High precision reading device of the graduations of a precision ruler

Claim 1

A measuring machine comprising a supporting stand carrying a movable member which can be displaced with respect to the supporting stand in at least one direction, a motor having a servo-control device for imparting to said movable member its movements, means alternately controlling said servo-control device, said alternate control means comprising a numerical control device comprising a reversible counter and an analog control device that acts on said servo-control device through a commutation device, said analog control device comprising a precision scale carried by the movable member, said precision scale having graduation marks thereon, a photoelectric microscope having an optical sighting device having a rotating field diaphragm movable between positions corresponding to portions of the measuring field centered on two different graduation marks of the precision scale, an impulse generator rotatable with the field diaphragm and supplying control signals to the reversible counter producing the algebraic subtraction from the value recorded in said counter of a number of units equal to the linear displacement of the movable member corresponding to the angular displacement of the rotating diaphragm, a coupling controlled by the reversible counter for coupling the motor and movable member to the rotating field diaphragm, means rigidly coupling the motor to the field diaphragm when the reversible counter is at zero and uncoupling the motor from the field diaphragm when the counter is at otHer than zero, and means connecting the servo-control device to the numerical control device through the commutation device when the reversible counter is at zero and connecting said servo-control device to the analog control device when the counter is at other than zero.