# Research on Measurement of body position and pose change of breakwater protection block based on IMU inertial sensor

Usually, there is a layer of protective block on the breakwater of the harbor. This layer of protective block can protect the breakwater from being damaged by the wave impact, but also can play the role of eliminating the wave. The shield block will be swayed instantaneously by the wave impact, and the displacement change page will occur during the long-term impact. It takes a lot of manpower, material resources and financial resources to study the swing of the protective block in the actual environment. Therefore, it is a common choice for scientific research to conduct physical model tests in experimental flume. At present, there is only qualitative analysis but no quantitative evaluation on the collision between the Angle change of the shield block and the surrounding objects caused by wave impact. Wang Tiening et al. studied the stability of the blocks with different wave levels and qualitatively evaluated the changes in the position and posture of the blocks. Li Heqing et al. gave the stability coefficient (4) of the moment the block is affected by waves. It can be seen that there is insufficient research on the measurement of block posture and there is no specific value to describe the change law of block subjected to wave action. In this paper, the IMU inertial sensor is used to directly study the shield block, which can reflect the instantaneous acceleration and angular velocity of the block under impact. An IMU inertial sensor is installed on the outside of the shield block, which will sway with the shield block at the moment of wave impact and output angular velocity signals. The impact velocity and collision number are obtained by processing the diagonal velocity signal. This paper studies the relationship between the stability number and the collision number at different waterlines, and the variation trend of the maximum impact velocity at different waterlines. Compared with previous studies, this paper gives the variation rule of wave action of the block and records the pose changes of the block during the test by combining the IMU inertial sensor with the shield block. After the test, the specific value of the wave action of the block can be checked through the data in the sensor.

**1. Measurement principle of body position and pose transformation of face protector**

The accelerometer and gyroscope inside the IMU inertial sensor were used to measure the acceleration and angular velocity of the shield block at the moment of wave impact. Since the IMU inertial sensor is arranged on the shield block, the amount of exercise output during the test is the motion information in the carrier coordinate system. Carrier coordinate system ⑷ will change with the wave impact of the surface block position change, through the pose transformation can block relative to the carrier coordinate system under the motion information, through the attitude matrix conversion to the geographical coordinate system, at the same time the signal processing under the geographical coordinate system, can make the surface block movement information will not change with the change of its position.

The relationship between the carrier coordinate system and the geographic coordinate system can be divided into three simple coordinate transformations: first fix the Z axis and rotate the Angle 0 about the axis, then fix the X axis and rotate 0 about the axis, and finally rotate Y about the y axis, as shown in Figure 1.

The transformation relationship in space described by the change of attitude Angle includes three basic rotations. The rotation angles of each rotation are called course Angle, pitch Angle and roll Angle respectively.

The transformation matrix expression corresponding to each rotation is

Therefore, the attitude matrix can be expressed as:

And that simplifies it

Test layout and signal processing

The experiment was carried out in the energy dissipation area of large scale wave flume of Tianjin Water Transport Engineering Research Institute of Ministry of Transport. The tank is 456 m long, 5 m wide and 12 m high. Wave flume with large scale can simulate waves up to 3.5 m high, and the wave formation period is 2 ~ 10 s. The flume can well simulate the real wave impact process of the coastal protection block.

The integrated module of IMU inertial sensor is a high precision gyroscope, accelerometer and geomagnetic sensor. It adopts high performance microprocessor and advanced dynamic calculation and Kalman dynamic filtering algorithm. At the same time, it has the advantages of reducing measurement noise, improving measurement accuracy and strong anti-interference ability. Table 1 lists the features of the IMU.

Prior to the test, an SD memory card was inserted into the IMU inertial sensor and the IMU inertial sensor was placed in a waterproof box that had been secured to the protective block. And place the protection block to be tested near the water line of the energy dissipation area of the wave flume with large scale, as shown in Fig.4 and Fig.5.

Table 1 Feature specifications

Figure 4 IMU inertial sensor and SD card

The specific conditions of irregular waves in the test are shown in Table 2.

Table 2 Wave height and waterline position

After the test, the IMU inertial sensor is taken out and the built-in SD card is inserted into the card reader. After associating with the JY901 PC software developed by Shenzhen Witt Company, acceleration and angular velocity signals can be obtained. The software is capable of real-time dynamic display of signals in the test process, as well as acceleration and angular velocity signal curves, as shown in FIG. 6 and FIG. 7. 25.

The ordinate unit of acceleration signal is g, the ordinate unit of angular velocity signal is (°)/s, and the abscissa is the acquisition time interval. Each square represents 1s.

The acceleration signal will be affected by the gravitational acceleration when it is affected by the collision, resulting in inaccurate results. It is difficult to correct the generated signal when it is processed by the existing technical means. Therefore, the angular velocity signal is selected as the processing signal, which has the advantage that it is not affected by the gravitational acceleration. The absolute angular velocity signal can be synthesized from the angular velocity components generated by the z, y and Z axes.

Assuming that the protective block moves around a point in pure rotation, the nominal diameter of the block is, so the impact velocity can be obtained.

According to the impact velocity, the impact peak at the moment when the shield block is impacted by the wave can be drawn, and the impact peak can directly reflect the velocity distribution of the block when it is impacted by the wave and the maximum and minimum impact velocity when it is impacted by the wave, as shown in Figure 8 and 9.

**3 Analysis of experimental results**

According to the irregular wave condition, the corresponding relation between the stability number and the collision number can be obtained at different waterline positions. At the same time, the variation trend of the maximum impact velocity at different waterline positions is analyzed.

**3.1 Stability number and collision number**

The ratio of wave height Hs to the nominal diameter Dn of the shield block is denoted by the stability coefficient H"/Dn, and the collision number NgjN is the ratio of the number of collisions N® received by the block to the number of waves N. The number of collision N penalty can be defined as: In the impact velocity generated by wave impact, the average number greater than the impact velocity can be calculated, as shown in Figure 10.

FIG. 10 Correspondence between stability number and collision number

Under the condition that the nominal diameter D" of the block is kept constant, the stability number will increase with the increase of wave height, and the collision number will also increase. As can be seen from FIG. 10, the increase in the number of collisions is not large, which is reflected in the small change trend of the curve in the figure. By comparing 9 groups of operating conditions in Table 2 with different waterline and wave height positions, the relationship between stability coefficient and collision number will increase with the increase of stability coefficient, collision number will also increase. However, the number of collisions at the waterline z/Dn = -2 is significantly greater than that at the water line z/D "= 0 and z/Dn =2, indicating that the rocking of the block at the waterline z/Dn =2 is more intense than that at z/Dn = 0 and z/Dn =2, and the collision frequency with the surrounding block is also higher. Under the condition of constant stability coefficient, the number of collisions at z/Dn =-2 is greater than that at z/Dn = 0 and z/Dn = 2. This indicates that the location of the waterline will affect the intensity of the block rocking when it is affected by the wave impact.

**3.2 Maximum impact velocity at different waterline positions**

The shield block plays a key role in the protection of breakwater when it is placed at different waterline positions. At the same time, the maximum impact velocity of the shield block is different at different waterline positions. It is helpful to choose the appropriate waterline position during the placement of the shield block to prolong the life of the shield block. In order to study the maximum impact velocity of the block at different waterline positions, 9 groups of test conditions were carried out, and the variation trend of the maximum impact velocity at different wave heights and waterline positions was shown in FIG.

11.

FIG. 11 Trend of maximum impact velocity at different wave heights and waterlines

It can be seen from Figure 11 that the maximum impact velocity increases first and then decreases at different waterline positions, and there is a maximum impact velocity at z/Da = 0. At the same time, the larger the wave height, the greater the maximum impact velocity. If the protection block is affected by the maximum impact speed for a long time, the instability and damage of the protection block will be accelerated.

**4 Conclusion**

The inertial sensor was used to carry out experiments in the energy dissipation area of the large scale wave flume in Tianjin Institute of Water Transport Engineering, Ministry of Transport. Under the conditions of wave movement close to the real environment, the relationship between the stability number and the collision number and the maximum impact velocity at different waterline positions were studied. In the study of stability number and collision number, it is found that the intensity of the collision between the test block and the surrounding block is smaller when z/Da = 0 than when z/Dn =-2, and the maximum impact velocity appears at z/Dn = 0. The research can provide reference for breakwater protection.

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