1. Field of the Invention
The present invention relates to a printing apparatus, for example to an inkjet printing apparatus which prints by discharging a liquid such as ink.
2. Description of the Related Art
In inkjet printing apparatuses, to realize a high-quality image, a high conveyance accuracy of a sheet-like printing media (in the present specification, simply referred to as a “sheet”) is required. Recently, to control conveyance with a higher degree of accuracy, direct sensors are being realized which directly detect a movement amount of the sheet by imaging the sheet surface and then performing image processing. For example, U.S. Pat. No. 7,104,710 discusses a technique for controlling conveyance by using a direct sensor. In the apparatus discussed in that document, the direct sensor is provided on a carriage which mounts a print head, or at a position facing a discharge port surface of the print head.
However, in this configuration the sheet can only be imaged at a position where the direct sensor is fixed in the sheet conveyance direction. Therefore, during sheet conveyance, the sheet may not be present at a sensing position of the direct sensor in a certain period (hereinafter, referred to as “sensing disabled period”). For example, in a case where an image is printed by a multipass method during printing on a trailing edge or a leading edge of the sheet, if the sheet edge portion deviates from the sensing position during printing, thereby making sensing impossible, conveyance cannot be controlled with a high degree of accuracy. As a result, there is the problem that the quality of that portion cannot be guaranteed.