GB1492401A - Surface measuring instruments - Google Patents

Abstract

1492401 Determining the centre of curvature of a body RANK ORGANISATION Ltd 18 Sept 1975 [24 Sept 1974] 41561/74 Heading G1N The invention relates to a method of determining the co-ordinates 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 comprising a table on which the workpiece is mounted and the sensor, the table and sensor being relatively rotatable about a fixed axis. The method comprises the steps of traversing the sensor over the workpiece surface and deriving from the sensor output three signals representing the value of the radial distance of the workpiece from the fixed axis at three points along the traverse of the sensor the middle point lying at the intersection of one of the axes of the predetermined frame of reference and the workpiece surface, and the other two points extending an angle Ï with the middle point at the fixed axis, and deriving from these values two output signals representing the expressions, and where x and y respectively represent the coordinates of the centre of curvature of the workpiece parallel to and perpendicular to the said one axis and r 1 , r 2 , r 3 represent the radial distance at the three points in order along the traverse. For a workpiece with a rough surface the value of the radial distance of each point may be the average distance over a range about the point. These ranges should not overlap but are preferably contiguous. The method may be used for centring a workpiece on the table, the centring being effected automatically on the co-ordinates of the centre of curvature thereof being evaluated. For accuracy after the first "centring" the method may be again carried out, followed by a second "centring" the process being repeated as required. The apparatus for carrying out the method comprises a potentiometer 21 which monitors the rotation of the motor spindle and four connected potentiometers 22, 25, 33, 34, the first 22 setting the angle Ï and with the potentiometer 21 controls the motor, the second 25 providing the value of 0 to a computing portion of the apparatus and the third 33 and fourth 34 deriving the values sin Ï and 1 - cos Ï respectively. Integrators 30 and 31 under the control of integrator switching and scaling circuit 27 derive from the range of sweep of the sensor about each point the values of r 1 , r 2 and r 3 , the output of integrator 30 representing the expression 2r2 - (ri + r3) and the output of integrator 31 representing the expression r 3 -r 1 . These representations are scaled by the outputs of potentiometers 34 and 33 respectively and the values of y and x so produced are then displayed.