The present invention relates to an ink tank for containing ink and an ink jet printer incorporating the ink tank as an ink supply source, and more particularly to an ink tank having a mechanism capable of precisely detecting a condition where ink has run out (an ink end).
Among those for use in ink jet printers is a known ink tank of such a type having the ink absorbed by and held in an ink absorbent material such as foam and felt. A foam-type ink tank, for example, has a container in which foam that has absorbed and held ink is contained therein, an ink outlet communicating with the foam container, and an vent port communicating with the atmosphere for opening the foam container into the atmosphere. When ink is sucked from the ink outlet by the ejection pressure of an ink jet head, air corresponding to the sucked amount of ink is caused to flow into the foam container.
In the case of such a foam-type ink tank, the calculation of the consumed amount of ink is carried out according to the number of ink dots ejected from the ink jet head, the sucked amount of ink through an ink pump for sucking ink from the ink jet head and so forth, so that the detection of the presence or absence of ink therein is made according to the calculated results.
Incidentally, a condition where ink in the ink tank has almost run out is generally called a “real end” and a condition where a residual amount of ink in the ink tank has decreased to an amount smaller than the predetermined amount is called a “near end.” However, an “ink end” used in this specification includes both the conditions above unless otherwise specified.
However, the method of detecting the ink end by calculating the consumed amount of ink and the like has the following problem. Since the ejected amount of ink from the ink jet head and the sucked amount of ink through the ink pump undergo wide variation, the consumed amount of ink that has been calculated according to the above amounts also shows a variation far greater than that of the actually consumed amount of ink. Therefore, a great margin needs setting in order to settle the ink end. Consequently, a greater amount of ink may be left at a point of time that the ink end is detected, whereby ink may often be wasted.
Therefore, with a back surfaces of a reflective face of a prism as an interface with respect to ink, it is conceivable to directly detect the ink end by an optical detection system utilizing optical characteristics in that the reflective face of the prism is restored as its was when ink is used. For example, Japanese Patent Publication No. 10-323993A and U.S. Pat. No. 5,616,929 disclose such a detection system.
In the case of a foam-type ink tank, however, ink absorbed by and held in the ink absorbent material (foam) is always kept in contact with the reflective face of the prism even though the back surfaces of the reflective face of the prism is so arranged as to be exposed in the foam container, the reflective characteristics of the prism remain unchanged even when ink has run out. Consequently, the above disclosed detection system is not directly applicable to the foam-type ink tank.
It is also conceivable to adopt an arrangement wherein air is introduced into a sub ink chamber under pressure control with ink in the main ink chamber consumed to a certain degree by forming such a sub ink chamber that is small in capacity and capable of storing ink between the main ink chamber (foam container) and an ink outlet, and by disposing the reflective face of the prism in the sub ink chamber to make the back surfaces of the reflective face an interface with respect to ink.
Accordingly, when the amount of ink left in the main ink chamber decreases, bubbles become introduced from the main ink chamber into the sub ink chamber every time ink is supplied from the ink outlet into the ink jet head. When ink in the main ink chamber is completely used, the residual amount of ink in the ink tank comes to be substantially equal to only the amount of ink left in the sub ink chamber. As the residual amount of ink in the sub ink chamber decreases in amount further, the back surfaces of the reflective face of the prism as the interface with respect to ink is exposed from the liquid level of ink and the reflective condition of the reflective face changes. In other words, the reflective face kept from serving as a reflective face while the back surfaces thereof is covered with ink gradually recovers its reflective function with the liquid level of ink going down. Therefore, the condition where the residual amount of ink has decreased to the predetermined amount or smaller is detectable according to the amount of reflected light on the reflective face. Consequently, the ink end is detectable at a point of time the residual amount of ink has substantially completely used by making the capacity of the sub ink chamber sufficiently small.
However, the air introduced into the sub ink chamber causes bubbles to be generated in the sub ink chamber. In case there exists a condition where bubbles are adhered to or floating around the back surfaces of the reflective face of the prism, a condition where the reflective face of the prism is covered with the ink held among bubbles is maintained even when the liquid level of ink becomes lower than the reflective face of the prism. Consequently, the reflective condition of the reflective face of the prism will not change even though the liquid level of ink lowers. As it takes much time until bubbles covering the reflective face of the prism fade out, there occurs nonconformity in that the ink end is not detected until then. Hence, the detection timing of the ink end is delayed and this causes a harmful effect such as dot missing because bubbles are sent to an ink jet head as a result of lost suction of ink.