應用室內GPS技術進行史都華平台位置與姿態量測

Abstract

史都華平台利用六個驅動器以及連桿裝置來進行承載平台的六個自由度驅動（三維位置、三維姿態）。由於平台定位所需的位置感測器僅能裝置於驅動器上而非承載平台上，因此連動機構的誤差（例如：連桿變形、移動件的間隙等）將造成承載平台的定位誤差。本論文探討利用室內GPS定位系統進行承載平台定位／姿態判定的可行性，希望藉由將感測裝置直接置於承載平台上，來提高定位精度。 室內GPS包含超音波訊號的發射端與接收端，利用超音波訊號傳播的時間來做距離的量測，進而計算出待測物的距離與姿態。利用超音波訊號獲得距離的方式有二：一是利用到達時間差法（Time Difference Of Arrival）來獲得發射端與接收端的距離；二是利用載波相位差(Carrier Phase Difference)來獲得同一發射端到不同接收端的距離差。利用第一種方式獲得6DOF資訊的訊號處理較為簡單，距離量測的誤差約在公分等級；利用第二種方式的訊號處理較為複雜，但是距離量測誤差約在0.01公分等級。 本研究利用室內GPS技術中的達時間差法進行史都華平台定位與姿態判定。完成工作包括：硬體部分整合了史都華平台、室內GPS系統（CRECKET）、雙軸角度儀；軟體部分完成史都華平台逆向運動學控制介面的實現、使用「接收時間」法進行6DOF的訊號處理。藉此軟硬體整合，我們完成雙軸角度儀輸出與CRECKET六軸姿態判定的比較。比較結果與結論將於本報告中詳細說明。

The Stewart platform manipulators are parallel-type linkage systems that have been widely used in applications requiring accurate positioning and attitude determination. The platform, where the payload is located, is actuated by six single-axis actuators and the linkage system. And, the position sensors of the platform are the encoders accompanied with each actuator. Therefore, the positioning accuracy of the platform are often suffering from the manufacturing tolerances, installation errors, link offsets, and etc. This thesis studies the feasibility of adapting indoor GPS technology to the steward platform to improve the accuracy of the payload positioning and attitude determinations, In the GPS technology, one can use either “time difference of arrival” or “carrier phase difference” technique to determine the range between the beacon and listener. For most of the indoor GPS systems, the former approach is simple but the position accuracy is around 1cm. The later approach is accurate (around 0.01cm) but needs to resolve wave length ambiguity. This thesis proposed using the “time of arrival” technique of the GPS system to improve the positioning accuracy of the steward platform. Several works has been done so far to evaluate the feasibility, which includes system setup for the steward system, indoor GPS system (CRICKET, an indoor GPS system form commercial), and two-axes angle sensors; human-machine interface for operating the steward platform system; algorithms for the plate positioning and attitude determination. With the integration of these hardware and software, we are able to compare the output of the angle sensors and indoor GPS system. The positions accuracy of the proposed measuring system is discussed in detail in this thesis.