| 摘要 |
PURPOSE:To achieve a highly accurate measurement, by comparing, with a threshold, received signal levels of reflected ultrasonic waves from an object to be measured and an object to be compared to prevent adverse effect from the reception of a secondary reflected wave positively. CONSTITUTION:An ultrasonic transmitter/receiver 5 transmits or receives an ultrasonic wave intermittently and received signal levels of reflected ultrasonic waves from an object to be measured and an object to be compared are compared with a threshold by a comparator 4 to produce an output when the received signal level is higher than the threshold. An automatic threshold adjusting means is made up of an integrator 1 comprising a resistance R1, an operational amplifier OP1 and a capacitor C1, a one-shot multivibrator 2 which receives an input of a sensing gate signal GS to output a one-shot signal OS at the rising thereof and an adder 3 comprising variable resistances R31, R32 and DELTAR3, an operational amplifier OP2 and a resistance R4. The threshold shall be set higher than the received signal level of a secondary reflected wave from the object being measured based on the preceding wave transmission at the timing of receiving the wave but lower than the received signal levels of a primary reflected wave from the object being measured and a reflected wave from the object being compared based on the current wave transmission at the timing of receiving the waves.
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