Automated bot transfer arm drive system

A method for operating an automated storage and retrieval system, the method comprising:

• providing a storage space with at least one level having at least one storage rack with rack storage locations;
• providing at least one autonomous transport vehicle having a transfer arm including at least one finger and a movable finger support member, the at least one finger being movably coupled to the movable finger support member, the at least one autonomous transport vehicle and the at least one storage rack being disposed so as to define random access storage to each rack storage location of the at least one storage rack;
• moving the movable finger support member in a first direction relative to a reference frame of the at least one autonomous transport vehicle and moving the at least one finger linearly relative to the movable finger support member in a second direction substantially perpendicular to the first direction in the reference frame of the at least one autonomous transport vehicle, the at least one finger having set different incremental indexed positions of the at least one finger in the second direction between ends of a range of motion in the second direction;
• and
• selectably varying, with a controller, a size of the transfer arm conforming to pickface size of a pickface transferred to and from the rack storage locations by indexing the at least one finger between the set incremental indexed positions between ends of the range of motion in the second direction that is different than the incremental indexed position set for another of the at least one finger movably coupled to the movable finger support member.