“The measurement of underwater pressure signals has important military significance. The traditional signal acquisition is mainly to tow the cable on the ship and transmit the signal to the PC. This measurement method can only be carried out in the case of good sea conditions. When the sea conditions are poor, this method is dangerous and difficult. In this paper, the acquisition system designed with MSP430 micro-power single-chip microcomputer combined with SD card memory has the characteristics of high accuracy, low power consumption, large storage capacity, and the written files can be read and written by the Windows operating system. The actual test shows that the system has low power consumption, With high precision, reliable performance and easy operation, it solves the problem of high danger and high difficulty in marine surveying.
Author: Li Nan, Hu Shian, Han Wei
The measurement of underwater pressure signals has important military significance. The traditional signal acquisition is mainly to tow the cable on the ship and transmit the signal to the PC. This measurement method can only be carried out in the case of good sea conditions. When the sea conditions are poor, this method is dangerous and difficult. In this paper, the acquisition system designed with MSP430 micro-power single-chip microcomputer combined with SD card memory has the characteristics of high accuracy, low power consumption, large storage capacity, and the written files can be read and written by the Windows operating system. The actual test shows that the system has low power consumption, With high precision, reliable performance and easy operation, it solves the problem of high danger and high difficulty in marine surveying.
1 System hardware design
The signal data acquisition system is mainly composed of sensors, signal conditioning circuits, analog-to-digital conversion, control circuits, SD memory and power circuits. The system principle block diagram is shown as in Fig. 1.
Figure 1 System principle block diagram
1.1 Signal conditioning circuit
The signal conditioning circuit is mainly used for impedance matching, signal amplification, level conversion and low-pass filtering. The impedance matching circuit is mainly composed of a resistor divider, a voltage follower and a low-pass filter. The conditioning circuit is shown in Figure 2.
Figure 2 Signal conditioning circuit
Since the sensor outputs a 0-5V signal, and the supply voltage of the subsequent circuit is 3V, the sensor output signal must be converted to within 0-3V. Considering the small output impedance of the sensor, select two identical resistors in series to output the sensor The signal is divided by 1/2 voltage, and the voltage divider resistance is 10k. The output resistance of the divided water pressure field signal is very large. In order not to affect the next-level circuit, a voltage follower is used for impedance matching. The filter circuit adopts a second-order active low-pass KRC filter. Since the main purpose of this measurement is to obtain a slowly varying pressure signal, the cut-off frequency of the low-pass filter is 0.4 Hz, and the system sampling frequency is 1 Hz. The amplifier chip selects OP281 integrated with two amplifiers. This amplifier has the characteristics of low power consumption, high precision, and single power supply. The maximum operating current of each amplifier is 5 A, and its voltage noise peak-to-peak value is 10μV. Take a pressure sensor with a full range of 100m as an example. After partial pressure, the voltage generated per 1mm of water column is 25μV. Therefore, the amplifier noise meets the measurement requirements.
The transfer function of the low-pass filter in Figure 2 is:
The amplitude-frequency curve and delay characteristics are shown in Figure 3 and Figure 4 respectively. It can be seen from Figure 3 that the amplitude characteristic has better flatness below 0.2 Hz, and the deviation is small. It can be found from Figure 4 that within the passband of 0.4 Hz, the delay deviation is small, and the delay deviation of 0-0.2 Hz is 0.102s.
Figure 3 Filter amplitude characteristics
Figure 4 Filter delay characteristics
1.2 Analog-to-digital conversion
AD7799 is a 24 bit ΣΔADC introduced by Analog Devices, which works with a single power supply of 2.75-5.25V. The typical operating current is 380μA, and the minimum voltage noise RMS is only 27nV. AD7799 has three selectable differential input buffers (buffer can be connected or not), the output data rate can be set by software, and the allowable rate is 4.17-470Hz. It can provide 50Hz and 60Hz synchronization suppression under the condition of 16.6Hz default conversion rate, which is suitable for low-power analog front end for low frequency measurement. The digital interface circuit of AD7799 and MSP430F1611 is shown as in Fig. 5.
Figure 5 AD7799 and MSP430F1611 digital interface circuit
AD7799 is connected with the single-chip microcomputer through the SPI serial port. The 3-wire method is used here. The serial synchronous clock SCLK, the data input line DIN and the data output DOUT/RDY pin are connected with the single-chip microcomputer. The chip selection signal CS is independently controlled by the P3.0 port of the microcontroller. MSP430 reads and writes the data of each register in AD7799 through SPI, CS should be kept low during the read and write process.
1.3 Storage module
In order to enable the data acquisition system to record the measured physical quantity for a longer period of time, the data memory should have a larger capacity and lower power consumption. At the same time, in order to match the sampling frequency in the field environment, there are corresponding requirements for the read and write speed of the data memory. This system adopts the SD card (capacity 2G) produced by SanDisk Company. The SD card has the characteristics of high capacity, high performance and high safety, and its working voltage is 2.7-3.6V. The SD card works in SPI mode. Its SPI interface uses the SD card’s CS, SCLK, DATIN, DATAOUT to communicate with the MSP430. Among them, DATIN and DATAOUT are the data input and output signal lines, and CS is the chip selection signal line of the SD chip. During the entire SPI operation, CS must remain active low, and SCLK is the clock signal provided by the external controller. The interface circuit of SD card and MSP430F1611 is shown as in Fig. 6.
Figure 6 Interface circuit between SD card and MSP430F1611
2 System software design
The software design of this system is mainly the software design of the SCM system. The core CPU structure of MSP430 is designed in accordance with the tenet of simplified instruction set and highly transparent instructions. Therefore, the development of the single-chip microcomputer adopts the integrated development environment IAR Embedded Workbench designed specifically for the MSP430 series of single-chip microcomputers, and the programming adopts C language. The program execution flow chart of the one-chip computer is shown as in Fig. 7. In order to facilitate the reading of the collected data, the SD card file system format uses the FAT16 file format. Since the SD card takes a little longer to create a file in accordance with the FAT16 file format, if the file is created during the acquisition process, the subsequent data is easy to lose. Therefore, the file is created after the program is initialized. After the file is created, the MSP430 works in LPM3 mode. When the timer is up, the AD7799 is enabled, and the data after AD conversion is first placed in the MSP430F1611 memory. When it reaches 512 bytes, it will be programmed into the SD card by sector. After the data collection is over, the Windows operating system reads the data into the PC through the card reader.
Figure 7 Program execution flow chart of single chip microcomputer
3 Test verification
In order to test the performance of the acquisition system, the hardware power consumption and the actual test are tested.
3.1 Power consumption test
The power consumption test here is mainly for the acquisition system. During the measurement, the sensor is powered separately. The power supply of the acquisition system is a lithium battery (3.6V), and a 10 ohm resistor R1 is connected in series to the power supply. A 16-bit acquisition card (PCI-1716) is used to measure the voltage difference between both ends of R1 when the system is working. The sampling frequency is 1KHz. Obtain the current change of the system, and the system current change is shown in Figure 8.Figure 8 System current changes
It can be found from Figure 8 that the current presents a periodic change, which is caused by the continuous conversion of the single-chip microcomputer between the operating mode and the low-power mode. In the low-power mode, the average system current is 2.3mA. The proportion is 69.8%. In the operating mode, the average value of the system current is 3.1mA, so the average current of the system in 1s is:
3.1× 69.8%+ 2.3× 30.2% = 2.8814mA (6)
That is, the power consumption is 10.4mW. Taking 3.6V, 1A/h lithium battery as an example, the system can work continuously for 14 days.
3.2 Actual test
In order to test the performance of this system, take the water pressure signal collection as an example, the water pressure sensor uses a piezoresistive absolute pressure sensor, the power supply voltage is 8-32V, the maximum measurement depth is 70m, the full-scale output is 5V, and the conversion frequency of AD7799 is 16.6Hz. The peak-to-peak resolution is 19 bits. This system uses lithium batteries for power supply, using 2 sets of batteries, respectively 14.4V and 3.6V, 14.4V power supply to power the sensor, 3.6V after the voltage is converted into 3V to supply power to AD7799, MSP430F1611 and SD card. The actual test data is measured in a laboratory environment. The test water depth is 10m, the static pressure is about 10KPa, and the water is suddenly added to 5cm (about 500Pa) at 100s. The waveform diagram is shown in Figure 9. As can be seen from Figure 9, under the condition of 10m static water depth, the system successfully measured the dynamic pressure change of 5cm on the water surface, and the measurement accuracy is high, which meets the requirements.Figure 9 Measured pressure data
This article adopts the MSP430F1611 micro-power single-chip microcomputer combined with 24-bit A/D conversion chip AD7799 and SD card to design the acquisition system with the characteristics of high accuracy, low power consumption, large storage capacity, etc., which solves the low power consumption and large capacity of the underwater measurement system There are two major storage problems. The power consumption test and the actual pressure in the laboratory show that the system has low power consumption, high accuracy, and reliable performance. Sea trials to further verify the performance of the system will be the focus of the next step.
The author’s innovation is: In order to collect and store the pressure signal, a pressure signal data acquisition and storage system based on MSP430 single-chip microcomputer and SD card memory is proposed. This system successfully solves the two major issues of low-power consumption and large-capacity storage of underwater measurement systems. problem.