| 摘要 |
PURPOSE:To prevent chattering phenomenon by switching a frequency immediately in response to a threshold level higher than a body detection level, and holding the level above the body detection level as far as a body is present even if a reflected wave and a detouring wave becomes opposite phase with each other momentarily. CONSTITUTION:A power amplifier 5 drives an ultrasonic wave transmitter 6 with a signal of one frequency of an oscillation circuit 4. The 2nd comparator 11 discriminates the output of an ultrasonic wave receiver 7 with a detection level highe than the detection level of the 1st comparator 10 and supplies a switching signal to a switch circuit 3. When the body 12 comes to before a transmission part, an ultrasonic wave transmitted by the transmitter 6 is reflected by the body 12 and when the signal after amplification 8 and detection 9 increases above a set threshold level, comparators 11 and 10 generate outputs according to respective threshold levels. Thus, the frequency is switched to obtain the level higher than the body detection level as far as the body 12 is present even if the reflected wave and detouring wave becomes opposite phase with each other, preventing chattering. |
