Closed gantries are similiar to a very precise crane. They have two X-beams that support a moving Y bridge. The Y bridge has a moveable carriage that supports a vertically moveable rigid Z-beam. The end of arm tooling attaches to the bottom of the Z-beam, The EOA tool, or End Effector, can be automatically controlled within the XYZ travel range.
Legs support the X-beam. Closed gantries can operate over large work spaces, perform many tasks in that envelope and position parts to an accuracy of typically 0.3mm. Payload capacity can be thousands of pounds.
Sage Engineers have pioneered the designs of four generations of gantry systems since the mid 80's, and have continually improved flexibility, reliability, speed, payload, and precision. Your project will benefit from this experience.