Method for controlling operation of cylinder apparatus

A method for controlling operation of a cylinder apparatus comprising a piston which is provided displaceably in a cylinder body, a ball screw which is engaged with said piston, and a driving section which drives and rotates said ball screw, wherein said piston is displaced by a driving force of said driving section and a pressure of a pressure fluid supplied to a cylinder chamber of said cylinder body to move a workpiece attached to said piston, said method comprising the steps of:

• applying a weight load to said piston, said weight load applied to said piston fluctuating during a displacement range of said piston;
• displacing said piston along said cylinder body by said ball screw under a driving action of said driving section;
• performing, during a portion of said displacement range, a step of supplying said pressure fluid to said cylinder chamber at a pressure corresponding to fluctuation of said weight load before said weight load applied to said piston is fluctuated so that a pressing force, which is substantially balanced with said weight load, is applied to said piston, and performing, during another portion of said displacement range, a step of detecting a driving torque of said driving section to compare said driving torque with a preset driving torque of said driving section set in a control unit, and calculating a pressure of said pressure fluid to be supplied corresponding to a difference between said preset driving amount and said driving amount detected to supply said pressure fluid to said cylinder chamber based on the calculated pressure;
• and moving said piston by said driving force brought about by said driving section and said pressing force brought about by said pressure fluid.