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Parker 590 speed error

Dear All, 

I have Parker 590P it working with motor very well when working with normal speed as speed demand 8 % and FBK was 8 % and error 0.01 to 0.03 

but when speed demand 0.4% speed error increase to playing from 0.2 to - 0.1 that affect on machine accuracy and motor sound 

any one have an idea why when working on low speed error increase ????????