Robotic service device and handling method

A grid frame for a storage system, the grid frame comprising:

• a top level comprising:
• a plurality of X-direction rails configured to engage a first pair of wheels and a second pair of wheels of a first load handling device to guide movement of the first load handling device in an X-direction along the top level, the plurality of X-direction rails comprising a first X-direction rail and a second X-direction rail, the first X-direction rail comprising a first part for travel on by the first pair of wheels and a second part for travel on by a pair of wheels of a second load handling device, the first part and the second part of the first X-direction rail each having a width that is at least a width of one of the first pair of wheels;
• and
• a plurality of Y-direction rails perpendicular to the plurality of X-direction rails and configured to engage a third pair of wheels and a fourth pair of wheels the first load handling device to guide movement of the first load handling device in a Y-direction perpendicular to the X-direction along the top level, the plurality of Y-direction rails comprising a first Y-direction rail and a second Y-direction rail, the first Y-direction rail comprising a first part for travel on by the third pair of wheels and a second part for travel on by a pair of wheels of a third load handling device, the first part and the second part of the first Y-direction rail each having a width that is at least a width of one of the third pair of wheels;
• and
• a three-dimensional storage structure configured to store stacks of containers beneath the top level,
• wherein:
• the first X-direction rail, the second X-direction rail, the first Y-direction rail, and the second Y-direction rail define an opening above one of the stacks of containers, the opening being on an opposite side of the first part of the first X-direction rail from the second part of the first X-direction rail and being on an opposite side of the first part of the first Y-direction rail from the second part of the first Y-direction rail,
• the first X-direction rail has a first X-direction lip and a second X-direction lip that extend in the X-direction and protrude from the first X-direction rail on opposite sides of the first X-direction rail, and
• the first Y-direction rail has a first Y-direction lip and a second Y-direction lip that extend in the Y-direction and protrude from the first Y-direction rail on opposite sides of the first Y-direction rail.