Surface flatness measuring apparatus

A flatness measuring apparatus for measuring flatness of a surface of an electrically conductive disk comprising:

• a measuring table for removably holding thereon a disk, said measuring table having a projection for being engagedly received in a central through hole of said disk;
• a sensor head arranged along said measuring table with a predetermined gap between itself and the surface of said disk held on said measuring table so as to be able to linearly reciprocate relative to said disk along lines parallel to the surface of said disk;
• a plurality of eddy current undulation sensors arranged in said sensor head to detect undulations on a surface of said disk as the sensor head is linearly reciprocated relative to said disk;
• a control section for drawing a flatness map for the surface of said disk based on signals from a position detector for detecting the position of said sensor head and on output signals from the eddy current undulation sensors;
• each of said eddy current undulation sensors having an excitation coil excitable by an AC current to create a magnetic field passing through and creating eddy currents in said disk, and also having a detection coil arranged so that the magnetic field also passes through the detection coil and generates in the detection coil said output signals;
• and
• an inverting means adapted to be able to horizontally reciprocate to a position right above said measuring table and invert said disk by holding said disk so as to automatically measure flatness of each of opposite sides of said disk.