Surface measurement instruments

Apparatus for determining the coordinates of the centre of curvature of the curved surface of a workpiece with respect to a predetermined frame of reference the origin of which coincides with the centre of relative rotation between the workpiece and a sensor of a surface measurement instrument of the type having a table on which a workpiece is mounted and a sensor which is drawn in contact with the surface of the workpiece to produce output signals representative of surface variations upon relative rotation of the sensor and the table on which the workpiece is mounted,

• said apparatus comprising:
• means for traversing the sensor over the workpiece surface whereby to provide an output signal representing said surface,
• means for deriving from said sensor output signal three signals representing the value of the radial distance of said surface of said workpiece from said centre of relative rotation at three points along the traverse of said sensor, the middle point lying at the intersection of one of the axes of said predetermined frame of reference and the workpiece surface, and the other two points subtending at the centre of relative rotation an angle .phi. with the middle point, and
• means for deriving from said three signals two output signals representing the expressions ##EQU10## where x and y respectively represent the coordinates of the centre of the workpiece parallel to and perpendicular to said one axis and r.sub.1, r.sub.2, r.sub.3 respectively represent the radial distance determined at said three points in order along said traverse.