1.  Field of the Invention
The present invention relates to a sheet feeding device used in a sheet processing apparatus such as a copier or a printer, and in particular to the improvement of a sheet feeding device and a sheet processing apparatus using the same in which a stack of sheets lifted up by an elastic support member is successively fed beginning with the uppermost sheet. 
2.  Description of the Related Art
Generally, a sheet processing apparatus such as a copier or a printer is ordinarily disposed with a sheet feeding device that feeds sheets, such as paper sheets, to an image forming section serving as a sheet processing section.  An image formed in the image forming section is transferred to the sheets. 
Conventionally, as this kind of sheet feeding device, there has been a device that includes a sheet tray in which the sheets are accommodated, a sheet feeding unit is disposed above the sheet tray, and the sheets are successively fed by this sheet feeding unit from above. 
Here, as the sheet feeding unit, a unit disposed with a pickup roll for sending the sheets and a separating mechanism (e.g., configured by a feed roll and a retard roll) that separates the sent sheets one sheet at a time is often used. 
Incidentally, in this type of sheet feeding device, a bottom plate lift system is used in order to feed the sheets in the sheet tray. 
The bottom plate lift system is a system where a bottom plate is liftable disposed at the bottom portion of the sheet tray and the sheets are pushed up and supported by the bottom plate, whereby the uppermost sheet is pushed against the pickup roll of the sheet feeding unit, the sheet is sent by the pickup roll, and thereafter the sent sheets are guided to the separating mechanism so that they are separated one sheet at a time. 
JP-A-5-4733 (Examples, FIG.  5), JP-A-5-229674 (Configuration of the Invention, FIG.  1) and JP-A-11-29226 (Embodiments of the Invention, FIG.  1) disclose such a configuration. 
With respect to this type of bottom plate lift system, technology has already been proposed where a drive source such as a motor and a driving force transmission mechanism for transmitting the driving force from the drive source to the bottom plate are disposed, and in which the bottom plate is lifted up in accordance with the remaining amount of the sheets stacked on the bottom plate (e.g., see JP-A-5-4733 and JP-A-5-229674).  However, the drive source such as the motor, the driving force transmission mechanism and a sheet remaining amount detection system are required, which results in the system becoming expensive. 
Also, technology has already been proposed where the drive source such as a motor is replaced with an elastic member such as a spring (e.g., see JP-A-11-29226).  However, when handling sheets of various sizes and quality, the lift amount of the bottom plate becomes uneven due to differences in sheet weight, and the sheet feeding operation by the pickup roll easily become unstable. 
Thus, in order to effectively eliminate this drawback, the lift amount of the bottom plate must be minutely controlled, and there has been the technical problem that this control mechanism ends up being complicated.