外文原文
ULTASONIC RANGING IN AIR
G. E. Rudashevski特大事故 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 parate 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 purp
o has been known for a long time,and at first glance appears very simple,nevertheless at the prent time there are only a small number of developments using this method that are suitable for practical purpos.The main difficulty here is in providing a reliable acoustic three-dimensional contact with the test object during vere changes in the air's characteristic.
Practically all acoustic arrangements prently known for checking distances u 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 themlves a source of information.In order to transmit some informational communication that can then be lected at the receiving end after reflection from the test member,the radiated vibrations must be modulated.In this ca the ultrasonic vibrations are the carrier of the information which lies in the modulation ,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 lection of parameters for the carrier vibrations is of minor importance.On the contrary,the frequency of the carrier vibrations is linked in a very clo 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.
西城杨柳弄春柔
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 ca of reflection from an ideal plane surface located at right angles to the acoustic axis of the transducer comes to
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桂圆干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.三救姻缘
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 cas 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 cas 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 d/λ as found from the following expression:
(2)
投资方案计划书where Jl is a Besl function of the first order and α is the angle between a normal to the piston and a line projected from the center of the piston to the point of obrvation(radiation).
From Eq.(2)it is readily found that a t w o-t o-o n e reduction in the nsitivity of a radiator with respect to sound pressure will occur at the angle
（3）
For angles α≤20.Eq.(3)can be simplified to
（4）
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