PURPOSE: To improve the accuracy of discrimination, by detecting thrust of a drill, differentiating the detected signal, and detecting the disappearance of annual rings at a deteriorated part through wave shaping.
CONSTITUTION: To detect a deteriorated part, the disappearance of a signal pattern in response to annual rings is detected. Thus, an output signal of an amplifying circuit 51 of the detected signal is differentiated with time to obtain differentiated output, this is compared with a reference level L2 or L3 to detect whether the output is lower than the L3 or more than the L2, shaping is made, a pulse wave is obtained and the periods Ti, Ti+1, Ti+2W are detected at a period detection circuit 54. That is, the maximum value of the period is predetermined and if the period Ti is greater than the value is detected. Thus, when the signal pattern in response to the annual rings is disappeared, a deterioration detecting signal (d) is outputted. Then, the accuracy of discrimination can be increased.
KAMEOKA TAKAHITO
TAKAHASHI KENICHIROU
