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#### Analysis of Asymmetry of Received Signal Strength of Ultrasonic Sensor and Countermeasures

2016-05-23

Due to the directivity of the ultrasonic wave, that is, the existence of a pointing angle, and the laminar and turbulent effects in practice, there is a time difference between the arrival of the ultrasonic wave at the center and the periphery of the receiving surface. Therefore, there is a problem of non-uniformity and asymmetry of the received signal strength of the sound receiving sensor.

This paper proposes an improved method. Based on the existing sensor technology, the piezoelectric element is divided into a two-body structure. Under the premise of not affecting the effective received signal strength, the uniformity and symmetry of the received signal are improved, and the information is improved. The noise ratio is of great significance to ensure the measurement accuracy of the ultrasonic flowmeter.

2 Problems with current technology

2.1 Ultrasonic beam angle

The ultrasonic beam shape is shown in Figure 2. An ultrasonic wave with a velocity of c at a certain point is emitted at a certain pointing angle θ, and the ultrasonic wave is emitted toward the receiving side in a disc shape, and the disc-shaped cross-sectional area is from small to large. 2.2 Analysis of uniformity and symmetry under ideal conditions

The ideal state is that the flow velocity of the fluid in the pipeline is consistent.

Assume that when the ultrasonic wave propagates in the downstream direction, the time for the wave to propagate on the center line is , The time of propagation along the θ direction is ,then:   Because c >> v, in the ideal state, when the ultrasonic wave propagates in the downstream direction, the time difference between the ultrasonic wave propagating along the center line and the direction of a certain angle to the receiving surface for: (1)

When the ultrasonic wave is propagated in the countercurrent direction, according to the calculation principle of equation (1), it can be known that the time difference between the ultrasonic wave propagating along the center line and the direction of a certain pointing angle to the receiving surface for: (2)

From the comparison of formula (1) and formula (2), we can know: .

It shows that when the ultrasonic wave is in the same direction with the flow velocity, the time difference between the ultrasonic wave traveling along the center line and the direction with a certain pointing angle and reaching the receiving surface is greater than when the ultrasonic wave is inconsistent with the direction of the flow velocity, the ultrasonic wave is traveling along the center line and reaching the receiving surface with a certain direction. Time difference. That is, when the fluid flows in the pipeline, the received ultrasonic waves have non-uniformity and asymmetry.

2.3 Analysis of uniformity and symmetry under non-ideal conditions

For the non-ideal state, the liquid is viscous and has two flow patterns. The state in which liquid particles move in an orderly manner and are not mixed with each other is called laminar flow, and the state in which liquid particles do irregular movements, are mixed with each other, and the path is chaotic is called turbulence. Figure 3 shows the comparison of laminar and turbulent flow velocity distributions. Is the axial velocity, and v is the surface velocity. The relationship between the two is:

Laminar flow: Turbulence: (Take 0.825 for simplified calculation) 2.3.1 Uniformity and Symmetry Analysis in Laminar Flow

When the ultrasonic wave is propagated in the downstream direction, according to the calculation principle of equation (1), it can be known that the time difference between the ultrasonic wave propagating along the center line and reaching the receiving surface in a direction of a certain pointing angle for: (3)

When the ultrasonic wave is propagated in the countercurrent direction, according to the calculation principle of equation (2), it can be known that the time difference between the ultrasonic wave propagating along the center line and the direction of a certain pointing angle to the receiving surface for: (4)

The comparison of equations (3) and (4) shows: .

2.3.2 Analysis of uniformity and symmetry under turbulent flow When the ultrasonic wave is propagated in the downstream direction, according to the calculation principle of equation (1), it can be known that the time difference between the ultrasonic wave propagating along the centerline and in a certain direction to the receiving surface for: (5)

When the sound wave propagates in the countercurrent direction, according to the calculation principle of equation (2), it can be known that the time difference between the ultrasonic wave propagating along the center line and the direction of a certain pointing angle to the receiving surface for: (6)

The comparison of formulas (5) and (6) shows: .

According to the above analysis: , , It can be seen that no matter whether the flow velocity direction of the fluid is consistent with the ultrasonic transmission direction, there will be a time difference between the time when the ultrasonic waves reach the center of the receiving side and the surrounding area. That is, because there is a pointing angle θ, the arrival time of the ultrasonic waves along the centerline direction is shorter than the time that the ultrasonic waves reach the surrounding area, and the forward time difference is greater than the reverse time difference.

In this way, there are the following two problems: ① The energy received by the ultrasonic sensor is not concentrated, the main reason is that there is a difference in the received signal strength between the central part and the peripheral part. At the same time, there is non-uniformity in the strength of the received signal, which will reduce the signal-to-noise ratio of the received signal. ② There are non-uniformities in the forward and reverse directions, and the non-uniformities in the forward and reverse directions are inconsistent, which causes two The asymmetry of the received signal strength of each sensor directly affects the signal processing effect of the signal generating circuit and the signal receiving circuit.

Both of these issues affect the accuracy of flow detection.

3 Data analysis and improvement measures

Aiming at the shortcomings of the prior art, the improvement method proposed in this paper is to divide the piezoelectric element of the existing structure into a piezoelectric element module and a virtual piezoelectric element module, the piezoelectric element module is arranged in the outer ring, and the virtual piezoelectric element The module is set on the inner ring, and the virtual piezoelectric element has no piezoelectric effect. As shown in Figure 5. Because the difference between the inside and outside diameters of the piezoelectric element module is much smaller than the outside diameter of the piezoelectric element, this limits the effective angle of the received signal to a small range, thereby reducing the forward and reverse ultrasonic signals reaching the receiving side. The time difference greatly improves the non-uniformity and asymmetry of the received signal, and improves the signal-to-noise ratio. At the same time, the outer ring is used as a piezoelectric element module to ensure the receiving area of the ultrasonic signal and the strength of the received signal.

This article uses the difference between the maximum and minimum values divided by the flow rate as the relative deviation to illustrate the superiority of the improved method. As shown in Table 1, 10 measurement points were selected as the comparison data in the full flow rate section. For comparison, this paper compares the time difference between the received signal time with zero flow rate and the received signal strength according to Table 1 to obtain a comparison curve, as shown in Figure 6. It can be seen from Table 1 and FIG. 6.

Relative deviation of the time difference of the received signal in the prior art:

(611.76-101.56) /356.66≈143%

The relative deviation of the time difference of the received signal after improvement:

(466.93-266.39) /356.66≈51%

Relative deviation of received signal strength in the prior art:

(1252.55-207.93) /356.66≈293%

Relative deviation of received signal strength after improvement:

(477.52-284.62) /356.66≈54%

It can be seen that the relative deviation of the time difference of the received signal after improvement has been reduced from 143% to 51%, and its mean square error has been reduced from 161.34ns to 57.08ns. The relative deviation of the received signal strength has been reduced from 293% to 54%, and its mean square error has been reduced by 304.96 dropped to 60.47. It is shown that this method greatly improves the symmetry and uniformity of the time difference and strength of the received signals, and the accuracy of flow detection can be greatly improved.

Article excerpts: Wang Yan, Cui Xiaozhi, Hou Chunlei. Analysis of the asymmetry of received signal strength of ultrasonic sensors and countermeasures [J]. Journal of Transduction Technology, 2015 (1): 81-85.