Sheet stacker and image forming apparatus

A sheet stacker comprising:

• a lifting mechanism;
• a tray configured to stack a plurality of sheets;
• a conveyor disposed above the tray to convey a sheet in a conveyance direction;
• processing circuitry configured to control, via the lifting mechanism, a height position of the tray on which the plurality of sheets conveyed by the conveyor is stacked;
• an upper-limit sensor configured to detect that an uppermost sheet of the plurality of sheets stacked on the tray has reached an upper-limit position;
• a lift pause sensor configured to detect that the uppermost sheet of the plurality of sheets stacked on the tray has reached a lift pause position lower than the upper-limit position;
• a lower-limit sensor configured to detect that the tray has reached a lower-limit position;
• a lowering pause sensor configured to detect that the tray has reached a lowering pause position higher than the lower-limit position, the upper-limit sensor, the lift pause sensor, the lower-limit sensor, and the lowering pause sensor disposed in a space in which the tray is lifted and lowered;
• and
• a lift operation device configured to instruct the processing circuitry to lift and lower the tray, and issue an operation instruction to the processing circuitry in response to a predetermined operation, the operation instruction including an instruction to lift the tray and an instruction to lower the tray,
• wherein the processing circuitry is configured to:
• cause the tray to be lifted or lowered in accordance with the operation instruction;
• continue lifting or lowering the tray even if the operation instruction is not issued after the instruction to lift the tray or the instruction to lower the tray is once issued based on one operation of the lift operation device;
• stop lifting or lowering the tray when the lift pause sensor or the lowering pause sensor detects that the tray has reached the lift pause position or the lowering pause position;
• and
• after stopping lifting or lowering the tray, cause the tray to be lifted beyond the lift pause position or lowered beyond the lowering pause position only while the operation instruction is issued.