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ULTASONIC RANGING IN AIR

G. E. Rudashevski and A. A. Gorbatov UDC 534,321.9:531.71.083.7

One of the most important problems in instrumentation technology is the

remote,contactless measurement of distances in the order of 0.2 to 10 m in air.Such a

problem occurs,for instance,when measuring the relativethre edimensional position of

separate machine members or structural units.Interesting possibilities for its solution

are opened up by utilizing ultrasonic vibrations as an information carrier.The physical

properties of air,in which the measurements are made,permit vibrations to be

employed at frequencies up to 500 kHz for distances up to 0.5 m between a member

and the transducer,or up to 60 kHz when ranging on obstacles located at distances up

to 10 m.

The problem of measuring distances in air is somewhat different from other

problems in the a -pplication of ultrasound.Although the possibility of using acoustic

ranging for this purpose has been known for a long time,and at first glance appears

very simple,nevertheless at the present time there are only a small number of

developments using this method that are suitable for practical purposes.The main

difficulty here is in providing a reliable acoustic three-dimensional contact with the

test object during severe changes in the air's characteristic.

Practically all acoustic arrangements presently known for checking distances use

a method of measuring the propagation time for certain information samples from the

radiator to the reflecting member and back.

The unmodulated acoustic(ultrasonic)vibrations radiated by a transducer are not

in themselves a source of information.In order to transmit some informational

communication that can then be selected at the receiving end after reflection from the

test member,the radiated vibrations must be modulated.In this case the ultrasonic

vibrations are the carrier of the information which lies in the modulation 24411dLtgePPLradrsignal,i.e.,they are the means for establishing the spatial contact between the

measuring instrument and the object being measured.

This conclusion,however,does not mean that the analysis and selection of

parameters for the carrier vibrations is of minor importance.On the contrary,the

frequency of the carrier vibrations is linked in a very close manner with the coding

method for the informational communication,with the passband of the receiving and

radiating elements in the apparatus,with the spatial characteristics of the ultrasonic

communication channel,and with the measuring accuracy.

Let us dwell on the questions of general importance for ultrasonic ranging in

air,namely:on the choice of a carrier frequency and the amount of acoustic power

received.

An analysis shows that with conical directivity diagrams for the radiator and

receiver,and assuming that the distance between radiator and receiver is substantially

smaller than the distance to the obstacle,the amount of acoustic power arriving at the

receiving area Pr for the case of reflection from an ideal plane surface located at right

angles to the acoustic axis of the transducer comes to

where Prad is the amount of acoustic power radiated,B is the absorption

coefficient for a plane wave in the medium,L is the distance between the

electroacoustic transducer and the test me -mber,d is the diameter of the

radiator(receiver),assuming they are equal,and c~is the angle of the directivity

diagram for the electroacoustic transducer in the radiator.

sinsin21ddJW

Both in Eq.(1)and below,the absorption coefficient is dependent on the amplitude

and not on the intensity as in some works[1],and therefore we think it necessary to

stress this difference.

In the various problems of sound ranging on the test members of machines and

structures,the relationship between the signal attenuations due to the absorption of a

planewave and due to the geometrical properties of the sound beam are,as a

rule,quite different.It must be pointed out that the choice of the geometrical

parameters for the beam in specific practical cases is dictated by the shape of the

reflecting surface and its spatial distortion relative to some average position.

Let us consider in more detail the relationship betweenthe geometric and the

power parameters of acoustic beams for the most common cases of ranging on plane

and cylindrical structural members.

It is well known that the directional characteristic W of a circular piston vibrating

in an infinite baffle is a function of the ratio of the piston's diameter to the wavelength