Issue 7 of Electronic Technology Applications, 2016
Authors: Yu Haibin, Yuan Jiuyi, Li Guanbao, Yan Haiyu
Abstract: The acoustic characteristics of underwater sediments are of great significance for underwater resource exploration, military applications, and geophysical scientific research. This paper addresses the limitations of existing acoustic sediment in-situ detection devices, which operate in a single mode. We designed and implemented a control system for an acoustic in-situ detection device that supports real-time monitoring mode, self-contained mode, and delayed self-contained mode. The system is centered on an STM32 processor and achieves functions such as motor control, acoustic signal and sensor signal acquisition and transmission, underwater video monitoring, and work log storage. Test results show that the system can operate stably and reliably in different working modes, meeting design requirements.
0 Introduction
The acoustic properties such as sound speed and sound attenuation coefficient of underwater sediments, as well as their spatial distribution, are extremely important for both military and geophysical scientific research. Representative measurement systems internationally include the Acoustic Lance (funded by the U.S. Navy), the In Situ Sediment Acoustic Measurement System (ISSAMS) developed by the U.S. Navy Laboratory, and the Sediment Acoustic and Physical Properties Apparatus (SAPPA) from the UK. Most of these systems rely on their own weight to quickly insert acoustic transducers into sediments, which can significantly disturb the sediment. Furthermore, the aforementioned measurement devices can only operate in modes with communication cables or self-contained modes, which limits their usability.
To overcome the technical shortcomings of the above systems, this paper develops a multi-mode deep-sea sediment acoustic in-situ measurement and control system. The system will use hydraulic drive penetration to insert four 1.4 m long acoustic probe rods into the underwater sediments. Compared to the above systems, the hydraulic operation greatly reduces the disturbance to the sediments compared to gravity-driven penetration. Moreover, users can choose between self-contained mode or real-time monitoring mode based on the actual equipment situation of the measuring vessel, significantly enhancing the versatility of the device.
1 Overall System Structure
1.1 Overall Design Requirements
The entire system includes four parts: the deck operation terminal, underwater control system, underwater data acquisition and storage system, and acoustic probe rod drive and beam acquisition system. This paper will mainly introduce the measurement and control system part. According to the design needs of the system functions, the following four problems need to be solved:
(1) The entire system will be designed to adapt to different research vessels, compatible with real-time monitoring mode with communication cable connections and self-contained mode without communication cable connections.
(2) Under the condition of having a communication cable connection, this system adds video monitoring functions.
(3) During the operation with a communication cable connection, the system can adjust the working parameters of the acoustic transducer in real-time based on the situation of the underwater sediments, such as gain and voltage. The collected acoustic signal can be transmitted back to the deck operation platform through the communication cable.
(4) When operating in self-contained mode, the system can automatically complete related measurement tasks on the seabed according to pre-set parameters, and the measurement data will be stored in the underwater measurement and control system.
1.2 Overall System Framework
The overall structure of the underwater multi-mode control system mainly consists of the motor and its drive part, video monitoring and measurement control part, and acoustic emission and reception acquisition part. The video monitoring and measurement control part includes the instrument mounting frame, underwater battery compartment, underwater measurement and control cabin, underwater camera, underwater altimeter, underwater lights, and necessary waterproof connectors, etc. The connection relationships of each part of the system are shown in Figure 1.
The measurement and control system is the core of the entire system and is key to the efficient, long-term, and stable operation of the underwater sediment acoustic in-situ detection system. This measurement and control system is mainly divided into two working modes: self-contained mode and real-time monitoring mode. The self-contained mode does not require a communication cable connection; the system will automatically complete the work based on the working parameters set before the device is deployed. Here, the self-contained mode is further divided into delayed working mode and water contact working mode. The real-time monitoring mode requires a communication cable connection; during system operation, the upper computer can monitor the underwater operating status of the device in real-time. After the device contacts the seabed, the depth of penetration of the acoustic probe rod can be set, and the working state of the acoustic measurement system can be controlled. The collected acoustic characteristics of the sediment will be stored and transmitted back to the deck communication terminal through the communication cable after being ASK modulated by the MODEM in the measurement and control system, using corresponding upper computer software for data analysis.
2 System Hardware
2.1 Measurement and Control System Circuit Design
The measurement and control system circuit mainly includes 5 switch control quantities, 6 channels of 12-bit analog-to-digital conversion circuits, 3 digital level detection ports, 3 serial ports, and 1 Ethernet interface, as shown in Figure 2. These 5 switch control quantities are divided into two types: one type outputs 24V voltage to control the underwater camera and underwater lighting when closed, providing them with power; the other type serves as a switch contactor to control the motor of the acoustic transducer probe, allowing for lifting and insertion functions. The 12-bit A/D acquisition circuit can collect voltage signals from 0V to 3V and current signals from 4mA to 20mA, which can be used to collect displacement signals of the acoustic transducer probe, pressure signals of the hydraulic cylinder, and signals from other underwater sensors. The voltage range of the underwater power battery pack is from 0V to 120V. Since directly using resistor voltage division to collect voltage is unstable and unsafe, a WBV342D01 voltage transformer is used to measure the direct current voltage in real-time, converting it to a standard direct current voltage output of 0V to 5V for the A/D circuit to collect. The system provides 3 USART communication interfaces, one for RS485 to receive information sent by the underwater altimeter. The other two are RS232, one for communication with the transducer drive and beam acquisition system via Modbus protocol, and the other for sending log data in self-contained mode to the data acquisition and storage system. At the same time, the system also provides 3 digital level detection ports for detecting water entry sensors, bottom contact sensors, and motor fault detection sensors. When the system operates in real-time monitoring mode, the data status frame and underwater video signal will be transmitted through the network via MODEM or optical terminal, modulated into DSL signals or optical signals, and transmitted to the deck.
2.2 Data Acquisition and Storage System Circuit Design
In order to collect various underwater environmental parameters as much as possible, and considering that most underwater sensors have RS232 data interfaces, while the main control system has insufficient serial port resources, a separate data acquisition and storage system needs to be designed. This system can connect with various underwater sensors, such as CTD (salinity, temperature, depth), DO (dissolved oxygen), etc.
This paper uses the 32-bit processor STM32F407VET6 with Cortex-M4 as the core for the data acquisition and storage system. This data acquisition and storage system includes 5 USARTs, 2 12-bit A/D conversion circuits, and a microSD card, as shown in Figure 3. USART1 is connected to the main measurement and control system to receive the operating status of the main measurement and control system. In working status, the main measurement and control system will send a status frame every second. USART2 is used for communication with a PC, which can be used for system time calibration before underwater operation and for exporting system status data after measurement completion (when the device operates in self-contained mode). The other 3 USARTs can be used to receive data from other underwater sensors, and each sensor’s data will be stored in files named by time and serial port number.
3 System Software Design
3.1 Real-Time Monitoring Mode Software Design
3.1.1 Software Design for Underwater Unit in Real-Time Monitoring Mode
The software design for the underwater unit in real-time monitoring mode can be divided into the following 4 steps:
(1) System initialization, entering real-time monitoring mode. After powering on the system, initialize the system clock, I/O ports, USART, and A/D ports. Receive mode selection instructions to operate in real-time monitoring mode.
(2) Receiving sensor status. During operation, the system will continuously collect data from underwater sensors through the ADC acquisition circuit and USART serial ports.
(3) Data frame integration and transmission. The system will integrate the collected sensor data and the operating status of the system into a data frame. The data frame contains fixed frame headers, frame tails, and data formats, with each sensor status separated by commas. The data frame format is shown in Figure 4.
(4) Receiving control data instruction frames sent by the upper computer. The upper computer software on the deck can send working instructions to the underwater unit, and the unit will execute immediately upon receiving the instruction.
The software design process for the underwater unit in real-time mode is shown in Figure 5.
3.1.2 Software Design for Upper Computer in Real-Time Monitoring Mode
The real-time monitoring platform includes a video display area, related status display area, and command operation area. The video display area will display the underwater working environment and the execution status of related mechanical actions. The status display area is used to show the system communication connection status and related sensor information. The command operation area is mainly used to control the underwater lights, camera, and related operations of the acoustic transducer probe, including turning on the motor and lifting or inserting the probe. The human-computer interaction interface is shown in Figure 6.
3.2 Self-Contained Mode Software Design
3.2.1 Software Design for Underwater Unit in Self-Contained Mode
The software design for the underwater unit in self-contained mode can be divided into the following 3 steps:
(1) System initialization, entering self-contained mode. After powering on the system, initialize the system clock, I/O ports, USART, and A/D ports. Receive mode selection instructions to operate in self-contained mode.
(2) Sequential execution of related operations. The self-contained mode is further divided into bottom contact self-contained mode and delayed self-contained mode. The system will sequentially execute related actions.
(3) Sending system status data frames to the data acquisition and storage system. The data frame format in self-contained mode is the same as that shown in Figure 4.
The software design process for the underwater unit in self-contained mode is shown in Figure 7.
3.2.2 Software Design for Data Acquisition and Storage System
The core processor of the data acquisition and storage system has the FatFs file system embedded. The FatFs file system has high configurability, with a minimum configuration requiring only 1 KB of RAM space, making it very suitable for embedded systems. The execution process of the data acquisition and storage system is as follows:
(1) System initialization and configuration of the RTC system clock.
(2) Naming files according to different ports (USART, ADC).
(3) Once the USART ISR interrupt is triggered and the data frame format is correct, the data will be saved in the file, and
(4) After the device is retrieved from the water, data extraction software can be used to export the data to the PC, and related status can form data curves.
4 Debugging and Summary
To verify the reliability of this measurement and control system, experiments were conducted in the laboratory workshop. Through repeated tests, it was found that the system can operate safely and stably according to design requirements and complete the specified measurement tasks. The entire measurement and control system successfully passed sea trials in the Yellow Sea, where the upper computer software on the deck could normally display the working status of the system at various times. In self-contained mode, the system could also strictly follow the pre-set working sequence to complete measurement tasks stably.
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