Solar street light charging and discharging monitoring system based on LabVlEW

　　1 Introduction

　　Solar street lights are mainly composed of four parts: solar photovoltaic cell components, batteries, charge and discharge controllers, and lighting fixtures. The bottleneck for the popularization of solar street lights is no longer a technical problem, but a cost problem. In order to improve the stability of the system and maximize the efficiency on the basis of reducing costs, it is necessary to reasonably match the output power of solar photovoltaic cells, battery capacity, and load power. For this reason, theoretical calculations alone are not enough. Because the intensity of sunlight changes rapidly, the charging current and discharge current are constantly changing, and theoretical calculations will bring large errors. Only by automatically tracking and monitoring the charging and discharging current can the maximum power output of photovoltaic cells in different seasons and different directions be accurately determined. In this way, the battery and load are determined to be reliable.

　　On the LabVIEW software platform, combined with data acquisition card, the use of virtual instrument technology can realize automatic monitoring and data analysis tasks. This paper describes the monitoring system developed by ourselves.

　　2 Design Principles

　　The conditioning circuit is used to sample the output current of the photocell, the discharge current of the battery, and the charge and discharge working voltage of the battery at a certain frequency, and the USB data acquisition module sends the collected data to the computer. The LabVIEW software platform is used for timely display and statistical analysis. The principle block diagram is shown in Figure 1.

Figure 1 System block diagram

　　3 Data Acquisition Hardware Configuration

　　3.1 Conditioning Circuit

　　The battery output signal is a floating signal. Using differential measurement will minimize the impact on the measured circuit and also reduce the measurement error. In this conditioning circuit, the differential amplifier made of high-precision, low-drift integrated operational amplifier ICL7650 uses the differential signal on the precision small resistor in series in the charging and discharging circuit as the current detection signal. The small signal obtained by the large resistor voltage divider incorporated into the battery is used as the voltage signal. To eliminate interference, two equal value resistors are connected between the two input terminals of the differential amplifier and the signal ground. The output signals of the two differential amplifiers are sent to the two input channels of the USB acquisition module to complete signal conditioning and acquisition. The conditioning circuit is shown in Figures 2 and 3.

Figure 2 Current signal conditioning

Figure 3 Voltage signal conditioning

　　This system uses a USB 1:3 data acquisition module produced by Canada HYTEK, model iUSBDAQ-U120816. Although it is not a high-end acquisition card, it is sufficient for acquiring DC signals, especially when the sampling frequency is not very high. After the collection is completed, select the "Exit and Playback" button, and the acquisition requirements will be displayed in the playback waveform window. The USB interface can be easily used for hot plugging and plug-and-play. Connecting a laptop can realize on-site data monitoring. [page]

　　3.3 Module configuration and driver

　　Execute the device driver iUSBDAQ-Driver.exe provided by HYTEK on the computer to activate the acquisition module. Then copy the iUSBDAQ_ALL_Vis.LLb dedicated VI function library provided by the company to the User subdirectory under the vl subdirectory of the LabVlEW7.1 installation directory, and then call User Functions and User Controls in the BlockDiagramt and Front Panel editing environments.

　　4. Software Development

　　LabVIEW is a graphical programming language developed by National Instruments (NI) and widely used in instrument control, automated testing, data analysis and processing, and is an excellent virtual instrument development software. It is increasingly widely used in the field of automated measurement and control. This system was developed using LabVIEW 7.1.

　　4.1 Program flow chart (see Figure 4)

Figure 4 Program flow chart

　　4.2 Panel Design

　　In the panel diagram shown in Figure 5, two switches are set to select whether to monitor current, voltage or both. The real-time current acquisition curve and the real-time voltage acquisition curve are displayed separately, and are equipped with digital display. In the acquisition cycle setting window, the sampling cycle is set in seconds. The "Start Acquisition" button also has a pause function. After all acquisitions are completed, select the "Exit and Replay" button, and the acquired current waveform will be displayed in the playback waveform window, and the area under the curve will be calculated as the ampere-hour (energy value) of current charging and discharging. A list of all data with date and time information is displayed at the bottom right of the panel, and the maximum and minimum values ​​of current and voltage are displayed.

Figure 5 Virtual instrument panel diagram

　　The overall structure is a flat sequence consisting of two frames. The first frame completes data acquisition, display, and storage; the second frame completes data calculation, recovery, and display of operation results. Figure 6 shows the graphical program of the first frame. [page]

Figure 6: Graphical program flowchart of the first frame

　　This frame uses a timed loop structure (Timed Loop). Before entering the loop structure, use OpenCreate/Replace File VI to establish the path and file name of the data file (.txt) to be stored, and call the iUSBDAQ_OpenDevice function to open the acquisition device. When the acquisition branch structure is selected, the two input channels of the acquisition device will be opened to collect data in turn to obtain a two-dimensional array. The array is separated by the Delete From Array function, and the two branch structures are used as current and voltage selection switches respectively, and sent to the waveform chart after numerical scaling transformation. The date and time information output of Format Date/TimeString and the string output converted from the two arrays through Number To FractionalString are combined by Concatenate Dtrings and sent to the specified disk data file through Write File VI for analysis and calling.

　　When the "Pause Collection" or "Exit Collection" button is selected, the loop should be exited quickly regardless of the collection cycle setting. To this end, an algorithm for determining the number of cycles and collection times is used.

　　Number of acquisitions = number of cycles/acquisition period.

　　The result of integer division is the number of acquisitions, and the remainder of zero is used as the judgment condition for the end of one acquisition. The two branch structures can be used to accumulate multiple discontinuous acquisition times until you choose to exit the timing loop. Since the set timing loop period is 1 second, the loop always runs at a period of 1 second regardless of whether it is in the acquisition state. When the "Exit Acquisition" button is pressed, the maximum waiting time is 1 second before exiting, instead of waiting for a sampling period set by the user.

　　After the data acquisition is finished, before exiting the first frame structure, close the file through Close File VI and close the device through iUSBDAQ-Release Defice. Figure 7 shows the second frame of the graphical program.

Figure 7 The second frame of the graphical program flowchart

　　This frame first uses the Read Characters From File VI function to read the data in the disk data file. Since the data read is a string, it must be converted into a three-dimensional array through the Spreadsheet StringTo Arrary function, and then separated into three one-dimensional arrays through two Delete From Arrays. The current information data is sent to BuildWaveform to generate a waveform, which is displayed by Chart. At the same time, the group of data is integrated through NumericIntegration.vi to obtain the area under the curve, and the ampere-hours of charge and discharge are displayed in the output energy window.

　　While replaying the current curve, the String window on the screen displays each sampling data. The current and voltage information arrays are then reshaped using Reshape Array and Array max&min to find the maximum and minimum values ​​and display them.

　　5 Conclusion

　　By using LabVlEW software and USB acquisition module combined with the developed virtual instrument, the whole process monitoring of the battery charging and discharging of the solar street lamp is completed well.

　　The operation is simple, the interface is friendly, and the measurement data is highly accurate. The data that is difficult to obtain with the ammeter and voltmeter is obtained. The measurement efficiency is improved and the instrument expenses are saved. The data measured in this way can reasonably match the output power of the solar cell with the battery capacity and the load size to maximize the benefits. At the same time, it has practical significance for extending the service life of the battery, improving the brightness of the street lamps, and budgeting the reserve energy for continuous rainy days.

　　This system is also valuable for other rechargeable batteries. On this basis, the control unit can be further developed, and the PID toolkit of LabVlEW can be used to realize automatic control of battery overcharge, over discharge, short circuit, reverse charge and time switch.

References:

[1]. ICL7650 datasheet http://www.dzsc.com/datasheet/ICL7650_989776.html.