Robotic service device and handling method

A pair of service devices for operation on a robotic picking system, the robotic picking system including two substantially perpendicular sets of rails including a first set rails and a second set of rails, the sets of rails forming a grid and at least one robotic load handling device operable thereon, the pair of service devices each comprising:

• a body mounted on two sets of wheels including a first set of wheels and a second set of wheels, the first set of wheels being arranged to engage with at least two rails of the first set of rails, the second set of wheels being arranged to engage with at least two rails of the second set of rails, the first set of wheels being independently moveable and driveable with respect to the second set of wheels such that only one of the first set of wheels and the second set of wheels is engaged with the rails at any one time thereby enabling movement of the service device along the rails to any point on the grid by driving only the set of wheels engaged with the rails;
• wherein the pair of service devices are connected by a connecting member, allowing the pair of service devices to act as a single unit,
• the connecting member being provided with connecting, latching and lifting means operable to lift a load handling device clear of the grid and tracks such that the pair of service devices assumes control of the load handling device and moves the load handling device to a position off the grid under control of the robotic picking system to enable recovery of the load handling device.