True AC Root-mean-square Measurement Based on the Calculation of Cycle Double-end Sampling Values

When non-synchronous sampling measurement method was applied to the measurement of AC RMS, the increase in the measurement error was resulted from the existence of the RMS calculation error. A new non-synchronous sampling true RMS measurement method and implementation of the program was presented in this paper, and RMS calculation error greatly reduced through distributing the sampling values of cycle double-end. Analysis and calculation showed that: for an AC signal with the fundamental frequency of 50 Hz, the RMS calculation error was less than 0.00012% when the frequency of period sampling was more than 128 times, and with the increase in the frequency of sampling, the error decreased faster. This error was much smaller than the quantization error of an A / D converter in measuring. The first advantage of the measurement meter designed based on this method is fast response speed, which can be seen in the result that the true RMS of the former cycle can be obtained at first time sample of the next cycle. Additionally, the second advantage is high measurement accuracy, and the third advantage is that it suits for the situation of AC frequency fluctuation and so on.