Improper selection of zero return switch; while selecting block switch, because of different performances of block switch, some are tight and some loose on the machine, which makes the upspring sometimes fast and sometimes slow and further the positioning deviate. As for servo system, normally zero return ends after it detects the first revolving signal. But due to the slow response of the machine, it might stops after the second or third revolving signal is detected, causing positioning error. Magnetic switch with poor sensitivity may cause incorrect zero return.
Select quality zero return switch. If it is mechanical switch, check its mechanic performance; if it is inductive switch, then select one with high sensitivity so as to make sure reliable zero return signal