An Innovative Designed Velocimeter Application for Set Net Fishery

Abstract

This article is aimed at the problems encountered by the fishing ground of set nets in Taiwan when the set net operators need to work when the weather is bad at sea. We developed a velocimeter that can be placed in the ocean for a long time and change sensing directions with the flow direction. The self-designed velocimeter has advantages, such as simple operation, low cost, and easy maintenance. With wireless monitoring and an early warning system, it can monitor current velocity, flow direction, sea temperature, and displacement. The data is transmitted back to the operator through the radio transmission module as a basis for dispatching personnel to go out to sea for fishing. To prevent the set net from drifting into the open sea due to bad weather, a GPS module is used to monitor the current location of the set net. If the waves wash away the set net, the warning signal can be received the first time to prevent the floating set net from endangering the safety of ship navigation and the survival of marine life. This innovative technology aligns with SDG 14, which aims to “conserve and sustainably use the oceans, seas, and marine resources for sustainable development”.

1. Introduction

The cost of the set net is quite expensive. If the set net is damaged by wind and waves, especially the super-strong wind and waves caused by typhoons passing through the border every summer in Taiwan, fishermen will suffer heavy losses. If the waves wash away the set net, it will not only affect the navigation safety of the ship, it will also affect the ecological environment of the ocean. If the winds are fast, and the waves are big, the current velocity will be fast. Therefore, it is important to monitor the current velocity and the position of the set net [1]. In today’s setnet fishery, it is impossible to understand the status of the set net effectively, not to mention that changes at sea are happening all the time. Therefore, it is necessary to develop an integrated system to transmit information, such as current velocity and set net displacement. Operators can avoid huge losses by understanding the current status of the set net through the integrated system and taking immediate measures.

The set net structure is mainly composed of four parts: stopper net, play ground cage, ladder net, and cage, as shown in Figure 1 [2,3]. Before sailing for fishing, it is necessary to decide whether to sail according to the conditions of the sea surface and current velocity. The entire set net will be damaged and washed away if the current velocity is too high. If the current velocity can be known immediately, the anchor’s weight can be increased to stabilize the set net or retract the set net in advance to avoid it being washed away by strong currents. While several velocimeters are on the market, they are costly and cannot be placed under the water for a long time. The sensing direction of the velocimeters will not be changed with the current. In addition, since the equipment is built at sea, communication is also a big problem. Therefore, a device capable of locating and detecting current direction, current velocity, and temperature at sea was developed in response to these problems.

The biggest problem with existing industrial velocimeters is the inability to immerse devices with high-precision sensors in seawater for a long time [4]. Reference 4 uses the self-developed robotic arm to put the sensor into the water when the value needs to be measured to avoid leaving the sensor in water for a long time. If exposed to seawater for a long time, the sensor will be affected by the salinity of the seawater and become inaccurate. Since the surface of the velocimeter is metal, it is easily corroded and rusted by seawater, as shown in Figure 2. It can easily cause measurement errors in the velocimeter and even damage it [5,6].

There are many kinds of velocimeters on the market, including traditional propeller velocimeters, acoustic velocimeters, Doppler velocimeters, radar wave velocimeters, and rotor velocimeters [7,8,9]. When the propeller velocimeter is placed in the water channel, the water flow via the propeller of the instrument and the propeller will produce a revolution. A functional relationship exists, $\mathrm{V}=\mathrm{f}\left(n\right)$, between the revolution rate “n” and the flow velocity “V”. At the same time, according to the revolution and duration of the measurement record, the measurement point flow velocity can be calculated by the formula. The main disadvantage of the propeller velocimeter is that it is easily damaged, due to long-term mechanical rotation. The measurement error is also easily caused by the relationship of mechanical kinetic energy, which requires manual flow velocity conversion and regular calibration. If debris is entangled once, the traditional velocimeter cannot be used. The only advantage is that the equipment cost is cheap.

The latest generation of Doppler ultrasonic velocimeters has replaced the traditional propeller velocimeter [10]. The flow velocity, flow, and water level can be output simultaneously without manual conversion, which saves much time and has more accurate measurement accuracy. The accuracy of the flow velocity can reach $1\%\pm 1$ cm/s, which is currently the most precise velocimeter. However, in terms of equipment cost, the price is not affordable for all general set net fisheries.

The radar velocimeter uses a planar microstrip array antenna at the K-band, which has concentrated energy and low power consumption. It integrates vertical angle compensation, average flow velocity, signal strength detection, serial communication, wireless communication, and other functions. The equipment of the noncontact radar velocimeter is not affected by sewage corrosion and sediment during velocity measurement, and the construction is simple and easy to maintain. It can be used for daily environmental monitoring and is especially suitable for undertaking urgent, difficult, and dangerous observation tasks.

When designing a velocimeter, the relationship between the surface velocity and the average velocity must first be considered. Suppose the flow velocity is too fast, causing the contact velocimeter to be difficult to control or to exceed its measurement range. In this case, a buoy is generally used to observe the surface velocity. There are three types of buoys, as shown in Figure 3. One is the surface float, another is the canister float, and the other is the rod float. The floats that are commonly used include glass bottles, PET bottles, Makino bamboo, and a floating ball. The current standard flow velocity observation method is the buoy method, and the surface velocity of the water flow measured by the buoy is converted into the average velocity. There are different types of standard buoys, including surface buoys, 0.5 m buoys, 1.0 m buoys, 2.0 m buoys, and 4.0 m buoys, designed according to the water depth measurement environment.

The main body of the velocimeter in this article does not require expensive structures and complicated designs. It only needs to use rigid polyvinyl chloride (PVC) water pipes to complete the design. The rigid PVC pipe is light in weight, strong in corrosion resistance, and good in insulation, and it is commonly used in fluid transport, the insulation of power cables, and many other applications. Because the rigid PVC pipe is not metal, it will not easily be corroded by seawater for a long time and will not rust [11]. Compared with metal materials, it can save a lot of maintenance costs and only needs regular clean-up for the attachments on the velocimeter about every three months. The signal amplification circuit module is placed inside the rigid PVC pipe to avoid the influence of seawater salinity. The GPS module locates the position of the set net to prevent the set net from drifting away due to strong wind and waves. Because water temperature, current velocity, and flow direction will all affect the number of fish harvested, the operator can choose whether to go to sea to collect the fishing net according to the monitoring data [12,13].

2. Design of the Self-Designed Velocimeter

2.1. Design Principle of the Self-Designed Velocimeter

Figure 4 is an entity diagram of the velocimeter designed to change the sensing direction with the ocean current. It has the advantages of simple structure, easy access to data, and easy cleaning and maintenance at sea to maintain measurement quality. The overall shape is similar to a wind vane. When a stream of water flows through the velocimeter, the head of the sensor faces the direction of the flow, as shown in Figure 5. The covers at the front of the velocimeter will expand arbitrarily when the current velocity is moderated. Conversely, if the current velocity is fast, the covers of the velocimeter will close tightly. The load cells are placed inside the covers of the velocimeter, and a signal amplification circuit module is designed and placed in the rigid PVC pipe. As the current squeezes the metal sheets, the strain gauge built into the load cell will be deformed. The strain gauge will change the impedance according to the magnitude of the force and then determine the current velocity.

Figure 6 shows the internal structure of the self-designed velocimeter front-end design. Due to the design of the structure, the signal is pulled to the floating platform via the pipe and transmitted to the shore by wireless communication. In order to prevent the rotation of the velocimeter from causing knots in the pipe, a conductive slip ring is added to the upper end of the pipe [14]. A rotary encoder is added to the stainless steel pipe to convert the rotational displacement into a series of digital pulse signals. The velocimeter will judge the current direction according to the corresponding signal. Two stainless steel pipes of different diameters are at the upper end of the velocimeter. The inner ring is directly welded on the velocimeter, and the outer ring is connected to the floating platform, as shown in Figure 7. When the current changes direction, the velocimeter oscillates as the current flows. There is a place reserved for external sensors on the floating platform. In order to make the entire velocimeter function more perfectly, a temperature sensor and a GPS positioning system are added to measure the seawater temperature and locate the position of the set net. In order to reduce the power required by the floating platform, a solar panel is installed on the platform to provide an independent power supply to ensure the regular operation of the entire velocimeter system, as shown in Figure 8.

2.2. Principle of the Load Cell

The load cell is a unique form of the force sensor, as shown in Figure 9 [15,16,17]. It is composed of a strain gauge and a bridge circuit. When subjected to tension or compression, it generates a voltage output proportional to the applied force. Depending on the gravitational load, the load cell deforms elastically, which is converted proportionally to an electrical signal by the internal strain gauge. A high precision of $\pm 0.1\%$ FS can be achieved. Sensing types include compression, tension and compression, shear beam, single-point load cell, and loop load cell. Typical areas of application are the gravimetric monitoring of containers and silos, as well as the platform and basic weighing in craft and metering plants, which can be calibrated in grams (g), kilograms (kg), or tons (t).

The principle of the strain gauge is that the resistance value of a wire is proportional to its resistivity ($\mathsf{\rho }$) and length (L) and inversely proportional to its cross-sectional area (A) [18]. Therefore, if the length of this wire is elongated or shortened, its resistance value will change. If there is a conductor with L length, the corresponding resistance is R, as shown in Figure 10. When the conductor is subjected to an external force, in addition to the geometric shape change, the resistivity will also change. When the material is subjected to pressure, the resistivity will increase for general materials. On the contrary, when the material is subjected to tension, the resistivity will decrease.

The internal resistance of a conductor can be determined using the following formula:

$$R_0=\mathsf{\rho }\times \frac{L_0}{A_0}$$

(1)

• $R_0$: The resistance of the conductor.
• $L_0$: The length of the conductor.
• $A_0$: The cross-sectional area of the conductor.
• $\mathsf{\rho }$: The resistivity of the conductor.

If an external force is applied to the conductor, the length of the conductor will change. If the length change is $\Delta L$ then the new length will be $\mathrm{L}=L_0+\Delta L$. However, its volume remains almost unchanged, and the volume without force is $\mathrm{V}=L_0\times A_0.$ If the volume remains unchanged but the length increases, the cross-sectional area must decrease, and the change is $\Delta \mathrm{A}$.

$$\mathrm{A}=L_0\times A_0=\left(L_0+\Delta L\right)\left(A_0-\Delta A\right)$$

(2)

Assuming that the resistivity is not affected by force, the conductor resistance also changes because both the length and the cross-sectional area are changed:

$$\mathrm{R}=\mathsf{\rho }·\left(L_0+\Delta L\right)\left(A_0-\Delta A\right)$$

(3)

From Equations (2) and (3), it can be proved that the new resistance value is about:

$$\mathrm{R}=\mathsf{\rho }·\frac{L_0}{A_0}\left(1+2\frac{\Delta L}{L_0}\right)$$

(4)

From this, the resistance change value can be obtained as follows:

$$\Delta \mathrm{R}=2\frac{R_0\Delta L}{L_0}$$

(5)

When the load cell is deformed by pressure, the voltage will change with the resistance value of the load cell. Then, the voltage value is amplified by the instrument amplifier circuit, as shown in Figure 11.

When the current is passed through the self-designed velocimeter, its front cover is pressed against the load cell by the flow force. After the value of the load cell is obtained, the flow velocity can be calculated using the formula:

$$\mathrm{F}=\mathsf{\rho }\mathrm{A}{V}^2$$

(6)

• F: The flow force of the liquid.
• ρ: The density of the liquid.
• A: The cross-sectional area of the water flow.
• V: The flow velocity of the liquid.

3. Set Net Displacement Monitoring System

If the anchorage of the set net is damaged, the set net will be washed away by the current, which will cause huge losses for the operators. Therefore, there is an urgent need to build a positioning system for the set net. The positioning part uses GPS to perform the positioning action and to monitor the displacement of the set net [19]. The GPS provides accurate positioning, speed measurement, and high-precision standard time. After GPS positioning, when the displacement of the buoy exceeds the set range, it indicates that the current may wash away the set net, as shown in Figure 12. At this time, the displacement-monitoring system will send a signal to remind the operator.

This system used the GPS module, radio transmission module, and microcontroller unit (MCU) [20,21]. The position of the GPS was represented by two values: longitude and latitude. Finally, the position and displacement information were transmitted to the NI myRIO through the radio transmission module, and the NI myRIO transmitted the information back to the server of the shore management station through the radio transmission module. The server can send information to the provider through the network service. In this way, even if the anchor is damaged and the set net is washed away by the current, the location of the set net can be effectively judged.

Figure 13 is a base station, which shows the radio transmission architecture of the system. The base station was about 1.6 km away from the set net. The power consumption was 1.6 W. The radio transmission module was the information exchange medium in this architecture, and NI myRIO was the data integration end. The system will send a message alert if the current velocity is too large or if the set net displacement is too much. The server can then transmit the data to the operator’s mobile device through the network to realize the fishery Internet of Things (IoT). The design of the transmission architecture requires the consideration of long-term placement in seawater and a continuous supply of electricity. It may be necessary to set up long antennas to enhance the receiving ability. When the signal to be tested is at a reasonable value, the device enters sleep mode to reduce power consumption. However, the monitored values will still be reported periodically. When the signal to be tested exceeds the allowable range, the system will transmit a message to the mobile device of the set net operator to inform the user of the status of the set net.

Data transmission was mainly based on radio 2.4 G, and the frequency used was 915 MHz.

Table 1. The table of each transmission module.

Attribute	Radio 2.4 G	NB-IoT	Sigfox	LoRaWAN
Range	3 km~10 km	1 km~10 km	3 km~50 km	2 km~8 km
Throughput	250 kbps	50 kbps	100 bps	50 kbps
Power Consumption	Low	Low	Low	Low
Module Cost	Under $100	$8~20	Under $5	$8~15
Topology	P2P, Star, Relay network	Star	Star	Star
Bandwidth	415 MHz~933 MHz	180 KHz	100 Hz	125 KHz~500 KHz

is the data table of each transmission module. It has the advantages of long transmission distance, low power consumption, and high security. Digital signals can be converted into radio signals using modulation techniques and demodulated at the receiving end to restore the original data. Radio data transmission modules are widely used in various wireless communication systems, including drones, remote controls, and Internet of Things devices [22,23]. Their primary function is to receive and send radio signals to realize wireless data transmission. The main features are high-efficiency data transmission, long-distance communication, broad applicability, and safety. The node network structure of the data transmission module is usually determined according to the specific usage scenarios and requirements. This article used the peer-to-peer (P2P) structure, which is a standard network structure. Each node can directly communicate with other nodes in this structure, unlike traditional centralized or hierarchical structures. In those structures, specific nodes are responsible for coordinating and forwarding communications. In a P2P structure, nodes can be easily added or removed without reconfiguring the network or changing the settings of other nodes. This makes the P2P structure suitable for dynamically changing environments and scalable applications.

4. Temperature Monitoring System

With the rapid advancement of wireless technology, numerous wireless transmission modules have found extensive applications. Previous research has yielded several published papers concerning intelligent control and monitoring systems [24]. The marine water quality can be monitored for parameters such as temperature, salinity, pH value, and more [25,26]. This study aimed to monitor the temperature of seawater, aiming to assist personnel responsible for setting up stationary nets in determining the optimal placement of net openings. Because seawater temperature is one of the important indicators of fish behavior and habitat selection, different species exhibit distinct preferences for seawater temperature. Additionally, the temperature variations in seawater can be used to predict the migratory pathways of fish populations. In order to measure seawater temperature, Pt100 was selected, according to the unique environment and convenience of the ocean. Pt100 is a resistance temperature detector with a resistance of 100 Ω at 0 °C. Therefore, it can provide excellent stability and accuracy. Pt100 has a linear relationship in the range of 0~100 °C, as shown in Figure 14. This article used NI myRIO, the self-designed module, and Pt100 to measure seawater temperature. Since the temperature change in seawater below 5 m must be monitored, the top end was tied to the floating platform, and then a heavy object was tied to the tail end so that it was not washed away by the current. When cleaning, we only pulled up from the top to clean.

5. Results

5.1. Laboratory Testing

To evaluate the performance of the self-designed velocimeter, we conducted a comparison with an industrial velocimeter. The industrial velocimeter used in this article was the SN450 series hot wire water flow switch. Its accuracy was $\pm 1\%$. The datasheet of the SN450 series is shown in

Table 2. The datasheet of the SN450 series water flow switch.

Type	SN 450 GA	SN 450 GA-3M	SN 450/1 GA	SN 450/1 GA-3M	SN 450/1 GAN-S
Detection range (cm/s)	5~150	5~300	5~150	5~300	5~150
Output	4~20 mA
Supply voltage	24 DC ±10%
Current consumption	<100 mA
Ambient temperature	−20~+70 °C
Protection	IP65
Connection	M12 connector

. Multiple tests were carried out on the two velocimeters using a pumping motor, with 25 cm/s flow rates. It can be concluded that by using an industrial velocimeter as a calibration sensor, the self-designed velocimeter will have an error of $\pm 5\%$. Figure 15 presents the results of the self-tests, where the white curve represents the line chart of the self-designed velocimeter, and the red curve represents the line chart of the industrial velocimeter.

5.2. Field Trial

The test location was Sizihwan, Kaohsiung, Taiwan. The self-designed velocimeter was placed on the bay area platform for the experiment. The velocimeter took a flow velocity every four hours. In this way, it was compared with the velocimeter of the Taiwan International Ports Corporation in the open sea, as shown in Figure 16. Figure 17 is a flow velocity measurement diagram of two kinds of velocimeters in the bay area and the open sea of Kaohsiung. The x-axis is the flow velocity, and the y-axis is time.

6. Conclusions

The core system of this article utilized the NI myRIO [27] to receive information on the platform, including flow velocity, flow direction, GPS, and sea temperature, as shown in Figure 18. The data were transmitted back to the onshore base station through a radio transmission module. Using LabVIEW 2019, the monitored data were displayed on the front panel. With this, operators can instantaneously ascertain the current status of the set net, enabling them to determine whether to deploy or retrieve the set net. The cost estimation for the self-designed velocimeter sensor in this article was approximately 300 dollars. After using this system, fishermen can judge whether to go fishing through the current conditions, and they can also know the current status of the set net at home. If the waves wash away the set net, the warning signal can be received the first time to avoid the safety of the navigation of the ship caused by the floating set net. Additionally, it can also avoid the set net that drifts away and entangles and kills marine life.