以FMCW RADAR實現高精準度液位量測

Abstract

儲油槽、油船以及液體儲藏槽都需要液位量測系統對液體進行監控與計量，高精準度液位量測對於各種液位自動控制系統的正常運作以及液體的安全儲藏均有重要的意義。壓力式液位測量系統原理簡單、成本低，但測量精度與應用場合均有一定的限制。超音波液位儀精準度高，但設備複雜且安裝維護麻煩。微波液位量測系統是非接觸性方式，不受液體狀態限制，因此可在高溫、高壓、有毒以及腐蝕性強之液體環境下工作。FMCW雷達測距系統，具有距離分辨率高、發射功率小等優點。因此我們主要研究FMCW液位量測雷達基於FFT訊號的高精準度測距原理。 FMCW雷達的測距誤差受有限長度FFT頻譜估測精準度所影響，FFT過程柵欄效應和頻譜洩漏影響頻率的精準度，而在高精準度要求較高的情況下，直接利用有限長度FFT頻譜估測是無法達到精準度的要求，因此本文將提高頻率估測精準度從而提高距離量測精準度。採取的方法則是利用FFT得到FMCW雷達差頻訊號峰值的粗略範圍，再利用Gradient Search Method訊號處理方式從而實現高精準度之頻譜。

Oil storages tank, Oil tanker and other process tanks all need liquid level measurement system to monitor and take measurement on liquid. High accuracy measurement of liquid level has great significance on varieties of formal operation on liquid automatic control systems and secure store of liquid. The principle of pressing liquid measure system is uncomplicated and lower-cost, but it has established limitation on measured accuracy and applied fields. The accuracy of liquid level on Ultra-sonic is high, however, the internal parts of equipment is complex and installment and maintainability are troublesome. Measurement liquid level of microwave system is a way of isolation and not to be limited on the state of liquid. Therefore, it is able to perform on the environment of high-temperature, high-pressure, toxic and corrodible liquid. Measurement by FMCW radar system includes the high resolution of measurement range, the lower transmitter power, and so on. For these reason, our research is mainly involved in FMCW level radar which is based on the high accuracy of ranging of FFT signals. Error in measurement range of FMCW radar is affected from the high accuracy of the estimate of FFT spectrum that is finite length FFT. It will produce picket fence effects and leakage effects. On the condition that the high accuracy is requested, using the estimate of FFT spectrum directly is unable to achieve the request for the accuracy. Hence, we will increase the accuracy of estimate frequency to increase the accuracy of range estimation. The method we take using FFT to get the rough scope of the peak value on beat frequency signal. And then, use the digital signal processing method to achieve the high accuracy of spectrum.