| 摘要 |
PURPOSE:To make it possible to prevent error detection, by a method wherein the reference voltage value of a detector circuit is progressively reduced from a predetermined value in a predetermined ratio with the lapse of time and this operation is repeated at every generation of the clock pulse. CONSTITUTION:The reference voltage Vs of a detecting circuit is set to formula Vs=f(t)-Vs2 (0<=t<=T1) (wherein f is exciting frequency, T1 is a definite cycle and Vs2 is positive constant) from a distance (l) to an obstacle 6, a time (t) required in reciprocating ultraviolet waves over the distance (l) and a relation formula t=2l/v (v is acoustic velocity). Directly after the clock pulse 2a of the aforementioned reference voltage Vs is generated, that is, when (t) is zero, the value is shown by a formula Vs=Va(L1+a)-Vsa and reduced from this value with the lapse of time and, when (t) is T1, the value comes to formula Vs= Va.a-Vsa. In the formula, Va and (a) are constant L1=VT1/2. As a mentioned above, because the reference voltage Vs is reduced with the elapse of time corresponding to the reducing ratio of an output signal 8a after the clock pulse is generated, a signal conventionally detected in error is not detected. |
