Data acquisition and application of a new temperature, salinity and depth sensor

0 Introduction

With the development of industrial and national defense technology, diving equipment has higher and higher requirements for high, precise and advanced detection technology in various applications, especially in military applications. In the technical development of various diving equipment, the important submersible fully reflects this point. In order to improve the deep diving ability of submersibles, submersibles are equipped with integrated navigation display control systems to ensure that submersibles can accurately dive and successfully complete various underwater operations. Due to the size and load limitations of submersibles, the sensors used in the integrated navigation display control system must be small in size, light in weight, and resistant to high pressure, and they must be able to quickly and accurately obtain the navigation parameters of the submersible in terms of function. In terms of measuring the temperature, conductivity, pressure, depth, salinity, sound speed, density and other state parameters of seawater, the SBE 37-SI MicroCAT temperature, salinity and depth sensor can measure the seven state parameters of seawater in one, and can well meet the requirements of system design in terms of accuracy, volume and weight. It is a very suitable sensor for use on submersibles. The following is a discussion on the data acquisition process of the SBE 37-SI MicroCAT temperature, salinity and depth sensor in a submersible. As the transmission format does not correspond and is not convenient for direct transmission, a feasible design scheme and idea is proposed, and an example that has been applied is given to illustrate.

1 Application of SBE 37-SI MicroCAT Temperature, Salinity and Depth Sensor in Submersibles

The integrated navigation display and control system of the submersible mainly uses the combined navigation technology and information fusion technology to organically combine various navigation devices. Without changing the navigation devices, the filtering technology is used to process various navigation information, so that various navigation information can complement each other and improve the navigation positioning accuracy. Through the centralized display, centralized control and comprehensive processing of navigation information, the role of each navigation information can be maximized, and comprehensive and optimal navigation information can be provided to the equipment in real time. The role of the temperature, salinity and depth sensor is to measure the various state parameters of seawater in real time, such as temperature, conductivity, pressure, depth, salinity, sound speed, density, send them to the integrated navigation display and control system, and then display them or use them for other equipment. In order to ensure the safety of the submersible's navigation and the completion of the salvage and search and rescue mission, the measurement of the sensors on the submersible must meet the requirements of navigation; the refresh rate of the data must meet the requirements of the submersible; the measured data must be stable and reliable. The performance of the SBE 37-SI MicroCAT temperature, salinity and depth sensor in these aspects can meet the requirements of the navigation system.

Figure 1 SBE 37-SI MicroCAT temperature, salinity and depth sensor
SBE 37-SI MicroCAT temperature, salinity and depth sensor is currently mostly used in submersibles and submarines abroad, and is rarely used in China. As shown in Figure 1, its overall dimensions are 43.69cm long, 13.97cm maximum height, 10.8cm maximum width, and weighs about 2.9kg. The measurement accuracy of the SBE 37-SI MicroCAT temperature, salinity and depth sensor is very high. For example, the temperature range is -5 to 30°C, and the accuracy reaches 0.0001°C. It can measure the required accurate navigation information under high pressure conditions at a depth of 7000 meters. SBE 37-SI MicroCAT and other devices mainly communicate serially asynchronously, exchange data, and transmit bidirectionally based on the NMEA-0183 standard. Its external interface is EIA RS-232, and the transmission baud rate can be selected from 1200bit/s to 38400bit/s. The character code sent is ASCII code, and the transmission rate of the measurement data can also be selected by the output parameters. The time interval for each measurement data output ranges from 1.15 seconds to 22.19 seconds. [page]

2 Design of SBE 37-SI MicroCAT Interface Circuit

The output data format of the SBE 37-SI MicroCAT temperature, salinity and depth sensor is similar to the common format of general navigation equipment, such as the data format of GPS: $PASHR, POS, n, ..., tt.t, vvvv * cc [CR] [LF], which contains the start character $, data content, code and mark *, code and cc, and data terminator [CR] [LF]. However, the output data format of the SBE 37-SI MicroCAT does not have the start character, code and mark, and code and. It also has a great feature: the output of the SBE 37-SI MicroCAT temperature, salinity and depth sensor has a level protection function. When it is powered on again after a power outage, the output level is zero, and it will not send measurement data out autonomously. It must be triggered by sending an excitation signal (data sending command). Its trigger signal is two carriage return and line feed bytes (i.e. 0x0D, 0x0A), but it cannot be sent continuously. It must be sent twice with an interval of two seconds, so that the sensor can be triggered and send data out according to the set format and rate.

In order to make the data format of the temperature, salinity and depth sensor output consistent with the data format of the commonly used navigation equipment, a data format conversion interface circuit must be designed. The tasks it needs to complete are: first, obtain external instructions or generate a trigger command to send to the sensor to trigger the sensor; then receive external instructions to change the baud rate of the SBE 37-SI MicroCAT output data and the update rate of the transmission data; and receive data from it, add the start character, code and flag, and code and; if there is a space in the data, replace the space with zero; finally, output the data to the integrated navigation display console. In this way, the data format conversion interface circuit must handle the interruption of the control instruction and realize the change of the baud rate and data update rate of the sensor on the one hand, and communicate serial data with the sensor on the other hand.

3 Practical application examples

Since the RS232 data format provided by the SBE 37-SI MicroCAT temperature, salinity and depth sensor is different from the commonly used navigation equipment data format, it cannot be directly connected to the submersible integrated navigation display console system for data exchange. In order to facilitate communication, the author of this article designed a data format conversion circuit board in actual engineering applications. The circuit structure is shown in Figure 2.

Figure 2 Design of SBE 37-SI MicroCAT data format conversion circuit board
In Figure 2, the communication control microcontroller is responsible for receiving data, converting the received ASCII characters expressing the state parameters of seawater into digital types, judging the positive and negative of the digital type, replacing the corresponding spaces with zeros, and then converting it into ASCII character type, adding the starting symbol $ and the first letter of each data (i.e. T, C, P, D, S, V, R), calculating the code and all characters after $, and then adding the code and symbol and the code and carriage return and line feed, and finally outputting it to the integrated navigation display console through RS232. On the other hand, it receives commands from the integrated navigation display console and then sends them to the SBE 37-SI MicroCAT temperature, salinity and depth sensor, and returns the response of the sensor completely to the integrated navigation display console. Therefore, the integrated navigation display console can change the output data, data output interval and status of the temperature, salinity and depth sensor at any time, and can also receive data in a universal format.

The hardware used in this example is the commonly used and reliable AT89C51 microcontroller, which is a low-voltage, high-performance CMOS 8-bit microprocessor with 4K bytes of flash programmable and erasable read-only memory (FPEROM). The device is manufactured using ATMEL's high-density non-volatile memory manufacturing technology and is compatible with the industry-standard MCS-51 instruction set and output pins. Because it combines a multifunctional 8-bit CPU and flash memory in a single chip, ATMEL's AT89C51 is an efficient microcontroller that provides a flexible and inexpensive solution for many embedded control systems. [page]

Since the program design is relatively simple, in order to improve development efficiency, the program is not written in high-level language, but directly in assembly language. In an environment with high reliability requirements, the programming of the serial port RS232 only uses three signal lines, namely, sending, receiving and grounding, to ensure that data can be sent and received at any time, thus improving reliability.

The circuit shown in Figure 3 is a SBE 37-SI MicroCAT data format conversion circuit that has been designed and used in practice. The circuit is exquisitely designed, small in area, easy to install, and the signal line is easy to plug and unplug.

The data format directly measured by the SBE 37-SI MicroCAT temperature, salinity and depth sensor in the air (if the data is negative, add one more bit):

xxx.xxxx,xx.xxxxx,xxxx.xxx,xxxx.xxx,xxxx.xxxx,xxxxx.xxx,xxx.xxxx[CR][LF]

24.7798, 0.00068, -0.310, -0.307, 0.0130, 1496.123, -2.8873[CR][LF]

The interface circuit receives the data from the sensor (three negative values) and the data output by the interface (replacing spaces with zeros):

$Txxx.xxxx,Cxx.xxxxx,Pxxxx.xxx,Dxxxx.xxx,Sxxxx.xxxx,Vxxxxx.xxx,Rxxx.xxxx＊cc[CR][LF]

$T024.7798,C00.00068,P-0000.310,D-0000.307,S0000.0130,V01496.123,R-002.8873＊66[CR][LF]

Figure 3 SBE 37-SI MicroCAT format conversion circuit board
4 Conclusion

The author's innovation: The performance, volume, quality and reliability of the SBE 37-SI MicroCAT temperature, salinity and depth sensor can meet the design requirements of the comprehensive navigation and display console system in the submersible, and can accurately measure the temperature, conductivity, pressure, depth, salinity, sound speed, density and other state parameters of the seawater required by the submersible. Since the data format of its output is different from the commonly used format of the navigation equipment and cannot be directly connected, a data format conversion circuit must be connected to its interface to obtain the conventional data output format, so that the communication and data acquisition software can be developed more quickly, and the control and communication of the data acquisition system can be completed through the human-machine interface. The implementation method of the software and hardware of the example system introduced in this article is relatively simple, highly reliable, and has wide practicality.