Method and apparatus for retrieving units from a storage system

A load-handling device for lifting and moving containers stacked in stacks in a storage system, the system having a plurality of rails or tracks arranged in grid pattern above the stacks of containers, the grid pattern having a plurality of grid spaces, each stack being located within a footprint of only a single grid space, the load handling device being configured to move laterally on the rails or tracks above the stacks, the load handling device comprising:

• a lifting mechanism, the lifting mechanism having a gripper device configured to grip a container from above;
• a vehicle a body having an upper portion and a lower portion, the upper portion for housing components such as power components, and/or control components, and/or drive components and/or lifting components, and the lower portion arranged directly beneath the upper portion, the lower portion having a cavity for accommodating a container and a wheel assembly, the wheel assembly having a first set of wheels for engaging with a first set of rails or tracks to guide movement of the device in a first direction and a second set of wheels for engaging with a second set of rails or tracks to guide movement of the device in a second direction, wherein the second direction is transverse to the first direction;
• the lifting mechanism being configured to raise and lower the gripper device relative to the cavity, the lifting mechanism being located above the cavity;
• and
• the wheel assembly having a wheel positioning mechanism, the wheel positioning mechanism having means for selectively engaging either the first set of wheels with the first set of rails or tracks or the second set of wheels with the second set of rails or tracks thereby enabling the load handling device to selectively move in either the first or second direction across the grid.