Patent ID: 7565841

Claim:
A method for selecting an operating frequency for a hardness measurement system for measuring a hardness of an object from a change in frequency that occurs according to the hardness of the object and comprising a hardness sensor having a vibrator for applying vibrations to the object and a vibration detection sensor for detecting signals reflected from the object, and a phase shift circuit, which is connected in series with an amplifier to the hardness sensor, for changing a frequency and shifting a phase difference thereof to zero when a phase difference occurs between an input waveform to the vibrator and an output waveform from the vibration detection sensor, the method comprising: a peak detection step for detecting a plurality of peaks and distinguishing each peak in amplitude characteristic with respect to frequency or phase characteristic with respect to frequency, under a condition that the hardness sensor is in a free end state and not in contact with a test piece; a free end characteristic acquisition step for measuring and storing in a memory at least one of frequency, phase, or amplitude that changes at each peak position for the distinguished peaks; a contact characteristic acquisition step for measuring and storing in a memory at least one of frequency, phase, or amplitude that changes for each peak of the distinguished peaks when the hardness sensor is placed into contact with the test piece; a peak selection step for selecting a peak for use in hardness measurement on the basis of at least one of frequency change, phase change, or amplitude change between the free end characteristic acquisition step and the contact characteristic acquisition step for each peak of the distinguished peaks; and an operating frequency setting step for setting the frequency of the free end state of the selected peak as the operating frequency of the hardness sensor and setting a frequency separated by an arbitrary frequency width from the operating frequency of the hardness sensor as the operating frequency of the phase shift circuit.