1、PDF外文：http:/ 2 中英文翻译 英文原文 ： A New Level Measurement Method based on Acoustic Standing Wave Wang ZhiWen Cheng-linXu Xiao-bin Li Zhi-liang Hangzhou DianzhiUniversity Email: Abstract A new level measurement method based on acoustic standing wave is presented. Firstly, the system model about
2、 the frequencies of the acoustic standing waves is established and Kalman Filter is used to preprocess the standing wave frequency. It will increase the accuracyof level measurement by reducing the influence of environment noise and measurement error, which are usually ignored by most of
3、 acoustic level measurement methods. Secondly, the new conversion technique can be given to calculate the level height with estimates of standing wave frequencies from Kalman Filter. And it can overcome the strict limitation about frequency range in existing classical method. Moreover, the experimen
4、ts show the proposed method effective. 1. Introduction The level measurement methods based on sound reflection phenomena have been used successfully in some areas of the process industry 1-2 (waste water treatment, chemical, petroleum, etc.). And the method based on the acoustic standing
5、 wave reflection is important among them. The sound waves are emitted vertical to the liquid surface and the echo signals will be sent to the controller. Then the level height will be calculated by terms of the character of the standing waves extracted from the echo signals 3. The common standing wa
6、ve methods are simply equipped with single waveguide tube. It is necessary to measure sound velocity for accurate calculation of level height. However, the exact value of sound velocity can hardly computed, because its a complex physical quantity related mainly to temperature, gas composition, and g
7、as pressure in changeful measurement environment. In order to avoidappearance of sound velocity, reference 3 presents a standing wave method with double waveguide tubes, which can compensate sound velocity. Although this method increases the accuracy in some degree, itbrings other problem that the a
8、coustic frequencies ofemitted waves must start strictly from 0Hz so as to capture the first standing wave, by which, the level height can be computed. So, the microphone receiving the echo must be very sensitive to the low frequency sound wave. Whereas, the lowest frequency that the common microphon
9、e can capture is about 20Hz. High-quality microphone that can feel the frequency below 20Hz are 10 times more expansive than the former. Considering the cost of level gauge, it is undesirable to choose the latter. For overcoming this strict limitation a new height conversion technique is given to en
10、sure that the user can choose a suitable range of frequencies, which make the requirement that that wave be emitted from 0Hz unnecessary. On the other hand, most of the present acoustic level measurement methods directly use the measurementsof the standing wave frequencies to compute lev
11、el height. The measurement error and environmental noise often lead to decline of measuring accuracy which is a common shortage for them. This paper establishes the system model about the frequencies of the acoustic standing waves and uses Kalman Filterto preprocess the standing wave fre
12、quency. The new conversion technique can be used to calculate level height with estimates of standing wave frequencies from Kalman filter. The proposed method will increase the accuracy of the result by reducing the influence of environment noise and measurement error, meanwh
13、ile overcome the strict limitation about frequency range. Moreover, the experiments show the proposed method effective. 2. The level measurement method with double tubes based on acoustic standing wave theory If two waves propagate in the opposite directionwith the same frequency, amplitude an
14、d the velocity, standing waves will be formed 7. When the sound waves travel in the direction vertical to the level, the above conditions can be satisfied. Let y1 be the acoustic wave from the transmitter and y2 be the reflection of y1 through the liquid surface. Expressionsof both are respect
15、ively y 1 = A cos 2 ( ft L ) (1) y 2 = A cos 2 ( ft + L ) (2) And their synthesized wave is y = 2 A cos(2 L )cos(2 ft) (3) Where A is the amplitude of acoustic wave, f is the frequency, L is the distance from the liquid surface to the top of tube, is the wavelength of the acoustic wave. L is the lev
16、el height needed to be calculated. When and L satisfy the relation below L = 2 n , n = 1,2,. (4) the amplitude of the synthesized wave will reach the peak. Its wavelength is =c/f , and then L = nc (2f) (5) Where c is the sound velocity, f is the acoustic frequency and T is the centigrade. Figure.1 Level gauge with single tubeFigure.3 Level gauge with double tubes
