LENGTH MEASURING APPARATUS USING ULTRASONIC

PURPOSE:To achieve a higher reliability of measurement, by forbidding the sampling of results of measurement with a time measuring circuit when a reflected wave is not received within a specified time range to remove an abnormal measured value. CONSTITUTION:A probe 11 is driven with a transmitting circuit 12 to transmit an ultrasonic wave. The reflected ultrasonic wave is received with the probe 11, the output of which is fed to a time measuring circuit 14 through a receiving circuit 13. A received wave monitoring circuit 31 monitors the presence of a receiving trigger from the receiving circuit 13 due to the received wave until a fixed time passes after the reception of a transmitting wave and outputs a received wave detection signal when the receiving trigger is inputted. In response to the received signal detection signal, a sample holding circuit 16 starts the sampling of the output from the time measuring circuit 14. In addition, malfunctioning due to a jamming wave can be prevented with a disturbance counter measure circuit 40.