  This paper describes the design and construction of the automatic calibration
unit (ACU) for the JUNO experiment. The ACU is a fully automated mechanical
system. It is capable of deploying multiple radioactive sources, an ultraviolet
(UV) laser source, or an auxiliary sensor such as a temperature sensor, one at
a time, into the central detector of JUNO along the central axis. It is
designed as a primary tool to precisely calibrate the energy scale of detector,
aligning timing for the photosensors, and partially monitoring the
position-dependent energy scale variations.
