A power assist device (110) for assisting a user to operate an object and a method of operating a power assist device (110) are provided. The power assist device (110) includes a pressure sensor (70) configured to detect an operating force applied to the object by the user; a motor (90) configured to output and apply a motive force to the object; and a controller (80). The controller (80) is configured to record a history of the operating force detected by the pressure sensor (70); detect a periodically fluctuating component of the operating force based on the history of the operating force; calculate a component of a current operating force caused by motion of the user operating the object; calculate a correction value that excludes the component caused by motion of the user from the current operating force; calculate a power assist force based on the correction value; and output the power assist force as the motive force of the motor (90).
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