Image forming apparatus and control method for controlling sheets fed from a detachable sheet feeding unit using detected sheet intervals

An image forming system comprising;

• an image forming device configured to form an image on a sheet;
• a sheet feeding device connected with the image forming device, and configured to feed a sheet to the image forming device;
• a first sensor provided at the sheet feeding device and configured to detect a sheet fed to the image forming device at a first position on a sheet conveyance path and output a signal regarding an existence or nonexistence of the sheet;
• a second sensor provided at the image forming device and configured to detect a sheet fed from the sheet feeding device at a second position which is downstream of the first position in a sheet conveyance direction and output a signal regarding an existence or nonexistence of the sheet;
• and
• a controller provided at the image forming device and configured to control operation of the image forming device based on the signal outputted from the first sensor and the signal outputted from the second sensor,
• wherein the sheet feeding device is further configured to feed a first sheet and a second sheet following the first sheet to the image forming device such that there is an interval between a trailing edge of the first sheet and a leading edge of the second sheet in the sheet conveyance direction, and
• wherein the controller is further configured to continue the operation of the image forming device in a case where the controller can not detect the interval based on the signal outputted from the first sensor and the controller can detect the interval based on the signal outputted from the second sensor even though the first sheet and the second sheet pass the first position and the second position while maintaining the interval.