TELKOMNIKA Indonesian Journal of Electrical Engineering Vol.10, No.6, October 2012, pp. 1157~1162 1157
Ultrasonic Wind Velocity Measurement Based on Phase Discrimination Technique Chunyu Yu*, Chao Guo, Jia-yi Liang, Tong Wang Higher Educational Key Laboratory for Measuring &Control Technology and Instrumentation of Heilongjiang Province Harbin University of Science and Technology, China *corresponding author, e-mail:
[email protected]
Abstract According to the shortcomings of the existing ultrasonic wind velocity measurement device, for instance, complexity of circuit and difficulty of signal processing, a new ultrasonic wind velocity measurement is put forward based on phase discrimination and a new sensor configuration as well. Thus, a system model is built. Firstly, an equilateral triangle should be constituted by an ultrasonic emission sensor and two ultrasonic receiving sensors. Then by high-precision phase discrimination circuit, the lag between the ultrasonic which is received by two ultrasonic receiving sensors during the traveling time in the upwind and downwind is converted to the phase difference. After that, the wind velocity is measured. Besides, a mathematical model is established among the wind velocity, the ultrasonic velocity, and the structural parameters with ambient temperature. The factors which influence the precision of the wind velocity measurement are analyzed and the solutions are given as well. The experimental results show that in view of the phase discrimination technology, the system has a good numerical stability and the resolution is one order magnitude better than that of the cup anemometer. Keywords: ultrasonic, phase discrimination, phase difference, wind velocity Copyright © 2012 Universitas Ahmad Dahlan. All rights reserved.
1. Introduction Wind velocity is an important meteorological parameter closely related to many fields, such as industry, agriculture, aviation, marine, and so on. Therefore, wind velocity measurement technology has always been a hot suubject for many researchers both at home and abroad [1-3]. At present, the common wind velocity measurement methods are known as mechanical, Pitot tube and ultrasonic measurement. The mechanical measurement is primarily in wind cups and wind wheel, which is characterized by: (1)requirement of starting up wind velocity; (2)slow response; (3)due to the existence of the movable components, mechanical friction, the accumulation of the sediment dust and other factors result to the unreliability of the instrument or even the error [4]. It is suitable for the environment of less precision. The main drawback of the Pitot tube measurement is that while the wind velocity is tiny, the error is big [5]. Besides it is not suitable for the wind velocity measurement which contains dust and gases. The existing ultrasonic wind velocity and direction measuring device usually adopts the speeddifference method, which is divided into the direct time-difference method, the frequencydifference method, and the phase-difference method. The basic principle of the direct timedifference method is that by tracking the maximum peak of the received ultrasonic, the propagation time difference is reached by subtracting the propagation time of the downstream and countercurrent [6]. The disadvantage of this method is that when the ultrasonic wave deforms going through the wall and fluid, or there’re strong interfering signals, it is prone to ultrasonic detection error and causes the wind velocity measurement error. Moreover, this means is influenced by temperature considerably, which leads to the inaccuracy of the measurement results. As for the frequency difference method, it is vulnerable to the environmental interference ring, and it works flexibly. Therefore, the application of these methods mentioned above is all restricted. In order to solve the problem above, the scholars both foreign and domestic have made lots of researches. Document [7] is an ultrasonic wind velocity measurement which was designed and researched based on FPGA and DSP. In [8], ultrasonic wind velocity and direction detection system was designed based on SOPC technology. In [9], ultrasonic wind velocity Received June 14, 2012; Revised September 8, 2012; Accepted September 20, 2012
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measurement system was studied based on DSP. The three documents mentioned all adopt the transit-time of the speed difference to measure the wind velocity, and rely on the hardware to improve the accuracy of measurement. In [10], wind velocity measurement and analysis of uncertainty were studied through the ultrasonic sensor and the Kalman filter. Ultrasonic vortex detection system was evaluated in [11]. In this paper, a novel ultrasonic wind velocity measurement method has been pointed which is based on phase discrimination, and a new sensor configuration. By high-precision phase discrimination module, the lag caused by the signals from the two wind velocity receiving sensors is converted to phase difference. This method overcomes the shortcomings of the difficulty in signal processing and circuit complexity of anemometer compared with the time difference method.
2. Principle of Ultrasonic Wind Velocity Measurement Based on Phase Discrimination Technique The wind velocity direction measurement method adopted in this paper belongs to the phase difference method of the speed difference method. The phase difference result of signals received by the receiving sensor reflects the time difference. That is, to make the measurement, the time difference is converted to the phase difference. 2.1 System Configuration The ultrasonic wind velocity and direction measurement system based on phasediscrimination technique consists of ultrasonic sensors, the transmitting circuit, the receiving circuit, phase discrimination circuit, single-chip system, the data display and the measuring temperature circuit, as is shown in Figure 1.
Figure 1. Block diagram of ultrasonic wind velocity measurement system
Microcontroller-driven ultrasonic emission sensor continuously launches ultrasound signals which are received by the ultrasonic receiving sensors, and then are converted to analog signals passed through zero-crossing phase-discrimination detector phase difference and wind velocity value can be received after data processing. In this process, the impact caused by the drop of the signal amplitude is very small. The key point of the phase discrimination based ultrasonic wind velocity measurement method lies in the design of the sensor configurations and the phase discrimination circuit. 2.2 Sensor Configurations Traditional sensor configurations consist of Z method (permeation), V method (reflection method), X method (interior extrapolation method) [12]. However, the three sensor configurations all have many deficiencies [5]. In this paper, a new form of sensor configuration has been proposed. In this system, an equilateral triangle is formed by an emission sensor T and two receiving sensorsR1and R2, as follows in Figure 2. L is regarded as a slide length of the equilateral triangle, that is, the acoustic distance. t1 is the time of the ultrasonic travels from transmitter T to receiver R1. t2 is the time from transmitter T to receiver R2. TELKOMNIKA IJEE Vol. 10, No. 6, October 2012 : 1157 – 1162
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Figure 2. The sensor configurations
It can be deduced: L t1 = c − v cos θ L t = 2 c + v cos θ
(1)
where: v : Wind velocity c : Ultrasonic propagation velocity without wind θ : Included angle of equilateral triangle(60°) By Eq. (1) ∆t = t1 − t2 =
where:
L L 2 Lv cos θ − = c − v cos θ c + v cos θ c 2 − v 2 cos 2 θ
(2)
∆t : Jet lag. When v is very small, and v
