the gear gl or g2 in engagement with power conveying means such as a gear or sprocket, so that the direction of the ventilation outlet of the housing in association with contact or separation of the right and left members R and L is modified m accordance with the rotary frame 322. In case that tne left member L contacts tne board plate 111, ventilation from the forced drier 260 is directed to the left board plate 111 of the left member L. In case that the right member R contacts the ooard plate 111, ventilation from the forced drier 260 is directed to the right board plate 111 of the right member R through the ventilation patn 228 between the washing liquid guide means 226 and the plate 322c of the rotary frame 322. Operation of the erasing apparatus 200 is performed as the first embodiment. Referring to Fig. 12B, m case of erasing operation by moving the carriage 201 in the right direction, the left member L is contacted with the board plate 111 by rotating the motor M2, and the carriage is moved m the right direction. Rotation of the erasing devices 230R and 230L, supply of the washing liquid, and ventilation on the board plate 111 from the rear portion of left member L, thereby achieving the right erasing operation. In case of erasing operation by moving the carriage 201 m the left direction, the right member R is contacted with the board plate 111 by rotating the motor M2, and the carriage 201 is moved m the left direction. Rotation of the erasing devices 230R and 230L, supply of the washing liquid, and ventilation on the board plate 111 from the rear portion of right member R, thereby achieving the left erasing operation. In case of non-erasing operation such as suspension or movement of the carriage 201 to the desired position, the variable contact means 320 becomes a neutral state and the right and left members R and L keep separated from the board plate 111. The effect is the same as the first embodiment. Referring to Figs. 13 to 15, the third embodiment will be explained. Fig. 13 is a vertical sectional view taken from the front direction of an erasing apparatus according to a further preferred embodiment of the present invention; Fig. 14 is a vertical sectional view taken from right direction of an erasing apparatus according to a further preferred embodiment of the present invention; and Figs. 15A and 15B are plan views showing the operating state of an erasing apparatus according to a further preferred embodiment of the present invention. This embodiment is distinguished from the configuration of the variable contact means for contacting the erasing devices and wipers on the board plate. Reference numeral 420 indicates a variable contact means m the third embodiment. The variable contact means 420 is mounted on the carriage 201 including the housing and the upper and lower support frames 211a and 211b and axially supports a couple of erasing devices 230R and 230L in rotation. The wipers 240R and 240L are mounted at opposite ends of the erasing devices 230R and 230L. At the upper and lower support frames 211a and 211b of the carriage 201, the hmge pins 421a and 421b are protruded on each side. At opposite of the hmge pin 421a of the support frame 211a, as shown Figs 15A and 15B, right and left through holes 413R and 413L m a circular arc shape are longitudinally extended with respect to the hmge pm 421a. The variable contact means 420 includes an upper plate 422a and a lower plate 422b m rectangular between the support frames 211a and 211b, and a gear housing 430 axially supported with respect to the hmge pm 421a on the upper support frame 211a and the rotary frame 422 integral with the rear plate 422c between the upper plate 422a and the lower plate 422b. Around the upper side of the upper plate 422a of the rotary frame 422, a couple of cylindrical connectors 423L and 423R are mounted m protrusion for connecting the upper plate 422a to the gear housing 430, and the through holes 413R and 413L are protruded from the upper support frame 211a to insert to the through holes 413R and 413L of the upper support frame 211a. As to the connectors 423R and 423L, right and left shafts 431R and 431L protruded from the lower portion of the gear housing 430 are connected to the lower portion of the gear housing 430. The lower plate 422b of the rotary frame 422 in engagement with the bearing 425 axially supports the hmge pm 421b of the lower support frame 211b. The rotary frame 422 and the gear housing 430 are integrally rotated m clockwise with respect to the axle of the hmge pins 421a and 421b. Between the lower plate 422a and the lower plate 422b of the rotary frame 422 of the variable contact means 420, the erasing devices 230R and 230L are oppositely disposed at front side of the board plate 111. The upper portion of the support shaft 231 of the erasing devices 230R and 230L are inserted to the cylindrical connectors 423R and 423L to be axially supported. The lower portion of the support shaft 231 of the erasing devices 230R and 230L are axially supported on the lower plate 422b. In the variable contact means 420, the washing liquid guide means 226 surrounding the rear end of the erasing devices 230R and 230L is mounted between the upper plate 422a and lower plate 422b of the rotary frame 422. The wipers 240R and 240L are mounted on the protrusions 227 formed at opposite of the washing liquid guide means 226. On the gear housing 430, a motor Ml for driving rotations of the erasing devices 230R and 230L is mounted. A plurality of gears 230a to 230e for delivering driving power of the motor Ml to the erasing devices 230R and 230L are held in the gear housing 430. On the upper support frame 211a, the motor M2 is mounted m which the wire W is wound for rotating the gear housing 430 of the variable contact means 420 with respect to the hmge pm 421a. The upper portion of the wire W is connected to the left rear portion of the gear housing 430 (Fig. 14 and Figs. 15A and 15B) . On the upper support frame 211a, a coil spring S is mounted at the right rear end of the gear housing 430 to actuate the gear housing 430 m clockwise (Figs. 15A and 15B) . One end of the coil spring is connected to the right rear end of the gear housing 430, and the other end of the coil spring is connected to the upper support frame 211a. When the motor M2 is actuated to draw the wire W from the neutral state (shown as an imaginary line in Fig. 15A) of the gear housing 430, the gear housing 430 and rotary frame 422 connected thereto are rotated by the coil spring with respect to the axle center of the h ge pins 421a and 421b, and the left portion of the rotary frame 422 moves forward the board plate 111. The left member L consisting of the left erasing device 230L and the left wiper 240L at the left portion of the rotary frame 422 contacts the board plate 111. At the neutral rotating position (of the variable contact means 420) of the gear housing 430 and the rotary frame 422, the right member R consisting of the right erasing device 230R and right wiper 240R, and the left member L are separated from the board plate 111 for a predetermined distance. Referring to Fig. 15B, when the motor M2 is actuated to wind the wire from the neutral rotating position of the gear housing 430, the gear housing 430 and the rotary frame 422 are resistant to the coil spring and rotated in counterclockwise with respect to the axle center of the hmge pins 421a and 421b and the right portion of the rotary frame 422 moves forward the board plate 111, which is contrary to the above case. The right member R consisting of the right erasing device 230R and the right wiper 240R contacts the board plate 111. By drawing and winding the wire W by the motor M2, either of the right member R and the left member L selectively contacts with the board plate 111. The right and left members R and L are completely from the board plate 111 at the neutral state of the rotary frame 422. Except for the above configuration, the other parts m the erasing apparatus 200 including the variable contact means 420, the washing liquid supplier 250, forced drier 260 or controller 270 show the same configuration as the first embodiment. The housing of the forced drier 260 is axially supported between the upper and lower support frames 211a and 211b of the carriage 201 at left of the rotary frame 422 as m the first embodiment. Depending on clockwise or counterclockwise rotation of the rotary frame 422 and the gear housing 430, the power delivering means by the gear or sprocket to change the direction of the ventilation hole of the housing m combination with the contact or separation of the right and left members R and L is connected to the driving shaft of the motor M2. In case of contact of the left member L with the board plate 111, the ventilation hole of the forced drier 260 directs the left board plate 111 of the left member L. In case of contact of the right member R with the board plate 111, the ventilation hole directs the ventilation path 228 between the rear plate 422c of the rotary frame 422 and the washing liquid guide means 226 to ventilate toward the ventilation path 228 of the right board plate of the right member R. The erasing apparatus 200 will be operated as the first or second embodiment. Fig. 16 to Figs. 18A and 18B show the fourth embodiment of the invention. Fig. 16 is a vertical sectional view taken from the front direction of an erasing apparatus according to still another preferred embodiment of the present invention; Fig. 17 is a vertical sectional view taken from the right direction of an erasing apparatus according to still another preferred embodiment of the present invention; and Figs. 18A and 18B are plan views showing the operating state according to still another preferred embodiment of the present invention. The fourth embodiment utilizes the same configuration with the first embodiment except for the variable contact means for contacting and making separation of the erasing device and wipers with/from the board plate. Operations will be described as the second or third embodiment. Referring to Fig. 16 to Figs. 18A and 18B, reference numeral 520 is a variable contact means m the fourth embodiment. The variable contact means 520 has an outer housing 210 mounted on the carriage 201 mcluding the upper and lower support frames 211a and 211b, so as to rotatably support the erasing devices 230R and 230L. The right and left wipers 240R and 240L are mounted exterior to the erasing devices 230R and 230L. A couple of circular through holes 513R and 513L are mounted with respect to the hmge pm 512a at the right and left h ge pins 521a and 521b protruded on the upper side of the upper and lower support frames 211a and 211b of the carriage 201 (Figs. 18A and 18B) . The variable contact means 520 includes a couple of rectangular upper and lower plates 522a and 522b spaced between the support frames 211a and 211b, a rotary frame 522 for forming a rear plate 522c integrally and vertically between the rear end of the upper plate 522a and 522b, and the gear housing 530 axially supported on the hinge pm 521a on the upper support frame 211a. On the upper side of the upper plate 522a of the rotary frame 522, a couple of cylindrical connectors 523R and 523L are prudently formed through which through holes 513R and 513L of the upper support frame 211a are connected. The shafts 531R and 531L are made through with the connectors 523R and 523L to connect with the gear housing 530. The lower plate 522b of the rotary frame 522 is supported by the hmge pm 521b of the lower support frame 211b, m engagement with the bearing 525. The rotary frame 522 of the variable contact means 520 m integral with the gear housing 530 is rotated m clockwise and counterclockwise with respect to the axle of the hmge pins 521a and 521b. The right and left erasing devices 230R and 230L adjacent to the board plate 111 between the upper plate 522a and the lower plate 522b of the rotary frame 522 of the variable contact means 520 are m parallel located to axially support the upper end of the support shaft 231 of the erasing devices 230R and 230L to the connectors 523R and 523L. The lower end of the support shaft 231 of the erasing devices 230R and 230L is axially supported on the lower plate 522b. The washing liquid guide means 226 surrounding the rear portion of the erasing devices 230R and 230L is located between the upper plate 522a and the lower plate 522b of the rotary frame 522. At the protrusions 227 m opposite of the washing liquid guide means 225 the wipers 240R and 240L are formed respectively. In the gear housing 530, the motor Ml for driving rotation of the right and left erasing devices 230R and 230L is mounted and a plurality of gears 230a to 230e for delivering the driving power of the motor Ml to the right and left erasing devices 230R and 230L. On the upper support frame 211a, as shown m Fig. 17 and Figs. 18A and 18B, the motor M2 for rotating m clockwise and counterclockwise the gear housing 530 of the variable contact means 520 with respect to the hmge pm 521a is mounted at the rear end of the gear housing 530. A semicircular pinion 534 is mounted on the driving shaft of the motor M2. The pinion 534 is engaged with the rack 533a formed at the rear portion of the protrusion 533 on the gear housing 530. In the variable contact means 520 of the third embodiment, the configuration of the driving device for rotating the gear housing 530 and rotary frame 522 connected thereto is distinguished from the third embodiment. Clockwise rotation of the gear housing 530 and the rotary frame 522 is accomplished by rotating the pinion 534 by the motor M2. By clockwise rotation of the motor Ml, either of right member R and the left member L may selectively contact with the board surface 111. The right and left members R and L are separated from the board plate 111 at the neutral state of the rotary frame 522 and the gear housing 530. Other configuration of the fourth embodiment is the same as the third embodiment. Even though each embodiment does not illustrate the curved blackboard, the invention does not limit the above-described embodiments and includes the curved blackboard as well. In case of the curved board, modification will be applied to the carriage to the necessary curvature, and the erasing devices and the wipers are modified to the curved board. All this modification is a scope of the claimed invention. The effect of the invention is explained. The present invention employs the washing liquid guide means for guiding the washing liquid to be rotated in spiral with respect to the erasing device, which prevents from scattering, thereby improving erasing efficiency. The invention also improves the operating speed of the erasing device, making the whole erasing apparatus m compact . In case of the dual erasing devices, either erasing device is m a direct contact with the board plate to perform erasing operation by friction, and the washing liquid is sprayed on the board plate by rotating the erasing device m indirect contact with board plate, improving erasing efficiency. The erasing direction of the erasing device and the wiper can be selected at the user' s need, m which any direction of movement guarantees the erasing operation on board. Further, the invention allows selective contact of the wiper. The wiper following the erasing device m accordance with the operating direction contacts with the board plate to wipe out the washing liquid, thereby preventing the wipers or board plate from abrasion. Further, the invention enables the wipers and the erasing device to be separated from the board plate m moving the erasing apparatus. This allows the partial erasing operation on the board. The erasing device is operated m engagement with the wipers m contact or separation, thereby making the erasing apparatus compact. Those skilled m the art will readily recognize that these and various other modifications and changes may be made to the present invention without strictly following the exemplary application illustrated and described herein, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.

Claims: CLAIMS 1. An apparatus for automatically erasing a blackboard, the blackboard erasing apparatus comprising: a carriage laterally supported on the board; erasing device (s) mounted on the carriage and rotated in contact with the board plate in erasing operation; washing liquid supplier for supplying the washing liquid on contact area of the erasing device and the board plate; washing liquid guide means for guiding the washing liquid to be rotated in spiral with respect to the erasing device; and wiper (s) mounted on the carriage to move on the board plate, for wiping out the remaining washing liquid following the erasing device. 2. An automatic blackboard erasing apparatus according to claim 1, wherein the wipers are installed in opposite of the erasing device (s) and the wiper actuating means enables the right and left wipers to perform contact or separation operation during erasing operation in accordance with the direction of erasing operation and to be separated from the board plate during movement and suspension period. 3. An automatic blackboard erasing apparatus according to claim 1, further including an erasing device actuating means for making the erasing device contact with the board plate during erasing operation and be separated from the board plate during movement and suspension period. 4. An automatic blackboard erasing apparatus according to claim 1, further including a forced drier for drying the remaining washing liquid on the surface of the board plate after wiping operation by the wipers. 5. An automatic blackboard erasing apparatus according to any of claims 1 to 4, further mcluding a variable contact means for controlling contact and/or separation operation of the wipers and the erasing device. 6. An automatic blackboard erasing apparatus according to claim 2 or 3, further including an actuating control means for controlling the wiper actuating means or erasing device actuating means to make a wiper following the erasing device m contact with the board surface, and to make the other wiper ahead of the erasing device separate from the board surface during erasing operation. 7. An automatic blackboard erasing apparatus according to claim 5, further including an actuating control means for controlling the variable contact means to make a wiper following the erasing device m contact with the board surface and the other wiper ahead of the erasing device separate from the board surface. 8. An automatic blackboard erasing apparatus according to claim 6, wherem the actuating control means includes a means for moving the carriage to the desired position m non-erasmg operation movement. 9. An automatic blackboard erasing apparatus according to claim 5, wherem the forced drier holding a rotating fan the housing with the ventilation outlet is mounted on the carriage by axially supporting the upper and lower portions of the housing, the ventilation outlet of forced drier is selectively changed by rotating the housing to ventilate on the right board plate and the left board plate, and the actuating control means includes a means for actuating ventilation operation by rotating the housing of the forced drier m the ventilation direction on the board plate from any wiper following the erasing device . 10. An automatic blackboard erasing apparatus according to claim 6, wherem the forced drier holding a rotating fan m the housing with the ventilation outlet is mounted on the carriage by axially supporting the upper and lower portions of the housing, the ventilation outlet of forced drier is selectively changed by rotating the housing to ventilate on the right board plate and the left board plate, and the actuating control means includes a means for actuating ventilation operation by rotating the housing of the forced drier m the ventilation direction on the board plate from any wiper following the erasing device . 11. An automatic blackboard erasing apparatus according to claim 7 or 8, wherem the forced drier holding a rotating fan m the housing with the ventilation outlet is mounted on the carriage by axially supporting the upper and lower portions of the housing, the ventilation outlet of forced drier is selectively changed by rotating the housing to ventilate on the right board plate and the left board plate, and the actuating control means includes a means for actuating ventilation operation by rotating the housing of the forced drier m the ventilation direction on the board plate from any wiper following the erasing device. 12. An automatic blackboard erasing apparatus according to claim 5, further including a first eccentric axle having an eccentric rod rotatably mounted between the upper and lower ends on the carriage, a second eccentric axle disposed in parallel with the first eccentric axle, and variable plates outwardly inserted on the opposite eccentric axles and being close to the board plate depending on the rotational direction at the neutral state of the first and second eccentric axles, wherein the right wiper is mounted at the right of the variable plates only when the right portion of the variable plates is moved to be close to the board plate, the left wiper is mounted at the left of the variable plates only when the left portion of the variable plates is moved to be close to the board plate, and the actuating control means controls rotation of the first and second eccentric axles to contact the right and left wipers. 13. An automatic blackboard erasing apparatus according to claim 12, wherein the variable plates are mounted on the upper and lower position of the first and second eccentric axles, and the erasing device is separated from the board plate in the non-actuating state of the variable plates at the neutral state of the first and second eccentric axles, and contacted in the actuating state of the variable plates to rotate the erasing device on the variable plates. 14. An automatic blackboard erasing apparatus according to claim 12, wherein a couple of erasing devices are mounted in parallel between the right and left wipers, a couple of variable plates are movably mounted in integral at the upper and lower portions of the first and second eccentric axles, the erasing devices are separated from the board plate in the non-actuating state of the variable plates at the neutral state of the first and second eccentric axles, the upper and lower ends of the erasing device are rotatably mounted at the intermediate position between the right and left ends of the variable plates to contact the left erasing device with the board plate and make separate the right erasing device from the board plate when the left variable plate is moved to the board plate and to contact the right erasing device with the board plate and make separate the left erasing device from the board plate when the right variable plate is moved to the board plate. 15. An automatic blackboard erasing apparatus according to claim 5, wherein the variable plate is axially supported on the carriage with respect to the axial center from the board surface and is moved forward and backward in accordance with the rotational direction at a predetermined neutral position, the right wiper is fixed on the variable plate at the right variable plate to move forward and contact with the board only when the variable plates are rotated at the neutral position; the left wiper is fixed on the variable plate at the left variable plate to move forward and contact with the board only when the variable plates are rotated at the neutral position; and the actuating control means controls rotation of the variable plates to contact the right and left wipers. 16. An automatic blackboard erasing apparatus according to claim 15, wherem a couple of the erasing devices are mounted in parallel between the right wiper and the left wiper and rotated m integral with a couple of the variable plates, the erasing devices are separated from the board plate at the neutral position of the variable plates, the upper and lower ends of the erasing device are rotatably mounted at the intermediate position between the right and left ends of the variable plates to contact the left erasing device with the board plate and make separate the right erasing device from the board plate when the left variable plate is moved to the board plate and to contact the right erasing device with the board plate and make separate the left erasing device from the board plate when the right variable plate is moved to the board plate. 17. An automatic blackboard erasing apparatus according to any of claims 1 to 4, further employing the actuating control means to control the erasing device actuating means or wiper actuating means by which the dual erasing devices are performed such that the forward erasing device m non-contact (or partial contact) with the board plate disperses the chalk powder and the backward erasing device in contact with the board plate erases the chalk powder . 18. An automatic blackboard erasing apparatus according to claim 5, further mcluding the actuating control means to control the variable contact means by which the dual erasing devices are performed such that the forward erasing device non-contact (or partial contact) with the board plate disperses the chalk powder and the backward erasing device m contact with the board plate erases the chalk powder. 19. An automatic blackboard erasing apparatus according to any of claims 1 to 18, wherem the carriage is moved or stopped by carriage moving means comprising a couple of rack gears mounted on upper and lower ends of the board, a couple of pinion gears mounted on upper and lower ends of the carriage, motor, and power transmission means m which power from the motor is divided to two paths to rotate the pinions in the same speed. 20. An automatic blackboard erasing method, comprising the steps of: supplying the washing liquid on the contact area of the board plate and the erasing device; rotating the erasing device m contact with the board plate with respect to the axle center; guiding the washing liquid by spiral rotation with respect to the erasing device to lower cohesion of the chalk powder from the board; detaching the chalk powder by fπctional rotation of the erasing device on the board plate; moving m lateral on the board the carriage on which the erasing device and the washing liquid supplier are mounted; and removing the washing liquid on the board plate by moving the wipers following the erasing devices to wipe the washing liquid. 21. An automatic blackboard erasing method according to claim 20, further including a step of drying the remaining washing liquid on the board plate after removing the washing liquid by the wipers. 22. An automatic blackboard erasing method according to claim 20, further including a step of controlling the wiper following the erasing device to be in contact with the board and the wiper ahead of the erasing device to be separated from the board. 23. An automatic blackboard erasing method according to claim 21, further including a step of controlling the wiper following the erasing device to be contact with the board and the wiper ahead of the erasing device to be separated from the board. 24. An automatic blackboard erasing method according to claims 21 to 23, further including a step of controlling the wipers and the erasing device to be separated from the board plate during non-erasing movement and suspension period. 25. An automatic blackboard erasing method according to claims 20 and 23, further including a step of controlling the ventilation direction depending on the erasing direction such that ventilation is made at the rear of the wiper following the erasing device m the drying step. 26. An automatic blackboard erasing method according to claims 20 and 23, further including a step of controlling the dual erasing devices to lower cohesion such that one erasing device at front m the erasing direction is separated to lower cohesion of the chalk powder and the other erasing device is to detach the chalk powder. 10 -

Inventors: JANG CHANG-HO, JANG, CHANG-HO
Applicants: JANG CHANG HO, JANG, CHANG-HO
Application year: 2000
Main classification: B43L21
IPC codes: B43L1, B43L21