Load handling device

A method of controlling movement of a load handling device on a grid structure, the grid structure including a plurality of grid members arranged in a grid pattern, and the load handling device including:

• a) a container receiving space arranged to be located above a grid structure when in operation;
• b) a lifting mechanism configured and arranged to lift a container from a stack into the container receiving space, said lifting mechanism including a driving mechanism configured for driving the lifting mechanism;
• c) a vehicle body including an upper part and a lower part, said upper part of the vehicle body housing the driving mechanism;
• d) a wheel assembly configured and arranged to support the vehicle body, the wheel assembly including a first set of wheels configured for engaging with a first set of grid members to guide movement of the load handling device in a first direction, and a second set of wheels configured for engaging with a second set of grid members to guide the movement of the load handling device in a second direction, wherein the second direction is transverse to the first direction, the first and second sets of wheels being arranged around the container receiving space in the lower part of the vehicle body, wherein each wheel of the first set of wheels and the second set of wheels is rotatable about a respective first axis of rotation and a respective second axis of rotation, wherein the respective second axis of rotation is radially offset from the respective first axis of rotation;
• and wherein said radial offset is less than or equal to a radius of each wheel of the first set of wheels and the second set of wheels, and
• e) a drive mechanism for selectively driving rotation of the first set of wheels and the second set of wheels about their respective first axes of rotation;
• wherein each wheel of the first and second set of wheels includes a wheel positioning mechanism configured to rotate the wheel about its respective second axis of rotation to selectively lower or raise the first set of wheels or the second set of wheels relative to the vehicle body, and thereby to selectively engage or disengage the first set of wheels with a first set of grid members or the second set of wheels with a second set of grid members when in operation, the method comprising:
• rotating each wheel of the first or second set of wheels about its respective second axis of rotation so as to raise or lower the respective wheels relative to the vehicle body and engage or disengage the first set of wheels with the first set of grid members or the second set of wheels with the second set of grid members.