Design of multi-band spectroradiometer acquisition system based on C8051F020 microcontroller

introduction

Our country launched its own water color satellite in 2001. In order to meet the needs of the development of water color remote sensing, we developed our country's offshore ocean optical buoy. The anchor chain underwater multispectral radiometer is one of the main observation equipment planned to be equipped. The data acquisition system is the work control center of the equipment.

Overall design ideas and working principles

In view of the specific working environment and working cycle of the anchor chain underwater multispectral radiometer, during the design process, the design of the data acquisition card must follow the principles of high accuracy and low power consumption. First of all, since the buoy uses independent power supply in the sea to operate continuously for more than three months, the energy of the power supply battery must be rationally utilized to ensure that the battery has enough power to supply the instrument with normal operation. To this end, effective measures must be taken to improve power utilization. Secondly, the optical signals in the depth of sea water are relatively weak. For the detection of weak optical signals, on the one hand, a high-sensitivity photodetector with a preamplifier must be used (the internal preamplifier uses a carrier auto-zeroing operational amplifier, and its low-frequency The characteristics are close to those of an ideal operational amplifier), and on the other hand, the overall accuracy of the instrument must be improved. At the same time, the data measured when the buoy is working must correspond one-to-one with the current position (including depth, azimuth, etc.). In addition, optical windows must be protected against contamination.

The data acquisition card is designed as an independent measurement unit with its own CPU. Its internal clock interrupt triggers the CPU to start sampling control, power on the data acquisition card, and complete the work required by the buoy control center (such as decontamination, sampling, and communication). wait). After receiving the stop command from the buoy control center, the CPU module actively writes the next working time to the clock chip on the data acquisition card, and then turns off all power supplies to put the entire instrument into a low-power state.

Hardware components of data acquisition card

The hardware circuit part of the data acquisition card is shown in Figure 1. It is mainly composed of three parts: CPU module, A/D conversion module and battery management.

CPU module

The CPU module serves as the control center of the entire spectroradiometer, controlling the data collection and transmission of the instrument, the antifouling and decontamination of the window, and the power supply process of the instrument.

The microcontroller used in the CPU module is C8051F020 from Cygnal Company of the United States. It is a fully integrated mixed-signal system-level MCU chip with a speed of up to 25MIPS and powerful control functions. Since it integrates I2C, SPI, UART and other serial communication methods, when choosing a clock chip based on I2C communication and a high-precision ADC based on SPI communication, there is no need to design complex software simulation programs to simulate the corresponding SPI or I2C communication.

After the data acquisition card is powered on, the CPU module first calibrates the clock time on the data acquisition card according to the GPS (Global Positioning System) time, and then waits for the buoy control center to send a work command. After receiving the work command, the CPU will select the current work of the data acquisition card based on the received work command, including controlling ADC sampling, controlling the anti-fouling device to clean the optical window, uploading the ADC results to the buoy control center, etc. . After completing all the work specified by the buoy control center, the CPU module automatically controls the drive motor to rotate the protective cover of the anti-pollution device back to the top of the optical window and shut down all power supplies in the system except the clock power supply. The clock interrupt time on the CPU module can be the eight fixed-point interrupt times set by the CPU module, or the encrypted interrupt time given by the buoy control system as needed. After completing the last sampling of the day, the CPU module will actively write the dark current measurement time point to the on-board clock and perform a dark current calibration on the instrument. The dark current data will be saved in the flash memory of C8051F020 and reserved for the next day. Calibration data of instrument measurements, which can ensure the accuracy of instrument measurements.

Using the built-in 8-channel 12-bit ADC of C8051F020, the depth, inclination and azimuth angle of the radiometer in seawater can be measured. At the same time, it can be used to detect the power of the instrument battery pack and provide timely and accurate information to the overall control system of the buoy.

During the design process, the built-in watchdog of C8051F020 is used, and there is no need to add other reset sources to the CPU module.

A/D conversion module

The A/D conversion module mainly converts the pre-amplified photoelectric signal detected by the photodetector into a digital signal. In order to improve the measurement accuracy of the instrument and achieve the predetermined design accuracy, a high-precision, wide dynamic range, Δ-Σ type 8-channel 24-bit ADC was selected in the illuminance meter. Based on the characteristics of SPI communication, when communicating between the ADC and the microcontroller C8051F020, only three simple wires (SCLK, Din, Dout) are needed to connect the hardware, which greatly simplifies the design of peripheral circuits.

Since there are a total of 24 optical signals that need to be detected, this design uses four ADCs to realize the sampling and conversion of photoelectric signals through the control of C8051F020. When the sampling point arrives, the ADC of the radiometer waits for the CPU to send a sampling command after completing the initialization work. The CPU module will send the corresponding control command to the ADC after receiving the sampling command from the overall buoy control system. The ADC will follow the instructions of the CPU. Convert the optical signal. The conversion result is first transmitted to the C8051F020. After averaging and correction are completed inside the microcontroller, it waits for the host computer to send a sampling command. After receiving the sampling command from the host computer, the CPU module transmits the data to the shore station through the UART interface. on the buoy control center. The analog ground and digital ground of the ADC are connected together at a point outside it.

Battery pack and power management

Because the instrument uses independent power supply to work continuously for a long time in seawater. Therefore, in order to improve the effective utilization rate of the battery, in the design, the low power consumption requirement of the instrument is achieved through two ways. First of all, when selecting devices, we must strictly control their power consumption, and try to choose low-power devices among similar products; secondly, exercise reasonable control over the use of power supplies. Since the buoy has been working continuously in the sea for more than three months, it works 8 times a day for about 5 minutes each time. Therefore, the working time of the buoy is much shorter than the time when it is not working. In order to save power, this design designs a high-precision clock in the radiometer. , allowing it to work uninterrupted (clock power consumption is less than 1mW, using a button battery for continuous power supply). The clock is controlled by software to generate interrupts at 8 working points every day.

The radiometer comes with three sets of rechargeable batteries, which are 3.6V, +14.4V and -14.4V.

Figure 2 shows the workflow diagram of the data acquisition card. The relevant software subroutines are written in assembly language. The watchdog inside F020 is turned on in the program to ensure the stability and reliability of the instrument.

Conclusion

This data acquisition card is used in the buoy underwater multi-band spectroradiometer. It is mainly used to measure the downward spectral amplitude illuminance of the underwater true light layer and the profile measurement of the uplink spectral amplitude brightness. It uses the measured water body optical radiation data to deduce the water separation Water optical parameters such as amplitude brightness and water body can be used to estimate ocean photosynthesis and its primary production to meet the needs of on-site optical radiation measurement technology for water color satellite remote sensing.