Using LabVIEW software and NI PXI hardware to develop monitoring systems

Challenge: Build a custom real-time monitoring system capable of evaluating the performance and characteristics of grid-connected PV systems using a variety of measurement devices in a short time and at a low cost.

Solution: Use the open NI LabVIEW software platform to design the monitoring system and dedicated interface software. Input the outputs of multiple measurement devices into the PXI system through the serial interface. Use the ready-made User Datagram Protocol (UDP) functions to transmit the data to the PC, providing real-time display of different measurement results.

"The core of the whole set is the NI PXI-8184 real-time controller, which provides data storage capabilities, high system reliability, compactness, ruggedness and easy configurability."

The Singapore government has invested SGD 350 million to transform the city into a global clean energy hub, with a focus on solar energy. To achieve this goal, the Clean Energy Research and Experimentation Program (CERT) under the Singapore government and the Economic Development Board has developed a number of measures. To this end, we opened the Solar Technology Center with five different photovoltaic panels, with a total power of 14.2 kW. The center is used to demonstrate different grid-connected photovoltaic systems and serve as a test laboratory center to study the performance and characteristics of different photovoltaic modules in long-term operation under tropical climate conditions.

To study the performance and characteristics of these PV modules, we asked a team from the Electronic Engineering Department to build a custom real-time monitoring system capable of measuring different parameters of the PV array under test and actual climate conditions in a short time and at a low cost.

The team used LabVIEW software for development because it provided an open development platform, versatility, and an original graphical user interface. The software allowed them to quickly develop a non-standard Modbus serial communication protocol, which is a key interface for transmitting multiple measurement device outputs to the PXI real-time controller system. Afterwards, using UDP communication functions, the data was transferred to a PC for real-time display, analysis, and storage.

Photovoltaic system measuring instruments

The core of the whole set of equipment is NI PXI-8184 real-time controller, which provides data storage function, high system reliability, compactness, ruggedness and convenient configurability. The software development is based on LabVIEW and LabVIEW Real-Time Module. The PV measurement and monitoring system is shown in Figure 1.

Develop Modbus serial communication function

The weather monitoring system consists of seven measurement parameters, including global solar irradiance, diffuse solar irradiance, temperature, humidity, wind speed, wind direction, and rainfall. The measurement parameters of the photovoltaic monitoring system include DC voltage, DC current, and panel temperature. AC power and power generation are collected using electronic power meters. In addition, a total of 22 measurement data points are collected from the measuring instrument and five electronic power meters, totaling 23 measurements. In order to be able to handle multiple measurement data and minimize the wiring between the equipment and the PXI controller, a serial interface is used.

The team used Modbus Remote Terminal Unit (RTU), which is an open serial (RS232 or RS485) protocol that provides master/slave communication between devices connected over a network. It is simple to use, reliable, low-cost, and can exchange data in binary format, which improves throughput. However, each Modubs device manufacturer implements this protocol with different function codes, data formats, and cyclic redundancy check (CRC) codes. Therefore, it is necessary to develop a Modbus driver to allow users to modify the implementation of the protocol according to the requirements of different manufacturers.

Since LabVIEW provides Virtual Instrument Software Architecture (VISA) serial functions, it is simple to design a program to communicate between these devices. In addition, since data operation functions such as dividing numbers, right shift with carry, exchanging bytes, type conversion, etc. are provided, we can easily implement the Modbus message structure using the corresponding function code, data format, and CRC error verification algorithm. In this way, we developed and tested the Modbus RTU interface program and it was able to work for devices from different manufacturers.

Data transfer from PXI real-time controller to PC

The team used the UDP function in LabVIEW to transfer the collected data to the PC. UDP is simple, convenient, and can broadcast messages to multiple addresses at the same time. Although it does not guarantee reliable data transmission, this does not cause any impact in this system because the system only needs to transmit data to the PC at a time interval of 10 seconds, so even if some data points are lost, it will not cause any problems.

Designing graphical user interfaces

One of the system requirements was to provide a comprehensive display of all information related to the monitoring system measurement data and the Solar Technology Center settings. The tab control in LabVIEW provides the user with a practical feature to place all the information in a single application without making the user interface too complicated.

Another challenge is to display the different measurements in a graph. Graphs allow users to visually see the relationship between one parameter and another, as well as the trend of any parameter. At any point in time there are 22 measurement data points, including weather parameters, PV DC voltage and current, and panel temperature for each type of solar panel. It is impossible to list all the data on a single graph. This can be solved by controlling the graph drawing programmatically. The program is designed using item selection and graph property nodes to achieve this. Users can view up to four data curves and measurement parameters in one graph at a time. Figure 2 shows a screenshot of the user interface.

in conclusion

LabVIEW helped our team quickly develop the Modbus interface program within three months and design an excellent user interface that is rich in information. Using NI PXI hardware ensured the reliability of the system, which has been running 24 hours a day, seven days a week since September 2008. The next task for our team is to evaluate the performance of the PV system, including evaluating the efficiency of the PV array, the relationship between energy generation and climatic conditions and module conditions.

Figure 2: Screenshot of the user interface