1、超声波测距外文文献加中文翻译毕业设计说明书附录A 英文原文ULTASONIC RANGING IN AIRG. 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 instan
2、ce,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 vibr
3、ations 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 ultr
4、asound.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 difficu
5、lty here is in providing a reliable acoustic three-dimensional contact with the test object during severe changes in the airs characteristic. Practically all acoustic arrangements presently known for checking distances use a method of measuring the propagation time for certain information samples fr
6、om 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
7、member,the radiated vibrations must be modulated.In this case the ultrasonic vibrations are the carrier of the information which lies in the modulation signal,i.e.,they are the means for establishing the spatial contact between the measuring instrument and the object being measured.This conclusion,h
8、owever,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
9、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 po
10、wer 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 r
11、eflection 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 -m
12、ber,d is the diameter of the radiator(receiver),assuming they are equal,and cis the angle of the directivity diagram for the electroacoustic transducer in the radiator. Both in Eq.(1)and below,the absorption coefficient is dependent on the amplitude and not on the intensity as in some works1,and the
13、refore 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,q
14、uite 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 geom
15、etric 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 pistons diameter to the wave
16、length d/ as found from the following expression: (2)where Jl is a Bessel function of the first order and is the angle between a normal to the piston and a line projected from the center of the piston to the point of observation(radiation).From Eq.(2)it is readily found that a t w o-t o-o n e reduct
17、ion in the sensitivity of a radiator with respect to sound pressure will occur at the angle （3）For angles 20.Eq.(3)can be simplified to （4） where c is the velocity of sound in the medimaa and f is the frequency of the radiated vibrations.It follows from Eq.(4)that when radiating into air where c=330
18、 m/s e c,the necessary diameter of the radiator for a spedfied angle of the directivity diagram at the 0.5 level of pressure taken with respect to the axis can befound to be （5） where disincm,f is in kHz,and is in degrees of angle.Curves are shown in Fig.1 plotted from Eq.(5)for six angles of a radi
19、ators directivity diagram.The directivity diagrm needed for a radiator is dictated by the maximum distance to be measured and by the spatial disposition of the test member relative to the other structural members.In order to avoid the incidence of signals reflected from adjacent members onto the aco
20、ustic receiver,it is necessary to provide a small angle of divergence for the sound beam and,as far as possible,a small-diameter radiator.These two requirements are mutually inconsistent since for a given radiation frequency a reduction of the beams divergence angle requires an increased radiator di
21、ameter.In fact,the diameter of thesonicatedspot is controlled by two variables,namely:the diameter of the radiator and the divergence angle of the sound beam.In the general case the minimum diameter of thesonicatedspot Dmin on a plane surface normally disposed to the radiators axis is given by （6）wh
22、ere L is the least distance to the test surface.The specified value of Dmin corresponds to a radiator with a diameter （7）As seen from Eqs.(,6)and(7),the minimum diameter of thesonieatedspot at the maximum required distancecannot be less than two radiator diameters.Naturally,with shorter distances to
23、 the obstacle the size of thesonicated surface is less. Let us consider the case of sound ranging on a cylindrically shaped object of radius R.The problem is to measure the distance from the electroacoustic transducer to the side surface of the cylinder with its various possible displacements along
24、the X and Y axes.The necessary angleof the radiators directivity diagram is given in this case by the expression （8）where is the value of the angle for the directivity diagram,Ymax is the maximum displacement of the cylinders center from the acoustic axis,and Lmin is the minimum distance from the ce
25、nter of the electroacoustic transducer to the reflecting surface measured along the straight line connecting the center of the m e m b e r with the center of the transducer.It is clear that when measuring distance,therunningtime of the information signal is controlled by the length of the path in a
26、direction normal to the cylinders surface,or in other words,the measure distance is always the shortest one.This statement is correct for all cases of specular reflection of the vibrations from the test surface.The simultaneous solution of Eqs.(2)and(8)when W=0.5 leads to the following expression: (
27、9)In the particular case where the sound ranging takes place in air having c=330 m/sec,and on the asstunption that L minR,the necessary d i a m e t e r of a unidirectional piston radiator d can be found from the fomula (10)where d is in cm and f is in kHz.Curves are shown in Fig.2 for determining th
28、e necessary diameter of the radiator as a function of the ratio of the cylinders radius to the maximum displacement from the axis for four radiation frequencies.Also shown in this figure is the directivity diagram angle as a function of R and Yrnax for four ratios of m i n i m u m distance to radius
29、.The ultrasonic absorption in air is the second factor in determining the resolution of ultrasonic ranging devices and their range of action.The results of physical investigations concerning the measurement of ultrasonic vibrations air are given in1-3.Up until now there has been no unambiguous expla
30、nation of the discrepancy between the theoretical and expe -rimental absorption results for ultrasonic vibrations in air.Thus,for frequencies in the order of 50 to 60 kHz at a temperature of+25oC and a relative humidity of 37%the energy absorption coefficient for a plane wave is about 2.5dB/m while
31、the theoretical value is 0.3 d B/m.The absorption coefficient B as a function of frequency for a temperature of+25oCand a humidity of 37%according to the data in2can be described by Table 1.The absorption coefficient depends on the relative humidity.Thus,for frequencies in the order of 10 to 20kHz t
32、he highest value of the absorption coefficient occurs at 20%humidity3,and at 40%humidity the absorption is reduced by about two to one.For frequencies in the order of 60 kHz the maximum absorption occurs at 30.7o humidity,dropping when it is increased to 98% or lowered to 10%by a factor of approximately four to one.The air temperature also has an appreciable effect on the ultrasonic absorption1.When the temperature of the medium is increased from+10 to+30,the absorption for frequencies between 30 and 50 kHz increases by abou
