Patent ID: 7757526

Claim:
A method for changing a force control gain expressed as a ratio of a force error to a velocity command value in a control circuit performing a force control of a servo motor driving a die cushion disposed opposite to a slide in a press machine, said method comprising: obtaining a difference between a displacement of said slide and a displacement of said die cushion in a state in which said slide and said die cushion collide with and are exerting a force on each other; detecting said force when said difference between said displacement of said slide and said displacement of said die cushion is obtained; calculating a spring constant of a structural portion including said slide and said die cushion based on a proportionality between said difference between said displacement of said slide and said displacement of said die cushion and said force, when the structural portion is regarded as an elastic structure; and changing said force control gain based on said spring constant using a proportionality between said force control gain and a reciprocal of said spring constant.