於海流作用與不同載重下自主式水下無人載具之運動與深度控制

Abstract

自主式水下無人載具(Autonomous Underwater Vehicle)長時間以頭向控制方式航行時，一旦遭遇海流，若無母船(以水下通信機制)或外界(GPS)提供其精確之位置資訊以修正其位置之情況下，將造成其航行路徑之漂移，且隨著航行時間之增加而加大其漂移量。因無精確之位置資訊，若使用一般水下無人載具所慣用之轉折點(way point)與橫向跟蹤誤差控制法(cross track error control method)，其實際之路徑控制效果，將明顯受限於載具之位置精度。 自主式水下無人載具之深度控制迴路若採用比例控制器，則載具於水下航行期間，可能會由於載具之鰭、舵設計、佈局不同或物性(載具重量、浮力、重心或浮心位置)變異等因素，而造成載具於等深度航行時，在特定平衡攻角與平衡俯仰舵角下，一旦達成了載具俯仰向之力與力矩平衡，則於此深度控制上，將有可能被迫存在某種程度之誤差量，而無法透過原先設計之控制迴路作用以進行消除。一般而言，吾人可透過加入積分控制器之方式以消除穩態誤差，然而不當之積分控制器雖可消除穩態誤差，卻也可能造成暫態響應變差。 本論文之研究分為二個主軸，分別為1)針對於海流作用下，載具位置精度對其路徑控制成效影響之問題，本文提出以速度向量合成之觀念，藉由修正載具之航行頭向角、航行時間或推進速度等之方式進行路徑控制，因無需考慮載具之位置資訊，故可避免如同轉折點與橫向跟蹤誤差控制法對載具位置資訊精確度之依賴。2)針對載具深度控制迴路僅有比例控制器時，因不同之載重而可能造成不同程度之深度控制穩態誤差現象，本文提出透過載具配置可移動之質量塊機構，以改變其重心位置的方式，從而減小載具於等深度控制時所需之俯仰舵平衡角度，此方式不但可以改善其等深度控制時之穩態誤差，亦可加大載具俯仰舵角之可操作範圍，也因而提昇了載具縱向運動之操縱能力。另一方面，本文提出亦可透過將深度穩態誤差視為一偏差量，並於深度控制命令中扣除或於深度控制迴路中加入一條件式積分器之方式，電腦模擬結果證實此兩種方式均可有效地改善載具之深度穩態響應而不影響其暫態響應。

When an autonomous underwater vehicle (AUV) operates in the ocean, it is influenced by the ocean current, which leads to deviation of the AUV position. Hence, if there is no reference system able to correct the position drift and deviation, the error of estimated positions will grow with time. In such circumstances, cross track error control (CTEC) is no longer suitable for motion control. To obtain a more accurate position estimation of an AUV, it is necessary to eliminate the position error by fusing the message from other reference systems, such as a global positioning system (GPS), depth gauge, Doppler velocity log (DVL) with bottom track or a underwater acoustic positioning system. Since GPS signals cannot penetrate the ocean depth, AUVs have to rely on an acoustic tracking system such as long baseline, short baseline or ultra-short baseline combined with the ship GPS. Both the travel time and phases are employed to calculate ranges and bearings to a transponder mounted on the vehicle to determine a more accurate position and ocean current estimation. During a constant depth maneuver of an AUV, its pitch attitude and stern plane deflections create forces and moments to achieve equilibrium in the vertical plane. If an AUV has a proportional controller only in its depth control loop, then different weights or centers of gravity will cause different steady-state depth errors at trimmed conditions. In general, a steady-state depth error can be eliminated by adding an integral controller in the depth control loop. However, an improper integrator may lead to a bad transient response, even though the steady-state depth error can finally be eliminated. This study can be divided into two main parts, which include 1) Controlling the heading, navigation duration and longitudinal speed of an AUV in compensating ocean current effect during navigation by means of velocity summation of the heading and ocean current. 2) Eliminating the steady-state depth error of an AUV caused by different payloads by shifting the center of gravity of the vehicle along the body-fixed horizontal axis through moving a mass attached to a lead screw mechanism mounted in AUV. Other two methods alter the depth control loop by deducting the steady-state depth error from the depth command or by adding a conditional integrator in the depth control loop. Computer simulation results show that both methods can also improve the steady-state response effectively but do not worsen the transient response.