Software System for Impedance Measurement Unit
Small-signal impedances of components in an electronic power system can be used to judge the stability of the system. Measuring impedances of the system needs a sophisticated control and software system. Such a system consists of a state-of-the-art data acquisition unit, several high-precision sensors, and a PC with the software for control and user interface.
The main purpose of the data acquisition unit is to sample the signal and improve it by removing the noise, addressing possible aliasing issues, and removing small dc offset issues due to imprecise components in the signal conditioner. The digitally conditioned signal is also used for phase and frequency identification and then sent to the PC for off-line impedance calculation and system stability estimation (offline data analysis).
The hardware of the acquisition unit is a National Instrument PXI computer (NI PXIe-8840) with a high-performance DAQ card installed. The main design challenge for this unit is the software. To coordinate the real-time task, the PXI computer is set to run the real-time operating system. The LabWindows/CVI with real-time support module is used for real-time software development. It is also selected for the development of high-level measurement control PC software. Using the same development platform for PXI and PC software makes development easier since code can be easily shared between platforms, which saves the cost of development. The measurement control application includes all the functional units for data acquisition, storage, and analysis, realized on the host computer with a non-real-time operating system.
Different software techniques are used to ensure that an acquired digital signal will not be lost on its way to storage and to pre- serve required precision. The software solutions used to develop this system are discussed in this paper.