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Until well into the twentieth century, most devices developed for measuring distance worked on the same principle comparison of the measure distance with a standard unit of length. Other means are now available. One of these is the measurement of time taken by a sound wave to cover a certain distance. This sound normally lies beyond human hearing.

          The ultrasonic range finder presented here is suitable for measuring distance between 30cm and about 3m. The measured distance is shown on a 2line-16digit LCD.

Operation:

                This schematic block shows the four major parts of the meter a sender a receiver timing and time reference section and a counter with display.

            The transduction element emits burst of 12 pulses at a frequency of about 40 KHz. The frequency is roughly identical with the resonance frequency of the two transducer, so that some sort of selectivity is obtained

At the sensing element. As soon as the first burst is emitted a bitable is actuated which enable the counter. Immediately after the burst has been

Emitted, the unit is switched to reception. The sensitivity of the receiver is a function of time. During and immediately after emission of the burst, the sensitivity is low.

          Crosstalk between, the transduction and sensing elements has therefore no effect on the operation of the unit. If an echo is received very soon after cessation, of the emitted burst, it will be sufficiently strong to be processed by the receiver in spite of the very low sensitivity. An echo that takes a longer time to reach the sensing element will be weaker but by then the sensitivity of the receiver has become higher.

          The upshot of this arrangement is that reliable measurement, unaffected by spurious reflections and crosstalk, may be made with relatively simple means. At the instant the echo is sensed, the bistable is reset and counter state transferred the to the output latch. Since the clock frequency is 17.05 KHz and the velocity of sound under normal atmospheric conditions may be taken as 341m the period of the clock is equal to the time taken by the burst to travel 2cm i.e. 1 cm forward and 1 cm back. This means that the number of clock pulses counted between the onset of emission of the burst and the sensing of the echo is equal to the number of centimeters between the transducers and the reflecting surface.

 thanks
R.ASHOK KUMAR
MEGATECH
7,KANNAN AVENUE 1ST STREET
MUDICHUR ROAD, OLD PERUNGALATHUR
TAMBARAM-CHENNAI-63
(NEAR HERO SHOWROOM)
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044-22760866

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