本文提供了一種基于 PLC 開關量輸入單元的轉速信號測量方法，該方法的特點在于測量成本低、可靠性高、測量精度和刷新時間穩定。文中舉出了用PLC 開關量輸入單元測量頻率的成本優勢，并討論了常見的頻率測量算法存在的問題，然后引入了一種變時間
窗多周期頻率測量算法，從而克服了常見的頻率測量算法測量精度和刷新時間不穩定的缺點。針對轉速信號測量的硬件，分析了PLC 自身特點對頻率測量的影響，以及如何解決。最后給出了算法的程序流程。
關鍵字：PLC；開關量輸入；頻率測量；時間窗
Abstract: This paper presents a approach measuring the rotate speed with digital IO input unit
of PLC. This method characters high reliability, low cost and stable precision and fresh time. It
illustrates the advantage of this method in low cost, discusses some common technique to measure frequency and recommends a arithmetic of MVTW ( multicycle and vary timing window ) that implements the stable precision and fresh time. As the hardware of measurement, It discusses the influences on measuring frequency from the feature of PLC, and how to deal with these influences. At last, It provides the flow of the approach.
Keywords: PLC; input IO unit; frequency measurement; timing window