This paper describes the correction of 600 tons gantry system. The principle is the use of absolute encoders crane frame measuring the deflection of the orbit, through PLC-controlled inverter drive motor to adjust the speed on both sides. The frame automatically correct the deviation. Position detection for the encoders, absolute multi-turn encoder is the most suitable, it does not accumulate error. Power will not be lost after removal of the location information, the other encoders do not have this feature. The signal transmission is also an important component of the system, optical fiber has unparalleled advantage， such as communication capacity, transmission distance, Low loss，Signal crosstalk small, confidential performance, so that the operation of carts quickly sent to ABB encoder module. The gantry has three corrective methods: automatic correction, manual correction, correcting electromagnet.
