本文為改良基於過門檻時間的Time of Flight(T-O-F)量測方式。這個方法綜合T-O-F以及相位差量測之優點，同時具備長距離量測以及高精度量測之優點，本文有以下數點創新：1.使用多次跨門檻時點資料整理出到達時間資訊。2.用不同頻率之超音波之多次跨門檻時間，推算峰谷到達時間差，其以時間差之變動反推第一波到達時間。 本文同時提供用來實現雙頻率超音波峰谷到達時間測量的具體電路介紹。為實測用於量測距離的創新雙頻超音波測矩電路，我們也設置完成PC-BASED測試平台，此一測試平台在文中我們會一併介紹。運用這一個測試平台，我們完成了最少需求發射波數研究實驗、 可用穩態接收波段研究實驗、最小雙頻間隔時間研究實驗、間矩與介質微變異影響與反制研究實驗、以及最終的雙頻超音波測矩實驗。實驗的結果不但驗證了雙頻峰谷到達時差量測距離可行，同時在1450mm的測試距離內，使用波長8.6mm的超音波，實驗的標準差也達到0.097mm精度。由實驗結果，我們建立了一個使用雙頻超音波來測距條件參數表格，希望能有益於有興趣使用這種方法量測矩離的朋友。本研究所提出之測矩電路，僅用到微控制器，數位邏輯電路以及簡單功率放大電子元件，無需ADC等成本高之電路元件，不僅成本上具有優勢，且很容易IC化。 This paper proposes an innovative distance measurement method based on the time of flight (TOF) of ultrasound. By introducing received ultrasonic wave peak time sequences (PTS) of two slightly different frequencies, the relative distance can be accurately identified with resolution much better than a wavelength. The new PTS distance measurement is achieved in two steps. Firstly, a peak time sequence is built for received ultrasound signal of each frequency according to the arrival time of the wave peaks by calculating the mean value of the adjacent crossover time. Secondly, the arrival time of the wave front is rebuilt by estimating the common initiation time of the peak time sequences for the received waves of slightly different frequencies. A mathematical model is derived to describe the signal reception, from which the TOF estimation algorithm was derived. A simulation model describing the characteristics of the ultrasonic transducer and the ultrasonic wave propagation physics is developed to verify the new algorithm. An experimental system was implemented to confirm the feasibility. A PC-based test bench was built to test the characteristics. The characteristics of the transient behavior of the PTS were studied to determine the implementation parameters. Obtaining the PTS by averaging repeated tests was found to be effective in enhancing the precision, as well as the robustness, of the algorithm. In a TOF measurement over the distance of 145cm , a STD of 0.0113 of a period was achieved by a nominal driving wave period of 25.6us (39KHz) and frequency difference factor of 0.0048. When applied to distance measurement, the worst STD of 0.097mm was achieved with a relative distance ranging up to 1450mm, given the nominal driving wavelength of 8.6mm. This new dual frequencies PTS based TOF measurement system can be economically embedded in a micro controller together with floating point gate array (FPGA), and some simple transistors suitable for positioning mobile units indoors or in small open field environments.
