Abstract: A method and apparatus are disclosed for modifying command signals representing predetermined positions of a machine element relative a workpiece as a function of measurable unprogrammed changes in the relative position therebetween. The machine element is responsive to a servomechanism circuit which in turn responds to a control device producing the command signals to command motion of the machine element to programmed points relative to the workpiece. The servomechanism circuit produces interrupt signals for transferring the command signals thereto. A measuring circuit is responsive to the unprogrammed change in relative position between the machine element and the workpiece and is operative to produce feedback signals representing the direction and magnitude of the change in relative position. A first circuit is responsive to the measuring circuit for producing a first signal representing a current unprogrammed change in relative position with reference to a predetermined point.