自航水下載具縱移運動參數鑑定

PARAMETER IDENTIFICATION FOR SURGE EQUATION OF AN UNDERWATER VEHICLE

An estimating process is proposed to estimate parameters of the surge equation of an underwater vehicle. The parameters to be identified are: the thrust deduction factor, wake coefficient, and resistance coefficient. A free running underwater vehicle testbed is designed and constructed. A propeller with known data is controlled to propel the vehicle. The estimation process uses the surge speed and the propeller rotational rate as the input information. The surge speed is measured by a Doppler velocity sensor, and the rotational rate of the propeller is measured by an encoder. The surge speed conversion from the Doppler velocity sensor is corrupted by noises. A model based observer is designed to filter the surge speed out of noisy signals. Non-dimensional thrust force coefficients of the propeller are used as initial estimates of the model coefficients. Asymptotic convergence and boundness in errors of the filtered speed are proved by the linear system theory. The testbed vehicle is accelerated from rest, and it is propelled forward by the propeller with a controlled rotational rate. Accelerations are derived from estimated surge speeds. Least squares algorithm is further applied to derive the values of parameters. The identification procedure consistently predicts the aforementioned parameters at various surge speed conditions. Thrust loading coefficients are calculated from trials data based on identified parameters, and the derived rotational rates of the propeller are compared with the measured rotational rates. Propagation of parameter uncertainties are discussed based on the motion equation and the functional relationship between model coefficients and the thrust deduction factor, wake coefficient, and resistance coefficient. 本研究建立參數估測器，以估測一水下載具自航模型縱移方程式之阻力與推進參數。本研究製作一具自航載具，於水槽中驗證本文提出之參數估測方法、速度量測濾波器設計、參數解法，以及參數不確定性傳遞特性等。水下載具使用都卜勒速度聲納、方向計、深度計及螺槳馬達編碼器等記錄載具從靜止，加速至等速航行之運動數據。為了量測正確的速度，本文建立依據水下載具縱移模式所設計之觀測器，此觀測器對縱移速度無偏差之估測性質，可得到正確的速度量測值，並推求其加速度，以及對應之螺槳轉速，代入最小平方法估測縱移模式之阻力係數、推減因數、跡流因數及螺槳推力，所得參數值亦可用來正確反算螺槳轉速。本研究發現參數標準差與實驗採用的縱移速度有關，且參數之變異──共變異矩陣與水下載具之縱移方程式，以及量測雜訊有關。最後，本研究根據實驗數據討論參數誤差傳遞受參數誤差共變異值的影響。