A method for calibrating a positional relation between a main spindle and a contact tool sensor attached to a table in a machine tool. The method includes: mounting a reference tool on the main spindle and obtaining measurement position coordinates as respective tool sensor measurement values, the measurement position coordinates being of a distal end of the reference tool in at least two different measurement regions on an upper surface of the contact tool sensor; outputting predetermined difference values based on the respective tool sensor measurement values; determining an abnormality when the difference values are compared with preliminarily set acceptable values and at least one of the difference values are out of the acceptable values; and calibrating a positional relation between the main spindle and the contact tool sensor based on the respective tool sensor measurement values when the abnormality is not determined at the determining of abnormality.

A sensor detection error at a joint of a robot arm is correctly detected. A joint structure that joins links and of a robot arm includes a sensor for determining force acting between the links. A driving apparatus that generates a driving force of a joint includes first and second driving parts. A constraining part that constrains the joint movable in a driving direction of the joint and be unmovable in another direction includes first and second supporting parts that are movable relative to each other in the driving direction of the joint. The driving part of the driving apparatus is fixed to the link, and the supporting part of the constraining part is fixed to the link. Also, the supporting part of the constraining part is fixed to the driving part of the driving apparatus. The sensor is fixed so as to link the supporting part and the link.

A sensor detection error at a joint of a robot arm is correctly detected. A joint structure that joins links and of a robot arm includes a sensor for determining force acting between the links. A driving apparatus that generates a driving force of a joint includes first and second driving parts. A constraining part that constrains the joint movable in a driving direction of the joint and be unmovable in another direction includes first and second supporting parts that are movable relative to each other in the driving direction of the joint. The driving part of the driving apparatus is fixed to the link, and the supporting part of the constraining part is fixed to the link. Also, the supporting part of the constraining part is fixed to the driving part of the driving apparatus. The sensor is fixed so as to link the supporting part and the link.